The Acadian orogeny in the North Atlantic region is assessed in this chapter in the light of mid-Paleozoic tectonics; throughout, plate tectonic nomenclature is used, and cycles are avoided. In North America nine regions bearing the imprint of the Acadian orogeny are recognized. In Newfoundland, in the Maritime Provinces of Canada, and in Vermont and New Hampshire a continuous sequence of lithotectonic belts correlates along the orogen. The Bronson Hill belt, although a continuous structure in southern New England, is not recognized as such but splits into two structures northeast of the Maine-New Hampshire border: the Boundary Mountain anticlinorium and the Lobster Mountain anticlinorium. Other lithotectonic belts are partly continuous from Canada into the United States; they include: (1) North-Central Maine belt, (2) Aroostook-Matapedia belt, (3) Miramichi belt, (4) Fredericton-Central Maine belt, (5) Richmond belt, (6) Casco Bay belt, (7) Benner Hill belt, (8) St. Croix-Ellsworth belt, (9) Mascarene belt, and (10) Avalon belt. The decision as to whether each of these belts represents a separate terrane is at present reserved. In the coastal Maine zone the situation is particularly complex, and belts 6 through 10 can be recognized there. In Massachusetts, we interpret the Merrimack Trough belt as in fault contact with both the Kearsarge-Central Maine and Bronson Hill belts to the northwest, and in Connecticut, with the Bronson Hill belt alone. Additionally, the Merrimack Trough belt is in fault contact with the Putnam-Nashoba belt to the southeast. The latter shows mainly a Taconian metamorphism and extensive intrusion of granites; clear evidence for Acadian orogenic effects in the Putnam-Nashoba belt is lacking. In Newfoundland the main orogeny appears to be Silurian in age, and the same is true of New Brunswick, whereas in the Meguma of Nova Scotia the Devonian deformation and intrusive activity continue from the Devonian to the Carboniferous. Correlations with the south-central Appalachians indicate a possibility of significant Acadian transpressional effects. The most recent evidence of a new microfossil find, however, implies that considerable Acadian deformation occurred in the Southern Appalachians, although it may have been directly continuous with earlier Taconian events. The Acadian metamorphism in the Northern Appalachians is associated with numerous granites, in general ranging in age from the Silurian to the Carboniferous. The earlier Silurian granites may have originated along the Iapetus suture or may be associated with transcurrent faults. The plate tectonic interpretation of the orogenic system is based on a model of successive blocks (terranes) approaching and colliding with North America and squeezing intervening sediments and volcanics. This took place over a fairly prolonged period of time.

Paleomagnetic and tectonostratigraphic data for the northern Appalachians record Silurian closure of a major ocean, the Iapetus Ocean, that was bordered by the Laurentian craton and the Avalonian microcontinent. In Ordovician times this ocean consisted of at least two basins (Iapetus I and II) and extended from a paleolatitude of 10 to 20°S (Laurentian margin) to ca. 50°S (Avalonian margin); Gondwana was located yet farther south. Paleomagnetic data from the Middle Ordovician Robert’s Arm, Chanceport, and Summerford groups in north-central Newfoundland, which represent intraoceanic arcs and ocean islands, yield paleolatitudes of 30 to 33°S. In contrast, pillow lavas of the upper part of the Late Cambrian to Lower Ordovician Moreton’s Harbour Group, which are currently juxtaposed to the Chanceport Group along the Lobster Cove-Chanceport Fault, acquired their remanence at 11 °S in a marginal island arc setting. Subaerial deposits of the mid-Silurian Botwood Group that unconformably overlie marine sequences in northeastern Newfoundland yield a primary magnetization with a paleolatitude of 24°S, which is indistinguishable from the Early Silurian position of the southeast-facing Laurentian margin. Silurian closure of Iapetus is supported by the timing of thrusting and folding and by the age of angular unconformities in the Central Mobile Belt. The combined paleomagnetic and tectonostratigraphic data present a working hypothesis for the geometry and tectonic evolution of the northern Appalachians. In Early Ordovician times, a volcanic arc and a back-arc basin (Iapetus I) were located near the Laurentian margin. Following Middle Ordovician obduction of ophiolites onto the Laurentian margin when Iapetus I closed, convergence of Avalon and Laurentia by northward subduction continued until closure of Iapetus II was complete by the Late Silurian. In view of these data, the traditional subdivision of Ordovician “Taconic” and Devonian “Acadian” orogenies needs to be revised. Maintaining the terminology of orogenic phases, one either has to expand the time interval of the Acadian orogeny to include the Silurian or add an orogenic phase (Caledonian?) in between Taconic and Acadian. In either case, Early to Middle Paleozoic closure of Iapetus should be viewed in terms of a progressive deformation history with peak deformation pulses rather than temporally discrete orogenies.

