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 localiz
