Paleo-Redox Conditions During Deposition of the Devonian New Albany Shale, Illinois Basin: Results of a Multi-Proxy Investigation

Ovidiu Remus Lazar  and  Juergen Schieber
Indiana University, Bloomington, IN

The Middle to Late Devonian New Albany Shale of the Illinois Basin consists of organic-rich (up to 20% TOC), and variably bioturbated dark shales, and strongly bioturbated, lighter-colored, organic-poor (< 2% TOC) shales. Water column oxygenation is considered a major control on the formation of black shale successions such as the New Albany Shale. Sedimentary features, trace fossils, petrographic characteristics, and geochemical data were examined and compared to assess the degree of seafloor anoxia at the time of carbonaceous mud deposition.

Abundant evidence of bottom current activity (e.g. silt laminae with sharp boundaries and cross-laminae) suggests water column mixing and oxygenation rather than stagnation. This view is supported by the presence of numerous intervals rich in benthos-substrate interactions (fabric disruption, trace fossils such as Zoophycos, Teichichnus, Chondrites). The latter suggest persistent bottom water oxygenation that permitted bioturbation of soupy to firm muds by worm-like organisms. Measurement of pyrite framboid size distributions shows variable paleo-oxygenation of bottom waters, but only rarely indicates anoxic conditions. Geochemical oxygenation proxies (e.g., DOP, V/Cr, V/(V+Ni), Ni/Co, U/Th) suggest, for single samples even, conditions ranging from anoxic to oxic, and are in many instances at odds with conclusions from sedimentary features, trace fossils, and framboid diameters.

From an integrative perspective, it appears that the New Albany Shale accumulated under a variably oxygenated water column and that the trace element enrichments underlying oxygenation proxies are at least as strongly influenced by depositional parameters (sedimentation rate, reworking, resuspension) as by the degree of oxygenation of bottom waters.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005