STRATIGRAPHY, PALEONTOLOGY, AND PALEOGEOGRAPHY OF THE ELLIPTOCEPHALA ASAPHOIDES STRATA IN CAMBRIDGE AND HOOSICK QUADRANGLES, NEW YORK

Abstract

An Elliptocephala asaphoides fauna from rocks of the “Taconic sequence” of eastern New York is a characteristic “Schodack” faunule comparable in age and composition to those described by Ford from the vicinity of Troy, New York, and by Walcott from northern Washington County. Several new species and a revision of described species show that this fauna is a transitional assemblage composed of some typical Atlantic province genera, some typical Pacific province genera, and several unique genera. Eleven trilobite genera are present, of which the two belonging to Dorypygidae are new. Two new trilobite and one new molluscan species are described. A detailed population study of Calodiscus lobatus (Hall) was possible on the large number of specimens. One limestone pebble with a diagnostic Middle Cambrian fossil and several with Tremadocian fossils were found in Prindle's collection. Outcrops in the Cambridge-Hoosick quadrangles give a limited stratigraphic section; but on the basis of distribution of several trilobite species two faunules of stratigraphic significance can be distinguished within the Elliptocephala asaphoides fauna. Lack of appreciation of the concept of lateral gradation of lithofacies has caused considerable stratigraphic confusion in the Slate Belt; yet this principle was emphasized by Dale and has been reaffirmed by all recent studies. Fossils and careful attention to details of the lithofacies units in this region show three consecutive Lower Cambrian phases or times of deposition during which one or several major lithofacies units were deposited. When the lithologic units were given formational names by Ruedemann a time-stratigraphic significance was incorrectly attached to them, and the sequences from Rensselaer County and from the Slate Belt were erroneously correlated. The position of first appearance of the lower Elliptocephala asaphoides faunule is the only significant datum level for age correlation in this Lower Cambrian sequence, indicating as it does the approximate age equivalence of all beds at this position and above, regardless of their particular lithofacies expression. The mixed composition of the Elliptocephala asaphoides fauna demands a revision of paleogeographic concepts. To account for mixing, the enclosing beds must have been deposited in essentially the same geographic relationship they now hold to the Lower Cambrian rocks of adjoining areas. The faunal assemblage also indicates that the major barrier between Atlantic province faunas and Pacific province faunas was a fundamental difference in environment and not, as previously assumed, a continuous land barrier. The Pacific province genera were normal inhabitants of an environment, a shallow submerged coastal shelf, which bounded the western, northern, and eastern shores of the old North American continent, Laurentia. The outer edge of this environment was the landward boundary of the Atlantic province environment, and the intermingling of faunal elements observed in the Elliptocephala asaphoides fauna could have occurred only in such a boundary area. It is postulated that in eastern New York and adjoining parts of Vermont one or more deeply downwarped basins developed on the shallow coastal shelf and that these basins had narrow connections eastward and southward with the Atlantic Ocean. The benthonic environment of these basins was similar to that of the open ocean and contrasted sharply with the biofacies of the surrounding shallow coastal shelf. Consequently, it would be inhabited by an assemblage primarily adjusted to ocean temperatures, pressures, and salinity and showing affinities to Atlantic province faunas dwelling to the south and east. Few normal benthonic species of the coastal shelf could establish themselves in such an alien environment, although, because of the geographic proximity of the two areas, sporadically drifted individuals might be found.