THE QUATERNARY GEOLOGY AND SEQUENCE STRATIGRAPHY OF LAKE BONNEVILLE DEPOSITS IN THE MATLIN QUADRANGLE, BOX ELDER COUNTY, NORTHWESTERN UTAH

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1 THE QUATERNARY GEOLOGY AND SEQUENCE STRATIGRAPHY OF LAKE BONNEVILLE DEPOSITS IN THE MATLIN QUADRANGLE, BOX ELDER COUNTY, NORTHWESTERN UTAH A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Master of Science Matthew P. Cavas November 2003

2 This thesis entitled THE QUATERNARY GEOLOGY AND SEQUENCE STRATIGRAPHY OF LAKE BONNEVILLE DEPOSITS IN THE MATLIN QUADRANGLE, BOX ELDER COUNTY, NORTHWESTERN UTAH BY Matthew P. Cavas has been approved for the Department of Geological Sciences and the College of Arts and Sciences by Gregory C. Nadon Assistant Professor of Geological Sciences Leslie A. Flemming Dean, College of Arts and Sciences

3 Cavas, Matthew P. M.S. November Geological Sciences THE QUATERNARY GEOLOGY AND SEQUENCE STRATIGRAPHY OF THE LAKE BONNEVILLE DEPOSITS IN THE MATLIN QUADRANGLE, BOX ELDER COUNTY, NORTHWESTERN UTAH. (78 pp.) Director of Thesis: Gregory C. Nadon The Matlin quadrangle, located in Box Elder County, northwestern Utah, was selected for detailed geologic study because of the extensive Quaternary deposits of Lake Bonneville present in a region with no major sediment sources or tectonic activity. The mapping showed a well-preserved record of deposits laid down as the lake filled and emptied. The Transgressive Systems Tract is composed of a series of backstepping barriers formed as the lake rose to the Bonneville shoreline. The Highstand Systems Tract consists of the Bonneville shoreline. The subsequent catastrophic drop in lake level associated with the Bonneville flood stabilized at the level of the Provo shoreline, which is interpreted as a Falling Stage Systems Tract deposit. The Bonneville sequence then records a drop in lake level that marks the end of the sequence. The data collected during this study and from literature sources suggest that Lake Bonneville should be considered a Balanced-Fill lake. Approved: Gregory C. Nadon Assistant Professor of Geological Sciences

4 4 Table of Contents Page Abstract... 3 List of Tables... 5 List of Figures... 6 Chapter 1 Introduction Introduction and Purpose Rationale Study Area Chapter 2 Previous Work Introduction Pennsylvanian-Permian Tertiary Quaternary Fundamentals of Sequence Stratigraphy Lacustrine Sequence Stratigraphy Chapter 3 Methodology Chapter 4 Results Geologic Map Map Units Introduction Tertiary Volcanics Overview Basalt Tuff Bentonite Quaternary Deposits Overview Undifferentiated Alluvium and Colluvium Alluvial Fan Undifferentiated Lacustrine and Alluvial Deposits Lacustrine Fines Lacustrine Gravel Lacustrine Lagoon Deposits Lacustrine Marl Lacustrine Sand Chapter 5 Quaternary History and Sequence Stratigraphy Quaternary Lake Bonneville Sequence Stratigraphy of Lake Bonneville Deposits Chapter 6 Conclusions References Appendix minute Quaternary Geologic Map of the Matlin Quadrangle... 78

5 List of Tables Table Page 2.1 Table detailing the evolution of the name Oquirrh Group

6 List of Figures Figure Page 1.1 Location map of Lake Bonneville within the Great Basin that includes previous study locations and current study location Location map of the major depocenters during the Pennsylvanian throughout the western United States Location of the Oquirrh Basin within Utah and the surrounding states Figure of Erskine s model of the evolution of the Oquirrh Basin Figure showing the extent of Lake Bonneville at its two most prominent heights The generally accepted hydrograph for Lake Bonneville The modified hydrograph for Lake Bonneville Fundamentals of sequence stratigraphy The Falling Stage Systems Tract Previous sequence stratigraphic interpretation for Lake Bonneville minute Quaternary geologic map of the Matlin quadrangle and legend Photos of the vesicular basalt Photo of amygdaloidal basalt Photos of the tuffs Photos of the bentonite Photo of the undifferentiated alluvium and colluvium unit Photo of the alluvial fan and the undifferentiated lacustrine and alluvial units Photo of the lacustrine fines unit Photo of the lacustrine gravel and lacustrine lagoonal units Photo of the lacustrine marl unit and lacustrine sand unit Photo of the Bonneville shoreline with a smaller barrier behind Measured section showing a deep-water representation of the Bonneville lake cycle and sequence stratigraphic interpretation Figure showing the nearshore representation of the Bonneville lake cycle and sequence stratigraphic interpretation Bohacs et al. (2000) classification of lake basins

7 7 CHAPTER 1 - INTRODUCTION 1.1. Introduction and Purpose The western United States has a long and complex geologic history (e.g., Dickinson, 1977). The most recent tectonic forces to affect this area are the extensional forces that created the Basin and Range physiographic province, which is characterized by a series of grabens and half grabens (Sonder and Jones, 1999). During the last 150,000 years variations in climate have also altered the landscape by creating conditions that allowed the formation and desiccation of large lakes within these basins (McCoy, 1987; Oviatt et al., 1999). The largest paleolake in western North America during the late Pleistocene was Lake Bonneville, which covered 51,000 km 2 at its greatest extent (Gilbert, 1890) (Figure 1.1). Lake Bonneville existed from approximately 30 to 12 ka (Oviatt et al., 1992). Subsequent aridity during the last 12,000 years has left a well-preserved and exposed record of shorelines around the basin margins (Gilbert, 1890). The principal shorelines left by the large lake have been mapped and primarily used for reconstructing the lake s chronology (e.g., Gilbert, 1890; Oviatt et al., 1992), but it should also be possible to use Lake Bonneville deposits to evaluate theories about the sequence stratigraphy of lacustrine basins (Shanley and McCabe, 1994; Posamentier and Allen, 1999; Bohacs et al., 2000). The few sequence stratigraphic studies that have been published on Lake Bonneville sediments have all been conducted along the eastern margin of the lake, north and south of Salt Lake City (Figure 1.1) (Oviatt et al., 1994; Anderson and Link, 1998;

8 Figure 1.1. Map showing the extent of Lake Bonneville within the Great Basin at its highest lake level (modified from Sack, 1999). Approximate locations are given for the Matlin quadrangle study area and various sequence stratigraphic studies mentioned in this thesis. 8

9 9 Milligan and Chan, 1998; Lemons and Chan, 1999). These locations all lie close to the eastern boundary fault (the Wasatch fault) and the main sediment sources exiting from the Wasatch Mountains. Large amounts of coarse material were deposited along the eastern margin of the lake, much in the form of Gilbert-type deltas (Gilbert, 1890). However, the expression of sequences varies with the rate of formation of accommodation space and therefore the question remains as to whether the deposits found along the basin margins, away from high-volume sediment sources, record the same sequence stratigraphic response to lake level change as those along the eastern edge of the basin. In order to test this hypothesis a location is required where the fault motion is minimal and the sediment input a minimum. The Matlin 7.5-minute quadrangle, located in Box Elder County of northwestern Utah (Figure 1.1), is an appropriate site to test the hypothesis that an area with limited sediment input preserves a more accurate record of the variations in lake level of the basin than do sites along the Wasatch Front. All of the major shorelines of the Bonneville lake cycle are present on this quadrangle and Quaternary deposits cover approximately 90% of the map area. The purpose of this thesis is to map the surficial deposits of the Matlin quadrangle at the scale of 1:24,000 so that the various Quaternary deposits can be interpreted in a sequence stratigraphic context. The sequence stratigraphic model developed here is compared with those developed along the Wasatch Front Rationale Geologic mapping provides geoscientists with information on the spatial distribution of the various deposits found within a given area. Cross-cutting relations and

10 10 structural data can also be described to allow a more detailed interpretation of the regional geologic history than can be gathered from any single outcrop locality. In addition, the spatial distribution data is important in ultimately being able to define the sequence architecture of the sedimentary facies present within the map area. With the addition of chronostratigraphic data geoscientists are then able to develop a sequence stratigraphic interpretation for the sedimentary facies in that area, which can later be tested in adjacent locations. The application of sequence stratigraphy to the marine realm resulted in a better understanding of the spatial and temporal distribution of sedimentary facies (Posamentier and Allen, 1999). The spatial distribution of sedimentary facies has an economic importance because of its predictive abilities, while the temporal distribution of facies allows for a better understanding of the dynamics of ancient depositional systems and the various controlling factors that must be considered. In either lakes or marine basins this can be done within a sequence stratigraphic context, allowing a more detailed interpretation of the water level dynamics. With a sequence stratigraphic interpretation information about sediment supply, subsidence, and water level can be gained to ultimately provide insight into the surrounding climate Study Area The Matlin 7.5-minute quadrangle contains a diverse record of geologic deposits. The quadrangle is located in northwestern Utah among a series of low hills collectively called the Matlin Mountains. Today, this area is located in the northern part of Great Salt Lake desert. The climate is generally hot and dry throughout the summer months. North

11 11 of the Matlin quadrangle is Park Valley, the nearest town, and to the east is Snowville with Tremonton lying farther to the east. To the west is Grouse Creek and ultimately Elko, across the border in Nevada. The Matlin quadrangle is primarily Bureau of Land Management (BLM) controlled with a small portion controlled by the state. During the winter months the Matlin quadrangle and surrounding areas are used by the neighboring ranchers to winter their herds. Water and food are trucked in as the sheep and cattle roam over the area. Access to the Matlin and surrounding quadrangles is afforded by either BLM or state maintained dirt and gravel roads. In addition to these roads there is an old railroad grade that runs east-west through the quadrangle. This railroad grade was once the Transcontinental Railroad that continued from the Matlin quadrangle on to Promontory Point where the golden spike was driven to join the two tracks in This railroad grade is now a dirt road maintained by the Bureau of Land management as a scenic throughway.

12 12 CHAPTER 2 - PREVIOUS WORK 2.1 Introduction The area surrounding the Matlin quadrangle has been previously mapped at varying scales. The Matlin quadrangle was mapped at a scale of 1:500,000 by Hintze in 1980 as part of the data collection to produce the latest geologic map of Utah. Subsequent to that the area has been mapped by several authors at varying scales as part of larger works concerning northwestern Utah. These works include Compton et al., (1977), Jordan and Douglas (1980), and Todd (1983), and all focused on the bedrock outcrops within the Grouse Creek, Raft River, and Matlin Mountains. These areas contain strata ranging in age from the Latest Proterozoic through the Mesozoic and include the Matlin quadrangle. For all of the works cited above some detailed mapping was completed, but never in the area of the Matlin quadrangle. The previous work conducted in or around the Matlin quadrangle is outlined below. This is followed by an overview of sequence stratigraphy, in general, and then lacustrine sequence stratigraphy more specifically. This chapter concludes with a summary of the previous sequence stratigraphic studies conducted on Lake Bonneville sediments. 2.2 Pennsylvanian-Permian The sedimentary rocks found on the Matlin quadrangle are mainly Pennsylvanian to Permian in age (Todd, 1983). During the Late Devonian to Early Mississippian the western margin of North America was undergoing active deformation that formed a broad foreland basin during the Antler Orogeny (Ross, 1991; Trexler et al., 1991). After the Antler Orogeny ceased the area experienced continued small collisions during the

13 13 Pennsylvanian and Permian. This tectonic activity resulted in the deposition of a wide range in type and thickness of predominantly marine facies throughout the western United States (Jordan and Douglas, 1980; Ross, 1991; Figure 2.1). After the Antler orogeny several small tectonic events caused a change from basin-wide subsidence to localized loading and subsidence resulting in the formation of the Oquirrh basin (Figure 2.2). The activity that created these depocenters occurred along major structural zones that trended east-west, north-south, and northwest-southeast (Stevens, 1991). The basins were filled with sediment shed from the surrounding highlands, but by the Late Permian the topography of the western craton was more subdued (Stevens, 1991). The Oquirrh basin, located throughout Utah and extending into Idaho, is a large basin that was actively subsiding throughout the Pennsylvanian and Permian (Bissell, 1962). According to Bissell (1962), Gilluly (1932) first described 5,200 m of alternating limestones and sandstones in the Oquirrh Mountains as the Oquirrh Formation. Since its original definition several members have been described from various locations throughout the Oquirrh basin. Welsh and James (1961) raised the Oquirrh Formation to Group status with four formations and 18 members. Other workers have refined the stratigraphy of the Oquirrh Group and in the process have expanded the area considered to be a part of the Oquirrh basin (Table 2.1). Early work on the Oquirrh basin focused exclusively on describing sedimentation patterns found in the eastern part of the Cordilleran geosyncline (e.g., Bissell, 1962; Roberts et al., 1965; Ross, 1991; Erskine, 1997). Those authors considered the Oquirrh

14 Figure 2.1. Paleogeographic reconstruction showing the locations of various depocenters and adjacent highs active throughout the Pennsylvanian in the western United States (from Ross, 1991). 14

15 Figure 2.2. Reconstruction centering on Utah and the surrounding western states during the Pennsylvanian showing the Oquirrh basin of central Utah and southern Idaho (from Bissell, 1962). 15

16 Table 2.1. The evolution of the term Oquirrh Group. 16

17 17 basin to have been affected by local tectonic events as deposition into the larger geosyncline continued (Bissell, 1962). A later model of basin formation proposed by Erskine (1997) suggested that the Oquirrh basin formed as the result of gravity spreading driving the formation of growth faults. In Erskine s (1997) growth fault model Pennsylvanian and older sediment moved basinward along a basal decollement while sediment continued to be deposited on the basin margins. This would have resulted in the movement of nearshore facies basinward, created a detachment where additional shallow marine sediment could be deposited, and caused an apparent rise in sea level due to the seaward stepping of shoreface sediments (Erskine, 1997; Figure 2.3). However no growth faults have been identified within the basin. Erskine (1997) argued that subsequent thrusting during the formation of the Cordillera reactivated many of the older growth faults. This, combined with Basin and Range extension, separated outcrops, covered strata, and potentially hid important fault surfaces making correlations difficult. A subsequent model for the formation of the Oquirrh basin was based on reactivation of the Roberts Mountain allochthon (Geslin, 1998). The Roberts Mountain allochthon was originally emplaced during the Devonian-Mississippian Antler Orogeny. Reactivation of the fault during the Late Pennsylvanian-Permian, which was a response to the Andean-style subduction occurring off the west coast of North America during the Middle Paleozoic, created a foreland basin by renewed thrust loading and deformation of the existing Antler orogenic belt beginning in the Middle Pennsylvanian and lasting until the Middle Permian (Geslin, 1998). Geslin (1998) suggested that the Oquirrh basin is related to, and was actually connected with, the Wood River basin in Idaho and proposed

18 Figure 2.3. Erskine s (1997) growth fault model for the development of the Oquirrh basin. The shaded regions represent non-oquirrh Pennsylvanian carbonates that the growth faults displaced basinward to allow for the Oquirrh Group sediments to be deposited in the space left behind.

19 19 that the two be combined to form the Oquirrh-Wood River basin (Figure 2.2). In northwestern Utah the Oquirrh Group has been studied most recently by Jordan (1979) and Todd (1983). Jordan (1979) concentrated on the upper part of the Oquirrh Group, measuring sections and describing 13 distinct facies as well as several subfacies in the upper Oquirrh Group. More recently, Todd (1983) divided the Oquirrh Group in northwestern Utah into three distinct parts; a lowermost limestone subunit, a middle sandstone subunit, and an uppermost interbedded limestone and sandstone subunit Tertiary During the Cenozoic the Matlin quadrangle, along with the southwestern United States, underwent a period of extensional tectonics (Sonder and Jones, 1999). This extension began as a result of a spreading center gradually nearing the western United States. The Farallon plate that was subducting under the west coast broke into two pieces, the Farallon and Vancouver plates, at approximately 52 Ma (Sonder and Jones, 1999). The plate continued to break up from 27 to 16 Ma with smaller fragments either subducted under the North American plate or captured by the Pacific plate. During this time subduction was completely replaced by right-lateral transform faulting as the Pacific and North American plates interacted (Sonder and Jones, 1999). Contemporaneous with the break up of the two plates was a switching of tectonic regimes from collisional to extensional tectonics that persisted throughout most of the Cenozoic (Sonder and Jones, 1999). The affected areas can be subdivided into four main regions that include from north to south: the Omineca extensional belt in northern Washington and British Columbia; the Rocky Mountains Basin and Range in Montana,

20 20 Idaho, and Wyoming; the Basin and Range in Nevada, Utah, Arizona, California, and northern Mexico; and the Rio Grande Rift of southern Colorado, Arizona, and northern Mexico as it merges with the southern end of the Basin and Range province (Sonder and Jones, 1999). The extension is thought to have begun in the northern areas during the Eocene with extension migrating south and eventually affecting the central areas of the Basin and Range province in the middle Miocene. Bimodal volcanism accompanied the extension in the Cenozoic, although no strong correlation exists between volcanic centers and areas of maximum extension (Sonder and Jones, 1999). This lack of extension in areas of high volcanic activity is generally interpreted to mean that extension ceased during peak volcanism. Best and Christensen (1991) considered that because of this, and the fact that no dikes are exposed at the surface, extension could not be caused by diking. Gans and Bohrson (1998) proposed that in these areas the extension interfered with the volcanism so that magma did not reach the surface. The result of this crustal extension would be that magmas would coalesce into smaller chambers before reaching the surface, crystallize at midcrustal depths, and/or increase their interaction with meteoric waters during the onset of extension (Gans and Bohrson, 1998). Sonder and Jones (1999) hypothesized that diking, caldera collapse, and minor faulting accompanied peak volcanism and that as volcanism waned on its own the extensional forces were accommodated by faulting and stratal tilting instead of those processes which dominated during active volcanism. 2.4 Quaternary The Quaternary record is a combination of lacustrine and volcanic processes.

