Leah Toms. established on the adjacent floodplain. Two styles of avulsion have been observed:

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2 Characterizing relationships between channel, floodplain, and avulsion deposits in the Wasatch Formation (Paleocene/Eocene, Colorado) and interpreting controls on avulsion style through the Paleocene-Eocene Boundary Abstract Leah Toms Avulsion is the process by which a channel pathway is abandoned and a new course is established on the adjacent floodplain. Two styles of avulsion have been observed: progradational and incisional avulsion. Conditions that might control avulsion style include floodplain slope, grain-size distribution, vegetation, sediment supply, and water-table levels. By studying ancient stratigraphic records of channel avulsion, we can compare numerous avulsion deposits in an effort to elucidate factors that control avulsion style. We consider progradational avulsion to be associated with transitional avulsion stratigraphy, wherein channel bodies are underlain by proximal overbank deposits and crevasse splays (referred to as avulsion deposits). Incisional avulsion may be associated with abrupt avulsion stratigraphy, where channel bodies directly overlie distal floodplain deposits. In order to understand the different floodplain and channel conditions associated with each style of avulsion, we evaluate the relationships between channel, floodplain, and avulsion deposits in the Wasatch Formation (Paleocene-Eocene, Colorado). The Wasatch Formation straddles the Paleocene-Eocene boundary and comprises three members: the lower Atwell Gulch Member and upper Shire Member which are both mud dominated, and the middle Molina Member which is sand rich. The Atwell Gulch Member contains small, single story channel sandstones that overlie well-developed, abundant clay-rich floodplain paleosols. The Molina Member exhibits large, sheet-like channel bodies that are generally preceded by coarsening upward avulsion successions associated with decreased paleosol development. The Shire Member contains large, multi-story channel sandstones 1

3 sometimes underlain by well-developed floodplain paleosols and other times underlain by coarsening upward, proximal overbank deposits. The scarcity of avulsion deposits in the Atwell Gulch Member suggests that the primary mechanism for channel relocation was incisional avulsion. In contrast, common avulsion deposits in the Molina Member imply that Molina avulsions were dominantly progradational. Shire Member channel bodies variably overlie avulsion deposits and floodplain deposits, which may indicate a mix of avulsion styles. Shifts in avulsion style through the Wasatch may be linked to changes in floodplain and channel characteristics resulting from climate change at the P-E boundary. 2

4 1. Introduction River avulsion, the process by which a flow establishes a new course in a river basin, is an important control on sedimentary basin filling, hydrocarbon reservoir quality and distribution, and can also be a hazard to people living near river systems. A deeper understanding of avulsion processes is necessary for prediction and management of modern river systems and subsurface reservoirs. However, because river avulsion typically has very long recurrence intervals (on the order of 1 kyr; Bridge and Leeder (1979), Mackey and Bridge (1995)) it can be difficult to constrain processes that control avulsion behavior from modern systems alone. Ancient fluvial deposits provide information about avulsion conditions, and although data are somewhat imprecise, can provide an important, long timescale perspective on avulsion behavior. Here we focus on comparing channel, floodplain, and avulsion deposits in the different members of the Wasatch Formation in order to better understand how floodplain and channel conditions influence the sedimentary record of avulsion. There are two styles of avulsion observed: progradational and incisional avulsion (Jones & Hajek 2007, Mohrig et al 2000, Slingerland & Smith 2004). During progradational avulsion, abundant crevasse splays and coarse over-bank deposits (collectively, heterolithic avulsion deposits ; Kraus and Wells (1999)) rapidly accumulate on the floodplain adjacent to the parent channel prior to avulsion (Figure 1A). Research on modern systems and Holocene deposits suggest that progradational avulsions can be associated with low floodplain slopes, relatively coarse suspended sediment, dense vegetation, and high water-tables, which all discourage rapid overbank flow and promote sediment deposition (Mohrig et al 2000). During incisional avulsion, new channels are established primarily through floodplain incision. Consequently few heterolithic avulsion deposits are produced during channel relocation (Figure 1B). Incisional 3

5 avulsions may be associated with steep floodplain slopes, fine-grained suspended sediments, sparse vegetation, and low water-tables, which all encourage rapid overbank flow and floodplain erosion (Jones & Hajek 2007, Mohrig et al 2000, Slingerland & Smith 2004). Figure 1. Diagram of progradational (A) and incisional (B) avulsion styles after Mohrig et al. (2000) Modern and Holocene avulsion deposits provide limited examples for observing avulsion processes; the ancient stratigraphic record, however, can provide abundant information on avulsion style and the floodplain and channel conditions that control avulsion style. Two characteristic avulsion patterns have been distinguished in the stratigraphic record that may reflect whether an ancient channel avulsed via incision or progradation (Jones & Hajek 2007). Transitional avulsion stratigraphy is identified by an overall coarsening upward sequence of proximal overbank deposits (i.e. crevasse splays) and a decrease in paleosol development below the main channel sand body. In contrast, fine-grained, distal-overbank deposits with welldeveloped paleosols directly underlying channel sand body deposits indicate stratigraphically abrupt avulsion deposits. Jones and Hajek (2007) propose that stratigraphically transitional avulsion deposits reflect progradational avulsion processes and stratigraphically abrupt avulsion 4

