Coelobiontic communities in neptunian fissures of synsedimentary tectonic origin in Permian reef, southern Urals, Russia
Vennin, E. (2007). Coelobiontic communities in neptunian fissures of synsedimentary tectonic origin in Permian reef, southern Urals, Russia, in: Álvaro, J.J. et al. (Ed.) Palaeozoic reefs and bioaccumulations: climatic and evolutionary controls. Geological Society Special Publication, 275: pp. 211-227 In: Álvaro, J.J. et al. (2007). Palaeozoic reefs and bioaccumulations: Climatic and evolutionary controls. Geological Society Special Publication, 275. Geological Society: London. ISBN 978-1-86239-221-2. viii, 291 pp., more In: Hartley, A.J. et al. (Ed.) Geological Society Special Publication. Geological Society of London: Oxford; London; Edinburgh; Boston, Mass.; Carlton, Vic.. ISSN 0305-8719; e-ISSN 2041-4927, more |
| Abstract | Sedimentary dykes in the Permian reef complexes of the Russian platform are well preserved and important in providing information about reef growth, the reef biota and, particularly, cavity-dwelling organisms and sediment sources. Two main fissure assemblages are recognized with N80° and N170° (late Asselian-early Sakmarian) and N130°-140° and N60° (Sakmarian-Artinskian) orientations. These contemporaneous orthogonal dyke sets present orientations corresponding to the regional tectonic fabric and a tectonic origin for fracturing associated with the foreland basin development. The largest dykes record eight lithofacies and several stages of fracture opening. Stromatoids and centimetre-thick deposits of peloidal grainstones-packstones (thromboids), which form in situ within microbial laminae on the fissure walls, preceded filling by skeletal and terrigenous sediments. The fissures contain a well-preserved biota similar to the Lower Permian cavity-dwelling organisms observed within the reef. The coelobiontic habitat was episodically enlarged by successive synsedimentary fracturing episodes reflecting several phases of encrustation and infill of recurrent lithofacies. The pioneer microbialites grew when nothing else was deposited in the fissures immediately after fracturing. Gastropods and ostracods occur as dense clusters, probably indicating in situ growth, or are considered as reworked material from the subsurface. Others organisms, such as crinoids, bryozoans, Tubiphytes, conodonts, brachiopods, ammonoids and spiculate sponges, are also found within fractures as reworked shells and skeletons, indicating that they were washed in from the overlying sea floor. The biota in the dykes is mainly represented by the dweller guilds living on the reef surface, whereas members of the binding and frame-building guilds are poorly represented. The dykes formed during reef development and the last stage of argillaceous wackestone sedimentation within the fractures is related to a major flooding event corresponding to upper Artinskian drowning of the Russian platform. Biostratigraphically significant fossils in the dykes date the ages of the host rock and of the fissure formation and filling from late Asselian to upper Artinskian. The Tratau reef thus provides an instructive example of the interaction between carbonate accumulation and tectonic events. |
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