Abstract: Regional Diagenesis in the Mt. Simon and St. Peter Sandstones, Illinois Basin: Evidence of Basin-Wide Fluid Flow

Neil S. Fishman, Janet K. Pitman

Diagenetic alterations in the Cambrian Mt. Simon Sandstone and Ordovician St. Peter Sandstone, two regionally extensive aquifers in the Illinois basin, Illinois and Indiana, record the nature and timing of fluids that have passed through them. Similarities and differences in the units' alteration histories provide critical information that, when viewed in a basin-wide context, helps constrain paleohydrologic models of lead-zinc mineralization and hydrocarbon migration.

Although the paragenetic sequence of mineral cements is similar in both units, there is noticeable interformational variability in cement abundance. Quartz and K-feldspar overgrowths are volumetrically minor (<5% each) in the St. Peter but significantly more abundant (20% and 10%, respectively) in the Mt. Simon. In contrast, multiple generations of carbonate cements (dolomicrospar, planar dolospar, baroque dolospar, and calcite) occur throughout the St. Peter, but these cements are uncommon in the Mt. Simon. Early, illitic clay rims and late, illitic pore-filling clays are widespread in both units. Fluorite cement, some of which contains hydrocarbons in fluid inclusions, occurs sparsely in both units, principally in northern Indiana.

The similar diagenetic histories of the two units suggest that many alterations are due to pore fluids of similar compositions. However, variability in cement abundance points to additional intraformational controls on diagenesis. Nevertheless, both units experienced K-feldspar and illite authigeneses--alterations that occurred throughout the mid-continent at about 400 Ma and 270 Ma (ages based on work of others), respectively. Other workers have postulated that lead-zinc mineralization in the region occurred at about 270 Ma; therefore, the areally extensive Mt. Simon and St. Peter aquifers may have hosted the mineralizing solutions as they moved in the basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994