Summary
Lower Ordovician (Arenig) rocks of the Cow Head Group in western Newfoundland represent a well preserved fragment of the continental slope of the early Paleozoic Iapetus Ocean. Cow Head Group strata are characterized by conglomerate beds intercalated with deep water carbonates and minor siliciclastics. Three megaconglomerate units (beds 10, 12 and 14) on Cow Head Peninsula with lateral and distal equivalents exposed at Lower Head, on Stearing Island, at St. Pauls Inlet, Western Brook Pond and Martin Point yield clasts and boulders from shelfedge and upper slope of the Iapetus Ocean. These clasts represent superbly preserved remains of the inner continental margin, a rarely preserved facies around the North American craton. This study comprises a documentation of carbonate lithofacies deduced from fragments of the shelf margin for the purpose of determining nature and evolution of the inner marginal environment through the Arenig interval. Among clasts in the megaconglomerates local (lower slope derived), foreign (shelf-, shelfbreak- and upper slope-derived) and older clasts were distinguished. Study of facies relationships and abundances of the different clast lithologies combined with conodont biostratigraphy enable reconstruction of the upper slope/shelfbreak sedimentary environment.
Bed 10 foreign clasts are interpreted as being derived from an incipiently drowned shelf margin with a low angle shelfbreak situated in relatively deep water. The foreign lithologies found in Bed 10 are interpreted as remains of mud banks and mounds formed with the aid ofGirvanella algae and lithistid sponges.
Foreign clasts in Bed 12 record a drowned shelf edge where muddy calcarenites were deposited during lower Bed 11-time and possibly exposed with shallowing of the water during upper Bed 11-time. Boundstones and massive wackestones developed from low diversity algal dominated types to high diversity buildups withPulchrilamina-like organisms and several algal species. Lumpy wackestones which encrouched upwards onto the upper slope and platform edge during the high sea level stand recorded in lower Bed 11 retreated with the regression during upper Bed 11-time.
In Bed 14 complex organic buildup facies accumulated at the shelfedge probably with steep slopes surrounded by coarse talus clastics. Shallower water on the shelf itself (Aguathuna Formation) forced continuous seaward growth of the shelfedge buildups which led to overloading and finally failure of an entire segment of the margin. The Bed 14 megaconglomerates record this episode with their high abundance of old clasts and huge boulder sizes. The event may be related to synsedimentary faulting on the platform recorded in the Aguathuna Formation.
About 300 conodont samples were collected principally from clasts of the three conglomerate horizons and processed. Faunal assemblages from clasts contained conodonts of North Atlantic and North American Midcontinent faunal aspects as well as endemic and cosmopolitan species.
Bed 10 clasts yielded conodonts of theParoistodus proteus andPrioniodus (P.) elegans zones of Balto-Scandia, together with faunal elements of faunas D and basal E of the Midcontinent zonal scheme.
Bed 12 clasts yielded older clasts with conodonts of thePrioniodus (O.) evae Zone and younger clasts with dominantlyPeriodon aculeatus together with faunal elements typically found in Fauna E-1 (e.g.Protoprioniodus aranda) of the Midcontinent faunal scheme.
Boulders from Bed 14 also produced abundantP. aculeatus and the occurrence ofTripodus laevis andPteracontiodus cryptodens suggests basal Whiterock age.
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Pohler, S.M.L., James, N.P. Reconstruction of a lower/middle ordovician carbonate shelfmargin: Cow Head Group, Western Newfoundland. Facies 21, 189–261 (1989). https://doi.org/10.1007/BF02536836
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DOI: https://doi.org/10.1007/BF02536836