THE ENGLISH BASIN: A STRUCTURALLY COMPLEX LAYERED PROTEROZOIC DEPOCENTER BENEATH THE ILLINOIS BASIN

The English Basin is interpreted as a Meso- and Neoproterozoic-aged depocenter, forming a portion of the East Continent Rift Complex in Kentucky and Indiana. Subsequent tectonism during the Middle Cambrian resulted in rift style sedimentation within the English Basin.

Seismic data demonstrate the existence of a complex depocenter with a depth to crystalline basement exceeding 6100 m. Acoustically bright reflectors are traced continuously over a large lateral area, depicting a layered Proterozoic sequence. Potential field data has been tied to regional seismic data to interpret the lateral geometry of the basin and associated structural features. Borehole penetrations of the Proterozoic sequence suggest that the depocenter is filled with strata similar in lithology to the Mesoproterozoic Middle Run Formation (Centralia Group) described in Ohio and Kentucky, and locally overlain by Neoproterozoic sediments.

A thrust-fault system developed in the lowermost Mesoproterozoic-aged Centralia Group is referred to as the Hoosier Thrust Belt. Above a major angular unconformity truncating Centralia strata, the interpreted volcaniclastic and clastic sediments of the Wyandot Formation (proposed) and the Neoproterozoic-aged Marengo Formation are locally deposited.

The eastern basin margin is defined by the Louisville Uplift, a foreland-style basement thrust exhibiting approximately 8 km of vertical uplift dated at 600 mya. The margins of the uplift are expressed in Paleozoic strata by a series of shallow fault systems, extending to the northern Mt. Carmel Uplift. The remobilization of bounding fault systems appears to have controlled the development of hydrocarbon fields occurring along the margins of the basin.

Area earthquake epicenters may be localized along detachments within the Precambrian thrust belt, suggesting that contemporary seismicity may be related to the current east-west horizontal maximum principle compression of the Hoosier Thrust Belt. Significant large-scale structures within the English Basin are interpreted to provide excellent potential for hydrocarbon exploration.