COMMUNITY PALEOECOLOGY OF THE BIRD SPRING GROUP, ARROW CANYON, NV

This study examines the fine scale structure of Early Pennsylvanian brachiopod communities of the Bird Spring Group, Arrow Canyon, NV. Arrow Canyon consists of laterally extensive, well-exposed carbonate deposits representing continuous, shallow-water marine, cyclic, shoaling-upwards packages. The lower Bird Springs Formation in Arrow Canyon has been assigned as the Global Stratotype Section and Point (GSSP) for the Mid-Carboniferous boundary, allowing for easy correlation with equivalent sections. Despite the fact that Nevada in the Pennsylvanian is considered to be representative of Panthalassan global Mid-Carboniferous oceanic conditions (Saltzman, 2003), surprisingly few studies have examined the paleocommunities of the region quantitatively, even though fossils are abundant. We examined brachiopod assemblages from the Mississippian-Pennsylvanian boundary to the Upper Pennsylvanian. Surface counts (~1m2 areas) were performed for 33 samples, totaling 2112 brachiopods from 17 genera. The effects of time-averaging or transportation on these deposits appears to be minimal (low abrasion, minimal size sorting, low fragmentation), each surface probably representing a single community. Bray-Curtis Q- and R-mode ordination analyses were run to explore community structure, and identify possible environmental gradients. The results suggest that community patterns do not correlate with stratigraphy, which is as expected due to the cyclic nature of the packages. The Q-mode analysis revealed a broad discrimination between fine (shale and lime-mud stone) and coarser (wacke-pack stone) grained sediments, with axis 1 accounting for 45% of the variation, and axis 2, for 26%. The distinction is not a gradual transition, but rather an abrupt break. This lithological differentiation may also indicate that depth and/or productivity may be factors structuring the community. The R-mode analysis lead to axis 1 accounting for 46% of the variation, and axis 2 for 29%. While the R-mode data were not consistent with any obvious gradient, larger concavo-convex taxa tended to group together. These results are broadly consistent with previous work, and may be useful for comparison with Pennsylvanian sections elsewhere.