AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Sedimentological and Sequence Stratigraphic Analysis of the Marcellus Formation and Associated Strata
(1) Department of Geosciences, Pennsylvania State University, State College, PA.
This study uses approximately 1000 wireline well logs from across Pennsylvania and 8 cores in Central PA to construct a depositional model of the Middle Devonian Marcellus Formation and associated strata. Although previous studies invoked tectonic forces to explain shifts in black shale deposition, we conclude that facies shifts in the proximal to medial portions of the basin are due to eustatic sea level fluctuations. Chronostratigraphic surfaces defined using sequence stratigraphic principles demonstrate that much of the Marcellus Formation is genetically related to the overlying, less organic-rich, more proximal facies of the Mahantango Fm. Genetically related facies between chronostratigraphic surfaces indicate a primarily clastic eastern depocenter with carbonates dominating the flexural forebulge in western Pennsylvania. Relatively continuous carbonate deposition on the forebulge indicates a slowly subsiding, but non-migrating forbulge throughout Onondaga and Hamilton Group deposition. Onondaga and Hamilton Group sediments are classified as lowstand (LST), transgressive (TST), or highstand systems tracts(HST) by observing parasequence and facies stacking patterns. The upper Onondaga facies are interpreted as a TST. Continued deepening led to interfingering of Onondago limestones with Marcellus black shales as indicated by through-going volcanic ash beds. A maximum flooding surface, picked at the highest gamma ray value observed in the lower Marcellus Fm. marks the start of the HST. In the HST, black shales generally coarsen upwards into sandy siltstones, and parasequences become less organic. Overlying the highstand systems track is a sharp basinward progradation in facies, marking a generally conformable third-order sequence boundary and start of the next LST of the Purcell member of the Marcellus Fm. These basin-wide systems tract classifications combined with parasequence and facies correlations indicate that organic- rich, Hamilton Group black shales were deposited during three major transgressions associated with third-order depositional sequences.