Conodonts from the Cobbs Arm Formation (Middle Ordovician) north-central Newfoundland

• Main Author: Hunter, David Roy
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 1978
• Subjects: New World Island; Cobbs Arm; Newfoundland
• Online Access: https://research.library.mun.ca/6842/; https://research.library.mun.ca/6842/1/DavidRoyHunter.pdf; https://research.library.mun.ca/6842/3/DavidRoyHunter.pdf

On New World Island, north central Newfoundland, the Middle Ordovician Cobbs Arm Formation constitutes only a small proportion of the dominantly volcanic and volcaniclastic strata of the island are terrains characteristic of the Dunnage Zone. The Cobbs Arm Formation is predominantly composed of limestones that are dark grey in colour when mixed with volcanic detritus and light grey and more coarsely crystalline when purely carbonate. The purer carbonate accumulations of the formation would appear to have been deposited in an environment free of terrigenous detritus between emergent volcanics and shallow water volcanic sandstones to the north, and trench deposits to the south. The result was a carbonate grainstone barrier that subdivided parts of the Middle Ordovician coastline into a landward restricted lagoonal environment and an oceanward open marine environment. -- Because of the presently deformed nature of the New World Island rocks, several small incomplete sections of the Cobbs Arm Formation were measured and sampled. Detailed investigation of the lithologies of the samples revealed a sedimentological pattern indicative of a slightly erratic transgressive phase of deposition. It is possible that the emergence of the arc terrains, which preceded the deposition of the Cobs Arm Formation, and the subsequent subsidence of the island arc terrains, which accompanied and succeeded Cobbs Arm Formation deposition, was the fore and aftermath of a ridge-trench interaction. The transgression resulted in a landward shift of oceanward facies, now recorded in the verticle sequences of the Cobbs Arm Formation. -- The conodont fauna of the Cobbs Arm Formation appears to be divisible into two species associations. One relates well with sediments of the proposed restricted environment and the other relates well with sediments of a more open marine environment. On a finer scale, there appears to be a relationship between substratum characteristics and certain conodont species. Because of this apparent ecological control on ...