Abstract

Thick mudstone successions contain a wealth of elemental proxy information than can be readily accessed by hand-held X-ray fluorescence (XRF) spectroscopy. Mudstones of the Upper Ordovician Trenton Group and Utica Shale record drowning of a carbonate-shelf in a peripheral foreland basin during the Taconic Orogeny. Two discrete episodes of shelf erosion, the Thruway and Honey Hill disconformities, followed by deepening occurred during this period. XRF-based bulk-rock elemental data, coupled with detailed lithologic descriptions and XRD analysis, highlight previously unidentified changes in the composition of siliciclastic detritus across these disconformities. This likely relates to variation in the relative contribution of sediment derived from the cratonic interior, eroded carbonate-shelf, and rising orogenic wedge, as well as possible exhumation of the Adirondack Massif. The data also indicate that mudstones of the lower Indian Castle Shale, the interval of the Utica Shale deposited between the Thruway and Honey Hill disconformities, are more carbonate-rich than the underlying mudstones of the Trenton Group. This is surprising given the large volume of limestone in the Trenton Group, and attests to the considerable amount of intrabasinal carbonate winnowed from the shelf during formation of the Thruway Disconformity. Drowning, following erosion along the Honey Hill Disconformity, diminished this sediment source and led to a significant reduction in the amount of carbonate mudstone present in the upper Indian Castle Shale. The concentrations of redox-sensitive trace elements normalized to Al increase sharply across the Thruway Disconformity, suggesting increased bottom-water anoxia and organic-matter preservation in a deepening basin. Increases in the S and Fe concentrations above the Honey Hill Disconformity suggest a shift to even more strongly reducing conditions. Results of this study are consistent with previous interpretations, indicating that higher-resolution XRF-based elemental concentration data may reveal more cryptic changes in sediment provenance and bottom-water redox conditions in the Taconic foreland basin, and in other thick mudstone successions.