Black shale–gray shale transitions in a Late Devonian shale succession, Central Appalachian Basin (Northern Ohio): Sedimentary and geochemical evidence for terrestrial organic matter input driving anoxia events

Abstract

In a 140 m-thick sediment core through Upper Devonian shale units in the Central Appalachian Basin, gray shale units (TOC ≤1%) and black shale units (TOC ≤9%) are separated by “intercalated zones” (ICZs) of thinly interbedded gray and black shales. In accordance with Walther’s law of facies successions, ICZs imply environments producing black shales were coeval and adjacent to environments producing gray shales. To understand these facies transitions, we studied event layer sedimentology, organic geochemistry, and trace elements. This study found black shales to be shallow-water deposits (with tempestites and wave-modified turbidites), associated with significant terrestrial input (with abundant hyperpycnites, and terrestrial-derived humic and fulvic acids). The shales were deposited under varying redox conditions, with extensively bioturbated intervals alternating with intervals preserving intact micro-lamination, and demonstrating significant variations in trace element V/Cr, Ni/Co, and Th/U ratios. These observations are reconciled by attributing shale TOC values to sediment dynamics of deltas draining the Neo-Acadian Highlands. Progradation of individual delta distributary lobes resulted in localized offshore regions receiving increased terrestrial nutrient input and causing freshwater-induced water column stratification and oxygen stress (similar to the “dead zones” of many modern deltas), producing black shales. In contrast, gray shales formed between deltas or after delta distributary lobe abandonment. The intercalated black shale-gray shale transition intervals (ICZs) are interpreted to represent dynamic water and sediment conditions during the time interval of delta distributary lobe avulsion events. This model contrasts with previous models calling for basin-wide anoxia or assigning black shales to basinal deposits.