6 deposits may indicate incisional avulsions. Here we compare avulsion stratigraphy to channel and floodplain characteristics in an effort to reveal association between channel and floodplain conditions and avulsion style. We observe avulsion stratigraphy in the three members of the Paleocene-Eocene Wasatch Formation (western Colorado), which contain abundant avulsive channel sand bodies (Lorenz & Nadon 2002, Mohrig et al 2000). By characterizing the avulsion stratigraphy for each member into transitional or abrupt and recording the channel and floodplain deposits related to these avulsion deposits, observations can be made about the floodplain and channel conditions that relate with these patterns. Additionally, the Wasatch Formation straddles the Paleocene-Eocene Thermal Maximum Event, which allows us to investigate how climate change may influence channel avulsion behavior. 2. Overview of avulsion and possible controls on avulsion style 2.1. Avulsion Processes When observing avulsion deposits in the ancient stratigraphic record, it is necessary to understand avulsion processes. Avulsion can generally be divided into 3 stages: initiation, finding, and stabilization (Hajek & Wolinsky 2012). The initiation stage refers to the steps for setting up an avulsion and the trigger necessary to begin an avulsion. Research has proposed that the conditions necessary for setting up channel avulsion is a steeper cross-valley water-surface slope compared to down-valley slope and/or critical super elevation of channel waters above mean floodplain height (Mohrig et al 2000, Slingerland & Smith 2004). Cross-valley watersurface slope refers to the slope created by perching of the channel via levees above the floodplain, also known as an alluvial ridge (Slingerland & Smith 2004). A higher cross-valley slope to down-valley slope encourages an unstable system, which can ultimately lead to 5

7 avulsion. A trigger is then needed to initiate avulsion of the unstable channels and cause flow diversion (Slingerland & Smith 2004). The trigger, such as flooding or log jams, will cause a change in local channel flow dynamics that serves to reroute water and sediment through a new path. Over geologic timescales, triggers are ubiquitous; consequently differential aggradation between the channel and floodplain is typically the rate-limiting factor for avulsion (Jerolmack & Paola 2007, Reitz et al 2010). The diverted flow will seek a suitable path adjacent to the parent channel in a step referred to as the finding stage of channel avulsion (Hajek & Wolinsky 2012). During this phase, the avulsion will proceed either progradationally or incisionally across the floodplain depending on a number of channel and floodplain conditions (see Section 2). In some cases, flow is quickly diverted and small quantities of sediment are deposited over the floodplain (Hajek & Wolinsky 2012, Tooth et al 2007). In other cases, this finding phase is associated with large quantities of sediment deposited across the floodplain (Hajek & Wolinsky 2012). Once the flow chooses a suitable path, it stabilizes and becomes confined in a new channel on the floodplain, during which the new channel may incise and deepen. Ultimately, a successful avulsion over geologic timescales will stabilize in a new location on the floodplain where there is constant flow potential and a favorable downstream path (Hajek & Wolinsky 2012, Slingerland & Smith 2004) Channel and floodplain controls on avulsion style During the finding phase of channel avulsion, the path a flow takes may be dependent upon two factors: the speed of overland flow across the floodplain and overland flow confinement. These two factors may be controlled by a multitude of channel and floodplain conditions including down-valley floodplain slope, water-table level, grain-size distribution, and 6

8 vegetation. The speed of overland flow and overland-flow confinement may directly affect whether a system avulses progradationally or incisionally Floodplain Slope Floodplain slope is a first order control on overland flow speed (steeper slopes result in faster flows), and consequently influences local water flux, flow shear stress, and sediment transport across floodplains. An increased floodplain slope results in faster down-valley overland flow which in turn promotes sediment transport and can subsequently inhibit deposition of coarse sediment on floodplains and/or promote sediment entrainment from the floodplain. The result is floodplain incision (or lack of deposition) which may help promote corralling of overland flows, further increasing shear stress in focused pathways across the floodplain (Tooth et al 2007). This tendency towards fast flows and incision may promote incisional avulsion style. In contrast, a low floodplain slope results in slower down-valley overland flow thereby promoting overbank deposition (Mackey & Bridge 1995). A low floodplain slope prevents incision and therefore inhibits flow corralling and incision into the floodplain. Slower overland flows and sediment deposition, including crevasse splays and anastomosed channel networks, may promote progradational avulsion style Water-table levels The water-table level (or floodplain drainage) in a system may also influence avulsion style by either speeding up or slowing down overland flow. When flow from a channel runs onto a well-drained floodplain (i.e. a low water-table), the speed of the overland flow may be relatively fast. Rapid overland flow across the floodplain may encourage floodplain erosion and consequently incisional avulsion style may be more dominant in these systems. In contrast, when overbank or crevasse flow from a channel encounters higher water-table levels on the floodplain 7

