Lower Devonian ichnofacies at Highland Mills, New York and their gradual replacement across environmental gradients

A sequence of Lower Devonian strata from the Green Pond-Schunemunk Outlier was examined at three localities in Highland Mills, New York. Nearly the entire Onesquethawan Series is represented by the Esopus Formation (Mountainville, Quarry Hill, Highland Mills and Eddyville Members) and the Pine Hill Formation (Woodbury Creek and Kanouse Members). Trace fossils found include Zoophycos, Skolithos, Monocraterion, Scalarituba, Chondrites, Planolites, Phycosiphon, Imbrichnus, Phycodes, Scolicia, Obliquus (n. gen.), and Endaulites (n. gen.). These include 16 species, four of which are newly named. Ichnofossils occur in discrete ichnofacies each associated with a specific environmental setting as interpreted from lithofacies, sedimentary structures, and previously known paleoenvironmental affinities of the various trace fossil types found. Between ichnofacies are ecotones or zones of overlapping ichnofacies margins. The Skolithos-Planolites Ichnofacies occurs within the major part of the Highland Mills Member and is associated with environments ranging from very shallow water below wave base to intertidal mud flats. The Zoophycos Ichnofacies occurs twice in the stratigraphic column, once in the Mountainville and lowermost Quarry Hill Members and again in the uppermost Highland Mills, Eddyville, and lowermost Woodbury Creek Members. Environments represented are somewhat deeper water than the above Ichnofacies. The Scalarituba and Chondrites Ichnofacies are found within the middle two-thirds of the Woodbury Creek Member and represent shallow water environments similar to those found within most of the Skolithos-Planolites Ichnofacies. Along with presence-absence distributions within the stratigraphic column, greater abundances of particular trace fossils and generally larger forms of animals creating the ichnofossils are postulated to indicate an increased suitability to a given environment. Within this framework, faunal replacements in time can be better understood in the context of facies shifts along environmental gradients.