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.

Boston Basin is an outcrop of metasedimentary and metavolcanic strata bordered by faults mainly against late Precambrian and Paleozoic granitoids. No unit-by-unit stratigraphic correlations can be made between this basin and others of the same general age in the circum-Atlantic region. Its sedimentary, mainly clastic rocks, named the Boston Bay Group, contain a diamictite frequently referred to as the Squantum tillite. The Boston Bay Group is underlain and partly interbedded with mainly felsic meta-volcanics. In the Boston Basin the sediments are of Vendian age determined by microfossils and isotopic dating. The surrounding calc-alkaline granitoids are closely associated with gabbro-diorites that often show close (“acid-basic”) relations between products of coexisting mafic and felsic magmas. These lithological associations are typical of late Proterozoic Avalonian terranes in the North Atlantic. H. Williams’ original concept of the Avalon domain in Newfoundland was that of a platform at the edge of a continent. Thereafter, the platform was successively referred to as a prong, a microcontinent, a plate, and then a terrane. Keppie (1985) and ourselves advocated that the terrane, in turn, consists of a collage of linked (accreted) blocks. Yet the broad lithological similarity of rock associations among such blocks in Precambrian and possibly earliest Cambrian time indicates that they were parts of an originally distinct major lithotectonic unit such as an island arc. The collage of the closely related blocks is referred to as the Avalon superterrane. It is conjectured that the superterrane broke up in late Proterozoic and possibly Early Cambrian time into several blocks, which drifted apart during the opening of the Iapetus Ocean. From mid-Paleozoic time onward the ocean started closing. The blocks were assembled and accreted to the Laurentian continent as a new collage, referred to as the Avalon composite terrane. It is proposed that the thick terrigenous-volcanic Boston Bay Group accumulated in a graben-like structure within the late Proterozoic superterrane, although now it is a fragmented part of the Boston block that is a constituent of the composite terrane.