PROTEROZOIC SEQUENCES IN WESTERN KENTUCKY AND SOUTHERN INDIANA

Contrary to the traditional view, the easternmost Granite Rhyolite Province is dominated by sedimentary rocks. Proterozoic sedimentary and interbedded volcanic rocks have been identified from drilling and reflection seismic data in western and central Kentucky and southern Indiana. These Proterozoic rocks can be divided into seismic sequences based on seismic reflector and boundary indicators. Two groups of seismic sequences are separated by the north-south-oriented Louisville basement thrust fault that bounds the western edge of the Louisville High.

The group to the east of the fault lies in the East Continent Rift Basin and is composed of five seismic sequences. The lower three sequences are well defined and appear to be regional in extent. Drilling in western and central Kentucky has shown that two of the lower sequences consist of lithic arenite. The lowermost sequence has not yet been drilled. The upper two sequences are known only from a single seismic line and can be only speculatively correlated to drill data. Direct and indirect evidence shows the East Continent Rift Basin sequences to be Mesoproterozoic in age and older than the last Grenville thrusting event. They generally represent a northeastward-thickening wedge, ranging from less than 2 km on the east flank of the Louisville High to more than 6 km near the Grenville Front.

The group west of the Louisville Fault lies in the English Basin and is composed of four well-defined regional seismic sequences. Though these sequences remain undrilled, they show demonstrable sequence boundaries, suggesting sedimentary and possibly volcanic origins. Indirect geochronological and fission track evidence suggests that the youngest and comparatively undeformed sequence of the western group is Neoproterozoic in age. The older sequences of this group are deformed by thrust and wrench faulting and have been overthrusted by the eastern group. The western group pinches out to the west and to the south along the northern edge of the Rough Creek Graben, but attains a thickness in excess of 2 km to the northeast. The older Proterozoic, east-vergent Hoosier Thrust Belt underlies the western group. Uncorrelated layered Proterozoic rocks in the Rough Creek Graben of Kentucky likely relate to the western group, but have not yet been fully delineated.