Thick, relatively complete successions of deformed Carboniferous strata are present in a narrow (16 mi maximum width) elongate zone trending from Cape Anguille on the Cabot Strait northeast to White Bay on the east side of the Great Northern Peninsula. Patches of thin, relatively undeformed Carboniferous strata are found west of this zone between Stephenville Crossing and Cape St. George, and east of this zone, bordering Red Indian Lake. Sedimentary facies within the zone indicate that the southeastern margin of the narrow zone was a major high-angle fault periodically active during Carboniferous time.

The oldest post-Acadian strata in western Newfoundland (Anguille Group, Early Mississippian age) are thickest in the deformed zone. They are more deformed than younger strata there, and locally reach a slate grade of metamorphism. Strata of Late Mississippian age (Codroy Group and lower part of the Deer Lake Group) are present within and outside of the deformed zone. The Codroy Group is present only in southwestern Newfoundland and the Port au Port Peninsula, where it contains the only proved marine units in the Carboniferous of Newfoundland. The Deer Lake Group (Late Mississippian and Early Pennsylvanian) is present only in the northeastern part of the deformed zone, where it unconformably overlies the Anguille Group and adjacent pre-Carboniferous basement. The Upper Codroy, Deer Lak , and Barachois Groups show fanglomerate facies nearest to, and finer grained facies farthest from, the Long Range and Grand Lake faults that bound the deformed zone on the southeast.

A period of deformation occurred during either late Hortonian (Early Mississippian) or late Windsorian (Late Mississippian) time in north-central Newfoundland, because early Cansoan strata (Deer Lake Group) unconformably overlap both the basement and the Anguille strata and have been deformed only mildly. These movements are reflected in the fanglomerate facies (Upper Codroy and Barachois Groups) next to the Long Range fault in southwestern Newfoundland.

Two areas of pre-Acadian mafic plutons surrounded by lower Paleozoic metasedimentary and felsic intrusive complexes have been recognized within the basement rocks on different sides of the Long Range fault in southwestern Newfoundland. These mafic areas may represent a single region split into two parts by dextral strike-slip movement of approximately 60 mi. The movement would have been post-Middle Devonian (age of Acadian granite cut by the faults) and partly pre-Cansoan (age of the Deer Lake Group that overlaps the northernmost fault). This hypothesis is currently under investigation.

In County Mayo, western Ireland, Visean (approximately Windsorian) flat-lying limestone beds unconformably overlie structurally deformed pre-Carboniferous basement and Tournaisian and Late Devonian nonmarine redbeds. These pre-Visean strata were deposited in a basin-and-range environment; the Ox Mountains represent an upthrown fault block that formed alluvial fans on the northwest. Early Devonian and Silurian redbeds in the west of Ireland apparently are not related to a fault-block framework of deposition.

The Tournaisian and/or Late Devonian strata of western County Mayo are entirely of fluvial and alluvial-fan origin and contrast with the section on the northeast in Northern Ireland (Ulster), where Tournaisian lacustrine and/or lagoonal (cementstone) facies are present above Late Devonian fluvial redbeds and below marine Visean limestone beds. These lacustrine and/or lagoonal facies persist northeastward into the Midland Valley of Scotland, and are interpreted to be strata developed in a rift valley. Facies similar to those in Northern Ireland and southern Scotland were developed in the Carboniferous rift valley of eastern Canada. The similarity of facies, tectonic framework, and age of rifting in eastern Canada and the northern British Isles are entirely consistent with the concept t at the two continents once were joined and were split apart some time after the Carboniferous.