Abstract

Lower Ordovician (Canadian) strata in the Mohawk Valley, New York, are represented by the Lower Canadian Tribes Hill formation, the Lower Canadian Chuctanunda Creek formation (new), and the Upper Canadian Cranesville formation (new). Perhaps the most striking revelation is the existence of beds younger than Gasconade (Lower Canadian) age lying south of the Adirondack Mountains. The name Tribes Hill is restricted to the limestone phase of the Lower Canadian, and the formation has profitably been subdivided into the Fort Johnson, Palatine Bridge, Wolf Hollow, and Fonda members.

The basal Fort Johnson member consists of slightly sandy dolomisiltite and dolomarenite changing upward through calcitic dolomite to dolomitic calcilutite. This, in turn, grades imperceptibly upward into thin-bedded silty fucoidal dolomite with many intercalations of shaly calcilutite (Palatine Bridge member). A recurrent lithofacies of dolomitic calcilutite constitutes the Wolf Hollow member characterized by an abundant cephalopod-gastropod fauna. Glauconitic calcarenites with a profuse development of gastropods, ribeiroids, hystricurids, and Clelandia characterize the Fonda member. Increase in clasticity, together with many shallow-water features, reflects the crustal instability and steady uplift which occurred in late Tribes Hill time. An unconformity, disclosed by truncation of the lower and upper Ribeiria limestone zones of the Fonda member, separates the Tribes Hill and Chuctanunda Creek formations.

The Chuctanunda Creek formation is a siliceous dolomite, very cherty, and finer-grained in the upper portion, and presumably the youngest Lower Canadian deposit in the Mohawk Valley. Accurate age assignment is difficult owing to the scarcity and poor preservation of its fossils. An unconformity separates the formation from the overlying Upper Canadian Cranesville dolomite.

The Cranesville dolomite is noncherty, more arenaceous, and lighter-colored than the older Canadian dolomites. It is remarkably barren of fossils, and on the chance discovery of two specimens of Eccyliopterus rests its Upper Canadian designation.

A preliminary petrographic study of the rocks suggests that seaweeds supplied adequate magnesium to the lithotope to effect local dolomitization, which gave rise to fucoidal dolomites and mottled limestones.

Restored structural diagrams illustrate that the Lower Ordovician strata were folded prior to Mohawkian (Lowville) sedimentation which makes the unconformity atop the Canadian a major one. This folding was prophetic of more severe structural deformation which occurred subsequently during the Ordovician period.

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