Bivalves from the Rio del Penon formation (Late Carboniferous-Early Permian), La Rioja province, Argentina

BIVALVES FROM THE RfO DEL PENON FORMATION (LATE CARBONLFEROUS-EARLY PERMIAN), LA RIOJA PROVINCE. Taxonomic, taphonomic and paleoecologic analysis of fossil concentrations were carried out in the middle part of the Rio del Penon Formation, western Jague (La Rioja, Argentina). Five species of bivalves are described and figured: Aviculopecten n. sp. and Acanthopecten jaguelensis Gonzalez, Nuculavus levatiformis (Walcott), Leptodesma (Leptodesma) cf. potens Hall and Palaeolima sp. The last three are recorded in Argentina for the first time. Besides, Phestia sp., Parallelodontinae indet., Ptychopteria (Ptychopteria) liagracielae (Leanza), Streblopteria sp., Schizodus sp., Edmondia sp. and Wilkingia riojana Gonzalez are also identified. The associated brachiopods belong to the latest Carboniferous-Early Permian Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone. Based on biostratinomic indices two types of bioclastic deposits are recognized: allochthonous and autochthonous-parautochthonous fossil concentrations. A detailed taphonomic analysis of the upper autochthonous-parautochtonous fossil concentration was carried out: pavements A and B, interpreted as biogenic-sedimentologic in origin, are distinguished. A third pavement, sedimentologic in origin is also recognized (pavement C). These deposits were probably formed under low rates of detritial sedimentation in a prodelta environment. Fossiliferous interval D was subsequently developed; it accounts for a dilution of the concentration due to the increment in elastic supply. This increment may be associated with the deltaic progradation that took place in the upper part of the Rio del Penon Formation. From a paleontological viewpoint the assemblage of pavement B represents a relatively mature paleocommunity. Ecospace utilization was good, as infaunal and epifaunal tiering levels were largely exploited. In the upper part, the faunal distribution and composition were affected by new conditions of substrate consistency and water turbidity, probably caused by a deltaic progradation.