287 / 2019-08-19 14:44:19

THE SIGNIFICANCE OF AUTOMICRITE OF ORDOVICIAN CALATHID-DEMOSPONGE CARBONATE MOUNDS

reef evolution,organomineralization,GOBE,automicrite,microbial

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.
By using a spreadsheet with six parameters and seventeen 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.
Carbon and oxygen stable isotopic composition of automicrite plot within a range of 0.28‰ to 0.84‰ PDB for δ13C and -7.23‰ to -5.23‰ PDB for δ18O. A total of 70% of the automicritic samples plot in or near to the field defining Middle Ordovician (Darriwilian) marine calcites. In addition, 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.
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. 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.