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The Boston Basin is a late Precambrian structural and depositional basin that is thought to be a fragment of Avalon terrane. Basement rocks consist of the calc-alkaline Dedham Granite and a bimodal volcanic assemblage that is, in part, interbedded with basal sedimentary rocks of the Boston Bay Group. The sedimentary succession within the basin has traditionally been described in terms of the Roxbury Conglomerate and the Cambridge Argillite. The Roxbury, in turn, has been subdivided into three members: the basal Brookline, the medial Dorchester, and the upper Squantum.

Recent study of the Boston Bay Group by the authors and their students indicates that redefinition of these rocks in terms of lithofacies and lithofacies assemblages provides a clearer picture of the late Precambrian sedimentary history of the basin. As many as 17 lithofacies types, composing 4 lithofacies assemblages, have been described from the Boston Bay Group. While indicating that the original definition of rock units is broadly consistent with lithofacies assemblages, this work further indicates that lithofacies variability is greater, and facies interrelations more complex, than was originally thought. We propose that the rocks of the Boston Bay Group make up a sedimentary facies assemblage consisting of proximal debris flows and high- and low-density turbidites deposited in a marine slope/fan setting under the strong influence of glacial processes. Deposition began with the development of the Boston Basin as a rifted successor(?) basin, either just prior to the closure of the Cadomian(?) Ocean or during the opening of Iapetus.

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