Cenozoic seismites and soft-sediment deformation structures in the Losero Formation, southern Sierra de Guanajuato, Mexico

Abstract

Siliciclastic deposits of the Eocene-Oligocene Losero Formation, which crop out in the Sierra de Guanajuato in central Mexico, show several types of soft-sediment deformation structures that are interpreted as seismites. The Sierra de Guanajuato is a complex tectonic pile of thrust sheets composed of Mesozoic lithostratigraphic successions. During the Paleogene, the Sierra de Guanajuato was affected by extensional tectonism that occurred in a short time interval, which was contemporary with clastic sedimentary depositional processes. Eocene-Oligocene deposits are characterized by a red bed sequence represented by the “Guanajuato Conglomerate” and the Losero Formation, which has many syn-sedimentary faults and microfaults oriented NE 35°– 45°SW, with a dip of 70°–87°SE. These sediments are the result of a braided fluvial system characterized by plane parallel laminae, desiccation cracks, ripples, and cross bed sets, antidunes, groove and flute casts. In the study area, two stratigraphic sections (Caídos y Socavón) were measured in detail (facies distributions, primary structures and stacking patterns). Several types of soft-sediment deformation structures were recorded in Losero deposits and these occur interbedded with undeformed strata throughout the entire stratigraphic interval. The lateral extent of the deformation is of several hundred meters long and the deformed bed involves the mid-upper part of the 30 meter thick succession. Deformation affects about two meters of coarse-grained sandstone, fine-grained sandstones, and mudstones. This deformation is represented by layers with asymmetric and symmetric folds, dish structures, pseudonodules, sandstone dikes, flame structures, syn-sedimentary normal microfaults and convolute laminae. It can be described as a gradient system of unstable density layers. Soft-sedimentary deformation structures observed in the Losero Formation are interpreted as seismites, developed as a result of increased pore pressure and vertical or horizontal stresses induced by seismic activity.