Sand tectonics – sand mobility linked to faulting and the influence on
depositional systems
Kristine Halvorsen
Thesis submitted for the degree of Master of Science in Geology
Department of Geoscience
Faculty of Mathematics and Natural Science UNIVERSITY OF OSLO
June 2018
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Sand tectonics – sand mobility linked to faulting and the influence on depositional
systems
Thesis submitted for the degree of Master of Science in Geology
Department of Geoscience
Faculty of Mathematics and Natural Science UNIVERSITY OF OSLO
June 2018
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© Kristine Halvorsen
Supervisors: Alvar Braathen (Primary), Ivar Midtkandal (Co), Valentin Zuchuat (Co) 2018
Sand tectonics – sand mobility linked to faulting and the influence on depositional systems This thesis is published digitally through DUO – Digitale utgivelser ved UiO
http://www.duo.uio.no/
All rights reserved. No parts of this publication may be reproduced or transmitted, in any form or by any means, without permission
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Abstract
Mobility of sand by fluidization, so-called sand tectonics, can drive surface movements and thereby have a significant impact on depositional environments. Deformation related to sand mobilization is a typical understudied component in geology, yet important because it has implications for sediment distribution and sand body geometry. Previous studies imply regional, large-scale impacts as triggering events. This study consider the potential of local, smaller-scale drivers for sand mobilization. Improving the understanding of sand tectonic related processes is therefore necessary to better comprehend their dynamics and impact on sedimentary systems.
Three outcrops in the Middle Jurassic Entrada Sandstone and Upper Curtis Formation in Utah (USA) have been used to characterize the structural and sedimentary response to the underlying mobilization of sand. Data show that mild sand mobilization results in meter-scale, gentle sags and up-warps, which can be linked to distinct depositional environments and erosional processes. With an increased localization of deformation, faults nucleate and small fault-bound grabens develop. Overall, growth sequences attest to fault movement events, demonstrating the structural control on the basin. Many of the faults in upper parts of the grabens are however removed by erosion and overlain by late basin fill.
The results from this study may provide important input on deformation in near surface settings, applicable to both CO2 storage operations and the petroleum industry.
Keywords: Sand tectonics, Sand mobility, Fluidization, Fault-bounded basins, Sediment distribution, Sand body geometries, Erosional surfaces
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Acknowledgement
First, I would like to thank my principal supervisor, Professor Alvar Braathen for all of his effort and time regarding guidance the last year. I have learned a great deal from him, and I am very thankful for the knowledge and experience I have gained from fieldwork and excursions in Utah, Spain and Svalbard. Next, I would like to thank my co-supervisor, Associate Professor Ivar Midtkandal for sharing his knowledge in sedimentology and for his great ability to motivate throughout this process. I would also like to thank my other co-supervisor, PhD-student Valentin Zuchuat, for guidance during fieldwork and helpful discussions at Blindern.
Big thanks goes to Sigrid da Costa, Nikoline Bromander and Susanne Tveterås for some incredible weeks of fieldwork in Utah, September 2017. We made memories for a lifetime. In addition, I am very grateful for all of the people I have met during my master`s degree who have made these last two years of studying to such an adventure. Especially all my friends at ZEB and all the great people I met on Svalbard.
I would also like to thank the Department of Geoscience and the COPASS-project for financial support, making it possible to carry out the needed fieldwork for this thesis.
Finally, I would like to thank my family and especially my parents who have supported me endlessly throughout my studies - you are the best.
June 2018.
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Content
Abstract
Acknowledgment
1 Introduction ... 1
2 Background ... 5
2.1 Fault induced deformation ... 5
2.2 Growth successions ... 9
2.3 Mobilized and intrusive sands in Jurassic formations, Central-Utah ... 13
3 Geological Framework ... 15
3.1 Tectonic setting... 15
3.2 Stratigraphic Units ... 17
3.2.1 Introduction ... 17
3.2.2 Glen Canyon Group (Lower Jurassic) ... 17
3.2.3 San Rafael Group (Middle- to Upper Jurassic) ... 18
3.2.4 Morrison Formation (Upper Jurassic) ... 22
4 Methods ... 23
4.1 Fieldwork ... 23
4.1.1 Introduction ... 23
4.1.2 Sedimentological logging ... 23
4.1.3 Mapping of mobilized sand ... 24
4.1.4 Structural measurements ... 24
4.2 Photogrammetric models ... 24
5 Result: Sedimentary systems ... 25
5.1 Introduction ... 25
5.2 Depositional environments ... 29
6 Result: Site description ... 39
6.1 Humbug Flats ... 39
6.2 Smith`s Cabin ... 45
6.3 Interstate 70 ... 53
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7 Result: Deformational structures ... 61
7.1 Introduction ... 61
7.2 Description of structures ... 61
7.3 Style of faulting ... 67
7.4 Deformation mechanisms ... 69
8 Discussion of sand tectonics ... 71
8.1 Introduction ... 71
8.2 Structural style vs. sand mobilization ... 73
8.3 Driving mechanisms for deformation ... 77
8.4 Deformation mechanisms ... 85
8.5 Fault development and surface geometries ... 89
8.6 Depositional patterns and late basin infill ... 93
9 Conclusions ... 97
References ... 99
Appendices ... A Appendix A – Log from Humbug Flats ... C Appendix B – Log from Smith`s Cabin ... G Appendix C – Log from Interstate 70 ... K Appendix D – Facies photographs with log-position ... O
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1 Introduction
Deformation related to sand mobilization is a typically understudied component in geology, yet it is critically important because of its implications on surface movements, and thereby the impact on depositional patterns and sand body geometries. Mobilization of sand by fluidization, of various scales and dimensions, represent deformation of poorly lithified sediments, commonly occurring in water-saturated environments. This is verified through the sedimentary response of syn-depositional deformation features such as liquefaction structures, rotated blocks and slumps and multiformation-scale clastic pipes, for example as documented in studies from Jurassic eolian sediments throughout the Colorado Plateau (Alvarez et al., 1998; Chan et al., 2007; Netoff, 2002; Wheatley et al., 2016). Triggering events for soft sediment deformation are often ascribed to regional, large-scale impacts, such as fault induced seismics waves (Netoff, 2002), meteor impact shaking (Alvarez et al., 1998) or potentially a combination of multiple triggers (e.g. ground shaking due to earthquakes, explosive volcanism, bolide impact, etc.) as proposed by Chan et al. (2007). Although such processes are viable explanations for sand mobilization, local triggering events needs consideration. Such local drivers include; (1) lateral variations in sedimentary facies and permeability, (2) instability triggered by top-side loading of sediments (3) instability triggered by localized top-side removal of sediment, (4) top-side load from water load in a shallow marine or tidal setting, and (5) differential compaction.
An increase in sand mobilization may nucleate faults and potentially the formation of small fault-bounded grabens. Such graben structures are documented to arrest high amount of sediments, as seen in the delta collapse of the Last Chance delta in Utah (Braathen et al., 2017).
