HARDGROUND FORMATION IN THE RUST FORMATION OF THE UPPER TRENTON GROUP IN THE TACONIC FORELAND BASIN, MOHAWK VALLEY, NEW YORK STATE

Deposition of the Trenton Group (middle-upper Ordovician) in the Taconic Foreland Basin recorded a transition from a carbonate platform to carbonate slope in the Central Mohawk Valley of New York. While numerous studies agree this occurred during a marine transgression and the rise of an orogenic belt, questions regarding the details of this transition remain. Of particular interest to this study is the Rust Formation, an interval that exhibits significant lateral thinning and expansion compared to the surrounding units, and in part represents a transgressive lag. In the study area, this formation is primarily composed of brecciated, fossiliferous wackestones interbedded with mudstones. The question to be addressed in this study is whether some of the features seen in the Rust Formation represent subaerial exposure and karsting or were formed as a subaqueous carbonate hardground. To answer this question, detailed sedimentologic descriptions, stratigraphic correlations, and XRF-based elemental abundance data was collected from five cores.

The presence of encrusting marine organisms, borings filled by calcite cement, and extensive surfaces mineralized by phosphatic allochems are consistent with hardground formation, a feature associated with rapid sea level rise. The lack of typical karst features, such as cavernous porosity or terra rossa helps to rule out subaerial exposure as a depositional process. XRF analysis confirmed the presence of phosphatic grains which are common in marine lag deposits. Additionally, and unlike the underlying units, cores located towards to the basin center, in the east side of the study area, contain higher concentrations of siliciclastic detrital proxies (Al, Si, K, Zr, Rb), less Ca, and slightly higher concentrations of some redox proxies (Cu, Mo, Fe, S, and V) relative to cores in the west. The lateral distribution of redox proxies is consistent with an eastward dipping margin, and the shift towards siliciclastic-rich deposits suggests either a reduction in carbonate material shed from the platform to the west or the delivery of siliciclastic detritus from the orogenic wedge to the east. The Rust Formation is a key interval in the evolution of the Taconic Foreland Basin as represents deposition during relative sea level rise on a submarine ramp setting.