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The stratigraphy of the region of Los Colorados along the western border of the Cordillera Oriental in northwest Argentina overlies Cambrian quartzous sandstones and siltstones of the Mesón Group covered by a thick shaly section of the Santa Victoria Group that embraces Tremadoc to Arenig ages. This unit, at the base of the section, shows progressively coarser beds toward the top in the Acoite Formation. The Alto del Cóndor Formation (nov.nom.), the Sepulturas Formation and the Zapla glacial horizon overlie it. The last two respectively assigned to the Middle and Late Ordovician. A first meter-scale iron-rich horizon follows concordantly covered by the Lipeón Formation, that although of reduced thickness would embrace a great deal of the Silurian. Above, a second iron-rich horizon (4 m thick) separates it from a purplish sandy-muddy unit with remains of Siluro-Devonian fauna comparable with that present in the Arroyo Colorado Formation in the subandean ranges and equivalent to the more widespread Baritú Formation in subsurface. The paleozoic stratigraphy in the area is erosively truncated by Cretaceous continental red beds of the Salta Group, for which the thickness stripped off by erosion is unknown. The relatively complete record in this area of the Cordillera Oriental allows rethinking some assumptions about the existence of topographic highs structured from the Early Ordovician onwards. Continuity of basin fill would have been periodically interrupted by relative sea-level fluctuation, most probably of eustatic origin, which would be responsible for the observed boundaries between most units as well as of sequence and supersequence boundaries. Nevertheless, the only discontinuity that has partly proved eustatic origin and partly tectonic is the Ocloyic unconformity. The good access and excellent exposition of the units described allow considering this area as an obligated locality for gaining knowledge on the development of the early-middle Paleozoic foreland in the Cordillera Oriental and a reference area concerning regional stratigraphic correlation in northwest Argentina.
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