Abstract

The upper Brushy Canyon and lower Cherry Canyon formations (Permian, Guadalupian), West Texas, were deposited as a basin-restricted sandstone and siltstone wedge. To date, the sandstones have been studied preferentially to address the question of depositional environment. The sandstones, however, are interbedded with organic-rich siltstones, which contain critical information regarding conditions in the water column, depositional environment, and mode of deposition. The interaction and relations between the siltstones and sandstones is not well understood.

Preliminary work suggests there are at least three distinct siltstone facies within the upper Brushy Canyon Formation and lower Cherry Canyon Formation (“genetic top of the Brushy”). These are: Facies 1) Discontinuous siltstone drapes and lenses within amalgamated sandstone channels. Beds are commonly graded with scoured, loaded bases, and laminations are due to fining upward of silt grains and an increase in the amount of organic matter. TOC is generally less than 0.3 weight percent (wt%), visual kerogen is mixed algal-herbaceous, with up to 90% herbaceous input, and gamma-ray readings tend to be low. Facies 1 is interpreted to be deposited by sediment gravity flows while the channel system is active. Facies 2) Thick, laterally continuous siltstone bodies that occur between sandstone units. Silt is distributed uniformly throughout the beds, showing no grading, and laminations are due to compaction of organic matter. Average TOC is 0.72 wt%, visual kerogen dominantly marine algal, and gamma-ray readings are generally high. Facies 2 is interpreted to be the result of hemipelagic processes during times of abandonment of the channel system. Facies 3) Thin to moderately thick, laterally continuous, organic-rich siltstones interbedded with organic-poor, sandy siltstones. Organic-rich beds seem to have uniform silt distribution while organic-poor beds tend to be graded. TOC in organic-rich beds averages 2.36 wt% and in organic-poor beds averages 0.52 wt%. Visual kerogen is primarily marine algal and amorphous, but occasionally shows significant shallow-water organic input, and gamma-ray readings are high in organic-rich beds and low to moderate in organic-poor beds. Facies 3 also contains large amounts of radiolarians and zooplanktonic fecal pellets. Facies 3 is interpreted to be intermittently deposited by hemipelagic processes (organic-rich beds) and sediment gravity flows (organic-poor beds) during basin starvation. Current work indicates that additional siltstone facies can be differentiated within the above three facies in the studied interval based on elemental, biomarker, and detailed stratigraphic analyses.