Geomorphology of Eridania Basin, Mars: A Study of the Evolution of Chaotic Terrain and a Paleolake

Transcription

1 Wesleyan University Geomorphology of Eridania Basin, Mars: A Study of the Evolution of Chaotic Terrain and a Paleolake By Keenan B. Golder Faculty Advisor: Dr. Martha S. Gilmore A Thesis submitted to the Faculty of Wesleyan University in partial fulfillment of the requirements for the degree Master of Arts Middletown, Connecticut May, 2013

2 Acknowledgements There are many people I would like to thank in regards to this thesis and all of the work that has gone into it, but there is simply not enough space to cover them all. First, I want to thank my committee members, Dr. Peter Patton and Dr. James Greenwood, for their patience and guidance in this project, and especially my advisor, Dr. Martha Gilmore. Without Dr. Gilmore s advice and direction I would not have found such a wonderful and engaging project, nor been able to extract so much interesting material from it, and for that I am extraordinarily grateful. Next I would like to thank the entirety of the faculty and staff in the Department of Earth and Environmental Sciences at Wesleyan. Virginia Harris and Joel Labella have been steadying presences and offered moments of calm with their chats. All of the faculty members have been open and welcoming, even when I would repeatedly pester them with questions. I am also thankful for the great friends and colleagues I have made while at Wesleyan, they have made this entire experience even more enjoyable. I also want to thank the Foye Fund for partial funding of my research and travel expenses to present my findings at conferences. Lastly, I want to thank my mother, Sandra Lesch, for her ever present love, encouragement, and support of my chosen to pursuits. No matter the situation or the difficulties faced, she has been there to ensure that I stayed focused and succeeded. Without her I would not be the person I am today, and I could not ask for anything more. i

3 Acknowledgements i Table of Contents ii List of Figures v Abstract vii I. Introduction 1 II. Background Previous Geologic Studies History of Water on Mars Water Sources Fluvial Systems Ma adim Vallis and Eridania Basin Basin Morphologies Hypsometry Chaotic Terrain and Possible Formation Mechanisms 20 III. Methodology Data Sets and Processing Thermal Emission Spectrometer (TES) Thermal Emission Imaging System (THEMIS) Mars Orbiter Laser Altimeter (MOLA) High Resolution Stereo Camera (HRSC) Context Camera (CTX) High Resolution Imaging System Experiment (HiRISE) ArcMap Software Suite Analyses Regional Mapping Unit Descriptions Unit Contacts Topographic Distribution and Topographic Profiles Elevation Contours Regional Stratigraphy Regional Surface Hydrology and Drainage Basin Delineation 28 ii

4 Basin Hypsometry Channel Distribution Channel Termini Distribution 30 IV. Results Primary Unit and Structure Descriptions Mountainous Terrain Electris Terrain Bright Electris Terrain Etched Terrain Chaotic Terrain Chaos Modification Basin Topographic Profiles Gorgonum Benches Hummocky Terrain and Mantling Deposit Crater Rims and Ejecta Tensional and Compressional Structural Deformation Minor Unit Descriptions Volcanic Edifices and Platforms Striated Terrain Glacial and Periglacial Terrain Geologic Map Stratigraphy Basin Stratigraphy Major Unit Elevation Distribution Catchment Basin Delineation Channel Analyses Summary of Results 80 V. Discussion Geologic Mapping and Stratigraphy 81 iii

5 Eridania Stratigraphy and Relative Timing Geologic History within Eridania Basin Paleolake Indicators Presence of Paleolake and Impact on Mineralogy of Bright Electris Terrain Chaotic Terrain Formation Mechanisms Models of Chaos Terrain Formation Mechanisms Aquifer Collapse Release of Water from Hydrated Sediments Basal Ice-Lens Melting Deflation and Erosion Chaotic Terrain within Eridania Eridania Chaos Formation Mechanism 97 VI. Conclusion 101 VII. References 103 VIII. Appendices Appendix A: Image Processing and Arc Analyses Appendix B: HRSC Image List Appendix C: CTX Image List Appendix D: HiRISE Image List Appendix E: Map Formatted Unit Descriptions Appendix F: Geologic Map Insert iv

6 List of Figures Figure 1: Global image indicating location 3 Figure 2: Mars geologic and chemical timescale 7 Figure 3: 1:15M global map mosaic 10 Figure 4: Map by Irwin et al. (2004) delineated 15 Figure 5: Type Section for mountainous terrain 34 Figure 6: Type section for Electris terrain 36 Figure 7: Type locality for Bright Electris Terrain 38 Figure 8: Type section for Etched terrain 40 Figure 9: Gorgonum Chaos exhibiting transitional terrain 42 Figure 10: Areal distribution of chaos terrain 43 Figure 11: SW margin of Atlantis Chaos 45 Figure 12: Location of chaos concentrations 47 Figure 13: Topographic transects across basins 48 Figure 14: Contiguous bench deposit with lobate scarps 50 Figure 15: Hummocky terrain draping Etched outcrop 52 Figure 16: Distribution of degraded crater rims 54 Figure 17: Distribution of tensional (Sirenum Fossae) 56 Figure 18: Two small volcanic edifices with flows 58 Figure 19: Subtle surface lineations 59 Figure 20: Lineated valley fill in Mountainous terrain 61 Figure 21: Individual distribution of the major mapped units 63 Figure 22: Final synthesis of the regional Eridania map 64 Figure 23: Chaos knobs in Gorgonum 66 Figure 24: Simplified local stratigraphy 67 Figure 25: Elevation distribution of major unit types 68 Figure 26: Distribution trends of Electris, Bright Electris and Chaotic terrains 70 Figure 27: Catchment basin delineation 71 Figure 28: Non-standardized basin hypsometry 74 v

7 Figure 29: Channel distribution within Eridania 76 Figure 30: Channel termini elevation distribution 78 Figure 31: Overlay of potential highstand lake levels 79 Figure 32: CRISM frames with mineralogy 90 Figure 33: CRISM image with Electris plateaus and Etched terrain 91 Figure 34: Proposed model of flood and collapse mechanism 93 Figure 35: Ice partially filling a crater 95 Figure 36: Emplacement of sill and dike 95 Figure 37: Process of deposition through deflation 96 vi

8 Abstract The Eridania basin in the southern hemisphere of Mars is hypothesized to have contained a large paleolake that has been identified as the source of the large outflow channel, Ma adim Vallis. Found within the confines of the paleolake boundaries are 5 degraded impact basins which contain concentrations of chaotic terrain, including: Ariadnes Colles, Atlantis Chaos, Gorgonum Chaos, and two unnamed basins. Chaos is typified by accumulations of angular mesas and knobs separated by fractures, which have been interpreted to be the result of subsurface collapse due to breaching of a confined aquifer with a general spatial association to large outflow channels. Chaos within Eridania are not directly related to any outflow features, and also lie at the lowest elevation points within the basins, therefore require a different interpretation for their formation mechanism. We have prepared a geologic map using CTX, HRSC, HiRISE, and MOLA data in order to better understand the history of water, the paleolake, and to constrain models for chaos formation. vii

9 I. INTRODUCTION Mars is a planet that has undergone extensive surface modification through time: from impacts, volcanism, tectonism, fluvial, lacustrine, and eolian activity. The evidence for these processes is preserved within the present-day surface, found in extant unit morphologies and mineralogy from exposed materials. Relationships between the various surface units, tectonic and volcanic structures, and modification or alteration mechanisms are well preserved. Extracting the stratigraphy of these geological markers allows for a reconstruction of the sequence of events as they occurred in the past, along with previously dominant environmental conditions. Through a thorough analysis of the available evidence, a reconstruction of the processes that shaped the surface of Mars can be determined. A growing body of evidence suggests that Mars was once a world that had significant quantities of surface water, as indicated by the presence of sinuous incised channels, large outflow channels, and possible lacustrine or oceanic shorelines in both the northern lowlands and southern highlands (e.g. Carr, 1979; 1996; Head et al., 1999; Carr and Head, 2003; Irwin et al., 2004; Fassett and Head, 2008). Tracing the evolution of standing bodies of water offers insight into changes to Mars climate over time and the impact this would have had on surface and subsurface hydrology. Climatic transitions on Mars would have also impacted the deposition or alteration of numerous mineral species, such as hydrated phyllosilicates or sulfates, while also having a significant impact of the formation and modification of surface features (e.g. Poulet et al., 2005; Mustard et al., 2008; Gendrin et al., 2005, Bibring et al., 2006). 1

10 Some of the most enigmatic regions on Mars are related to chaotic terrain. These are areas where the surface has been severely disrupted by various processes, generally associated with either gentle or catastrophic subsidence events (e.g. Sharp, 1973; Carr, 1979). Chaos knob fields are characterized by concentrations of jumbled angular mesas and blocks dissected by curvilinear fracture planes (Sharp, 1973), and are often associated with large outflow channels such as Ares Vallis (Carr, 1979). Understanding the relationship between the chaos fields and the outflow channels can offer an explanation for both groundwater and surface-water systems that were active early in Mars history. The presence of chaotic terrain can also be used as an indicator of the type(s) of stress that occurred within regions. Ma adim Vallis is one of the largest outflow channels on Mars, stretching for ~900 km; it originates along the margins of the Eridania basin, which has been postulated to have contained a large paleolake, and terminated along the dichotomy boundary where it flowed into Gusev crater (Fig. 1A; Irwin et al., 2004). Found in the eastern subbasin of Eridania are large chaos fields (Fig. 1B), originally identified by Scott and Tanaka (1986) and Greeley and Guest (1987), which are generally constrained within the interiors of heavily degraded basins, including: Ariadnes Colles, Atlantis Chaos, and Gorgonum Chaos (Grant and Schultz, 1990). These fields are atypical in that they are not located directly at the head of the Ma adim Vallis outflow channel, but at significant topographic depth within the basins, often with greater than ~1km of vertical relief. This indicates that the dominant formation 2

11 mechanism must be different from the generally accepted catastrophic collapse model. A B Figure 1: A. Global image indicating location of study area within Eridania basin. B. Eastern margins of the Eridania basin including the primary locations of chaotic terrain (white boxes.) 3

12 The purpose of this study are to create a geologic map of the eastern margins of the Eridania paleolake basin based on unique geomorphological characteristics to refine local stratigraphy and reconstruct a geologic history to determine how the surface evolved over time. This will include the reconstruction of the paleolake to clarify the parameters of highstand levels and how that relates to the filling and drawdown of the lake, while also determining the transition in the hydrologic system from precipitation-fed to one dominated by groundwater input. This study will also attempt to constrain the formation and/or triggering mechanism(s) responsible for the local chaotic terrain. Our work provides a new look at Eridania with improved data, which leads to a better understanding of the region and the processes that shaped it. The project goals are divided into six different tasks, including: (1) identify and classify units based on diagnostic geomorphological characteristics; (2) create a geologic map based on unit descriptions; (3) reconstruct the local stratigraphy to determine or refine the history of each unit and structure formation; (4) map the distribution of channel incisions and their termini; (5) identify possible paleolake highstand levels utilizing topography and channel distribution constraints; (6) identify potential mechanism(s) responsible for the formation of the atypical chaotic terrain fields. By mapping the eastern sections of the Eridania basin in detail and combining it with the above described analyses, a synthesis of the major geologic events that occurred within the basin can be constructed. 4

13 II. BACKGROUND The Martian surface is divided into three major chronostratigraphic divisions; the Noachian, Hesperian, and Amazonian. Each is based on the major geological processes that dominated those epochs, with absolute ages determined by crater superposition relationships (Fig. 2; Tanaka et al., 1992). The Noachian is characterized by rugged, heavily cratered basement material represented by the terrain in the Noachis quadrangle; the Hesperian is defined as the base of the ridged plains material represented by Hesperia Planum; and the Amazonian includes relatively smooth, moderately cratered plains materials and polar deposits represented by Amazonis Planitia (Scott and Carr, 1978; Tanaka, 1986; Tanaka et al., 1992). These classifications are further subdivided into Upper, Middle, and Lower series based on refined crater density data (Tanaka, 1986; Tanaka et al., 1992). In conjunction with the major geologic epochs, the prevailing environmental conditions had a significant impact on the deposition of mineral suites (Fig. 2). Early in the history of the planet conditions were most conducive to the deposition of phyllosilicates during aqueous alteration (Bibring et al., 2006). This era, the phyllosian, ended prior to transition from the Noachian into the Hesperian, where conditions then became conducive to the deposition of sulfates, during the theiikian. This is indicative of the transition of the planetary chemistry being dominated by evaporative and acidic conditions (Bibring et al., 2006). The final era, the siderikian, is dominated by anhydrous ferric oxide weathering, lacking significant surface water interaction (Bibring et al., 2006). These changes in the 5

