Tectonic Factors Controlling Initiation and Distribution of Silurian Reefs in Illinois Basin, Southwestern Indiana

Lloyd C. Furer

In 1980, John B. Droste and Robert H. Shaver discussed the distribution, growth, geometry, and age of Silurian reefs in the Illinois basin. Questions still exist as to the relative effect that differential compaction between reef and nonreef rocks vs. post-Silurian tectonic movement has had on the structural geometry of overlying strata. In 1973, David L. Stevenson pointed out that tectonics, rather than drape, better fit his data for many reefs in southwestern Illinois.

The working hypothesis proposed here is that both factors have had a significant effect, each at different times. The effect of significant differential compaction occurred during a short time geologically and nearly ceased development by the end of the Devonian. In the Illinois basin, both patch and pinnacle reefs are present, and the effect of differential compaction is pronounced over the latter. This is well demonstrated by the Linton field structure in Greene County, where compaction of the nonreef rocks is 15% greater than that of the reef rock. At the Plummer field, also in Greene County, the differential compaction factor applicable to a single patch reef is approximately 1%.

Most of the structural closure seen at various horizons in Chesterian (Mississippian) rocks in southwestern Indiana was probably a result of tectonic movement on northeast-southwest-trending basement faults during the Morrowan (Pennsylvanian). Many of these faults experienced, during a long period of geologic time, several episodes of movement which have been referred to as recurrent tectonics. During the Silurian, these faults, which probably are a northeast extension of the well-known Wabash Valley fault system, experienced small movements that partially controlled the growth and distribution of the reefs.

Preliminary testing of this hypothesis has begun. Ten known Silurian reefs are clearly aligned with the Mt. Carmel fault. The fault and reefs are most likely related to vertical movement on basement faults. Published structural maps of the Plummer field and mapping in progress in the Marts field area show that dip steepens rapidly on the southeastern side of both of these fields. This steep dip is likely controlled by structural draping over an underlying horst block in the crystalline basement.

The small closures (20-60 ft) seen in Desmoinesian (Pennsylvanian) strata are probably a result of either minor folding associated with the same fault zones or differential compaction over thick underlying Morrowan sandstones. This idea minimizes the effect of any continued differential compaction between Silurian reefs and contemporaneously deposited rocks.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.