Sequence Stratigraphic Framework of the Middle Devonian Marcellus Shale
Gary G. Lash
Dept. of Geosciences, SUNY Fredonia, Fredonia, NY, 14063, [email protected]
Sequence stratigraphy allows the explorationist to place a sedimentary succession into a chronostratigraphic framework of unconformity bounded packages of strata. Our ongoing stratigraphic analysis of the subsurface stratigraphy of the Marcellus shale involves the generation of a composite wireline log “type section” that serves as a comparative tool in basinwide sequence stratigraphic correlation. The Marcellus comprises two depositional sequences, each one spanning perhaps 1 MY. The transgressive surface of the lower sequence is found in the upper part of the Onondaga Limestone. The overlying transgressive systems tract, which spans the Onondaga- Marcellus contact, terminates at a marine flooding surface in the lower Union Springs shale defined by a radioactivity maximum and bulk density minimum. Overlying highstand systems tract deposits record diminished accommodation volume and accumulation of progradational parasequences. The upper sequence boundary is an erosional contact separating the Union Springs shale from the overlying Cherry Valley limestone, an inferred lowstand systems tract of the upper sequence. The upper Cherry Valley and lower Oatka Creek shale comprise a thin transgressive systems tract, the peak in gamma-ray count marking the marine flooding surface. Overlying highstand systems tract deposits recognized by diminishing radioactivity and increasing bulk density record lowering relative sea level. The sequence boundary is placed at the erosional contact of the Oatka Creek shale and overlying Stafford Limestone, an inferred lowstand systems tract. Extension of this sequence stratigraphic framework across the basin reveals that locally, sequence boundaries cut well down into underlying sequences, especially in the northwest region of the basin, reflecting a complex history of eustacy and tectonics.
AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009