14 hydrologic processes on the planet would have shaped much of what is found in the geology and mineralogy of the studied regions, and offer insight into the timing of emplacement of these units. Murchie et al. (2009) identified the preserved record of chemical environments, and placed them in their respective chronological order based on geomorphic and mineralogical data (Fig. 2). This chemical stratigraphy constrained the timing of the deposition of many of the observed mineral species found on the surface of Mars today. Within the context of the available dating schema described above, hypotheses for the major surface processes can be made. Ehlmann et al. (2011; and references therein) compiled a list of the primary processes that would affect the availability of water for direct mineral precipitation or alteration, coupled with the primary surface modification events (Fig. 2). Early in Mars history the planet had a global magnetic field as the core dynamo still operated. The loss of this protective field would have caused continued and rapidly increasing loss of atmospheric gases and affected the conditions that were required to keep surface waters stable over a long period of time. Mobilization of volatiles would have been greatly increased caused by the impact of large bodies, especially those responsible for the formation of the Hellas, Argyre, and Isidis basins. Not long after the formation of the crust, volcanism became a dominant and long-lived process that shaped the surface of the planet, but would have also been significantly affected by the presence of water, leading to alteration of the minerals present. Water itself would have had a relatively short-lived period in which it would have been stable on the surface of the planet, 6

15 Figure 2: (left) Mars geologic and chemical timescale. Mineralogical species delineated based on mineral species and morphological context. Murchie et al., 2009, after Bibring et al., 2006 and Tanaka, (right) Chronostratigraphic and mineralogical timeline: A. Retention age for a global magnetic field. B. Global-scale impact events. C. Large-scale volcanic provinces, with younger limited scope eruptive events. D. Depiction of the changes in environmental conditions and their impact on the presence of liquid water. E. Timing associated with valley network and outflow channel formation. F. Key mineral species generally formed by aqueous alteration. 7

16 likely not extending past the late-noachian. Post-dating this time period, surface waters would have shifted to a more ephemeral nature and valley network incision would have decreased. Outflow channels would later become the dominant expression of surface flows and would have resulted from the catastrophic release of confined aquifer systems. Utilizing the general constraints on the age of units through stratigraphic relations, morphological characteristics, and generalized mineral compositions can further refine the sequence of events on a regional or local scale, contributing to the description of specific processes and events that happened throughout Mars history Previous Geologic Studies Mapping of the surface of Mars has been performed since the earliest images were returned by orbiting satellites, including Mariner 9 and Viking 1 & 2. Early global maps were created which identified the major morphological units and structures, at a global scale of 1:25M (Scott and Carr, 1978) and 1:15M (Scott and Tanaka, 1986; Greeley and Guest, 1987). Regional maps were also created at a scale of 1:5M (De Hon, 1977; Scott, 1978; Mutch and Morris, 1979; Howard, 1979). These early workers placed the major surface units and structures in a global context, allowing for a comprehensive reconstruction of global stratigraphy. The early global maps by Scott and Tanaka (USGS I-1802-A) and Greeley and Guest (USGS I-1802-B) included the Eridania basin, and identified the major units and structures found within (Fig. 3). The units in the region are presented in stratigraphic order: Noachian Plains Hills (Nplh), Noachian Plains 1 (Npl 1 ), Noachian 8

17 Plains Etched (Nple), Noachian Plains 2 (Npl 2 ), Hesperian Plains 3 (Hpl 3 ), Hesperian Ridged Plains (Hr), Hesperian Chaotic Terrain (Hcht), Amazonian Knobby Plains (Apk), intermingled Smooth Crater Floor (s), Crater Rim & Ejecta (c), and Volcanoes (v). A further unit, Electris, was identified and described by Grant and Schultz (1990). Nplh is typified by a rough, hilly morphology resembling in part the Noachian basement material and older fractured units. This was interpreted as ancient highland volcanic rocks and impact breccia uplifted by tectonism and impact-basin formation. Npl 1 is described as a cratered unit within the plateau and high-plains sequence consisting of highly cratered terrain with moderate relief; fractures, faults and small channels. This material was likely formed during high impact flux, and is a mixture of lava flows, pyroclastic material, and impact breccia. Nple is an etched unit similar to Npl 1, but deeply furrowed by sinuous, intersecting, curved to flat-bottomed grooves that etch or sculpt the surface. It is interpreted as the cratered unit that has been degraded through eolian processes, decay and collapse of ground ice, and minor fluvial processes. Npl 2 is a subdued cratered unit, forming plains marked by subdued and partly buried old crater rims, with rare flow fronts present. This area is interpreted to consist of thin interbedded lava flows and eolian deposits that bury underlying deposits (Scott and Tanaka, 1986; Greeley and Guest, 1987). 9

18 Figure 3: 1:15M global map mosaic bounding the eastern Eridania basin, showing the early interpretations of unit types and structures found in the region. Geologic units described in text. Modified from Scott and Tanaka (1986), and Greeley and Guest (1987). 10

19 Hpl 3 is a smooth unit forming large areas of flat, relatively featureless plains which embay other units of the plateau sequence. Faults and flow fronts are rare in the unit. It is interpreted as thick interbedded lava flows and eolian deposits. Hr consists of ridged plains materials consisting of broad planar surface with flow lobes, containing mare-like wrinkle ridges. The unit is interpreted as extensive flows of low-viscosity lava erupted from numerous sources at high rates, with the ridges having been formed as volcanic constructions or compressional features. Hcht and Apk are mapped as different units between the two maps, but have similar morphology and are correlated. They are typified by a mosaic of highland blocks within depressions forming irregular conical hills or knobs, occurring at source areas and along margins of channels and within some chasmata and craters. These likely formed from a variety of processes, primarily erosion of older units (Scott and Tanaka, 1986; Greeley and Guest, 1987). Several broad unit types or structural features are present in the basin as well, including: (s) and (c) which are the remnants of crater materials including rims and ejecta, with varying relative ages with superposed or partly buried ejecta blankets. Volcanic edifices (v) are present and may have a central caldera and radial channels, with unknown relative ages in relation to surrounding terrain. The region is also extensively dissected by East-West trending, curvilinear parallel graben, along with North-South trending mare-like wrinkle ridges (Scott and Tanaka, 1986; Greeley and Guest, 1987). The combination of various terrain types including large impact sites, volcanic edifices, plains materials, hilly locations, chaotic terrain, extensional graben, 11

20 and compressional wrinkle ridge features indicate a complex geologic history within the basin and in the surrounding region History of Water on Mars Water on the surface of Mars was once prevalent with standing bodies of water present in both the northern and southern hemispheres. These may have been either ephemeral or stable lakes, or transient oceans (Fassett and Head, 2008; Head et al., 1999). These potential bodies of water were identified based on channel termination points, contacts with broad and flat regions that may represent shoreline sedimentation, and topographic analyses of possible terraces along the boundaries of these bodies (Parker et al., 1989; 1993; Head et al., 1999; Carr and Head, 2003). At the end of the Noachian period, at ~3.7 Ga, a climatic shift occurred that reduced overall atmospheric temperature and pressure resulting in the loss of atmospheric volatile content to space or through changing weathering and depositional processes, creating a much more arid environment (Fanale et al., 1992; Carr, 1999). These atmospheric changes reduced precipitation and caused large portions of Mars' water inventory to be incorporated into the cryosphere, including permafrost and polar deposits (Clifford, 1993; Carr, 1996; Clifford and Parker, 2001). After this environmental change, during the remainder of the Hesperian and continuing into the Amazonian, fluvial activity was limited to the formation of outflow channels sourced from local cryosphere melting through volcanism or impacts (Carr, 1979, 1996; Clifford, 1993; Tanaka, 1999; Clifford and Parker, 2001). Late fluvial activity was likely caused by hydrothermal groundwater mobilization or 12

21 localized precipitation events caused by condensation from volcanic venting (Gulick and Baker, 1990; Gulick, 1998, 2001; Dohm and Tanaka, 1999). Today, water based processes are a function of the cryosphere Water Sources Ephemeral and standing bodies of water were once common, fed by intricate and well-developed tributary systems along with extensive groundwater and atmospheric precipitation contributions (Baker et al., 1992). During the Noachian there were extensive valley network systems incised through the heavily cratered southern highlands (Carr, 1996). The highlands had been heavily modified, with extensive infilling and degradation of craters and superposition of channels with characteristic dendritic morphology (Carr, 1996). The major contribution of water into these systems would have been dominated by precipitation-fed groundwater aquifers, with periodic surface water input (Andrews-Hanna et al., 2007). Groundwater movement to these bodies of water would have been controlled by the regional topography, especially through growth of the Tharsis rise which would have altered regional hydraulic head, raising the water table to the point that it intersected with the surface (Andrews-Hanna and Phillips, 2007) Fluvial Systems Channel morphologies along with headwall incisions offer indicators for the source of the waters necessary for the resultant erosion. Integrated dendritic forms of valley networks suggest formation from precipitation fed runoff (Masursky, 1973; Milton, 1973). When channels initiate along the margins of a drainage divide, 13

22 especially when found within high-relief regions, direct atmospheric precipitation is the likely source for the water required to denude the highlands (Craddock and Maxwell, 1993; Hynek and Phillips, 2001; Mangold et al., 2004). Many channels also appear to originate without a clear source area. These are apparent within the mapping region, where they exhibit amphitheater morphologies of their headwalls. This, coupled with other planar attributes, is indicative of groundwater sapping processes feeding these channels, with further growth governed by headwall retreat as water is released (Pieri, 1980; Howard et al., 2005). Discharge rates determined from channel dimensions, including channel length and width, and depth of incision into Noachian surfaces imply that surface runoff from precipitation or rapid melting of accumulated snow or glacial terrain could not account for the channel morphologies associated with the oldest Noachian to earliest Hesperian terrains (Howard and Moore, 2005). Fluvial activity during most of the Noachian period was characterized by widespread erosion of the highlands and crater rims, with deep infilling of crater floors and intercrater plains, with the development of local drainage systems (Howard and Moore, 2005). Combining these channel characteristics with the groundwater evolution described by Hanna and Phillips (2006) suggests an integrated hydrologic system where the groundwater system dominated but also had contributions from an overland flow system Ma adim Vallis and Eridania Basin The Eridania basin covers an area of ~3 x 10 6 km 2, while the lake itself may have covered an area of upwards of 1.1 x 10 6 km 2 (Fig. 4, Irwin et al., 2004). The 14

23 head of Ma adim Vallis originates at two spillway points along the margins of the Eridania basin, indicating a breach of a previously self-contained lake that eroded the outflow channel (Irwin et al., 2002, 2004). The main tributary channel is bifurcated near its inception point, found within an unnamed intermediate basin between Ma'adim and Eridania, with the larger channel to the east (Irwin et al., 2004). The western branch of the valley is less deeply incised. The channel heads likely retreated headward during their formation, as their upstream position is within the body of the basin that was the source of the water, instead of at the spillway (Irwin et al., 2004). Figure 4: Map by Irwin et al. (2004) delineated the Eridania drainage basin (white contour) and the approximate highstand level of the paleolake at ~1100 m (blue contour). White box denotes mapping area for this study. The source of the waters that filled Eridania is likely groundwater recharge, with minor contributions from overland flow (Andrews-Hanna et al., 2007). While 15

24 many large outflow channels on Mars originate directly from chaotic terrain, which is not the case in the Eridania region. In contrast, the water required to fill the Eridania basin and initiate the overspill event that formed Ma adim Vallis may have come from the raising of the regional water table during growth of the Tharsis rise. If the water intersected the surface, groundwater discharge could fill deep surface depressions (Andrews-Hanna and Phillips, 2007). Although there are large concentrations of chaotic terrain located within the lowest reaches of several heavily degraded impact basins contained in the eastern reaches of the Eridania basin they are unlikely to be the direct source for Ma adim Vallis because of their elevation, which places them at least ~600 m below the spillway points at the valley head. Irwin et al. (2004) identified three elevation constraints to determine the presence of a paleolake in the Eridania basin; (1) overflow elevations into Ma adim Vallis, (2) the elevation of boundary/slope transitions from high-relief regions to basin interior plains, and (3) the termination elevation of valley networks that flow into the basins. Elevations of the spillways range between 1100 and 1250 m. The transition point between high-relief outcrops and the basin interior plains is between ~1000 and 1100 m. Valley networks had a common termination between 950 and 1100 m. Some channels were found to extend down to 750 m, but did not extend farther into the basin interiors. Based on these elevation measurements, Irwin et al. (2004) constrained the upper elevation limits of the lake between 1100 and 1250 m. 16

