ARE DECIMETER-SCALE CYCLES OF CORAL BIOSTROMES IN THE CARBONIFEROUS-PERMIAN BIRD SPRING FORMATION, NYE COUNTY, NEVADA, A BIOLOGICAL RESPONSE TO GLACIO-EUSTATIC SEA-LEVEL OSCILLATIONS?

An exposure of the Carboniferous-Permian Bird Spring Formation near Pahrump, Nevada contains a cliff-forming interval with multiple, low-diversity biostromes dominated by solitary rugose corals. This coral-biostrome interval, which is approximately 1.2 m thick, contains three to five biostromes. It is laterally continuous for at least 2.5 km. The tabulate coral Syringopora and crinoids occur in associated horizons, but they typically do not occur within the rugose corals biostromes.

In this study we are testing the hypothesis that this cyclicity of coral biostromes is a far-field response to glacio-eustatic sea-level oscillations as the late Paleozoic, Southern Hemisphere ice cap expanded and contracted. The Bird Spring carbonate platform, which developed in a low-latitude setting, is well known for its alternating cliff-and-slope erosion pattern. This pattern is commonly interpreted to record sea-level oscillations. However, we are not aware of previous studies that have documented sub-meter-scale cyclicity in marine invertebrate community structure, such as we observe at our “triplehorn” site. Such an interpretation would imply that the rugose corals and other taxa lived within a surprisingly narrow range of water depths.

We are using standard techniques of carbonate sedimentology and facies analysis to reconstruct water depth and wave energy conditions at successive levels within this interval of the Bird Spring Formation.