Geochemistry and depositional environment of the Losero Formation in the Mesa Central, México

Resumen

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.