25 2.4. Basin Morphologies The morphologies of basins are initially controlled by the original impact event, as material is excavated from the point of impact and deposited in an ejecta blanket. A study of ~7000 fresh impact sites on Mars led to the determination that the diameter of the basin was directly proportional to the depth of excavation, represented by the equation: d e = 0.36D 0.49 a (Garvin et al., 2000a, 2000b). Here d e is the final depth of excavation, 0.36 is a constant related to the transition from simple to complex craters (at ~6km dia.), D a is the final crater diameter at the rim crest, and 0.49 is the distribution exponent. Later modification of craters generally leads to a degradation of the initial bowl-shaped interior, resulting in a relaxed shallow-slope interior (Forsberg-Taylor et al., 2004) The degraded basins found within the eastern margins of Eridania exhibit unusually high internal vertical relief along with high slope gradients. These were identified by Howard (2000) when characterizing crater populations. He found three distinct populations of degraded crater morphologies in the southern highlands within a latitudinal range of 20 N to 60 S (Howard, 2000). Degraded basins under ~60 km in diameter slope symmetrically inward towards the basin center, with a gradient of 0.5 to 1.5 (Howard, 2000). The depth of the basin concavity generally increases with the size of the crater, to a maximum of ~250 m for 60 km craters (Howard, 2000). This morphological group is interpreted as having formed as alluvial fans composed of debris removed from the interior crater 17

26 walls, though ephemeral playa lakes may have also been present in the interiors of these craters (Howard, 2000; Craddock and Howard, 2002). Craters in the 60 km to 100+ km range show a decrease in crater floor relief with increasing diameter, often with a flat-floor consisting of less than 100 m of vertical relief (Howard, 2000). This is typical for craters up to ~200 km in diameter. Two explanations for this morphology present themselves: (1) flooding of the basin interiors by lava; and/or (2) the basins contained shallow ephemeral or perennial lakes that through wave action redistributed sediment (Howard, 2000). Howard preferred the second hypothesis because of the prevalence of large basins with this morphology. Basins with diameters of 150+ km occasionally have high internal relief, in excess of 500 m (Howard, 2000). A cluster of these atypical basins is found between S and W (this includes the eastern margin of Eridania and the chaotic terrains). These craters have retained high internal relief even though they are highly degraded as evidenced by the heavily eroded rims. Some of the basins contain well-defined concentric terraces surrounding the basin interiors which are interpreted as marginal terraces such as sedimentary fans, deltas, or shelves formed within perennial seas (Howard, 2000; Craddock and Howard, 2002). The main subbasins within Eridania; Gorgonum, Atlantis, Ariadnes, and several unnamed basins exhibit this atypical basin morphology. Gorgonum and Atlantis exhibit the greatest internal relief, in excess of 1 km. Ariadnes and the unnamed basins have an internal relief up to ~500 m. These large basins have 18

27 retained much of their original internal relief, and this may be in part due to redistribution of sediment within the basin margins instead of concentration within the deepest portions of their interiors during infilling Hypsometry Through the use of topographic data, the morphology of a basin can be used to distinguish between runoff or precipitation as the source of the water that drives the primary drainage mechanism, as well as an assessment of which drives the basin degradation process (Luo, 2002). This measure of the basin area versus elevation characteristics can be used to identify processes influencing valley formation, and the subsequent characteristics of those valley networks, (Luo, 2000, 2001, 2002). Martian basins are dominated by impact mechanics forming the drainage basins with minimal fluvial modification affecting efficient drainage, suggesting that hypsometric analyses may be a poor indicator of the style of erosion (Grant and Fortezzo, 2003; Fortezzo and Grant, 2004). Aharonson et al. (2002) argued a similar point based on a lack of topographic equilibrium of terrains, indicating that surface runoff did not occur over a prolonged period of time. Irwin et al. (2004) argue that due to dissimilarities between the Earth and Mars, Martian drainage inefficiencies due to physical, climatic, and surficial properties could be accounted for with a homogeneous runoff model. Another aspect of Irwin s work identified the noncumulative hypsometric curves of standard degraded basins peak at the lowest elevations, where the area is concentrated, while those in Eridania have infilling material that concentrated around the margins of the crater floors, so that the internal 19

28 relief of the craters was maintained. Stepinski and Coradetti s (2004) model inferred a heterogeneous water input but it favored neither runoff nor sapping as the source Chaotic Terrain and Possible Formation Mechanisms Chaotic terrain was identified from early Mariner 6 images (Sharp et al., 1971), and is characterized by an irregular jumble of angular blocks of varying size, which often preserve the remnants of the smooth upland surfaces (Sharp, 1973). The blocks range in size from <1 km to several tens of km across, and are bounded by steep escarpments (Sharp, 1973). There is an apparent transition of the chaotic terrain from a slightly fractured upland zone into a progressively more degraded and fractured zone, terminating in a highly fractured, degraded jumble of pyramidal knobs (Sharp, 1973). Chaotic terrain is hypothesized to be the result of surface collapse filling an evacuated subsurface cavity formed during the catastrophic release of a confined aquifer (Carr, 1979). The typical mechanism would have involved the aquifer being filled through groundwater recharge, and capped with a resistant ice layer. Infiltration from overland flow would have contributed to aquifer recharge prior to the capping event (Andrews-Hanna et al., 2007). Through erosive processes, the capping layer would have been exposed to the atmosphere, where it would have melted and sublimated. This would have weakened the cap leading to a breach and the catastrophic release of water from a high-pressure aquifer system. This flood then caused rapid scouring, eroding and transporting large quantities of sediment that ultimately formed the outflow channels. 20

29 Chaotic terrain is typically associated as the source area for many of the large outflow channels found on Mars (Carr, 1979; 1996). The basic processes involved the breaching of a subsurface aquifer with direct catastrophic surface discharge. The chaotic terrain in this region is atypical in that it cannot be tied to a concrete location for a breach, or an outflow channel this would have sourced. Instead, this terrain is concentrated within the deepest points of the region, especially within the interiors of the bowl-shaped degraded subbasins that formed the main body of this portion of the Eridania paleolake. Additional models have been proposed to explain the presence of chaos, including: (1) release of water from hydrated sediments, (2) melting of a basal ice-lens, and (3) deflation and erosion of surface sediments. These hypotheses are described in greater detail in section

30 III. METHODOLOGY To determine the interrelationship between units, various analyses were chosen to highlight each aspect of the basin and its evolution. Mapping of the geomorphological units was used to identify the major geological components in the region and reconstruct the stratigraphy to delineate relative ages for the dominant processes. Thermal Emission Spectrometer (TES) and Thermal Emission Imaging System (THEMIS) analyses were performed to clarify compositional differences between units, such as if the material is loose eolian sediment or indurated bedrock material. Mars Orbiter Laser Altimeter (MOLA) analyses were used to highlight topographic variability and unit boundaries. Combining similar analyses provides insight into processes of chaotic terrain formation. Regional surface hydrology and basin hypsometry were used to refine previous work on the presence of a paleolake and the upper limits on its depth. Approximations of the lake highstand levels were made using channel distribution and channel termini distribution Data Sets and Processing Various visible (VIS), near-infrared (NIR), thermal IR (TIR), and altimeter data sets were used in the investigation of the Eridania basin. Each offered an expanded perspective on the interpretation of the region and the delineation of primary morphological units and structures. The VIS image data offers a significant improvement in spatial resolution over the previous data sets (e.g. Mariner and Viking) that were utilized in early mapping and interpretations of the basin. 22

31 Each data set required varying levels of pre-processing prior to use in the Arc software suite. Once imported into Arc, numerous analyses were performed to determine the morphological characteristics of each unit, aiding in the final determination of the distinctive characteristics for each unit type. These analyses offered new criteria for identifying possible highstand levels for the paleolake that occupied the basin along with constraints on the mechanisms that resulted in the formation of the chaotic terrain within the subbasins Thermal Emission Spectrometer (TES) TES instrumentation onboard the NASA spacecraft Mars Global Surveyor (MGS) is utilized to characterize the thermal properties of both the Martian surface and atmosphere by: (1) determining the mineral composition of surface rocks, minerals, and condensates; (2) identifying the composition and distribution of atmospheric dust and condensate clouds; and (3) and creating temperature profiles of atmospheric CO 2. This is accomplished using three instrument components, (1) a Michelson interferometer, (2) a solar reflectance sensor, (3) and a broadband radiance sensor. The spectrometer covers a wavelength range of 6 to 50 µm, the solar reflectance band covers 0.3 to 2.7 µm, and the broadband radiance channel covers 5.5 to 100 µm. Each sensor has a spatial resolution of 3 km at the nadir (Christensen et al., 1992) Thermal Emission Imaging System (THEMIS) THEMIS instrumentation onboard the NASA spacecraft Mars Odyssey is utilized to characterize the surface mineralogy and physical properties through the use 23

32 of thermal-infrared, near-infrared, and visible image bands. The multi-spectral instrumentation produced images in three categories: (1) 9 band thermal-infrared images forming day- and nighttime mosaics with ~100% global coverage at 100 m/pixel resolution; (2) 5 band visible/near-infrared forming 5-band color images with a global area coverage of several percent; (3) 1 band visible images with ~60% global coverage at 18 m/pixel resolution (Christensen et al., 2004) Mars Orbiter Laser Altimeter (MOLA) MOLA instrumentation onboard the NASA spacecraft MGS is used to map the topography of Mars surface. The main instrument consists of a 1064 nm laser transmitter that acquires data with a maximum vertical accuracy of ~1.5 m, with an along track spatial resolution of ~300 m (Zuber et al., 1992) and gridded topographic data at ~463 m/pixel (Smith et al., 2001). The MOLA global mosaic data set was integral to identifying the topographic relationship between units and structures, clarifying the distinctions between structurally modified regions, and those that were not heavily modified. These data were also used in the creation of basin transects to identify any commonality in chaos outcrop elevations High Resolution Stereo Camera (HRSC) HRSC instrumentation onboard the ESA spacecraft Mars Express is utilized to characterize the surface structure and morphology, surface topography and atmospheric phenomena. The camera unit consists of a VIS stereo color scanner and the Super-Resolution Channel (SRC). This enables the acquisition of high resolution 24

33 color images and stereo pairs used in the creation of digital terrain models (DTMs). The horizontal resolution ranges from ~2.5 m with area coverage of 1%, to ~15 m with area coverage approaching or exceeding 50%; vertical resolution is on the order of ~10 m (Neukum et al., 2004). HRSC images (Appendix B) were acquired through the Planetary Data System (PDS) geosciences node administered by Washington University in St. Louis at: Context Camera (CTX) CTX instrumentation onboard the NASA spacecraft Mars Reconnaissance Orbiter (MRO) is utilized to provide regional context of surface features for data acquired by other MRO instruments while also providing high-resolution images for the determination of geologic, geomorphic and meteorological processes on Mars. The instrument consists of a VIS imager with a channel range of µm, providing ~6 m spatial resolution (Malin et al., 2007). CTX images (Appendix C) were acquired through the PDS geosciences node High Resolution Imaging System Experiment (HiRISE) HiRISE instrumentation onboard the NASA spacecraft MRO is used to characterize surface geology at very high spatial resolution while acquiring stereo image data to create high precision topographic profiles. The instrument consists of a primary camera unit acquiring data in the VIS spectrum with a horizontal spatial resolution of ~ m with global area coverage of ~1%, with a vertical precision within ~25 cm (McEwen et al., 2007). HiRISE images (Appendix D) were acquired through the PDS geosciences node. 25

34 3.2. ArcMap Software Suite Analyses ArcMap versions 9.3 and 10.1 were used to create a geologic map covering the region of interest, along with thermal, topographic, and hydrologic analyses used to characterize and identify distinct units and structures. ArcScene was used to create three-dimensional representations of the region to highlight topographic variations Regional Mapping The study area is bounded between 170 E E and 30 S - 45 S. This encompasses the eastern margins of the Eridania paleolake basin identified by Irwin, et al (2004). Contained within the region are 5 subbasins with large chaotic terrain concentrations. Once the primary geomorphologic units were identified and described, mapping of the areal extent of the subbasin commenced. Contacts between the primary units were identified and mapped in the ArcMap software suite. Units are represented by polygons and structures by polylines. A geodatabase (.gdb) file was created to store the unit and structure polygons and polylines, and set with a standard projection containing the parameters outlined above Unit Descriptions Units were delineated using the above described datasets, with CTX images used as the basemap to refine the descriptions put forth in the global 1:15M maps by Scott and Tanaka (1986) and Greeley and Guest (1987). Having utilized higher resolution images allowed for refinement of morphological units and contacts Unit Contacts 26

