THE PROTEROZOIC TECTONIC HISTORY OF SOUTHERN INDIANA

A model for the Proterozoic tectonic history of southern Indiana and nearby parts of Illinois and Kentucky has been developed by integrating interpretations of seismic reflection data with analyses of gravity, magnetic and well data. Two pre-Mount Simon basins have been delineated in southern Indiana. The English Basin contains up to 3000 meters of mildly-deformed sedimentary rock assigned to the newly defined Marengo sequence. Seismic data indicate that the English Basin covers an area of at least 20,000 square km in Indiana; potential-field models and inversion results suggest the basin may be five times this size. Sediments of the English Basin onlap a prominent angular unconformity on the top of the Centralia sequence. The age of the English Basin is poorly constrained, with alternate interpretations suggesting that it may be either latest Proterozoic (~600 Ma) or late-Middle Proterozoic (~1.0 Ga) in age. The smaller Vincennes Basin covers at least 1000 square km in west-central Indiana. This basin contains up to 1000 meters of undeformed sedimentary rock, possibly of Lower Cambrian age and assigned to the newly defined Vandalia sequence, onlapping a prominent unconformity inferred to be the boundary between the Proterozoic and Phanerozoic eras.

Both of the newly discovered basins appear to overlie the layered Centralia sequence, a probable Middle Proterozoic unit filling a large basin underlying most of Indiana and Illinois. At its maximum, the Centralia is 10 km thick and may consist of sedimentary and metasedimentary rocks, probably interbedded with volcaniclastic rocks and intruded by a significant volume of igneous material. Thrust faults and folds imaged by the seismic data indicate that the Centralia has undergone extensive compressional deformation.

The presence of multiple pre-Mount Simon basins indicates a complicated Proterozoic tectonic history for the region. This history includes two episodes of Middle Proterozoic rifting (~1.5 Ga and ~1.3 Ga), Middle-Late Proterozoic orogeny (~1.0 Ga), Late Proterozoic uplift and erosion, and Late Proterozoic-Early Cambrian rifting. The Phanerozoic tectonic history can be largely explained by the reactivation of Proterozoic crustal boundaries in response to distant plate margin interactions.