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Sequence Stratigraphy of Fluvial and Lacustrine Deposits in the Lower Part of the Chinle Formation, Central Utah, United States: Paleoclimatic and Paleoecologic Implications
www.d.umn.edu/~beer0048/Prospectus.htm
Joseph John Beer
Thesis proposal submitted in partial fulfillment of the requirements for the degree of
Masters of Science (Geology)
University of Minnesota Duluth May 2004
ABSTRACT
The Chinle Formation, exposed throughout New Mexico, Colorado, Arizona, Utah, and Nevada, is characterized by terrestrially deposited mudstones, sandstones, limestones, and conglomerates preserved in a back arc basin during the Late Triassic. The depositional facies represented in the Chinle include a variety of fluvial, lacustrine-deltaic, lacustrine, paludal, and minor eolian deposits each representing deposition in an entirely continental setting. Erosional unconformities and extensively pedogenically modified horizons found within the Chinle indicate a complicated depositional history involving alternating times of landscape aggradation and degradation (Demko, 2003).
The goal of this project is to construct a detailed depositional model and sequence stratigraphic framework for fluvial and lacustrine deposits in the lower part of the Chinle Formation (Shinarump, Temple Mountain, Monitor Butte, and Moss Back Members) exposed in the San Rafael Swell, Waterpocket Fold, and Monument Valley Uplift areas in southern Utah and to evaluate the paleoecological data and paleoclimatic proxies preserved.
Figure 1: Paleogeography of the Upper Triassic (from Dubiel 1991)
INTRODUCTION
The Chinle Formation was deposited during an important time in Earth history. During the Triassic, as the supercontinent Pangaea neared its maximum extent, a volcanic arc was developing along its western margin (Figure 1). This tectonic setting provided not only the source for a large portion of Chinle sediment but also created the subsidence necessary to preserve those deposits. During this time the large Pangaean landmass combined with the basin’s subequatorial position (roughly 10 degrees north latitude) resulting in humid/wet megamonsoonal conditions (Parrish, 1993, Dubiel et al., 1991). The Chinle fluvial and lacustrine deposits and poorly drained and well developed paleosols reflect these fluctuating wet/dry conditions. These deposits and paleoclimatic conditions stand in sharp contrast to the arid coastline deposits found below (Moenkopi Formation) and the large eolian deposits (Wingate Sandstone) located above the Chinle Formation.
Not only was the Chinle deposited during an important paleoclimatic interval, it also coincides with a critical period of biologic evolution. The Chinle Formation is paleontologically significant because it preserves a record of the continuing diversification of terrestrial life following the largest extinction event in Earth’s history, the Permo-Triassic extinction. The Triassic marks the first appearance of mammals and the diversification and ascension of dinosaurs. Numerous studies (Ash and Creber, 1992, Gottesfeld, 1972) of outstandingly preserved plant material within the Chinle Formation (especially in Petrified National Park) provide detailed insights into the paleoecology and paleoclimatology of subequatorial Pangaea.
The Chinle also provides a unique opportunity to apply sequence stratigraphic concepts and methods to terrestrial deposits far upstream from the affects of eustatic sea level fluctuations. Placing the lower part of the Chinle Formation into a regional depositional framework and into a definitive sequence stratigraphic context will provide a way to understand the depositional circumstances under which the Chinle was deposited. In addition to drawing conclusions concerning the controls on large scale landscape evolution, the lacustrine deposits in the lower Chinle contain an important, underexamined archive of paleoecological data and paleoclimatic proxies recorded during a critical time in the development of the western margin of subequatorial Pangaea.
BACKGROUND
Location
The Chinle Formation is present in outcrop and in the subsurface throughout much of the southwestern United States (Figure 2). The most extensive exposures of the Chinle Formation in southern Utah are found in large uplifts (San Rafael Swell, Waterpocket Fold, and Monument Valley Upwarp) (Stewart et al., 1972) formed during the Laramide Orogeny (Late Cretaceous – Eocene) where the Chinle Formation has been brought to the surface as part of large antiforms that have since been cut by deep canyons.
Figure 2: Triassic Outcrop Extent (from Dubiel et al. 1991)
Geologic Setting
The Chinle was deposited during the Late Triassic in a backarc basin (Lawton, 1994, Dubiel, 1994) in western Pangaea. The Pangaean continent was at its maximum extent during the Triassic; the continent stretched from pole to pole and was centered on the equator. The large exposed landmass combined with the Chinle basin’s subequatorial position on the western portion of the continent was favorable for the preservation of evidence of megamonsoonal conditions (Dubiel, 1991). Geologic and paleontologic evidence from the Chinle supporting fluctuating periods of wet and dry conditions are well documented (Dubiel et al., 1991).
The Chinle is made up of continental mudstones, sandstones, limestones, and conglomerates representing alluvial, lacustrine, paludal, and eolian environments (Stewart, 1972, Dubiel, 1991). Studies of paleocurrent and provenance (Riggs, 1996, Stewart et al.,1986, Stewart, 1972, Albee, 1957) suggest that the Chinle fluvial systems flowed northwest from the Ancestral Rockies in Colorado and the Mogollon Uplands near the U.S.-Mexico border (Figure 1). The Chinle Formation lies unconformably within large paleovalleys cut into the Lower-Middle Triassic Moenkopi Formation below (Stewart, 1972, Blakey and Gubitosa, 1984, Dubiel, 1994, Demko et al., 1998). This study focuses on the Chinle deposits that fill the Painted Desert and Eagle paleovalleys in southeastern Utah (Figure 3) (Dubiel, 1994).
