Summary

Analysis of the prominent regionally diachronous Black River-Trenton hardground indicates that it is an amalgamated unconformity formed as a result of multiple diagenetic overprinting in submarine, and perhaps subaerial, environments. The Black River-Trenton hardground occurs atop a heavily burrowed and bored mudstone (uppermost Platteville Group). However, the character of the unconformity itself is highly variable across the Upper Mississippi Valley region.

The most extreme hardground morphology observed is where the underlying mudstone has been cemented and sculpted into a “swiss cheese-like” latticework. Low plate-like pillared galleries 10's of cm's in lateral dimension locally extend 15 cm down into the underlying rock. Geochemical reaction rims and at least two generations of calcite cements variably coat these sculpted surfaces. The voids remained open on the Ordovician seafloor, and were filled by two later generations of marine carbonate and siliciclastic sediments. Locally the surface is overlain by thin organic matter-rich (~ 40 wt.% TOC) mudstones interbedded with fossiliferous shallow water limestones containing dark phosphatic grains. The other end member in hardground morphology is an almost planar surface with less than 1 cm of relief. The uppermost surface of the underlying mudstone is covered by a thin layer of lighter colored wackestones, the uppermost bored surface of which is variably coated with oxides, calcite cements, and phosphate grains. These are in turn overlain by a thin layer of marine lime mudstone and 1.5 cm-thick bladed to prismatic calite cements. The ongoing petrographic and geochemical analyses in this study suggest that the Black River-Trenton unconformity formed primarily due to the complex interplay of changes in sea level and/or sediment accumulation rates, in addition to corrosive seawater chemistries.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah