FOURTH ORDER EARLY PENNSYLVANIAN SEA LEVEL CYCLES, ELY LIMESTONE NORTHEASTERN NEVADA

Cycles within the mixed carbonate-siliciclastic successions of the Ely Limestone at Carlin Canyon, Pequop Mountains, and Grindstone Mountain, in northeastern Nevada record 4th order sea level changes. The 10-20 meter thick cycles occur as coarsening upward sequences with varying abundances of siliciclastic detritus (chert, monocrystaline quartz, and silt) and brachiopod, pelmetazoan, bryozoan allochems. The abundance of chert stringers and nodules appears to be related to the relative abundance of silt. Samples recovered from the stratigraphic sections yielded small foraminifera Millerella and Globivalvulina and conodonts indicating an approximate age of early Morrowan (earliest Pennsylvanian). Although it is difficult to precisely estimate the duration of each cycle, when placed within the overall Morrowan stratigraphic succession, each cycle fits with the generally accepted definition of 4th order. Carlin Caynon (CC), Grindstone Mountain (GM), and Pequop Mountains (PM), sections provide a west to east transect of the Ely Basin. Cycles at CC tend to be comprised of a basal silty micrite that shift upward to wackestone and packestone, generally capped by a brachiopod-pelmetazoan grainstone. The cycles are sometimes topped by a chert-limestone conglomerate. Fine to very-fine calcareous arenite occur in the middle of some cycles and may represent local events or possible 5th order overprints. Stratification includes foresets up to 2.5 m high that suggest tidal bedforms. The basal facies at GM are typically micrite to sandy micrite, these are capped by pelmetazoan packstone or grainstone. The last GM cycle studied contains a 1.5 meter thick bed of coarse, cross-stratified calcareous arenite. Cycles at P M tend to have basal micritic siltstones and micritic arenites respectively topped by allochemic arenites (mainly brachiopod and pelmetazoan) and pelmetazoan-bryozoan packstones, respectively. There is a west to east decrease in sedimentary structures, such as cross-stratification, and detrital chert input. The bimodal character of the sediments at these sites suggests two different sources of siliciclastic detritus for the basin. Future, more extensive studies of these 4th order cycles may lead to a better understanding of climatic and tectonic processes that affected the Ely Basin.