The Tully Clastic Correlatives (Upper Devonian) of New York State: A Model For Recognition of Alluvial, Dune (?), Tidal, Nearshore (Bar and Lagoon), and Offshore Sedimentary Environments in a Tectonic Delta Complex

Abstract

ABSTRACT A spectrum of sedimentary environments, including alluvial, tidal, dune (?), nearshore (bar and lagoon) and offshore, existed side by side in central New York State during early Late Devonian time. These environments, part of the great Catskill deltaic complex, are represented in the stratigraphic record by the Tully Limestone and its eastern clastic correlatives. This very limited interval of the rock record was studied in order to develop associations of criteria for recognition of sedimentary environments in ancient tectonic delta complexes. Recognition criteria include sedimentary structures. lithology, geometric relationships of rock units, and character of biologic content. The Tully Limestone and associated strata are subdivided into four lithosomes. Lithosome A is the thinned eastward extension into the study area of the Tully Limestone. The unit consists of argillaceous calcilutite and sandy biocalcarenite. Lithosome B, which lies directly above Lithosome A and is co-extensive with the tongue-shaped eastern extension of the Genesee Shale, consists of very dark fissile shale grading eastward into shaly siltstone. Lithosome C forms a massive clastic wedge which in cross-sectional view envelops Lithosomes A and B. It is comprised of interlensing gray siltstone and slightly lighter gray, very fine-grained sandstone which contains flow-rolls, trace fossils, coquinite lenses and, at the eastern margin of the lithosome, fossil seed-ferns. Lithosome D consist of red and green siltstone and mudstone with interbeds of gray, fine to medium grained, texturally very immature sandstone. The spectrum of Tully interval sedimentary environments includes sandstone bodies of alluvial channel origin which truncate underlying beds, contain basal shale-pebble lag-concentrates, are well trough cross-bedded, texturally immature, and display a fining-upwards. The alluvial strata of overbank origin are horizontally laminated, red and green siltstones. Strata of the marsh facies consist of black organic lenses containing abundant plant remains some of which are coalified. At the distal margin of the alluvial plain, just below the Tully interval, a swamp environment is represented by three levels of abundant fossil seed-ferns. Sedimentation that resulted in strata of tidal origin within the Tully interval was of the Wadden-type. The tidal flat facies consists of gray, very finely cross-laminated muddy siltstone and very fine-grained sandstone, which contain allochthonous brachiopods and locally well developed mud-cracks. Sedimentary structures of the tidal channel facies are essentially identical to those of the alluvial channel facies, but can be distinguished by the unique character of the basal lag-concentrate, which is coquinite or coquinoid pebble conglomerate consisting largely of allochthonous brachiopod shells. Strata of the nearshore facies consist of thick bedded bar sandstone bodies interbedded with very thinly bedded lagoonal siltstone which is rich in wood fragments. The offshore facies consists of very thinly bedded and laminated, fossiliferous siltstone that becomes increasingly calcareous westward and grades into the very argillaceous eastern extension of the Tully Limestone. Well developed trends of change in texture, general biologic character, and type and scale of sedimentary and biologic structures are present in both the nearshore (bar and lagoon) facies and the offshore facies. Synthesis of the environmental pattern of the Tully interval indicates that (1) Tully sedimentation occurred during the transgressive phase of a transgressive-regressive cycle, (2) terrigenous material was trapped east of a submarine topographic high, thus permitting deposition of carbonate material in a basin that was for the most part being overwhelmed by clastic influx, and (3) landward migration of the strandline during the transgressive phase caused river mouth drowning and resulted in more widespread estuarine (tidal) conditions than were present immediately prior to and following Tully time. In addition, the transgression raised the base level of streams that were flowing westward across the deltaic plain, thus causing alluviation of fine-grained sediment in quantity greater th n that deposited directly preceding and following Tully time.