Recognition of a Transgressive Carbonate Sequence Within an Epeiric Sea: Helderberg Group (Lower Devonian) of New York State
Léo F. Laporte, 1969. "Recognition of a Transgressive Carbonate Sequence Within an Epeiric Sea: Helderberg Group (Lower Devonian) of New York State", Depositional Environments in Carbonate Rocks, Gerald M. Friedman
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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 northward and westward. The resultant stratigraphie section comprises several hundred feet of fossiliferous limestones which exhibit several distinctive sedimentary facies.
Early workers intepreted each of the major facies as a separate time-stratigraphic lithologie unit or formation. However, from detailed field examination Rickard (1962) demonstrated that some of these formations are in fact time-transgressive and interfinger laterally with each other. Detailed paleoecologi-cal study of the Helderberg Group supports this interpretation and shows that each of the formations represents a major environment within the transgressive interval as a whole. These formations (facies) are: (1) Manlius Formation (25 to 50 feet), a complex of rock types which are interpreted as representing supratidal, intertidal, and shallow subtidal environments within a broad shelf lagoon (Laporte, 1964; 1967). (2) Coeymans Formation (20 to 100 feet), crinoidal-brachiopod skeletal sandstones and siltstones which are frequently burrow-mottled toward the base of the unit but which show increasingly greater evidence of current re-working toward the top (high- and low-angle cross-stratification and sheet deposits; Anderson, 1967). The Coeymans environment is interpreted as a wide belt of shallowly submerged crinoid mounds and banks which served as an effective, though discontinuous, barrier to circulation separating the more open marine environments to the east with the restricted shelf lagoon of the Manlius to the west. (3) Kalkberg Formation (50 to 100 feet), highly burrow-mottled lime mud-stone with a very abundant, diverse, and well preserved biota. The Kalkberg is interpreted as a shallow-water, open marine environment which developed on an extensive shelf seaward of the Coeymans crinoid banks and meadows. (4) New Scotland Formation (50 to 150 feet), highly argillaceous and siliceous lime 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 came from a distant easterly or northeasterly source.
Variations in constituent carbonate grain types, mud-sparite ratios, fossil abundance and diversity, and presence of primary sedimentary structures provide criteria for recognizing the lateral relations, of the major sedimentary facies of the Helderberg Group. The stratigraphie superposition of the facies records their temporal transgression 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.
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Depositional Environments in Carbonate Rocks
One of the principal tasks of the geologist is to determine the depositional environments in which rocks are deposited. Although regional environmental interpretations of transgressions and regressions, movements of shoreline, and gross aspects of continental and marine sedimentation have been understood since stratigraphy became an established branch of geology, only recently has the science of sedimentology come up with criteria for environmental recognition of specific outcrops, wells, or even hand samples. This observation is especially true of carbonate rocks. The papers in this volume will provide a key to the subject of recognition of depositional environments in carbonate rocks. Based on a symposium held in Los Angeles, California, on April 1967, at the joint meeting of AAPG and SEPM.