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Fluvial to Shelfal Strata of the Late Cretaceous to Paleogene Dorotea and Tres Pasos Formations, Magallanes Basin, Patagonia, Argentina

Abstract

Late Cretaceous to Paleocene 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 belt thrust and were studied approximately 50 km east of the leading thrust. The Tres Pasos Formation consists of upward coarsening siltstones and sandstones, with hummocky and swaley cross stratification. These are interpreted as distal to medial shelfal deposits (lower shoreface to offshore). 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. Although the Magallanes Basin has been the subject of many studies, details on the depositional environments of the Dorotea Formation are limited, despite quality outcrop exposure in the Santa Cruz Province. The Dorotea Formation is divided into two informal stratigraphic units, both dominated by fluvial strata and overbank deposits. Upward-fining channel-fill sandstones in the lower unit average 5–10 m in thickness and are dominated by trough cross stratification. Those channels at the base of the section have marine influence, and contain abraded marine fossils (bivalves, gastropods) and marine trace fossils (Ophiomorpha, Schaubcylindrichnus). Channelized sandstones are separated by overbank fines interpreted as flood plain deposits with poorly developed paleosols. Lateral accretion deposits were uncommon, and most channel-fill sandstones are interpreted as straight to low-sinuosity systems. The upper unit contains channel-fill sandstones that reflect an increase in fluvial energy. In addition to an overall increase in average grain size from fine/medium to medium/coarse sand, many of the channelized sandstones are conglomeratic at the base. Clast size averages 1 cm, but ranges up to 7 cm in diameter. Channelized sandstones in the upper unit are 10–15 m thick, some of which are amalgamated. No lateral accretion was seen in the upper unit, and the majority of these channel-fill sandstones are interpreted as deposits of high-energy fluvial systems. Individual channelized sandstones can be traced for hundreds of meters across the outcrop exposure. No significant lateral facies changes were observed across the 10 km extent of outcrop exposure.