Abstract
Abstract The Upper Cretaceous-Palaeocene Paso del Sapo-Lefipán Basin of west-central Patagonia (southern Argentina) is an intracratonic depocentre located to the east of the Andean magmatic arc. The depression is related to uplifted basement blocks and was generated by a complex system of strike-slip faults. The sedimentary infill comprises two stratigraphical units: the Paso del Sapo and the Lefipán Formations. The Campanian-Maastrichtian Paso del Sapo Formation (145 m thick) is mainly composed of quartz-rich sandstones and conglomerates associated with minor heterolithic intervals, mudstones and coal beds. The Maastrichtian-Palaeocene Lefipán Formation (200 m thick) is composed in its lower part of gypsiferous mudstones and shales with isolated cross-bedded and plane-bedded sandstone bars. The middle and upper sections of this unit are distinguished by cross-bedded sandstone multi-storeys associated with several coquina beds. Sediments from Paso del Sapo Formation show features of fluvial and tidally influenced systems. Sandstones and conglomerates were deposited as subtidal and intertidal estuarine bars. Heterolithic sections, mudstones and coal beds represent the more restricted inter- to supratidal marginal estuarine deposition. Most of the Lefipán Formation was formed under open marine conditions. Offshore fine-grained transgressive deposits accumulated during basin starvation. Later on, sandstone multi-storeys and coquinas were deposited in a wave and tidally influenced lower to upper shoreface environment. The sedimentary record of the K-T basin is in the range of a second-order eustatically controlled cycle. Based on facies arrangement and stratal geometries, the estuarine deposits of the Paso del Sapo Formation and the basal section of the Lefipán Formation are interpreted as a retrogradational systems tract. Later on, during a highstand period, the middle and upper Lefipán Formation was deposited as a progradational systems tract.
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