ABSTRACT: Gravity Field and Tectonic History of the Junction Area of the Cottage Grove, Shawneetown, and Wabash Valley Fault Systems, Near Equality, Illinois

STRUNK, KEVIN LEE, Wabash Resources, Inc., Indianapolis, IN

Equality is the junction area of three major fault systems: the Shawneetown (SFS; complexly faulted), the Cottage Grove (CGFS; right-lateral wrench) and the Wabash Valley (WVFS; normal). Between the faults is a gravity low, here termed the equality gravity low (EGL). The SFS and CGFS systems lie along the 38th Parallel Lineament, a reactivated Precambrian deformation zone. The EGL is interpreted as a deep-seated structural transitional zone exhibiting a remnant lineament gravity signature bordered by later structural overprints. The SFS makes a southwest bend near the western side of the EGL prior to incorporating into Fluorspar area structures. The CGFS begins west of the EGL and north of the bend. East of the EGL is the Ridgeway fault of the WVFS. The newly (here) designated Omaha l neament connects the SFS bend, the CGFS eastern end, the Omaha dome lacolith, and the Albion fault, the westernmost WVFS fault. This feature has regional significance as the reactivated western margin of the Precambrian Southern Indiana rift. The corner of the SFS is the Omaha and 38th Parallel lineament intersection, and the focal point of Appalachian Orogeny compressional forces. Near equality, these forces resulted in early Permian high-angle reverse faulting along the reactivated lineaments. The CGFS formed via compressional forces continuing west of the Omaha lineament. Jurassic extensional forces resulted in SFS scarp "decompression," and renewed rifting via reactivation basement faults. Stronger rifting south of SFS allowed the emplacement of Fluorspar area deposits, while weaker orthern rifting formed the WVFS.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)