Fault-controlled sedimentary architecture often display various stacking patterns and geometries (Osmundsen et al., 2014). Hence, they are important when determining local sediment dispersal and accumulation (Wignall and Best, 2004). Although growth successions are independent of scale, most literature addresses them based on seismics or large-scale outcrop analysis (e.g., Bhattacharya and Davies, 2001; Childs et al., 2002; Osmundsen et al., 2014; Fielding, 2015; Braathen et al., 2017). In this study, we aim to address such successions on smaller outcrop scales and subsequently discuss them in comparison to previous work (e.g., deltas in the Ferron Sandstone, Utah).
2 This study addresses faulting by mobilization of sand in the substrate, and the sedimentary response to such processes, using well-exposed outcrops in the Middle Jurassic Entrada Sandstone and Upper Jurassic Curtis Formation (east-central Utah, USA) as a basis for broader discussions. The presented work goes deeper into the structural and sedimentological response than complimentary studies of other syn-sedimentary deformation studies in the Jurassic formations in Utah. The analysis is based on a broad dataset to address several questions around sags and upwarps formed by mild sand mobilization, deformation style and depositional patterns.
Located on the eastern flank of the San Rafael Swell (see Figure 1.1 for map location), Smith`s Cabin was targeted as a foremost example of sand mobilization. This area offers some of the best exposed outcrops, allowing highly detailed investigations of sag and upwarps formed by sand mobilization, coupled with information on growth faults and graben infill geometries. The study also address two less mature accessible outcrop-examples found in the study area, Humbug Flats and Interstate 70 (see Figure 1.1 for map location), respectively located on the northern tip of the crestal zone and on the eastern part of the swell. There, mild sand mobilization shows very gentle sag-like structures. The overall development is less than that of Smith`s Cabin. However, structural and sedimentological observations attest to faulting and channelized infill patterns due to sand mobilization in the substrate. This thesis seek insight into five main aspects:
1. Are these sag-like structures and grabens related to sand tectonics from fluidization of sand, and not for instance landslide or erosion?
2. What drivers may potentially have triggered sand mobilization?
3. What are the mechanisms, geometry and style of faulting bounding the grabens?
4. How does lithology control deformation mechanisms?
4. How are the sag/graben geometries influencing sedimentary body geometries?
5. What characterizes erosional contacts and sedimentary facies of the later infill successions?
3 Figure 1.1: USA mini map: Location of Utah (UT) and bordering states.(A): Map of Utah, showing key cities, towns and roads.(B): Map of study area. Green dots with corresponding locality name represent investigated sites (modified from Zuchuat et al., in press).
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2 Background
2.1 Fault induced deformation
The following section describes micromechanical processes that leads to rock failure, and subsequent structural discontinuities that form as a result of these processes, with the focus area being upper crustal conditions. Deformation in poorly consolidated sediments is the norm in the study area. Accordingly, this section brings necessary information to develop an understanding on how fault-induced deformation appear in unconsolidated to poorly lithified sediments.
Micromechanics of failure in granular rocks
Granular rocks are typically made up of grains, pores and cement, whereas deformation involves a change in size and/or shape of one or more of these elements. The composition of the grains play an important role in the failure processes due to the differences in shape, strength, chemical stability and susceptibility to cleavage fracture. Grains can deform in three basic modes; (1) translation and rotation (primarily controlled by the Hertzian grain-to-grain interaction), (2) dissolution (dependent on the overburden thickness or the burial depth), and (3) internal strain dominated by fracturing in brittle states (Aydin et al., 2006). If the stresses at grain contacts exceed the tensile strength, they break, which leads to fragmentation and comminution. These elements relate to cataclastic deformation. The amount of pore space will influence the volumetric deformation and control the strength of the rock. If the rock is under a high pore fluid pressure, it will be considerable weaker and therefore easier to break (Aydin et al., 2006). The fundamental micromechanical processes that characterize the failure modes and related structures in granular rocks (under upper crustal conditions), are therefore characterized as intergrain sliding (grain translation and rotation), grain fracturing, pore volume change and pressure solution. Such micromechanical processes are strongly influenced by diagenetic reactions, affecting the host rock prior to deformation. Reactions related to cementation and
6 dissolution control shape, strength, amount and distribution of cement, and is therefore crucial for failure processes (in granular rocks). The cement in a rock will exert resistance to intergranular movement, distribute contact forces, and implicitly reduce the stress concentrations, leading to grain breakage. Consequently, an increase in burial depth and cementation will affect the deformation behavior of granular materials (Aydin et al., 2006). A critical physical property is the porosity of the rock, which exert a primary influence on the type of structures that form, and consequently, the resulting permeability relative to that of the parent rock (Schultz and Fossen, 2008). Deformation bands, described below, are restricted to porous granular rocks, notably porous sands and sandstones. If the porosity is too low, fractures, stylolites and/or slip surfaces will preferentially develop (Fossen et al., 2007).
Classification of failure structures in sandstones
Failure of soil, sand and granular rocks involves a variety of micromechanical processes. It is therefore important to understand what type of structures one can predict from these processes.
Aydin et al. (2006) proposed a classification of failure modes in granular rocks, which include two main categories, and several sub-divisions.
(1) Deformation bands
(1.1) Shear deformation bands (1.1.1) Isochoric shear bands (1.1.2) Compactive shear bands (1.1.3) Dilatant shear bands (1.2) Volumetric deformation bands
(1.2.1) Compaction bands (1.2.2) Dilation bands
(2) Sharp discontinuities
(2.1) Fractures with predominantly shearing/slip surfaces
(2.2) Discontinuities with predominantly volumetric deformation (2.2.1) Joints (dilatant/opening fractures)
(2.2.2) Pressure solution surfaces (compaction/closing fractures)
7 The first category, (1) deformation bands, includes structures that form by localization of strain into narrow tabular bands. Such narrow zones of shearing and volumetric deformation are termed deformation bands (Aydin, 1978). The formation and evolution of deformation bands are associated with a significant amount of grain rotation and translation. Whether if it is characterized with grain crushing, or merely rotation and frictional gliding along the grain boundaries, the process requires a certain amount of porosity (Fossen et al., 2007). The category is subsequently sub-divided in two, considering two different types of deformation bands based on their kinematic attributes. The first group (1.1) are bands predominated by shearing, referred to as shear bands. The second group (1.2) are bands predominated by volumetric deformation.
The latter group (1.2) are sub-divided into compaction bands, characterized by volume decrease with respect to the undeformed parent rock, and dilatation bands, also characterized by a decreased volume. Shear bands (1.1) have either a dilatant or volumetric deformation compared to the undeformed parent rock (Aydin, et al, 2006). In granular rocks, shear bands have a limited offset, observed commonly in the order of a few millimeters to centimeters. Consequently, it is necessary to form new shear bands in order to widen, or lengthen, a shear band structure. This leads to formation of a zone of shear bands, often more resistant to weathering, and thus forming geomorphic features in outcrops (Aydin et al., 2006).
