Multiple controls on the preservation of organic matter in Devonian–Mississippian marine black shales: geochemical and petrographic evidence

Abstract

Geochemical and petrographic analysis of an interval of black shales between the Huron Shale Member of the New Albany Shale (Upper Devonian, Famennian) and the Sunbury Shale (Lower Mississippian, Tournaisian) suggests that multiple controls influenced the accumulation of organic matter. Stratigraphic trends were determined following detailed analysis of a core from the central Appalachian Basin (D6, Montgomery County, KY). Specifically, C–S–Fe relationships, Mo, V/(V+Ni), Ni/Co, and V/Cr were used to assess paleo-redox conditions during sediment accumulation; the role of clastic input was evaluated using Ti/Al, K/Al, and Si/Al; and C/P was used as a proxy for paleoproductivity. The terrestrial-to-marine maceral ratio (T/M)—(vitrinite plus inertinite)/(alginite plus bituminite)—was used to evaluate changes in OM source. C–S–Fe relationships and trace-element ratios suggest anoxic conditions prevailed during deposition of much of the upper part of this interval. However, bottom-water conditions were intermittently dysoxic during accumulation of the Huron Shale Member, and possibly dysoxic to marginally oxic during accumulation of the lowermost Huron Shale Member. Productivity appears to have been an important factor in the accumulation of C org, and high C/P ratios are consistent with a productivity–anoxia feedback mechanism. In addition, nutrients from increased terrestrial weathering may have contributed to enhanced productivity. Variations in sediment influx may also have influenced C org concentrations through sediment starvation. One of the more significant observations in this work is an increase in terrestrial OM (inertinite and vitrinite) up-section, which may reflect vegetative changes occurring on land during this time interval. Most of this increase is due to inertinite, much of which appears to be fusinitic in origin and hence may be fossil charcoal. This latter observation has implications for the expansion of land plants into drier niches and for Late Devonian atmospheric oxygen levels. Introduction of more resistant terrestrial OM to anoxic basins may have been an important factor in the preservation of high-carbon (>10%) intervals, especially in the Cleveland Member of the New Albany Shale (Upper Devonian, Famennian) and the Sunbury Shale (Lower Mississippian, Tournaisian).