In considering the tectonic evolution of the Acadian orogen in New England and Maritime Canada, account should be taken of the fact that from the Early Cambrian through the Mississippian the Coastal Acadia marine faunas of eastern North America are very distinctive biogeographically from those occurring to the west and northwest within the Northern Appalachians and adjacent parts of the continent. The limited amount of boundary mixing can be largely ascribed to dispersible larval stages of a few taxa. This biogeographic situation suggests that the surface-current circulation pattern that maintained this type of reproductive isolation and biogeographic integrity from the Early Cambrian through the Mississippian implies a certain level of geographic remoteness as well, although specific distances cannot be derived from such data. Paleogeographies and plate tectonic concepts need to be consistent with the available biogeographic information. Early-through-Late Cambrian biogeographic units have been recognized, since early in this century, in the Acadian orogen and adjacent regions. Baltic Realm (= Atlantic Realm, = Acado-Baltic Realm) faunas are restricted to the coastal regions of New England (Boston region), the Maritimes (St. John, New Brunswick), Nova Scotia, and eastern Newfoundland. On the other hand, Laurentian Realm (= Pacific Realm) faunas are restricted to a belt that extends from western Newfoundland down the valley of the St. Lawrence River and through Vermont and eastern New York. Ordovician biogeography is less well documented, with earliest Ordovician (Tremadocian) Baltic Realm-type faunas also occurring in eastern Newfoundland, Nova Scotia, and Coastal Acadia, whereas on the North American Platform to the west, Laurentian Realm-type faunas occur. Baltic Realm faunas of the later Early Ordovician (Arenigian) are known in eastern Newfoundland. In central Newfoundland, the few shelly faunas are largely of the Laurentian Realm. There is no useful Middle to Upper Ordovician (upper Arenigian-Ashgillian) fauna from Coastal Acadia, whereas fauna of that age-span on the northwestern edge of the Northern Appalachians is strictly Laurentian Realm in character. During this time interval in central Newfoundland, central and northern New Brunswick, and northern Maine, the few shelly faunas presently available are largely of the Laurentian Realm, with a Baltic admixture in some cases, although more study is required for a definitive statement on this matter. In the latest Ashgillian (Hirnantian) there is at least one good occurrence of the globally very extensive cold water, Gondwana (=Malvinokaffric) Realm Hirnantia fauna in eastern Gaspe. Silurian European Province faunas occur in Coastal Acadia, including parts of mainland Nova Scotia, southern New Brunswick, coastal Maine, and the Boston area. North American Province Silurian occurs farther to the west and northwest, including northern New Brunswick, northern Maine, and the Connecticut River Valley region, as well as in areas farther to the west. Marine Devonian units of Old World Realm type occur in Nova Scotia, coastal Maine, and southern New Brunswick. Eastern Americas Realm faunas, on the other hand, are present to the west and northwest in central and northern New Brunswick, northern Maine, eastern Quebec, and New Hampshire. Mississippian marine European Province faunas are present in Newfoundland, Nova Scotia, and coastal New Brunswick. By contrast, both North American and Southeastern Province faunas are known well to the west of Greater Acadia in the Southern Appalachians and the Mid-Continent region. Later Paleozoic European and North American nonmarine biota are all of Euramerian Province type; that is, there is no evidence for provincialism within the Northern Appalachians. These biogeographic data are of concern in tectonic analysis, particularly of the Acadian orogen, because belts yielding biogeographically similar organisms are unlikely to have been geographically remote from each other, and vice versa. Examples of boundary biogeographic mixing also indicate greater proximity—such examples are known in a few areas within the Northern Appalachians, particularly in eastern Quebec for the Early and Middle Devonian and in Nova Scotia for the Early Devonian. Early Devonian mixing is also present in northern Maine and adjacent New Brunswick. Boundary biogeographic mixing is also known in central Newfoundland and a few locales to the southwest in northern Maine and New Brunswick for the Middle and later Ordovician. The timing of the Acadian orogeny deduced from datable fossils preserved above and below the post-Acadian unconformity is Middle Devonian, with a Givetian date for the maximum being most likely. This statement applies to all parts of the Northern Appalachians except for Newfoundland, where datable beds overlying Acadian-deformed strata are scarce.

