CARBONATE-CARBON ISOTOPE STRATIGRAPHY AND THREE-DIMENSIONAL SUBSIDENCE HISTORY OF THE PENNSYLVANIAN ELY-BIRD SPRING BASIN, NEVADA AND UTAH

The Ely-Bird Spring basin (EBSB) contains cyclothemic limestone deposited on an equatorial shallow shelf during Pennsylvanian time. The stratal cyclicity was driven in part by eustatic sea-level changes during the late Paleozoic ice age, and also by active but unevenly distributed tectonic subsidence. EBSB strata are now exposed in mountain ranges of eastern Nevada and adjacent Utah. This study focuses on early to middle Pennsylvanian (Morrowan to Atokan) EBSB strata. We use detailed carbonate-carbon isotope data as a fine-scale correlation tool, built upon existing biostratigraphy, to elucidate changes in accommodation space generated across the EBSB through time.

We present data and correlations from six EBSB sections, including (from north to south) Grindstone Mountain (n=243), Illipah Reservoir (n=227), Mountain Home Range (n=183), Syncline Ridge (n=159), Arrow Canyon (n=107), and Mountain Springs Pass (n=183). The bulk of these data are dominated by 0.5 to 2‰ positive shifts toward cyclothem tops built on longer-scale/lower-frequency isotopic and stratigraphic cycles. Correlations between sections were investigated using Match-2.3 (Lisiecki and Lisiecki, 2002), a dynamic programming algorithm developed for stratigraphic signal correlation. For each correlation, Match also produces a “relative accumulation rate” curve, interpreted here as a proxy for relative accommodation space generation. All correlations are referenced to the mid-basin section at Illipah Reservoir (IR) in the west-central EBSB.

In lower Morrowan time, the EBSB sections to the south and north have similar to lower accumulation rates than IR, whereas the Mountain Home Range section to the east accumulated sediment ~1.5 times faster. Relative accumulation rates surpass IR throughout upper Morrowan time around the EBSB, except in the south where accumulation rates remain lower than IR. During Atokan time, relative accumulation rates in the central and southern EBSB are similar to lower than that at IR, but are >2 times faster than IR at Grindstone Mountain in the northwestern EBSB. Atokan accommodation space generation in the northwestern EBSB may be the first signal of a tectonic event that culminated in angular unconformities and overturned folding that deformed the northern EBSB during middle Pennsylvanian time.