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

Hunter, David Roy (1978) Conodonts from the Cobbs Arm Formation (Middle Ordovician) north-central Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

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 many of the conodont species, the conodont zones and subzones of the Middle Ordovician were difficult to accurately apply. -- Conodonts are found in abundance in the Cobbs Arm Formation and are represented by 39 multielement species and five residual forms including 8 new species and 2 new genera. The two new genera are described on the basis of the two new species Pseudobelodina n. sp. A. and a New Genus A n. sp. A. The fauna is dominantly of North Atlantic Province affinity yet the most abundantly represented species is more commonly found in the Midcontinent Province, i.e. Panderodus gracilis (Branson and Mehl). This species, along with Periodon aculeatus Hadding, Pygodus anserinus Lamont and Lindstrom, Baltoniodus variabilis (Bergstrom), Protopanderodus varicostatus (Swee and Bergstrom), and Pydogus serrus (Hadding) constitutes more than 75 percent of the total number of recovered conodont elements. Other important conodont species include: Baltoniodus prevariabilis-B variabilis transition, distacodus venustus (Stauffer), Drepanoistodus n. sp. A., Eoplacognathus lindstroemi (Hamar), E. robustus Bergstrom, Scalpellodus cavus (Webers), Strachanognathus parvus Rodes, Walliserodus ethingtoni (Fahraeus), Walliserodus nakholmensis (Hamar) and Walliserodus n. sp. A.

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