Figure 3: Paleovalley distribution and nature of the sequence boundary at the base of the Chinle Formation (from Dubiel et al. 1999)
Stratigraphy
Historically the Chinle Formation has been divided into numerous members based upon large scale lithological characteristics and stratigraphic relationships (Stewart et al., 1972, Dubiel, 1989). The members recognized in the study region include the Shinarump, Temple Mountain, Monitor Butte, Moss Back, Petrified Forest, Owl Rock, and Church Rock Members (Stewart et al., 1972). It is important to note that this study will use the existing nomenclature when referring to large scale lithostratigraphic trends (familiar packages of rocks), however correlations will be based upon chronostratigraphically significant surfaces which may cross lithologic boundaries. (Refer to the following section for more on sequence stratigraphic terminology, concepts, and methods.)
Shinarump Member. The Shinarump Member of the Chinle Formation is recognized as a resistant, highly silicic, light colored sandstone conglomerate usually less than 10 meters thick (Blakey and Gubitosa, 1984). The Shinarump Member rests unconformably upon the Moenkopi Formation in incised valleys as large as 10 miles wide and 175 feet deep (Stewart et al., 1972). The Shinarump represents the first of a series of deposits which fill these large paleovalleys. In all cases within the study area the Shinarump is in direct contact with the Moenkopi Formation. In some cases the Shinarump is found on top of the Moenkopi proper, while in other places it is found atop a pedogenically modified Moenkopi Formation. This well developed paleosol is often referred to as the “mottled strata” (Stewart et al., 1972).
The Shinarump is interpreted to have been deposited by braided streams which deposited “broad thin sheets of interconnected sandstone bodies” within the aforementioned paleovalleys (Figure 4) (Blakey and Gubitosa, 1984). Demko (2003) has interpreted the unconformities above and below the Shinarump in the White Canyon area, characterized by erosional truncation of beds and paleosol development, as sequence boundaries (Figure 5). According to Demko (2003) the Shinarump Member paleovalley fill represents deposition during a lowstand systems tract.
Temple Mountain Member. The Temple Mountain Member of the Chinle Formation is characterized by cross-bedded and rippled sandstones, and laminated mudstones. In the southeastern part of the San Rafael Swell at Chute Canyon the Temple Mountain lies unconformably above the Moenkopi Formation and is topped by the Moss Back Member. Further west in the San Rafael Swell at Hidden Splendor Mine the Monitor Butte Member and the Temple Mountain Member (relationship unclear) are found between the Moenkopi Formation and the overlying Moss Back Member. In the Painted Desert paleovalley in the White Canyon region the Temple Mountain Member is not present, and the Moss Back is underlain by either just the Monitor Butte or both the Monitor Butte and Shinarump (Figure 6).
Figure 6a-c: Hypothesized Lithostratigraphic relationships in the Petrified Forest Paleovalley. In Figure 6a the Temple Mountain Member is conformable with the Monitor Butte Member. In Figure 6b the Temple Mountain represents a stranded terrace deposited either before or after the Shinarump, but before the Monitor Butte. In Figure 6c the Temple Mountain represents a period of fluvial incision and subsequent deposition after the Monitor Butte.
Figure 7: Large lateral accretion sets (just above the geologists) in the Temple Mountain Member. Chute Canyon, southern San Rafael Swell, Utah.
Large lateral accretion surfaces (Figure 7) indicate the Temple Mountain represents deposition within a high sinuosity fluvial system which also deposited finer grained levee and overbank deposits. These deposits show evidence of extensive pedogenic modification at numerous horizons within the Temple Mountain Member (Figure 8) (Kowalweski and Demko, 1996).
Figure 8: Crayfish burrows in a Temple Mountain Member paleosol. Chute Canyon, southern San Rafael Swell, Utah.
Monitor Butte Member. The Monitor Butte is made up of green-gray mudstones and siltstones, thin sandstones, and occasional thin coals and limestones (Demko, 2003). The Monitor Butte Member is recognized throughout much of southeastern Utah in the Monument Upwarp, Waterpocket Fold and in the western portion of the San Rafael Swell. The Monitor Butte overlies either the Moenkopi Formation or the Shinarump Member where present. Stewart et. al. (1972) state that the Monitor Butte either interfingers with the Shinarump Member or is separated by an unconformity depending upon location within the basin. Demko (2003) interprets the contact as wholly unconformable. The Monitor Butte rocks were deposited within a lacustrine system and contain deltaic, nearshore, and deep water facies (Figure 9) (Dubiel, 1992).
Figure 9: Paleogeography of the Monitor Butte Member (from Blakey 2003)
Demko (2003)has interpreted the boundary between the Shinarump and Monitor Butte to represent a flooding surface sequence boundary followed by deposition of the Monitor Butte in highstand systems tract. In Demko’s (2003) model, the boundary between the Monitor Butte and the overlying Moss Back Member also represents a sequence boundary (Figure 5). (For further discussion of this boundary refer to the next section.)
Moss Back Member. The Moss Back Member is made up of well sorted, fine to medium grained quartz sandstone deposited by meandering streams (Figure 11) (Stewart et al., 1972) occupying the Eagle and Cottonwood paleovalleys (Dubiel, 1991). At White Canyon, the thickness of the Moss Back varies from 100 feet to zero from the middle to the edge of individual fluvial channels (Demko, 2003). Where present, the Moss Back lies unconformably above either the Monitor Butte or Temple Mountain Member depending upon position within the paleovalley.
