PRELIMINARY REPORT ON ACROTRETID AND PATERNIID BRACHIOPODS FROM THE EMIGRANT FORMATION (MIDDLE CAMBRIAN) AT BIG MIKE CANYON, ESMERALDA COUNTY, NEVADA

The proposed Esmeraldan Series (Middle Cambrian) for international Cambrian chronostratigraphy is located at Split Mountain in Esmeralda County, Nevada. The basal boundary is marked by the first occurrence of Oryctocephalus indicus, which is a widespread trilobite species known in North America and in south China. The Split Mountain section is complicated by at least one fault (possible two), and faunal control is poor above the basal 20-meter shale of the Emigrant Formation. It is debatable whether or not the section at Split Mountain is a condensed interval that represents nearly the entire Middle Cambrian within a sequence less than 40 meters in thickness. A previously unknown locality of the Emigrant Formation was discovered about two miles northwest of the boundary stratotype at Big Mike Canyon. Although trilobite faunas at Big Mike Canyon are not diagnostic, abundant brachiopods recovered from limestone residues are herein utilized in an attempt to establish biostratigraphic correlations between the Big Mike Canyon section and the Split Mountain site.

Scanning electron photomicrographs of some 27 relatively complete brachiopods were captured digitally and identified. By comparison with previously reported inarticulate brachiopod ranges, our preliminary results indicate that the age of the brachiopod-bearing interval is no older than the Lower Cambrian Olenellus Zone and no younger than the Middle Cambrian Ehmaniella Zone.