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TAFONOMIA COMPARADA DOS CONULATAE (CNIDARIA), FORMAÇÃO PONTA GROSSA (DEVONIANO), BACIA DO PARANÁ, ESTADO DO PARANÁ

2003; Sociedade Brasileira de Geologia; Volume: 33; Issue: 4 Linguagem: Inglês

10.5327/rbg.v33i4.1084

2317-4889

Sabrina Coelho Rodrigues, Marcello Guimarães Simões, Juliana de Moraes Leme,

Geological formations and processes

A high resolution taphonomic analysis of the Conulatae from the Ponta Grossa Formation, Devonian (Pragian-Emsian), Parana Basin, Parana State is presented. The study is based on the occurrences found in rocks that are lithostratigraphically coeval to the Jaguariaiva Member. The taphonomic data gathered indicate that the conulariids [ Conularia quichua Ulrich in Steinmann & Doderlein 1890 and Paraconularia africana (Sharpe 1856)] are preserved according to three distinct taphonomic classes. These classes were established according to: a- three-dimensional distribution of fossils in the matrix; boccurrence of isolated or clustered specimens; c- degree of bioturbation; and d- degree and type of deformation showed by thecae. Taphonomic class 1 includes isolated and clustered conulariids, vertically oriented to bedding. Commonly, they are inflated, nonfragmented, and preserved in massive or laminated siltstones, sometimes with discrete and isolated ichnofossils. Taphonomic class 2 encompasses isolated or clustered conulariids that are inclined to bedding. Specimens assigned to this class are preserved in intensely bioturbated siltstones or including traces fossils, such as Zoophycus . Taphonomic class 3 is represented by isolated conulariids that are horizontally oriented to bedding, including four subclasses (I to IV) with distinct and complex taphonomic history. Vertically oriented conulariids (class 1) are interpreted as in situ (autochthonous) occurrences, in which mud clouds during storm events abruptly bury living organisms. Inclined (class 2) and horizontally oriented conulariids preserved in deeply bioturbated rocks (class 3-I) indicate autochthonous to parautochthonous occurrences. Horizontally oriented conulariids with inflated thecae, found in close association with brachiopod-dominated pavements ( i.e ., Australospirifer inheringi Kayser 1900) or with hummockies (class 3-II) are interpreted as parautochthonous to allochthonous records. Conversely, horizontally oriented conulariids, that are found incomplete and strongly compressed in massive fine-grained sediments (class 3-III) or in bioturbated siltstones, sometimes with climbing ripples (class 3-IV), record of complex history, including exposure at the sediment/water interface, transport and loss of skeletal hard parts. Conulariids exhibit nonrandom stratigraphic distribution in rocks of the Sequence B. In the geological section of Jaguariaiva, Parana State, conulariids are preferentially preserved in two well-defined intervals, located at 29 to 32 and at 44 to 48 meters from the top of the Furnas Formation. Layers with in situ occurrences of conulariids (class 1), representing obrution deposits or distal tempestites, located a few centimeters below the marine flooding surfaces characterize these intervals. Finally, the taphonomic data gathered strongly support the idea that the conulariid thecae are not hard structures, but rather are flexible ones and thus able to folding.