The Central Maine Terrane (CMT) includes the rocks that extend northeasterly from Connecticut to Maine and from the Monroe Fault on the west to the Campbell Hill-Nonesuch River Fault Zone on the east. A four-phase sequence of Acadian regional deformation is recognized for the CMT cover sequence. D 1 , the earliest phase, is characterized by F 1 nappes that have east or west vergence; the sense of vergence switches at the Central New Hampshire anticlinorium (CNHA). D 1 is also characterized by early, rarely observed, low-angle and “blind” T 1 thrust faults. The CNHA (or “dorsal zone”) is analogous to a “pop up” structure and is the likely root zone for both east- and west-verging Acadian D 1 thrust-nappes. D 2 is characterized by abundant F 2 tight to isoclinal, inclined to recumbent folds with northeast-trending axes and east-southeast vergence. Most of these folds face downward, a reflection of D 2 refolding the inverted limbs of D 1 structures, and these structures are identifiable chiefly in eastern New Hampshire. F 2 folds define a regional map-scale fold, the Lebanon antiformal syncline. During D 3 broad, open, upright to inclined F 3 folds with west- or northwest-trending axes were developed across the entire belt. F 3 map-scale syntaxial folds are well defined by the outcrop pattern of the metasedimentary rocks. D 4 , the last phase of deformation, is characterized by F 4 , tight to isoclinal, inclined folds with north-northeast-trending axes and east vergence and is restricted to the western part of the CMT. F 4 folds refolded the earlier structures and significantly modify the map pattern, tightening some of the earlier major structures in the CMT, for example the Kearsarge-Central Maine synclinorium. D 2 and D 4 are similarly oriented but spatially and temporally distinct. Deformation phases D 1 through D 4 are geographically restricted. This uneven distribution of structures is critical to correlations of deformation sequences across the orogen. Any local sequence of deformation in the CMT of central New Hampshire will commonly have only three of the four regional phases preserved in outcrop.

New insight into the nature of the Acadian orogeny in eastern Maine has been gained by combining detailed field studies in six lithotectonic belts with geochemical data from the igneous rocks of the region. Revised stratigraphies and deformation histories of the tracts reveal their sedimentological and structural evolution from Ordovician through Early Devonian times, and variations in the isotope geochemistry of the igneous rocks permit delineation of the basement blocks beneath the supracrustal belts. Combined, these results yield a model for plate interactions that followed Taconian deformation and culminated in the Acadian orogeny. Large basins (e.g., Aroostook-Matapedia, Central Maine) formed immediately after the Taconic orogeny on the recently accreted eastern margin of ancestral North America. These filled with thick clastic sequences derived from post-Taconian highlands during Late Ordovician through at least Middle Silurian times and characteristically preserve complex facies patterns at their margins. At the same time, sedimentation continued in the Fredericton Trough, inferred to be the only remaining oceanic crust in the region. This ocean basin separated the composite North American terrane from an equally complex Avalonian continent. Closing of this basin resulted in the Acadian orogeny. The onset of the Acadian suturing of Avalon to North America is indicated by a change from local basin filling to a more homogeneous blanket of sandstones whose deposition appears to have begun in the east (Flume Ridge Formation) and migrated westward. Collision of basement blocks led first to westward thrusting of parts of the Avalonian continent over the Fredericton belt. Later Acadian thrusting caused by final collision between Avalon and ancestral North America transported supracrustal Miramichi belt strata eastward over the Fredericton belt and parts of the Fredericton belt eastward over the western edge of the Avalonian allochthon. Acadian thrusting has displaced the original boundaries between supracrustal belts in southeastern Maine so that they no longer coincide with boundaries between the basement blocks that originally lay beneath them.

