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Two or more generations of buried Silurian reefs are present in southwestern Indiana. Possibly all the larger reefs grew until Devonian time. Some may have attained thicknesses greater than those of any other group of Silurian reefs. Strata topping the reefs range from Middle or Late Silurian to Middle Devonian in age. Deep drilling is sparse in southwestern Indiana, and limited geophysical surveys are mostly confidential. Interpretation of the reef province then relies heavily on evaluation of structural deformation (broad sense) of the rocks both encasing and overlying these and other Silurian reefs and of the reefs-proper and their flanks.
The amounts of suprareef drape in strata as young as Pennsylvanian are related to reef thickness, kind of reef, erosion of reef and postreef rocks, height above reef, and counterproductive subreef sagging. Both suprareef draping and subreef sagging are expectable for any given reef. Long-continued diagenesis, even to the present time, was the most significant cause of such structural deformation; subreef soft-sediment deformation penecontemporaneous with reef growth also was a factor. Differential compaction between reef and contemporaneously deposited interreef rocks was the most important diagenetic process. Differential compaction in rocks far above the reefs, acting in concert with lithologic and thickness differences brought about by continued growth of drape structure, had a min r role. Differential solution and recrystallization could have contributed especially to subreef sagging. These interpretations temper some ideas that localized tectonic uplifts influenced both reef siting and suprareef draping and that early cementation resulted in structural stabilization of substrate, reef, and reef flank penecontemporaneous with growth.
The ranges in geologic circumstances that apply--in setting, reef genesis and abortion, erosion or nonerosion and burial of reefs, and postreef attainment of structural clues to reef recognition--suggest that southwestern Indiana has a reef-related reservoir potential that applies differentially within the reef province. The differential extends to the reefs themselves and to individual formations draped over the reefs. This province, surely, is inadequately explored for hydrocarbon potential in sub-Mississippian rocks.
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