35 Unit contacts were determined primarily using CTX images, supplemented with HRSC and THEMIS IR-daytime mosaics where necessary. Contacts were based on the unit descriptions and clear delineation between two unit types. Clear morphological distinctions were the primary method for determining the transition point from unit to unit. The areas where the contacts were not distinct required the placement of an implied-contact polyline Topographic Distribution and Topographic Profiles Topographic distribution of the units within the basin is representative of their areal extent. This offered further evidence to the stratigraphic order, by placing the units into context with each other, and clarifying the placement of one unit in relation to another. This also placed limits on the areal and/or elevation extent for specific units, refining descriptive characteristics. Investigating the transition between units, or locating distinct breaks in slope required transects to be performed throughout the region. This was performed using the 3D Analyst tool in Arc. Profile locations were determined depending on the context of the investigation point. Point (e.g. A, B, etc.) locations were selected along with the corresponding Point (e.g. A, B, etc.). A transect histogram was then generated to provide a visual representation of the location profile. These were then used to identify any primary surface characteristics and their orientation with relation to the defined transect. 27

36 Elevation Contours Elevation contours were created using the 3D Analyst tool in Arc. Several iterations were performed depending on the required scale of contours for image interpretation. Using MOLA topography, 50 and 100 m contour interval rasters were created. This was performed for several reasons. It was necessary to ensure that the vertical relief of units was clear, and that there was no ambiguity in the relationship between a high vertical relief unit and the transition into broad shallowly dipping plains. It was also necessary when trying to determine if there were common elevation points related to this high-relief to basin transition, along with if there were common elevation points related to complete channel or channel termini distributions. This was also used in the development of the common lake elevation levels based on the channel termini, channel distribution, and slope transition points Regional Stratigraphy The stratigraphic relationship between units was determined through image interpretation of unit types and their corresponding contact points. Any indications of superposition, onlapping, embayment, or cross-cutting were used in the identification of the relative age of the units. Once the relative ages were determined, a framework for the stratigraphy was constructed Regional Surface Hydrology and Drainage Basin Delineation The manner in which water flowed along the surface offers insight into the behavior of the water and sediment within the basins. Modeling of the basin hydrology allows for the determination of surface flow direction and the delineation 28

37 of individual drainage subbasins, which are used to develop a profile of basin hypsometry Basin Hypsometry The drainage basin shapefiles created using the Arc hydrology tools were used to calculate the hypsometry of the primary subbasins in the region. This was calculated by extracting a mask of MOLA topography using the boundary of each basin shapefile. As a non-standardized hypsometric curve, the resulting graph is an indication of the areal distribution within the basin. Spikes in the profiles indicate high surface area within the basin, and allow for the identification of any potential shorelines. Due to the uncertainties between the models described in Section 2.4., we offer a non-standardized hypsometric analysis of the drainage basin areal distribution, coupled with full channel and channel termini elevation distributions to look for common points of area increase within the individual subbasins to identify putative shoreline features Channel Distribution The presence of valley networks within the region is indicative of extensive overland flow of running water. These were mapped based on several criteria, including: (1) a clearly identifiable sinuous incision into underlying surface materials; (2) identifiable initiation point (e.g. source area located along drainage divide or within plains) or clear headwall morphologies (e.g. amphitheater style headwalls); (3) a continuous channel body, or one that can clearly be inferred based on large fragmentary components; (4) removal of short channel bodies around crater rims (e.g. 29

38 recent gulley formations); (5) rejection of fragmentary channel bodies without clear initiation or termination points; and (6) rejection of flow features that originate at potential volcanic structures. Channel distributions within the region offer another avenue for investigation. By mapping the location of the channel bodies from their initiation points to their termini and extracting their elevation characteristics a regional distribution can be reconstructed. This approach is used to identify any elevation points that correspond to significant declines in the presence of these channel bodies. Such a decline would be indicative of the termination of incisions made by surface flow, likely corresponding to a contact point with a standing body of water. To ensure that the channel distribution was not skewed due to the area of the basin, the data were normalized based on the total basin area per channel elevation point. This allowed for a generalized trend for channel distribution increases or decreases at each elevation point in the basin Channel Termini Distribution The elevation points of the channel termini could offer insight into any common elevation points between the basins, which may be indicative of the presence of a standing body of water. To determine the point at which a channel actually terminated was difficult, due to the highly fragmentary nature of the channel remnants within the basins. Several criteria were required to ensure that a valid termination point was used. These included: (1) a clear and common termination of channels along the contact between unit types (e.g. Electris plateau scarps along 30

39 Etched terrain, Gorgonum 0 m proposed shoreline); (2) removal of fragmentary channel points that had no clear origination or termination point; (3) identifying the termination points based on the break in slope associated with the transition from mountainous terrain into low-lying flat basin interiors. By identifying common elevation points for the termination of channels, a determination of the possible highstand levels of the lake could be made. 31

40 IV. RESULTS The various analyses described above were used to create a synthesis of the geologic and hydrologic history of the eastern Eridania basin, while also attempting to discern a primary mechanism for the formation of chaos within the basin interiors. These included initial unit descriptions, creation of the geologic map, refining local stratigraphy, analysis of channel distribution in the basin, analysis of channel termini elevations, and transects across basins with chaos fields Primary Unit and Structure Descriptions Based on the geomorphological characteristics of each unique unit found within the basin, refined criteria for descriptions were determined, thus ensuring proper mapping was performed on each unit contact. Unit descriptions formatted for the map are located in Appendix E; full unit descriptions are as follows, in stratigraphic order from oldest to youngest: Mountainous Terrain Distributed throughout the region, the massifs and ridges cover a relatively small proportion of the area, approximately 1.3 x 10 5 km 2 within the basin. Morphologically distinct from the low-lying surrounding terrain, mountainous material is high standing, often exceeding m or more in vertical relief above the plains units (Fig. 5). With this high relief, the exposures of this unit can exhibit either a shallow or steep slope, oftentimes both on the same outcrop. These regions are saturated with small impact craters, many of which are significantly degraded. The surface texture is irregular, hilly and knobby. Scarps within the unit 32

41 are generally subdued and have shallow slopes, which appear to be the result of burial by eolian deposits. The ridge crests associated with the largest basins are heavily degraded, no longer preserving the sharp ridges associated with younger fresh craters. The lowest extent of this unit has irregular margins that are generally embayed by various units, dominated by Electris and Etched terrains. These mountainous units are surrounded by younger plateau and plains materials along with impact ejecta, placing the outcrops of this unit as the stratigraphically oldest components within the region. We interpret the outcrops of this unit to be comprised of the remnants of heavily degraded large impact basins and/or isolated high standing peaks of uplifted crustal material, which agrees with the interpretations set forth by Scott and Tanaka (1986), and Greeley and Guest (1987). These units may have also been stripped of hundreds of meters of surface material, providing the sediment for crater infilling or plains materials (Hynek and Phillips, 2001; Craddock and Howard, 2002). This unit contains a high proportion of the mapped valleys and channels. The valleys are generally linear to sub-linear and comprised of branching tributaries feeding into steep-sided v-shaped valleys. They are concentrated on, but not confined to, steep slopes. Initiation points for these channels are often located near drainage divides, indicating direct precipitation as the source for the waters feeding the valley networks. Many of the networks associated with this unit terminate at the break in slope that corresponds with the transition into the surrounding plains units. 33

42 A B C Figure 5: A. Type Section for mountainous terrain. Characterized by high vertical relief, high crater density, clear ridge crests, and dissection by high density valley networks. B. Isolated massif in the center of the region, with heavily denuded upper margins and regular valley incisions along flanks (blue lines). Dissected by large fault running N-S. Likely remnant of a volcanic edifice or ancient crustal material. C. Isolated massif in the East, with margin embayed by Electris and Etched terrains, indicating that the Mountainous unit predates the other units. All CI = 500 m. 34

43 Electris Terrain An extensive unit with a wide distribution covering an area of approximately 3.9 x 10 5 km 2, Electris is a thick plateau sequence that fills much of the lower-lying plains and is exposed along the margins of the older mountainous terrain. This terrain is characterized by either contiguous or isolated plateaus, often exceeding several hundred meters in thickness (Fig. 6). This unit is morphologically distinct due to steep arcuate, scalloped scarps. Areal distribution of the material is concentrated within low-lying plains where it onlaps the margins of mountainous terrain, though limited exposures are found atop mountainous outcrops. This signifies that Electris material is younger than mountainous basement material. The margins are distinguished by their distinct scarps, while the contiguous portion of the material is moderately cratered, relatively smooth in texture, and has a shallow slope which mirrors the underlying basement material. Individual scarps can be highly degraded with shallow angles of repose and a degradation of the scalloped margins; these may be identified by talus piles or slumps. Originally identified by Grant and Schultz (1990), and later supported by Grant et al. (2010), Electris terrain is described as an ash airfall deposit. Terraces along scarp boundaries and within crater interiors and indications of periodic erosion of the unit by various fluvial, lacustrine, or eolian processes suggest separate periods of deposition. The unit is deeply dissected by valley networks that are characterized by steep-sided walls, relatively high sinuosity well-developed tributary systems with flat floors. The headwalls of the valley networks generally begin within the plains or 35

44 along the margins of the highlands, and have amphitheater-head morphologies indicating groundwater sapping as the dominant erosional process (Howard et al., 2005). The channels have a tendency to widen significantly as length increases, with the channel width often exceeding 5 km at the mouth. Fresh exposures of Electris deposits along the scarps or upper surfaces reveal underlying bright material. A B C Figure 6: A. Type section for Electris terrain. B. Contiguous Electris deposit with dendritic channel incision. Numerous well-developed tributaries feed the main body of the channel, which then terminates at the contact with Etched terrain. C. Isolated Electris plateau exhibiting typical lobate, scalloped scarp, standing approximately 300 m. All CI = 100 m. 36

45 Bright Electris Terrain Associated with Electris and Etched terrains, bright material exposures are concentrated within individual Electris plateaus, along scarps, within eroded chaos knobs (see section ), and beneath Etched surfaces (Fig. 7). This terrain covers an area of approximately 1.8 x 10 4 km 2. The material has a very high albedo and a high thermal inertia in comparison to surrounding material indicating it is well consolidated. In HiRISE images large polygonal fractures on the scale of tens of meters are visible. These fractures are common to most of the bright material exposures, though they vary between small polygonal fracture types and large fractures dissecting chaos knobs that may reach hundreds of meters in length. Those exposures that are not heavily fractured exhibit an apparently well-indurated surface that is generally smooth, though regular pitting and small knobs are present. These surficial features may be erosional, as they are prevalent in areas where overlying material has been removed, exposing the underlying bright material. Outcrops of Bright Electris terrain decrease dramatically above ~1100 m in elevation, a possible constraint on any formation process of the bright materials as this elevation correlates to the maximum highstand level within the paleolake (see section 4.7). This unit was named Bright Electris due to its intrinsic near-inseparability from Electris terrain and may be an alteration product related to the original ash airfall deposit. Previous work on chaos knobs in the region point to the presence of phyllosilicates, sulfates, and chlorides within comparable bright material exposures (Noe Debra et al., 2008; Grant et al., 2010; Gilmore et al., 2011; Annex and Howard, 37

46 2011; Wendt et al., 2012). Grant et al. (2010) have revised their earlier interpretations of the Electris to include the chaos knobs as possibly reworked Electris material, rather than a separate unit that postdates it. A B C Figure 7: A. Type locality for Bright Electris Terrain. Bright material exposed along the scarp and the upper extremities of an Electris plateau. B. Bright Electris exhibiting meter-scale polygonal fractures (SE of red line), and a more consolidated underlying member of bright material. C. Bright Electris exposed from beneath Etched terrain in an unnamed basin in the East of Eridania. 38

47 Etched Terrain This terrain type is composed of various members that are prevalent over a large portion of the basin. Primarily constrained within the plains, though not restricted exclusively to low-lying terrain, this unit is found to cap or embay older underlying units. This terrain type covers an area of approximately 5.2 x 10 5 km 2. The various members are stratigraphically overlapping indicating periodic resurfacing processes over time. The Etched terrain overlies nearly all of the units, except the most recent and spatially limited expanses of the Gorgonum, Hummocky and Mantling deposits. Members embay the margins of older units while also filling depressions resulting in subdued topography. This terrain is further characterized by outcrops of variable brightness which range from low to intermediate, along with a variable thermal inertia. The surface texture is dominated by irregular knobs and hills that generally follow topography (Fig. 8). Etched members often terminate in lobate scarps when capping other materials, including other Etched members, and the scarps rarely exceed 10 m in height. Mare-like wrinkle ridges are common throughout which generally have circumferential orientations within the large basin interiors. This unit also has a preponderance of well-preserved small craters with limited evidence of extensive modification post-impact. We interpret this unit to be comprised of various lava eruptions, ranging from early regional resurfacing events that filled the entirety of the basin, to late-stage localized eruptions with no apparent source that only filled the interiors of individual sub-basins. Gorgonum basin also 39