Regional deformation encountered in Ordovician rocks of the Ascot Complex and the Magog Group of the Dunnage zone is synchronous with the development of the La Guadeloupe fault. Structural analysis indicates that along most of the length of this fault, Silurian and Devonian rocks of the Saint-Francis Group were thrust over the Dunnage zone and its post-Ordovician cover sequence. Structural fabrics of these units are thus related to the Acadian Orogeny, and pre-Acadian deformations are of much less importance. Major Acadian faults in the northeastern part of the Québec Appalachians are dextral strike-slip faults, whereas they are largely thrust faults in its southwestern parts. The northern part of the Beauce area lies within the transition zone between these two fault regimes. Transected folds on both sides of the La Guadeloupe fault suggest a dextral oblique-slip tectonic transport. Northwest of the Magog Group, Upper Silurian rocks of the Cranbourne Formation unconformably overlie the Baie Verte-Brompton Line. The Cranbourne Formation is folded and cut by the same regional cleavage as the rocks below the unconformity. This demonstrates that the Acadian Orogeny is the major regional phase of deformation of all these units.

The Middle Paleozoic rocks deformed by the Acadian orogeny in the Gaspé Peninsula are divided into three major structural zones, from north to south: (1) the Connecticut Valley-Gaspé synclinorium, (2) the Aroostook-Percé anticlinorium, and (3) the Chaleurs Bay synclinorium. These three structural zones were part of a single depositional belt, the Gaspé Belt, located mainly to the south of the Baie Verte-Brompton Line over the Dunnage Zone. The Gaspé Belt comprises four broad temporal and lithological packages: (1) Upper Ordovician-lowermost Silurian deep water fine-grained siliciclastic and carbonate fades, (2) Silurian-lowermost Devonian shallow to deep shelf fades, (3) Lower Devonian mixed siliciclastic and carbonate fine-grained deep shelf and basin facies, and (4) upper Lower to Upper Devonian nearshore to terrestrial coarse-grained facies. Upper Ordovician to Middle Devonian rocks of the Gaspé Belt are bracketed between the Taconian and the Acadian unconformities, whereas the Salinic unconformity is well recorded in the Gaspé Belt sequence. Three shallowing-upward phases, separated by two transgressive episodes, are recorded by the sequence, the shallowing phases occurring more or less in response to three tectonic pulses: the Middle to Late Ordovician Taconian orogeny, the Late Silurian Salinic disturbance, and the Middle Devonian Acadian orogeny. The major structural trend of the Late Ordovician to Middle Devonian rocks of the Gaspé Belt is oriented roughly northeast. Major easterly striking dextral strike-slip faults (in the southeastern Aroostook-Percé anticlinorium) and northwesterly striking faults (in the northeastern Connecticut Valley-Gaspé synclinorium) transect this trend, whereas northeasterly striking high-angle reverse faults are present in the western and central regions. Two phases of folding are recorded in the Gaspé Belt. Northwest-southeast-striking F 1 folds recognized mainly in the Aroostook-Percé anticlinorium and synsedimentary faulting along the northwesterly trending faults of the northeastern Connecticut Valley-Gaspé synclinorium that affect the Silurian part of the sequence may be related to the Salinic disturbance. The northeast-southwest-striking F 2 folds correspond to the major Acadian structural trend of the Gaspé Peninsula. Structural features related to the D 2 deformation can be integrated in a model of strike-slip tectonics. Shortening of the cover rocks of the Gaspé Belt and transcurrent motion along the major easterly striking strike-slip faults are related to local transpression in the northeastern part of the Québec Reentrant during the continued continental convergence and accretion of outboard terranes to the North American craton in post-Middle Devonian and pre-Carboniferous time.

