Recognition of Transgressive Carbonate Sequence Within Epeiric Sea: Helderberg Group (Lower Devonian) of New York State: ABSTRACT

Abstract

The regional Late Silurian-Early Devonian marine transgression of the central Appalachians is represented in New York State by a shallow-water carbonate rock sequence (Helderberg Group) which locally transgressed north and west. The resultant stratigraphic section comprises several hundred feet of fossiliferous limestone which has several distinctive sedimentary facies. Early workers interpreted each of the major facies as a separate time-stratigraphic lithologic unit or formation. However, from detailed field examination Rickard (1962) demonstrated that these formations are in fact time-transgressive toward the west and interfinger laterally with each other. Paleoecological study of the Helderberg Group supports this interpretation and shows that each of the formations represents a local sub-environment within the transgressive interval as a whole. These formations (facies) are: (1) Manlius Formation (25-50 feet), a complex of rock types interpreted to represent supratidal, intertidal, and shallow subtidal environments within a broad shelf lagoon (Laporte, 1964; 1967). (2) Coeymans Formation (20-100 feet), crinoidal-brachiopod skeletal calcarenite and carbonate siltstone which are commonly burrow-mottled toward the base of the unit but which show increasingly greater evidence of current reworking toward the top (high-and low-angle cross-stratification and sheet deposits). The Coeymans is interpreted to have been deposited in a wide belt of shallow, submerged crinoid mounds and banks which served as an effective, though discontinuous, barrier to circulation separating the more open-marine environment on the east from the restricted shelf lagoon of the Manlius on the west (Anderson, 1965). (3) Kalkberg Formation (50-100 feet), highly burrow-mottled carbonate mudstone with a very abundant, diverse, and well-preserved biota. The Kalkberg is interpreted to be a shallow-water, open-marine deposit which developed on an extensive shelf seaward from the Coeymans crinoid banks and meadows. (4) New Scotland Formation (50-150 feet), highly argillaceous and siliceous carbonate mudstone with a somewhat less diverse and abundant biota than the Kalkberg. The New Scotland is interpreted as having developed on a broad shelf like the Kalkberg (and marginal to it), but with significantly greater influx of terrigenous detritus which probably came from a distant easterly source. Lateral and vertical variations in constituent carbonate-grain types, mudstone-sparite ratios, fossil abundance and diversity, and presence of primary sedimentary structures provide criteria for recognizing the transgressive nature of the major sedimentary facies of the Helderberg Group. The inferred depositional framework, moreover, is very similar to that postulated by Shaw (1964) and Irwin (1965) for clear water sedimentation within an epeiric sea and demonstrates the predictive validity of their generalized sedimentary model. End_of_Article - Last_Page 473------------