Abstract

Petrographic studies of Mississippian echinoderm-bryozoan limestones indicate that compaction significantly reduced intergranular porosity in over 60 percent of the packstones and grainstones throughout the 30,000 square kilometers of study area. Compaction was accomplished by chemical and mechanical processes that commonly reduced intergranular volumes by up to 50 percent, and more rarely by up to 75 percent. Mechanical compaction processes included the rearrangement of grains and the plastic deformation and breakage of grains. Chemical compaction comprised pressure solution between grains, between grains and syntaxial cement crystals, and within bryozoan grains. Bryozoan grains were more susceptible to mechanical and chemical compaction than echinoderms; thus bryozoan deformation (chemical and mechanical) is the most common compaction feature in these rocks. In contrast, echinoderm grains typically underwent neither mechanical deformation nor intragranular pressure solution, although they commonly underwent intergranular pressure solution. Petrographic study of the relationship between compaction features and cherts, calcite cements, and clasts at the Post-Lake Valley unconformities reveals the timing of compaction. Much of the mechanical and chemical compaction occurred before Morrowan time and some before Meramecian time. All non-stylolitic compaction was probably completed before Permian time. These observations imply: 1) that compaction began under a few tens to a few hundreds of feet of overburden and was completed under less than 6500 feet (2000 m) of overburden, and 2) that chemical compaction occurred during cementation and thus could have been an important source of dissolved CaCO 3 for the cements. The model proposed is that compaction began under low to moderate overburden pressure by rearrangement and repacking of grains, accompanied and followed by breakage of bryozoan fragments and other microcrystalline grains. Plastic deformation of bryozoans and chemical compaction began under still moderate overburden, the latter as intergranular pressure solution first of bryozoans and then of bryozoans and echinoderms. Compaction was retarded but not stopped by pre-Pennsylvanian syntaxial cementation. Mechanical and chemical compaction continued as intrabryozoan pressure solution; as intensified intergranular pressure solution; as development of drag fabrics; and as minor shell breakage. The chemical compaction was probably promoted by undersaturated groundwaters in the recharge regions of the large phreatic lenses of meteoric water that migrated northward and southward in response to late Mississippian sea-level changes, the seaward portions of which precipitated the pre-Pennsylvanian cements.

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