ABSTRACT

Wood-grained chert is nodular chert which has internal light- and dark-colored, concentric, three-dimensional, compositional banding resembling grain in wood. The light-colored bands are dolomitic and the dark-colored bands are quartzose with carbonaceous matter. Wood-grained chert is recognized in the Mississippian Lodgepole, Deseret, and Great Blue Limestones and some correlatives of these formations in Montana, Utah, and Idaho. The Deseret starved basin model (Sandberg and Gutschick 1980; Gutschick and Sandberg 1983) provides a paleogeograpic framework that restricts the wood-grained cherts in this basin to the foreslope between the oxygenated carbonate platform margin (< 50 m water depth) and the anaerobic-dysaerobic deep water (> 300 m) basin. Wood-grained chert may mark th approximate position of the lower limit of the pycnocline in the Deseret starved basin.

Upwelling of nutrient-rich water along the foreslope generated high biological productivity which in turn supplied abundant biogenic silica and organic matter to the sediments of the foreslope and basin. Siliceous microfossils dissolved and provided a source of silica for the formation of porcellanite (opal-CT) nodules within the foreslope sediments during burial and compaction. Calcium carbonate diffused into the nodules from the surrounding mixed calcareous/siliceous muds and early diagenetic conversion of opal-CT to quartz released magnesium. Reduction of sulfate, a dolomite inhibitor, in the associated anoxic, organic-rich sediments below the pycnocline permitted dolomite formation. Dolomite precipitated in bands as a result of a periodic supersaturation-precipitation-nucleation-d pletion process (Ostwald-Prager theory of Liesegang band formation), possibly augmented by competitive particle growth. Petrographic evidence and the apparent inverse relationship between band spacing and time support our theory.