Sedimentary evolution and biostratigraphic framework of Carboniferous glaciation in Barreal Hill, Calingasta-Uspallata basin

MARTÍNEZ, M.; CISTERNA, G.A.; SIMANAUSKAS, T.

Bahía Blanca

Congreso; VII Congreso Argentino de Paleontología y Bioestratigrafía; 1998

Understanding of the Carboniferous sedimentary record in Calingasta-Uspallata basin has resulted from studies conducted using different approaches. Detailed lithofacies mapping in combination with new sampling of marine benthic invertebrate assemblages and taphonomic/paleoecological studies provided critical information to evaluate and discriminate between our interpretation and previous ones. Carboniferous glacial successions scattered across of the Barreal hill are characterized by distinctive diamictite-bearing deposits and faunal concentrate horizons traditionally assigned to "Zona de Levipustula" indicative of Late Carboniferous age. The outcrops analyzed exhibit a complex suite of glacigenic facies recording the advancing and retreating phases of the ice masses that ended with the flooding of the depositional site, and are associated to the installation of an open marine system. Rocks of the El Paso Formation, the main subject of this work, are some of the earliest diamictite-rich deposits that unconformably overlaps pre-Carboniferous sedimentary substrate. Two diamictitic packages occurs in lower and upper parts of the sections and vertical alternation of pebbly sandstones, boulder conglomerates, pebbly (dropstone) shales, sandstones, siltstones and shales occur elsewhere. The internal architecture of facies associations distinguished reveals that bedrock topography played a major role in the pattern of glacigenic sedimentation. Therefore, ice-marginal and subglacial deposition are localized and discontinuous. In contrast, thick proglacial basin-fill sequences exhibit complex facies relationships and comprise subaqueous fining-upward outwash sequences punctuated by bioturbated marine mudstones containing a diverse fauna composed fundamentally for brachiopods, gastropods, bivalves and others invertebrates (cnidaria, cephalopods orthocones and crinoids) that appeared locally. This association is notably different with those identify in others glacimarine sequences inside of the some basin (Hoyada Verde and La Capilla formations), reflecting different adaptative patterns to environmental conditions including important fluctuations in salinity, oxygen content and sedimentation rate. It is possible therefore that an oscillatory retreat of the glacial margin, related to deglaciation stage, must have lead to reorganization and relocation of species associations and may have enhanced species turnover in isolated areas probably delimited by morainal banks or preexisting saddle bedrock. The lateral variability of the basin-fill succession and fossil associations occurrences cause significant problems in defining major stratigraphic boundaries and unequivocal correlations of events. However, in the preliminary approximation, the brachiopods identified in this section (Aseptella aff. patriciae Simanauskas, Neochonetes sp., Tuberculatella sp. and Rhipidomella sp.), suggests a strong affinity with forms of Late late Carboniferous, whence to mark the age of the last pulse of the glaciation.