Outcrop Expression of Late Devonian Transgressive-Regressive Cycles along Catskill Delta Complex Basin Margin of Central West Virginia
Robin John McDowell, Ray M. Boswell, Alan C. Donaldson
The Upper Devonian Catskill delta complex prograded westward across the central Appalachian basin in response to eustatic fluctuations in sea level and high rates of sediment supply associated with the Acadian orogeny. Excellent outcrops of Catskill delta complex basin-margin facies, located near Elkins, West Virginia, record a hierarchy of transgressive-regressive (T-R) cycles that formed as the Catskill shoreline regressed across east-central West Virginia during the middle Famennian.
Subsurface mapping of intervals studied in outcrop indicates three distinct scales of T-R cycles based on the magnitude and lateral continuity of shoreline shift and the thickness of enclosed strata. Major cycles represent regional shifts of 30-50 mi (48-80 km), and are roughly 350 ft (100 m) thick. Intermediate cycles indicate more localized shoreline shifts of 15-25 mi (25-40 km), and are approximately 100-150 ft (30-50 m) thick. Minor cycles are expressed as local and minor shifts of less than 5 mi (8 km), and are from 10 to 30 ft (3-10 m) thick.
Major cycle boundaries are expressed in outcrop as thick intervals of shale separating sequences of shoreline, delta-front, or delta-plain facies. Cycles of this order correlate well with eustatic sea level fluctuations. Intermediate cycles are exposed in outcrop only in the lower portion of major cycles, and are expressed in the subsurface by regional shale marker beds. Minor cycles are distinguished in outcrop as small fining-upward sequences of river-mouth bar and tidal-channel sandstones overlain by tidal-flat facies or delta-front shale, and are associated with episodes of distributary switching.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.