The base of the Moss Back has been interpreted (Demko 2003) as a sequence boundary marking an unconformable basinward shift in facies (Figure 5). There may be two ways to interpret this boundary. The Moss Back fluvial system, viewed at any one location, may represent the fluvial source to more distal Monitor Butte deltas. Alternatively, the Moss Back system may not be linked to the Monitor Butte lacustrine system and the sequence boundary separating the two may represent a larger scale depositional hiatus. Both models present unique paleoclimatic implications which can be resolved by analyzing and correlating the sequence boundary which marks the transition between the poorly drained Monitor Butte lacustrine system and the more well drained Moss Back fluvial system.
Petrified Forest, Owl Rock, and Church Rock Members. These three members represent fluvial, lacustrine, and eolian deposits, respectfully, located stratigraphically above the aforementioned members where present. The Petrified Forest Member is the first of the Chinle deposits to overlap the Painted Desert paleovalley. The transition between the lower deposits making up the incised valley fill and the overlying, relatively unrestricted deposits is marked by a change in fluvial style (Demko pers. comm., 2004). While recognition of the upper members of the Chinle is vital to the success of this study, detailed analysis of these members is beyond the scope of this project.
Sequence Stratigraphy
Sequence stratigraphy was developed in the late 1970’s promoting the widespread analysis of sedimentary deposits within a time-based stratigraphic framework. Whereas traditional lithostratigraphy attempts to correlate groups of rocks based upon similar physical characteristics, sequence stratigraphy acknowledges Walther’s law† of facies by grouping genetically related packages of rocks separated by chronostratigraphically-significant surfaces (sequence boundaries, flooding surfaces, transgressive surfaces). The identification and correlation of these ‘timelines’ allows the construction of a depositional model for a given rock record and permits speculation concerning the relative importance of various driving mechanisms (Keighley, 2003).
Sequence stratigraphy has been most successfully applied to nearshore sedimentary deposits affected by eustasy (Posamentier and Allen, 1999) where the affects of variations in base level and accommodation space through time can be readily recognized in the rock record. While traditional sequence stratigraphic methods have been successfully applied to fluvial deposits isolated from the affects of base level, interpretations become difficult due to the uniquely dynamic nature of fluvial systems. Unlike nearshore deposits where accommodation space (depositional potential) is dependant upon relative sea level, accommodation space in fluvial systems is either gained or lost by positive and negative shifts in the equilibrium profile of a stream system, respectfully (Shanley, 1994). Changes in basin length, sediment supply, sediment size, discharge, and base level drive shifts in the fluvial profile, which ultimately leads to either aggradation or incision of the fluvial surface (Blum and Tornqvist, 2000, Allen, 1990). Overall, fluvial systems are more sensitive to upstream forces (especially sediment supply) than are marine deposits (Shanley, 1994).
The Chinle Formation of southeastern Utah was deposited too far upstream, roughly 600 km (Dubiel, 1994), to be affected by eustatically forced base level shifts. As a result, this study will be able to develop a model that isolates the upstream parameters (namely climate and local tectonics) controlling the stratal architecture which are more ambiguous eustatically controlled deposits. That said, the stratal architecture of portions of the Chinle formation, namely the Monitor Butte lacustrine deposits (and potentially time equivalent fluvial facies), were affected by fluctuations in local base level (lake level). Sequence stratigraphy has successfully been applied to similar lacustrine deposits by considering limnostatic base level fluctuations (Lemons and Chan, 1999), however several distinct differences exist between limnostatically and eustatically controlled systems.
First, unlike rivers that flow into oceans, the discharge of inflowing streams partially controls lake level.
Secondly, lakes are relatively short-lived features and lake level fluctuations occur on shorter time scales (Keighley, 2003).
Lastly, tectonic forces acting to either raise or lower the sill (elevation of the spillway) have an affect on lake level of open basin lakes (those with an outflow) (Keighley, 2003).
† Walther’s Law (Fritz and Moore 1988): Different sediment types (facies) accumulate beside each other at the same time. (For example: Trough cross-bedded sandstones can be deposited in a stream, laminated mudstones can be deposited on the adjacent floodplain, and carbonates can be deposited just downstream in a shallow lake.) Assuming continuous deposition, vertical lithologic changes in the rock record reflect lateral shifts of facies through time.
RESEARCH PLAN
Problem
1. Traditional depositional histories of the Chinle Formation based upon lithostratigraphic correlations are ambiguous.
2. The relationship between the Monitor Butte Member and the Temple Mountain
Member in the San Rafael Swell is unclear.
3. A multitude of independent paleoclimatic, paleohydraulic, and paleoecological
indicators preserved in the Chinle are largely unexplored.
4. The factors influencing large scale shifts from landscape aggradation to degradation
and smaller scale changes in depositional style are poorly constrained.
Hypotheses/Approach
1. The development of sequence stratigraphic methods has allowed stratigraphers to
group genetically related sequences of rocks by making time-based correlations. This
study will develop a detailed depositional history of the lower portion of the Chinle by
placing the alluvial/lacustrine deposits a time-based sequence stratigraphic context.
2. Figure 4 depicts the Temple Mountain/Monitor Butte contact three different ways and
outlines four hypothetical depositional histories to explain their relationship. The
primary hypothesis (Figure 6a) makes the Temple Mountain time equivalent to the
Monitor Butte. In this scenario the Temple Mountain represents a fluvial system
flowing into the Monitor Butte lakes from the northeast (Figures 6a, 10). Unlike
previous lithology/facies based methods, the correlation of chronostratigraphically
significant surfaces (with emphasis on pedogenically modified strata) using sequence
stratigraphy concepts and methods will allow proper testing of these hypotheses.
