Faunal analysis of Permo-Carboniferous shales, North-Central Texas




Carew, James L.

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For the purpose of studying the nature of shelled marine invertibrate faunas that inhabited muddy bottom environments during a segment of the geologic past, fossil collections containing approximately 30,000 specimens were obtained from shale beds at thirty-two localities in North-Central Texas. The collections come from rocks ranging in age from Desmoinesian (Middle Pennsylvanian) to Wolfcampian (Early Permian). Brachiopods, bivalves, gastropods, and proifera were systematically studied and identified to species level. All other specimens were identified at higher taxonomic levels. A new species of pseudozygopleurid gastropod is identified. Fossils collected from bulk samples were identified and tabulated for statistical analyses. Sediment parameters were also entered into the computations, and various elements of faunal composition were compared with sediment composition. Q-mode cluster analysis shows that these faunas can be statistically grouped into three general community-types or assemblages. One is an association of taxa numerically dominated by sponges and with a low diversity of other species. These assemblages are laterally associated with carbonate banks. A second grouping includes assemblages dominated by brachiopods and containing an unusually large number of bryozoa. These faunas are of moderate diversity and abundance. The third community-type is the most variable in composition but is usually characterized by the dominance of mollucs. These assemblages are the most diverse and have the highest abundances. All of these assemblages or community-types can be compared to communities of other authors. Q- versus R-mode cluster analysis delineates those taxa most influential in the clustering of faunas. Comparison of the faunal data with sediment parameters reveals a strong correlation between increased carbonate content of the shale and increased brachiopod content of the fauna. A strong negative correlation exists between carbonate content and faunal diversity. As the depositional environment of the study area at that time consisted of delta complexes prograding onto a shallow carbonate platform, these trends are interpreted as reflecting faunal variation that is a function of the rate of influx of fine grained clastic material. At least in this area, the distribution of organisms and faunal communities that has previously been correlated with distance from shore and increasing water depth can be more accurately correlated with the rate of supply of fine grained sediment. Thus, animal communities previously assumed to represent an on-to offshore gradation may be distributed in any orientation to the shoreline along a gradient of decreasing fine grained sediment influx. Faunas rich in brachiopods seem to have preferred clear, less turbid, more calcareous environments, while the molluscs (especially gastropods) thrived in the more turbid muddy areas. One species of brachiopod, Crurithyris planoconvexa, has been interpreted to behave as an explosive opportunist; occurring in huge numbers after theorized reductions in the salinity of the environment as a result of increased fresh water run-off. Although the units sampled span the Pennsylvanian-Permian boundary, the statistical studies indicate that among these faunas there is no significant change in the megafossil fauna