a b s t r a c tRecent field-based studies documented deformation bands in porous carbonates; these structures accommodate volumetric and/or shear strain by means of pore collapse, grain rotation and/or sliding. Microstructural observations of natural deformation bands in carbonates showed that, at advanced stages of deformation, pressure solution helps to reduce the grain size, enhancing comminuted flow and forming narrow cataclastic zones within the bands. In contrast, laboratory studies on the mechanics of deformation bands in limestones identified grain crushing, pore collapse and mechanical twinning as the micromechanisms leading to strain localization.Here, we present a multidisciplinary field and laboratory study performed on a Cretaceous carbonate grainstone to investigate the microprocesses associated to deformation banding in this rock. A quantitative microstructural analysis, carried out on natural deformation bands aimed at defining the spatial distribution of pressure solutions, was accompanied by a force chain orientation study. Two sets of triaxial experiments were performed under wet conditions on selected host rock samples. The deformed samples often displayed a shear-enhanced compaction behavior and strain hardening, associated with various patterns of strain localization.We constrained the pressure conditions at which natural deformation bands developed by reproducing in laboratory both low and high angle to the major principal stress axis deformation bands. The comparison among natural and laboratory-formed structures, allowed us to gain new insights into the role, and the relative predominance, of different microprocesses (i.e. microcracking, twinning and pressure solution) in nature and laboratory.
The enormous concentration of marine vertebrates documented within the Pisco Formation is unique for Peru and South America and places this unit among the prime fossil Lagerstätten for Miocene to Pliocene marine mammals worldwide. In order to provide a robust stratigraphic framework for the fossil-bearing locality of Cerro Los Quesos, this study presents a 1:10,000 scale geological map covering an area of about 21 km 2 , a detailed measured section spanning 290 m of strata, and a refined chronostratigraphy for the studied succession well constrained by diatom biostratigraphy and high-resolution 40 Ar/ 39 Ar isotopic dating of three interbedded ash layers. Within the apparently monotonous, diatomite-dominated sedimentary section, the Pisco Formation has been subdivided into six local members, with stratigraphic control over the different outcrops facilitated by the establishment of a detailed marker bed stratigraphy based on 15 readily distinguishable sediment layers of different nature.
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