9 (i.e. poorly drained floodplains) flow will tend to slow down. Poorly drained floodplains may result in progradational avulsion because slower flow across the floodplain may encourage sediment deposition. Systems with higher water-table levels may also be prone to standing bodies of water, such as floodplain lakes. When a channel avulses and overland flow runs into one of these features via tie channels, for example, it promotes sedimentation similar to how deltas form at the margins of larger water bodies (Rowland et al 2008). Consequently, progradational avulsion may be more common in poorly drained, wet flood basins, whereas dry floodplain may be associated with incisional avulsion Sediment Grain-size Distribution The proportion of coarse bedload and fine suspended sediment and washload in a system may also be an important factor governing avulsion style by way of controlling overland flow confinement and the potential to distribute coarse sediment across the floodplain. A system with abundant fine-grained sediment (silt and especially clay) will typically have a cohesive floodplain which may promote confinement of overland flow. Confined flows cannot readily widen and are associated with higher bed shear stresses, resulting in increased erosion and further confinement (Edmonds & Slingerland 2010). A system with a higher proportion of finegrained washload may therefore tend to promote incisional avulsion due to increased erosion. In contrast, a system that is dominated by sand (with relatively little clay) bedload may have a less cohesive floodplain and be less confined, leading to channel widening and enhanced deposition (especially near the channel margins)(edmonds & Slingerland 2010). Non-cohesive systems bay be more prone to progradational avulsions. 8

10 Vegetation Vegetation affects both overland flow speed and overland flow confinement on floodplains; therefore, the degree of influence that vegetation has on avulsion style may be interrelated with other floodplain conditions previously mentioned. Dense vegetation slows down overland flow across floodplains, which leads to the deposition of suspended sediment, and potentially promotes progradational avulsions. However, dense vegetation, particularly plants that help bind sediment together, may result in increased overland flow confinement due to its tendency to corral flows, encouraging erosion into the floodplain. In this case, vegetation would promote incisional avulsion style. Generally, however, sparse vegetation should encourage rapid overland flow and therefore inhibit sediment deposition and promote floodplain erosion Interplay of channel and floodplain controls on avulsion The floodplain conditions mentioned above all have an effect on overland flow confinement and/or the speed of overland flow, which we consider the main controls on avulsion style. We hypothesize that progradational avulsion is most likely when deposition is common, i.e. when overland flows are slow and/or unconfined (Figure 2). A paucity of cohesive sediments (clays) and vegetation can lead to unconfined flows, and low floodplain slopes, standing water, or dense vegetation can lead to slow overland flows. When flow is diverted from the main channel, there is a decrease in sediment carrying capacity which causes coarser sediments to deposit immediately onto the floodplain, while fine sediments are carried further out onto the floodplain (Slingerland & Smith 2004). Because there is a low degree of overland flow confinement, these sediments are deposited into a wedge shaped fan that progrades across the floodplain (Slingerland & Smith 2004). The slow overland flow speed reduces shear stress across 9

11 the floodplain and so incision is less likely to occur prior to avulsion. An example of a modern progradational avulsion system is the Saskatchewan River in Canada (Perez-Arlucea & Smith 1999, Smith et al 1989). We suggest that incisional avulsions will be more common in systems with relatively rapid, confined overland flows (Figure 2). Overland flow confinement may be enhanced by abundant fine sediment supply and/or dense vegetation, and rapid overland flow may be a result of steep floodplain slows, low water-table level, or sparse vegetation. These conditions may lead to rapid runoff which encourages incision and sediment entrainment (Slingerland & Smith 2004). Modern systems that produce incisional avulsions include the Klip River in South Africa (Tooth et al 2007), the Columbia River (Makaske et al 2002), and the Australian rivers studied by Nanson (1986). Figure 2. Conceptual diagram of controls on avulsion based on overland flow confinement and the speed of overland flow 10

