Abstract

Recent published attempts at developing a sequence stratigraphic model for the Marcellus Formation of the Appalachian basin show a bias towards the Black Sea depositional model for black shale sedimentation. Such an approach ultimately leads to the conclusion that the organic-rich facies of the Marcellus shale were deposited in dysoxic to anoxic conditions found in deep water environments. It is evident, however, from textural analysis that while some of the Marcellus shale lithofacies were probably deposited by suspension settling out of low-energy buoyant plumes, others were very likely deposited by sediment dispersal mechanisms that involved a combination of storm-winnowing and re-suspension, gravity-driven processes and storm-driven unidirectional currents in a more bathymetrically subdued setting, where the key controls on black shale sedimentation will be tectonics, nutrient sourcing of algal blooms, substrate and clastic influx rates, rather than water depth as implied by the Black Sea model.

This paper is thus an attempt at a review of the sequence stratigraphic framework of the Marcellus shale based on an approach that defines cyclicity without bias towards any particular depositional model. 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 were used for this study. Information on total organic carbon (TOC) content, mineral composition, sedimentary structures and shale facies fabric as well as log data were integrated to define cyclicity. The approach required the introduction of new terms to describe important zones and surfaces. These terms include Preservation Shut-down Surface (PSS), Preservation Initiation Surface (PIS) and Maximum Preservation Surface (MPS). Accordingly, the various shale packages enclosed by these surfaces are described as Preservation Shutdown Tract (PST), Preservation Initiation Tract (PST) and Preservation Decline Tract (PDT).

The results suggest that up to four cycles can be recognized from the base of the Selinsgrove/Seneca Member of the Onondaga Limestone, which underlies the Marcellus to the top of the Stafford Limestone, which overlies the Marcellus Formation. This work shows that it is possible to define cyclicity in organic matter-rich mudrocks-dominated successions without bias towards the Black Shale depositional model.

URTeC 1563723

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