PROTEROZOIC LAYERED SEQUENCES IDENTIFIED IN SEISMIC DATA FROM CENTRAL AND WESTERN KENTUCKY AND SOUTHERN INDIANA

The easternmost Eastern Granite-Rhyolite Province has been shown to be in large part a sedimentary province. Some of the drilled sedimentary rocks have been directly related to Proterozoic layered sequences evident from reflection seismic data in central Kentucky. These and other similar, but undrilled, layered reflectors in western Kentucky and southern Indiana can be divided into two groups of layered sequences. The eastern group, east of the western edge of the Louisville Geophysical Anomaly, is composed of six seismic stratigraphic sequences, three of which are well defined and regional in extent. Three other seismic sequences known only from a single seismic line appear to be younger and in a deeper part of the basin. Drilling in central Kentucky has shown that two of the six sequences consist of lithic arenite, but the deepest and the three shallowest sequences remain undrilled. Other drilling in the area has revealed both mafic and felsic volcanic rocks as well as other lithic arenite units, but their relationships to the defined sequences are not well established. Direct and indirect evidence shows the eastern group to be Mesoproterozoic in age. The eastern group represents an eastward thickening wedge with an aggregate thickness ranging from less than 2 km on the Louisville Geophysical Anomaly to more than 6 km near the Grenville Front. Much of the group appears to be part of a large thrust plate with internal thrusting cut by later wrench faults.

The western group is composed of four well-defined, regional seismic stratigraphic sequences lying mainly west of the Louisville geophysical anomaly. These sequences also show sequence boundaries, suggesting sedimentary and possibly volcanic origins, but they remain undrilled. Indirect geochronologic and fission track evidence suggests that the youngest sequence, which onlaps onto the Louisville Geophysical Anomaly is Neoproterozoic in age. The older sequences of this group are cut by thrusts and later wrench faults and have been overthrusted by the eastern group. The western group ranges from a feather edge in western Kentucky and southwestern Indiana to more than 2 km along the western edge of the Louisville Geophysical Anomaly, where it rapidly thins and pinches out to the east. The older Proterozoic, east-vergent Hoosier Thrust Belt underlies the western group.

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