Diagenetic Model for Carbonate Rocks in Mid-Continent Pennsylvanian Eustatic Cyclothems: ABSTRACT

Diagenetic patterns in cyclic Mid-Continent Pennsylvanian carbonates are readily explained in terms of a predictive diagenetic model derived logically from the eustatic depositional model for widespread Pennsylvanian cyclothems. Transgressive shoal-water calcarenites are characterized by overpacking of grains, discernible neomorphism (with excellent preservation of structure) of originally aragonitic grains (ooids, green algae, mollusks), and ferroan calcite and dolomite cement, which indicate movement from the marine phreatic environment of deposition and diagenesis into the low-oxygen deeper burial connate zone, with substantial compaction before any cementation. Offshore invertebrate calcarenites associated with offshore (“core”) shales also are characterized by overpacking of grains and ferroan carbonate cements, which indicate a similar diagenetic history. Regressive shoal-water calcarenites show a much greater variety of diagenetic features, including early marine cement rims and large-scale leaching of originally aragonitic grains, commonly with subsequent collapse of micrite envelopes, grain fragments, and overlying material in samples insufficiently stabilized by early cement rims. This was followed by pervasive cementation by blocky calcite before much further compaction, then by ferroan calcite, and finally ferroan dolomite in remaining voids. This pattern indicates replacement of depositional marine phreatic water by meteoric water, which dissolved unstable carbonate grains and then deposited stable carbonate cements in enviroimients that eventually became increasingly oxygen-depleted and otherwise chemically changed, probably as mixing-zone and deeper connate water moved back into the rock and replaced the meteoric water during and after the succeeding transgression. Trends in calcilutites are essentially similar to those in calcarenites of equivalent phase of deposition, with evidence of subaerial exposure and meteoric vadose soil formation in strata at the top of many regressive limestones. It is apparent that with the regression of the sea and emergence that terminated deposition of a cyclothem, meteoric water penetrated the permeable parts of the regressive carbonate and left its distinctive diagenetic patterns of early leaching and cementation before much compaction occurred, but rarely did meteoric water penetrate the impermeable offshore shale, which acted as a seal and allowed associated deep-water and underlying transgressive carbonates to become more deeply buried and substantially compacted before cementation, with unstable grains undergoing slow neomorphism in the absence of leaching.