Paleogeography and Tectonic Regime During Deposition of Muddy Sandstone Formation Rzger Abdula Fall 2008

Content Paleogeography Definition of Foreland Basin System Development of a Foreland Basin Patterns of Strata Forebulge Migration Source Area and Transported Distance Conclusion References

Paleogeography Early Cretaceous In early lower Cretaceous time a sea covered part of eastern Mexico and from this area it invaded the United States; however, it did not reach Colorado until late in Lower Cretaceous time. This invading sea gradually extended north-ward over what is now the mountain zone. (William L. Effinger, 1988)

Early Cretaceous Paleogeography, Southwestern US Colorado (The Natural American)

Simple, restored cross section of stages of Sevier Orogenic Farallon Plate Kula Plate (Wikipedia, the free encyclopedia)

Definition of Foreland Basin System (a) an elongate region of potential sediment accommodation that forms on continental crust related to subduction and the resulting fold-thrust belt. (b) it consists of four discrete depozones; wedge-top, foredeep, forebulge and back-bulge. (c) the longitudinal dimension of the foreland basin system is roughly equal to the length of the fold-thrust belt.

Types of Foreland Basin Foreland basins can be divided into two categories: Peripheral (Pro) foreland basins, which occur on the plate that is subducted or underthrust during plate collision. Examples include the North Alpine Foreland Basin of Europe Modified after DeCelles and Giles 1996

Types of Foreland Basin “Continued’’ Retroarc (Retro) foreland basins, which occur on the plate that overrides during plate convergence or collision (i.e. situated behind the magmatic arc that is linked with the subduction of oceanic lithosphere) Examples include the Mesozoic to Cenozoic Rocky Mountain Basins of North America Modified after DeCelles and Giles 1996

Development of a Foreland Basin (Colorado) Subduction and the formation of the Sevier fold-thrust belt in Nevada and western Utah led to the development of a foreland basin system that migrated eastward toward Colorado. (Sharwan Smith, 2002)

Types of Depozones The wedge-top depozone is the mass of sediment that accumulates on top of the frontal part of the orogenic wedge. The foredeep depozone consists of the sediment deposited between the structural front of the thrust belt and the proximal flank of the forebulge. The forebulge depozone is the broad region of potential flexural uplift between the foredeep and the back-bulge depozones. The back-bulge depozone is the mass of sediment that accumulates in the shallow but broad zone of potential flexural subsidence cratonward of the forebulge. (Peter G. DeCelles & Katherine, 2000)

Diagram showing a hypothetical foreland-basin system Muddy Sandstone Central & eastern Colorado Eastern Utah & Western Colorado West-Central Utah Nevada Modified after Brian S. Currie, 1997

Patterns of Strata Lower Cretaceous Regional isopach patterns of Lower Cretaceous strata define foreland-basin system depozones. Lower Cretaceous strata in west-central Utah that thicken westward were deposited within the flexurally subsiding foredeep depozone. Correlative units in eastern Utah and western Colorado were deposited in the forebulge depozone while Lower Cretaceous rocks in central Colorado were deposited in the back-bulge depozone. (Brian S. Currie, 2002)

Forebulge Migration Forebulge migration can be inferred from erosional surfaces. Those surfaces occurred as a result of faulting that affected the area. Based on field observation the thickness of outcrops are different. The lower part (Fort Collins Member) is absent in all three locations while very thin layers are present in Byers Field core. This phenomena indicates that influence of fault decreased toward East.

Source Area and Transported Distance From the morphology and mineral composition, the sediments in the measured sections can be classified as mature because they are: well sorted well cemented fine grained size rounded grain Those are evidence for a deposition that was laid down far from the source area and transported in a low topographic relief. Further evidence for this is the low feldspar content which is unstable relative to quartz. In mature sandstones that have been transported great distances, most of the feldspar has weathered. The vertical variation reflects the change of tectonic activity during the time of deposition because tectonic uplift occurs at different rates at different times in different parts of a mountain belt, which may result in diversions of rivers within mountain belts. (Posamentier and Walker, 2006)

Conclusion Subduction and the formation of the fold-thrust belt led to the development of a foreland basin system that migrated eastward toward Colorado during deposition of Muddy Sandstone Formation. Lower Cretaceous rocks in central Colorado were deposited in the back-bulge depozone. Influence of fault decreased toward East. Deposition was laid down far from the source area and transported in a low topographic relief.

References William L. Effinger, 1988, The Geology of Rocky Mountain National Park, U.S. Department of the Interior; National Park Service. The Natural American is a collecting of information about Arizona, Colorado, Utah and New Mexico htt://www.thenaturalamerican.com/ Wikipedia, the free encyclopedia Sharwan Smith, 2002, Possible Signatures of an Active Foreland Basin System in the Early Cretaceous Dakota Sandstone, Northern San Juan Basin, Southwestern Colorado, Rocky Mountain – 54th Annual Meeting. Ron Boyd, Robert W. Dalrymple, and Brian A. Zaitlin, 2006, Esturine and Incised-Valley Facies Models, Henry W. Posamentier and Roger G. Walker (Editors), Facies Models Revisited, Oklahoma, pp. 171-236. Robert Francis,2006, Allogenic and autogenic influences, v. 38, No. 4, pp. 453- 464 Peter G. DeCelles & Katherine A. Giles, 2000, Foreland basin systems, Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA, Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003, USA Brian S. Currie, 2002, Structural Configuration of the Early Cretaceous Cordilleran Foreland-Basin System and Sevier Thrust Belt, Utah and Colorado, The Journal of Geology, v. 110, pp. 697–718.

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