OUTER SHELF TO SLOPE DEPOSITS OF THE LOWER CAMBRIAN POLETA FORMATION, WESTERN GREAT BASIN, NEVADA

The shelf margin to slope break environment is critical to understanding the Cambrian paleogeography of western Laurentia, although there are only a few examples of this environment in the geologic record. Therefore, our study on the earliest Phanerozoic carbonate platform deposits of the Lower Cambrian Poleta Formation is of special interest, because carbonate slope and slump deposits are present within at least two mountain ranges outboard of the typical shallow carbonate platform deposits of the outer shelf. Although the Lower Cambrian slope deposits of both Miller Mountain and the Benton Range are metamorphosed to hornfels and garnet skarn, the stratigraphy and sedimentology remain straightforward, and trilobite faunas at Miller Mountain provide for precise correlation to shelfal sections.

The tripartite subdivision of the Poleta Formation, consisting of upper and lower limestone members separated by a siliciclastic member, is also present in the slope sections. The carbonate members at Miller Mountain are metamorphosed to a uniform-appearing, crystalline marble. The lower carbonate member is comparable in thickness to its shelfal counterparts, but the upper carbonate member is several times thicker in the slope environment. The middle clastic member in the shelfal environment consists of a coarsening-up sequence of shales and thin interbedded bioclastic limestones, grading upward into quartz sandstones, while the slope environment is dominated by argillitic and siliceous turbiditic deposits and chert horizons, overlain by polymictic megabreccias and fine-grained quartzitic layers. The distinct faunal change from nevadiid to olenellid trilobites (base of Dyeran Stage) occurs within the middle member of the Poleta at Miller Mountain as it does in the shelf areas to the east.