Abstract

NAVARRO, Edgardo  and  ASTINI, Ricardo. An exceptional example of brecciation (seismite?) in the Chubut Group, Patagonia. Rev. Asoc. Geol. