143-19 Paleoenvironmental Interpretation of the Upper Devonian Gordon Sand: Possible Record of the Dasburg Event

Abstract:

The Gordon Sand is an informal unit of the Upper Devonian Venango Group that has been extensively drilled in several oil fields across northern West Virginia (e.g., Wileyville, Jackson-Stringtown), permitting a high-resolution stratigraphic correlation of these units across this part of the Acadian basin. To extend this correlation ~80 km toward the south within the context of a depositional model, we performed a detailed examination of a 41 ft Gordon Sand core extracted from a wildcat well (API# 4700701140 ) located in Braxton county, West Virginia.

Analysis of the core was performed through detailed cm-scale measurement and description.

Ten lithofacies were identified, with most lithofacies composed primarily of thick bedded (~0.5 m) fine-grained well-sorted sandstone, ranging from gray to purple to tan color. These sandstone beds often exhibit horizontal to inclined cross-lamination, and contain layers of rounded quartz pebbles and dark mud rip-up clasts at the base of beds. Sandstones were interbedded with thick (~0.3 to ~1m) dark wavy-lenticular laminated very fine sandstone and siliciclastic mudstone, along with thinner beds of black to purple laminated mudstone ranging from <2 cm to several centimeters in thickness. Mud-dominated intervals exhibited rare small marine fossils (brachiopods) along with a few trace fossils. In addition, mud-dominated intervals contained rounded granule to pebble clasts in thin horizontal laminations and thin conglomerate beds of 1-2 cm thick, along with rare load structures.

Overall the interpretation of these facies indicate a tidally-influenced shallow marine shelfal environment positioned along the southern extent of the Gordon Sand shoreline, with conglomerate representing coarse-grained influx during increased energy associated with storm weather conditions. Depositional environments record environmental change in response to sea level fluctuations, with sandstone beds deposited during lower sea level and interbedded mudstones deposited in anoxic offshore environments that developed during shoreline transgression. Based on prior conodont biostratigraphy of the Venango Group, the Gordon Sand records environmental conditions of the Dasberg event, a period associated with a minor extinction, ocean hypoxia and sea level fluctuations.

Further work to identify metal enrichment in shales through XRF analysis could support the proposition that the Gordon Sand is a record of the Dasberg event.

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