Grain fracturing in initial stages can be detected (in thin sections), where grains are damaged but not demolished. Progressively deformation leads to grain fracturing and grain crushing, reflected by grain size reduction or comminution, and consequently a change in grain shape within the localized zone of deformation. These processes are usually referred to as cataclasis (Aydin et al., 2006). Although rare, there are shear bands without any volumetric components of deformation, referred to as isochoric shear bands. This type of deformation is known as simple shear in geology. Mixed mode bands are referred to as either “compactive shear bands”
or “dilatant shear bands”. They are predominantly characterized by shearing, but can also be associated with volume decrease or increase, respectively (Aydin et al., 2006). Volumetric deformation bands lack evidence of macroscopic shear offset, and form predominantly by volumetric deformation. They are characterized, as mentioned above, by either a volume decrease (compaction bands) or a volume increase (dilation bands) (Aydin et al., 2006).
8 The second category (2) encompasses structures formed as sharp discontinuities. Sharp discontinuities are structural discontinuities that have a discontinuous change in stiffness or strength, which occurs between a pair of discrete planar surfaces (Schultz and Fossen, 2008).
These are initially made up of two surfaces that move either perpendicular or parallel to the surface. If the movement of the surfaces are parallel to each other, shear fractures form (2.1).
These fractures may grow in plane in either a sliding mode (mode-II) or a tearing mode (mode- III). However, the motion of the surfaces can also be perpendicular to each other, moving either away from, or towards each other, generating joints, or anti-cracks, respectively. The latter are characterized by mineral dissolution at grain-to-grain contacts, and by removal of dissolved materials (Aydin et al., 2006).
The type of deformation bands described above are based on a kinematic classification.
Although this is logical, it can be useful to classify them in terms of deformation mechanism.
Fossen et al. (2007) classified them as; (1) granular flow, (2) cataclasis, (3) phyllosilicate smearing, and (4) dissolution and cementation. This classification is especially useful when permeability and fluid flow are the issue (Fossen et al., 2007).
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2.2 Growth successions
An important controlling factor in this study is the nucleation of growth faults. Such faults are characterized as syn-sedimentary faults, meaning they develop as sediments are filling the basin (Gabrielsen et al., 1990; Nottvedt et al., 1995; Cohen and McClay, 1996; Wignall and Best, 2004; Braathen et al., 2017), affecting the available accommodation space and overall infill architecture and stacking pattern. Therefore, the following section introduces commonly observed features of such growth packages. Although growth faults are independent of scale, most literature addresses this based on seismics or large-scale outcrop analysis (e.g., Gabrielsen et al., 1990; Prosser, 1993; Nottvedt et al., 1995; Cohen and McClay, 1996).
In larger-scale rift-basins, growth successions are commonly characterized by a four-fold division, related to rift initiation, rift climax, immediate post-rift, and late post-rift stages (Gabrielsen et al., 1990; Prosser, 1993; Nottvedt et al., 1995; Childs et al., 2002; Hooper et al., 2002; Bjørlykke, 2015). In the study of Prosser (1993), these stages are linked to distinct depositional systems and distinctive expressions on seismic profiles. Large-scale patterns will however show different expressions on smaller scales, depending on climate, source rock composition and potentially sea level fluctuations. Figure 2.2 shows how each stage may correlate to particular systems tracts in order to determine the stage of basin evolution. An ideal rift basin might show each of these systems tracts clearly, although, in practice, it is not always possible to distinguish them completely (Prosser, 1993).
10 Although there are numerous studies of large-scale growth successions (e.g. from seismic sections), briefly discussed above, these are often not adequate when interpreting smaller-scale growth successions. The Last Chance delta in the Ferron Sandstone, Utah, has been studied by several authors (Bhattacharya and Davies, 2001; Braathen et al., 2017) and allows investigations of growth successions at outcrop-scale. Recordings from this delta-collapse resemble some of the observed structures in the study area, and may therefore serve as an analogue to a certain degree.
The cliff exposures of the Ferron Sandstone attest to syn-sedimentary growth faults, in which fault-bounded half-grabens arrested a large quantity of sands. In the investigation of syn- kinematic sandstones in hanging wall rollover folds, Braathen et al. (2017) described the following features; (1) rapid < 1 meter slip increments on faults, (2) followed by a mild erosion along the crest of the fault blocks, and (3) sedimentary infill of the adjacent accommodation space. In the investigated sandstone packages, internal stratification was locally gently fan- shaped, although most commonly, it was observed as parallel, displaying very gentle onlap towards the crest of the fault blocks. Here, the bedding interfaces were mildly undulating, which
Figure 2.1 An idealized section of a line drawing demonstrating an ideal growth basin where each tectonic systems tract can be recognised (Prosser, 1993).
11 coincided with down-section truncation of lamination. The erosion at the fault block crest is important for the formation of a wedge-shaped geometry. Both of these observations indicate that the faulted and folded beds were eroded prior to the deposition of the next section (Braathen et al., 2017). Figure 2.1 show an interpretation of faults and internal geometries of syn- (yellow and orange tints), and post-kinematic (grey) identified in two of the studied fault blocks. Many of the defined packages are separated by thin (1-3 cm) mud interlayers (green lines). Block F show truncated/eroded units that either thicken or thin towards the fault, which attest to both syn-kinematic deposition and temporal changes between drag-folding and rollover folding (Braathen et al., 2017).
Figure 2.2 Line drawing of two investigated fault blocks in the study of Braathen et al., (2017). Syn-kinematic beds are seen as yellow and orange tints, whereas post-kinematic beds are coloured grey.
12 Investigated growth faults in the study of Bhattacharya and Davis (2001) show fault patterns and associated facies changes that, similar to the study of Braathen et al. (2017), demonstrate a complex kinematic history and style of fault development. Cross-bed sets in the hanging wall are observed to decrease from meter to decimeter scale away from the recorded faults, interpreted as a result from either decreasing flow velocity or as a result from decreasing accommodation space in the distal hanging walls. Upstream and downstream accretion of mouth bar sands are evidenced by shifts in depositional loci, which contributed to an internal heterogeneity and development of isolated sand bodies. Growth faults initiated as a response to deposition of these mouth bar deposits, and therefore, as the channels avulsed, faults show similar patterns, indicating a complex relationship between internal changes of depositional environments and growth-faulting (Bhattacharya and Davies, 2001).
