STRATIGRAPHIC LEAKAGE INTO THE LOWEST MARINE BEDS OF THE LOWER CODROY GROUP (MIDDLE MISSISSIPPIAN), PORT AU PORT PENINSULA, SW NEWFOUNDLAND, CANADA

The Diplognathodus Biozone, characterized by the conodonts Diplognathodus and Clydagnathus n. spp., occurs in correlative carbonate mound and laminite facies of the basal lower Codroy Group of SW Newfoundland. However, at four localities on the Port au Port Peninsula, thin marine beds below the mound facies contain, in addition to conodonts of the Diplognathodus Zone, the conodonts Gnathodus girtyi, ?Mestognathus sp., and Vogelgnathus spp. and the brachiopods Martinia spp.

The occurrence of these taxa in the lower Codroy Group is anomalous because elsewhere, the brachiopods in particular, identify younger strata. The four occurrences have since the 1930s, been mostly regarded as secondarily emplaced. However, more recently, lower Codroy Group Martinia spp. were interpreted as contemporaneous, being characterized as unreliable guide fossils. Subsequent interpretations, accompanying a hypothesis of hydrothermal vent origin for the overlying mound facies, also argued for primary emplacement for Martinia spp. and the sediment in which they occur.

A primary origin, one not involving stratigraphic leakage, is difficult to sustain for these basal sediments and their contained fossils. The lowest marine beds directly overlie and infill a well-developed L. Dev.- E. Miss. paleokarst that includes extensive fissure and cave fillings, features that likely relate to a secondary origin for the Martinia beds and their contained fossils.

The lower Codroy Group was recently correlated with the Late Chadian substage of Britain and Ireland using palynoflora. The fact that the conodonts Gnathodus girtyi and Vogelgnathus spp. are unknown from the Chadian, Gnathodus girtyi first appears in the late Early Asbian, and Vogelgnathus spp. first appear in the Late Arundian suggests these conodonts were emplaced secondarily.

Karstification of lower Codroy Goup evaporites, and the L. Chadian to L. Asbian ‘age-spread' between the two lowest conodont zones, suggest a erosional hiatus in the lower Codroy Group, similar to one postulated in Nova Scotia. Thus, there was likely time for the creation of a Middle Mississippian ‘plumbing' system, for the emplacement of Brigantian sediment and fossils into pre-Mississippian karst below the basal lower Codroy Group. Martinia spp. either inhabited this ‘plumbing' system, or was washed into it.