CONODONT PALEOECOLOGY WITHIN A SEQUENCE STRATIGRAPHIC FRAMEWORK: MIDDLE DEVONIAN OF THE ILLINOIS BASIN

Subtle paleoenvironmental and stratigraphic gradients have been recognized through paleoecological analyses of conodonts from the lower part of the Middle Devonian Paraspirifer Biofacies (Muscatatuck Group of Indiana). Bulk limestone samples were collected along a north-south transect in southern Indiana directly above and below a correlative marine flooding surface. This marine flooding surface is the maximum flooding surface (MFS) of a depositional sequence. The northernmost sample locality lies directly over a basement structure that manifested itself as a paleotopographic high and relatively shallow water conditions during the Middle Devonian, and the southernmost locality lies in a probable paleotopographic low representing an area of deeper-water under normal open-marine conditions.

This south-to-north environmental gradient is also supported by the lateral distribution of fossil biota within the Paraspirifer Biofacies. Multivariate statistical analyses of percent-abundance and presence/absence data of conodonts and other microfossils suggest a south to north shallowing trend. This environmental trend is not, however, the only gradient suggested by the multivariate statistical analysis of samples of this fossil assemblage. Cluster analysis and gradient analysis of microfossil presence/absence data and conodont percent-abundance data suggest a south-north stratigraphic gradient with respect to degree of condensation and developmental stages within deposits above a marine flooding surface. Samples from the northern or updip locality are less condensed and the degree of condensation appears to increase progressively towards the central and southern or downdip localities. The delineation of environmental and stratigraphic gradients using the distribution patterns of conodonts and other microfossils may have implications for the ability to predict updip versus downdip areas in regions with limited stratigraphic information and as such allow the construction of a preliminary sequence stratigraphic framework in these frontier regions.