SPATIOTEMPORAL WATERMASS EVOLUTION IN THE LATE DEVONIAN APPALACHIAN SEAWAY: EXPLORING THE LINK BETWEEN SEDIMENTATION, REDOX, AND SALINITY

The interiors of stable cratons were flooded by epeiric seas during many intervals of the geologic past. Few modern analogues exist for these environments, however, and physical and chemical properties of epeiric watermasses such as redox, salinity, and restriction are difficult to reconstruct in deep time. Despite these challenges, constraining epeiric watermass properties is critical because much of our preserved and accessible sedimentary record was deposited in epeiric seas. Here, we present a case study of watermass reconstruction in the Late Devonian Appalachian Seaway of North America. Geochemical data are presented for six cores from the Cleveland Shale in environments ranging from proximal to the Catskill Delta in northeastern Ohio, to environments in the deeper central axis of the basin in south-central Ohio and northern Kentucky, to environments that shoaled towards the Cumberland Sill in southeastern Kentucky. The B/Ga paleosalinity proxy records a pronounced gradient from brackish conditions proximal to the Catskill Delta to fully marine conditions in the basin interior. In addition, there are strong relationships between paleosalinity, detrital sediment input, and redox indicators such as TOC, Mo, and U in the cores proximal to the Catskill Delta, indicating that freshwater input promoted bottom-water oxygenation. In the lower Cleveland Shale, iron speciation indicates a gradient from intermittently oxic conditions at the most proximal site, to persistently anoxic and periodically ferruginous conditions in the deep basin center, to persistently euxinic conditions approaching the Cumberland Sill. Mo/TOC is consistently between ~13 and 18, indicating a moderate degree of basin restriction. In the upper Cleveland Shale, iron speciation indicates expansion of euxinia to all six cores, accompanied by a pronounced pattern of Mo and U drawdown from north to south. Mo/TOC averages ~19 in the most proximal core but progressively decreases to ~3 in the southernmost cores, indicating highly restricted conditions. These data suggest that the lower and upper Cleveland Shale represent distinct intervals in the hydrographic evolution of the Appalachian Basin, reinforcing the notion that redox reconstruction in black shales should be considered in the context of basin hydrography and paleosalinity.