Abstract: Buried Alive! The Demise of the Upper Ordovician Carbonate Platform in the Illinois Basin and Cincinnati Arch Areas

HOHMAN, JOHN C., and BRIAN D. KEITH

The Upper Ordovician Trenton Limestone, Lexington Limestone, Maquoketa Group and equivalent lithologic units in the Illinois Basin and Cincinnati Arch areas present a distinct stratigraphic section notable for its complex assemblage of carbonates and siliciclastics. Utilizing cores, outcrops, and wireline logs, a sequence stratigraphic framework of unconformities and marine-flooding surfaces was constructed to establish the stratigraphic relationships between the carbonate-dominated lower part of the section and the siliciclastic-dominated upper part of the section.

These stratigraphic relationships record the collapse and flooding of a stable, extensive carbonate-dominated platform that was eventually buried by siliciclastic-dominated deposits of a foredeep basin. The initial phase of the collapse commenced with the tilting of the platform down to the northwest. The resulting transgression and flooding drove the evolution of the shallow-water low-energy Black River platform into the higher energy transgressive Trenton platform on the flanks of the Trenton high located in the area of the Cincinnati Arch. The next phase of collapse migrated from the eastern edge of the platform leaving the highstand lower Lexington platform over the flooded Trenton high as the only remnant of the original extensive platform. Shaly carbonates of the upper Lexington platform struggled to persist over this increasingly areally restricted platform that was being inundated by siliciclastic-dominated deposition from the Maquoketa foredeep basin to the east. The final stage of collapse of the platform was characterized by subsequent episodes of siliciclastic input that progressively inundated and further buried the underlying carbonates from east to west until the final remnants of the platform were buried.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky