Geologija (Jun 2017)
Characterization of silicifid fossil assemblage from upper Carnian “Amphiclina beds” at Crngrob (central Slovenia)
Abstract
The village of Crngrob (central Slovenia) is known to geologists for the silicifid fossils found in the thick soil covering the “Amphiclina beds”, a Carnian lithostratigraphic unit of the Mesozoic Slovenian Basin. Due to silicifiation, the assemblage may provide an important insight into Carnian marine life; however, the nature of the fossil assemblage, as well as the timing and mode of silicifiation are unknown. A detailed sedimentological section, which covers the uppermost part of the “Amphiclina beds” and their transition to the “Bača Dolomite”, was recorded at one of the fossil-bearing localities. The section consists of limestone (including bioturbated fiament and radiolaria-fiament wackestones, laminated fiament packstone, bioclast wackestone, peloid packstone, flatstone with intraclasts and bioclasts, and intraclast flatstone and rudstone), marlstone and dolomite. Chert is locally present in nodules or as dispersed silicifid patches, giving a speckled appearance to the host rock. According to our interpretation, the deposition of the limestone occurred via hemipelagic settling, turbidite currents and debris flws in a slope-to-basin or outer ramp setting. The composition of the grains (which include green algae fragments, thick-shelled bivalves, corals and solenoporacean algae) clearly points to the allochthonous nature of the material, which was largely derived from shallow-water environments. Fossils, as well as the intraclasts and sometimes the matrix, are locally replaced by chalcedony and granular megaquartz. This type of silicifiation does not appear to be selective to particular microfacies. As the silicifiation occurred after the down-slope transport of shells and the deposition of the sediment, the Crngrob fossil assemblage represents a thanatocoenosis or even a taphocoenosis, and it is not representative of autochthonous Carnian associations.
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