Environmental implications of oncoids and associated sediments from the Remoredo Formation (Lower Jurassic) Mendoza, Argentina

Lower Jurassic deposits of the middle member (19 m) of the Remoredo Formation at Arroyo Montañesito, southwestern Mendoza Province (Neuquén Basin, Argentina), and its oncoids are described. Four lithofacies constitute the lacustrine deposits of the Remoredo Formation: (1) laminated green to gray calcareous shales; (2) laminated to massive dark gray mudstones; (3) massive to upward-coarsening dark gray oncolite wackestones; and (4) massive dark gray packstones with abundant macro-oncolites. According to layer configuration, two types of oncoids were recognized: (a) those having distinctly banded, well defined concentric and continuous lamination (Type I); and (b) those having a vertical arrangement of irregular and discontinuous laminae (Type I3–I). The continuous micritic laminations suggest growth during agitated conditions, and the discontinuous laminations reflect short periods of non agitation or tranquility in the water mass. The oncolites are associated with a well preserved faunal assemblage of ostracods (Darwinula sp. and possible members of the Subfamily Iliocypridinae) and some fragments of bivalves that are indicative of shallow water conditions. The facies association is a coarsening-upward sequence interpreted as the product of deposition in a shallow nearshore lacustrine environment, and a progressive infilling by pyroclastic subaereal flows.