LATE FAMENNIAN ROCK UNITS IN DYNAMICALLY UNSTABLE CONTEXT: CLEVELAND SHALE-BEDFORD FORMATION INTERVAL, NORTHERN OHIO

The northern Ohio upper Famennian geological succession is best exemplified by the classic Cleveland Member-Bedford Formation succession in the Cuyahoga Valley region. The transgressive Cleveland black shale is marked by a diachronous basal drowning unconformity, suggestive of regional southeastward unit onlap, as well as southward erosive truncation below the younger Bedford Formation. The lower Bedford records a rapid westward progradational pulse that followed an erosive “icehouse” lowstand event timed with onset of the Hangenberg biocrisis interval. A key marker unit, Euclid Member dramatically descends northwestward within the Bedford clinoform, where it nearly comes in contact with the Cleveland Member. However, west of Cleveland, the top-Cleveland contact becomes conformable, and lower Bedford units greatly thin westward through non-erosional condensation.

Superimposed on this stratigraphy were seismic episodes, which locally convulsed the upper Cleveland Member and lower Bedford. In sections near Parma and Berea, Ohio, the Euclid Member has locally foundered into the upper Cleveland as diapiric masses in association with cross-cutting clastic dikes and micro-faulting. Significantly, the deformed and complexly intruded top-Cleveland interval is unconformably overlain by an interval of undeformed basal Bedford strata at several localities, constraining the timing of the upper Cleveland-into-Euclid disturbances. Curiously, Big Creek in Parma, bounded to the east and west by coeval disturbed sections, entirely escaped this deformation, preserving intact the westernmost Euclid section and its stratigraphic context. The Cleveland Member particularly displays cross-cutting clastic dike swarms, with some dikes containing Bedford-sourced shelly fossils, and others displaying vertical, rhythmic ribbing suggestive of sudden sediment emplacement.