12 3. Sedimentary (stratigraphic) record of avulsion style Both the finding phase and the stabilization phase of channel avulsion produce deposits that are seen in the stratigraphic record (Hajek & Wolinsky 2012). Two end-member types of avulsion patterns in the stratigraphic record are considered related to the finding phase of avulsion and are classified based on the types of deposits directly underlying the paleochannel (Jones & Hajek 2007). In this study, we focus primarily on sediments deposited immediately below and surround channel-belt sandstones in the Wasatch Formation in order to identify channel and floodplain conditions that reflect differences stratigraphic record of the finding and stabilization phases of avulsion to produce a certain avulsion pattern observable in the stratigraphic record. Stratigraphically transitional avulsion deposits are noted by crevasse splays and other proximal overbank deposits (collectively heterolithic avulsion deposits ; Kraus and Wells 1999) underlying a main channel-belt deposit (Figure 3A). Proximal overbank deposits, found subjacent and laterally from a channel body, consist of fine-grained sediment associated with crevasse splays and crevasse channels (Jones & Hajek 2007). These deposits may contain ripples, bioturbation, rooting, and weak floodplain development, suggestive of intermittent flow conditions (Jones & Hajek 2007) and relatively rapid aggradation. Here we also use evidence of decreasing paleosol development as additional evidence of progradational avulsion (Hajek & Wolinsky 2012). We consider local sedimentation rate to be the primary control on paleosol development, consequently, weakly developed paleosols should be found on rapidly aggraded portions of the floodplain, i.e. near channels, and vice versa (Kraus 1987). Stratigraphically abrupt avulsion deposits are classified by channel deposits directly overlying distal overbank deposits (Figure 3B) (Jones & Hajek 2007, Kraus & Wells 1999). 11

13 Distal overbank deposits, found subjacent to the channel deposit, generally consist of finegrained mudstones associated with overbank flooding (Jones & Hajek 2007) and well-developed paleosols (Kraus 1999). The deposits associated with abrupt avulsion stratigraphy lack coarsegrained proximal-overbank and crevasse-splay deposits. ~ 2m ~ 2m Figure 3. Stratigraphic comparison between transitional avulsion stratigraphy (A) and abrupt avulsion stratigraphy (B) as seen in the Wasatch Formation of western Colorado. Yellow represents channel deposits and red represents crevasse splay deposits. The purple/red pattern underlying the abrupt avulsion pattern (B) is considered welldeveloped//distal floodplain. By categorizing avulsion deposits into stratigraphically transitional or stratigraphically abrupt, we may be able to infer differences in the processes that deposited these sediments and controlled avulsions in ancient systems (Jones & Hajek 2007). The transitional pattern reflects a phase of increased sediment deposition prior to channel avulsion whereas the abrupt pattern reflects non-deposition and perhaps incision directly into the floodplain during avulsion (Jones & Hajek 2007). Both end member types of avulsion stratigraphy seen in the rock record may reflect the style of avulsion that governs the system. 12

14 4. Study area and methods 4.1. The Wasatch Formation The Paleocene-Eocene Wasatch Formation is a fluvial dominated succession that was deposited during the end of the Laramide Orogeny, from about 70 Ma to 40 Ma, into the Piceance, Uinta, and Green River basins of the western United States (Lorenz & Nadon 2002, Mohrig et al 2000). Rivers carried sediments from the Rock Springs Uplift, the Wind River Uplift, the Umcompahgre Uplift, and the White River Uplift, as well as sediments from other minor uplifts, and deposited them into the three alluvial basins (Franczyk 1992, Lorenz & Nadon 2002, Mohrig et al 2000). The Wasatch Formation overlies the Upper Cretaceous Mesaverde Group (marked by an unconformity) and conformably underlies lacustrine deposits of the Eocene Green River Formation (Lorenz & Nadon 2002). The area used in this study is a segment of the Wasatch Formation that was deposited in the Piceance Basin of western Colorado (Figure 4). Figure 4. Field Area in western Colorado (from Mohrig 2000) The Wasatch Formation has three members: the lower Atwell Gulch Member, the middle Molina Member, and the upper Shire Member (Figure 5). All three members are composed of channel deposits, crevasse splay deposits, overbank deposits, and floodplain paleosols of varying 13

15 development, making it an ideal location to study varying patterns in avulsion stratigraphy. The Atwell Gulch and Shire members are both mudstone-dominated, whereas the Molina Member is sand-rich (Lorenz & Nadon 2002). Previous studies suggest that the Wasatch Formation is dominated by stratigraphically abrupt avulsion stratigraphy (Jones & Hajek 2007, Mohrig et al 2000). These studies, however, focus mainly on the mudstone-dominated Shire Member. This research focuses on the variable trends in avulsion patterns and related floodplain deposits we see in the record in each of the three members of the Wasatch Formation. Figure 5. Overview of The Wasatch Formation in western Colorado; ages from Foreman (2011) During the deposition of the Wasatch Formation, a large climatic shift occurred at the P- E boundary known as the Paleocene-Eocene Thermal Maximum event (PETM) (Foreman 2011, Koch et al 2003). The Molina Member was deposited during and after the PETM, during which channel and floodplain characteristics may have changed in response to climate forcing (Foreman 2011). A study by Smith et al (2009) in the Willwood Formation (Bighorn Basin, Wyoming) suggests that during and after the PETM, floodplain conditions were drier and allowed for longer periods of landscape stability and soil development. We can compare overall 14

