HOHMAN, JOHN C., Department of Geological Sciences, Indiana University, Bloomington, IN; and KEITH, BRIAN D., Indiana Geological Survey and Department of Geological Sciences, Indiana University, Bloomington, IN
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.
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