Abstract

Diverse conodont faunas recovered from the Grog Brook Group in northwestern New Brunswick indicate a Late Ordovician, probably Gamachian, age. The conodonts are of mixed provincial affinity including components of the North Atlantic Province (e.g., Hamarodus, Icriodella, Periodon, and Protopanderodus) and taxa representing shallow (e.g., Rhipidognathus) to deeper water environments (e.g., Phragmodus) in the Midcontinent Province. Elements of Amorphognathus ordovicicus Branson and Mehl numerically dominate the faunas that together with sparse representatives of Gamachignathus ensifer McCracken, Nowlan and Barnes suggest a latest Ordovician age. The beds from which the conodont faunas have been recovered are interpreted as distal debris flows that originated at the basin margin and brought Midcontinent Province conodonts down the slope to mix with indigenous North Atlantic Province faunas. The faunas are correlative with those from the Matapedia Group (previously thought to overlie the Grog Brook Group) and their occurrence suggests at least partial lateral equivalence of the two units.The conodonts recovered are only slightly thermally altered. Two alternative hypotheses are proposed to explain the juxtaposition of relatively unaltered conodonts of the Grog Brook Group and highly altered forms from the Matapedia Group. The first suggests that thrust faulting took place in the Late Ordovician – Early Silurian and that this process and later normal faulting account for the unusual distribution of conodont colour alteration. The second possibility is that the strata of the Grog Brook Group in the section examined were deposited on a structural high and overlain by little or no sediment of the Matapedia Group; however, such a structural high must have had access to a source of Midcontinent Province conodonts. Acadian thrusting then brought higher grade Matapedia Group strata into contact with this part of the Grog Brook Group.

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