Geology of the Corner Brook Lake area, western Newfoundland

Abstract

The Corner Brook Lake area, located at the eastern margin of the Humber zone in central western Newfoundland, was mapped on a reconnaissance scale in order to sketch the salient features of its geology, which until now has been very poorly understood. -- The area is underlain mainly by a metamorphosed and deformed sedimentary cover sequence deposited during Hadrynian-Ordovician time on a Grenvillian basement complex, part of which outcrops in the area. The cover consists of basal clastic rocks, which underlie the eastern half of the map area, stratigraphically overlain by carbonate rocks, which occupy the western half of the area and are continuous with the extensive Cambro-Ordovician carbonate bank sequence of western Newfoundland. The area also includes allochthonous Cambro-Ordovician clastic and ophiolite-derived rocks, a small Siluro-Devonian granitoid pluton, and Carboniferous clastic rocks. -- Five distinct deformation events (D1-D5) are recognized, representing the effects of three regional orogenic events - the Middle Ordovician Taconic (D1/D2), the Devonian Acadian (D3) and the Carboniferous to Permian Alleghenian (D4/D5) orogenies. The intensity of deformation decreased after a peak during the Taconic, and a single, lower amphibolite facies metamorphic peak reached during the Taconic and Taconic-Acadian interkinematic Interval was followed during the Acadian by lower greenschist facies conditions. Three major east-dipping thrust faults are delineated, and their long histories, involving initiation during Taconic and subsequent reactivations during Acadian and Alleghenian orogenies, are outlined. The thrusts superpose the highly deformed and metamorphosed basement and lowermost cover rocks on the Cambro-Ordovician carbonate sequence, and in combination with west-verging folds account for the general westward, tectonic transport in the area. -- The tectono-stratigraphic features of the area clearly reflect the Hadrynian-Ordovician construction and Middle Ordovician and later destruction of the ancient continental margin of eastern North America, the Number zone. -- The more significant results of this study include: delineation of the stratigraphic and structural features of the eastern part of the area; firm establishment of the Grand Lake Brook group as part of the Cambro-Ordovician carbonate sequence, and its recognition as a stratigraphic and structural link across the major thrust zones with the Hadrynian-Cambrian metaclastic sequence in the eastern part of the area; identification of the earliest deformation events (D1/D2) in both the carbonate and metaclastic terranes and the correlation of these events with the Taconic Orogeny, implying the earliest post-Grenvllle deformation in this part of the Humber zone was due to Taconic Orogeny in Middle Ordovician time.