--> Abstract: Making the Mold for Sedimentary Fill: How Fluvial Strath Terraces Indicate the Role of Climate Forcing and Bedrock Type in Developing a Variety of Valley Geometries, by Thomas H. Morris, James L. Eddleman, Tannner C. Hicks, William F. Christensen, Scott L. Morris, and Samuel C. Sorber; #90078 (2008)

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Making the Mold for Sedimentary Fill: How Fluvial Strath Terraces Indicate the Role of Climate Forcing and Bedrock Type in Developing a Variety of Valley Geometries

Thomas H. Morris1, James L. Eddleman2, Tannner C. Hicks1, William F. Christensen3, Scott L. Morris3, and Samuel C. Sorber4
1Department of Geological Sciences, Brigham Young University, Provo, UT
2Comanche Exploration Company, LLC, Oklahoma City, OK
3Statistics Department, Brigham Young University, Provo, UT
4ExxonMobil Exploration Company, Houston, TX

Strath terraces, or fluvial-cut terraces that are emplaced into bedrock, can indicate the processes by which fluvial systems create a variety of valley geometries. In Capitol Reef National Park, Utah, the Fremont River cuts through the relatively resistant sandstones of the Glen Canyon Group leaving a multitude of strath terraces (more than 90) over a distance of less than 20 km. By integrating terrace elevation population data, published cosmogenic absolute age dates, and digital elevation models (DEM), we are able to demonstrate a significant correlation (r-value of 0.61 over the past 120ky) between the number of terraces at a given elevation above the present streambed and the last Pleistocene climate cycle. This correlation suggests that floodplain widening and alluvial fill occurs during relatively high flow conditions and that downcutting occurs during relatively low flow conditions, creating a step-like valley geometry. High flow conditions likely result from the polar front moving into Utah during glacial maximums or some small phase shift from those maximums. Low flow conditions are experienced during interglacial (dry) conditions. Floodplain widening greatly increases the 3D volume of the incised valley. Our study also demonstrates that where the bedrock lithology is different, the incision processes change. This, in turn, affects valley geometries and post-incision valley-fill volumetrics on a number of scales. In conclusion, this case study focuses on climatic aspects of fluvial incision and not on base level or tectonic influences. Our study implies that fluvial-cut valley geometries are affected by 1- the phase of the glacial-interglacial climate cycle, 2- the latitudinal position of the fluvial system, 3- the flow condition, and 4- the bedrock lithology.

 

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas