Sequence stratigraphy of the Muscatatuck Group (Middle Devonian) in southern Indiana: A new framework for paleoecological analysis.

摘要

Subtle paleoenvironmental and stratigraphic gradients have been recognized through sequence stratigraphic and paleoecological analyses of the lower part of the Middle Devonian Paraspirifer Biofacies (Muscatatuck Group of Indiana). Bulk limestone samples were collected along a north-south transect in the lower part of the Paraspirifer Biofacies directly above a correlatable marine flooding surface. This marine flooding surface is the maximum flooding surface of the upper depositional sequence in the Jeffersonville Limestone. The northernmost sample locality lies directly over a basement structure that manifested itself as a paleotopographic high during the Middle Devonian, and the southernmost locality lies in a probable paleotopographic low. The influence of these paleotopographic features on Middle Devonian deposition is recorded by the lateral distribution and composition of lithofacies and genetic units. The southern part of this transect represents deposition in relatively deeper-water under normal open-marine conditions, whereas the carbonates in the north were deposited in shallower-water.; This south-to-north environmental gradient in the upper sequence of the Jeffersonville Limestone 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 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. Samples from the northern or updip locality are less condensed and the degree of condensation appears to progressively increase towards the central and southern or downdip localities. The composition of macrofossil samples do not reflect this stratigraphic trend possibly because of the greater amount of taphonomic loss experienced by these taxa, (as compared to conodonts) directly above an MFS. The ability to delineate environmental and stratigraphic gradients 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.