The sedimentology of the Late Precambrian Rencontre Formation, Fortune Bay, Newfoundland

Smith, Simon Andrew (1983) The sedimentology of the Late Precambrian Rencontre Formation, Fortune Bay, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Rencontre Formation is a sequence of Late Precambrian sediments outcropping around Fortune Bay in southeastern Newfoundland. The formation is variable, consisting of red conglomerates, coarse sandstones, red siltstones, with subordinate pale sandstones, and gray-green siltstones with fine sandstones. The exact thickness of the formation is uncertain as a complete section is not seen at any one locality, but the maximum possible thickness is in the order of 1500m. -- The Rencontre Formation is divided into facies associations on the basis of lithology and sedimentary structures. Five of these have been recognised in northern Fortune Bay, and four others on Brunette Island in the south. These associations reflect the vertical and lateral variability of the Rencontre Formation. -- The formation consists of fan delta, braided river, low energy marine, marginal marine, and tidal-marine deposits. In certain cases, there are no closely analogous modern environments with which to compare these sediments, in particular the basal 300m in the north of Fortune Bay. These are the deposits of fan deltas, but show a poor distinction between delta plain and delta front environments. -- A palaeogeographic model is developed in which deposition occurred in a northeast to southwest trending basin. Initially this was small, probably fault-bounded, with a marine connection to the northeast. Basin growth to the west and southwest is implied by the progressive overstepping of older deposits by younger strata, and led to a elongate basin with marine connections to the northeast and southwest. The basin history was complicated with alternating tectonic activity, quiescence, and sea-level changes. The upper part of the sequence signals a change from deposition in a narrow, elongate, basin, a motif characteristic of late Precambrian sedimentation in the Avalon terrane, to one of stable cratonic-type sedimentation, with broad depocentres, a motif characteristic of Early Palaeozoic sedimentation in the Avalon terrane.

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