16 climate conditions in the Bighorn Basin to the Piceance Basin based on a climate model of the Paleogene Laramide Foreland by Sewall and Sloan (2006) in which the Piceance Basin seems to have more arid conditions compared to the Bighorn Basin. In the Piceance Basin, it is thought that the rise in temperature resulting from the PETM caused a rise in seasonality, or more extreme periods of wetting and drying, which in turn caused increased sediment supply and greater in channel sedimentation rates (Lorenz & Nadon 2002). These changes may have had an effect on the style of avulsion that dominates throughout each of the three members of the Wasatch Formation Study methods A combination of measured section and photo panels were used to classify avulsion stratigraphy in each member of the Wasatch Formation. Approximately 30 channel deposits and the surrounding overbank and non-overbank deposits were studied. Channel thickness and width were noted for each measured channel, as well as channel stacking patterns (i.e. multi-story vs. single-story). We measured paleoflow depths using bar clinoforms (Mohrig et al 2000). Grainsize, sedimentary structures, and the contact between the sand body and the underlying deposits were also recorded. Overbank deposits surrounding each paleochannel were described and categorized as either distal or proximal overbank deposits. Distal overbank deposits for a particular member represent characteristic deposits found far from channel deposits. Paleosol development was graded based on the abundance and development of pedogenic features, including mottling, slickensides, horizon distinction and color, and overall thickness of the paleosols. We assume that weakly developed paleosols have little mottling, lack slickensides, have weak horizons, and are generally thin, and strongly developed paleosols have generally more and better developed 15

17 pedogenic features, have strong horizons, and are generally thick. Grain-size was also measured within proximal and distal floodplain deposits. Splay frequency (number of splays/thickness of section) in each member was noted, as well as the sizes and vertical trends of these splays (particularly whether splay frequency was highest below and surrounding channel deposits. 5. Results 5.1. Atwell Gulch Member The Atwell Gulch Member contains sand bodies that are broadly distributed throughout paleosol and floodplain deposits (Figure 6). The sand bodies are 1-2 meter thick and are dominated by thinly laminated mudstones and very fine-grained sandstones. The dominant sedimentary structures in these bodies are trough-cross stratification. The sandstone bodies are single story, with average paleoflow depths of 1m. The reddish-purple paleosols within the Atwell Gulch are similar both immediately under the channel deposits and at a distance from the channel deposits (Figure 7B). Typically, the Atwell Gulch paleosols have strong horizons and a high clay to silt ratio. The paleosols also have a low frequency of splay deposits which does not increase near the channel sandstones. The avulsion stratigraphy was determined to be 100% abrupt (Table 1). 16

18 ~ 20m Figure 6. Overview of the Atwell Gulch Member (yellow represents channel deposits) ~ 1m ~ 20m Figure 7. Typical Atwell Gulch channel sand body and underlying deposits (A) and typical floodplain deposits (B) 5.2. Molina Member In contrast to the Atwell Gulch Member, channel-belt sandstones comprise a large fraction of the Molina Member (Figure 8). Molina Member sandstone bodies are typically 3-5 meters thick and are multi-story. There are planar parallel laminations and trough cross stratification within the typically fine-upper, medium-lower grained sand bodies, as well as 17

19 grain-size variations within the channel sandbodies (Figure 9A). The paleoflow depths were measured to be about 1.7m (Foreman 2011). Distal paleosols in the Molina member are typically greyish-green paleosols (Figure 9B). These paleosols have weakly developed or no horizons and a low clay-to-silt ratio. They have a high frequency of crevasse splay deposits, which increases near the channel sandstones. The avulsion stratigraphy of the Molina member was found to be 10% abrupt and 90% incisional (Table 1). ~ 10m Figure 8. Overview of the Molina Member (yellow represents channel deposits) 18

20 ~ 10m ~ 1m Figure 9. Typical Molina channel sand body and underlying deposits (A) and typical floodplain deposits (B) 5.3. Shire Member The Shire Member contains mostly large, single-story sand bodies with some multi-story sand bodies that are typically 5-6m thick and contain very fine to fine-grained sand (Figure 10). These sand bodies can be very erosive (Mohrig et al 2000), cutting into underlying crevasse splay deposits, other channel bodies, or distal floodplain deposits. Rip up clasts and trough cross stratification are present in these bodies (Figure 11A). The paleoflow depths in the Shire channels averaged to about 1.1m (Foreman 2011). Distal floodplain deposits in the Shire Member are reddish-orange with strong horizons and occasional carbonate nodules. Proximal paleosols are either reddish-tan or grey and show fewer pedogenic features, transition deposits, and are sometimes found immediately below channel-belt sandstones. The reddish-tan and grey paleosols are typically located underneath the channels (Figure 11B). There is a high frequency of splay deposits within the paleosols approaching the channels, which then decreases away from the channels. The avulsion stratigraphy was found to be 30% abrupt and 70% transitional (Table 1). 19

