Terebratulid-echinoderm biostromes.

Abstract ref. 1836:
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 clastic 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 an example of this.  The Liard biostromes are composed primarily of whole and unabraded terebratulid brachiopods, articulate crinoid elements and cidaroid echinoid spines and interambulacral plates. Many of the terebratulid brachiopods are characterized by the pedicle attachment trace fossil Podichnus. Other organisms which occupied the biostromes include acrotretid and spiriferid brachiopods, bivalves, and gastropods, as well as rare bryozoans, oysters 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 (i.e. Asterosoma, Cylindrichnus, Palaeophycus, Rosselia, Teichichnus and Thalassinoides). This trace fossil assemblage contains a mix of horizontal and vertical deposit feeding organisms, consistent with deposition in a lower shoreface setting. Hiatal surfaces and laterally restricted bioclastic sandstone layers and lenses within these features suggests that frequent turnover of infaunal and epifaunal communities occurred at the edges of the Liard brachiopod-echinoderm biostromes.  Environmental/ecological criteria essential for the development of the Liard biostromes include: a) initial storm-induced concentration of shell material to create islands or zones of comparably stable substrate, thus promoting colonization by crinoids, terebratulid brachiopods, and cidaroid echinoids; b) a temporary decrease in siliciclastic input to the shoreface, allowing biostrome building organisms the chance to colonize these patches of stable substrate, and dominate deposition within the lower shoreface, and c) aerobic conditions in the lower shoreface allowing for a healthy and robust biostrome fauna.