Spinicaudata do triássico e jurássico das formações Santa Maria e Caturrita (Bacia Do Paraná): tafonomia e paleoambientes

Abstract

Due to the chitino-phosphatic nature of of Spinicaudata conchostracans, these organisms exhibit, as a rule, a low preservational potential when compared to other bivalve groups. Notwithstanding, recent studies point to their increased capacity to tolerate physical processes. Due to this biased characteristic, conchostracan have been utilized as precise temporal markers. Besides this, owing to this taphonomic particularity, spinicaudates can be also useful in the investigation of sedimentary processes and depositional environments in which these organisms inhabited in the past. The present work aims at providing a paleoenvironmental and stratigraphic analysis of conchostracans (Spinicaudata) from the Triassic-Jurassic of the Paraná Basin (Santa Maria and Caturrita formations). The adopted methodology consisted in the appraisal of facies, depositional systems and taphonomic signatures of the fossiliferous deposits. The results from the taphonomic study shows the presence of four distinct fossil assemblages, individualized based on the generation mechanism and intrinsic characteristics of the concentrations: 1a and 1b (facies Fl) is related to the decantation of fine grained sediments in floodplains; 2 (facies Sr-Fl-Sh) is related to overflow currents in floodplains; 3 (facies Fl-Sm) river mouth bars. In assemblages 1a and 1b, the high degree of preservation is associated to autochtony in the conchostracans, whereas the preservational condition of assemblages 2 and 3 point to authochthonoy, parautochthony and even allochthony. It is concluded, then, that the preservational quality of these organisms is probably related to transport duration, distance from original site of life and magnitude of the event of final burial. Within the observed species, the recognition of Eustheria minuta in the stratigraphic level of the Passo das Tropas creek corroborates an age for these deposits between the late Middle Triassic and early Upper Trisassic. The presence of a new form, likely related to the family Fushunograptidae, in sediments from the Caturrita Formation suggests a Jurassic age for these deposits.