48 has preserved populations of what appear to be small rootless cones, which are the result of localized volcanic extrusion in the presence of subsurface ice. Etched members are often found to be incised by spatially limited valley networks that are generally constrained within basin interiors. These are characterized by sinuous, dendritic valley networks that are not deeply incised into A B C Figure 8: A. Type section for Etched terrain. This exposure contains two distinct members, with the younger capping member exhibiting a lobate termination. B. Etched member intruding between individual chaos knobs in Atlantis Chaos. C. Two distinct etched members, with overlying younger member exhibiting lobate scarp, along with a prominent mare-like wrinkle ridge. 40

49 the unit. These late-stage, Hesperian to Amazonian Mid-Latitude Valleys (MLV) have been described by Howard and Moore (2011). The valleys do not begin within the Etched members, but are found either within the extensive Electris deposits or the Mountainous terrains. The channels extend from their source areas and drain to the lowest topographic point usually correlating to the interiors of the major subbasins. This unit is further includes outcrops that contain eolian or mantling deposits (see section ), but owing to the difficulty separating these surface textures in CTX images are partially included in limited Etched terrain outcrops. This is because of the similarities of the morphological textures in low-resolution images Chaotic Terrain Chaotic terrain is typified by a random assortment of flat-topped mesas and pyramidal knobs measuring between tens of meters to tens of kilometers. This terrain type covers an area of approximately 7.2 x 10 4 km 2 within the basin. Individual blocks are bounded by steep-sided linear to curvilinear scarps (Fig. 9A). The knobs are preserved to varying degrees, with some suffering little apparent degradation while others have been reduced to bare hills (Fig. 9B). Modification of the knobs is not concentrated in any specific portion of the knob field as degraded outcrops can be found along the outer margins or the interior of the fields. Chaos is generally concentrated within the interiors of the 5 major subbasins of Eridania, and found within the same stratigraphic interval. Within the subbasins most chaos distribution occurs below the ~500 m elevation mark (Fig. 10), and then generally follows the 41

50 A B C D Figure 9: A. Gorgonum Chaos exhibiting transitional terrain trending NW, from flat basin floor to severely disrupted and degraded knobs. Note bright exposure (red arrow). B. Isolated chaos knobs in Ariadnes. C. Knobs capped by Etched member in Gorgonum. D. Interior of individual knob in Atlantis exhibiting m-scale polygonal fracturing, and larger fractures that dissect the entirety of the knob proper. 42

51 basin topography. Etched terrain is often found filling the margins between the knobs, and where not present a mantling eolian fill is present. Many individual knobs retain a cap of competent material, identified as Etched material (Fig. 9C). When the capping material is missing bright material is exposed, which exhibits characteristics that are indistinguishable from Bright Electris terrain including meter-scale polygonal fracturing (Fig. 9D). Several locations along the margins of Atlantis Chaos and UB- 2 contain Electris plateaus dipping into the interior of the basin that display characteristics consistent with direct modification to form chaos knobs, suggesting they are the erosional or structurally deformed remnants of the original Electris deposits, consistent with Grant et al. s (2010) reinterpretation of the evolution of the Electris. Figure 10: Areal distribution of chaos terrain within Eridania. Knobs fields are greatly restricted to elevations below ~500 m with rare, small fields found above this elevation. 43

52 Chaos Modification Isolated portions of the chaos knob fields have an apparent correlation to the Electris and Bright Electris deposits. This relationship offers a potential direct connection to the surface deposits that ultimately were modified to form the chaos fields. One region where this connection is clearly exposed is along the SW margins of the Atlantis Chaos basin (Fig. 11A). Along this stretch of the basin isolated Electris plateaus appear to transition into chaos knobs. The transect across the plateaus indicates a gentle dip of the surface deposits following the basin topography (Fig. 11B). The plateaus are disrupted as they progress into the interior of the basin, forming distinct mesas and pyramidal knobs. Surfaces where the capping Etched member is missing expose outcrops of bright material, which have consistent morphologies associated with Bright Electris terrain. This further strengthens the relationship between the three units. 44

53 A B Figure 11: A. SW margin of Atlantis Chaos, with transition of dipping Electris plateaus into individual chaos knobs. B. Transect of SW margin of Atlantis Chaos illustrating the dip of the basin and the morphological relationship between the Electris plateaus and chaos knobs. 45

54 Basin Topographic Profiles The basins that contain chaos knob fields have varying topographic profiles that are independent from each and have distinguishing characteristics. Transect locations were determined based on finding representative regions that included the basin margins, interior flat plains (if present) and their associated chaos knob fields. This is important to illustrate the difference in the location that chaos formed within the basin, highlighting the differences between each of the fields. Transects were performed on Ariadnes, Atlantis, Gorgonum and Unnamed Basins 1 & 2 (Fig 12). The basin transects do not indicate a commonality between the initiation point of the chaos fields where they range from ~600 m in Ariadnes to 0 m in Gorgonum, nor their lowest elevations, as both Atlantis and Gorgonum extend below -600 m, Ariadnes barely reaches 0 m, UB-1 extends to ~ -200 m and UB-2 does not extend below ~100 m (Fig. 13). This variability in basin topography and the initial elevations at which chaotic knobs are found indicates that the basins were not interconnected in one mass subsidence event when the chaos formed. Due to their unique topography and elevation profiles, the formation mechanism associated with each chaos field was likely to have resulted from localized events, though they may have been triggered by a region wide phenomenon. 46

55 Figure 12: Location of chaos concentrations within the subbasins, and the location of the basin transects A through D. 47

56 Figure 13: Topographic transects across basins that contains chaos knob fields. Arrows indicate the location that chaos initiates within each basin. The fields each start at different elevations, and dip at varying degrees within the basins. 48

57 Gorgonum Benches Gorgonum Chaos contains a unique population of benches and terraces which ring the interior where they are restricted below ~0 m. They are typified by thick (~50 m) relatively smooth, flat-topped surfaces with steep scalloped margins that encompass individual chaos knobs or chaos fields (Fig. 14). Benches may form stepped terraces with varying elevation. The benches are the dissected remnant of a once continuous surface. The material that comprises these units is also found to infill earlier surface structure, including portions of Sirenum Fossae that crosscut Gorgonum. Outcrop surfaces have a very low apparent crater population, indicating a relatively young surface age within the early Amazonian (Howard and Moore, 2011). Lobate features along the basin floor, past the margins of the benches, may indicate ice- or sediment flow features. Along the basin floor there are populations of apparent small rootless cones (~200 m in diameter, Fig. 14D) indicating localized volcanism in the presence of subsurface ice, as described by Lanagan et al. (2001). The benches are hypothesized to have formed by lateral flow of sediment below the ice-water interface of a perennially frozen lake, with ice-thickness approaching 300 m (Howard and Moore, 2003, 2004, 2011). A B 49

58 C D Figure 14: A. Contiguous bench deposit with lobate scarps embaying margins of transitional chaotic terrain. Surface texture is generally smooth with limited pitting. B. Further examples of contiguous benches, embaying chaos knob field and onlapping an Etched member. C. Type section for Gorgonum Benches. Located only within the confines of Gorgonum Chaos and restricted below ~0 m. Benches form distinct lobate scarps and terraces that encompass chaos knobs. D. Populations of small rootless cones in the basin (red arrows), interior ward of benches. Note the lobate surface surrounding several cone clusters. 50

59 Hummocky Terrain and Mantling Deposit Hummocky terrain is dominated by smooth undulating plains material that drapes older units, while also infilling depressions such as craters and Sirenum Fossae (Fig.15A). Spatial distribution is not constrained to any specific area, though exposures are not as extensive as older plains materials. Unit thickness is variable, on the order of several to tens of meters. Thermal inertia properties are low, consistent with a fine-grained material with low heat retention capacity. Mantling deposits are dominated by irregular knobs, heavy pitting and grooves (Fig. 15B). There are distinct points of retreat from the upper margins of scarps, though onlapping appears to occur along the lower margins of many scarps. The mantling deposit has a large spatial distribution, but appears to be somewhat restricted to the southern portions of the region. Figure 15C shows a representation of the likely modification of Hummocky terrain into a correspondingly more degraded Mantling deposit. There is a clear transition from the smooth hummocky material within the valley interior where the surface is disrupted and pitting becomes the dominant texture. This could explain the limited expanse of the hummocky material as it may have been modified and replaced by the mantling deposit. These two unit types appear to be representative examples of post-noachian mantling processes acting within the Martian midlatitudes (Soderblom et al., 1973; Mustard et al., 2001; Berman et al., 2009). Hummocky terrain is composed of finegrained eolian sediments that drape underlying topography. This is likely composed 51

60 of surface dust cemented by atmospheric water vapor condensation and freezing during the last major Martian obliquity shift (Mustard et al., 2001). The mantling deposit consists of modified hummocky terrain that has undergone volatile loss owing to environmental instability (Mustard et al., 2001). A B C Figure 15: A. Hummocky terrain draping Etched outcrop (Etched within blue outlines), infilling Sirenum Fossae, and subduing topography. B. Mantling deposit draping Electris plateaus. C. Apparent transition of smooth hummocky material into pitted and degraded mantling deposit. 52

61 Crater Rims and Ejecta Major surface modification processes within the region are impactors and their related ejecta blankets and remnant rims. The surface of the southern highlands has been heavily modified because of impacts, and this region has preserved an extensive record of that process. Preserved crater morphologies are dependent on the initial diameter of the crater rim, and are categorized as simple or complex based on morphological characteristics. Early descriptions were based on Lunar craters (e.g., Schultz, 1976; Croft, 1981; Wilhelms, 1987); these were then applied to Martian craters (e.g., Hartmann, 1973; Wilhelms, 1973; Mutch et al., 1976; Wood et al., 1978). Further categorization of crater and ejecta morphologies were presented by Strom et al. (1992, and references therein). Within the basin, craters may be represented by remnant rims of early, large impacts where the rims are severely degraded and no ejecta blanket remains. This also applies to some of the smaller craters that are eroded or embayed, leaving only a marginal remnant behind. However, this does not preclude the presence of wellpreserved ejecta blankets (Fig. 16). These are common throughout, and often typified by lobate rampart morphologies. Rayed ejecta are rare in the basin. Most craters in the region fall into either the simple or complex categories, though there are limited examples of central peak basins, and pit craters. Lobate margins and ramparts (Fig. 16B, C) indicate impact occurred in locations with high concentrations of groundwater or ice mobilized during impact, where fluidization of the ejecta occurred due to entrainment of gas or impact-melted ices (Carr and 53

62 Schaber, 1977; Strom et al., 1992; Barlow, 2005). Rayed margins indicate impact occurred in locations lacking significant volatile concentrations. There is an apparent relationship to crater diameter and the depth of the groundwater or ice within the regolith (Kuzmin et al., 1988), where the impactor can excavate to a depth great enough to interact with the available volatile reservoirs. A B C Figure 16: A. Distribution of degraded crater rims (black) and various preserved ejecta blankets. B. Lobate ramparts associated with simple and complex craters. C. Large complex crater with central pit, exhibiting lobate rampart ejecta. 54

63 Tensional and Compressional Structural Deformation Dissecting the entirety of the eastern Eridania basin is the well-developed arcuate graben network, Sirenum Fossae, which trends to the NE directly towards the Tharsis rise. This simple graben network is part of the enormous radial fracture system centered on Tharsis (Wise et al., 1979). Sirenum stretches across nearly the entire mapped region (Fig. 17A), approaching 1500 km in length, with the troughs being several kilometers wide and often exceeding 100 m in depth. The network consists of three major parallel grabens, with smaller fractures paralleling the main trench. The Sirenum Fossae graben, associated with extension from the uplift of the Tharsis bulge, crosscut the region and is a useful indicator for determining the chronology of events, as its age is constrained to the latest-noachian/earliest- Hesperian (Anderson et al., 2001). There are numerous faults associated with tectonic compression running through the region (Watters, 1988, 1993); these range from local to regional scale features. Though each fault was not specifically characterized, most follow the same basic morphological constraints. Mare-like wrinkle ridges share several broad physiographic elements, including: (1) a broad, gentle rise; (2) a superposed hill or ridge; and (3) a crenulation (Banerdt et al., 1992, and references therein). The oldest of the large scale wrinkle ridges trend North-South, and dissect many of the oldest units, including Mountainous, Electris, Bright Electris, and Etched terrains (Figs.7C, 17D). These faults have been further disrupted by the development of the Sirenum Fossae graben, which bisect the faults (Fig. 17A). 55

