A modern vs. Permian black shale—the hydrography, primary productivity, and water-column chemistry of deposition.pdf

Chemical Geology 206 (2004) 177–197
ate/chemgeo
A modern vs. Permian black shale—the hydrography, primary
productivity, and water-column chemistry of deposition
. Piper*, . Perkins1
. Geological Survey, M/S 902, 345 Middlefield Road, Menlo Park, CA 94025, USA
Abstract
The sediment currently accumulating in the Cariaco Basin, on the continental shelf of Venezuela, has an anic-
2 À
carbon content of approximately 5%; is accumulating under O2-depleted bottom-water conditions (SO4 reduction); is
composed dominantly of foraminiferal calcite, diatomaceous silica, clay, and silt; and is dark greenish gray in color. Upon
lithification, it will e a black shale. Recent studies have established the hydrography of the basin and the level of primary
productivity and bottom-water redox conditions. These properties are used to model accumulation rates of Cd, Cr, Cu, Mo, Ni,
V, and Zn on the seafloor. The model rates agree closely with measured rates for the uppermost surface sediment.
The model is applied to the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, a phosphate deposit of
Permian age in the northwest United States. It too has all of the requisite properties of a black shale. Although the deposit is a
world-class phosphorite, it posed mostly of phosphatic mudstone and siltstone, chert, limestone, and dolomite. It has
organic-carbon concentrations of up to 15%, is strongly enriched in several trace elements above a terrigenous contribution and
is black. The trace-element accumulation defines a mean primary productivity in the photic zone of the Phosphoria Basin as
moderate, at 500 g mÀ 2 yearÀ anic carbon, comparable to primary productivity in the Cariaco Basin. The source of
nutrient-enriched water that was imported into the Phosphoria Basin, upwelled into the photic zone, and supported primary
productivity was an O2 minimum zone of the open ocean. The depth range over which the water was imported would have been
betw