Stratigraphy, sedimentology and paleogeography of Mississippian Strata of the Bay St. George Subbasin, western Newfoundland

Knight, I. (Ian) (1983) Stratigraphy, sedimentology and paleogeography of Mississippian Strata of the Bay St. George Subbasin, western Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Bay St. George subbasin is one of two major depressions filled by Carboniferous sediments in insular Newfoundland. It is the northeast extension of the larger Maritimes Basin. It contains approximately 10 km of sediments ranging in age from Late Devonian to Late Carboniferous of which strata of the Anguille Group (Famennian - Tournaisian age) and the Codroy Group (Visean age) are described here. Both groups contain mostly nonmarine terrigenous clastic sediments with marine strata only within the Codroy Group. -- The subbasin is believed to have formed as a pull-apart trough, rather than as a simple rift, adjacent to, and west of, the northeasterly trending Long Range fault, a major strike-slip structure that is part of the Hercynian Cabot Fault system in western Newfoundland. Dextral, strike-slip movements began in Middle or Late Devonian time and ended in Early Carboniferous (middle Visean) time. Three basin-fills, each approximately 3000 m thick, filled the pull-apart sequentially so that the oldest fill now occurs in the southwest and the youngest fill in the northeast. The first two basin-fills of Famennian and Tournasian age (Anguille Group), were deposited within an elongate, 30 km wide trough which initially formed between divergent faults near the southern margin of the Precambrian Steel Mountain anorthosite and which with time enlarged southwestward. The third basin-fill is made up of middle Visean strata (basal part of the Codroy Group). By this time, the subbasin had broadened to 60 km, and was irregular in shape, consisting of several subsiding depressions separated by fault-bounded archs. Wrench movements ceased in middle Visean time and the subbasin was subsequently influenced by block faulting when sediments of the upper part of the Codroy Group were deposited. -- The Anguille Group comprises Famennian redbeds (Kennels Brook Formation), and Tournaisian deepwater lacustrine black shales and mudstones, and turbidite and deltaic sandstones (Snakes Bight Formation), gray, fluvial-deltaic sandstones and shales (Friars Cove Formation), and red braided stream sediments (Spout Falls Formation). Gray conglomerates of the Fischells conglomerate member of the Spout Falls Formation formed a local alluvial fan on the northwestern margin of the subbasin. -- The Codroy Group of middle to late Visean age consists of marine and nonmarine rocks that appear to overlie conformably the Anguille Group. The basal Ship Cove Formation is a thin, subtidal to intertidal, laminated limestone (Windsor subzone A). Subsequent marine sedimentation includes sulphate and chloride evaporites that accumulated in sabkhas and shallow salinas (Codroy Road Formation and lower Jeffrey's Village Member of the Robinsons River Formation - Windsor subzones A and lower B). Associated carbonates and fine grained, gray to red siliciclastic rocks of the Codroy Road Formation and Jeffrey's Village Member formed in shallow seas, in lagoons, on shorelines and rarely as bioherms. Some nonmarine redbeds are intercalated with the basal marine deposits, and increasingly dominate the upper part of the Jeffrey's Village Member and the overlying Highlands Member of the Robinsons River Formation (Windsor subzones B and C). These redbeds were laid down on playa flats, on coastal and alluvial plains and on alluvial fans. Younger strata of the Codroy Group are confined to the southwest of the subbasin. Flood plain and alluvial fan were deposited there together with minor lacustrine and shallow marine rocks of the Mollichignick and Overfall Brook Members of the Robinsons River Formation and deltaic rocks of the Woody Cape Formation (Windsor subzones D and E). -- Detritus for the groups was principally derived from uplifted Lower Paleozoic crystalline and volcanic terranes southeast and northeast of the subbasin. Lower Paleozoic platformal carbonates and quartzites and Taconic allochthonous ophiolite and flysch sequences were an important local source area for the Anguille Group northwest of the subbasin. -- Evaporites and calcretes suggest that the Carboniferous climate was dominantly semiarid. Aridity was most intense during the middle Visean but humidity increased during the late Visean.

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