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2.3 Mobilized and intrusive sands in Jurassic formations, Central-Utah
In this thesis, the focus is on the structural and sedimentological response to mobilization of underlying sand. Therefore, a description on sand mobilization is required. Unconsolidated to poorly lithified sediments are most prone to subsurface remobilization, encompassing properties such as high porosity, low cohesion and low intergranular bonds, as well as high fluid production rate by compaction (Van Rensebergen et al, 2003). For liquefaction processes to happen, the sediments need to lose their internal friction and, consequently, act almost like a fluid (Van Rensebergen et al., 2003). This happens if the pore pressure is equal to the weight of the overlying strata. When sediments are deposited, they tend to have high content of water.
Due to this, the sediments are often packed in an unstable manner. Loading (stress) or earthquakes may cause the framework of grains to collapse and pack even more closely together. For this to happen, the water within the pore space must flow out of the beds, as the porosity decreases. The result is an upward flow of pore-water and fine sediment particles, and the process is called liquefaction (Van Rensebergen et al., 2003). The most susceptible sediments for liquefaction are silt and fine sand (see Figure 2.3). In coarser grain sizes, compaction will lead to such rapid outflow of water that the overpressure will drop too quickly for liquefaction to happen (Bjørlykke et al., 2015)
Figure 2.3 Presentation of the relationship between cohesion and permeability in sediments. In clay-rich sediments, cohesion prevents mobilization, and in coarse-grained sediments, the water escapes too quickly for overpressure to build up (Bjørlykke et al., 2015).
14 Several studies address how large bodies of fluidized sandstone occur in the Jurassic formations, especially in the Entrada, Carmel, Page and Navajo formations, in Central Utah (Alvarez et al., 1998; M. Chan et al., 2007; Netoff, 2002). Chan et al. (2007) studied a range of what they interpret as syndepositional deformation structures, present in Jurassic eolian and sabkha deposits. Across the Colorado Plateau, they classified and characterized the structures based on scale and style of deformation. The syndepositional structures represent a range of expressions at the time of deformation, including brittle failure, hydroplastic flow and liquefaction flow from fluidization. The most enigmatic structures in the study were the clastic- injection pipes. In the study of Netoff (2002), large bodies of fluidized sandstones are described, within the Jurassic Entrada Sandstone, Carmel Formation, and Page and Navajo sandstones, in south-central Utah. They are most abundant in the Entrada Sandstone. The structures are commonly arranged in clusters, with cylindrical shape and sharp contacts to their host rocks.
Recorded sandstone bodies have tongue-like projections into the host rock, considered a forcible injection of fluidized sand. Netoff (2002) advocates fluidization of the Entrada Sandstone to have occurred in a water-saturated environment during early diagenesis, most likely under significant pore water pressure. These observations are consistent with features described in this thesis. Alvarez et al. (1998) studied numerous intraformational folds that occur locally in the Carmel Formation and Slickrock member of the Entrada Sandstone. The folded beds have lateral thickness variations, interpreted to reflect lateral flowage. Some of the beds involved in folding were observed as brecciated, with blocks containing primary sedimentary structures. These blocks/clasts are suggested to be destroyed laminae and homogenizing of sand due to liquefaction. Intrusive plugs and pipes of sandstones, ranging from 1 cm to more than 10 meters, where also observed. However, the direct connection between the sand intrusions and trains of folds was not unraveled (Alvarez et al., 1998).
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3 Geological Framework
3.1 Tectonic setting
The sedimentary rocks presented in this study crop out on the north-eastern part of the Colorado Plateau (Marzolf and Morales, 1996). Several important tectonic events reflect the scenic modern landscape seen today, which has drawn geologists from the early days. The area of interest exhibit mainly tectonic features that relates to the development of the North American Cordillera (Dickinson, 2004), and the succeeding orogenies. The major ones are (1) the Late Jurassic – Early Cretaceous Nevadan Orogeny, (2) the Middle Jurassic Elko Orogeny, (3) the Lower Cretaceous to Paleogene Sevier Orogeny, (4) the Upper Cretaceous to Paleogene Laramide Orogeny and (5) Basin and Range faulting (Hintze and Kowallis, 2009).
The remains of the Nevadan Orogeny are seen as several small granitic intrusions, observed along the present day border between Utah and Nevada (Hintze and Kowallis, 2009). Contrary from the others, the Elko Orogeny is evidenced by alternating stress regimes of extension and compression (Thorman and Peterson, 2004). During the Sevier orogeny, Mesozoic, Paleozoic and Precambrian strata were folded and thrust-faulted towards the east, transforming the western Utah into an alpine upland. At the same time, western Utah subsided into a foreland basin, receiving sediments from the Sevier mountain belt in the west (Allmendinger and Jordan, 1981; Armstrong, 1968). The San Rafael Swell (see Figure 1.1) originated from the Laramide Orogeny; an orogeny of deep-seated, basement rooted reverse faults, driving multiple uplifts with adjacent basins (English and Johnston, 2004). The Uinta Mountains are an example of such an uplift, with Uinta Basin as its adjacent basin (Anderson and Picard, 1974). Less profound, although important episodes, relates to movements of the Paradox Basin salt deposits (i.e. Paradox basin halokinetics) (Trudgill, 2011), regional uplifts of the Colorado Plateau (Liu and Gurnis, 2010) and adjacent extensional events, as well volcanic activity during Oligocene into early Miocene (i.e. La Sal, Abajo and Henry Mountains igneous complexes) (Hintze and Kowallis, 2009).
16 Seen from the Wasatch Mountains westwards, Utah is situated in the Great Basin, a basin that is part of a larger segment of North America, the Basin and Range province. The province formed as a result of extensional tectonics that began around 17 Ma ago. It is characterized by a series of elongated ranges that are separated by mainly alluvial-fan dominated valleys (Hintze and Kowallis, 2009). The Wasatch Fault Zone (WFZ) forms the eastern boundary of the Basin and Range province, and is considered the longest continuous, active normal fault in the US (343 km) (Machette et al., 1991). The study area in this thesis is affected by a kilometer-scale, steep, ENE-WSW to NE-SW trending, and subordinate N-S trending, normal fault array, showing displacements from tens to hundreds of meters. The down-faulted hanging walls, locally, exhibit fine-grained alluvial/lacustrine deposits of the Early Cretaceous Cedar Mountain and the Upper Jurassic Summerville Formation (Ogata et al., 2014).
The studied localities in this thesis outcrops in the vicinity of the San Rafael Swell (see Figure 1.1), targeting the Middle Jurassic sediments of the Entrada Sandstone and the Upper Jurassic Curtis Formation, as illustrated in Figure 3.2. Jurassic strata accumulated within a retroarc basin, east of a subduction margin along the western side of the North-American continent (Anderson and Lucas, 1992; Brenner and Peterson, 1994; Marzolf and Morales, 1996). The deposits were laid down in the southern extent of an interior seaway and in marginal marine to continental areas adjacent to the seaway. Although much of the deposits within the basin were derived from the adjacent craton, and by reworking of sediments within the basin, sediments were also shed from the western highlands along the continental margin (Hintze and Kowallis, 2009). The climate was intensively hot and arid, generally within the trade wind belt. Lateral shifting of eolian sand seas, sabkhas, tidal flats and shallow marine settings characterize the depositional environment (Hintze and Kowallis, 2009). The following section introduces the Middle- to Upper Jurassic stratigraphic units within the study, from the lower Glen Canyon Group to the Morrison Formation. Seeing that the Entrada Sandstone and Curtis Formation are the focus of this study, a broad descriptions of these formations are given.
