LOWER MEDINA GROUP (UPPER ORDOVICIAN THROUGH LOWER SILURIAN) CHANNEL SANDS IN NEW YORK STATE, USA

The Hirnantian (Upper Ordovician) through Sheinwoodian (Wenlock; Silurian) Medina Group is a clastic interval spanning the Taconic foreland basin from proximal settings in east-central New York State (USA) to a more distal setting in southern Ontario Province (Canada). In the type region, the Medina consists of the Whirlpool Sandstone, Power Glen Shale, Devils Hole Sandstone, Grimsby Formation, Thorold Sandstone, Cambria Shale, and Kodak Sandstone. Despite extensive study along the outcrop belt, there have been few published subsurface mapping studies of either the Medina Group as a whole or of formations within the group. This study provides the first formation-scale subsurface correlations of the Medina Group in New York.

Geophysical logs from 217 wells were used in Geographix software to correlate units across western and central New York and northwestern Pennsylvania. Correlations based on logs were verified with gamma profiles taken from core (Niagara Gorge region) and outcrop (Seth Green Park, Rochester, NY). Isochore contour lines were created using minimum curvature in Geographix. The contours were hand edited and the maps created using Esri^TM ArcGIS software. Isochore maps for the Whirlpool, Power Glen, Devils Hole, and Grimsby are presented here.

Gamma-ray patterns indicative of channel facies in the Whirlpool and Devils Hole sandstones and the spatial distribution of the Whirlpool and Power Glen in isochore maps suggest pronounced incised valley fill and channel sands throughout west-central New York. Isochore maps of the overlying Devils Hole and Grimsby show increased thickness along parts of the “Whirlpool channel” and show channel-like thickness distribution in the eastern part of the study area. Incised valleys in western and east-central New York likely developed during eustatic lowstands during maximum glaciation in the Late Ordovician ice age. The eastward migration of valley fill and channel sand facies from western New York to east-central New York likely reflects renewed subsidence in the overfilled eastern part of the basin that allowed flooding of previously exposed land.