Abstract The Neoproterozoic diamictite-bearing Squantum Member is located in the Boston Basin in eastern Massachusetts, USA. The Boston Basin forms part of the Avalonia island arc terrane ( c. 650 Ma), and appears to have originated as a rift-type basin in an extensional setting along the northern margin of Gondwana, although its exact position is debated. Inferred palaeoenvironmental reconstructions of the Boston Basin have alternated between a fluvial basin where ice played a major role in transporting much of the coarse material and an evolving marine basin dominated by non-glacial subaqueous mass flow, submarine fans and turbidity-current deposition. The age of the Squantum is bracketed between c. 595 and 570 Ma, and is correlated by some to the glaciogenic diamictite succession of the Gaskiers Formation (eastern Newfoundland) as part of the putative global Gaskiers Glaciation c. 582–585 Ma. However, the Squantum Member consists of diamictite, graded sandstone and siltstone units, and fine-grained laminated argillite/mudstone units typical of debris flow and turbidite facies that accumulate in a submarine setting. A glacial influence is not readily identified and revolves around early interpretations of the diamictite as being ‘till-like’, the presence of laminated horizons that resemble glaciolacustrine ‘varvites’ and the disputed recognition of ice-rafted dropstones. There are no associated carbonates and, consequently, no geochemical data are available in connection with the Squantum Member.

Based on a detailed analysis of facies assemblages within the Boston Basin (Boston Bay Group), we have concluded, to a first approximation, that the Boston Bay Group was originally laid down as a sequence of inclined strata that appears to have formed a submarine fan/slope/apron at unknown water depths. There appears to be a systematic progression of facies and the imprints of depositional mechanisms from the southern (more proximal) portion of the basin, in the vicinity of Nantasket and Hingham, to the northern and northeastern (more distal) portion of the basin, in the vicinity of Newton and Somerville. In the more proximal areas of the basin, the facies assemblages are characterized by bouldery and cobbly debris-flow deposits and high-density turbidites; in the more distal reaches of the basin, the facies assemblages are characterized by high-to low-density turbidites, pebbly and cobbly high-density turbidites and/or debris-flow deposits, and traction-generated features. Syndepositional slump features are ubiquitous, and dropstones are present in most fine-grained facies investigated. The rocks appear to have been transported into the basin from the south and possibly from the west. Consequently, on the basis of our investigation of facies assemblages, we are forced to conclude that the historic stratigraphy and stratigraphic units of the Boston Basin are suspect. Insofar as this historic stratigraphy has at times been used to infer the presence of structure, and insofar as this stratigraphy has been used as the basis for interpreting the depositional history of the Boston Basin, we are also forced to conclude that many of the stratigraphically derived interpretations of the depositional history of the Boston Basin, as well as some unknown parts of the structure, are also suspect.

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.

The Boston basin is one of several late Paleozoic nonmarine sedimentary basins that developed in eastern New England subsequent to the Acadian revolution. Most of the sedimentary rocks in these basins are known to be Pennsylvanian in age; those in the Boston basin are presumably of this age. The principal map units—except for the Blue Hills and Nahant—are the Precambrian basement, the Mattapan and Lynn Volcanic Complexes (Mississippian?), and the Boston Bay Group (Pennsylvanian?). The Boston Bay Group consists of the Cambridge Argillite and the Roxbury Conglomerate. The Roxbury Conglomerate in turn is subdivided, from bottom to top, into the Brookline, Dorchester, and Squantum Members. During the past 25 years, a series of bedrock tunnels, driven for water supply and drainage purposes, have added greatly to our knowledge. The tunnels, 3 to 3.5 m in diameter, are at a depth of 30 to 90 m below the surface. The total length of these tunnels is 39.57 km; the Dorchester Tunnel, under construction, is another 10.19 km long. New observations and interpretations are as follows: (1) The maximum thickness of the Boston Bay Group is 5,700 m. (2) The Boston Bay Group thins to the south. (3) The Roxbury Conglomerate, with a maximum thickness of 1,310 m, is a southerly facies of the lower part of the Cambridge Argillite. (4) The Cambridge Argillite reaches a maximum thickness of 5,700 m in the northern part of the basin. (5) The sedimentary rocks were derived from a highland to the south. The most important new results that bear on structure are that (1) the Northern border fault, where exposed in a tunnel, dips 55°N; (2) the Charles River syncline, exposed in two tunnels 10.5 km apart, plunges 19° in a direction N84°E; and (3) many minor folds and faults complicate the structure. Although no tunnel crosses the Blue Hills, a new interpretation of the structure is presented. The volcanic complex of that area was erupted onto flat-lying Cambrian sedimentary rocks. The Quincy Granite and Blue Hill Granite Porphyry were injected into the horizontal Cambrian strata and volcanic complex. After a period of uplift and erosion, the Pennsylvanian strata of the Norfolk basin were deposited. All the rocks were then folded into a syncline, the vertical north limb of which is now the Blue Hills. The Blue Hills were then thrust northward over the Boston basin.

