Abstract

Detailed mapping of the predominantly argillitic Cambro-Ordovician “Taconic sequence” in the Kinderhook quadrangle, New York, indicates that the Chatham thrust divides these rocks into two distinctive sequences. To the east slates predominate and form a hilly upland, but west of this thrust less resistant shales underlie a gently rolling lowland. Stratigraphically, the eastern sequence is composed of a transgressive black Trentonian (Middle Ordovician) slate, locally with a thin basal limestone, resting upon a thick series of Lower Cambrian green slate, graywacke, quartzite, limestone, and limestone conglomerate (Rensselaer-Nassau-Schodack). Canadian carbonate rocks crop out intermittently along the Chatham thrust, and they are interpreted as slivers trapped during thrusting. The western sequence is primarily Normanskill (Middle Ordovician) and Deepkill (Lower and Middle Ordovician) shale, unconformably underlain by Lower Cambrian shale and overlain by Siluro-Devonian limestone.

Though many small thrusts are present, folding is the principal mode of deformation. The Chatham thrust, however, is a major structural break and continues at least 20 miles southwest of the quadrangle and an unknown distance northward. It is a post-middle Trentonian feature, and maximum displacement is a few miles rather than tens of miles. Unconformities are very important in the geologic history of this area, but some have gone undetected because of the structural complexity and poor exposure. The following formations are believed to have basal unconformities in the Kinderhook quadrangle: the Upper Silurian Manlius limestone, the Trentonian black slate (locally with a basal limestone), the Deepkill shale, and the Canadian carbonate rocks.

Evidence for the existence of the “Taconic klippe” was not found in mapping this quadrangle. Analysis of the development of the klippe hypothesis indicates that it is based principally upon stratigraphic considerations; available structural evidence weighs against this interpretation. While the klippe hypothesis seems to explain well the relations at the north end of the Taconic Range, the problem of adequately defining the boundaries of this “klippe” causes serious doubt about its existence.

An alternative interpretation of the regional relations is suggested, involving unconformities and facies changes in a single indigenous sequence. Trentonian rocks lie unconformably on rocks as old as Precambrian from Vermont to Pennsylvania and pass indiscriminately in and out of the “Taconic klippe.” The Normanskill and Deepkill rocks (mainly shale) are interpreted as passing transitionally into limestone to the west; an analogous situation exists in the Middle Ordovician rocks of Virginia. The Deepkill is believed to rest unconformably on rocks of Early Cambrian to middle Canadian age. Middle Canadian formations in the Kinderhook quadrangle are carbonate rocks and appear to rest unconformably upon Lower Cambrian slates; their striking similarity to equivalent rocks in the near-by “autochthonous” series suggests they have not been displaced any great distance. The Lower Cambrian is a thick series of argillite, gray-wacke, and quartzite with some thin carbonate rocks near the top. The thick, lower part of this series is considered a southward continuation of the Mendon Series of Vermont. The upper strata are interpreted as the offshore equivalents of shallow-water quartzites and carbonate rocks deposited marginal to an eastern welt in later Early Cambrian time.

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