The lower Paleozoic rocks that extend from northwestern Newfoundland, through the Gaspé Peninsula, the south shore of the St. Lawrence River as far west as Quebec City, the Champlain Valley, western New England, eastern New York, and north-central New Jersey to southeastern Pennsylvania were deformed markedly by the Ordovician Taconic orogeny. This belt is bordered to the north and west by the little-disturbed foreland; in Canada the boundary includes Logan’s Line. To the south and east, the identity of this belt is lost in rocks that have been more severely deformed and metamorphosed by later, principally Acadian, orogeny. Rocks of this identifiable Taconic orogenic belt are here termed the Taconides.
During Cambrian and much of Early Ordovician time, sedimentation within the Taconide belt was arranged in parallel zones: to the northwest, on the craton, was a shelf environment of shallow subtidal, intertidal, and supratidal carbonate deposition. Southeast of this shelf across a steep slope and an abrupt facies change, that probably reflects a sharp increase in water depth, was an area of clastic sedimentation; between these two zones was a zone of carbonate-clast slump conglomerates. The zone of clastic sediments, called the transitional zone, in turn passed seaward into one of typical eugeosynclinal sedimentation where the rocks are poor in carbonate, but rich in volcanic components.
Another major facies change is preserved within the area of the shelf sequence deposition. Depending on the geographic location, this second facies change is late Early Ordovician to Middle Ordovician in age; it is marked by the regional unconformable overlap of a syntectonic black-mantling shale sequence on the carbonate rocks of the shelf sequence and older units.
The shelf and mantling shale sequences are divisible into two tectonic zones: a foreland to the northwest, and a zone of deformed autochthonous rocks to the southeast, in which the intensity of deformation and metamorphism increases to the southeast. Rocks of the transitional sedimentary facies are even more intensely deformed, and over wide areas have been moved bodily northward and westward for long distances. These moved rocks are preserved in three forms: (1) klippe, now surrounded entirely by rocks of the shelf or mantling shale sequence across structural contacts; (2) allochthonous rocks only partly surrounded by rocks of the shelf or mantling shale sequence across contacts; (3) allochthons now eroded to a mere structural stump, but whose former extension and large movement are recorded by distinctive syntectonic sedimentary rocks. To the first category belong the Hare Bay and Humber Arm klippen of Newfoundland, small klippen in the Quebec City area, the Taconic klippe in New England and New York, and small scattered areas of allochthonous sedimentary rocks as well as a large klippe of Precambrian crystalline rocks in New Jersey and Pennsylvania. To the second category belong the rocks immediately southeast of Logan’s Line, from the tip of Gaspé Peninsula to the Vermont-Quebec border. To the third category belong the rocks of the Hinesburg thrust in northern Vermont, where large-scale Ordovician movement at the surface level is recorded in the wildflysch-type sedimentary rocks exposed along the Lake Champlain shore.
South and east of the zone of allochthons and of the deformed shelf sequence is a zone of structurally high ground, part of the axial region of the composite Berkshire–Green Mountain–Sutton Mountain–Notre Dame Mountain–Shickshock Range–Indian Head Range–northern Long Range anti-clinoria. Where the structural relief is especially great or where epeirogenic uplift has caused erosion to reach sufficient depth, the zone is marked by Precambrian basement rocks; these Precambrian rocks approximately mark the southeast limit of basement rocks of 1 (±) b.y. (billion years) age and appear to be the edge of a lower Paleozoic craton.
Coincident with the zone of structural highs is a zone of Bouguer gravity highs; the coincidence extends from northwestern Newfoundland through the Gulf of St. Lawrence, as far south as the north end of the Berkshire massif. From here south to Long Island Sound, the gravity ridge is displaced east of the structural ridge. It passes through the Coastal Plain deposits and reappears in the area of the Glenarm Series in Maryland. Northwest of the gravity ridge is a coextensive zone of Bouguer gravity troughs. The troughs follow the zone of deformed shelf sequence; where the allochthons occur, the troughs coincide with these features.
Lower to Middle Ordovician ultramafic rocks occur near the western boundary of the eugeosynclinal facies, in a narrow belt parallel with and just east of the zone of structural highs. From Gaspé Peninsula southwest, these ultramafic rocks are apparently strictly intrusive. In Newfoundland, however, intrusive ultramafic bodies may be genetically related to an apparently extrusive ultramafic-mafic ophiolite complex preserved in the allochthons.
I suggest that the process leading to the locations of the structural, igneous, and gravity features was the interaction of an oceanic segment of the crust with the adjoining craton. The location of this junction of crustal segments originally determined the location of the sedimentary facies junction between the shelf and basin sequences; compressive plunging of the oceanic crust under the craton caused rafting of the lighter cratonal margin, thus accounting for the structural uplift of the outermost (southeasternmost) known belt of 1-b.y.-old Precambrian basement rocks through much of the length of the Taconides. Farther into the craton (west and north), the compressive forces caused a gentle downwarp of the crust, leading to the subsidence of the former shelf area and, therefore, to a bathymetric reversal. The reversal allowed a black mantling shale sequence, whose sediments were derived in large part from the uplifted cratonal margin to the east, to be deposited over the former shelf area; continued uplift of the cratonal margin and subsidence of the former shelf area led eventually to wholesale emplacement of allochthons by gravity sliding of rocks of the transitional facies off the uplifted cratonal margin into the basin that was the former shelf area.
Continued compression in the Taconides after the initial submarine gravity sliding led to northwestward thrusting of consolidated rocks, including Precambrian crystalline rocks of the uplifted cratonal margin, in a more deep-seated environment, probably in Late Ordovician or Early Silurian time. Regional metamorphism accompanied this last stage of diastrophism.
The underthrusting of oceanic crust, in a process that probably involved the upper mantle as well, was accompanied by intrusion of the ultramafic bodies and the extrusion of siliceous, mafic, and ultramafic rocks on the surface. These igneous rocks are preserved today mainly in the eugeosynclinal sequence formerly deposited on the oceanic crust, but they are found also among the gravity slides and as volcanic ash in the shelf sequence.
The addition of a mass of relatively dense oceanic material under the margin of the craton, as well as the concomitant introduction of dense intrusive rocks, resulted in the belt of positive Bouguer gravity anomalies. Where the gravity ridge is southeast of rather than coinciding with the belt of Precambrian rocks in the Taconide zone, the Precambrian rocks have undergone large lateral transport toward the craton.
Compared to the Taconic orogeny, the Acadian orogeny in the northern Appalachian region was of wider regional extent, developed larger systems of nappes, led to more intense regional metamorphism, and was accompanied by larger scale plutonism. Despite these facts, however, the Taconic orogeny appears to have defined structural trends in the lower Paleozoic rocks that effectively controlled structural evolution of the northern Appalachian orogen during later Paleozoic orogenies, including the Acadian orogeny.