64 A Figure 17: A. Distribution of tensional (Sirenum Fossae) and compressional (marelike wrinkle ridges) throughout eastern Eridania. Sirenum Fossae graben network trends to the NE, towards the Tharsis rise. Large wrinkle ridges trend nearly N-S (red), while smaller localized wrinkle ridges are found related to most, but especially plains units (black). The localized wrinkle ridges also tend to have a circumferential orientation. B. Sirenum Fossae dissecting Mountainous, Electris, Etched and Chaos units. C. Circumferential wrinkle ridges disrupting Etched terrain within a large unnamed basin. D. Large wrinkle ridge east of Ariadnes Chaos disrupting Etched terrain and curving west to disrupt chaos knob field. B C D 56

65 Smaller scale wrinkle ridges are generally constrained within the interior of the largest degraded basins and are generally related to Etched terrain members, though there are limited examples disrupting Electris terrain. The ridges have a general circumferential orientation within the basins (Fig. 17C) and are often restricted to only one member of Etched, without breaching the margins of that unit and extending into older underlying members (Fig. 8C) Minor Unit Descriptions The following units are minor components of the geologic framework, but offer further context for both stratigraphy and basin evolutionary processes Volcanic Edifices and Platforms Several isolated mountainous edifices measuring several kilometers in diameter and several hundred meters in height were found, each with a centrally located caldera (Fig. 18). Distinct flows are present from the caldera, spreading out and around the main body of the edifice. These edifices appear to be the source area for localized lava flows which became progressively more viscous, ultimately forming platforms around the edifice structures. The platforms are relatively thick units surrounding volcanic edifices, superposing surrounding lowland terrain with similar texture and albedo characteristics to Etched terrain (Fig. 18). The platforms decrease in height distally from edifices with greatest apparent thickness proximal to source (exposed throat of volcano). These platforms may be representative of a late stage eruption of high viscosity lavas forming thick deposits surrounding the source edifice. 57

66 Figure 18: Two small volcanic edifices with flows sources from their central calderas. Thick platforms encompass the main body of the individual edifice Striated Terrain This unit typically has a surface texture indistinguishable from Etched terrain, except for the presence of parallel, sub-parallel or offset linear striations. This terrain subtype is found along the margins of Mountainous or Electris terrain with prominent linear or curvilinear structures. Orientation of the units suggest a gentle dip in the surface basinward, therefore the lineations may be erosional remnants exposing underlying stratified layers with more resistant material forming the basis for the lineations (Fig. 19A). Localized disruption of the structures resulted in offsets similar in form to those generated by strike-slip faulting (Fig. 19B). The one example of a striated morphology within Electris terrain (Fig. 19C) resembles a complex graben, though on a far smaller scale than typical examples (Banerdt et al., 1992). The limited presence of these features, coupled with their distribution and morphological characteristics, make them unlikely indicators of shoreline features associated with the paleolake basin. 58

67 A B C Figure 19: A. Subtle surface lineations found along the interior margins of a large basin; unit is dipping basinward (west). B. Lineations with angular offsets found along the margins between plains material and mountainous terrain; unit is dipping basinward (east). C. Example of a small localized complex graben system Glacial and Periglacial Terrain These terrain types are characterized by highly pitted and grooved material found primarily within crater interiors and along scarps, generally latitudinally restricted above ~35-40 S. Lineated valley fill (LVF) is present in Mountainous terrain and limited Electris outcrops, with apparent flow downslope (Fig. 20A, B). The outcrops generally originate at high elevations and extend downslope following valley incisions. Individual LVF may also merge with other extant LVF systems forming a larger body. LVF generally terminate in a lobate structure similar to delta structures or lobate debris aprons. Those outcrops restricted to crater interiors and along scarp margins have typical thermokarst structures, lobate margins, pressure ridges and possible crevasses (Fig. 20C). Protalus ramparts form along crater walls, merging into the greater body 59

68 of the unit. When this outcrop does not fill the entirety of a crater interior, push moraines appear to be present. The exposures are generally draped by fine eolian deposits with similar textures to Hummocky terrain, or a rubbly, pitted material similar to the Mantling deposit. These materials contain few craters of any size, and the vast majority of those that are present are highly deformed. This deformation represents the warping of a fresh crater structure to form an oblate shape. The lack of significant cratering, and the deformation of those that are present indicates a young surface material that is currently, or recently, undergoing active modification. One interpretation is that they are glacial terrains characterized by the LVF and lobate debris aprons (LDA), described by Sharp (1973) and Squyres (1978, 1979, 1989). LVF and LDA may be relict features, or indicative of modern resurfacing processes at work within the Eridania basin. The observed characteristics also agree with recent analyses of LVF deposits which indicate local sources within valley-wall alcoves, down-valley flow, merging of flow into trunk valleys and termination in lobate deposits (Head et al., 2005, 2006). 60

69 A B C Figure 20: A. Lineated valley fill in Mountainous terrain, exhibiting characteristics of down-valley flow following the curvature of the land. B. LVF sourced near highstanding Electris terrain, with protalus ramparts along wall margin. C. Interior of a crater filled with glacial terrain exhibiting protalus ramparts, downslope flow and apparent push moraines. 61

70 4.3. Geologic Map The initial global geologic map (Fig.3) was not sufficient to extract localized stratigraphic relationships. A new map was developed based on the unique geomorphological characteristics of the unit exposures and structures, and used to refine local stratigraphy. The above described unit types categorized the major divisions used in the mapping of the region. Unit contacts, be they distinct or inferred, were determined through VIS and IR data sets (HRSC, CTX, HiRISE, and THEMIS) and mapped in the Arc software suite, based on the distinct morphologies of each unit and their boundaries (Figs. 16A, 17A, 21). This was then combined in a final synthesis, overlaying each unit and structure to create a regional representation of the geology in the basin (Fig. 22, Appendix F). The synthesis of the final map involved the incorporation of each of the unique unit and structural component layers found throughout the basin. Each layer was displayed in its proper stratigraphic order to ensure that the correct contacts and overlapping relationships were preserved in the final product. The mapping of this region was confined to the eastern portion of the Eridania basin because of the concentration of chaotic terrain in the subbasin. Mapping of the western Eridania basin was not done because of the lack of significant chaos terrain limited its utility for this study. Once the mapping was completed, all unit and structure contacts were identified, these finding were incorporated into a refined local stratigraphy. 62

71 Figure 21: Individual distribution of the major mapped units; from left to right, top to bottom: Mountainous, Electris, Bright Electris, Etched, Chaos, Hummocky/Mantling Terrains. 63

72 Figure 22: Final synthesis of the regional Eridania map, including all units and structures. 64

73 4.4. Stratigraphy Establishment of the stratigraphy was based on the interrelationship of the above described and mapped units. Contextual analyses of the timing of initial unit emplacement and subsequent structural deformation offered a framework for reconstruction of the temporal sequence of events and processes Basin Stratigraphy The earliest unit represented in the basin consists of the Mountainous terrain. All other units and structures in the basin are found to sit atop or dissect these outcrops, indicating the greatest possible age (examples: Figs. 5, 17). Electris terrain has a wide spatial distribution, and drapes or embays the margins of mountainous outcrops (Figs. 5, 17). Bright Electris is found to be intrinsically associated with Electris and Chaos terrains, outcrops along fresh scarp exposures or eroded margins (Figs. 7, 9). Due to the mineralogical nature of the Bright Electris (Figs. 33, 34), it is seen as a likely alteration product of the original ash material comprising Electris terrain, placing its formation post-electris. Etched terrain is broadly expressed throughout the basin (Fig. 21), and its multiple members require a broad temporal range for deposition. Etched terrain pre-dates Chaos formation, as members cap the chaos deposits in all of the basins (Fig. 9), but also post-dates the Chaos as evidenced by embayment by later Etched members (Fig. 8). The structural process forming the chaotic terrains occurred post-deposition/alteration of the Electris and Bright Electris deposits, as evidenced by the morphological continuity between the units (Figs. 7, 9). It also followed the deposition of multiple Etched members, as the caps were 65

74 dissected by the same process forming the chaos. The formation time for the chaotic terrains can be constrained by crosscutting relationship associated with Sirenum Fossae, which was dated to the latest-noachian/earliest Hesperian by Anderson et al. (2001). Within Gorgonum and Unnamed Basin-1, the graben dissect individual chaos knobs (Fig. 23), placing all previous units and the chaos formation process timing prior to the formation of the graben network. Etched members continued to be emplaced throughout the basin, with flows appearing to drape the margins of graben, while also infilling the spaces between chaos in the basin interiors. Late-stage units such as the Gorgonum benches, Hummocks and Mantling deposits have had their ages constrained to the relative recent past, during the Amazonian (Howard and Moore, 2004; Mustard et al., 2001). A reconstruction of the timing of events led to a simplified local stratigraphy based on relative ages (Fig. 24). Figure 23: Chaos knobs in Gorgonum (left) and Unnamed Basin-1 (right) that were dissected by graben associated with Sirenum Fossae. This relationship offers a specific constraint on the timing of early unit formation. 66

75 Figure 24: Simplified local stratigraphy of the major units and structures, including surface water occurrences Major Unit Elevation Distribution Placing the major units in topographic context further refines their stratigraphic placement, by teasing out interrelationships centered on those units that were emplaced first, and later embayed or draped by younger units. Mountainous terrain is the only unit that extends to the uppermost elevations within the basin, and is generally found above 1000 m (Fig. 25A). Electris deposits have limited exposures above ~2500 m, giving way to mountainous materials. There are also few examples 67

76 Mountainous Electris A B Bright Electris Etched C D Chaos Hummocks and Mantle E F Figure 25: Elevation distribution of major unit types; A. Mountainous, B. Electris, C. Bright Electris, D. Etched, E. Chaos, F, Merged Hummock and Mantle below ~500 m, which is along the transition point into the deepest parts of the large subbasins (Fig. 25B). Both Electris and the Hummocky Mantling terrain (Fig. 25F) follow a similar distribution trend to that of Mountainous terrain, corresponding to the fact that both of them drape the older unit, though they are skewed to lower elevations as neither of them are present at the highest points in the region. Etched material has a wide spatial extent, and drapes or embays many of the older units. Its greatest 68

77 concentrations are centered within the plains and basin interiors, though not restricted to them, which is indicated by its distribution trend to lower elevations (Fig. 25D). The distribution of Electris, Bright Electris and Chaos are of particular interest. Overall their distributions are different, but the trends associated with them are consistent with a common origin. Electris terrain covers a significant expanse within the basin, where it is found to drape both Mountainous and older Etched terrain units. Neither Bright Electris nor Chaotic terrains have extensive areal coverage. However, these two terrains have been found to have significant morphological characteristics in common with Electris (Figs. 6,7,9,11), and their distribution further highlights these similarities (Fig 26). Bright Electris terrain distribution is found to fall within the area covered by Electris terrain, which is consistent with the modification process involved in its formation. Chaotic terrain does not follow the same distribution of Electris, but is found to increase as Electris distribution decreases. This corresponds to the lack of distinct Electris deposits within the interiors of the basins and the dominance of Chaotic terrains found within their lowest depths. Furthermore, Bright Electris and Chaotic terrain distribution closely match, in part due to their stratigraphic and morphological linkages, as Bright Electris is consistently found within chaos knobs. There is a discontinuity between chaos and Bright Electris terrain distributions found above ~700 m, and this is explained by the exposure of widely distributed Bright Electris materials within several basins, outcropping from beneath removed Etched terrain members. 69

78 Elevation Point Count Distribution of Electris, Bright Electris, Chaos Electris 800 Chaos 600 Bright Electris Elevation (m) Figure 26: Distribution trends of Electris, Bright Electris and Chaotic terrains. Bright Electris and Chaos fall within the bounds of Electris terrain and follow similar distribution patterns for their respective unit types Catchment Basin Delineation The individual catchment basins were delineated based on the performed analyses described in Appendix A. Each catchment basin encompasses the drainage area that would have fed surface water flow into the individual basins, and is based on the topography that is currently found in the region (Fig. 27). This method is based on present topography and may not be indicative of surface modification that has occurred during the past. This reconstruction is at best an idealized representation of the basins and their connection to the surrounding terrain. This was necessary for comparative analyses of the non-standardized hypsometric curves of the basins, to determine if there were specific elevations indicative of a common evolution. 70

79 Ariadnes Atlantis Gorgonum UB-1 UB-2 UB-4 UB-3 Figure 27: Catchment basin delineation based on ArcHydro tools, exposing the major distinction between surface flow constraints based on current topography. 71

