Analysis of carbonate microfacies in the Chachao Formation (cretaceous), Barda Blanca-malargüe, Mendoza province-Argentina: A cluster analytic approach

Abstract

The Chachao Formation (Valanginian), which occupies a middle position in the Mendoza Group, is a carbonate unit consisting of packstones and grainstones, with subordinate wackestones and rare mudstones. Its average thickness is 35 meters. Pelecypods, echinoderms, gastropods, crinoids, ostracods, worm tubes, sponge spicules, and foraminifera dominate the fauna in the studied sequence. Rare remains of bryozoans, ammonoids, brachiopod shells, coral fragments, and algae are present too. Phosphatic debris occurs in minor quantities. Pyrite, glauconite and phosphate minerals commonly occlude foraminiferal chambers, ostracods shells, as well as borings in skeletal particles. Taphonomic features enable this association to be considered as parautochthonous in response to compositeevent concentrations. The Barda Blanca sequence was divided into six microfacies in order of relative increasing energy level. Each one is characterized by the dominance of depositional textures, bioclasts and sorting. In order to determine the degree of interrelationships among bioclasts, data were subject to cluster analysis. Two isolated nuclei (calcispheres & sponge spicules/fish scales & coprolites), and a big group integrated by three subgroups were recognized. Each one was distinguished by the existence of pelecypods, echinoderms, gastropods, annelids, ostracods, and foraminifers. Changes in microfacies types and in fauna content reveal considerable variation in the frequency-distribution curves of bioclasts. These variations point out discontinuities, which may indicate that water depth increased with time. These discontinuities suggest the presence of different parasequence boundaries and they could be used as time lines in the reconstruction of the depositional events. The fossil assemblage and deposit suggest an open marine condition below the influence of fairweather waves, and reworking by action of smaller storms (gradient currents) eventually.