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Evidence of former equilibrium line elevations on Mount Olympus, Greece, coupled with estimates of uplift rate, point to more extensive Pleistocene glaciation and far colder climates than previous studies have indicated. These findings are supported by the record of glacial deposition both on the mountain and across the adjacent piedmont. The data not only provide evidence of significant equilibrium line altitude depression from a present-day elevation as much as 600 m above the mountain's summit (2917 m), but also show that Mount Olympus was glaciated on several occasions, and that the first episode of glaciation significantly predated the late Pleistocene. Piedmont sediments east and west of Mount Olympus record three discrete stages of deposition, each of which can be related to glacial activity on the mountain. Soils that separate these sedimentary units correspond to nonglacial intervals and can be correlated to a dated soil succession south of Olympus. This correlation suggests that the oldest soils correspond to the isotope stage 7 (Mindel/Riss) interglacial event (ca. 210,000 yr before present; U/Th disequilibrium) and that the oldest Pleistocene sediments record isotope stage 8 (Mindel) glaciation in the Olympus region. Subsequent stages of deposition are interpreted to record glaciation on the mountain during the isotope stage 6 (Riss) and isotope stages 4–2 (Würm) glacial events. Sedimentary units defined on the piedmont are also recognized on the Olympus upland and within valley-head cirques, where they correspond to three stages of cirque development. The distribution of these materials, as well as the occurrence of glacial erosional and depositional landforms, indicates that Mount Olympus supported upland ice during the first and second episodes of glaciation and that the first glaciation was sufficiently extensive to produce piedmont ice lobes that covered parts of the eastern, northern, and western piedmont of the mountain. Uplift-corrected cirque floor elevations, coupled with the distribution of glacial sediments, indicate that the Pleistocene equilibrium line altitude during each episode of glaciation was depressed at least 1400–1500 m, to elevations of 2000–2100 m above present sea level (asl). Assuming that lowering of the equilibrium line was solely a function of temperature, this would correspond to a mean annual temperature decrease of 8–9 °C. This contrasts with previous studies of the Mount Olympus region, which suggest that glaciation was restricted to small valley glaciers in upland and valley-head positions, that the regional Pleistocene equilibrium line was lowered only to elevations of 2200–2400 m asl, and that active glaciation was restricted to the latest Pleistocene (Würm). Study of the neotectonic history of Mount Olympus suggests that uplift persisted throughout the mid-Pleistocene–Holocene at a rate of ∼1.6 m/k.y.
Morainal banks and the deglaciation of coastal Maine
Quaternary glacial history of Mount Olympus, Greece
Facies architecture and grounding-line fan processes of morainal banks during the deglaciation of Coastal Maine
Late Precambrian–early Paleozoic arc-platform transitions in the Avalon terrane of the Northern Appalachians; Review and implications
The Avalon terrane of the Northern Appalachians is best defined by (1) the presence of latest Precambrian (c. 600 Ma) volcanic-sedimentary successions and cogenetic granitoid bodies widely attributed to the development of ensialic arc(s), and (2) early Paleozoic platformal sequences bearing Acado-Baltic fauna believed to define the “European” margin of the Iapetus Ocean. The tectonic transition from magmatic arc to stable platform lacks evidence for a major collisional event and is recorded in the development of arc-related successor basins taken to reflect the transform termination of oblique subduction. In southeastern New England, within-plate mafic volcanism and thick marine clastics of the late Precambrian Boston Basin suggest that Early Cambrian platformal conditions were preceded by rapid extensional or transtensional subsidence within the former magmatic arc. Late Precambrian volcanics in eastern Newfoundland follow an evolutionary path analogous to that of the Basin and Range, and culminate in bimodal and peralkaline activity interpreted to have accompanied terrestrial sedimentation in strike-slip basins prior to the development of shelf conditions during the earliest Paleozoic. In mainland Nova Scotia, wrench-related Cambrian successions have been interpreted to follow strike-slip closure of late Precambrian arc-rift basin(s) that may have formed in response to oblique subduction. In southern New Brunswick, an “Eocambrian” continental rift basalt/red-bed association suggests subduction was replaced by extension within the former magmatic arc. Avalonian arc-platform transitions that are widely defined as extensional strike-slip features rather than major compressional events suggest that late Precambrian subduction terminated through transform activity rather than collision. The tectonic transitions coincide with the break-up of a late Precambrian supercontinent that was responsible for the inception of the Iapetus cycle and may reflect the major plate reorganizations that would follow such an event.
