Systematics and biostratigraphy of halobiid bivalves from the Martin Bridge Formation (Upper Triassic), northeast Oregon

Abstract

A diverseHalobiafauna with rare ammonoids and conodonts is described from the type Martin Bridge Formation (Upper Triassic) of northeast Oregon. The composite section of black shale, mudstone, bedded micrite, limestone conglomerate, and halobiid lumachelle spans the Carnian–Norian boundary without apparent sedimentologic break or pronounced biotic extinction. The occurrence ofHalobia radiata radiataGemmellaro,Halobia? cf.teltschenensis(Kittl),H. superba superbaMojsisovics, andH. superba ornatissimaSmith (nomen translatum) with the ammonoidsDiscotropitessp. andArietoceltites? sp. low in the section indicates a late Carnian age (Dilleri?, Welleri to Macrolobatus Zones). These fossils occur below beds containing early Norian (Kerri Zone)H. beyrichi(Mojsisovics) and middle Norian (Rutherfordi?, Columbianus Zone)H. haloricaMojsisovics. The described fauna increases the halobiid and ammonoid diversity previously known from the Martin Bridge Formation despite synonymy of species previously reported from the section.