TURRITELLINE-DOMINATED ASSEMBLAGES FROM THE MONTE LEÓN FORMATION (LOWER MIOCENE), PATAGONIA, ARGENTINA: PALEOECOLOGICAL AND PALEOENVIRONMENTAL IMPLICATIONS

Abstract

Fossil concentrations are a conspicuous feature of the Monte León Formation (Lower Miocene), providing the opportunity to carry
out combined sedimentological and paleontological studies that allow elucidating the processes involved in its genesis and contributing to the knowledge of the depositional paleoenvironment. With these objectives, the stratigraphic, sedimentological, and paleontological features of fossil concentrations containing turritelline gastropods were analyzed in rocks exposed at Restinga Norte (Santa Cruz Province). Four representative samples were extracted, and the taphonomic features of recovered turritelline specimens were analyzed in the laboratory. The textural analysis of the sediments that make up the matrix of the concentrations and the adjacent beds was also carried out. The results allowed them to be classified as “turritelline-dominated assemblages” constituting mixed biogenic-sedimentologic concentrations, with characteristics given by the behavior of the turritellines and their final reworking within the habitat. The abundance of turritellines and the diversity of bryozoans as sclerobionts suggest a habitat of nutrient-rich marine waters, probably associated with upwelling. The good preservation and the high percentage of empty shells—together with the characteristics of the sediment—suggest that their accumulation on the bottom was favored by a low-energy environment. These shells would have been oriented by a unidirectional current that promoted —favored also by a high sedimentary supply— their rapid burial and preservation. The results established that the turritelline-dominated assemblages were formed in a shallow marine shelf in the transition between the retrogradational and progradational facies of the transgressive-regressive cycle proposed for the Monte León Formation in the study area.