PALEOENVIRONMENTAL INTERPRETATION OF THE "BOARD BEDS" UNIT OF THE ENTRADA FORMATION, WESTERN COLORADO, USA

The “board beds” unit of the middle Jurassic Entrada Formation occurs throughout parts of western Colorado and eastern Utah. While the Slick Rock Member of the Entrada Formation has been studied extensively, the paleoenvironment of the “board beds” unit is still something of a mystery, and it has only been described informally. Therefore, the purpose of this project is to describe the unit and interpret the depositional environment.

Measurement of stratigraphic sections, petrographic analysis, and evaluation of sedimentary structures provide a basis for interpreting the paleoenvironment of the “board beds” unit.

Five outcrop locations were chosen in western Colorado: Rough Canyon, Mesa Co., CO; Escalante Canyon, Delta Co., CO; Black Canyon – Duncan Trail, Montrose Co., CO; Rabbit Valley, Mesa Co., CO; and Bridgeport, Mesa Co., CO. A total of 136 feet was measured, samples were taken (n = 15), and 12 thin sections were prepared. Supplemental thin sections from Colorado National Monument (Mesa County, CO) were also obtained.

Grain size distributions and point counts are being performed on the thin sections, and research and analysis are continuing.

Work to date suggests a shallow, fluctuating water table due to regional sea level rise. Data indicate the presence of two and possibly three distinct facies in the study area as the area transitioned from a wind-dominated regime to a sub-aqueous environment.

The Rough Canyon, Escalante, and Black Canyon sections are generally gray to tan, fine to medium-grained, bioturbated, massive sandstones with differential cementation which gives the unit a distinct “board-like” appearance. This indicates reworking, either by marine influence or in an interdune/sabkha setting.

The Rabbit Valley section displays planar lamination, fluid escape structures, ripple marks, non-marine trace fossils, and micro-trough cross-bedding and seems to be indicative of a sheet sand on the downwind leading edge of the erg.