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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.

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