3. A detailed dataset of paleoclimatic and paleoecological indicators such as fluvial
channel geometries, trace fossil assemblages, and paleohydraulic indicators preserved
within paleosols will be compiled.
4. The stratal architecture of the Chinle Formation reflects the depositional
circumstances under which it was deposited, therefore it is possible to back out these
boundary conditions through detailed study of the stratal architecture. The
independent paleoclimatic data also preserved in the rock record will be used to
further constrain the model in order to draw conclusions concerning the relative roles
of paleoclimatic and paleotectonic forcing during the Late Triassic. In other words,
independent paleoclimatic data can constrain the boundary conditions (more
constants, less variables) permitting the calculation of a unique solution to the inverse
problem.
Field Work Summer 2004
My undergraduate research assistant (Corey Wendland) and I will be spending approximately 60 days during the summer of 2004 conducting field work. Work will focus on the lower portion of the Chinle Formation deposited within the Painted Desert Paleovalley (Blakey and Gubitosa, 1984, Dubiel et al., 1999) and exposed in the Circle Cliffs, Capitol Reef, San Rafael Swell, and White Canyon areas in southeastern Utah. Field methods include measuring sections, conducting detailed facies analysis, identifying and describing pedogenic fabrics and ichnofossils, analyzing paleohydrologic indicators, interpreting photopans, and collecting samples for petrographic analysis.
Measured sections will be used to make correlations across the strike of the paleovalley along two lines: the first trending approximately N45OE from Capitol Reef to the southern San Rafael Swell, the second trending roughly N80OE from the Circle Cliffs the White Canyon region. Geophysical data will be used to augment field data between the Waterpocket Fold in the west and the San Rafael Swell and Monument Valley uplifts in the east where the Chinle Formation is in the subsurface (see Appendix II).
I estimate that of the total dataset 35% will be collected in the San Rafael Swell, 35% in White Canyon, 15% in Capitol Reef, and 15% in the Circle Cliffs. I have developed an itinerary based upon these estimates; however, due to many logistical unknowns, most notably outcrop accessibility, it remains unclear how much time will be devoted to each region. For a tentative itinerary see Appendix I.
Long Term Plan
Financial Support
UMD Department of Geological Sciences: $800 Awarded
AAPG: $2000 Pending
Colorado Scientific Society: $1200 Pending
Sigma Xi: $1000 Denied
GSA: $3000 Denied
REFERENCES CITED
Albee, H.F., 1957, Comparison of the pebbles of the Shinarump and Moss Back
Members of the Chinle Formation: Journal of Sedimentary Petrology, v. 27, p. 135-42.
Allen, P.A., and Allen J.R., 1990: Basin Analysis: Principles and Applications.
Blackwell Science, London.
Ash, S.D., and Creber G.T., 1992, Palaeoclimatic interpretation of the wood structures of
the trees in the Chinle Formation (Upper Triassic), Petrified Forest National Park, Arizona, USA:
Palaeogeography, Palaeoclimatology, Palaeoecology. v. 96, p. 299-317.
Blakey, R.C., 2003, Stratigraphy and Sedimentology on the Colorado Plateau:
jan.ucc.nau.edu/~rcb7/RCB.html.
Blakey, R.C., and Gubitosa, R., 1984, Controls of sandstone body geometry and
architecture in the Chinle Formation (Late Triassic), Colorado Plateau:
Sedimentary Geology, v. 38, p. 51-86.
Blum, M.D., and Tornqvist T.E., 2000, Fluvial responses to climate and sea-level change:
a review and look forward: Sedimentology, v. 47, suppl. 1, p. 2-48.
Demko, T.M., 2003, Sequence Stratigraphy of a Fluvial-Lacustrine Succession in
Triassic Lower Chinle Formation, Central Utah, USA: GSA Conference, poster session 173-2.
Dolling, H.H., Blackett, R.E., Hamblin, A.H., Powell, J.D., Pollock, G.L., 2000, Geology
of Grand Staircase-Escalante National Monument, Utah. In: Geology of Utah’s Parks and Monuments.
(ed.) Sprinkle, D.A., Chidsey, T.C., Anderson, P.B. Utah Geological Association, publication 28. p. 189-232.
Demko, T.M., Dubiel, R.F., and Parrish, J.T., 1998, Plant taphonomy in incised valleys:
Implications for interpreting paleoclimate from fossil plants: Geology, v. 26, p. 1119-1122.
Dubiel, R.F., Hasiotis, S.T., Demko, T.M., 1999, Incised valley fills in the lower part of
the Chinle Formation, Petrified Forest National Park (PEFO), Arizona: complete measured sections
and regional stratigraphic implications of Upper Triassic rocks: Technical Report NPS/NRGRD/GRDTR-99/3.
Dubiel, R.F., 1989, Sedimentology and revised nomenclature for the upper part of the
Upper Triassic Chinle Formation and the Lower Jurassic Wingate Sandstone, northwestern New Mexico and
northeastern Arizona: New Mexico Geological Society Guidebook, 40th Field Conference, Southeast Colorado Plateau.
Dubiel, R.F., Parrish, J.T., Parrish, J.M., Good, S.C., 1991, The Pangaean Megamonsoon
– Evidence from the Upper Triassic Chinle Formation, Colorado Plateau: Palaios, v. 6, p. 347–370.
Dubiel, R.F., 1992, Sedimentology and Depositional History of the Upper Triassic Chinle
Formation in the Uinta, Piceance, and Eagle Basins, Northwestern Colorado and Northeastern Utah:
USGS Bulletin, B1787-W, p. W1-W25.
Dubiel, R.F., 1994, Triassic deposystems, paleogeography, and paleoclimate of the
Western Interior: in Caputo, M.V., Peterson, J.A., and Franczyk, K.J., eds., Mesozoic Systems of the
Rocky Mountain Region, USA: SEPM (Society for Sedimentary Geology), Rocky Mountain Section, Denver, p 133-68.
Fritz, W.J., and Moore J.N., 1988: Basics of physical stratigraphy and sedimentology.
John Wiley and Sons, Inc.
Gottesfeld, A.S., 1972, Paleoecology of the lower part of the Chinle Formation in the
Petrifed Forest. In: Breed, C.S. and Breed, W.J. (eds.) Investigations in the Triassic Chinle Formation.
Museum of Northern Arizona Bulletin, vol. 47.
Kowalweski, M., and Demko, T.M., 1996, Trace fossils and population paleoecology –
Comparative analysis of size-frequency distributions derived from burrows: Lethaia, v. 29, p. 133-124.
Lawton, T.F., 1994, Tectonic setting of Mesozoic sedimentary basins, Rocky Mountain
region, United States. In: Mesozoic Systems of the Rocky Mountain region, USA, Mario V. Caputo,
James A. Peterson and Karen J. Francyk, eds.
Lemons, D.R., Chan, M.A., 1999, Facies architecture and sequence stratigraphy of fine
grained lacustrine deltas along the eastern margin of Late Pleistocene Lake Bonneville, Northern Utah
and Southern Idaho: American Association of Petroleum Geologists Bulletin, v. 83, no. 4, p. 635-65.
Parrish, J.T., 1993, Climate of the Supercontinent Pangea: The Journal of Geology, v.
101, p. 215-233.
Posamentier, H.W., Allen, G.P., 1999: Siliciclastic sequence stratigraphy – Concepts and
applications. SEPM Concepts in Sedimentology and Paleontology #7.
Riggs, N.R., Lehman, T.M., Gehrels, G.E., Dickinson, W.R., 1996, Detrital zircon link
between headwaters and terminus of the Upper Triassic Chinle-Dockum paleoriver system:
Science, July 5, v. 273, p. 97-100.
Riggs, N.R., Ash, S.R., Barth, A.P., Gehrels, G.E., and Wooden, J.L., 2003, Isotopic age
of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: an example of dating
a continental sandstone: Geological Society of America Bulletin, v. 115, p. 1315-1323.
Shanley, K.W. and McCabe P.J., 1994, Perspectives on the Sequence Stratigraphy of
Continental Strata: AAPG Bulletin, v. 78, no. 4, p. 544-568.
Stewart, J.H., Poole, F.G., and Wilson, R.F., 1972, Stratigraphy and origin of the Chinle
Formation and related Triassic strata in the Colorado Plateau region with a section on Sedimentary
Petrology by R. A. Cadigan and on Conglomerate Studies by Thordarson, W., Albee, H.F., and Stewart, J.H.:
U.S. Geological Survey Professional Paper 690, p. 1-336
Stewart, J.H., Anderson, T.H., Haxel, G.B., Silver, L.T., Wright, J.E., 1986, Late
Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic
detrius in the Chinle Formation of the Colorado Plateau region: Geology, v. 14, p. 567-70.
Figures:
www.d.umn.edu/~beer0048/Prospectus.htm
Joseph John Beer
Thesis proposal submitted in partial fulfillment of the requirements for the degree of
Masters of Science (Geology)
University of Minnesota Duluth May 2004
ABSTRACT
The Chinle Formation, exposed throughout New Mexico, Colorado, Arizona, Utah, and Nevada, is characterized by terrestrially deposited mudstones, sandstones, limestones, and conglomerates preserved in a back arc basin during the Late Triassic. The depositional facies represented in the Chinle include a variety of fluvial, lacustrine-deltaic, lacustrine, paludal, and minor eolian deposits each representing deposition in an entirely continental setting. Erosional unconformities and extensively pedogenically modified horizons found within the Chinle indicate a complicated depositional history involving alternating times of landscape aggradation and degradation (Demko, 2003).
The goal of this project is to construct a detailed depositional model and sequence stratigraphic framework for fluvial and lacustrine deposits in the lower part of the Chinle Formation (Shinarump, Temple Mountain, Monitor Butte, and Moss Back Members) exposed in the San Rafael Swell, Waterpocket Fold, and Monument Valley Uplift areas in southern Utah and to evaluate the paleoecological data and paleoclimatic proxies preserved.
Figure 1: Paleogeography of the Upper Triassic (from Dubiel 1991)
INTRODUCTION
The Chinle Formation was deposited during an important time in Earth history. During the Triassic, as the supercontinent Pangaea neared its maximum extent, a volcanic arc was developing along its western margin (Figure 1). This tectonic setting provided not only the source for a large portion of Chinle sediment but also created the subsidence necessary to preserve those deposits. During this time the large Pangaean landmass combined with the basin’s subequatorial position (roughly 10 degrees north latitude) resulting in humid/wet megamonsoonal conditions (Parrish, 1993, Dubiel et al., 1991). The Chinle fluvial and lacustrine deposits and poorly drained and well developed paleosols reflect these fluctuating wet/dry conditions. These deposits and paleoclimatic conditions stand in sharp contrast to the arid coastline deposits found below (Moenkopi Formation) and the large eolian deposits (Wingate Sandstone) located above the Chinle Formation.
