The New Bedford area is among the least studied in eastern New England, yet an understanding of its tectonic history is essential to the development of a viable paradigm for the assemblage of Avalonian terranes in New England. We review previous work and present the preliminary results of an ongoing program whose goal is to systematically study the structural, petrographic, and geochemical relations in the New Bedford area of southeastern Massachusetts. The region can be divided into two suites of rocks. Group I includes variably deformed granitoids that bear close resemblance to the Proterozoic- to Devonian-aged granitic basement of the Esmond-Dedham terrane. Of particular interest is a newly recognized unit of slightly deformed ~600-Ma, alkalic granite plus diorite, in which contact relations imply comingling of magmas of contrasting composition. In contrast, Group II consists of alaskitic and banded gneisses and schist that are reminiscent of gneissic rocks in western Rhode Island and adjacent Connecticut. Trace-element analyses for both groups are presented, and form the basis for discussion of their possible correlation with other granitoids in southeastern New England. The dominant structural feature in the New Bedford area is a steeply dipping to vertical, east-west–trending foliation that becomes more pervasively developed to the south. This tectonic fabric is strongly oblique to the northeast-trending schistosity of the southern Narragansett Basin, and geometrically more comparable to the east-northeast–trending folds found in the Carboniferous rocks of the northern Narragansett Basin. At present, the geometric and temporal relations among structural features in the New Bedford area and the adjoining Carboniferous rocks remain unclear. Although several interpretations of the regional setting of the lithologies of the New Bedford area are possible, currently the most plausible considers the lithologies of Group I to be a continuation of the Esmond-Dedham terranes, while Group II may be correlative with either the Hope Valley terrane or the Bass River Complex.

Mélanges interpreted as olistostromes are present in the late Precambrian Westboro Formation, Blackstone Group, and Newport Neck Formation of the Avalonian terrane of southeastern New England. All of these metasedimentary sequences were intruded by plutons dating 600 to 640 Ma and were variably deformed during late Precambrian(?) to late Paleozoic orogenic events. These Avalonian olistostromes contain quartzarenite and carbonate olistoliths, from 10 to 1,000 m in length, and are interstratified with turbidites and laminated and deformed mudstones. The Blackstone and Westboro sequences are associated with mafic volcanics, whereas the Newport Neck Formation contains interstratified felsic tuffs. Olistostromes and interstratified rocks are interpreted as submarine-slide, debris-flow, and sediment gravity flow deposits of midslope to proximal-fan environments. These deposits provide evidence for an episode of significant Late Proterozoic (650 to 800 Ma) rifting and marginal collapse of continental crust.

The Narragansett Basin contains a calculated thickness of 12,000 feet of clastic sedim entary rocks including conglomerates, sandstones, and shales. Sedimentary structures indicate deposition in an alluvial environment for many of the sediments. The lowermost formation in the western and northern portion of the basin is the Pondville, characterized by polymict conglomerate overlying older igneous and metamorphic rocks with sharp unconformity. Basal Pennsylvanian rocks along the eastern margin of the basin are highly altered arkoses that locally grade imperceptibly into underlying weathered granite. Overlying the basal rocks is the Rhode Island Formation, a thick and extensive unit characterized by sandstone with numerous beds of conglomerate and siltstone. Granitic detritus, prominent in many of the sandstones and conglomerates, was probably derived from western highlands. A quartzite source region lay to the east and contributed detritus to rocks of the southeastern part of the basin. The Wamsutta Formation interfingers with the Rhode Island Formation in the northwestern part of the basin. It contains reddish sedimentary rocks, some of which are poorly sorted, contain considerable felsite debris, and may have been emplaced by subaerial mudflows. Occurrence of extrusive and intrusive igneous rocks within the Wamsutta provides further evidence that volcanic activity attended deposition of the sedimentary rocks. The uppermost formation in the basin is the Dighton Conglomerate. The lower part of the basin sequence has been florally dated as Pennsylvanian. Radiometric age calculations raise the possibility that deposition may have continued into the Permian. Composition of conglomerates within the Narragansett Basin contrasts sharply with that of vein quartz-pebble conglomerates deposited along the western front of the central Appalachians from Ordovician through Pennsylvanian time. The differences are attributed to a combination of different sources and different depositional histories.