Conodonts and phosphatic problematica from the Cambro-Ordovician Cooks Brook and Middle Arm Point formations, Bay of Islands, western Newfoundland

Roy, Kaustuv (1989) Conodonts and phosphatic problematica from the Cambro-Ordovician Cooks Brook and Middle Arm Point formations, Bay of Islands, western Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Cooks Brook and Middle Arm Point formations, situated in Bay of Islands, western Newfoundland mainly consist of deep-water carbonates and shales ranging in age from middle Cambrian to early Ordovician. Previous studies have shown that these rocks were deposited as a "base-of-slope sediment apron", downslope from a carbonate platform. -- A total of 143 samples from five sections spanning the Cooks Brook and Middle Arm Point formations were collected for conodonts and other microfossils. Of these, 38 samples yielded identifiable conodonts and 19 samples yielded various types of phosphatic problematica. Systematic study of the conodonts have resulted in the identification of 68 species which are assignable to 36 genera. Three new unnamed genera have been described. -- The conodont fauna permits the recognition of six standard uppermost Cambrian-lowermost Ordovician conodont zones within the Cooks Brook Formation. These zones are: (i) Proconodontus tenuiserratus Zone, (ii) Proconodontus muelleri Zone, (iii) Eoconodontus Zone, (iv) Cordylodus proavus Zone, (v) Cordylodus angulatus Zone and the (vi) Loxodus bransoni Interval. -- The conodonts from the lower part of the Middle Arm Point Formation are assigned to Fauna D of Ethington and Clark (1971) while conodonts from the uppermost Middle Arm Point Formation are assigned to the Prioniodus elegans Zone. -- Based on the conodont fauna the age of middle and upper Cooks Brook Formation ranges from uppermost Franconian to middle(?) Tremadocian. The Middle Arm Point Formation, on the other hand, ranges in age from middle(?) Tremadocian to lower Arenigian. Within the Cooks Brook Formation, the Cambro-Ordovician Boundary can be placed either at the base of the Cordylodus proavus Zone or at the base of the Cordylodus angulatus Zone. -- The abundance and distribution of conodonts within the Cooks Brook and Middle Arm Point formations show strong environmental control and suggest that most of these conodonts were benthic or nektobenthic in habit. -- The phosphatic problematica recovered from the Cooks Brook and Middle Arm Point formations exhibit a variety of morphologies and have been divided into four broad, informal groups, namely (i) phosphatic plates and related microfossils, (ii) spherical microfossils, (iii) tubular microfossils, (iv) miscellaneous microfossils and (v) nauplius-like larvae. This large fauna, most of which is previously undescribed, have been described in detail using open nomenclature. Of the previously described forms, three new types (species?) of Anatolepis Bockelie and Fortey are described and the significance of this type of fossil in the earliest history of vertebrates is discussed. The presence of nauplius-like larvae in the Middle Arm Point Formation is of special interest as this is the first reported find of such fossils in North America. -- (KEY WORDS: Conodonta; Cambro-Ordovician; Bay of Islands; western Newfoundland; Cooks Brook Formation; Middle Arm Point Formation; Curling Group; Biostratigraphy; Cambro-Ordovician Boundary; Paleoecology; Taxonomy; Phosphatic problematica; Nauplius-like larvae).

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