CONTROLS ON THE DISTRIBUTION OF SADDLE DOLOMITE IN THE ST. LOUIS LIMESTONE (MISSISSIPPIAN) FROM INDIANA

The distribution of coarse-crystalline, void-lining saddle dolomite cement in the strata of the St. Louis Limestone (Blue River Group; Valmeyeran, Mississippian) was examined along an SE–NW trending outcrop belt that extends from south-central into west-central Indiana. These strata were deposited along the eastern margin of the Illinois basin on a shallow subtidal to supratidal carbonate ramp of an epiric sea, which occupied a cratonic embayment that formed above a rift complex (the Reelfoot rift and the Rough Creek graben). Regionally these strata dip and thicken to the SW towards the basin depocenter in southern Illinois and western Kentucky.

Saddle dolomite was observed only in the southeasternmost parts of this outcrop belt. Petrographic and geochemical evidence supports the origin of dolomite by precipitation from warm burial fluids. The restricted geographical distribution of saddle dolomite in southern Indiana is interpreted to be a function of the: 1) distribution of vuggy porosity; 2) proximity to the Illinois basin depocenter; and 3) location of faults extending northwards from the Rough Creek graben. The cm-scale dolomite-lined voids likely formed by dissolution of: 1) evaporite nodules in extensively dolomitized, poorly fossiliferous deposits; and 2) skeletal fragments in wackestone and packstone lithofacies. These voids locally provided pore space for the migration of fluids and precipitation of dolomite cement. The fluids most likely migrated up-dip (SW to NE) from the deeply buried strata of the Illinois basin. The sedimentary succession of the Illinois basin reaches its maximum thickness south of the Rough Creek-Shawneetown fault system, which trends W–E from southeastern Illinois into west-central Kentucky and defines the northern boundary of the Rough Creek graben. A series of faults extends northeastward from the eastern part of the Rough Creek-Shawneetown fault system. Saddle dolomite was observed in the area that corresponds with the northeastern projection of these faults into southern Indiana. The faults may have provided a localized pathway for the migration of dolomitizing fluids from the Rough Creek graben into the porous St. Louis Limestone strata.