21 ~ 10m Figure 10. Overview of the Shire Member (yellow represents channel deposits) ~ 2m ~ 3m Figure 11. Typical Shire channel sand body and underlying deposits (A) and typical floodplain deposits (B) 20

22 Table 1. Table of General Results Member (# of measured channels) General Channel Characteristics General Floodplain Characteristics Avulsion Stratigraphy Atwell Gulch (5) -Small, single story channels -Spread out -Paleoflow Depths = 1m -Well-developed throughout -Thick -Infrequent splays 100% abrupt Molina (9) -Broad, sometimes multi-story channels -Compensational -Paleoflow Depths = 1.7m -Weakly developed throughout -Thin -Frequent splays 10% abrupt 90 % transitional Shire (10) -Large, single-story channels -Clustered -Erosive bases -Paleoflow Depths = 1.1m -Well-developed in background -Can be weakly developed or well-developed immediately under channel 30% abrupt 70% transitional 6. Discussion Here we take avulsion stratigraphy as evidence of the tendency for aggradation prior to avulsion (Figure 12A, C) and channel deposits lacking evidence of subjacent heterolithic avulsion deposits as potentially indicating incisional avulsion processes. As a channel stabilizes, even during progradational avulsion, there is a tendency for channel enlargement and incision (Figure 12B, D). However, we note here that channel deposits showing abrupt avulsion stratigraphy, e.g. those in the Atwell Gulch Member, are not simply cases where channel formation and deepening occurred at the end of a progradational avulsion. Excellent exposures and multiple observations confirm that aggradational deposits associated with avulsion were not readily produced by Atwell Gulch rivers. This suggests that characteristic channel and floodplain conditions in the Atwell Gulch inhibited progradational avulsion. Estimates in the Willwood Formation and the Saskatchewan River indicate that up to 40% of flood basin accumulation 21

23 results from avulsion processes (Perez-Arlucea & Smith 1999). While this may be true for systems that prograde via avulsion (e.g. the Molina) it is not the case for the Atwell Gulch Member, where aggradation associated with crevasse-splays and avulsion deposits can scarcely be identified. Figure 12. Avulsion style as it relates to avulsion stratigraphy: progradational avulsion (A) is related to transitional avulsion stratigraphy (C) whereas incisional avulsion (B) is related to abrupt avulsion stratigraphy (D) The Atwell Gulch Member, dominated by abrupt avulsion stratigraphy, may represent a fluvial system that tends to avulse by incision. In contrast, the Molina member, dominated by transitional avulsion stratigraphy, may reflect dominantly progradational avulsion style. Avulsion stratigraphy in the Molina and Atwell Gulch members may indicate that sediment supply was an important control on avulsion in these two systems. The mud-rich Atwell Gulch Member may have been highly confined, leading to relatively rapid overland flow and potentially incision, whereas the coarser Molina systems may have been less confined and more prone to rapid progradation (this is consistent with weaker paleosols in this interval). 22

24 The Shire member has both transitional and abrupt avulsion stratigraphy and consequently Shire fluvial systems may have avulsed both progradationally and incisionally. We observe that the Shire channels are very clustered (i.e. they are sometimes grouped together in one portion of the basin (Hajek et al 2010)), and therefore transitional avulsion deposits may not always reflect progradational avulsion processes. Some Shire channel bodies incise directly into underlying channel and crevasse splay deposits. In these cases, the crevasse splays are not associated with avulsion, but are levee deposits of underlying channels that are unrelated to the overlying channel, which results in a transitional pattern. The clustered channels tell us that the avulsion off-set distance was short resulting in a higher concentration of coarse overbank deposits, which could have resulted in the transitional deposits we see in the field regardless if Shire channels avulsed by incision. It was noted that progradational avulsions are more common near the top of the Shire member. This may be associated with a shift towards a lacustrine environment, which eventually deposited the Green River Formation. Progradational avulsion would be dominant in this type of system because it becomes more of a deltaic environment. The onset of progradational avulsion style in the Molina Member may be due to the compositional change from mud dominated to sand rich, which may have been a response to the PETM (Foreman 2011). Studies of the Willwood Formation in the Bighorn Basin have shown that the onset of the PETM brought a very arid environment and improved paleosol development due increased floodplain drainage (Smith et al 2009). A climate model of the Paleogene Laramide Foreland (Sewall & Sloan 2006) shows both the Bighorn Basin and the Piceance Basin aridity levels, and it was determined that the Piceance Basin is more arid. We expect the floodplains to be better drained and contain more well-developed paleosols than in the Bighorn Basin. Our observations indicate that the paleosols are weakly developed throughout the Molina 23

25 Member and so we interpret that the Molina Member floodplain development was controlled by sediment flux into the system. A higher sediment flux into the system most likely suppressed paleosol development closer to the channels. Also, the influx of coarser sediments into the system may have resulted in lower floodplain slopes and weaker channel stability, which promotes progradational avulsion. 7. Conclusion Channel avulsion is an important process to understand because it controls how fluvialdominated basins fill through time. The two styles of avulsion observed are progradational and incisional, which may be reflected in the stratigraphic record. We relate incisional avulsion, where flow is diverted from the main channel and incises directly into the floodplain, to abrupt avulsion stratigraphy (Jones & Hajek 2007). Progradational avulsion, where coarse overbank sediments (i.e. crevasse-splays) accumulate adjacent to the parent channel prior to avulsion, can be related to transitional avulsion stratigraphy (Jones & Hajek 2007). By identifying the different stratigraphic avulsion patterns throughout the three members of the Wasatch Formation, we can begin to determine what floodplain and channel conditions may promote the style of avulsion that dominates in a system. In the Wasatch Formation, the lower Atwell Gulch Member was found to be dominated by abrupt avulsion deposits, and so we interpret the system to be controlled by incisional avulsion. The Molina Member contains mainly transitional avulsion deposits, and so we infer it to be controlled by progradational avulsion. The Shire Member contains both transitional and abrupt avulsion stratigraphy, and therefore we believe that the member was dominated by both progradational and incisional avulsion. The onset of the PETM, which occurs at the beginning of the Molina Member deposition (Foreman 2011), may have resulted in the progradational style of 24

26 avulsion we found to dominate the Molina Member. Weaker paleosol development is found closer to the channels, and so we interpret this to be related to higher sediment flux into the system. Increased seasonality, during and after the PETM, could have resulted in this higher sediment flux we observe which ultimately may have resulted in lower floodplain slopes and weaker channel stability, encouraging progradational avulsion. Implications can be made about the importance of coarse-grained sediment flux into a system and the resulting style of avulsion that occurs. In order to better understand avulsion processes and related avulsion stratigraphy, future work includes performing grain-size analysis and distribution throughout the three members of the Wasatch Formation (and other similar basins) in order to justify the that sediment flux is an important control on avulsion style and related stratigraphy. 8. Acknowledgements This work was funded in part by the National Science Foundation, an Undergraduate Research Grant from the Geological Society of America NE Section, and the Penn State Department of Geosciences. I would like to thank my advisor, Liz Hajek, for helping me with this thesis and always being there to answer my questions. I would also like to thank Ellen Chamberlin for assisting in the field. Insight from Brady Foreman, Paul Heller and his graduate students as well as David Mohrig and Andy Petter was also greatly appreciated. 25

27 9. References Bridge JS, Leeder MR A simulation model of alluvial stratigraphy. Sedimentology 26: Edmonds DA, Slingerland RL Significant effect of sediment cohesion on delta morphology. Nature geoscience 3: Foreman BZ EVALUATING CLIMATIC AND TECTONIC CONTROLS ON FLUVIAL DEPOSITION SPANNING THE PALEOCENE-EOCENE BOUNDARY IN THE PICEANCE CREEK BASIN (WESTERN COLORADO, USA) GSA Annual Meeting Franczyk KJ Cretaceous and Tertiary paleogeographic reconstructions for the Uinta- Piceance basin study area, Colorado and Utah. [Reston, Va.?] :: U.S. Dept. of the Interior, U.S. Geological Survey ; Hajek EA, Heller PL, Sheets BA Significance of channel-belt clustering in alluvial basins. Geology (Boulder) 38: Hajek EA, Wolinsky MA Simplified process modeling of river avulsion and alluvial architecture: Connecting models and field data. Sedimentary geology : 1-30 Jerolmack D, Paola C Complexity in a cellular model of river avulsion. Geomorphology (Amsterdam, Netherlands) 91: Jones HL, Hajek EA Characterizing avulsion stratigraphy in ancient alluvial deposits. Sedimentary geology 202: Koch PL, Clyde WC, Hepple RP, Fogel ML, Wing SL, Zachos JC Carbon and oxygen isotope records from paleosols spanning in the Paleocene-Eocene boundary, Bighorn Basin, 26

