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Ordovician K-bentonite Illitization and Tectonic Compression in Eastern New York: Applying K/Ar Dating to Examine a Potential Link between Development of Within-Ash Slickensides and Alleghanian Far-Field Tectonics

James J. Zambito IV1 and Gordon C. Baird2

1University of Cincinnati, Department of Geology, Cincinnati, OH 45221

2SUNY College at Fredonia, Department of Geosciences, Fredonia, NY 14063

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. 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.

Directional data suggest the influence of Alleghanian events, indicating that this study may 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. Slickenside formation may possibly have occurred synchronously with the illitization of volcanic ash beds; which can be tested using K/Ar dating to determine the timing of illitization of ash beds displaying similar, different, and multiple slickenside groove orientations. Results from this testing may show that slickenside genesis concurrent with, or subsequent to, hydrothermal tuff-to-illite transformation at depth within an Alleghanian far-field stress regime, closely parallel widespread early joint and cleat development observed elsewhere in the Appalachian Basin by others.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York