Shell bed stacking patterns in the Chachao Formation (Early Valanginian) in Nialargide Area, Mendoza Province, Neuquen Basin-Argentina

The Lower Cretaceous Chachao Formation in the Malargue anticline area consists of wackestone, packstone, and minor grainstone and mudstone rich in benthic fauna which were deposited in a shallow carbonate ramp. These shell beds are composed of a low-diversity molluscan fauna mostly dominated by oysters, other benthic pelecypods (Eryphila sp., Ptychomya sp., Pecten sp., Pinna sp., Trigonia sp., Cucullaea sp.), serpulids (Sarcinella sp., Parsimonia sp.), and occasional ammonites (Olcosthepanus curacoensis). Most skeletal accumulations range from 15 to 70 cm in thickness. Shell beds exhibit a sharp, flat and commonly undulating base. In some cases the tops were bioturbated principally by Thalassinoides isp. Some shell beds show a densely packed biofabric (bioelastic-supported) but most of them have dispersed or loosely packed biofabric (mud-supported). Bioclasts include articulated or disarticulated shells, fragmented valves and shell debris. The analysis of shell concentrations of the Chachao Formation allow to recognize four taphofacies according to their taphonomic attributes. Different kind of processes, such as low-energy storm currents and waves, were interpreted, whose deposits are arranged into a coarsening- and thickening-upward and then fining- and thinning upward trends. According to their biostratinomic features shell beds of Chachao Formation have been interpreted as parautochthonous to autochthonous skeletal concentrations, mainly of sedimentologic origin. Towards the end of the Valanginian, the drowning of the carbonate ramp took place, related to a relative sea level rise and environmental stress. This drowning event was recorded by the alternating thin- to medium bedded dark gray carbonate and black shales (Agrio Formation) of LateValanginian-Late Hauterivian age.