A TRILOBITE FAUNA FROM THE POTSDAM FORMATION (CAMBRIAN; NEW YORK) AND ITS CONTRIBUTION TO STRATIGRAPHY

The Potsdam Formation is a Middle to Upper Cambrian conglomerate to sandstone that is preserved along the fringes of the Adirondack Uplift. Deposition was on a high relief erosional surface into the 1.1-1.3 billion year old basement complex forming a regionally significant unconformity. Where present, a basal conglomerate contains clasts from the Precambrian basement. The Potsdam Formation ranges in thickness from tens of meters near the southeastern margin of the Adirondacks to over 650 m in the study area in northeastern New York. Quartz arenite makes up the bulk of the Keesville Member of the Potsdam Formation. The lower Ausable Member is limited to northeastern New York and the adjacent region of Quebec and is a relatively immature feldspathic sandstone. The Potsdam Formation is overlain unconformably by dolostone and sandstone of the Lower Ordovician Theresa Formation.

The age and depositional environment of the Potsdam Formation has been a problem since it was first described by Emmons in 1838 due to the rarity of macrofossils. Documentation and description of trilobites is therefore significant. Trilobites were first reported from the Potsdam Formation (possibly the Ausable Member) by Walcott in 1891 but they were described only superficially in 1912. Since Walcott, trilobites have been reported from the Keesville Member, but identifications are not definitive and specimens were not illustrated. Here, we document a trilobite fauna from the Keesville Member of the Potsdam Formation that indicates a Late Marjuman age.