ABSTRACT On this trip, we review and examine depositional environments, sediment routing, and compositional trends of Carboniferous strata in northern Arkansas and southwest Missouri, USA. We demonstrate the likelihood that, during the Pennsylvanian, Appalachian sediment was routed southward through Missouri within incised channels and into Arkansas. Some of these channels are still preserved. They are sub-parallel and oriented approximately north to south; cross-bedding and clast imbrication consistently indicate southerly flow. Sandstone within the channels has very little feldspar with almost no igneous rock fragments but does contain significant amounts of metamorphic rock fragments. This composition points to the Appalachians as a major source area. The composition of the channel sandstones is close to those of Morrowan and Atokan fluvio-deltaic sandstones in the Boston Mountains, which were deposited by southward flowing streams. It takes but little interpolation to connect these deposits and conclude that the channels in southwest Missouri transported sediment to the Boston Mountains and possibly as far as Arkoma basin.

Recent geologic work in eastern Massachusetts and Connecticut has demonstrated that the rocks of the area are broken into a series of fault blocks. From east to west, the faults bounding these blocks are the North Boston Basin fault (in Massachusetts and possibly extending south into Rhode Island), the Bloody Bluff (Massachusetts) and the Honey Hill-Lake Char (Connecticut) fault system, and the Clinton-Newbury fault (Massachusetts). All of these faults, except possibly the North Boston Basin, trend southwest toward a common area in southern Massachusetts that has not yet been mapped in detail, so the relationship between them has not yet been established. Although early work in the area lumped various rock units across the faults, the modern work indicates that distinct groups of rocks occur within each fault block, and repetition of units across the major faults has not been demonstrated. The ages of the rocks in the area are not yet established. Fossil-bearing rocks from Cambrian to Pennsylvanian in age occur in fault-bounded blocks; their relationship to the metamorphosed rocks cannot be determined.

Five mafic and four intermediate to felsic swarms have been recognized among the dikes of the Avalon Boston terrane on the basis of their field, petrographic, and petrochemical characteristics and a limited number of K-Ar ages. The Boston terrane in Massachusetts is divided into three dike zones from north to south as follows: the Cape Ann, Danvers-Dedham, and Boston Basin zones. A new nomenclature is introduced for classifying dike systems into sets, simple swarms, swarms, compound swarms, and dike complexes. Cross-cutting relations and three K-Ar ages suggest most of the NE-trending dolerites and lamprophyres of the Boston terrane are Mesozoic and, as such, are related to Atlantic rifting. Unlike the eastern North American tholeiites (ENA) of Weigand and Ragland (1970), the Boston-terrane Mesozoic composite swarm also includes alkaline dolerites, transitional alkalic dolerites, and olivine-normative dolerites (latter absent in ENA dikes north of Maryland). Boston-terrane dikes also have much higher average TiO 2 and K 2 O contents and lower average SiO 2 and MgO contents (for quartz- and olivine-normative dikes, respectively). These differences indicate the Mesozoic Bostonterrane dolerites were derived from different magma sources than the ENA dikes and are probably a southern extension of the Mesozoic Coastal New England igneous province of McHone and Butler (1984). Unlike the rest of New England, the majority of the Boston-terrane dikes trend northwest rather than northeast. This, along with field evidence and 7 K-Ar ages, suggests most of the NW-trending dikes are Paleozoic, with some possibly late Proterozoic. A number of dikes (andesites, diorites, hybridized dolerites, and a few dolerites) appear to have intruded into mobile granitic magmas and are undoubtedly the same age as the late Proterozoic and Silurian or Ordovician granites they intruded. Other dike segments and irregular bodies of diorite and hybridized dolerite occur as xenoliths in the Cape Ann Granite and themselves contain very coarse to pegmatitic anorthosite xenoliths. A NW-trending swarm of felsic dikes in the Cape Ann zone is probably associated with the Cape Ann Granite. Dacite and andesitic dikes in the Danvers-Dedham zone appear to be late Proterozoic to early Paleozoic in age. Most of the remaining NW-trending (and E-W–trending) dolerite dikes are believed to be middle to late Paleozoic. The dominant northwest trend is consistent with intrusion into fissures opened during oblique NE-SW extension associated with Paleozoic (and Proterozoic?) pulses of right-lateral movements within the NE-trending Nashoba thrust belt (and bounding Bloody Bluff fault) immediately west of the Boston terrane. Alternatively, the NW-trending swarms may have been intruded during Paleozoic (and Proterozoic?) episodes of crustal rifting (or aborted rifting) along northwest trends.

