Abstract

Late Ordovician bryozoan carbonate mounds are present in the upper part of the Deschambault Formation (Trenton Group) at the Montmorency Falls locality, northeast of Quebec City. These mounds are local features within a stratigraphic interval otherwise characterized by a bedded sequence of bryozoan-rich deposits. From the core of the mound to its margins and into the well-bedded off-mound sediments, there is a taphonomic gradient in terms of preservation, automicrite formation, fragmentation, transport, growth versus shelter porosity, and marine cementation. By ruling out both local seepage of nonmarine fluids (nonspecialist fauna, normal-marine carbon and oxygen stable isotopes, near PAAS rare earth element distribution patterns) and differential growth rate (bryozoan zooecium size), mound formation is explained by a positive taphonomic feedback mechanism. Centimeter-size patches of automicrite in mound cores are considered crucial in explaining subtle variations of microtexture, microfacies, and subsequent mound formation. Automicrite (M1) is dark gray, contains microbioclasts, has sharply defined, scalloped outer margins, is commonly gravity defying, and occasionally joins up to bridge multiple bryozoan skeletons. Clusters of microtubules are present within M1 and a first generation of infiltrated microcrystalline carbonate sediment (M2). The size, geometry, and arrangement of these microtubules suggest an origin either from the attachment of nonspicular, keratosan sponges, or from the assimilatory action of marine fungi (so-called Wedl tunnels). In conclusion, a subtle and small-scale secondary reinforcement of bryozoan skeletons by cryptobiontic ephemeral substrates is critical for the development of mounds out of an essentially level-bottom bryozoan community.

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