Most published models for the Marcellus shale of the Appalachian basin show a great bias towards the Black sea depositional model for black shale sedimentation. Such an approach implies that organic-rich Marcellus shale facies were deposited in dysoxic to anoxic conditions in deep water environments. Recent sedimentological and stratigraphic work involving eight core data, over 100 thin sections, SEM and XRD data, field studies of Marcellus shale exposures in New York and Pennsylvania as well as over 800 electrical wireline logs, however, suggest that such a paleobathymetry-driven depositional model may not necessarily be appropriate for the Marcellus shale. The occurrence of sedimentary structures such as starved ripples, graded beds, bioturbation, burrows, reworking of authigenic minerals like pyrites, aligned fossils, basal fossil lag deposits and the presence and enrichment of silt grains even in organic-rich facies, while not indicative of the water depth, were all interpreted to be indicative of current activity. Total organic carbon (TOC) content was found to increase from the eastern margins of the basin towards the western craton-ward side of the basin. This can be attributed to the increased clastic influx to the east as a result of increased sedimentation rates from the Catskill delta. Marcellus black shale facies were thus probably deposited in a bathymetrically subdued setting akin to present-day continental shelves and not in the deepest part of the basin. In Marcellus times, such a setting occurred towards the western side of the Appalachian basin, away from the Acadian Mountains.
The key controls on Marcellus Shale deposition in such settings would be a combination of local geologically rapid subsidence/uplift events, seasonal variations in nutrient sourcing of algal blooms, changes in salinity and clastic influx rates, rather than water depth. Thus, organic-rich laminated Marcellus Shale lithofacies would have been deposited during periods of increased algal bloom and reduced clastic influx, increased organic preservation as a result of changes in bottom water chemistry to favor the deposition of organics and siliciclastics over carbonates. The less organic-rich Marcellus Shale lithofacies in turn were deposited during periods of episodic tectonic quiescence, increased dilution of organic matter as a result of increased clastic influx from the Acadian Mountains. In the same way, the interbedded limestone facies were deposited during times of reduced algal bloom, low sedimentation rates and changes in bottom water chemistry to favor carbonate deposition over deposition of organics and siliciclastics.
URTeC 1563700
