Resumo

PUY-ALQUIZA, María Jesús et al. Geochemistry and depositional environment of the Losero Formation in the Mesa Central, México. Bol. Soc. Geol. Mex [online]. 2014, vol.66, n.3, pp.413-430. ISSN 1405-3322.

The Losero Formation is exposed in the Guanajuato Mining District, conformably overlying the Guanajuato conglomerate. Most of the up-to-date studies of the Losero Formation have indicated that this formation corresponds to a Cenozoic volcanic sequence. However, sedimentological, petrographic and geochemical evidence that supports a sedimentary or volcanic origin has not been discussed extensively. We present a comprehensive geological analysis which includes collection of samples and elaboration of eight stratigraphic columns, followed by laboratory studies including petrography, trace element determinations using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and mineralogy by X-ray diffraction and Scanning Electron Microscopy (SEM). The petrographic analysis revealed that the Losero Formation is constituted by litharenite and arkose, mostly derived from granitic and metamorphic basement rocks typical of the interior of a craton. Primary sedimentary structures include ripples, antidunes, flute and groove casts, cross and parallel lamination, desiccation cracks and soft-sediment deformation structures interpreted as seismites. These findings demonstrate that the sandstones were deposited by supercritical and subcritical flows as well as by unidirectional flows within channels and bars in a river system. Elemental ratios critical of provenance (La/Sc, Th/Sc, La/Cr, Th/Cr and Ba/Sc) are similar to sediments derived from weathering of mostly felsic rocks. Using QmFLt and QtFL ternary diagrams allowed to infer that the sandstones of the Losero Formation were deposited in an active continental margin. Modal analysis of the samples indicates a recycled orogen and dissected arc. Most evidence suggests that the deposit was accumulated in a fluvial environment with conditions ranging from shallow to deep water. Based on these data and field observations, the sandstones from the Losero Formation can be considered as part of the distal facies of the Guanajuato Conglomerate.

Palavras-chave : Losero Formation; Guanajuato; Geochemistry; Provenance; Tectonic setting.

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