Fluvial to shelfal strata of the Late Cretaceous to Paleogene Dorotea and Tres Pasos Formations, Magallanes Basin, El Calafate, Argentina

Abstract

Late Cretaceous to Paleogene strata of the Tres Pasos and Dorotea formations, exposed along the Andean margin in the Santa Cruz Province of Southern Argentina, were deposited in the Magallanes Basin, a retro-arc foreland basin. These strata are well exposed east of the Cordillera de los Andes thrust belt and were studied approximately 50 km east of the leading thrust. Discontinuous exposure of the upper 190 m (~10%) of the Tres Pasos Formation contains at least 7 parasequences containing upward-coarsening siltstones and sandstones, with hummocky and swaley cross-stratification. These are interpreted as distal to middle shelfal deposits of the lower shoreface. The top of the Tres Pasos Formation is an erosional unconformity, and overlain by over 200 meters of Dorotea Formation. Outcrops of the Dorotea Formation can be traced laterally for over 15 km of dip-oriented section. The Dorotea Formation is divided into two informal stratigraphic units, separated by a sequence boundary. Both are dominated by fluvial strata and overbank deposits. At least 14 parasequences are identified in the lower unit and average 5.5 m in thickness. Parasequences are comprised of upward-fining channel-fill sandstones and associated overbank fines, which are interpreted to be low sinuosity meandering stream and floodplain deposits. The upper unit contains 19 parasequences that average 7.6 m in thickness. These parasequences also contain channel fill sandstone, which are coarser and are interpreted as the deposits of higher energy fluvial systems. In addition to an overall increase in average grain size, many of the channelized sandstones are conglomeratic at the base. Unlike the lower unit, no lateral accretion is seen in the upper unit, and the majority of these channel-fill sandstones are interpreted as deposits of high-energy, braided fluvial systems. No significant lateral facies changes were observed across the 10 km extent of outcrop exposure.