Abstract

The Molina Member is a Paleogene, sand-rich interval, up to 125 meters thick, found within the otherwise mud-dominated, nonmarine Wasatch Formation of west-central Colorado. Sandstone beds of the Molina Member form sheet-like units interbedded with subordinate volumes of mudstone. The sandstones contain a low-diversity assemblage of sedimentary structures that is commonly dominated by horizontal, planar laminae, and bedding is locally contorted by large-scale soft-sediment deformation. A braided fluvial model best explains the characteristics of these strata, yet the existing braided-river models do not fully account for the assemblage of sedimentary structures or the relatively high volumes (up to half of the section) of interbedded mudstones. Two modern braided rivers, Medano Creek (Colorado) and the Prairie Dog Town Fork of the Red River (Texas), have rapid, shallow flows that form horizontal, planar bedding. In both systems the presence of large volumes of easily mobilized sandy sediment, a flashy discharge regime, and a lack of reworking between flood stages lead to the development of high-energy, shallow-water bedforms and the preservation of the resulting high-energy sedimentary structures. Similar conditions and processes are suggested to have controlled deposition of the sand-dominated Molina Member, which is a depositional anomaly within the otherwise muddy Wasatch Formation. Short-term availability of readily eroded, second-cycle sandstones from a newly elevated source area to the southeast is the most likely reason for the abrupt change from a mud-dominated fluvial system to the sandy Molina system.

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