Taconic Stratigraphy in Northern Washington County, New York

Abstract

A lithostratigraphic sequence consisting of Lower and Upper Cambrian and Lower and Middle Ordovician clastic rocks has been mapped within the Taconic sequence in northern Washington County, New York, and an adjacent part of Rutland County, Vermont. The several lithostratigraphic units are dated by fossils collected from 28 localities, 20 of which are new. There are two Lower Cambrian formations. The lower, the Bull Formation, consists of two members: the Bomoseen Graywacke, dominantly of olive-green graywacke, and the overlying Mettawee Slate, dominantly of purple and green slates containing limestone conglomerates correlated with the type Schodack south of Schodack Landing, New York, and locally bearing the Elliptocephala asaphoides fauna. The overlying Lower Cambrian formation, now named West Castleton Formation but earlier named Schodack, consists essentially of black shales and limestone conglomerates locally bearing the Elliptocephala asaphoides fauna and, at one locality, an overlying Paedeumias-Bonnia fauna. Subdivision of this Lower Cambrian sequence into chronostratigraphic units is not possible at present because (1) there are lateral lithofacies gradations, (2) the first appearance of Lower Cambrian fossils is determined by the occurrence of a calcareous biotope within the time-span of the fauna, and (3) lack of evidence prohibits subdividing the Elliptocephala asaphoides fauna into upper and lower faunules. The fact that the Lower Cambrian faunas are mixed, including typical Atlantic province, Pacific province, and endemic genera, is not incompatible with a Taconic klippe hypothesis; the Taconic rocks could have been deposited anywhere between their present location and the nearest known occurrence of a typical Atlantic province fauna in eastern Massachusetts. Black shales with associated rotten-weathering bluish-gray sandstones, named the Hatch Hill Formation, overlie the West Castleton Formation unconformably. The Hatch Hill Formation contains a probable latest Cambrian graptolite fauna. The overlying Poultney Slate is divided into three members: (1) the A member, consisting of black shales with local interbedded limestone conglomerates and thin sublithographic limestones, overlain by (2) the B member, dominantly of bluish-gray argillites with thin beds of orange dolomitic quartzite locally in its lower part, and (3) the C member of dark-gray to black siliceous argillite, partly coeval with the B member in the northern part of the area mapped. The A member and, locally, the B member are lithologically identical with part of the Schaghticoke Shale at its type locality in the gorge of the Hoosick River at Schaghticoke, New York. The Poultney Slate contains Early Canadian (Late Tremadoc) graptolites in the lower part of the B member and Normanskill graptolites in its upper part. It passes conformably into the Indian River Slate of red and bluish-green slates. The Indian River Slate is overlain unconformably by the Pawlet Formation of graywackes and dark-gray or black shales with Normanskill graptolites. The Taconic rocks in northern Washington County occur in several approximately north-south-trending overturned folds whose axial planes dip eastward. The westward margin of the area is an overthrust from the east, which probably has a displacement of at least 20 miles.