With the recognition of the Hope Valley shear zone (HVSZ) as a terrane boundary, the Esmond-Dedham terrane (EDT) was subdivided, and the western division was named the Hope Valley terrane (HVT). The oldest rocks of the HVT consist of schist, gneiss and quartzite (Plainfield Formation), and metavolcanic and metaplutonic gneisses and amphibolites (Waterford Group), some of the latter yielding a radiometric age of 620 Ma. Members of the Sterling Plutonic Suite, consisting of granite gneiss and alaskite gneiss, intrude these older units. An exact radiometric age could not be determined for the alkaline pluton, Joshua Rock Granite Gneiss, but is assigned to the broad age range from c. 380 to 280 Ma. The Narragansett Plutonic Suite yields a radiometric age of c. 273 Ma, is a terrane-linking plutonic sequence cutting through the HVSZ, and links the HVT to the EDT. The EDT has a stratigraphic sequence that in many respects is similar to that of HVT, but has pronounced differences that mainly consist of a wider range of rock units and ages represented. Additionally, the rocks of HVT, and especially those near coastal Connecticut, have been elevated more generally to higher metamorphic grades than the EDT. The Harmony Complex and the Blackstone Group predominantly consist of plutonic and volcanic rocks, and schist, quartzite, and basaltic volcanics, respectively, into which have been intruded members of the Esmond Plutonic Suite or rocks correlated with them. The Price Neck Formation, of the Newport Basin, contrasts notably with the Harmony and Blackstone, but is intruded by the Cliff Walk Granite, similar in age and composition to the Esmond, and consists predominantly of fine-grained graded sedimentary rocks with volcanogenic beds. Fossiliferous limestone, phyllite, and siltstone make up Lower and Middle Cambrian rocks of the Pirate Cave Formation and the Conanicut Group of the Newport Basin, rocks unknown in HVT. A large part of the EDT is underlain by alkaline plutonic and volcanic rocks of the Scituate Plutonic Supersuite, whose radiometrically determined age is c. 373 Ma. Fluvial coal-bearing sedimentary rocks (Rhode Island Group) of the Narragansett and related basins contain a rich floral assemblage, which permits accurate dating to Westphalian and Stephanian stages of the Carboniferous. These rocks are unrepresented in the HVT. On the basis of structural and metamorphic data for the above stratigraphic units, a pre-Mesozoic evolutionary history has been outlined from late Proterozoic through Permian events. Compressional tectonic events within the Avalon superterrane and the composite Avalon terrane include the late Proterozoic Avalonian orogeny and the Alleghanian orogeny; mid-Paleozoic rifting events are interpreted for the alkaline plutonic rocks. Collisions involving the Avalon composite terrane with terranes farther to the west were responsible for Acadian and possibly late-stage Taconian orogenic events elsewhere in southern New England.
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.
The late Proterozoic Mattapan Volcanic Complex south and west of Boston consists primarily of ash-flow tuff. Nine analyzed samples are all high-silica rhyolites with trace-element patterns typical of calc-alkaline suites. Based on petrographic characteristics and trace-element concentrations these are subdivided into the older Twin Pine Tuff and the younger High Rock Tuff. The Twin Pine Tuff ranges from vitric tuff containing only 5 percent crystals on the east side of the Stony Brook Reservation to crystal tuff averaging 20 percent crystals (plagioclase > quartz > perthite) in Westwood and Sherborn. The crystal tuffs become more crystal-rich (from 15 to 30 percent) and more plagioclase-rich upward. Flattened shards and pumice lapilli give rise to foliation in most outcrops. Twin Pine tuffs cluster at higher values of Nb/Zr, Y/Zr, and Ce/La than the High Rock Tuff in Needham, Newton, and the west side of the Stony Brook Reservation. This member is a massive or columnar-jointed crystal tuff containing 25 to 35 percent crystals (plagioclase > quartz > perthite) and rare relic pumice. Plagioclase increases upward. Both Twin Pine and High Rock Tuffs show major-element variations consistent with observed petrographic trends. The high-silica and pyroclastic character of these rhyolites, their compositional zonation, and their total thickness (measured in kilometers) allies them with Tertiary caldera complexes throughout the American West. Younger intrusives that regionally crosscut and engulf the Mattapan ash-flow tuffs may represent magmatic resurgence after caldera collapse. These are tentatively assigned to the Westwood Granite.
