Early Ordovician (Arenig) conodonts from St. Pauls Inlet and Martin Point, Cow Head Group, Western Newfoundland

Johnston, David Ian (1986) Early Ordovician (Arenig) conodonts from St. Pauls Inlet and Martin Point, Cow Head Group, Western Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Middle Cambrian to Middle Ordovician Cow Head Group, Newfoundland, was deposited at the toe-of-slope and continental rise of a passive margin adjacent to a carbonate platform. Early Ordovician (Arenig) strata are characterized by massive megaconglomerates interbedded with deep-water carbonates and siliciclastics and minor conglomerate. The St. Pauls Inlet and Martin Point sections are interpreted as intermediate and distal facies, respectively, of this sequence. Arenig strata in these and in other Cow Head sections of the St. Pauls Member of the Green Point Formation record the transition from a tectonically passive to a tectonically active margin. In the Middle Ordovician these rocks were thrust over coeval platform strata. -- Previous conodont investigations focused entirely on sections on the Cow Head Peninsula. This study is an investigation of the biostratigraphy, paleoecology, and the taxonomy of Arenig conodont faunas from bedded strata of both the St. Pauls Inlet and Martin Point sections.- Each section was measured and sampled intensively, with 3.5 kg. samples taken at approximately 1 m intervals. The majority of processed samples were limestones which were dissolved in 10-15% acetic acid, and then sieved, separated, and picked for conodonts. A total of 35.659 conodont elements were recovered from 64 limestone and shale samples. -- Apparatuses of Lenodus Sergeeva, Oistodella Bradshaw, and Spinodus Dzik are described. The occurrence of Oepikodus communis Ethington and Clark below the first occurrence of O. evae (Lindstrom) supports the hypothesis that these species belong to separate lineages. Morphologic evidence from Cow Head material supports the assignment of elements resembling 'Scolopodus' gracilis Ethington and Clark s.f. to monoelemental species of Parapanderodus Stouge. Periodon flabellum evolves gradually into Periodon aculeatus, with early, intermediate, and late forms of the latter species recognized. The triangulariform element of Protoprioniodus papiliosus van Wamel is recognized as belonging in the apparatus of this species in Cow Head collections. Elements representing an intermediate evolutionary stage of 'Scolopodus' peselephantis Lindstrom are recognized. A new species is described: Polonodus? peavyl n. sp.. -- Five faunal assemblages (A1-A5) are recognized in Arenig rocks at St. Pauls Inlet. An additional late Arenig fauna was recovered from a limestone clast in the Lower Head Formation overlying the Cow Head Group at Martin Point. The early to middle Arenig North Atlantic Province Paroistodus proteus, Prioniodus elegans, and Oepikodus evae zones are recognized in Beds 9-11 at St. Pauls Inlet. In these same beds North American Midcontinent Fauna D and the Oepikodus communis Zone are recognized. Fauna 2 is recognized in Beds 13-15. Beds 9-12 are dated using conodonts as uppermost Canadian and Beds 12-15 as Whiterockian. The Arenig-Llanvirn boundary may lie in uppermost Cow Head strata at St. Pauls Inlet. Arenig strata at St. Pauls Inlet can be correlated with those on the adjacent platform and equivalent sequences around the North American craton and in Argentina and Australia. The bases of the Tetragraptus approximatus and Paroistodus proteus zones coincide in both sections. Conodont-based correlations at St. Pauls Inlet support correlations based on macrofossil evidence. -- The size and robustness of conodont elements in bedded carbonates was found to be correlative with grain size. The abrupt turnover from Oepikodus-dominated to Periodon-dominated faunas is documented in Bed 11 at St. Pauls Inlet. Significant changes in paleoecological patterns and major influxes and extinctions of taxa commonly coincide with each other and with major lithologic changes representing the onset or termination of aerobic, condensed and anaerobic phases in Arenig Cow Head strata. The predominance of deep water taxa in Beds 9, 10, and the lower part of 11, and the predominance of shallower water taxa in upper Bed 11, and Beds 12-15 support the interpretation that lower to middle Arenig Cow Head strata at St. Pauls Inlet are transgressive and middle to upper Arenig strata are regressive.

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