21 21 Paleolake Bonneville is located in the eastern Great Basin and at its maximum extent reached throughout central and northern Utah and into neighboring Idaho and Nevada. Its two most prominent shorelines are named the Bonneville and Provo (Gilbert, 1890) (Figure 2.4). Gilbert (1890) first described many features of Lake Bonneville, including shorelines and what are now known as Gilbert-type deltas. He also published in his monograph a lake-level chronology, which shows playas at arid times expanding during more humid times and eventually forming Lake Bonneville (Gilbert, 1890). Gilbert (1890) also described erosional segments of the Provo shoreline as a twocomponent system. He interpreted the Provo as having formed after the Bonneville flood, and noted that at some erosional locations there was a small underscore terrace located 1.5 to 6 m beneath the larger, more conspicuous Provo terrace (Gilbert, 1890). The elevations of the Bonneville and Provo shorelines were each controlled by an external outlet (Gilbert, 1890). The outlet river for both lake levels flowed through Red Rock Pass at the north end of Cache Valley (Figures 1.1 and 2.4) and water eventually joined the Snake River. In the years since the publication of Gilbert s (1890) U.S.G.S. Monograph 1, the lake-level history has been well documented through shoreline mapping and by the radiometric dating of deposits (e.g., McCoy, 1987; Sack, 1989; Oviatt et al., 1992; Oviatt, 1997). The resulting hydrographs show the lake-level changes throughout the history of the lake with increasing refinement. A recently published hydrograph (Figure 2.5) shows that Lake Bonneville water level rose from approximately 28 ka until 22 ka. An oscillation occurred at 22 ka resulting in the formation of a major shoreline complex, termed the Stansbury shoreline by Gilbert (1890), at approximately 1,372 m

22 Figure 2.4. Map of Lake Bonneville showing the lake levels at its two most prominent shoreline elevations, the Bonneville and the Provo (modified from Sack, 1999). 22

23 Figure 2.5. The hydrograph for Lake Bonneville from Oviatt et al. (1994). 23

24 24 above sea level (asl). After the oscillation, lake level rose again until approximately 15 ka when the highest lake level was reached at 1,552 m asl. Deposits of white marl were deposited on all older sediments while Lake Bonneville stood at its maximum elevation (Gilbert, 1890). The lake overflowed intermittently from 15 to 14.5 ka until a failure at the threshold resulted in a rapid decline in lake elevation as the Bonneville flood occurred (Gilbert, 1890; Oviatt et al., 1992). The lake level dropped about 110 m in as little as 43 weeks while discharging 4,750 km 3 of water until it reached a bedrock threshold (Gilbert, 1890; O Conner, 1993). The Provo shoreline was then created at 1,444 m asl. After formation of the Provo shoreline the lake level fell even more ending the Bonneville lake cycle at about 12 ka (Oviatt et al., 1992). The hydrograph was further refined by Sack (1999) through a detailed analysis of the compound Provo shoreline complex, which was previously investigated by Gilbert (1890). Sack (1999) documented numerous locations where the white marl separated the Provo shoreline sediments from deposits formed during the transgression of Lake Bonneville at approximately the same elevation as the later Provo shoreline (Figure 2.6). Sack (1999) called the transgressive shoreline the subprovo. She suggested that the superposition of shorelines may have been due to the bedrock sill controlling the lake level during Provo time also controlling the lake level during subprovo time (Sack, 1999). She hypothesized that landsliding near Red Rock Pass after subprovo time may have allowed Lake Bonneville to continue to rise and form the Bonneville shoreline. Sack (1999) noted that Sewell and Shroder (1981) determined there to be evidence of both pre- and post- Bonneville flood mass wasting in the outlet area at Red Rock Pass.

25 Figure 2.6. The Lake Bonneville hydrograph from Oviatt et al. (1994) modified to show the approximate elevation of the subprovo shoreline (Sack, 1999) relative to the Provo shoreline. The lake level during subprovo time, dashed interval, was probably not static as shown here (Sack, 1999). 25

26 Fundamentals of Sequence Stratigraphy The current paradigm in interpreting controls on marine sediment deposition is sequence stratigraphy. This technique was originally developed in marine basins to interpret rock relationships within a chronostratigraphic framework. Beginning as the seismic stratigraphy of passive margins (Vail et al., 1977), this technique has evolved into high-resolution sequence stratigraphy done at the outcrop scale. A sequence is an unconformity-bounded package of sediments. The controlling factor of sequence development is the rate of relative sea level change, which is a function of the rates of change of eustasy, tectonics, and sediment supply, all of which can influence each other during deposition. The result is a change in the rate of formation of space available for sediment deposition, which is referred to as accommodation space. This space is generally measured from the basin floor to sea level and can be changed by either eustatic changes in water level or changes in the rates of basin subsidence and sediment input (Posamentier and Allen, 1999). Accommodation is important in understanding a depositional sequence, but so too is the rate at which accommodation space is created or lost. Changes in any of the three controlling factors can dramatically affect the rate of change of accommodation space producing rapid changes in relative sea level. Changes in relative sea level result in a hierarchy of sediment packages formed at different scales (Figure 2.7). The highest frequency package is termed a parasequence, which is a relatively conformable succession of related strata that are bounded by flooding surfaces (Van Wagoner et al., 1988). Parasequences at one location either thin and fine upward or thicken and coarsen upward depending on changes in relative sea level.

27 Figure 2.7. Sequence stratigraphic nomenclature for a ramp setting (from Van Wagoner et al., 1988).

28 28 Parasequences deposited on a basal unconformity and that backstep in a landward direction form a retrogradational parasequence set, which is used to characterize a Transgressive Systems Tract (TST). Above the TST is the maximum flooding surface, which is defined as the most landward sea level reached. Parasequences deposited above this surface are stacked one on top of another when the water level is stable and offlap into the basin as relative sea level falls. These aggradational to progradational parasequence sets characterize the Highstand Systems Tract (HST). Above the HST is the upper sequence boundary. Like the lower sequence boundary this surface is an exposure surface, or unconformity, marking the end of a cycle of relative sea level change. Once relative sea level reaches its lowest level, sediment commonly by-passes the shelf and deposition occurs on the slope and basin floor in the form of fans prior to the next transgression. Deposition continues in the deep basin areas until a rise in relative sea level and parasequences begin backstepping again onto the upper sequence boundary. Recently, a new systems tract has been added to describe sediment deposited during a relative sea level fall. These deposits have previously been incorporated into the late HST, but numerous authors have advocated the creation of a systems tract to complement the TST and break out these deposits (Hunt and Tucker, 1992; Posamentier et al., 1992; Plint and Nummedal, 2000). The nomenclature preferred is that of Plint and Nummedal (2000) to describe the Falling Stage Systems Tract (FSST) (Figure 2.8). The FSST lies above and basinward of the HST with the Lowstand Systems Tract (LST) deposited above (Plint and Nummedal, 2000). The FSST is characterized by stratal offlap or a basinward stepping of facies that can be difficult to identify due to subsequent

29 Figure 2.8. Figure redrafted from Hunt and Tucker (1995) showing the Falling Stage Systems Tract (FSST) that is formed after a Highstand Systems Tract (HST) as relative sea level falls. 29

30 30 subaerial or transgressive erosion (Plint and Nummedal, 2000). As a result, FSST deposits will not be found in all sequences, but when present they provide additional information on the rates of relative sea level change. 2.6 Lacustrine Sequence Stratigraphy Sequence stratigraphy can be extended into lacustrine basins (Bohacs et al., 2000). Early sequence stratigraphic studies of lake basins were primarily seismic stratigraphic studies looking at the geometry of the Pleistocene and Holocene fill in modern lake basins (e.g., Bartov et al., 2002; Colman et al., 2002). High-resolution studies using outcrop or well core have recently become more prevalent as the application of sequence stratigraphy to study climate change has proven to be very useful (Saez and Cabrera, 2002). Although these studies are being conducted throughout the world, the application of high-resolution sequence stratigraphic interpretations from lacustrine outcrop studies is relatively rare. Previous high-resolution sequence stratigraphic studies of the sediments deposited in Lake Bonneville concentrated on the delta deposits mainly along the eastern edge of the lake basin adjacent to the Wasatch Mountains and the Wasatch fault (Oviatt et al., 1994; Anderson and Link, 1998; Milligan and Chan, 1998; Lemons and Chan, 1999; Figure 1.1). This active normal fault provides these areas with a large sediment input and high rates of formation of accommodation space. The four studies cited produced generally similar results and will be discussed below, but only one sequence stratigraphic model will be reproduced (Figure 2.9). Oviatt et al. (1994) looked at sediments along the Sevier River located in central Utah. The Sevier River has cut down through the lake basin s Sevier River delta and the

31 Figure 2.9. Figure redrafted from Lemons and Chan (1999) showing a sequence stratigraphic interpretation of the Weber River delta located along the eastern shores of Lake Bonneville. 31

32 32 exposed bluffs contain sediments deposited during the Bonneville and previous lake cycle, termed the Little Valley lake cycle (McCoy, 1987; Oviatt et al., 1994). What was recorded from the various measured sections taken along the river was a transgression during the Little Valley lake cycle that deposited deep water marl in the study localities (Oviatt et al., 1994). Over the marl lies highstand, regressive-phase underflow fan deposits (Oviatt et al., 1994). The Sevier River entrenched into the delta during the regression of the Little Valley lake cycle and soils were formed on top of the post-little Valley alluvium that was deposited on top of the delta (Oviatt et al., 1994). Lake Bonneville rose to levels where deltaic sediments began to be deposited again in the same Sevier River delta area at about 20 ka (Oviatt et al., 1994). On top of soils, fine-grained deltaic deposits were capped by lacustrine marl as Lake Bonneville continued to rise. According to Oviatt et al. (1994), after the Bonneville flood the Sevier River entrenched again, this time at a different orientation, and deltaic sediments were reworked and redeposited as underflow fan sediments during the regression. Deposits of the Bonneville cycle at this site are capped by eolian sand and silt marking the upper sequence boundary (Oviatt et al., 1994). Much of the research conducted by Oviatt et al. (1994) incorporates numeric ages obtained from gastropod shells and volcanic ash deposits. Although a transgression and regression were recorded for both the Little Valley and Bonneville lake cycles, no notion of the rate of relative lake level change or style of sequence architecture are given. Oviatt et al. (1994) described no parasequences or flooding surfaces, which would have recorded punctuated rises in lake level triggered by a local change in tectonic accommodation,

33 33 sediment supply, or lake level. Oviatt et al. (1994) place the lower sequence boundary for the Bonneville lake cycle on top of the underlying soil and they place the upper sequence boundary beneath eolian sand and silt deposits. Aside from this, no other application of sequence stratigraphy, such as the identification of various systems tracts, was conducted. Anderson and Link (1998) interpreted measured sections taken along the Bear River as it passes through and cuts down into the Lake Bonneville Bear River delta in southern Idaho, just across the border from Utah (Figure 1.1). Like the Sevier River delta site, this location also contains a record of two separate lake cycles. Beneath deposits of the Bonneville lake cycle is the Pleistocene Main Canyon Formation, which was deposited prior to the diversion of the Bear River into Lake Bonneville at approximately 50 ka. Measured sections from the Bear River delta record the transgression of Lake Bonneville, but the study area does not preserve a record of Lake Bonneville above approximately 1,500 m. This omits a small portion of the Lake Bonneville hydrograph but the record begins again during the regression. From the measured sections along the Bear River in southern Idaho, Anderson and Link (1998) divided the section into five sequences containing a total of 21 parasequences. To interpret this data they assumed that sediment supply to the delta did not change and that the primary influence on facies change was the change in lake level. Anderson and Link (1998) interpreted the parasequences as representing autocyclic abandonment of specific delta-front lobes, which can be seen making characteristic stacking patterns. In addition, Anderson and Link (1998) described regional flooding surfaces as deposits of prodelta facies directly over delta-top facies and could trace these surfaces between

34 34 measured sections. Finally, the sequence boundaries, which are unconformities, were defined where delta-top facies overlie prodelta clay deposits and could be correlated between several sections. Overall, the sequence stratigraphy of that study describes the Lake Bonneville hydrograph in terms of a transgression, a brief highstand, and a rapid regression all seperated by unconformities. Milligan and Chan (1998) conducted a study of composite deltas located along the eastern margin of Lake Bonneville. Deposits at the three localities were described and combined to provide data to cover the entire span of Lake Bonneville history. From study of the deltaic deposits, Milligan and Chan (1998) identified two end-member types of delta that were interpreted as corresponding to parts of the known hydrograph. They described topset-dominated deltas, which are characterized by horizontally stratified gravels, corresponding to deposition during low and rising lake levels. The other end-member delta was termed foreset-dominated, and was linked to deposition as lake levels fell. Foreset-dominated deltas are the classic Gilbert-type delta with well developed, steeply dipping foresets. Milligan and Chan (1998) considered topset-dominated deltas as representing the TST and HST in these three localities and to have formed where accommodation allowed the thick accumulation of deltaic deposits. The foreset-dominated deltas were interpreted as representing the LST, deposited during the regression from the Provo shoreline. HST deposits could not be identified in their study area. A sequence boundary was placed between the TST/HST and the overlying LST deposited after the Bonneville flood. The most recent interpretation of Lake Bonneville sequence stratigraphy was made

35 35 by Lemons and Chan (1999), who also describe and interpret three separate deltas located along the eastern margin of Lake Bonneville. These authors describe the Weber River delta as composed of delta-front sheet sands, delta-margin deposits, fluvial channel deposits, beach gravels, and lacustrine clays. They determined that the Bear River delta is composed of delta-front sheet sands, beach gravels, and lacustrine clays. The Spanish Fork delta was found to consist of delta-front sheet sands, beach gravels, fluvial gravels, and loess. Lemons and Chan (1999) considered the interbedding of delta-front sheet sands and lacustrine clays found in the Weber and Bear River deltas to be parasequences that form a retrogradational parasequence set (Figure 2.9). The HST at all three localities was poorly exposed or mostly removed due to erosion. At the Bear River locality, basinally isolated, downstepping shoreline sediments were described and interpreted as having been deposited during the forced regression of the lake. The LST was only present at the Weber River and Spanish Fork delta complexes, where it was composed of sediments cannibalized from the TST/HST and redistributed into delta-front deposits during the forced regression of the lake. For all three deltas Lemons and Chan (1999) place the upper sequence boundary below the LST deposits and above the HST/TST according to the passive margin model of Van Wagoner et al. (1988).

36 36 CHAPTER 3 - METHODOLOGY During the fall of 2001 the Matlin 7.5-minute quadrangle was chosen for detailed geologic and geomorphic study. The Matlin quadrangle was selected because all of the major Lake Bonneville shorelines cross it and it is relatively isolated from large mountains, thus it had a much lower supply of sediment into the lake than the sites previously studied from a sequence stratigraphic framework (e.g. Oviatt et al., 1994; Anderson and Link, 1998; Milligan and Chan, 1998; Lemons and Chan, 1999). A major portion of the mapping was done through the use of 1:20,000 scale air photos obtained from the U.S. Bureau of Land Management. During the winter and spring of 2002 an initial air photo and topographic map interpretation was completed and a preliminary Quaternary geologic map was produced. The map units were taken from a list of acceptable Quaternary map units published by the Utah Geological Survey and previously published Quaternary maps (e.g. Sack, 1994a, 1994b). Any and all specific sediment accumulations that could be broken out from the surrounding deposits were mapped to produce a preliminary 1:24,000 scale geologic map. In the summer of 2002, six weeks were spent in the field in northwestern Utah checking all units and measuring sections within the Quaternary sediments. Sections were measured at stream cuts in Quaternary deposits where datable material was collected. A hand-held global positioning system (GPS) unit was used to determine sample locations. After the field season, corrections were made to the initial air-photo interpretations. This included redrawing of boundaries between map units and noting other minor features that were not apparent from the air photos. Once the final air-photo interpretation was

37 37 complete the line work was transferred to a base map by tracing over a light table. The line work was traced onto a printout of the Matlin quadrangle downloaded from the Utah Geological Survey website. In addition to the Quaternary units transferred from the air photos the Bonneville and Provo shorelines were added to the base map. Once this was completed the entire base map was scanned into a computer and traced digitally to produce an electronic file of the geologic map of the Matlin quadrangle.

38 CHAPTER 4 - RESULTS Geologic Map The field work conducted on both the Quaternary and the Tertiary volcanic rocks was compiled to produce a 7.5-minute geologic map of the Matlin quadrangle. The map shows the distribution of various lithologies and sediment accumulations along with the preserved sections of the major shorelines of Lake Bonneville (Figure 4.1; Plate 1). In the northern part of the Matlin quadrangle bedrock outcrops of the Pennsylvanian-Permian strata can be found, along with isolated occurrences in the central map area, while younger igneous deposits can be found in minor occurrences throughout the quadrangle. The map area is dominated by Quaternary deposits, mainly from Lake Bonneville. These Quaternary sediments, which were deposited in a wide variety of alluvial and lacustrine environments, are described in detail below Map Units Introduction This section describes the various lithologies and sediments found on the Matlin quadrangle Quaternary geologic map in order from oldest to youngest. Volcanic rocks are found sporadically throughout the Matlin quadrangle. Each volcanic unit is described separately. The eight Quaternary map units are discussed last Tertiary Volcanics Overview The igneous deposits found sporadically throughout the study area are all believed to be less than 65 Ma old. There are many reports of widespread volcanism for this area

39 Figure 4.1. The Matlin quadrangle 7.5-minute geologic map. Legend on the following page. 39

40 Figure 4.1 (continued). Legend for the Matlin 7.5 minute geologic map. 40

41 41 of the western United States with many eruptions occurring during the Tertiary and Pleistocene (e.g., Oviatt and Nash, 1989; Williams, 1994; Perkins et al., 1998; Perkins and Nash, 2002). On the Matlin quadrangle a total of five distinct volcanic deposits were found and described. Only two are included on the geologic map of the Matlin quadrangle, the amygdaloidal basalt and the vesicular basalt flow, as the others were all found in stream cuts and not exposed at the surface Basalt In outcrop the vesicular basalt flow weathers to shades of dark reddish brown but is grayish black on a fresh surface (Figure 4.2). Outcrops are limited to the extreme northeastern corner of the Matlin quadrangle and can be up to 1.5 m thick and locally several meters wide. The amygdaloidal basalt weathers very dusky purple, rvery dark red, and dusky blue green but on a fresh surface it is dusky yellowish green (Figure 4.3). Outcrops can extend over tens of meters, be up to 30 to 40 cm thick, and are heavily fractured. In hand specimen there appears to be plagioclase crystals present. In thin section there is abundant fine-grained plagioclase with what may be olivine being replaced by calcite. The crystal morphology appears to be olivine but no optic-axis figure could be found to prove this. There is a slight alteration product found throughout the olivine grains, which may be iddingstite, which is a common alteration product of olivine (G. Heien, 2003, personal communication) Tuff Three types of tuff could be recognized on the quadrangle, crystal lithic, crystal

42 A 42 B Figure 4.2. A is an outcrop photo of the vesicular basalt flow found on the Matlin quadrangle and B is a closer view of the same outcrop.

43 A 43 B Figure 4.3. A is a photo of a typical outcrop of the amygdaloidal basalt found throughout the central part of the Matlin quadrangle and B is a closeup view of this basalt in the field.

44 44 C D Figure 4.3 cont. C is a photomicrograph of the amydgaloidal basalt in plain light and D is a photo of the same field of view under polarized light. The large crystal in the center is what is thought to be olivine.

45 45 few meters at most within the streams (Figure 4.4). None of the described tuffs are exposed on the surface of the Matlin quadrangle. The crystal lithic tuff appears white to very light gray in outcrop and is generally highly fractured. Exposures of this tuff are generally poor and thickness is difficult to estimate, but it is not laterally extensive. On a fresh surface there are abundant lithic fragments with a smaller amount of quartz and biotite crystals and it is white in color. In thin section lithic fragments are the most common constituent, but the tuff also contains quartz, biotite, plagioclase (intermediate to calcic), and microcrystalline quartz. There are abundant glass shards present along with some neomorphic quartz. The crystal vitric tuff appears white to very light gray on weathered surfaces; it is dominantly white on fresh surfaces. In outcrop this tuff is moderately fractured. It is not laterally extensive, and is approximately 5 to 7 cm thick in the creek bed where samples were collected. In hand specimen there are lithic fragments and some quartz crystals but overall the sample is much finer grained than the crystal lithic tuff. In thin section crystals of quartz, biotite, minor microcrystalline quartz, and poikolitic quartz are present in a fine matrix of glass shards. This tuff is finer grained and contains more glass shards than the crystal lithic tuff from the field area. The welded tuff appears white to very pale orange in outcrop and is white on fresh surfaces. Exposures are generally fractured, not laterally extensive, and a few tens of centimeters thick. In hand sample the only crystals easily identified are biotite and quartz. In thin section plagioclase is easily identified along with quartz, biotite, and minor amounts of micrite filling in fractures. These all lie in a fine-grained matrix of welded glass shards.

46 46 A B Figure 4.4. A is an outcrop photo of the crystal-vitric tuff found on the Matlin quadrangle. B is a photomicrograph of the crystal-vitric tuff in polarized light.

47 47 C D Figure 4.4 cont. C is the same view as B but under plain light. D is a photomicrograph of the welded tuff taken under plain light.

48 48 E F Figure 4.4 cont. E and F are photomicrographs of the crystal-lithic tuff under plain and polarized light.

49 Bentonite A bentonite unit is found in stream cuts throughout the central and west-central parts of the Matlin quadrangle. The bentonite is generally weathered to shades of moderate greenish yellow with some white areas and is up to 1.5 m thick (Figure 4.5). Generally, exposures of this unit are made up of clay minerals that have weathered from the original bentonite ash layer. Locally the bentonite contains abundant biotite crystals, which appear to be unaltered Quaternary Deposits Overview Quaternary deposits dominate the study area covering over 90% of the quadrangle. The sediments were primarily deposited by Lake Bonneville during the late Pleistocene. Many of the deposits were later reworked by alluvial processes but the original depositional environment for these sediments can still be determined. The units are presented from generally youngest to oldest stratigraphic age Undifferentiated Alluvium and Colluvium (Qac) Poorly sorted, coarse-grained angular sediment made up of alluvially reworked colluvium or alluvium with a significant colluvial component is mapped as Qac (Figure 4.6). This map unit occurs in the upper piedmont zone of the quadrangle and generally separates bedrock from lacustrine deposits. In places this unit is up to 20 m thick. The alluvium and colluvium is late Pleistocene to Holocene in age since deposits can be found reworking Lake Bonneville sediments, but they also occur above the Bonneville shoreline where they could range in age from pre-bonneville to post-bonneville.

50 50 A Figure 4.5. A is a typical outcrop of the bentonite, which can be found throughout the central part of the Matlin quadrangle.

51 B 51 C Figure 4.5 cont. B and C are both closeup views of the bentonite showing two examples of what this unit looks like in the field.

52 Figure 4.6. A typical occurrence of the undifferentiated alluvium and colluvium (Qac) unit in the field. 52

53 Alluvial Fan () In the Matlin quadrangle, alluvial fan sediments consist of poorly sorted, coarse to fine-grained alluvium and debris-flow sediment deposited on piedmont slopes (Figure 4.7). The alluvial-fan deposits are up to 20 m thick and are late Pleistocene to Holocene and modern in age since deposits can be found reworking Lake Bonneville sediments Undifferentiated Lacustrine and Alluvial Deposits () Large areas of coarse-to fine-grained material deposited on the piedmont below the level of the Bonneville shoreline are mapped as undifferentiated lacustrine and alluvial deposits (Figure 4.7). In the upper piedmont zone this unit consists mainly of pre-bonneville alluvial-fan deposits that have been reworked by lacustrine processes and can be up to 10 m thick. In these upper piedmont zones the undifferentiated lacustrine and alluvial deposits can be found on top of Pennsylvanian/Permian bedrock. In lower piedmont areas the unit is made up of finer lake deposits that have been reworked by post-bonneville stream processes and are thinner than the deposits found in the upper piedmont. This unit is late Pleistocene to Holocene in age Lacustrine Fines () This map unit is made of varying amounts of sand, silt, clay, and marl (Figure 4.8). From air photo interpretation it appears that these deposits may be locally reworked by eolian and stream processes on lower elevations of the quadrangle. In places these deposits can reach 1-2 m in thickness. These sediments were deposited within Lake Bonneville and therefore are late Pleistocene in age.

