PETROGRAPHIC ANALYSIS OF BENTONITE BEDS TO INTERPRET VOLCANIC SOURCES IN THE GREENHORN FORMATION, SOUTH DAKOTA

The Greenhorn Limestone (Cretaceous) is located throughout the western interior and is enriched with bentonites which can be used for stratigraphic correlation and source volcano chemistry. Determining these factors within the bentonite, a weathering product of volcanic ash, can help determine the location of the volcanic source. There are four possible sources of the ash in the northwestern vicinity: the forearc, backarc, volcanic arc, and an anorogenic source. The forearc is closest to the subduction, the backarc is further in the interior, the volcanic arc is the actual volcanoes formed from the subduction, and the anorogenic source is associated with a hot spot. Shaping results from the bentonites is important because the actual source of volcanism may have been destroyed by the earlier collision. Bentonites of the Greenhorn Formation are rhyolitic with an island arc and a back arc source.