Not only was the Chinle deposited during an important paleoclimatic interval, it also coincides with a critical period of biologic evolution. The Chinle Formation is paleontologically significant because it preserves a record of the continuing diversification of terrestrial life following the largest extinction event in Earth’s history, the Permo-Triassic extinction. The Triassic marks the first appearance of mammals and the diversification and ascension of dinosaurs. Numerous studies (Ash and Creber, 1992, Gottesfeld, 1972) of outstandingly preserved plant material within the Chinle Formation (especially in Petrified National Park) provide detailed insights into the paleoecology and paleoclimatology of subequatorial Pangaea.
The Chinle also provides a unique opportunity to apply sequence stratigraphic concepts and methods to terrestrial deposits far upstream from the affects of eustatic sea level fluctuations. Placing the lower part of the Chinle Formation into a regional depositional framework and into a definitive sequence stratigraphic context will provide a way to understand the depositional circumstances under which the Chinle was deposited. In addition to drawing conclusions concerning the controls on large scale landscape evolution, the lacustrine deposits in the lower Chinle contain an important, underexamined archive of paleoecological data and paleoclimatic proxies recorded during a critical time in the development of the western margin of subequatorial Pangaea.
BACKGROUND
Location
The Chinle Formation is present in outcrop and in the subsurface throughout much of the southwestern United States (Figure 2). The most extensive exposures of the Chinle Formation in southern Utah are found in large uplifts (San Rafael Swell, Waterpocket Fold, and Monument Valley Upwarp) (Stewart et al., 1972) formed during the Laramide Orogeny (Late Cretaceous – Eocene) where the Chinle Formation has been brought to the surface as part of large antiforms that have since been cut by deep canyons.
Figure 2: Triassic Outcrop Extent (from Dubiel et al. 1991)
Geologic Setting
The Chinle was deposited during the Late Triassic in a backarc basin (Lawton, 1994, Dubiel, 1994) in western Pangaea. The Pangaean continent was at its maximum extent during the Triassic; the continent stretched from pole to pole and was centered on the equator. The large exposed landmass combined with the Chinle basin’s subequatorial position on the western portion of the continent was favorable for the preservation of evidence of megamonsoonal conditions (Dubiel, 1991). Geologic and paleontologic evidence from the Chinle supporting fluctuating periods of wet and dry conditions are well documented (Dubiel et al., 1991).
The Chinle is made up of continental mudstones, sandstones, limestones, and conglomerates representing alluvial, lacustrine, paludal, and eolian environments (Stewart, 1972, Dubiel, 1991). Studies of paleocurrent and provenance (Riggs, 1996, Stewart et al.,1986, Stewart, 1972, Albee, 1957) suggest that the Chinle fluvial systems flowed northwest from the Ancestral Rockies in Colorado and the Mogollon Uplands near the U.S.-Mexico border (Figure 1). The Chinle Formation lies unconformably within large paleovalleys cut into the Lower-Middle Triassic Moenkopi Formation below (Stewart, 1972, Blakey and Gubitosa, 1984, Dubiel, 1994, Demko et al., 1998). This study focuses on the Chinle deposits that fill the Painted Desert and Eagle paleovalleys in southeastern Utah (Figure 3) (Dubiel, 1994).
Figure 3: Paleovalley distribution and nature of the sequence boundary at the base of the Chinle Formation (from Dubiel et al. 1999)
Stratigraphy
Historically the Chinle Formation has been divided into numerous members based upon large scale lithological characteristics and stratigraphic relationships (Stewart et al., 1972, Dubiel, 1989). The members recognized in the study region include the Shinarump, Temple Mountain, Monitor Butte, Moss Back, Petrified Forest, Owl Rock, and Church Rock Members (Stewart et al., 1972). It is important to note that this study will use the existing nomenclature when referring to large scale lithostratigraphic trends (familiar packages of rocks), however correlations will be based upon chronostratigraphically significant surfaces which may cross lithologic boundaries. (Refer to the following section for more on sequence stratigraphic terminology, concepts, and methods.)
Shinarump Member. The Shinarump Member of the Chinle Formation is recognized as a resistant, highly silicic, light colored sandstone conglomerate usually less than 10 meters thick (Blakey and Gubitosa, 1984). The Shinarump Member rests unconformably upon the Moenkopi Formation in incised valleys as large as 10 miles wide and 175 feet deep (Stewart et al., 1972). The Shinarump represents the first of a series of deposits which fill these large paleovalleys. In all cases within the study area the Shinarump is in direct contact with the Moenkopi Formation. In some cases the Shinarump is found on top of the Moenkopi proper, while in other places it is found atop a pedogenically modified Moenkopi Formation. This well developed paleosol is often referred to as the “mottled strata” (Stewart et al., 1972).
The Shinarump is interpreted to have been deposited by braided streams which deposited “broad thin sheets of interconnected sandstone bodies” within the aforementioned paleovalleys (Figure 4) (Blakey and Gubitosa, 1984). Demko (2003) has interpreted the unconformities above and below the Shinarump in the White Canyon area, characterized by erosional truncation of beds and paleosol development, as sequence boundaries (Figure 5). According to Demko (2003) the Shinarump Member paleovalley fill represents deposition during a lowstand systems tract.
Temple Mountain Member. The Temple Mountain Member of the Chinle Formation is characterized by cross-bedded and rippled sandstones, and laminated mudstones. In the southeastern part of the San Rafael Swell at Chute Canyon the Temple Mountain lies unconformably above the Moenkopi Formation and is topped by the Moss Back Member. Further west in the San Rafael Swell at Hidden Splendor Mine the Monitor Butte Member and the Temple Mountain Member (relationship unclear) are found between the Moenkopi Formation and the overlying Moss Back Member. In the Painted Desert paleovalley in the White Canyon region the Temple Mountain Member is not present, and the Moss Back is underlain by either just the Monitor Butte or both the Monitor Butte and Shinarump (Figure 6).
