Abstract

Shallow marine strata of the Upper Devonian West Falls Group in south-central New York comprise centimetre- to decimetre-thick, sheetlike to lenticular beds of sandstone and siltstone interbedded with mudstone. Unidirectional current-scour marks and tool marks occur on the basal erosion surfaces of the sandstones: their internal structures are hummocky cross strata of various styles, planar laminae, and wave-ripple cross-laminae, with characteristic vertical and lateral transitions among them. Vertical burrows of the Skolithos ichnofacies dominate upper portions of sandstone beds, whereas mudstones and silt-stones contain mainly horizontal burrows of the Cruziana ichnofacies. Transported and in situ shelly fossils occur throughout, in places concentrated in “coquinite” lenses. Large-scale (15 to 30 in) bedding sequences are characterized by upward thickening and coarsening of sandstone beds, increase in amalgamated hummocky cross-strata, coquinites, plant debris and bivalves, followed by abrupt fining and thinning of beds.

The depositional environment is interpreted as a shallow-marine shelf episodically influenced by intense tropical storms (hurricanes). Individual sandstone beds were deposited by spatially decelerating combined flows where an initially strong offshore-directed flow component decreased in time relative to the oscillatory flow component. Large-scale bedding sequences reflect changes in storm (combined) current intensity and deposition rate. Strong eustatic control is unlikely, and there is no evidence of changing wave climate. Instead, these sequences may be associated with delta-lobe progradation and/or offshore bar development.

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