Deposition and Diagenesis of a Cratonic Silurian Platform Reef, Pipe Creek Jr., Indiana

Anthony Simo, Patrick Lehmann

Petrographic and geochemical characteristics of the Pipe Creek Jr. paragenesis record the stratigraphic and burial evolution of the cratonic Silurian platform of Indiana during Late Silurian to Pennsylvanian. A variety of several diagenetic fluids acting over geological time affected the reef. The paragenetic sequence is as follows: (1) precipitation of turbid, fibrous, blotchy cathodoluminescent (CL) cement; (2) dolomitization of mud-rich facies; (3) precipitation of clear, zoned CL equant calcite cements; (4) fracturing and karst formation, partially filled by geopetal silt and sandstone; (5) precipitation of clear, dull CL, ferroan to nonferroan equant calcite cement, ferroan dolomite overgrowth and equant dolomite cement in moldic porosity, caves and fractures; (6) microdissolution and hydrocarbon emplacement; and (7) stylolitization.

Carbonate grew and fibrous cements precipitated in an open marine environment (1). During Late Silurian an increasingly restricted environment stopped reef growth and dolomite replaced mud-rich faces (2). The reefs were then subaerially exposed and two meteoric cement sequences, non-luminescent to bright luminescent, precipitated prior to Mid-Devonian fracture-controlled karsting (3). Caves and fractures crosscut former cement stages and were filled by sandstones (4). Later, the platform was buried by the late Mid-Devonian organic-rich New Albany Shale, and clear, dull CL calcite cement and ferroan dolomite precipitated (5). Hydrocarbon migration postdates all cements and created minor moldic porosity (6) and predates stylolitization (7).

The New Albany Shale was both the hydrocarbon source and top seal to the "fossil" Pipe Creek Jr. oil field with original oil in place estimated at 11 million bbl. The level of organic metamorphism of the New Albany Shale, the oil residue, and the two-phase fluid inclusions in the burial cements suggest that sediments accumulated on the platform throughout Mississippian time.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.