Figure 6a-c: Hypothesized Lithostratigraphic relationships in the Petrified Forest Paleovalley. In Figure 6a the Temple Mountain Member is conformable with the Monitor Butte Member. In Figure 6b the Temple Mountain represents a stranded terrace deposited either before or after the Shinarump, but before the Monitor Butte. In Figure 6c the Temple Mountain represents a period of fluvial incision and subsequent deposition after the Monitor Butte.
Figure 7: Large lateral accretion sets (just above the geologists) in the Temple Mountain Member. Chute Canyon, southern San Rafael Swell, Utah.
Large lateral accretion surfaces (Figure 7) indicate the Temple Mountain represents deposition within a high sinuosity fluvial system which also deposited finer grained levee and overbank deposits. These deposits show evidence of extensive pedogenic modification at numerous horizons within the Temple Mountain Member (Figure 8) (Kowalweski and Demko, 1996).
Figure 8: Crayfish burrows in a Temple Mountain Member paleosol. Chute Canyon, southern San Rafael Swell, Utah.
Monitor Butte Member. The Monitor Butte is made up of green-gray mudstones and siltstones, thin sandstones, and occasional thin coals and limestones (Demko, 2003). The Monitor Butte Member is recognized throughout much of southeastern Utah in the Monument Upwarp, Waterpocket Fold and in the western portion of the San Rafael Swell. The Monitor Butte overlies either the Moenkopi Formation or the Shinarump Member where present. Stewart et. al. (1972) state that the Monitor Butte either interfingers with the Shinarump Member or is separated by an unconformity depending upon location within the basin. Demko (2003) interprets the contact as wholly unconformable. The Monitor Butte rocks were deposited within a lacustrine system and contain deltaic, nearshore, and deep water facies (Figure 9) (Dubiel, 1992).
Figure 9: Paleogeography of the Monitor Butte Member (from Blakey 2003)
Demko (2003)has interpreted the boundary between the Shinarump and Monitor Butte to represent a flooding surface sequence boundary followed by deposition of the Monitor Butte in highstand systems tract. In Demko’s (2003) model, the boundary between the Monitor Butte and the overlying Moss Back Member also represents a sequence boundary (Figure 5). (For further discussion of this boundary refer to the next section.)
Moss Back Member. The Moss Back Member is made up of well sorted, fine to medium grained quartz sandstone deposited by meandering streams (Figure 11) (Stewart et al., 1972) occupying the Eagle and Cottonwood paleovalleys (Dubiel, 1991). At White Canyon, the thickness of the Moss Back varies from 100 feet to zero from the middle to the edge of individual fluvial channels (Demko, 2003). Where present, the Moss Back lies unconformably above either the Monitor Butte or Temple Mountain Member depending upon position within the paleovalley.
The base of the Moss Back has been interpreted (Demko 2003) as a sequence boundary marking an unconformable basinward shift in facies (Figure 5). There may be two ways to interpret this boundary. The Moss Back fluvial system, viewed at any one location, may represent the fluvial source to more distal Monitor Butte deltas. Alternatively, the Moss Back system may not be linked to the Monitor Butte lacustrine system and the sequence boundary separating the two may represent a larger scale depositional hiatus. Both models present unique paleoclimatic implications which can be resolved by analyzing and correlating the sequence boundary which marks the transition between the poorly drained Monitor Butte lacustrine system and the more well drained Moss Back fluvial system.
Petrified Forest, Owl Rock, and Church Rock Members. These three members represent fluvial, lacustrine, and eolian deposits, respectfully, located stratigraphically above the aforementioned members where present. The Petrified Forest Member is the first of the Chinle deposits to overlap the Painted Desert paleovalley. The transition between the lower deposits making up the incised valley fill and the overlying, relatively unrestricted deposits is marked by a change in fluvial style (Demko pers. comm., 2004). While recognition of the upper members of the Chinle is vital to the success of this study, detailed analysis of these members is beyond the scope of this project.
Sequence Stratigraphy
Sequence stratigraphy was developed in the late 1970’s promoting the widespread analysis of sedimentary deposits within a time-based stratigraphic framework. Whereas traditional lithostratigraphy attempts to correlate groups of rocks based upon similar physical characteristics, sequence stratigraphy acknowledges Walther’s law† of facies by grouping genetically related packages of rocks separated by chronostratigraphically-significant surfaces (sequence boundaries, flooding surfaces, transgressive surfaces). The identification and correlation of these ‘timelines’ allows the construction of a depositional model for a given rock record and permits speculation concerning the relative importance of various driving mechanisms (Keighley, 2003).
Sequence stratigraphy has been most successfully applied to nearshore sedimentary deposits affected by eustasy (Posamentier and Allen, 1999) where the affects of variations in base level and accommodation space through time can be readily recognized in the rock record. While traditional sequence stratigraphic methods have been successfully applied to fluvial deposits isolated from the affects of base level, interpretations become difficult due to the uniquely dynamic nature of fluvial systems. Unlike nearshore deposits where accommodation space (depositional potential) is dependant upon relative sea level, accommodation space in fluvial systems is either gained or lost by positive and negative shifts in the equilibrium profile of a stream system, respectfully (Shanley, 1994). Changes in basin length, sediment supply, sediment size, discharge, and base level drive shifts in the fluvial profile, which ultimately leads to either aggradation or incision of the fluvial surface (Blum and Tornqvist, 2000, Allen, 1990). Overall, fluvial systems are more sensitive to upstream forces (especially sediment supply) than are marine deposits (Shanley, 1994).
