FOURTH ORDER PARASEQUENCES OF THE MOLEEN FORMATION, ELY LIMESTONE, ELKO, NEVADA

The Early to Middle Pennsylvanian Ely Limestone was deposited in a mixed carbonate-siliciclastic system within a tectonic basin, one of a series of such basins that occupied the Antler foreland of the western U.S. At Grindstone Mountain (near Elko, Nevada), the Ely Limestone is comprised of the Moleen Formation (Morrowan-early middle Atokan) and the overlying Tomera Formation (early middle Atokan to early Desmoinesian). The Ely here, as common in other areas of Nevada, is characterized by sedimentary cycles typically interpreted as eustatic parasequences. At Grindstone Mountain the Moleen portion of section studied is 30 m thick and contains 3 cycles, each 6 to 12 m thick. Unresolved questions about these cycles include the potential influence of tectonism on their architecture and lithologic composition, and the importance of autocyclic processes. The study focuses on describing the sedimentary characteristics of a few of these parasequences to better determine the influence of tectonic and local factors on the origin of these mixed system cycles.

The base cycle at Grindstone Mountain, GM1, is a transgressive cycle comprised of basal sandy micrites that grade to sandy grainstones. The central 8 m is a covered section assumed to be eroded basinal micrites and interpreted to represent a period of aggradation. The MG1/GM2 boundary is covered and interpreted to be abrupt deep water sediments such as micrites - the same applies to the MG3/MG2 boundary. The upper part of the second cycle (GM2) is a transgressive parasequence from sandy micrite to silty grainstones. MG3, has been interpreted to be a transgressive sequence dominated by carbonaceous arenites with a small period of carbonate dominated deposition marking the top of the cycle. The upper boundary has been delineated as the center of a 1 m thick bed of sandy packstone and is arbitrary. Samples from the base and top of the Grindstone Mountain section contain Millerella and Pseudostaffella ex gr. needhami, (among others) which indicate a lower Atokan age. Based on this fauna an estimated maximum time span for the studied section is 3 million years. Thus, assuming a linear deposition rate, the maximum duration of cycle one is approximately 1.3 m.y., the second cycle is 625,000 yrs., while the third is approximately 800,000 yrs.