Regional diagenetic patterns in the St. Peter Sandstone; implications for brine migration in the Illinois Basin

Diagenetic minerals and alteration patterns in the
Ordovician St. Peter Sandstone, Illinois Basin, record varied
hydrologic and chemical conditions during the basin?s long
and complex geologic history. Major diagenetic events modifying
the St. Peter Sandstone include (1) mechanical compaction,
(2) early K-feldspar overgrowth and dolospar
precipitation, (3) burial quartz, dolospar, anhydrite, and calcite
cementation, and (4) carbonate-cement and K-feldspar
grain dissolution. Radiometric age dates of authigenic
K-feldspar and illite in combination with the reconstructed
burial history of the St. Peter reveal that early-diagenetic
K-feldspar and dolospar precipitated at shallow to moderate
depths in the Devonian, whereas late-diagenetic quartz,
dolospar, anhydrite, and calcite formed during deep burial in
the Late Pennsylvanian to Early Permian. Stable-isotope
geochemistry and fluid-inclusion paleothermometry suggest
that burial cements precipitated from saline fluids over a
wide temperature range. In the southern part of the basin,
burial cements preserve a record of diagenetic effects that
were in part controlled by fractures and hydrothermal-fluid
circulation. Baroque dolospar cementation is the most significant
of these effects.