LAUSR

Calathid–demosponge carbonate mounds are a feature of Early to Middle Ordovician shallow-marine carbonate depositional environments of tropical to subtropical palaeolatitudes. These mounds contain an important amount of autochthonous non-skeletal microcrystalline calcium-carbonate (automicrite) conventionally considered microbial in origin. Here, the automicrite of calathid–demosponge carbonate mounds (Tarim Basin, north-west China) is broken down into five distinct fabrics: an in situ peloidal–spiculiferous fabric (AM-1), an in situ peloidal fabric (AM-2), an aphanitic–microtubular fabric (AM-3), a minipeloidal fabric (AM-4) and a laminoid–cerebroid fabric (AM-5). Type AM-1 occurs with AM-2 being succeeded by an assemblage of AM-3 and AM-4. Types AM-4 and AM-5 are separated by an erosional disconformity. A good correlation of fluorescence and cathodoluminescence of automicrites indicates that induced and supported organomineralization produced automicrite, probably via the permineralization of non-living organic substrates adsorbing dissolved metal–humate complexes. Using a spreadsheet with six parameters and 17 characters, AM-1 to AM-4 turn out to be non-microbial in origin. Instead, these automicrites represent relics of calcified metazoan tissues, such as siliceous sponges, non-spiculate sponges or the basal attachment structures of stalked invertebrates. Fabric AM-5 is a microbial carbonate but is post-mound in origin forming a drape within a reefal framework established by AM-4. The five automicritic fabrics, individually or as an assemblage, are a common element of Ordovician calathid–demosponge carbonate mounds in general. The reassessment of the origins of these automicritic fabrics holds consequences for understanding of the Great Ordovician Biodiversification Event in terms of community structure, reef ecology and reef evolution. Episodically, these fabrics are also present in other carbonate build-ups stretching from the Neoproterozoic over the entire Phanerozoic Eon. The massive calcification of metazoan soft tissue (AM-1 to AM-4) characterizes episodes and conditions of enhanced marine calcification and might be of value to refine secular trends of pCO2, Ca concentration and Mg/Ca ratio at the scale of individual sedimentary basins.