Figure 3.1: Schematic stratigraphic column of the studied area (modified from Ogata et al., 2014).
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3.2 Stratigraphic Units
3.2.1 Introduction
Jurassic strata can be sub-divided into non-marine sandstones of the Lower Jurassic, followed by the Middle- and Upper Jurassic deposits, recording transgressions and subsequent regressions of the epicontinental seaway. The latter is seen by Upper Jurassic deposits that attest to lake and stream environments in the non-marine Morrison basin (Hintze and Kowallis, 2009).
A more comprehensive description of Jurassic age formations in Central Utah follows below.
3.2.2 Glen Canyon Group (Lower Jurassic)
A three-part sequence make up the Lower Jurassic Glen Canyon Group (Gilluly and Reeside, 1928). These Lower Jurassic sandstone sequences thickens westward, and passes beneath overthrust plates along Utah`s hingeline. The eolian deposits of the Wingate Sandstone are situated at the base of this sequence, and can easily be recognized with its steep cliffs. The formation comprises a complex assemblage of facies that reflect initiation, periodic growth, and eventual destruction of a large Jurassic erg (Clemmensen et al., 1989), reaching a thickness of 100-120 meters (Shipton and Cowie, 2001). The 30-80 meter thick fluviatile Kayenta Formation conformably overlies the Wingate Sandstone (Steiner and Helsley, 1974). The Kayenta formation consist of a sandstone-siltstone-mudstone package, in the modern landscape seen as covered ledge-slope benches (Hintze and Kowallis, 2009). The group is topped by fine- to medium-grained, well-sorted, well-rounded eolian deposits of the Navajo Sandstone. This formation forms rounded cliffs with spectacular cross-bedding features (Blakey et al., 1988). It is a regionally developed Jurassic erg, covering most of the Colorado Plateau, reaching a thickness of around 500 meters in the Buckskin Gulch area (Fossen et al., 2011). Accurately dating the Lower Jurassic sediments is difficult as both the Wingate and Navajo sandstones are unfossiliferous, and fossils from the Kayenta Formation have been variously assigned.
However, recent discoveries of dinosaur tracks have allowed positioning of the Triassic/Jurassic boundary in the succession (Hintze and Kowallis, 2009).
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3.2.3 San Rafael Group (Middle- to Upper Jurassic)
The studied successions in this thesis, the Entrada Sandstone and the Curtis Formation, belong to the Middle- to Upper Jurassic San Rafael Group. Formations of the San Rafael Group are well exposed along the margin of the San Rafael Swell (Gilluly and Reeside, 1928). The base of the group consists of reworked sediments of the eolian Navajo Sandstone, forming the Page Sandstone. It is interpreted as a coastal erg system, deposited over the irregular, eroded J-2 surface, commonly marked by thin-granule lag deposits and polygonal fractures (Blakey et al., 1988). It is overlain by marine and sabkha deposits of the Carmel Formation, consisting of red mudstone, limestone and gypsiferous deposits (Blakey et al., 1988). The Entrada Sandstone conformably overlies the Carmel Formation and consist of eolian erg and wet coastal dune field deposits, characterized with a rusty red color. The uppermost formations of the San Rafael Group are the Curtis and Summerville formations consisting, respectively, of green marginal marine and red coastal plain deposits (Hintze and Kowallis, 2009). More facts on the Entrada Sandstone, Curtis Formation and Summerville Formation are given below.
Entrada Sandstone (Middle Jurassic)
In the late Early Callovian, there was a regional retreat of the Carmel Sea, accompanied by a northwest progradation of the eolian sand sea reflected by the Entrada Sandstone, formally defined by Gilluly and Reeside (1928). It consists of well-sorted, compound crescentic dunes, in places reaching heights of near 100 meters. There are broad, enclosed interdune areas. See Figure 3.2 for overview pictures of the Entrada Sandstone in the study area. Periodical flooding characterized the interdune areas near the paleocoast, whereas the interdune deposits farther inland were largely strictly eolian derived (Kocurek and Dott Jr., 1983). The presence of extensive interdune-flat deposits has led several authors to propose the Entrada Sandstone as a wet eolian system (Blakey et al., 1988b; Crabaugh and Kocurek, 1993; Kocurek, 1981; Eschner and Kocure, 1986). During the Middle Callovian, a southward retreat of the Entrada erg reflects a major transgression. An important aspect of this transgression was the re-sedimentation of the Entrada eolian sand deposits in marine environments. This is shown by abundant water-lain, rapidly deposited sediments in the upper parts of the Entrada Sandstone, which, in many places display fluid escape structures, contorted bedding and massive bedding. These characteristics are attributed to the Curtis sea transgression, over the Entrada erg (Kocurek and Dott Jr., 1983).
19 The Entrada Sandstone is commonly divided into the basal Slick Rock Member and the informally termed “earthy facies”. The Slick Rock Member consists of alternating dune and interdune intervals, representing the main body of the Entrada Sandstone, seen as arid- to semi- arid eolian erg deposits (Crabaugh and Kocurek, 1998). In the study area of this thesis, the Entrada Sandstone is 150-450 meters thick, showing a gradual increase of marginal marine influence towards the NW and up-section. This up-section increase of marginal marine sediments (e.g. coastal sabkha,- lagoonal- and intertidal mudflat-type deposits) defines the so- called, and as mentioned, informally termed “earthy facies” of Hintze and Kowallis (2009), reflecting marine encroachment from the NNE. Viewed as a whole, the Entrada Sandstone records four erg construction-deconstruction cycles (Mountney, 2006), governed by regional variations in the water-table, which is related to relative sea level fluctuations (Crabaugh and Kocurek, 1993). The regional J-3 Unconformity caps the whole system, marking the boundary between the Entrada Sandstone and the Curtis Formation.
Discolored, white zones are common throughout the Jurassic eolian deposits of the Wingate- Chinle-Navajo-Entrada formations, thought to represent ancient fluid migration-stagnation pathways (Chan, Parry et al., 2000; Ogata et al., 2014). These bleached zones, within otherwise reddish continental deposits form a noticeable feature of the Entrada Sandstone in the study area (Chan et al., 2000). Dome-shaped, diapir-like structures, reaching several tens of meters in size, are studied by several authors (Alvarez et al., 1998; M. Chan et al., 2007; Netoff, 2002).
