Middle Triassic biostromes from the Liard Formation, British Columbia, Canada: oldest examples from the Mesozoic of NW Pangea

Abstract

Mesozoic carbonate accumulations are rare in northwestern Pangea. This is due, initially to the demise of many carbonate-secreting organisms during the terminal Permian biotic crisis and subsequently, to increased siliciclastic sedimentation resulting from the tectonic development of western North America. Ecological voids resulting from the extinction, however, resulted in new opportunities for surviving taxa. Terebratulid brachiopod-echinoderm dominated biostromes within the primarily siliciclastic Liard Formation (Middle Triassic, Ladinian) of northeastern British Columbia provide a unique example. The Liard biostromes are composed primarily of whole and unabraded terebratulid brachiopods ( Aulacothyroides liardensis ), articulate crinoid elements ( Isocrinus sp.) and cidaroid echinoid spines and interambulacral plates ( Miocidaris sp.). Many of the terebratulid brachiopods are characterized by the pedicle attachment trace fossil Podichnus . Other organisms which occupied the biostromes include discinid (cf. Orbiculoidea sp.) and spiriferid ( Spiriferina abichi ) brachiopods, bivalves (including oysters), and gastropods, as well as rare bryozoans and possibly scleractinian corals. Sedimentary units deposited on the margins of the biostromes are characterized by an abundance of body fossils (decapod crustaceans, bivalves and ophiuroids) and trace fossils ( Asterosoma , Cylindrichnus , Palaeophycus , Rosselia , Teichichnus and Thalassinoides ). This trace fossil assemblage contains a mix of dwelling traces, horizontal deposit feeding traces, and vertical suspension feeding traces, consistent with deposition in a lower shoreface setting. Hiatal surfaces and laterally restricted bioclastic sandstone layers and lenses within these features suggest that frequent turnover of infaunal and epifaunal communities occurred at the edges of the Liard brachiopod-echinoderm biostromes. Development of the Liard brachiopod-echinoderm biostromes depended on initial storm-induced concentration of shell material to create patches or zones of stable substrate, promoting colonization by terebratulid brachiopods, discinid brachiopods, crinoids and cidaroid echinoids.