Stratigraphy and Depositional History of Late Precambrian Coelenterate-Bearing Rocks, Southeastern Newfoundland

摘要

In the map area, the Conception Group and the overlying St. John's Formation of the Cabot Group constitute a continuous, conformable succession of Precambrian sedimentary rocks about 8000 ft thick, beneath an overburden of glacial drift; the base of the sequence is unexposed. Varied rocks of the Conception Group are predominantly green and purple and the St. John's shales are various shades of gray. The Conception Group is divisible into three lithologic units: Drook, Freshwater Point, and Cape Cove Formations, in order of decreasing age. The Drook Formation is composed mainly of chert, siliceous argillite, and siltstone; the Freshwater Point Formation is siliceous argillite with minor sandstone; and the Cape Cove Formation is composed of cyclic beds that grade from graywacke at the bottoms through siltstone to argillite at the tops. The Cape Cove Formation contains, in its upper part, imprints of soft-bodied coelenterates represented by polyps as well as Medusae. These constitute a newly described fauna, the environment of which can be inferred from the sediments that enclose them. During late Precambrian time, deposition of the Conception Group began in isolated basins that subsequently joined to form a shallow marine environment. The sea continued to deepen and became deepest during deposition of the middle part of the Cape Cove Formation. Turbidity currents played an important role in the deposition of the Conception Group and the St. John's Formation, and reached a maximum during deposition of the Cape Cove Formation. After a substantial thickness of the Conception Group had been deposited, the sea became shallow again, perhaps during deposition of the uppermost part of the Cape Cove Formation, and remained shallow during post-Conception times. During deposition of the St. John's Formation, the intensity of turbidity currents decreased. Also, mild volcanism, here reported for the first time, occurred during early St. John's time.