SLICKENSIDE SURFACES WITHIN K-BENTONITE BEDS: A POTENTIALLY SENSITIVE TOOL FOR ASSESSING THE TIMING OF FAR-FIELD TECTONIC COMPRESSION, VOLCANIC ASH ILLITIZATION, AND DIAGENETIC ASH CEMENTATION

Preliminary outcrop examination of numerous K-bentonite beds (altered volcanic ashes) at several different stratigraphic horizons within the Late Ordovician basinal Utica Shale and coeval basin margin deposits in the Mohawk Valley region of eastern New York, shows that approximately one third of these beds display bounding and/or internal planes of horizontal to oblique structural displacement (slickensides) characterized by thrust-related grooves. Slickensides appear to be solely associated with illitized (clay) ash beds and have not, so far, been found to cross-cut carbonate cemented ash layers, although later pyrite replacement effects are observed to cross-cut slickenside features. Many slip planes floor K-bentonites, but others show a ramped (en echelon) pattern within ashes with structural and diagenetic patterns changing regionally, even at outcrop scale. Some slickensides yield only bidirectional data, but many present good unidirectional information. Moreover, discovery of one slickenside yielding two groove azimuths indicates that relict displacements may reveal a complex history of changing structural dynamics.

The presence of as many as sixty major and minor ash beds in the study area coupled with the availability of numerous, long outcrop sections provides a potentially large database for assessing the nature and timing of local and regional compression across the region. In particular, this information may allow for the discrimination of far-field effects associated respectively with the Taconian, Acadian, and Allegheny orogenies. Preliminary directional data suggest the influence of Alleghenian events, indicating that this study could possibly allow for a constraining mechanism for the timing of ash illitization processes relative to far-field tectonic compression as suggested by southern Appalachian Basin studies. Temporal constraint of diagenetic ash cementation may also be possible from this study. Pyritization of slickenside surfaces also have hydrogeological implications; implying that these ash beds once acted as local conduits for groundwater flow.