Thin-bedded Turbidite and Hyperpycnite (?) Mudstones in the Cretaceous Lewis Shale,Carbon County, Wyoming: Preliminary Results
Demola Soyinka, Roger M. Slatt, 2004. "Thin-bedded Turbidite and Hyperpycnite (?) Mudstones in the Cretaceous Lewis Shale,Carbon County, Wyoming: Preliminary Results", Depositional Processes and Reservoir Characteristics of Siltstones, Mudstones and Shales, Erik D. Scott, Arnold H. Bouma
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The Upper Cretaceous Lewis Shale in the Greater Green River Basin provides outcrop exposures of deepwater deposits that are linked to updip shelf margin deltas. A 6m thick section of thin-bedded, fine-grained mudstones along a well-exposed gully has been measured and described in detail, and grain-size of individual laminae have been analyzed by laser particle size analysis. The outcrop consists of three facies. In the upper 3m of the section, a gray laminated, graded mudstone is the dominant lithology. Stratigraphically beneath this, the laminated, graded mudstone and tan, massive mudstone are interbedded. The laminated, graded mudstone encases siltstone beds which increase in thickness upward from 0.3 to 10 cm and which are also graded. The size ranges for the facies are: massive mudstones (6.4μm – 10.0 μm), laminated, graded mudstones (6.5μm -37.97μm), and siltstone (45.75μm – 55.13μm).
At outcrop scale, two styles of mudstone grading are observed. The first type is a fining-upward sequence that typically ranges in thickness from 2.5-5cm. The second type is a compound of a basal coarsening-up unit and an upper fining-up unit, which combined are typically 3.8cm thick.
At the mm+ scale, laser grain size analysis of individual laminae and groups of laminae spaced mm’s apart indicate both fining-upward and coarsening-upward laminae sets are present. The fining-upward laminae sets are interpreted as the result of deposition from waning flow from dilute turbidity or hyperpycnal flows. The combined graded sequence described in outcrop, and coarsening-upward laminae sets identified from laser grain size analysis are typical of hyperpycnites found elsewhere in modern deep marine environments. They are believed to be the result of progressive increase in river discharge during rising and peak flood stage (upward increase in grain size), followed by waning flow after the flood begins to abate (upward decrease in grain size). The results reported here are preliminary, and additional research is required to fully confirm or modify these conclusions.