28 Wyoming. In Causes and Consequences of Globally Warm Climates in the Early Paleogene, ed. SL Wing, PD Gingerich, B Schmitz, E Thomas, pp Boulder: GSA Kraus MJ Integration of channel and floodplain suites: II. Lateral relations of alluvial paleosols. Journal of Sedimentary Petrology 57: Kraus MJ Paleosols in clastic sedimentary rocks: Their geologic applications. Earthscience reviews 47: 41 Kraus MJ, Wells TM Recognizing Avulsion Deposits in the Ancient Stratigraphical Record. In Fluvial Sedimentology VI, pp : Blackwell Publishing Ltd. Lorenz JC, Nadon GC Braided-River Deposits in A Muddy Depositional Setting: The Molina Member of the Wasatch Formation (Paleogene), West-Central Colorado, U.S.A. Journal of sedimentary research 72: Mackey SD, Bridge JS DIMENSIONAL MODEL OF ALLUVIAL STRATIGRAPHY - THEORY AND APPLICATION. Journal of sedimentary research. Section B, Stratigraphy and global studies 65: 7-31 Makaske B, Smith DG, Berendsen HJA Avulsions, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada. Sedimentology 49: Mohrig D, Heller PL, Paola C, Lyons WJ Interpreting avulsion process from ancient alluvial sequences: Guadalope-Matarranya system (northern Spain) and Wasatch Formation (western Colorado). Geological Society of America bulletin 112: 1787 Nanson GC Episodes of vertical accretion and catastrophic stripping: A model of disequilibrium flood-plain development. Geological Society of America bulletin 97:

29 Perez-Arlucea M, Smith ND Depositional patterns following the 1870s avulsion of the Saskatchewan River (Cumberland Marshes, Saskatchewan, Canada). Journal of sedimentary research 69: Reitz MD, Jerolmack DJ, Swenson JB Flooding and flow path selection on alluvial fans and deltas. Geophys. Res. Lett. 37: L06401 Rowland JC, Dietrich WE, Day G, Parker G Formation and maintenance of single thread tie channels entering floodplain lakes: Observations from three diverse river systems. J. Geophys. Res. 114: F02013 Sewall JO, Sloan LC Come a little bit closer: A high-resolution climate study of the early Paleogene Laramide foreland. Geology 34: 81-4 Slingerland RL, Smith ND RIVER AVULSIONS AND THEIR DEPOSITS. Annual review of earth and planetary sciences 32: 257 Smith JJ, Hasiotis ST, Kraus MJ, Woody DT Transient dwarfism of soil fauna during the Paleocene Eocene Thermal Maximum. Proceedings of the National Academy of Sciences 106: Smith ND, Cross TA, Dufficy JP, Clough SR Anatomy of an avulsion. Sedimentology 36: 1-23 Tooth S, Rodnight H, Duller GAT, McCarthy TS Chronology and controls of avulsion along a mixed bedrock-alluvial river. Geological Society of America bulletin 119:

30 10. Appendices Outcrop Locations Shire Member: , , , , , , , , Molina Member: , , , , , Atwell Gulch Member: , , , Figure 13. Map showing locations of measured sections 29

31 10.2. Data collected in the field Formation Channel Grain Size Strat. Pattern Splay Splay Size Grain Size General Paleosol Channel Paleosol Paleosol Size of Channel Number Thickness of Splay Dev. Shire 1 2 m FU-ML Transitional 1 30 cm FL red green Moderate Shire 2.5 m FU Abrupt 2 10 cm FL-VFU red red Strong Shire m ML *Transitional - red/grey red (scours) Strong Shire 4 4 m FL *Transitional 3 50 cm VFU grey/yellow/red grey Strong Shire 5 8 m FL Transitional 2 30 cm VFU gray/tan (silty) tan (silty) Strong Shire m ML Transitional cm FU-FL red gray/tan/yellow Strong Shire 7 2 m FU *Transitional - 30 cm FU-FL purple/red grey/green Moderate Shire m FU-ML *Transitional - 20 cm FU red red Strong Shire m FL-FU Abrupt - 10 cm VFU greenish/grey silty grey Weak Shire 10 4 m FU Abrupt - red/grey grey/purple Moderate Molina 1 3 m FU Transitional cm FL-VFU grey greenish (silty) Weak Molina 2 3 m FU Transitional 50 cm VFU grey grey Weak Molina 3 5 m FU-ML Transitional Molina 4 HUGE FU-ML Transitional 70 cm VFU green/red grey (silty) Moderate Molina 5 3 m ML Transitional grey/purple tan/grey Weak Molina 6 3 m FU Abrupt grey/tan grey/green Weak Molina m FU Transitional 40 cm VFU grey grey Weak Molina 8 7 m FU-ML Abrupt grey/purple grey/purple Moderate Molina 9 30 m FL-FU Transitional 35 cm VFU grey grey/green Weak Atwell Gulch 1 2m ML Abrupt 4 20 cm VFL red/grey red strong Atwell Gulch 2 2m VFL-VFU Abrupt 1 50 cm VFL red/grey grey strong Atwell Gulch 3 3m VFU Abrupt - - VFL-VFU grey/purple grey strong Atwell Gulch 4 2m FL Abrupt - - grey//purple grey/purple strong Atwell Gulch 5 1m VFU-FL Abrupt - - grey/red grey strong 30

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