Location The Boston Bay Group is preserved in the Boston Basin of eastern Massachusetts. The instructions below allow the reader tolocate four different stops within the basin. For driving convenienceit is easiest to visit the areas in either regular or reversenumerical sequence (Fig. 1). Persons not familiar with the greater Boston metropolitan area should have a detailed road map orroad atlas. All of the sites are open to the public without permissionduring daylight hours and ample parking is available. Stop 1. Old Mystic River Quarry, Somerville (Fig. 2). From I-93 (traveling north) exit onto Massachusetts 28; turn leftunder interstate and immediately right onto Mystic Avenue (Massachusetts 38); after 0.3 mi (0.5 km) turn left onto Butler Drive; enter public housing project (that now occupies quarry) and turn left into parking area. Park next to basketball courts and walk about 290 ft (100 m) southwest across the field to thequarry face. The best exposures are to the northwest along therock face. If heading to Stop 2, take Massachusetts 38 onramp to I-93 south; exit onto Storrow Drive (U.S. Route 1); exit onto Beacon Street at Kenmore Square. Continue west on Beacon Street. Stop 2. Webster Conservation Area, Newton (Fig. 3). Outcrops in the vicinity of the intersection of Beacon Street and Hammond Pond Parkway may be reached by traveling west on Beacon Street (about 1.2 mi [2 km] beyond Boston College) ornorth on Hammond Pond Parkway from Massachusetts 9. Parkon north side of Beacon Street (Fig. 3) and examine outcropsalong and

Abstract Boston lies near the geographic center of the Boston Basin, a roughly triangular area of sedimentary and volcanic rocks of late Precambrian and Cambrian age that is surrounded by older, contemporary, and younger granites and related rocks. The basin rocks are mostly argillites, sandstones, and conglomerates interlayered with rhyolitic and spilitic volcanics and volcaniclastic sediments. The rocks are deformed into large east-west folds and are much broken by faults of several orientations. Dikes and sills abound. Deep alteration of rock to a soft clayey aggregate is widespread. The irregular bedrock surface is the result of differential glacial erosion of rocks of varying hardness, and under most of the central Boston area, bedrock is buried by thick drift. The stratigraphy of the drift is complex and includes four stratigraphically distinct tills and outwash units, representing several advances and retreats of glacial ice. These are provisionally interpreted to consist of two glacial stades of the early Wisconsinan and two of the late Wisconsinan, including a read-vance that overrode much of the Boston area about 12,000 years B.P. The effects of the geology on local engineering practices are briefly described.

Late Proterozoic igneous rocks in the Boston-Avalon zone of eastern Massachusetts are dominated by voluminous calc-alkalic intrusives and extrusives that yield radiometric ages between 600 and 650 Ma. This “main phase” Avalonian volcano-plutonic series, which includes the Dedham Granite and Lynn Volcanics, was emplaced over a time span that was both preceded and followed by separate episodes of mafic volcanism. The Middlesex Fells Volcanic Complex temporally preceded the emplacement of the Dedham and Lynn, whereas the Brighton Volcanics, part of the stratified section of the Boston Basin, clearly postdate them. Geochemical study of the mafic members of the Middlesex Fells Volcanic Complex, which were regionally and thermally metamorphosed to the greenschist facies, reveals that these alkaiic and transitional basalts have a geochemical signature characteristic of modern continental rift zones. In contrast, flows, pillows, and pyroclastics of the basalts, basaltic andesites, and andesites of the Brighton Volcanics are high-alumina basalts and andesites of calc-alkaline character resembling those from modern subduction-related magmatic regimes. Magmatism thus provides evidence of the changing tectonic regime during the late Proterozoic in the Avalon terrane of southeastern New England. An initial period of continental rifting preceded the extensive calc-alkaline magmatism associated with the Avalonian orogeny. Subsequently, the terrane experienced a brief period of continental subduction-zone-related magmatism before becoming a stable shelf in the early Paleozoic.