54 54 A B Figure 4.7. A typical occurrence of the A alluvial fan () and B undifferentiated lacustrine and alluvial () units in the field.

55 Figure 4.8. A typical occurrence of the lacustrine fines () unit. 55

56 Lacustrine Gravel () Lacustrine gravel consists of granule to cobble-size sediment deposited as beaches and barriers forming shorelines of Lake Bonneville (Figure 4.9). The composition varies from poorly sorted gravel with fine silt sized particles to well sorted gravel with rounded cobbles. In places these deposits can be up to 20 m thick and overlie lacustrine marl deposits. This unit was deposited by Lake Bonneville during the late Pleistocene Lacustrine Lagoon Deposits (Qll) Clay, silt, and fine sand deposited in depressions behind lacustrine gravel barriers are mapped as lacustrine lagoon deposits and are generally 1-2 m thick (Figure 4.9). Qll was deposited by Lake Bonneville during the late Pleistocene Lacustrine Marl (Qlm) Lacustrine marl is a mixture of calcium carbonate precipitated out of lake water and fine-grained clastics (Figure 4.10). Gypsum is present locally. A complete section through the marl begins with transgressive-phase clastic-rich marl, followed by Gilbert s (1890) white marl representing deposition during the highest lake levels, and capped by regressive-phase marl that is clastic-rich. The latter is white marl that was redeposited at lower lake levels after the Bonneville flood. Qlm was deposited during the late Pleistocene Lacustrine Sand (Qls) Moderately sorted medium-grained to pebbly sand that may contains gastropods is mapped as Qls (Figure 4.10). These deposits are generally found lakeward of lacustrine gravel barriers and were deposited by Lake Bonneville during the late Pleistocene.

57 57 A B Figure 4.9. A is a typical occurrence of the lacustrine gravel unit () while B is a photo of one of the barriers formed by the lacustrine gravel deposits (center) with a small lagoon behind it.

58 A 58 B Figure A is an occurrence of lacustrine marl (Qlm) overlain by lacustrine gravel () and B is an occurence of lacustrine sand (Qls).

59 59 CHAPTER 5 - QUATERNARY HISTORY AND SEQUENCE STRATIGRAPHY 5.1 Quaternary Lake Bonneville Considering the entire Bonneville basin much has been said about the depositional history of Lake Bonneville. Most of what has been previously described can be found in the Matlin quadrangle, and there are features of the Matlin quadrangle that are not found widely throughout the basin. During the transgression of Lake Bonneville one of the first prominent features created was the Stansbury shoreline. The Stansbury shoreline is not well developed on the Matlin quadrangle. One reason may be that there was not enough sediment being supplied to the area for it to form. The Stansbury elevation, in the Matlin quadrangle, is less than a mile from the Matlin Mountains but there is no evidence of a river or stream to transport the sediment to where the Stansbury should have been deposited. On the adjacent quadrangle to the east the Stansbury can be found quite easily, but Dove Creek flows south through this area and probably supplied just enough sediment to construct the barrier. In addition, this area of the Bonneville basin has not been actively subsiding to create the needed accomodation space and, as a result, any material deposited at lower elevations could have been reworked later as the lake level rose overtop the Stansbury shoreline. Evidence for a possible subprovo shoreline can be found in the Matlin quadrangle, although it does not appear well developed. The shoreline identified in this study as the subprovo is actually at a lower elevation than at sites elsewhere in the basin. In addition, this subprovo does not underlie the Provo shoreline or occur just in front of it as it does in other locations throughout the Bonneville basin. The Provo shoreline is approximately

60 60 30 m to the north and 21 m stratigraphically above the hypothesized subprovo on the Matlin quadrangle. This differs from the Sack (1999) study that found similar stratigraphy but not at so low an elevation. One reason for this may be that there may be more than one subprovo shoreline in the Bonneville basin. What was shown as a time of stable lake level on hydrograph for subprovo time (Figure 2.6) may actually have been a time period where the lake level was oscillating. Further research is needed to refine this interval of the hydrograph because the subprovo shoreline may actually be a series of oscillations and the shoreline identified on the Matlin quadrangle not the same as what has been found by Sack (1998) in other areas of the Bonneville basin. The Quaternary deposits on the Matlin quadrangle record a variety of depositional subenvironments. The numerous areas of lacustrine shoreline gravel deposits found between the Stansbury and Bonneville shorelines record a rise in lake level up to the highest, Bonneville shoreline. In addition, a smaller barrier is present behind the Bonneville shoreline (Figure 5.1). This indicates that the Bonneville shoreline may be a compound feature related to threshold control. The well-defined barriers in the Matlin quadrangle also record a punctuated rise in lake level. 5.2 Sequence Stratigraphy of Lake Bonneville Deposits Lacustrine sequence stratigraphic architecture is controlled by the rate of formation of accommodation space, which in turn is influenced by the three factors of sediment supply, tectonics, and lake level. Previous studies of Lake Bonneville sequence stratigraphy have focused on areas where sedimentation rates were high and tectonic subsidence significant (e.g., Lemons and Chan, 1999). In contrast to those studies the

61 Figure 5.1. Photo from the Matlin quadrangle showing the Bonneville shoreline with a smaller barrier behind (arrow). 61

62 62 Matlin quadrangle is an area where the sedimentation rate was much lower and therefore not a significant factor affecting the rate of change in accommodation space. Similarly, the faults present on the Matlin quadrangle did not create a significant amount of accommodation space and therefore did not have a major influence on the lacustrine sequence architecture. The result is that this study is the only one undertaken thus far in which the change in absolute lake level is the primary factor that controlled sequence architecture in this portion of Lake Bonneville. The Matlin quadrangle contains a record of the lake level history in both deepwater and proximal settings. The southern half of the quadrangle contains a record of the Transgressive Systems Tract (TST), which is composed of gravel and sand deposits, in places overlain by transgressive marl (Figure 5.2). This marl contains some fine-grained clastic sediments that decrease in amount up-section. This minor change in the finegrained marl deposits appears to be the only indication of alternations of lake level in the more distal portions of the lake, but is only found at one locality on the Matlin quadrangle. In the deeper water settings the maximum rise of lake level is recorded by the white marl that represents the Maximum Flooding Surface (MFS). After deposition of the MFS deposits the Bonneville flood occurred during the late Highstand Systems Tract (HST). The flood resulted in a rapid lowering of water level, or a forced regression. In deep water settings this may have been recorded as a thin pebble lag deposited above the white marl. Such a lag was only found at the Coyote Den site where Figure 5.2 was measured, but it may be the deep-water representation of the Bonneville flood, deposited as lake level was lowered by approximately 100 m (O Connor, 1993).

63 Figure 5.2. Measured section taken on the Matlin quadrangle showing the deep-water representation of the Bonneville lake cycle. 63

64 64 In more landward settings, such as the major embayment found on the Matlin quadrangle that has the subprovo, Bonneville, and Provo shorelines all deposited within or at its mouth, the TST is represented by numerous shorelines that backstepped as lake levels rose (Figure 5.3). These shorelines developed in shallow waters as a result of wave action and longshore currents present in shallower waters, which could affect sediment that had been previously deposited. The currents and waves could do this since there are no major rivers or sediment sources entering the Matlin quadrangle. The first shoreline found in this area is the subprovo, which is interpreted as a parasequence since it is a rise in lake level that is separated from surrounding sediments by a flooding surface and not an unconformity. Above the subprovo in the area between the Matlin Mountains and Coyote Hill exists a series of shorelines between the Provo and Bonneville levels that are also interpreted as parasequences formed during the lake level rise. This is because some of these barriers are all or partly draped with marl. On the geologic map of the Matlin quadrangle they were not mapped as lm/lg because it was only found in minor amounts mixed in with alluvial fans on the front of the barriers or mixed in with lagoonal sediments deposited behind the barriers. In addition, there are no stillstands in lake level after the Bonneville flood which could have produced a shoreline between the elevation of the Provo shoreline and the Bonneville shoreline. This means that any shoreline or barrier that records a stillstand in lake level at these elevations had to be deposited on the transgression as Lake Bonneville rose to its highest levels. The shorelines that are between the subprovo and Bonneville shorelines comprise a retrogradational parasequence set.

65 Figure 5.3. Sequence stratigraphy of the Quaternary Lake Bonneville deposits preserved on the Matlin quadrangle, not to scale. Figure is a north-south cross section with the deep-water representation inserted approximately where it is found in the field.

