ORGANIC-RICH SILTSTONES IN THE PERMIAN BRUSHY CANYON FORMATION, WEST TEXAS: DOES GRAIN SIZE MATTER?

Outcrop exposures in the Delaware and Guadalupe Mountains have allowed extensive study of sedimentology and stratigraphy in the Upper Permian (Guadalupian) Brushy Canyon Formation. The resulting evidence supports interpretation of a large submarine fan system dominated by sediment gravity flow processes. In connection with this work, investigation of organic carbon-rich strata interbedded with sediment gravity flows has resulted in a large dataset of stratigraphic, sedimentologic, and geochemical observations. This dataset provides a unique opportunity to further investigate the controls on organic matter burial in ancient epicontinental settings. Unlike the overwhelming majority of organic-rich strata in such basins, Brushy Canyon black facies contain little or no terrigneous clay. On average they contain quartz silt (93 wgt. %), organic carbon (0-6 wgt. %), and calcium carbonate (<10 wgt. %), with trace disseminated pyrite and volcanic ash.

Geochemical proxy data have provided a basis for evaluation of processes controlling accumulation of organic matter in these “deep water” deposits. Unlike prior interpretations of the Delaware basin, redox proxies do not indicate dominance of reducing conditions. In addition, we have no evidence to indicate elevated productivity. Sedimentologic and stratigraphic criteria indicate that ORS units represent intervals of condensation (background suspension fallout) following sandy sediment gravity flows from the adjacent shelf. Variations in TOC appear to be controlled by the impact of changes in relative sea level on degree of condensation, as well as by location within the geomorphic divisions of the fan system. Both organic petrographic and rock-eval analysis of ORS samples indicate primarily degraded Type II (marine algal) organic matter with low and variable amounts of Type III kerogen, suggesting minor influence of terrestrial OM.

In this study, compilation of data from mud-rich organic facies (Cretaceous and Devonian black shale), and the silt–rich Brushy Canyon ORS facies allows a comparative analysis of the processes controlling organic matter burial. By fundamentally constraining one of the key variables – matrix grain size – a simple test of the mineral surface area hypothesis for organic enrichment is possible.