Detecting allocyclic signals in volcaniclastic fluvial successions: Facies, architecture and stacking pattern from the Cretaceous of central Patagonia, Argentina

UMAZANO, A.M.; BELLOSI, E.S.; MELCHOR, R.N.; VISCONTI, G.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2012 p. 94 - 115

0895-9811

The Castillo Formation and the overlying lower member of the Bajo Barreal Formation (Cretaceous) arethe principal hydrocarbon-producing units of the San Jorge Basin, Patagonia, Argentina. They are mainlycomposed of sandstone lenses interbedded with finer-grained, tuffaceous, sheet-like strata. Both unitsrecord fluvial systems influenced by voluminous pyroclastic influx via ash-falls mainly from a westernsource. These fluvial systems drained from the west toward a non-marine depocenter located in theeastern part of the basin. The units were studied in the Sierra de San Bernardo, a NNW-SSE oriented foldand thrust belt located in the western sector of the basin. The objectives of this study were: (i) to assessthe influence of allocyclic factors on fluvial dynamics and sedimentation, and (ii) to determine thepossible link between changes in tephra reworking and configuration of channel belts. The methodologyincluded facies and architectural analyses, as well as determination of the stacking pattern of the channeldeposits. The Castillo Formation represents permanent single-channel rivers with channel-margin bars.Floodplains were commonly constructed from aqueous reworking of pyroclastic substrates (sheet-floods,debris-flows and shallow lacustrine sedimentation) and, to a lesser extent, by preservation of ash-falldeposits. The lower member of the Bajo Barreal Formation generally records braided fluvial channelbelts with a more variable water discharge and, in one locality, single-channeled rivers. Constructiveprocesses of the floodplains were similar to the underlying Castillo Formation, although other types ofdeposits were detected in lower proportions including hyperconcentrated flows and crevasse-splays. Thedifferent pyroclastic sediment supply between both units explains the general evolution of the fluvialsystems. The stacking patterns, which are a response to base-level changes, are probably associated withthe common tectonic activity recorded in the eastern part of the basin. Significant climatic changes arenot detected during deposition of both units, although indicators of variability in water dischargerecognized in some paleochannels of the lower member of the Bajo Barreal Formation could be linked toseasonality in the catchments.