66 66 After lake levels reached their maximum, the HST was deposited. The HST in proximal areas is marked by deposition of the Bonneville shoreline and white marl at a depth of 110 feet, which was the first mappable occurance. The rapid drop in water level due to the threshold collapse that resulted in the Bonneville flood halted at the Provo level. The Provo shoreline could be placed into the late HST, but is more appropriately considered a forced regression shoreline and part of the Falling Stage Systems Tract (FSST) of Plint and Nummedal (2000; Figure 2.8). After formation of the Provo shoreline, the lake level continued to drop due to the prevailing climatic conditions (Madsen et al., 2001) until the Bonneville lake cycle was terminated just before a short-lived, low-level transgression of Great Salt Lake (Oviatt et al., 1992). The sequence stratigraphic architecture recorded on the Matlin quadrangle is markedly different from that described in previous work on the Bonneville lake cycle that was conducted along the eastern margin of Lake Bonneville, adjacent to the Wasatch Mountains (Oviatt et al., 1994; Anderson and Link, 1998; Milligan and Chan, 1998; Lemons and Chan, 1999). Lemons and Chan (1999) recognized only two retrogradational parasequences, whereas on the Matlin quadrangle eight parasequences are preserved in the retrogradational parasequence set. In addition, the studies along the eastern shores of Lake Bonneville make no mention of the subprovo deposits that have been shown to exist throughout the Bonneville basin (Sack, 1999). All that was recorded from those areas was a transgression to the Bonneville shoreline, interrupted by two unnamed standstills, and a drop in lake level during the Bonneville flood. These eastern study areas all occur where large sediment inputs, emanating from the Wasatch Mountains, could serve to mask or

67 67 overprint many of the punctuated changes in lake level that may be found elsewhere in the Bonneville basin. The sequence architecture found on the Matlin quadrangle records a different lake level history of punctuated rises in lake level up to the Bonneville level because the reduced sediment supply allowed the preservation of the subtle landforms associated with the lake level rise. The Lake Bonneville sequence is interpreted to represent the deposits of a predominantly Balanced-Fill lake as opposed to an Underfilled or an Overfilled lake (Carroll and Bohacs, 1999; Bohacs et al., 2000; Figure 5.4). Balanced-Fill refers to a lake basin where potential accommodation is equal to the water level and sediment fill during the deposition of a single unit (Carroll and Bohacs, 1999; Bohacs et al., 2000). At any given time the water level and sediment fill could be enough to raise the lake level to sill height and allow outflow. Conversely, a significant water level drop can occur and, as a result, alternating deep and shallow water deposits are common, along with a record of shoreline movement, in Balanced-Fill lakes (Carroll and Bohacs, 1999; Bohacs et al., 2000). The Lake Bonneville deposits do not exhibit the characteristics of an Underfilled lake where potential accommodation is greater than what the water level and sediment fill would ever be able to fill. Similarly, the deposits do not show evidence consistent with an overfilled lake basin where the total accommodation is not sufficient to contain the combination of water and sediment. Ultimately, the best evidence for Lake Bonneville being Balanced Fill is the length of preserved shorelines, a total of approximately 6,800 km (Currey in Sack, 1992). The shorelines record numerous rises and falls in lake level, which would be expected in a Balanced Fill lake basin.

68 Figure 5.4. Classification for lake basins from Bohacs et al. (2000). 68

69 69 Unlike other Balanced-Fill lakes, Lake Bonneville does exhibit a change in character as the lake level rose toward the Bonneville level. As the lake transgressed and deposited the TST, there was what might have been a significant stillstand in lake level marked by the subprovo shoreline between about 20.8 and 17.7 ka, maintained by the original bedrock threshold (Sack, 1999). This threshold may have subsequently become plugged by alluvium allowing Lake Bonneville to rise to its maximum height. Once at its highest water level the lake overflowed but only for a short while before the alluvium plug failed and the Bonneville flood occurred, after which lake level fell according to the prevailing climatic conditions. During this time the lake remained a Balanced Fill lake basin since there was no change in potential accommodation from the previous lake cycle to create an Underfilled lake basin, or a dramatic increase in sediment fill and water level due to climatic changes to create an Overfilled lake basin. This ultimately shows that changes in the hydrology of a lake basin can have dramatic effects on the lake but ultimately leave the lake basin s character unchanged. Previous studies have shown that lakes can alternate between different lake basin types due to tectonics, water level, and sedimentation during one lake cycle (Carroll and Bohacs, 1999; Bohacs et al., 2000). The damming of the threshold of Lake Bonneville after subprovo shoreline time would have allowed the lake to reach previously unattainable heights while remaining a Balanced-Fill lake basin. This is because, even though new heights were reached due to the new threshold height, the lake would still have been only filling the available accommodation. In addition, Lake Bonneville did not overflow for more than half of the total Bonneville lake cycle. The alluvial threshold ultimately failed and the Bonneville flood produced a

70 70 drastic drop in lake level to the bedrock threshold that may also have controlled the lake at the subprovo level. After formation of the Provo shoreline, Lake Bonneville continued to recede due to climatic conditions. This shows that within one lake cycle drastic changes of lake level can be controlled by an alteration of threshold height in addition to climate (Sack, 2001).

71 71 Chapter 6 - Conclusions Research conducted for this project on the Matlin quadrangle: Shows that there is a spatial distribution of Quaternary sediments found on the Matlin quadrangle dominated by post-lake alluvial fan and Lake Bonneville coastal gravel deposits near the piedmont junction in the north, mixed lake and fan deposits in the central part of the map, and alluvial fan and lacustrine fine-grained deposits in the south. Supports the notion that the Bonneville shoreline is at least a compound feature. Supports the notion that a transgressive-phase stillstand or oscillation occurred somewhat close below the elevation of the regressive-phase Provo shoreline. Identifies and describes formerly unreported bentonitic and tuffaceous deposits that lie beneath the surficial Quaternary sediments in the central part of the map. Research conducted for the sequence stratigraphic interpretation of the Lake Bonneville sediments: Shows that the sediments of the Bonneville cycle on the Matlin quadrangle are composed of a TST of many punctuated lake-level rises, oscillations, and pauses creating a retrogradational parasequence set up to the level of the Bonneville shoreline.

72 72 Shows that the HST is not well developed on the Matlin quadrangle due to the short occupation time at the Bonneville shoreline elevation and low sedimentation rates. Shows that the Bonneville flood initiated a forced regression and the FSST consists of the Provo gravels deposited on the subprovo platform in proximal settings and the reworked marl above the white marl in distal settings. Shows that the Bonneville cycle was predominantly a Balanced-Fill lake (according to Bohacs et al., 2000) even though it showed periods of outflow and low lake levels.

73 References 73 Anderson, S.L. and Link, P.K., Lake Bonneville sequence stratigraphy, Pleistocene Bear River Delta, Cache Valley, Idaho, in J.K. Pitman and A.R. Carroll, eds., Modern and ancient lake systems. Utah Geological Association Guidebook 26, p Bartov, Y., Stein, M., Enzel, Y., Agnon, A., and Reches, Z., Lake levels and sequence stratigraphy of Lake Lisan, the late Pleistocene precursor of the Dead Sea. Quaternary Research v. 57, p Best, M.G. and Christensen, E.H., Limited extension during peak Tertiary volcanism, Great Basin of Nevada and Utah. Journal of Geophysics Research v. 96, p. 13,509-13,528. Bissell, H.J., Pennsylvanian-Permian Oquirrh Basin of Utah. Brigham Young University Research Studies, Geology Series v. 9, part 1, p Bohacs, K.M., Carroll, A.R., Neal, J.E., and Mankiewicz, P.J., Lake-basin type, source potential, and hydrocarbon character: an integrated sequence stratigraphicgeochemical framework, in E.H. Gierlowski-Kordesch and K.R. Kelts, eds., Lake basins through space and time. AAPG Studies in Geology 46, p Carroll, A.R. and Bohacs, K.M., Stratigraphic classification of ancient lakes: balancing tectonic and climatic controls. Geology v. 27, p Coleman, S.M., Kelts, K.R., and Dinter, D.A., Depositional history and neotectonics ingreat Salt Lake, Utah, from high resolution seismic stratigraphy. Sedimentary geology v. 148, p Compton, R.R., Todd, V.R., Zartman, R.E., and Naeser, Charles, Oligocene and Miocene metamorphism, folding, and low-angle faulting in northwestern Utah. GSA Bulletin v. 88, p Davis, L.E., Dyman, T.S., and Webster, G.D., Correlation of the West Canyon limestones (upper Mississippian to middle Pennsylvanian), basal formations of the Oquirrh Group, northern Utah and southeastern Idaho. USGS Bulletin, 301. Dickinson, W.R., Paleozoic plate tectonics and the evolution of the Cordilleran continental margin. IN Stevens, C.H. (ed.), Paleozoic Paleogeography of the western United States, Pacific Coast paleogeography symposium I, p Erskine, M.C., The Oquirrh Basin revisited. AAPG Bulletin v. 81, no. 4, p

74 Gans, P.B. and Bohrson, W.A., Suppression of volcanism during rapid extension in the Basin and Range province, United States. Science v. 279, p Geslin, J.K., Distal ancestral Rocky Mountains tectonism: Evolution of the Pennsylvanian-Permian Oquirrh-Wood River basin, southern Idaho. GSA Bulletin v. 110, no. 5, p Gilbert, G.K., Lake Bonneville. U.S. Geological Survey Monograph 1, 427 p. Gilluly, J., Geology and ore deposits of the Stockton and Fairfield quadrangles, Utah. U.S.G.S. Professional Paper 173, 171 p. Hintze, L.F., Geologic Map of Utah. Utah Geological and Mineral Survey Map, scale 1:500,000. Hunt, D. and Tucker, M.E., Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level fall. Sedimentary Geology v. 81, p Hunt, D. and Tucker, M.E., Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level fall, reply. Sedimentary Geology v. 95, p Jordan, T.E., Lithofacies of the Upper Pennsylvanian and Lower Permian western Oquirrh Group, northwest Utah. Utah Geology, v. 6, p Jordan, T.E. and Douglas, R.C., Paleogeography and structural development of the late Pennsylvanian to early Permian Oquirrh Basin, northwestern Utah. IN Fouch, T.D. and Magathan, E.R (eds.), Paleozoic Paleogeography of West-Central United States. Rocky Mountain Section SEPM West-Central United States paleogeography symposium I, p Lemons, D.R. and Chan, M.A., Facies architecture and sequence stratigraphy of fine-grained lacustrine deltas along the eastern margin of late Pleistocene Lake Bonneville, northern Utah and southern Idaho. AAPG Bulletin v. 83, p Madsen, D.B., Rhode, D., Grayson, D.K., Broughton, J.M., Livingston, S.D., Hunt, J., Quade, J., Schmitt, D.N., and Shaver III, M.W., Late Quaternary environ mental change in the Bonneville basin, western USA. Palaeogeography, Palaeoclimatology, Palaeoecology v. 167, p McCoy, W.D., Quaternary aminostratigraphy of the Bonneville basin, western United States. Geological Society of America, v. 98, p

75 75 Milligan, M.R. and Chan, M.A., Coarse-grained Gilbert deltas: facies, sequence stratigraphy and relationships to Pleistocene climate at the eastern margin of Lake Bonneville, northern Utah, in K.W. Shanley and P.J. McCabe, eds., Relative role of eustasy, climate, and tectonism in continental rocks. SEPM Special Publication No. 59, p O Connor, J.E., Hydrology, hydraulics, and geomorphology of the Bonneville flood. GSA Special Paper No. 274, 83 p. Oviatt, C.G. and Nash, W.P., Late Pleistocene basaltic ash and volcanic eruptions in the Bonneville basin, Utah. GSA Bulletin v. 101, p Oviatt, C.G., Currey, D.R., and Sack, D., Radiocarbon chronology of Lake Bonneville, eastern Great Basin, USA. Palaeogeography, Palaeoclimatology, Palaeoecology v. 99, p Oviatt, C.G., McCoy, W.D., and Nash, W.P., Sequence stratigraphy of lacustrine deposits: a Quaternary example from the Bonneville Basin, Utah. GSA Bulletin v. 106, p Oviatt, Lake Bonneville fluctuations and global climate change. Geology v. 25, p Oviatt, C.G., Bright, J., Forester, R.M., Kaufman, D.S., and Thompson, R.S., Reinterpretation of the Burmester core, Bonneville basin, Utah. Quaternary Research, v. 52, no. 2, p Perkins, M.E., Brown, F.H., Nash, W.P., McIntosh, W., and Williams, S.K., Sequence, age, and source of silicic fallout tuffs in the middle to late Miocene basins of the northern Basin and Range province. GSA Bulletin v. 110, p Perkins, M.E. and Nash, B.P., Explosive silicic volcanism of the Yellowstone hotspot: The ash fall tuff record. GSA Bulletin v. 114, no. 3, p Plint, A.G. and Nummedal, D., The falling stage systems tract: recognition and importance in sequence stratigraphic analysis, in D. Hunt and R.L. Gawthorpe, eds., Sedimentary responses to forced regressions. Geological Society Special Publication no.172, p Posamentier, H.W., Allen, G.P., James, D.P., and Tesson, M., Forced regressions in a sequence stratigraphic framework: concepts, examples, and exploration significance. AAPG Bulletin, v. 76, p

76 Posamentier, H.W. and Allen, G.P., Siliciclastic Sequence Stratigraphy Concepts and Applications. SEPM Concepts in Sedimentology and Paleontology #7, Tulsa, OK, 210 p. Roberts, R.J., Crittenden Jr., M.D., Tooker, E.W., Morris, H.T., Hose, H.K., and Cheney, T.M., Pennsylvanian and Permian basins in northwestern Utah, northeastern Nevada, and south-central Idaho. AAPG Bulletin v. 49, no. 11, p Ross, C.A., Pennsylvanian paleogeography of the western United States. IN Copper, J.D. and Stevens, C.H. (eds.), Paleozoic Paleogeography of the Western United States-II. Pacific Section SEPM, v. 67, p Sack, D., Reconstructing the chronology of Lake Bonneville; an historical review. In K.J. Tinkler (editor), Binghamton Symposia in geomorphology: International series vol.19. Allen & Unwin, London. p Sack, D., Obliteration of the surficial paleolake evidence in the Tule Valley subbasin of Lake Bonneville. IN, Quaternary Coasts of the United States: Marine and Lacustrine Systems. SEPM Special Publication No. 48, p Sack, D., 1994a. Geology of the Coyote Knolls quadrangle, Millard County, Utah. Utah Geological Survey, 18 p. Sack, D., 1994b. Geology of the Swasey Peak NW quadrangle, Millard County, Utah. Utah Geological Survey, 16 p. Sack, D., The composite nature of the Provo level of Lake Bonneville, Great Basin, western North America. Quaternary Research v. 52, p Sack, D., Shoreline and basin configuration techniques in paleolimnology. Tracking Environmental Change Using Lake Sediments 1: Saez, A. and Cabrera, L., Sedimentological and paleohydrological response to tectonics and climate change in a small, closed, lacustrine system: Oligocene As Pontes Basin (Spain). Sedimentology v. 49, p Sewell, R.E. and Shroder, J.F., Red Rock landslip: Massive failure initiated by late Pleistocene Bonneville Flood. Geological Society of America Abstracts, v. 13, p Shanley, K.W. and McCabe, P.J., Perspectives on the sequence stratigraphy of continental strata. AAPG Bulletin v. 78, p

77 77 Sonder, L.J. and Jones, C.H., Western United States extension: How the west was widened. Annual Review of Earth and Planetary Science v. 27, p Stevens, C.H., Permian paleogeography of the western United States. IN Copper, J.D. and Stevens, C.H. (eds.), Paleozoic Paleogeography of the Western United States-II. Pacific Section SEPM, v. 67, p Todd, V.R., Late Miocene displacement of pre-tertiary and Tertiary rocks in the Matlin Mountains, northwestern Utah. Geological Society of America Memoir 157, p Trexler Jr., J.H., Snyder, W.S., Cashman, P.H., Gallegos, D.M., and Spinosa, C., Mississippian through Permian orogenesis in eastern Nevada: Post-Antler, pre- Sonoma tectonics of the western Cordillera. IN Copper, J.D. and Stevens, C.H. (eds.), Paleozoic Paleogeography of the Western United States-II. Pacific Section SEPM, v. 67, Vail, P.R., Mitchum, R.M., and Thompson, S. III, Seismic stratigraphy and global changes of sea level, part 3: relative changes in sea level from coastal onlap. IN Payton, C.W. (ed.), Seismic Stratigraphy application to Hydrocarbon Exploration. AAPG Memoir 26, p Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S., and Hardenbol, J., An overview of the fundamentals of sequence stratigraphy and key definitions, in C.K. Willgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross, and J.C. Van Wagoner, eds., Sea-level changes: an integrated approach. SEPM Special Publication No. 42, p Welsh, J.E. and James, A.H., Pennsylvanian and Permian stratigraphy of the central Oquirrh Mountains, Utah. Guidebook to the Geology of Utah, v. 16, p Williams, Late Cenozoic tephrostratigraphy of deep sediment cores from the Bonneville Basin, northwest Utah. GSA Bulletin v. 105, p

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