Ichnology of the Trenton Group in the Quebec City area

Abstract

Marine limestones of the Middle Ordovician (Caradocian) Trenton Group in the Quebec City area are extensively burrowed. Traces referable to Anconichnus horizontalis, cf. Biformites, Bifungites cf. halli, Chondrites, Cruziana cf. pudica, Cruziana, Isopodichnus, Palaeophycus, Planolites, ? Rhizocorallium cf. R. irregulare, Rusophycus, Teichichnus cf. rectus, Tomaculum and Trichichnus are described in detail. Miscellaneous traces, those not morphologically distinct and (or) sufficiently abundant to warrant formal taxonomic treatment, are briefly described. The trace fossil assemblage can be broadly equated with the Cruziana ichnofacies. Southwest of Quebec City the Trenton Group was deposited in a gradually deepening offshore bar to shallow inner shelf situation, whereas to the northeast deposition occurred in a shallow inner shelf to deeper outer shelf – upper slope environment. The spatial and temporal distribution and abundance of the trace fossils are examined in relation to these broad environmental changes. The observed patterns are perhaps best related to variation in energy conditions, substrate stability, and food availability, parameters all broadly related to water depth. The trace fossil density and diversity patterns are analogous to those exhibited by modem and ancient shelly macrobenthos, where in shallow subtidal regimes they are low, then peak in slightly deeper subtidal situations, and gradually decrease seawards with increasing depth and distance from shore.