Basinal and prodeltaic communities of the early Carboniferous Borden Formation in Northern Kentucky and Southern Indiana (U.S.A.)

Abstract

Basinal and prodeltaic rocks of the Early Carboniferous (Early Mississipian) Borden Formation each contain a distinctive community type. The stratigraphic and geographic occurrence of these communities was physically controlled by the level of dissolved oxygen in sea water and by the proximity of submarine fan valleys. The older, basinal community (Coral Ridge fauna) was dominated by deposit-feeding molluscs. A variety of evidence suggests this fauna lived in a reduced oxygen (dysaerobic) environment: (1) many stunted individuals, (2) low diversity, (3) dominance by deposit-feeders, and (4) replacement of CaCO 3 skeletons by pyrite, marcasite, and siderite. The younger, prodeltaic community (Button Mold Knob fauna) was a high-diversity community dominated by suspension-feeders living under conditions of normal oxygenation. Diversity and abundance data for this community were assembled from 74 bulk samples. Paleoecologic reconstruction of this community is based on these data. The dominant groups of crinoids were non-pinnulate, probably in response to the optimum mode of feeding in the low-energy prodeltaic environment. A population study of the brachiopod Rhipidomella oweni (Hall and Clarke) indicates high juvenille mortality that probably was caused by smothering in mud. The tabulate coral Cladochonus crassus (M'Coy) was an extremely abundant element of the community. The data set for C. crassus indicates that this coral had a three-dimensional growth form with a mean of 678 corallites per corallum. The patchy distribution of fossils at one site on the ancient prodeltaic sea floor was studied by use of contour maps and trend surfaces. The brachiopod R. oweni was distributed in clusters that in general were mutually exclusive of crinoids. The significance of this mutual exclusion is uncertain, but there is some evidence for biological interaction. Settlement sites on the sea floor had a large-scale patchy distribution. This may be a function of the inability of suspension-feeding larvae to settle directly in the mud. Once sites were fortuitously established, the skeletal debris of earlier generations provided larval settlement sites. The trophic structure of the prodeltaic community suggests partitioning of food resources by tiering above the substrate. Each trophic group is dominated by one or two species, except for the crinoids. The crinoids showed the greatest species equitability because the wide range of filtration fan morphologies allowed a complex division of food resources.