The structures are observed along the thickest sandy portions of the Entrada Sandstone. They are interpreted as post-depositional structures, likely due to liquefaction/fluidization, deforming marine-marginal eolian dunes (Eschner and Kokurek, 1986). Radial discontinuities (e.g.
compaction/deformation bands and fractures) are observed in the host rock around the structures, interpreted as a result from localized differential compaction (Ogata et al., 2014).
20 Figure 3.2 Overview of the Entrada Sandstone and Curtis Formation. See Fig.1.1 for
locations. (A): Entrada Sandstone and Curtis Formation, separated by the J-3 Unconformity, at Smith`s Cabin. (B): Entrada Sandstone and Curtis Formation, separated by the J-3 Unconformity, at the Interstate 70 site. (C): Overview of the Entrada Sandstone and Curtis Formation at Sven`s Gulch. Note that this is not a locality within this study. (D): Curtis Formation at Interstate 70. Photograph (B) and (C): by V. Zuchuat
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Curtis Formation (Upper Jurassic)
Several detailed studies have investigated and examined the development of the Curtis Formation (Eschner and Kocurek, 1988; Gilluly and Reeside, 1928; Zuchuat et al., in press.), named from its excellent exposure at Curtis Point, Utah. Gilluly and Reeside Jr. (1928) were the first to formally define it, distinguishing it from the over – and underlying formations by its characteristic greenish-grey color. The Curtis Formation was deposited in the foreland area of an evolving fold- and thrust system, during Mesozoic times. In the study area, around the San Rafael Swell, the succession is 30-80 meters thick, seen to pinch out southwards and eastward (Gilluly and Reeside Jr., 1928). Tidal features are recognized throughout the formation (Eschner and Kocurek, 1988), typically showing basal heterolithic beds, grading into trough cross-stratified and tidally bundled sand intervals, and heterolithic beds at the upper parts of the formation (Zuchuat et al., in press). See Figure 3.2 for overview pictures of the Curtis Formation from the study area. Recent work conducted by Zuchuat et al. (in press) state that the first evidence of the transgression of the Oxfordian Curtis Sea is seen by laterally restricted shoreface deposits. These deposits are restricted to certain localities, as a direct consequence of the pre-existing relief of the J-3 Unconformity. Subsequent transgression continued in a back- stepping manner, indicated by three coarsening-up parasequences. Although the cause of these parasequences are not fully understood, Zuchuat suggests potential the link to orbitally forced relative sea level changes. This is similar to what was documented by Boulila et al. (2010) in the Oxfordian Terres Norres Formation in south-eastern France. Deposition of sub – to intertidal proximal deposits and correlative distal subtidal, multi-incised channels in parasequences 3, represents two short-lived falls in relative sea level. The maximum extent of the Curtis Sea transgression was accompanied by sub – to intertidal sandstone deposits within a system undergoing significant variations in energy levels. In the following normal regression, inferring a sea level highstand, the supratidal strata of the Summerville Formation prograded northward into the Curtis Sea (Gilluly and Reeside, 1928).
22
Summerville Formation (Upper Jurassic)
The Summerville Formation (~30 meters in thickness) comprises alternating dark red, fine- grained sandstone and gypsiferous siltstone or mudstones, conformably overlying the Curtis Formation, making it the uppermost unit of the San Rafael Group (Anderson and Lucas, 1992).
These sediments were deposited in a coastal plain environment, easily distinguished from the underlying, green, tidally influenced marginal marine deposits of the Curtis Formation (Hintze and Kowallis, 2009). Thin beds and veinlets of white gypsum, observed within the siltstones, suggests a dry climate where ponded tidal water readily evaporated. These gypsum veinlets, most often, crosscut the beds. Dark red paleosoil layers and dinosaur tracks, common in the lower and middle sand beds attest to enough moisture to support vegetation and animal life (Mickelson et al., 2003).
3.2.4 Morrison Formation (Upper Jurassic)
The youngest deposits of Jurassic age in Utah consists of multi-colored mudstones, sandstones and conglomerates of the Morrison Formation, interbedded with channel deposits, as well as lacustrine limestones. The Morrison Formation (~115-230 meters thick), is famous for its dinosaur bones (Blakey et al., 1988b). The sediments of the Morrison Formation are of entirely continental origin, and were deposited rather uniformly over a large area, including environments such as flood plains, localized riparian systems, fresh waters, and saline/alkaline wetland and lakes (Hintze and Kowallis, 2009). The sediments of the Morrison Formation indicate an increase in clastic input during a gradual change to more humid conditions in the Late Jurassic (Anderson and Lucas, 1992). Dating of volcanic ash in the Morrison Formation show that the formation varies in age from 155 to 147 million years (Hintze and Kowallis, 2009).
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4 Methods
4.1 Fieldwork
4.1.1 Introduction
In order to describe observed structures and sedimentary units related to sand tectonics, fieldwork was conducted. Data were collected in a three-week period in September 2017, in collaboration with fellow students at the University of Oslo, Nikoline Bromander, Sigrid da Costa and Susanne Tveterås. Well exposed, accessible localities were selected in order to analyze areas where sand appeared to have been mobilized. Three outcrops were studied in detail (localities 1-3; Figure 1.1). The data used in this thesis were obtained directly from outcrop studies by the use of conventional sedimentological logging, structural techniques and digital imaging. The collected data include: (1) Sedimentary logs recorded at scale 1:100, (2) photographs taken at various observation sites at the localities, (3) structural measurements of faults and smaller discontinuities, and (4) rock samples. Additional analysis were gathered by the use of photogrammetric 3D models. A description of the approach used during fieldwork and further work is presented below.
4.1.2 Sedimentological logging
Detailed sedimentary logging, on a paper at 1:100 scale, was completed along steep to vertical cross-sectional exposures ranging from 6 meters to 45 meters in length. The data recorded include bed thickness, grain size, sedimentary structures, color, weathering color and character of boundaries. From the 14 facies descriptions, 5 facies associations (with 4 sub- facies associations) were determined for the studied sections. Facies and facies associations are presented in chapter 5. In total, 9 logs were gathered from the localities, with one representative log covering the entire succession, at each locality (see Appendix A-C). The additional 6 shorter logs targets specific sections, and include short logs from Interstate 70. The shorter logs were gathered to document and analyze growth sections.
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4.1.3 Mapping of mobilized sand
By mapping the contact between structureless sandstones and bedded host rock, determining the extent and confinement of mobilized sand were straightforward. This was done by walking the outcrop, as well as interpreting 3D photogrammetric models (see Chapter 4.2 below).
Interpreting units classified as mobilized sand were based on field-observations. These include observations of disintegrated- to fluidized sand containing no primary structures, fluid escape structures and diapirism.
4.1.4 Structural measurements
Direct measurements of faults were made using a clinometer-compass and measuring-stick in the field. 30 extensional and contractional faults were analyzed in this study: 10 from Humbug Flats, 9 from Interstate 70 and 11 from Smiths Cabin. Fault displacement ranged from 3.0 cm to 400 cm. The faults were mainly exposed in 2D sections, often in cliffs. Due to along-fault variations, strike/dip and displacement were measured in several places along individual fault outcrops.
4.2 Photogrammetric models
By acquiring a series of high-resolution, geo-referenced images, 3D photogrammetric outcrop models have been made from each outcrop. These images and models are gathered and developed by the work of Valentin Zuchuat, Arve Sleveland and Ole Rabbel (images from 2017), PhD students at the University of Oslo. The concept behind photogrammetric modelling is building point clouds from similar matched features in multiple images that portray related outcrop geometries that can be draped. 3D models were generated with Agisoft PhotoscanPro.
Subsequent 3D model interpretations for this thesis was undertaken in Adobe Acrobat Reader DC. The model units were set to meters. Detailed sedimentological and structural work were carried out using these models, consisting of outlining geometries and the extent of sag and sedimentary units, thickness measurements of mobilized sand and fault mapping.
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5 Result: Sedimentary systems
5.1 Introduction
Lithostratigraphic logs from each of the three localities (see Appendix A-C) form the basis for interpreting depositional environments within the study area, of which five facies associations (and four sub- facies associations) have been defined, as shown in Figure 5.1. They are termed FA 1 to FA 5. A schematic log, shown in Figure 5.1, represents the stratigraphic position, average grain size, sedimentary structures and general color of these facies associations.
The studied sections record an overall transgressive development from rusty red to dark purple coastal wet eolian dune and interdune/sabkha deposits of FA 1a and FA 1b, to the uppermost section of sub-to intertidal dune and channel flat complex within FA 5. However, a minor regressive stage is indicated by pink-colored, plane parallel-laminated beds of FA 4 (sandy sub- to supratidal flat). The J-3 Unconformity separates FA 1 from overlying deposits, characterized by various types of relief, seen as eolian deflation, fluvial or tidal currents (da Costa, 2018), whereas the lower boundary of FA 5 corresponds to the Major Transgressive Surface (MTS) (Zuchuat et al., in press). Figure 5.1 demonstrate the stratigraphic position of the J-3 Unconformity and the MTS. The recorded development is in consistence with the overall trend of the regional depositional environment, documented by several authors (Crabaugh and Kocurek, 1998; Crabaugh and Kocurek, 1993; Hintze and Kowallis, 2009; Kreisa and Moiola, 1986). Facies descriptions and interpretations are given in Table 5.1 (see Appendix D for photographs and log positions of each facies) and a closer description and interpretation of each facies association, FA 1 to FA 5, is given below. Table 5.2 summaries key facies association interpretations at the end of the chapter.
26 Figure 5.1 Schematic log displaying recorded architectural elements within the studied section, while facies associations (FA) are colour-coded in a summary column next to their respective measured section. Relative proximal or distal positions of each depositional environment are indicated to the right in the figure.
27 Table 5.1: Facies description for the Entrada Sandstone and Curtis Formation, recorded on the northeastern margin of the San Rafael Swell, Central-Utah.
Facies Description Observations Interpretation Formation
A Cross stratified eolian dune
Unidirectional cross-
stratification with tangential bottom sets. Rusty red color.
Very fine- to fine-grained.
Wet eolian dune deposits.
Entrada Sst.
B Silty sandstone with sand-lenses
Silty sandstone with very fine- to fine-grained
sandstone lenses. Red to dark purple color.
Wet eolian interdune/sabkha
deposits.
Entrada Sst.
C Laminated to mottled mudstone
Plane parallel laminations.
Occasional evaporitic layers.
Silt to mud grain size.
Alternating layers of dark red and white layers.
Wet eolian interdune/sabkha
deposits.
Entrada Sst.
D Rippled
sandstone
Light pink to rusty red, uni- and bi-directional rippled layers. Silt to clay rich grain size.
Wet eolian interdune/sabkha
deposits
Entrada Sst.
E Structureless disintegrated to
fluidized sandstone
Disintegrated to fluidized sandstone with erased structures. Pale red to rusty red color. Very fine- to fine- grained.
Disaggregated to fluidized wet eolian
deposits
Entrada Sst.
F Plane parallel- laminated mud-
to siltstone
Plane parallel laminations with scattered bi-directional current ripples. Grey to green color. Grain size is silt to clay.
Gentle flow activity.
Current reversals indicate tidal activity
Curtis Fm.
G Cross stratified gravelly sandstone
Erosive base, medium- to gravelly-grained sandstone.
Grey colored with cross stratifications. 1-5 cm large nodules are observed at the base.
High energy conditions, point bar
lateral accretion within a tidal
channel
Curtis Fm.
H Ripple-cross laminated sandstone
Climbing rippled cross- laminated. Light beige to pink-ish color. Very fine- to fine grained.
3D migrating ripples under unidirectional current conditions
Curtis Fm.
28 I Silty sandstone
with lenticular bedding
Light brown to grey, very fine sand lenses, isolated by facies F. Scattered
herringbone cross-
laminations and oscillation ripples.
Lower sub-tidal zone Curtis Fm.
J Silty sandstone with wavy
bedding
Silty, grey to greenish sandstone layers alternating with layers of Facies F.
Scattered herringbone cross- laminations and bi-
directional current ripples.
Middle sub-tidal zone Curtis Fm.
K Silty sandstone with flaser
bedding
Light grey to pale brown sandstone with scattered lenses of mud. Scattered herringbone cross- laminations, multi-
directional current ripples, single- and double mud drapes.
Upper sub- to lower tidal zone
Curtis Fm.
L Cross stratified sandstone
White to yellow, fine- to medium-grained sandstone.
Cross-stratifications with tangential bottom sets. Mud drapes and rip up mud-clasts are observed at some
locations.
Tidally influenced, potentially tidal bar
deposits
Curtis Fm.
M Planar – to low angle laminated
sandstone
Pink to yellow colored, very fine- to fine-grained
sandstone. A fresh cut display light grey to white color. Planar- to low angle cross-stratification, scattered current and oscillation ripples.
Upper shoreface to beach deposits with
tidal influence
Curtis Fm.
N Plane parallel- stratified sandstone
Pink to yellow color, although a fresh cut often display a grey to soft greenish color. Plane
parallel-laminated, very fine- to fine-grained sandstone with scattered current ripples.
Sandy tidal flat Curtis Fm.
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5.2 Depositional environments
FA 1 – Wet eolian dune and interdune/sabkha deposits
Description:
FA 1 makes up the base of this stratigraphic study, observed as the earthy facies of the Entrada Sandstone. FA 1a consists mainly of interbedded units of facies A (cross-stratified eolian dune), recorded only at Humbug Flats. Silty sandstone beds with sand lenses (facies B), laminated to mottled mudstone (facies C), and rippled silty sandstones (facies D), represents deposits of FA 1b. FA 1b occur at all localities. Facies E (structureless disaggregated to fluidized sandstone) is characteristic for all the studied sections, appearing as a rusty red sandstone where all primary structures are erased. The transition from FA 1 to FA 2 is characterized by an unconformable contact, namely the regional J-3 Unconformity (see interpretation of FA 1 below, Figure 5.2).
