GEOCHEMICAL INVESTIGATION OF CARBONIFEROUS CHERTS IN SOUTHERN NEVADA AND THEIR RELATIONSHIP TO THE ANTLER OROGENY

Mississippian and Pennsylvanian-age, shallow water limestones and dolomites of the Cordilleran continental margin of southern Nevada contain numerous horizons of nodular chert. The first appearance of chert is abrupt, occurring in the lower Mississippian, but then continuing throughout the rest of the Carboniferous.

Chert is present in a variety of shallow water depositional facies, ranging from sabkha to quiet water lagoons to shallow marine shelf, with an abundant macrofauna of crinoids, bryozoa, corals and brachiopods. The diversity of depositional settings suggests that there is no common early environmental factor controlling the formation of the chert.

The chert nodules were formed through replacement of original carbonate close to the sediment-water interface during early diagenesis. In some horizons, the chert appears as a replacement of original bioclasts, most commonly corals, suggesting that original mineralogical or textural features controlled porosity and permeability and strongly influenced chertification.

The source of silica for these cherts is poorly understood. There is little evidence for siliceous input from an external source of terrigenous siliciclastic sediment or from an internal, siliceous organism source, such as sponge spicules, which are often proposed as the silica source for nodular chert in shallow marine carbonate rocks.

We hypothesize that the source of the silica is igneous in nature. Geochemical analyses of the chert, including REE, trace element, and stable isotope, suggest the source of silica in these cherts is from volcanic ash fallout from eruptions associated with the Antler orogney. Geochemical characterization of Carboniferous carbonates and coeval cherts of the Western Cordillera in southern Nevada may provide an insight into sediment-seawater interactions in the Antler Foreland Basin and potentially into the evolution of the Antler orogenic belt with respect to the Cordilleran margin of the North American continent.