Bathymetry and paleoecology of silurian bethic assemblages, late llandoverian, New York State

Abstract

Late Llandoverian marine shales in New York State are highly fossiliferous and provide and opportunity to examine gradients of biofacies. The Williamson and Willowvale formations (Llandoverian C 6) were deposited during a major transgressive episodes in and adjacent to the axis of a narrow NE-SW trending, central Appalachian foreland basin. The Williamson Shale is a black to dark green, graptolite-bearing unit of the upper Clinton Group of western New York State. It grades eastward into gray fossiliferous shales and coquinoidlimestones of the Willowvale Formation. Orientation analysis of graptolites ( Monograptus clintonensis) indicates paleoccurents dominantly from the northeast to east. The presence of rippled, cross-laminated sandstone beds (tempestites) within dark gray, graptolitic facies of the lower Williamson indicates that the darl shale facies was deposited in dysaerobic to marginally aerobic water slightly below storm wave base. Poorly fossiliferous green shales, higher in the Willimason, appear to slow deposition in dysaerobic water and suggest a gradual deepening. Graded brachipod coquinoid limestones, interpreted as storm layers, occur near the middle and top of the Williamson and in the Willovale Shale, and indicate deposition of these sediments above maximum storm wave base. The faunas of the Williamson and Willowvale shales are subdivisible into five vertically and laterally adjacent associations as follows: (a) Ichnofossil ( Chondrites). (b) Monograptid- “ Chonetes” Cornutus, (c) Eopletodonta-Atrypta, (d) Palaeocyclus-bryozoan, and (e) Eocoelia-“ Chonetes” cornutus associations. This spectrum of fossil associations appears to parallel the increasing frequency of bottom disturbance by waves and currents from deeper basinal areas to higher-energy outer and low energy inner shallow, shelf settings. The fossil associations of the Williamson (a-c) probably belong to the deeper benthic assemblages 4–5, but developed in environments within to slightly below storm wave base, (about 50–80 m deep). The Willowvale fauna (c,d) representing benthic assemblages 3–4, inhabited somewhat shallower, better oxygenated environments of the photic zone, probably at depths of approximately 20–50 m. The easternmost to slightly southeastward, equivalent of Williamson-Willowvale fauna (e) evidently represents shallow water benthic assemblage 2. These conclusions indicate the need for a critical reevaluation of the bathymetric relations of benthic assemblages 3–5; a new interpretations of approximate depths for this range is presented herein.