Trace Fossils and Bioturbation in Peritidal Facies of the Potsdam-Theresa Formations (Cambrian-Ordovician), Northwest Adirondacks

The Cambrian-Ordovician Potsdam Sandstone, Theresa Formation, and Canadian correlatives in the St. Lawrence Lowlandsp reservet ide-dominatefda cies during the basal Cambrian transgression. Low intertidal sand flats in the upper Potsdam contain a Skolithos Ichnofacies dominated by Diplocraterion parallelum in clean, herringbone cross-beddeds andstones indicative of high tidal current energy. Wind-wave-drivenlo ngshore and tidal currents along a macrotidal coastline were funneled northeast-southwebsty P recambriant opographirce liefo f up to 65 m. This relief is now expresseda s the ThousandI slands of New York and Canada. The conformably overlying Theresa Formation preserves a shoaling-upward sequence of mixed clastic-carbonate facies. Shallow subtidal and peritidal facies contain a mixed Skolithos- Cruziana Ichnofacies in sharply alternatingl ithofacies consisting of gray, intensely bioturbated, poorly sorted calcareous sandstone, and meter-thick, white cross-bedded sandstone. The parallelism between ichnofacies and lithofacies indicates that environmental energy level and persistence rather than water depth controlled trace fossil distribution. Bioturbated sandstones contain a Cruziana Ichnofacies of abundant deposit feeders including: Fustiglyphus?, Gyrochorte?, Neonereites uniserialis?, Phycodes flabellum, Planolltes beverlyensis, Rosselia socialis, and Teichichnus. Suspension feeders are represented by D. habichi, D. parallelum, Skolithos, Monocraterion, and possibly Palaeophycus tubularis. Scavenging or deposit-feedinag ar hropods are represented by rare Cruziana? furrows. Cross-beddeds andstones contain a Skolithos Ichnofacies of shallow Skolithos and Monocraterion burrows, and an undescribed large epistratal eurypterid(?) trail. Protrusive D. parallelum in high-energy intertidal Potsdam facies show an average burrowing depth of-12.2 cm that agrees with interpretations of tiering structure for suspension feeders in the Early Paleozoic. Frequency analysis of bioturbation units, thickness intervals defined by ichnofabric index, shows a -3 to -5 cm depth of burrowing for deposit feeders in the Cruziana Ichnofacies. It is the thickness of episodically deposited sedimentation units, in relation to endobenthic burrowing depth, rather than sedimentation rate, that determines trace-fossil preservation potential and the resulting ichnofabric. This is especially important for many shallowly burrowed Early Paleozoic facies, but becomes less important after the advent of deeper burrowers later in the Paleozoic.