The Chinle Formation of southeastern Utah was deposited too far upstream, roughly 600 km (Dubiel, 1994), to be affected by eustatically forced base level shifts. As a result, this study will be able to develop a model that isolates the upstream parameters (namely climate and local tectonics) controlling the stratal architecture which are more ambiguous eustatically controlled deposits. That said, the stratal architecture of portions of the Chinle formation, namely the Monitor Butte lacustrine deposits (and potentially time equivalent fluvial facies), were affected by fluctuations in local base level (lake level). Sequence stratigraphy has successfully been applied to similar lacustrine deposits by considering limnostatic base level fluctuations (Lemons and Chan, 1999), however several distinct differences exist between limnostatically and eustatically controlled systems.
First, unlike rivers that flow into oceans, the discharge of inflowing streams partially controls lake level.
Secondly, lakes are relatively short-lived features and lake level fluctuations occur on shorter time scales (Keighley, 2003).
Lastly, tectonic forces acting to either raise or lower the sill (elevation of the spillway) have an affect on lake level of open basin lakes (those with an outflow) (Keighley, 2003).
† Walther’s Law (Fritz and Moore 1988): Different sediment types (facies) accumulate beside each other at the same time. (For example: Trough cross-bedded sandstones can be deposited in a stream, laminated mudstones can be deposited on the adjacent floodplain, and carbonates can be deposited just downstream in a shallow lake.) Assuming continuous deposition, vertical lithologic changes in the rock record reflect lateral shifts of facies through time.
RESEARCH PLAN
Problem
1. Traditional depositional histories of the Chinle Formation based upon lithostratigraphic correlations are ambiguous.
2. The relationship between the Monitor Butte Member and the Temple Mountain
Member in the San Rafael Swell is unclear.
3. A multitude of independent paleoclimatic, paleohydraulic, and paleoecological
indicators preserved in the Chinle are largely unexplored.
4. The factors influencing large scale shifts from landscape aggradation to degradation
and smaller scale changes in depositional style are poorly constrained.
Hypotheses/Approach
1. The development of sequence stratigraphic methods has allowed stratigraphers to
group genetically related sequences of rocks by making time-based correlations. This
study will develop a detailed depositional history of the lower portion of the Chinle by
placing the alluvial/lacustrine deposits a time-based sequence stratigraphic context.
2. Figure 4 depicts the Temple Mountain/Monitor Butte contact three different ways and
outlines four hypothetical depositional histories to explain their relationship. The
primary hypothesis (Figure 6a) makes the Temple Mountain time equivalent to the
Monitor Butte. In this scenario the Temple Mountain represents a fluvial system
flowing into the Monitor Butte lakes from the northeast (Figures 6a, 10). Unlike
previous lithology/facies based methods, the correlation of chronostratigraphically
significant surfaces (with emphasis on pedogenically modified strata) using sequence
stratigraphy concepts and methods will allow proper testing of these hypotheses.
3. A detailed dataset of paleoclimatic and paleoecological indicators such as fluvial
channel geometries, trace fossil assemblages, and paleohydraulic indicators preserved
within paleosols will be compiled.
4. The stratal architecture of the Chinle Formation reflects the depositional
circumstances under which it was deposited, therefore it is possible to back out these
boundary conditions through detailed study of the stratal architecture. The
independent paleoclimatic data also preserved in the rock record will be used to
further constrain the model in order to draw conclusions concerning the relative roles
of paleoclimatic and paleotectonic forcing during the Late Triassic. In other words,
independent paleoclimatic data can constrain the boundary conditions (more
constants, less variables) permitting the calculation of a unique solution to the inverse
problem.
Field Work Summer 2004
My undergraduate research assistant (Corey Wendland) and I will be spending approximately 60 days during the summer of 2004 conducting field work. Work will focus on the lower portion of the Chinle Formation deposited within the Painted Desert Paleovalley (Blakey and Gubitosa, 1984, Dubiel et al., 1999) and exposed in the Circle Cliffs, Capitol Reef, San Rafael Swell, and White Canyon areas in southeastern Utah. Field methods include measuring sections, conducting detailed facies analysis, identifying and describing pedogenic fabrics and ichnofossils, analyzing paleohydrologic indicators, interpreting photopans, and collecting samples for petrographic analysis.
Measured sections will be used to make correlations across the strike of the paleovalley along two lines: the first trending approximately N45OE from Capitol Reef to the southern San Rafael Swell, the second trending roughly N80OE from the Circle Cliffs the White Canyon region. Geophysical data will be used to augment field data between the Waterpocket Fold in the west and the San Rafael Swell and Monument Valley uplifts in the east where the Chinle Formation is in the subsurface (see Appendix II).
I estimate that of the total dataset 35% will be collected in the San Rafael Swell, 35% in White Canyon, 15% in Capitol Reef, and 15% in the Circle Cliffs. I have developed an itinerary based upon these estimates; however, due to many logistical unknowns, most notably outcrop accessibility, it remains unclear how much time will be devoted to each region. For a tentative itinerary see Appendix I.
Long Term Plan
Financial Support
UMD Department of Geological Sciences: $800 Awarded
AAPG: $2000 Pending
Colorado Scientific Society: $1200 Pending
Sigma Xi: $1000 Denied
GSA: $3000 Denied
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Figures:
