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Sedimentological and Stratigraphic Evidences for the Deposition of the Marcellus Shale in Much Shallower Bathymetric Settings than Previously Assumed

Emmanuel O. Olusanmi
Colorado School of Mines, Department of Geology and Geological Engineering, Golden, Colorado, USA
[email protected]

Recent sedimentological and stratigraphic work involving outcrops and core data suggests that a deep-basin depositional model may not necessarily be appropriate for the Marcellus Shale. The occurrence of sedimentary structures such as size-graded laminae, starved ripples, pinch and swell structures, sharp-based beds, thin silt lags, bioturbation, aligned fossils, basal fossil lag deposits and the presence and enrichment of silt grains even in organic-rich facies, while not indicative of water depth, are indicative of significant current activity. The presence of sharp-based beds, starved ripples and pinch and swell structures suggest episodic reworking and eroding of the sea floor during deposition. The thin silt lags were interpreted to be storm events in shallow marine settings. Size-graded laminae were also probably deposited by suspension currents or from decelerating storm currents, in relatively short periods of time. Alternating couplets of organic-rich and quartz-rich layers that make up much of the laminae are suggestive of seasonal variations in organic matter production, preservation and dilution by equally fluctuating clastic influx. The presence of abundant diminutive organisms and colonies of in-situ infauna such as brachiopods indicate that the bottom waters were never permanently anoxic. The relationships between different stratigraphic units and the observed local thickness variations within these stratigraphic units might also be better explained by relatively localized anoxic environments influenced by factors such as sediment supply, paleo-topography, climate and bottom water chemistry. These evidences suggest that the Marcellus Shale could have been deposited in a bathymetrically subdued setting similar to modern-day continental shelves.

AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects