Diagenetic Model For Carbonate Rocks In Midcontinent Pennsylvanian Eustatic Cyclothems

Abstract

ABSTRACT Diagenetic patterns in cyclic Midcontinent Pennsylvanian carbonates are readily explained in terms of a predictive diagenetic model derived logically from the eustatic depositional model for widespread Pennsylvanian cyclothems developed by Heckel (1977, 1980). Transgressive shoal-water calcarenites are characterized by pervasive overpacking of grains, neomorphism of originally aragonite grains (ooids, green algae, molluscs), and ferroan calcite and dolomite cement, which indicate movement from the marinephreatic 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 pervasive overpacking of grains an ferroan carbonate cements, which indicate a similar diagenetic history; their lack of discernible neomorphism relates to original absence of shallow-water aragonite grains (ooids, green algae), and probable removal of thin-shelled molluscs by dissolution during long periods of sediment starvation in cool water, which also corroded the edges of many calcite grains. Regressive shoal-water calcarenites show a much greater variety of diagenetic features, including early marine cement rims and large-scale leaching of originally aragonite grains, often 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 environments 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 pans of the regressive carbonate and left its distinctive diagenetic patterns of early leaching and cementation before much compaction took place, but rarely did meteoric water penetrate the impermeable offshore shale. which acted as a seal. This allowed associated offshore and underlying transgressive carbonates to become more deeply buried and substantially compacted before cementation took place, with unstable grains undergoing slow neomorphism in the absence of meteoric leaching.