Geochemical distinctions of late Proterozoic and Paleozoic volcanism in the Avalon zone of New England
Geochemistry permits distinction of two broad groups of volcanic rocks distinguished from the Avalon zone of southeastern New England. An alkalic suite is relatively enriched in K, Rb, Y, Zr, Nb, Zn, La, Ce, but lower in Al, Sr, and Ba compared to a calc-alkaline suite. The alkalic suite is compositionally similar to plutonic rocks that range in age from Late Ordovician to Devonian, whereas the calc-alkaline suite is similar to late Proterozoic plutonic rocks. Geochemical discriminant criteria developed in this chapter indicate that a number of the volcanic units previously have been incorrectly grouped and misinterpreted. The late Proterozoic igneous rocks are compositionally compatible with rocks formed by arc-building processes, and may represent outboard terranes of Africa formed during Pan-African orogenic events. In contrast, the alkalic igneous rocks probably reflect anorogenic or extensional regimes, following earlier crustal thickening processes and removal of first melts from such thickened crust; these rocks may reflect subsequent foundering and break-up of outboard Pan-African terranes that occurred episodically throughout much of the Paleozoic. The presence of alkalic bimodal volcanic rocks within the Narragansett Basin suggests that alkalic igneous activity in southeastern New England was more prolonged than previously recognized, and intermittently continued beyond the Devonian into Carboniferous time, perhaps accompanying the early stages of basin formation. These bimodal volcanics, the only known example of Carboniferous volcanism in the Appalachians of the U.S., are reminiscent of Late Devonian to Early Carboniferous volcanism that preceeded and accompanied the early stages of the Magdalen Basin in the Canadian maritimes.
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.
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.
The New Bedford area; A preliminary assessment
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.
All rock units, many fabrics, and two major faults mapped in the Hadlyme and Willimantic areas, Connecticut, are identified in a 1.45-km-deep research hole drilled at Moodus, Connecticut. Correlation of surface information and existing shallow drill-core data with the Moodus data was partially based on the description of core and cuttings from the deep hole, with supplemental chemical analyses, petrography, and a geochemical log. Rocks to a depth of 709.9 m are a mixture of quartz-diopside granofels and biotite-quartz-plagioclase schist identical to outcrops of Hebron Formation and are assigned to the Merrimack terrane. A 46.1-m section of biotite granitic gneiss within the Hebron Formation at Moodus correlates with the Canterbury gneiss. Below the Hebron Formation is 26.2 m of mylonitic muscovite-biotite-plagioclase-quartz schist, and 79.2 m of mylonitic, strongly layered garnet-sillimanite-muscovite-biotite-plagioclase-quartz schist and biotite-hornblende-plagioclase-quartz gneiss. These units correlate with the Yantic and Lower members, respectively, of the Tatnic Hill Formation, included in the Putnam-Nashoba terrane in eastern Connecticut. The rocks below 815 m in the Moodus hole are dominated by biotite- and hornblende-bearing plagioclase gneisses, but also contain thin layers of granitic gneiss, amphibolite, and pegmatite. The total lithologic assemblage is very similar to and correlated with the sequence of rocks of the Avalon terrane south of the Honey Hill fault. Subdivision of these rocks into the Hadlyme and Mamacoke Formations of the Waterford Complex is proposed. Chemical variability identified in the geochemical log is used to divide these gneisses into 44 layers 12 to 30 m thick, and is interpreted to reflect primary compositional fluctuations of volcanic and intrusive protoliths. Fabrics in the core and chips from the research hole indicate that mylonitic fault zones occur above (125 m thick) and below (145 m thick) the Canterbury gneiss. Phyllonitic fabrics overprint these mylonitic rocks and lesser deformed rock higher in the Hebron Formation, at the base of the Canterbury gneiss, and throughout the Avalon section. Brittle faults indicated by the presence of breccia, microbreccia, and slickensided chlorite surfaces with associated chlorite-zeolite-calcite-quartz–filled fractures occur throughout the well and are relatively common in the Avalon section. The range of temperatures and deformation mechanisms reflected by these rocks is similar to those identified in the Honey Hill fault zone. This suggests that the long history of reactivation of the Honey Hill fault zone extends as far northwest as Moodus and is represented there by a wider zone of deformed rock.
