Stratigraphy and sedimentology of Middle Cambrian to Lower Ordovician shallow water carbonate rocks, western Newfoundland

Levesque, Rene Joseph (1977) Stratigraphy and sedimentology of Middle Cambrian to Lower Ordovician shallow water carbonate rocks, western Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Detailed lithostratigraphic analysis and new fossil data have resulted in a revised and refined stratigraphic and sedimentologic framework for the autochthonous, Middle Cambrian to Lower Ordovician, shallow water sedimentary sequence in western Newfoundland. Study of five separate localities, spanning a distance of 285 km., indicates that much more of this 900 metre thick sequence Is of Cambrian age and much Less of Ordovician age than previously reported. -- The oldest units studied are of late Lower Cambrian age and can be recognized in three of the five localities where they are named the Degras Formation (at the Port-au-Port Peninsula), the Penguin Cove Formation (at Goose Arm), and the Hawke Bay Formation (at Bonne Bay). These thick-bedded, supermature, quartzose sandstones are interpreted to have formed as a system of barrier bars or beaches. -- An overlying succession of distinctive. Middle and Upper Cambrian, limestones, dolostones, and shales is recognized from Port-au-Port in the south to Hawkes Bay in the north. This succession is variously known as the March Point (revised) and overlying Petit Jardin (revised) Formations at Port-au-Port, the Wolf Brook (proposed) and overlying Blue Cliff (proposed) Formations at Goose Arm, the South Head (proposed) and overlying East Arm (revised) Formations at Bonne Bay, and the upper Hawke Bay Formation at Hawkes Bay. -- Middle and Upper Cambrian rocks are characterized by conspicuously cyclic, "high-energy" lithofacies and comprise two large scale sequences which repeat, in vertical succession, as many as three times: (I) thin-bedded sequences composed of flaser bedded limestone and shale intercalated with occasional beds of edgewise conglomerate, stromatolites, and oolite, interpreted as a mixed carbonate-sillciclastlc tidal flat and (2) thick-bedded sequences composed of intraformational conglomerate, in places oncolitic, cross-bedded oolite, and laminated, mud cracked calcilutite, interpreted as carbonate sand shoals or barrier islands. -- The succeeding Lower Ordovician St. George Formation (revised) is recognized along the length of western Newfoundland and is divided into three members: the lower cyclic member, of interbedded limestone and dolostone, the middle limestone member, a thick limestone unit locally overprinted by epigenetic dolomitization, and the upper cyclic member, of interbedded limestone and dolostone. Thickness of the St. George is about 550 metres, greatly reduced from previous estimates. -- A disconformity that dies out to the north separates the St. George from the overlying basal limestone of the Middle Ordovician Table Head Formation. -- Lower Ordovician lithofacies are characterized by "low energy" subtidal features, with few diagnostic intertidal features, and comprise two megarhythms: (1) carbonate cycles grading from burrowed, fossiliferous, subtidal limestone to microcrystalline, laminated, occasionally mud cracked, supratidal dolostone, interpreted as shoaling upward cycles on a protected tidal flat and (2) burrowed, fossiliferous, hackly weathering, subtidal limestone, representing deposition in a protected lagoonal environment. -- These rocks record a major marine transgression and may reflect a change in the form of the shallow water continental margin from a ramp or open shelf in the Cambrian to a mound-rimmed carbonate platform in the Ordovician. Superimposed on this major transgression are as many as five smaller transgressive/regressive events.

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