WATERMASS RECONSTRUCTION IN ANCIENT EPEIRIC SEAS: REDOX AND SALINITY ANALYSIS OF DEVONIAN-MISSISSIPPIAN BLACK SHALES OF THE APPALACHIAN BASIN, NORTH AMERICA

Whereas watermass reconstruction in modern systems is achieved by direct measurement, understanding the spatial and temporal evolution of watermass chemistry in ancient basins is more challenging. Watermass reconstruction is particularly important, however, in black shales—a lithology long thought to represent homogenous deep-water depositional environments, but increasingly recognized in recent years to record dynamic conditions of redox and watermass mixing in variably restricted epeiric seas. Here, we suggest a novel approach to watermass reconstruction in ancient black shales, employing a multi-proxy paleo-redox and paleo-salinity approach applied to long lateral transects of multiple drill cores through coeval units across a depositional basin. We present two case studies from the environmentally-dynamic foreland Appalachian Basin—the Upper Devonian Cleveland Shale and the Lower Mississippian Sunbury Shale. In both cases, strong lateral redox and salinity gradients are recorded that can be fundamentally related to depositional setting and basin hydrography. Oxygenated conditions (at least episodically) are recorded in environments proximal to the Catskill deltaic wedge, ferruginous conditions are recorded in the deeper central axis of the basin, and euxinic conditions are recorded as environments shoal towards the basin-bounding Cumberland Sill. In addition, a clear increase in salinity is recorded across the same NE-SW transect, with brackish conditions present proximal to the Catskill Delta and increasingly marine conditions recorded as environments approach the open ocean. Vertical watermass gradients are also apparent, with deeper basin trough deposits showing higher salinity than their shallower-water counterparts. Ultimately, we suggest that this approach may be useful for watermass reconstruction in a variety of ancient systems, and will allow us to further unravel the fundamental watermass conditions needed for black shale deposition.