CARBON ISOTOPE CHEMOSTRATIGRAPHY OF LOWER PENNSYLVANIAN CYCLOTHEMS AT GRINDSTONE MOUNTAIN, ELKO COUNTY, NEVADA

The Pennsylvanian Moleen Formation was deposited in the northern part of the early to middle Pennsylvanian Ely-Bird Spring (EBS) basin of eastern Nevada. The EBS is one of a series of stacked, tectonically-generated late Paleozoic basins in western Laurentia. EBS strata consist of cyclothemic carbonates with local chert-pebble conglomerate lenses.

This work is part of an on-going study to characterize the tectonic framework of the EBS basin by combining traditional basin analysis and other stratigraphic techniques. Here we combine lithostratigraphic, biostratigraphic and carbonate-δ¹³C chemostratigraphic data from lower Atokan (uppermost Bashkirian-lower Moscovian) strata at Grindstone Mountain near Elko, Nevada. The Morrowan-middle Atokan Moleen Formation rests conformably on the late Mississippian Tonka Formation. Ages of the Moleen Formation were constrained using fusulinid biostratigraphy. The Moleen strata were broken lithostratigraphically into 35 cyclothems, each 5-20 m thick. These are dominated by mudstones and wackestones, with packstones and/or grainstones at the cycle tops. A few cycles have basal chert conglomerates, generally deposited on exposure surfaces. Siliciclastic grains and chert nodules are common. Cycles 16-25 were sampled for carbonate-δ¹³C chemostratigraphy. Micrite samples (n=86) were analyzed at the University of Nevada, Reno stable isotope lab. δ¹³C values vary between +1.9 and +5.1‰ relative to vpdb. Generally, δ¹³C values are lighter at the cycle bases and heavier toward cycle tops, though this is not true for every cycle. The isotopic data vary cyclically at several scales, including within the cyclothems and across the whole data set. These variations are at a much finer scale than biostratigraphic control. Similar variations have been observed elsewhere in the EBS basin and are interpreted as being a function of Milankovitch cycles (Tierney, 2005). Assuming a well-mixed ocean, the systematic variation in δ¹³C values in Pennsylvanian strata should allow for fine-scale basin-wide correlations when combined with the litho- and biostratigraphic framework.