Interpretation:
Well sorted, rusty red sandstones consisting of very fine to fine grained sand, characterized by cross-stratifications with tangential bottom sets (facies A), are indicators of an eolian environment (Crabaugh and Kocurek, 1993). Interbedded units of facies B (silty sandstone with sand lenses), laminated silty mudstone deposits of facies C, and rippled silty sandstones of facies D, are considered eolian interdune deposits. This implies an alternation of the ground water table level, as proposed by Crabaugh and Kocurek (1993). Scattered evaporite layers within facies C and paleosoil horizons can be related to a relative rise in water table and/or partial marine flooding (Crabaugh and Kocurek, 1993). Consequently, the overall trend for the studied intervals indicate a vertical stacking of eolian and marine-sabkha deposits, where marine processes and water table fluctuations influence the sedimentary development of the Entrada Sandstone. FA 1 is therefore interpreted to represents a coastal wet eolian environment, subsequently divided in two, a coastal wet eolian dune component (FA 1a) and a coastal wet eolian interdune/sabkha component (FA 1b). Studied sections of Facies E (Structureless disaggregated to fluidized sandstone) are interpreted to result from mild fluidization of sand, erasing all primary structures and laminations. Similar conclusions around sand mobility within Jurassic eolian deposits are drawn by Alvarez et al. (1998), Netoff (2002) and Chan et al.
(2007).
30 Figure 5.2 Overview of the Entrada Sandstone. The J-3 Unconformity, separating the Entrada Sandstone and Curtis Formation is marked with a red line. (A-B): Overview of FA 1b (coastal wet eolian interdune/sabkha deposits) and channel infill of FA 3 in the uppermost section (subtidal heterolithic flat) at Humbug flats (C): Photograph that show FA 1b in the upper Entrada Sandstone at Smith`s Cabin. Photograph (A): Drone picture by V. Zuchuat.
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FA 2 – Beach to upper shoreface deposits
Description:
FA 2 consists primarily of very fine- to fine-grained sand deposits, with a low mud content.
Layers of FA 2 is recorded at Smith`s Cabin and Interstate 70, unconformably overlying FA 1, separated by the J-3 Unconformity. There is a clear change in color, from the rusty red to dark purple beds of the underlying FA 1, to a white, almost pink color of FA 2. Common sedimentary structures are observed as cross-stratifications (facies L), planar-to low-angled laminations (facies M) (see Figure 5.3 A), as well as oscillating ripples. At the studied Smith`s Cabin site, a 40 cm thick layer of climbing ripples are recorded, whereas mud drapes, rip up mud clasts and soft sediment deformation are common features at the Interstate 70 site. FA 2 show a clear upward fining trend, indicated by planar- to low angle laminations (facies M) and cross- stratified sandstone beds (facies L), overlain by climbing-ripple cross-laminated layers of facies H. The transition from FA 2 to FA 3 commonly display a gradually fining-upward trend, although it has been documented as erosive at other localities in the study area (Zuchuat et al., in press).
Interpretation:
A high content of fine-grained sand deposits, coupled with a low content of mud, suggests a marine environment, influenced by higher energy. The occurrence of oscillating ripples are a strong indicator of upper shoreface to beach deposits, seeing that this is the predominant wave- generated bedform (Clifton and Dingler, 1984). A deepening-upward trend is indicated by the transition from Facies M (planar – to low angle laminated sandstone) to ripple-laminated beds of Facies H. Observations of rip-up clasts and mud-drapes may imply a secondary tidal influence over the system, as stated by Zuchuat et al., (in press). Supporting previous work (Eschner and Kocurek, 1988), the overall fining upward trend suggests a deepening marine/tidal environment for FA 2, considered to form as a response to a gradual transgression of the Curtis Sea (Zuchuat et al., in press).
32 Figure 5.3 Overview of facies associations FA 1b (coastal wet eolian interdune/sabkha deposits), FA 2 (beach to upper shoreface deposits), FA 3 (subtidal heterolithic flat) and FA 5 (sub-to intertidal dune and channel flat complex). (A): Photograph demonstrates the transition from FA 1b to FA 2 at Interstate 70. The J-3 Unconformity is marked with a red line, separating the Entrada Sandstone and Curtis Formation. (B): Expression of FA 3a (mud-dominated heterolithic strata at Smith`s Cabin). (C): Photograph show FA 3b and FA 5 at Interstate 70, separated by the Major Transgressive Surface (MTS), marked with a white line.
33
FA 3 – Subtidal heterolithic flat
Description:
The heterolithic, green colored nature of FA 3 makes it easily distinguishable in outcrops throughout the study area, present at all localities visited for this study (see Figure 5.3 B-C).
FA 3 can be divided in a lower mud-dominated unit (FA 3a), grading into an upper sand- dominated (FA 3b) unit. FA 3a consists of facies F (plane parallel-laminated mud – mud to siltstone), facies I (silty sandstone with lenticular bedding) and facies J (silty sandstone with wavy bedding), whereas FA 3b records a higher sand content, comprising facies F (plane parallel-laminated mud-to siltstone), facies J (silty sandstone with wavy bedding), and units of silty sandstone with flaser bedding (facies K). FA 3 forms the base of the Curtis Formation at Humbug Flats, unconformable overlying the Entrada Sandstone, separated by the J-3 Unconformity, as shown in Figure 5.2 A-B. A gravelly cross-stratified sandstone channel fill (facies E) sits on top of an erosive contact, cutting 1 meter into the underlying strata, creating a notable topographic relief. At the base of the channel, a thin (3-15 cm) silty mud layer is observed. Clasts of coarse-grained sandstones interbed the mud. At Interstate 70 and Smith`s Cabin, FA 3 conformably overlies FA 2.
Interpretation:
Flaser, wavy and lenticular bedding reflect a varying sand-to-mud ratio within the heterolithic deposits. Although deposits like these may form by episodes of flooding on alluvial plains, or by weak storm events in an offshore environment, the dominate occurrence is associated with tidally influenced settings (Chakrabarti, 2005; Daidu et.al, 2013; Davis and Dalrymple, 2010).
The heterolithic appearance reflects a system with fluctuations in energy level (Davis and Dalrymple, 2010), where clay is deposited in still-standing waters. This is supporting the interpretation of a tidal environment. The gravelly, cross-stratified channel fill indicates high energies within the system at the time of deposition, whereas mud-deposits at its base suggest slack-water periods (Kvale, 2006). Observation of coarse-grained sand lenses within the mud may imply a significant variation and asymmetry in current velocities (Zuchuat et al., in press).