Anoxia and Euxinia (or not) in an Upper Devonian Black Shale: Woodford Shale, Permian Basin, West Texas

Abstract

Black shales are commonly assumed to have been deposited in bottom waters are generally anoxic, if not euxinic. However it is now recognized that rich accumulations of organic matter can form in the presence of low levels of oxygen. We examine three types of evidence for euxinia in long cores from the Upper Devonian black shale sequences, the Woodford Shale in the Permian Basin, west Texas, USA, which contains one of the longest continuous records of black shale deposition. Total organic carbon (TOC) averages 5 to 6% in two long cores, reaching a maximum of 14%. We address trace metal concentrations, pyrite framboid sizes, and in a low maturity Woodford core, the concentration of the biomarker isorenieratane, which records the presence of green-sulfur bacteria in the water column. In the Woodford Shale, all parameters clearly record euxenic conditions in one relatively narrow interval: greatly elevated isorenieratane concentrations, which indicate the presence of sulfidic waters extending into the photic zone; small pyrite framboid sizes, which indicate precipitation of pyrite in the water column, and elevated Mo/Al ratios, consistent with euxenic conditions. But outside of this ∼ 3 meter zone, evidence for strongly developed euxenia is generally lacking and is only supported by elevated Mo/Al ratios, despite the presence of strong enrichment in organic carbon. If euxenic bottom water developed, it was restricted to a thin layer at the base of the water column. We relate the development of bottom water euxinia to the flux of reactive iron into the basin, generally associated with detrital clays. Where the flux of iron is high, sulfide is scavenged from ocean water in the form of pyrite, preventing the development of a euxinic water column. Euxinia develops when H2S production outpaces reactive iron input, for example when clay flux is low during sea level transgressions or in carbonate-dominated depositional systems. Development of euxinia may reinforce the preservation of organic matter in sediments because of the toxicity of H2S to most burrowing organisms, and the euxinic interval of the Woodford has the highest TOC in the formation. However other intervals lacking a euxinic signature have comparable TOC, so that euxinia is not a prerequisite for development of rich source rocks.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016