Did Compaction Fluids Influence Paleozoic Reef Development in the Illinois Basin?

By

LONGMAN, MARK W.

Consulting Geologist, Denver, CO,

SUSAN M. LANDON

Thomasson Partner Associates, Denver, CO

     The importance of expelled fluids on the development of “vent” communities in modern oceans is now widely recognized. Many examples of Mesozoic “vent” communities have also been documented. Development of Paleozoic reefs, however, is typically attributed to growth along shelf margins or on subtle paleohighs, despite the fact that many of these Paleozoic reefs developed over thick shale sequences.

     In the Illinois Basin, two of the best known sets of hydrocarbon-bearing reefs occur in the Silurian, a hundred feet or so above the thick Upper Ordovician Maquoketa Shale, and in the Ullin Limestone, just above the thick New Albany Shale. It seems likely that compaction waters, perhaps combined with biogenic methane, from these shales, migrated out of the shales during burial and compaction. Thus, “vents” on the respective Silurian and Early Mississippian sea floors may have helped localize reef development. Such compaction fluids probably migrated preferentially along faults and fractures that provided permeable pathways for the compaction fluids.

     A similar scenario can be used to explain the development of the Lower Mississippian Waulsortian mounds in the Lodgepole Formation of Stark County, North Dakota (Williston Basin). There, surface lineaments intersect in a horseshoe-shaped pattern that apparently allowed vent communities dominated by crinoids, fenestrate bryozoans, and ostracods to form buildups more than 300 ft high. Abundant marine cements helped lithify these buildups, giving them reef walls that dip at up to about 60º.

     It remains to be proved if the development of Silurian and Lower Mississippian reefs in the Illinois basin can be related to migration pathways of compaction fluids generated in the underlying shales, but preliminary evidence suggests this idea. Silurian reefs in Washington County, Illinois, for example, formed where linear faults intersect a paleo-shelf margin.