The climactic, middle Paleozoic event that affected the Newfoundland Appalachians has been referred to traditionally as Acadian orogeny. The latest Field studies and isotopic ages indicate that it began in the Early Silurian and continued into the Devonian. It affected the Newfoundland Humber, Dunnage, and Gander zones and western parts of the Avalon Zone. Intensities of the effects of the orogeny decrease westward across the Humber Zone (Appalachian miogeocline) and eastward across the onland Avalon Zone. Offshore, the wide Avalon Zone is virtually unaffected by Paleozoic deformation. The most intense regional metamorphism coincides mainly with the Gander Zone. Plutonism affected a wider area—from the eastern Humber Zone of White Bay to the western Avalon Zone of Placentia Bay. Deformation affected the widest area, from the Appalachian Structural front, which defines the western boundary of the Humber Zone, to the Avalon Peninsula of the western Avalon Zone. A change from marine to terrestrial conditions preceded the Silurian-Devonian deformation. Uninterrupted shallow marine conditions prevailed in bordering regions outside the Acadian deformed zone. The Acadian orogen spans the eastern portion of the Grenville lower crustal block that underlies the Humber Zone and western parts of the Dunnage Zone. It spans the Central lower crustal block that underlies eastern parts of the Dunnage Zone and the Gander Zone, and it spans the western part of the Avalon lower crustal block. Orogenic effects are most intense above the narrow Central lower crustal block and diminish outward across the margins of the opposing Grenville and Avalon lower crustal blocks. This spatial relationship between the surface orogen and lower crustal blocks implies that collisional interaction among lower crustal blocks controlled the tectonothermal effects of Acadian orogeny.

Western Newfoundland underwent widespread deformation and metamorphism following emplacement of the Taconian allochthons but prior to deposition of onlapping Carboniferous sediments. Stratigraphic and isotopic data indicate that this deformation and metamorphism took place during the latest Silurian to early Devonian Acadian orogeny. Acadian deformation is characterized by an overall pattern of west-directed thrusting. Grenville basement is thrust above its carbonate cover sequence, and both are locally thrust over the Taconian allochthons. Between the Long Range and Indian Head inliers, cover and basement are probably delaminated with a series of east-verging folds and thrusts developed above an inferred west-directed passive roof duplex in Grenville basement. Metamorphism accompanying deformation locally reaches upper greenschist to lower amphibolite facies. West Newfoundland lies at the Acadian orogenic front. The intensity of deformation and metamorphism decreases and dies out westward. Structural and topographic relief across the frontal zone resulted in gravitational collapse and the generation of a series of extensional structures. The basal detachment of west Newfoundland ophiolites truncates Acadian compressive structures and probably reflects late-Acadian extensional tectonics rather than initial Taconian emplacement.

The autochthonous Appalachian basin is located in western West Virginia, southern Ohio, eastern Tennessee, and eastern Kentucky. It is bounded on the east and the south by detachment fronts formed in the Alleghenian orogeny and to the west by the uplifts of the Cincinnati arch and Waverly arch. The Wallbridge unconformity of Early Devonian age corresponds to the boundary between the Tippecanoe and Kaskaskia sequences. It resulted primarily from a major eustatic drawdown but also marks the initial effects of the Acadian orogeny. Subsequent development of a foreland basin included deposits of the Needmore, Marcellus, and Mahantango shales. In the Middle Devonian, growth of this initial foreland basin was terminated by the regional Taghanic unconformity. The Taghanic unconformity was developed during a period of eustatic sea-level rise and is tectonic in origin, resulting from peripheral bulge reactivation during the Acadian orogeny. Development of a successor foreland basin followed, including deposition of the Genesee through the Ohio shales. Maximum Catskill delta progradation in the latest Devonian was followed by a period of postorogenic delta destruction, accompanied by glacio-eustatic sea-level drawdown. Recognition of two distinct foreland basins suggests that the Acadian orogeny outboard of the autochthonous Appalachian basin resulted from two distinct impacts on the continental margin. The first occurred in the Early Devonian and was relatively small. The second occurred in the Middle Devonian and was large. This interpretation requires a three-plate tectonic model. One possible interpretation would attribute the first impact to obduction of an Avalonian microcontinent and the second to collision with Gondwanaland.

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.