Major-element chemical analyses of 130 samples of high-grade plagioclase gneiss, together with geologic data, suggest that metadacitic gneisses of the late Proterozoic Avalon and Putnam-Nashoba terranes include four extrusive and two intrusive units formerly mapped as Monson Gneiss and Middletown Formation. Evidence for an igneous rather than sedimentary protolith for these gneisses comes from the mineralogic and chemical homogeneity, and the consistency of the chemical variations within individual bodies of gneiss with fractional crystallization processes. Evidence for the extrusive origin of some units comes from interlayered contacts with metasediments and metarhyolitic (alaskitic) and metatholeiitic (amphibolitic) volcanics, and from a 20- to 50-m-scale chemical cyclicity, typical of modern ash-flow tuffs. Evidence for an intrusive origin of two bodies comes from abundant amphibolite and calc-silicate xenoliths, from a massive structure lacking compositional layering, and from kilometer-scale chemical zoning or homogeneity. The identification of some units as lithodemic orthogneisses indicates that intrusion as well as extrusion led to the lithologic sequences in the Avalon terrane of southeastern Connecticut. It also precludes the lithostratigraphic correlation of these orthogneisses with paragneisses. Given this geochemical and geologic evidence, we believe that previously advanced stratigraphic arguments that major recumbent folds relate gneiss bodies now interpreted to be intrusive and extrusive should be reexamined. We believe that the part of the Avalon terrane consisting of metavolcanic rocks may be a right-side-up volcanic pile, and that major recumbent folding may not be present.
Revised correlations of critical units in south-eastern Connecticut contradict earlier stratigraphic correlations that led to the interpretation of major recumbent fold napping in the Avalon terrane. The revisions are required in part by the discrimination between lithodemic and lithostratigraphic units. We find no compelling evidence that schistose Putnam-Nashoba rocks exist within the Avalon terrane. On the contrary, we interpret these blastomylonitic schists and gneisses within the Avalon terrane around the Lyme dome as ductile fault rocks, derived from an Avalonian granodioritic orthogneiss. Rocks originally mapped as Brimfield Formation (Merrimack terrane) west of Old Saybrook are shown to correlate with the Middletown Formation of the Bronson Hill terrane. This new correlation makes all plagioclase gneisses of the Killingworth dome isolated from rocks of other terranes by the Middletown Formation, and provides evidence against the assignment of any plagioclase gneisses south of the Bronson Hill terrane near the Killingworth dome to the Avalon terrane. In contrast to the interpretation of fold napping, we propose that a major fault nappe exists in the Avalon terrane. The fault zone separates rocks of the Lyme dome from overlying allochthonous rocks of the Waterford Complex in the “Selden Neck block,” and fault splays isolate paragneisses of unknown age. Splays of the fault extend west, where they cut an extension of the Honey Hill fault zone, narrow slivers of Merrimack terrane rocks, and units in the Bronson Hill terrane. Important deformation in the zone probably occurred from middle Carboniferous to middle Permian, although earlier deformation is possible. Subsequently, the entire zone was folded around the Lyme dome. The tuning of the deformation suggests that it was related to the Hercynian assembly of Pangaea.
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.