80 Irwin et al. (2004) identified the noncumulative hypsometric curves of standard degraded basins peak at their lowest elevations where the area is concentrated, indicating that infilling of basins occurred. Those in Eridania have a widespread distribution of their area where little of the basin is concentrated at one specific elevation, and have retained much of their internal relief and concave floor morphology (Fig. 13). The varied peaks in area throughout the basins, specifically found near 0, 800, and 1100 m (Fig. 28) suggest that any infilling material has been concentrated around the margins of the crater rather than filling the entire basin. This could have been accomplished in the presence of standing bodies of water within the basins, resulting in the deposition of sediment at the interface between channel bodies and the standing water (e.g. Howard, 2000). This would be consistent with the formation of depositional benches along the outer rims of the basin, associated with sediment loads being preferentially deposited along the contact with a standing body of water. Each basin has a distinct distribution of their internal area, suggesting a complicated series of geologic activity occurred within each. Found within these distributions are general correlations between peaks in the area of the basins along the 1100 m, 800 m, and 0 m elevation contours. There are exceptions to these trends specifically within Atlantis and Unnamed Basin 2 and 4. Atlantis basin has a relatively smooth area distribution throughout, with few noticeable increases in area, while Unnamed Basin 2 has a peak at 800 and ~400 m, but does not extend to a depth of 0 m. The 1100 m area increase is found within Ariadnes, Gorgonum, Unnamed 72

81 Basins 1 and 3, while Unnamed Basin 4 has an area increase offset to ~ m. The 800 m area increase is found within Ariadnes, Gorgonum, Unnamed Basins 1, 2, 3, and 4. An increase in area within the basins at 0 m is specifically found within Gorgonum where a morphological contact is clearly identified. There are regions of broad area increase at 0 m located within Ariadnes and Unnamed Basin 1. The data is consistent that within those basins that share common increases in area a shared formation process may have affected each basin. Due to the contact morphologies of the surrounding terrains (e.g. Mountainous and Electris) and the characteristics of channel structures and their associated termini (see section 4.6.), there is a suggestion that these area increases at these particular elevation levels may relate directly to highstand levels within the Eridania paleolake. 73

82 74 Figure 28: Nonstandardized basin hypsometry for the 7 major subbasins. A common increase in area is found for the basins at ~1100, 800, and 0 m (black lines). Shaded area corresponds to chaos elevation spread in the basins, which decreases the reliability of correlative measures between the basins below those elevations. UB-4 is the only basin in which noncumulative hypsometric curve peaks near the lowest elevations, indicating a relatively flat surface, likely formed due to infilling of the basin from sedimentary input. The rest of the basins internal relief has been preserved, with a concentration of infilling material apparent around the individual basin margins.

83 4.6. Channel Analyses The distribution of channels and their corresponding termini elevations were analyzed to determine if there is evidence of a common surface water evolution. The preservation of the channels within the region varies significantly, partly because of the erosion and burial due to ejecta blankets of large stretches of the channels. This results in a large population of fragmentary channels throughout the region, limiting the identification of continuous channels. Channel distribution throughout the mapped region (Fig. 29A) shows most channels are concentrated between ~0 m and ~2000 m. This distribution would correspond to the concentration of channels within the lower lying units (e.g. initiating in Mountainous terrain and expanding through Electris and Etched terrains). The greatest proportion of channels peaks around 1300 m, which corresponds to the approximate position when Mountainous terrain distribution decreases and gives way to Electris and Etched terrains (Fig. 25). The decrease in numbers of mapped channels at this elevation point may indicate a decrease in channel incision events when transitioning to younger terrain types. Fewer channels were being created in the younger, lower lying terrains, thus their channel distributions decrease accordingly. This change in incision rate may be indicative of a change in environment, as channel incisions initiated in Mountainous terrain likely formed due to direct precipitation while those in Electris were primarily sourced from groundwater. 75

84 A Figure 29: A. Channel distribution within Eridania basin. B. Normalized by area channel distribution within Eridania basin. B To find any correlation in the number of channels at a specific elevation, the data were normalized for the region (Fig. 29B). There are positions within the data that indicate a significant decrease in the numbers of channels, corresponding to ~1100, 800, and 0 m. This decrease can be explained by a cutting off of channel incision events as they terminated at the boundary with a standing body of water. 76

85 There are variations in channel distribution, however these three decreases match well with channel termini elevation distributions and morphological contacts associated with apparent highstand levels. Channel termini elevations were determined based on their relationship to the mouth of the channel and the surrounding terrain. This involved identifying clear trends of channels terminating along breaks in slope, or along scarps associated with the termination of specific unit types (e.g. Electris plateaus). With the highly fragmentary nature of many of the channels in the region, only obviously continuous channels with a clearly defined termination point were chosen. This decreased the possibility of random data points. Once these were identified, their elevations were determined from MOLA topography. The distributions of the termini indicate peaks at ~1200, 800, and 0 m (Fig. 30), which corresponds closely with the declines found in the distribution of channels. Additional peaks in the channel termini may be the result of later evolution in the basin during drawdown as each basin would have generally evolved uniquely once water levels fell below ~800 m and disconnected from one another. This disconnection and subsequent evolution would continue for the western subbasins as drawdown continued. 77

86 Figure 30: Channel termini elevation distribution, including basin communication cutoff point at 800 m. 50 m bin The channel and termini distribution indicate a possible link between several other elevations, which are represented in Figure 31 as continuous bodies of water filling the basin to those maximum levels. This offers a representation of how the basin would have appeared when filled. The channels are overlain on this map (yellow lines) and their termination points match the 800 m contour, with some terminations occurring around 1100 m in elevation. There is a sharp decrease in the number of channels below 500 m, indicating a decrease in surface flow. The 0 m contour within Gorgonum matches the observed termination points of most of the channels within that particular basin. Each of these elevations may correspond to different levels of a standing body of water within Eridania or the individual subbasins. 78

87 Figure 31: Overlay of potential highstand lake levels within the Eridania basin. Included are the mapped channels in the basin (yellow lines), highlighting their relationship to the proposed levels. 79

88 4.7. Summary of Results The mapping of the eastern Eridania subbasin yielded a clearer understanding of the stratigraphy in the region and culminated in a sequence of events in which the relative timing of geological processes were constrained (Fig. 26). The geologic history represented by this sequence will be discussed in greater detail in section 5.1. Relating to the development of chaos in the region, a clear relationship was identified between several terrain types, which represented a modification and alteration history within the basin (see sections 4.1.2, 4.1.3, ) in which Electris terrain ash deposits were altered to form Bright Electris terrain, and were later modified to form chaotic knob fields. Though the identification of this modification process appears to have no bearing on the formation of Chaos itself, it offers a window into the understanding of the paleolake in the region. Clarification of the evolution of the paleolake boundaries itself were extracted through the use of basin hypsometry, channel body and channel termini distributions, and morphological unit contacts. The area distributions in the basins (Fig. 28) indicate an increase in area corresponding to ~1100, 800, and 0 m elevations. Channel body distributions indicate a sharp decline at ~1100, 800, and 0 m elevations, while channel termini distributions peak at ~1200, 800, and 0 m. Coupling these data with the areal distribution of the Bright Electris terrain which increases dramatically below 1100 m elevations (Figs. 23C, 24), interpreted as the alteration product of Electris ash material in the presence of water, several levels can be placed with regards to previous highstand levels within the Eridania paleolake. 80

89 V. DISCUSSION Each of the units within the basin have unique formation mechanisms that would restrict their distribution and control their surface expression. This would have included the contributions made due to material composition and the prevailing environment during deposition or alteration. Surface modification processes shaped many of these units as well, removing large quantities of material while redistributing the newly formed sedimentary materials. Impact, volcanic, tectonic, fluvial, lacustrine, and eolian processes dominated much of the surface modification of the basins following their initial deposition Geologic Mapping and Stratigraphy A final result of this study was the creation of a 15 x 25 geomorphological map covering portions of the Eridania and Phaethontis quadrangles (Fig. 25, Appendix F). Generally displayed at a 1:2M-scale map, the individual 5 x 5 quadrants can be displayed at a ~1:100K-scale. This vastly increases the previous map resolutions, from 1:15M-global scale and 1:5M-regional scale maps. Not only did this increase the understanding of the unit types, but further refined the unit contacts, and the crosscutting relationship of the regional and local structural components. All of these components were used in the synthesis of a new localized stratigraphic order, which indicated some variance from previous work done in the region. This refinement clarified the relative timing of deposition, alteration, and modification of local surface features. 81

90 Eridania Stratigraphy and Relative Timing Stratigraphic constraints within the basin place unit ages in context, indicating that the majority of major geological processes occurred no later than the latest-noachian. Previous mapping of the region established that most of the units had relative ages placing them in the Noachian and Hesperian (Scott and Tanaka, 1986) with few extending into the Amazonian (Greeley and Guest, 1987). Our mapping relied on the Sirenum Fossae graben network to place boundaries on the ages of the major units within the basin. Anderson et al. (2001) dated the formation of this structural feature to the latest-noachian to the earliest-hesperian. With the graben network crosscutting Mountainous, Electris, Bright Electris, early Etched members, and Chaotic terrains, along with large N-S trending wrinkle ridges, it is understood that this superposition post-dated those specific emplacement activities. Our findings agree with many of the interpretations put forth by Scott and Tanaka (1986) and Greeley and Guest (1987) though the sequence timing of several units varies, but disagree with those of Grant and Schultz (1990), Grant et al. (2010), Capitan and Van De Weil (2011). Below we discuss our findings and interpretations of the geologic history of the Eridania basin Geologic History within Eridania Basin Initial emplacement of Noachian basement material (Mountainous terrain) occurred, and this material was heavily modified by impacts to form individual basins. Direct atmospheric precipitation occurred, feeding the incisions of early channels within Mountainous terrain, as evidenced by channel incisions that initiated 82

91 along drainage divides. This was followed by the emplacement of regional Etched terrain members, which we interpret as the result of extensive lava flows, which filled the interiors of the individual subbasins. After the filling of the basins interiors with Etched material, the area was blanketed by the regional Electris ash airfall deposits, which formed a valley filling unit that may exceed several hundred meters in thickness. The characteristics of this deposit described by Grant and (1990), Schultz (2002), and Moore and Howard (2005) are consistent with an eolian airfall deposits, rather than potentially being sourced from such settings as alluvial, periglacial, or impact (Grant et al., 2010). There was then a decrease in the rate of precipitation-fed valley networks, as the concentration of channels decreases after Electris deposition. There are apparently fewer channels present in Electris in comparison to the mapped channels present in Mountainous terrain, and this would further correspond to the apparent decrease in the overall number of channels found in later Etched members that are restricted to the interiors of the basins (Fig. 29). Incision of Electris terrain by groundwater sourced valley networks would have become the dominant source of water flowing on the surface, as evidenced by amphitheater style headwalls (Fig 6). Paleolake water accumulation in the basin was likely dominated by groundwater contributions as the rise of Tharsis altered regional hydraulic head (Andrews-Hanna and Phillips, 2007). This led to the alteration of Electris ash to form Bright Electris terrain (Fig. 7) in the presence of the paleolake waters, characterized by the presence of phyllosilicates and sulfates (Gilmore et al., 2011). Further Etched member 83

92 emplacement formed a capping layer above Bright Electris (Fig. 9B, C), possibly subaqueously due to the lack of channel incisions in the capping materials. Most channel incision activity occurred prior to the formation of the Sirenum Fossae graben network, as evidenced by the dissection of channel bodies by the graben. Several examples of channel incisions can be placed post-graben formation, as small channels have been located within the interior of the graben itself, but these occurrences are rare. This was then followed by the deformation of surface units resulting in the formation of Chaos knob fields (see section ). This was followed by late-stage Etched emplacement within subbasins, embayed the margins of the chaos knob fields. These localized lava flows have no apparent outside source area, and likely formed from direct intrusion into the basin interiors. Sirenum Fossae formation places a temporal constraint prior to the Noachian-Hesperian transition on the timing of Chaos and limited late-stage Etched formations. Most activity within the basin occurred prior to the formation of the Sirenum Fossae graben network. Later modification processes involved limited and localized Etched terrain emplacement within the interiors of the individual basins; localized small-scale wrinkle ridge formation restricted to late-stage Etched deposits boundaries; local surface incisions by small channels; the formation of the Gorgonum Benches within an Amazonian ice-covered lake (Howard and Moore, 2003, 2004, 2011); and the formation of a Hummocky mantling deposit which is primarily composed of eolian sediments which were cemented by atmospheric volatiles and 84

93 later degraded through volatile loss, likely dating back to the most recent major obliquity shift (Mustard et al., 2001) Paleolake Indicators The presence of a paleolake within the boundaries of the Eridania basin was hypothesized by Irwin et al. (2004). The parameters for the identification of the paleolake were: (1) the spillway elevations at the head of Ma adim Vallis, (2) the slope transition at the contact between basin rims and the flat-floored basin interiors, and (3) the general termination of valley networks between 950 and 1100 m. Their work categorized the interior of the entire Eridania basin and the surface characteristics that would be sufficient to propose the past presence of a lake. The conditions they set forth were convincing, but were further refined based on our higher-resolution mapping. The first step in identifying the hydrologic cycle within the basin consisted in locating and mapping the extent of valley networks. Using all available datasets, extensive channel populations were located. Most were concentrated within the Mountainous, Electris, and Etched terrains. Typically located along the flanks of highstanding regions, many channel incisions originate along ridge crests and drainage divides, and flow downslope into surrounding plains materials. These characteristics indicate an early stage of channel incision, initiated from direct atmospheric precipitation or from melt of high elevation ice accumulations (Craddock and Maxwell, 1993; Hynek and Phillips, 2001; Mangold et al., 2004). These channels incise local plains units, carving valleys through the underlying material. 85

