The discovery of a fossiliferous interval (Altona Formation, new unit) under the Potsdam Formation requires a new geological synthesis of a large part of the northeast Laurentian craton. Potsdam sandstones can no longer be regarded as the oldest sedimentary unit on the middle Proterozoic Grenville orogen in northern New York and adjacent Quebec and Ontario. The thickest Potsdam sections (to 750 m) in the east Ottawa–Bonnechere aulocogen have been explained by deposition with normal faulting possibly associated with Ediacaran rifting (c. 570 Ma) that led to formation of the Iapetus Ocean. However, sparse trilobite faunas show a terminal early Cambrian–middle middle Cambrian age of the Altona, and indicate much later marine transgression (c. 510 Ma) of the northeast Laurentian craton. Altona deposition was followed by rapid accumulation of lower Potsdam (Ausable Member) sandstone in the middle–late middle Cambrian. The Altona–Ausable succession is probably conformable. The Altona is a lower transgressive systems tract unit deposited on the inner shelf (sandstone, reddish mudstone, and carbonates) followed by aggradation and the deposition of highstand systems tract, current cross-bedded, in part terrestrial(?), feldspathic Ausable sandstone. Unexpectedly late Altona transgression and rapid Ausable deposition may reflect renewed subsidence in the Ottawa–Bonnechere aulocogen with coeval (terminal early Cambrian) faulting that formed the anoxic Franklin Basin on the Vermont platform. Thus, the oldest cover units on the northeast New York–Quebec craton record late stages in a cooling history near an Ediacaran triple junction defined by the Quebec Reentrant and New York Promontory and the Ottawa–Bonnechere aulocogen.
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