PALEOREDOX CONDITIONS AND THE ORIGIN OF BEDDED BARITES ALONG THE LATE DEVONIAN NORTH-AMERICAN CONTINENTAL-MARGIN

The most important controls of the redox state of natural waters are the flux of organic matter from the photic zone, the degree of physical mixing, and oxygen concentrations of any connecting water masses. In coastal upwelling zones, these factors can be described with a simple box model constrained by data from modern field settings. Onshore-offshore mixing is largely controlled by the intensity of coastal currents that generally are found at depths of <400 m. As a result, onshore-offshore chemical gradients are greater in deeper waters. Application of the mass balance models to Late Devonian paleogeographic reconstructions of western North America illustrates how bedded barites in this setting may have formed in conjunction with a sulfate-reducing coastal upwelling zone. Low-oxygen waters from an extensive equatorial undercurrent entered the upwelling zone during regressive periods of the Late Devonian. The steep bathymetry of emergent bank carbonates to the east meant that the most intense portion of the coastal upwelling system occurred cover deep water. Sinking organic matter was remineralized in these deep waters rather than being incorporated into sediments of the continental shelf and shelf break. The influence of coastal currents, and therefore onshore-offshore mixing, was minimal, thereby leading to the depletion of available oxygen and nitrate. Recent geochemical studies of particulate and dissolved barium in the modern ocean in conjunction with the mass balance model illustrate how waters with only minor sulfate-reduction and elevated dissolved barium concentrations may have led to extensive barite flux to the Late Devonian deep-water sediments off the North American continent.