"Moluscos bivalvos en la Formación Río del Peñón (Carbonífero Tardío-Pérmico Temprano), provincia de La Rioja, Argentina"

Abstract

"Taxonomic, taphonomic and paleoecologic analysis of fossil concentrations were carried out in the middle part of the Río del Peñón Formation, westem Jagüé (La Rioja, Argentina). Five species of bivalves are described and figured: Aviculopecten n. sp. and Acanthopecten jaguelensis González, Nuculaous levatiformis (Walcott), Leptodesma (Leptodesma) d. 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 rioiana González are also identified. The associated brachiopods belong to the latest Carboniferous-Early Permian Tivertonia jachalensis- Streptorhynchus inaeouiornaius 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 clastic supply. This increment may be associated with the deltaic progradation that took place in the upper part of the Río del Peñón 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."