94 Channels that initiate within Electris terrain generally exhibit a morphology that is not associated with direct atmospheric precipitation as is the case with mountainous valley networks. These Electris channels have limited tributary networks, with rounded amphitheater headwalls. This is characteristic of groundwater sapping processes (Pieri, 1980; Howard et al., 2005). This indicates a transition of the prevailing environmental conditions from direct atmospheric precipitation to a groundwater dominated system during Electris incision. This also corresponds to a steep decline in the overall number of channel bodies located in the lower elevation valley-filling units as opposed to the highstanding Mountainous terrains (Fig. 29A), indicating a transition in the source waters associated with the channel incision. Channels located in Etched terrain do not show any clear source areas within the unit. They are simply smaller-scale reactivated channel extensions of those that originate within Mountainous or Electris units. A large volume of water is required for a lake the size of Eridania. It is apparent that there was an integrated hydrologic system dominated by groundwater discharge, with contributions from overland flow from channels, where the altered regional hydraulic head associated with the Tharsis rise would have raised the water table to the point that it intersected with the surface (Hanna and Phillips, 2006; Andrews-Hanna and Phillips, 2007), and filled low-lying terrain. This would offer an adequate source to explain the filling of the basin through groundwater input rather than relying exclusively on precipitation. 86

95 With sufficient water available from multiple sources the basin could fill to its maximum extent, prior to the overspill event that formed Ma adim Vallis, as described by Irwin et al. (2004). The upper reach of the spillways at the head of Ma adim Vallis originate at ~1,200 m elevation and ultimately eroded the channel head 25 km south to its present elevation of 950 m (Irwin et al., 2004). This is below the maximum extent of the Eridania paleolake, which is at 1250 m, identified by Irwin et al. (2004). The disparity in elevations may be due to the migration of the spillway, structural deformation within the basin post-eridania, or the presence of several large impact craters located near the head of Ma adim Vallis which may be obscuring the original elevation of the spillway. The hypsometric curves for most of the Eridania subbasins peak along their margins indicating that infilling materials are concentrated there rather than in the interiors of the basins (Fig. 28). This is counter to typical degraded basins where infilling material would subdue the original topography by concentrating at the lowest elevations near their centers. There are peaks of areal extent within the basins concentrated around 1100, 800, and 0 m, while there may have been a stable body of water that existed as drawdown within the basin proceeded. These would have been formed by the construction of shoreline features are sediments loads were deposited along the margins of a standing body of water. The normalized distribution of channel bodies also indicates a decrease in their presence corresponding to 1100, 800, and 0 m (Fig. 29B). This can be explained by the secession of channel flow as they terminated at the contact with a standing 87

96 body of water. Further supporting this is the distribution of channel termini, where peaks are found at ~1200, 800, and 0 m (Fig. 30). To further correlate the highstand levels, the distribution of the alteration product Bright Electris terrain, is found in its near totality below 1100 m (Figs. 25C, 26), with limited exposures of this unit found above this elevation point concentrated within local topographic lows where water ponding would have likely occurred. The preponderance of data suggesting the ultimate highstand level would have been found at or near 1100 m. After the overspill that formed Ma adim Vallis subsequent water levels dropped below the lowest elevation of the spillways at 950 m, therefore further water loss from the lake would have been caused by infiltration into the groundwater system or from direct surface evaporation. A later 800 m level is strongly suggested by the data, and would have also been equivalent to the elevation at which the larger basin would begin to separate into individual bodies of water that would then evolve independently in each subbasin. As the waters within the basin were lost and drawdown continued, late-stage individual basin shorelines may have been formed, as evidenced by the oscillations in the channel and termini distributions. However, no clear data are found to suggest a strong candidate for these shorelines except in the case of Gorgonum basin, where an obvious morphological shoreline in present. The hypsometry of the subbasin and the distribution of channels and their termini were used as an indication of a standing body of water, coupled with the distribution of Bright Electris terrain. A careful examination of the geomorphological 88

97 data presents a maximum elevation for the lake at ~1100 m, with lower elevations corresponding to the location of late-stage lake highstand levels at 800 and 0 m, as drawdown occurred Presence of Paleolake and Impact on Mineralogy of Bright Electris Terrain Electris material is identified as an ash airfall deposit that blankets the region to a thickness of several hundred meters (Grant and Schultz, 1990; Grant et al., 2010). This material appears to have undergone significant alteration in the presence of water, which agrees with work performed by Bibring et al. (2006), Bishop et al. (2008b), Gilmore et al. (2011). Grant et al. (2010) also identified hydrated phyllosilicates within the Electris deposits, but did not interpret this as an alteration product. Gilmore et al. (2011) identified the presence of hydrated phyllosilicate phases and sulfates within chaos knobs, and their results correspond to those found by Grant et al. (2010). This further confirms a mineralogical connection between Electris, Bright Electris, and Chaos. Intrinsically these are the same initial material, but have been altered in the presence of water. The mineralogy of the chaos knobs and the Bright Electris terrain can offer further information about the environmental conditions that were dominant at the time of formation. Mineralogically the exposed interiors of the chaos knobs are comprised of phyllosilicates and sulfates (Fig. 32). These two components could be explained by the alteration of a large ash deposit in the presence of water (Bishop et al., 2008b). Since the knobs can exceed one hundred meters in thickness, and the bright material is exposed along the entire scarp height, it is possible a large volume 89

98 of water was required to saturate and alter the ash deposit. Sulfate minerals can be indicative of an evaporative environment, corresponding to conditions invoked for the nearby Columbus crater (Wray et al., 2011). Figure 32: CRISM frames with mineralogy of exposed chaos knobs indicating the presence of phyllosilicates and sulfates (modified from Gilmore et al., 2011). Additionally, Grant et al. (2010) identified light-toned material within Electris plateaus (Fig. 33A). Mineral composition corresponded to hydration and Fe/Mg bonding within phyllosilicates (Fig. 33B). Compositional data suggests that the Electris material and thus the Bright Electris within the scope of this study, are not comprised of pristine volcanic material and may include a hydrated phase (Grant et al., 2010). This may be caused by the incorporation of water ice in near surface deposits (Mellon et al., 2008) or alteration products (Bishop et al., 2008). 90

99 Figure 33: A. CRISM image with Electris plateaus and Etched terrain. Black dot denotes area that Grant et al. (2010) analyzed. B. CRISM spectra compared to lab reflectance spectra, indicating hydration and Fe/Mg bonding in phyllosilicates (modified from Grant et al., 2010). The mineralogical data in conjunction with the distribution of the Bright Electris terrain (Fig. 25C) lend further support to the presence of a paleolake within Eridania, while also placing additional constraints on the maximum highstand not having exceeded ~1100 m Chaotic Terrain Formation Mechanisms Chaotic terrains are deposits of enigmatic origin located within the interior of basins in Eridania. In the literature the preferred mechanism for the formation of chaos is related to the breaching of large, confined high-pressure aquifers releasing a catastrophic volatile reservoir. This causes an evacuated subsurface cavity to collapse because of declining hydrostatic or lithostatic support. The collapse of the overburden forms a jumbled chaotic field of surface remnants. Chaotic knobs fields found within the Eridania subbasins are atypical when compared to the largest concentrations of chaos terrain in the Valles Marineris region. 91

100 The chaos fields found in Ariadnes Colles, Atlantis Chaos, Gorgonum Chaos, Unnamed Basin 1 and Unnamed Basin 2 are found in the deepest parts of the region, and do not exhibit any clear outflow features, that suggests a difference in the formation process Models of Chaos Terrain Formation Mechanisms The following models describe previous hypothetical interpretations of chaotic terrain formation mechanism Aquifer Collapse The typical mechanisms responsible for chaotic terrain formation involve: (1) pressure build-up with pore spaces within a confined aquifer, (2) rupturing of the capping cryosphere layer, (3) release of the confined aquifer groundwater combined with outflow channel incision, (4) subsequent collapse of the overlying terrain into the evacuated cavity forming the jumbled chaotic knob fields (Fig. 34; Carr, 1979). The chaotic knob fields would have also undergone further weathering, reducing a near continuous, lightly fractured surface into a highly fractured mesa/butte field, with further weathering reducing the mesas and buttes to small irregular pyramidal hills (Carr, 1979). 92

101 Figure 34: Proposed model of flood and collapse mechanism by Carr (1979). Within highly brecciated old cratered terrain water is contained under high pressure by a permafrost layer, capping nonporous rock. Disruption of the permafrost layer would free the confined waters to access the surface Release of Water from Hydrated Sediments Another hypothesis for chaos formation involves the release of massive volumes of water from the heating of hydrated sediments by volcanic input (Montgomery and Gillespie, 2005; Kargel et al., 2007). This would involve thermally inducing the dewatering of thick evaporite deposits (Montgomery and Gillespie, 2005), which would have produced a volume of water sufficient to carve the Martian outflow channels. Miller et al. (2003) modeled fluid-pressure feedback systems during dehydration reactions that indicated a potential positive feedback would result in the rapid, catastrophic dewatering of thick sedimentary deposits. This was then incorporated into the conceptual model of Kargel et al. (2007) to explain the formation of chaotic terrain. With sufficient heat flux, kilometers thick sediment loads could be induced to dewater, releasing up to 75% of their water content 93

102 (Montgomery and Gillespie, 2005). This volume loss could be sufficient within an enclosed basin to cause significant subsidence Basal Ice-Lens Melting An alternative model calls for the burial by sediment of a preexisting ice sheet that gradually increases pressure and temperature conditions to the point of melting the ice lens (Fig. 35; Zegers et al., 2010). Continued melting would lead to overburden collapse, forcing high-pressure expulsion of the confined waters. A crater is the optimal site for this model, where a drying paleolake would provide the required ice lens as environmental conditions permit. A secondary aspect of this model would involve the intrusion of a volcanic dike and sill system below the surface, fracturing it. This would cause initial subsidence of the overlying material, while the increased heat flux would disrupt and destabilize the confined waters in the aquifer, causing further ruptures along the fractures, and subsequent release of water to the surface. This activity would be capped by late-stage volcanic activity, filling low-lying troughs formed during the initial subsidence events (Fig. 36; Maresse et al., 2008). These two methods could signify a method of gradual water loss where the expelled waters were not released catastrophically, and may have subsequently infiltrated into the groundwater system or been lost through evaporation within the basins. 94

103 Figure 35 (below): A. Ice partially filling a crater. B. Sediment filling crater interior, burying ice. Melting begins as pressure increases. C. Instability grows and melting continues, leading to collapse. D. Collapse forces water to the surface, where it will either carve a channel or evaporate. (Zegers et al, 2010). Figure 36 (right): 1. Emplacement of sill and dike, early fracturing. 2a. A shallow subsidence event with limited aquifer release. 2b. Reactivation of an outflow surge, following a secondary pulse of magmatic activity. 3. Surface discharge of lava following final subsidence associated with chaotic collapse, infilling the basin Deflation and Erosion Another proposed mechanism for the formation of chaos specifically within the Eridania basin relates to direct modification of the surface through deflation and erosion (Fig. 37), rather than modification of the subsurface (Wendt et al., 2012). This would be accomplished post-drawdown of the lake, as the deposited clays deflate and are further modified from fluvial and eolian erosion. Step-wise the process of chaos formation occurs as follows: (1) Deposition of Electris airfall deposit 95

104 on top of Noachian basement rocks; (2) Valleys carve into Electris and form paleolake; (3) Water level decrease to form individual basins; clays form; (4) Electris is dissected into individual knobs; (5) A hard outer layer develops; (6) Ridged plains material covers/embays knobs; (7) Wrinkle ridges form by pressure from Tharsis; (8) Localized, precipitation-fed valleys form; water ponds and deposits chlorides. Figure 37: Process of deposition (1) through deflation of clays to form knobs (8). Modified from Wendt et al., Chaotic Terrain within Eridania Chaos terrain in Eridania is not only an alteration product of the original Electris material; it has been modified to form Chaos knobs within the basin interiors. This is illustrated in Figures 9A and 11. Here one can identify direct disruption or erosion of Electris plateaus to form the jumbled chaos knobs (Fig. 11). Dipping into the basin, there is a clear continuity of Electris plateaus that are then dissected to form individual scarp-bounded chaos mesas and knobs. Exposures of bright material are 96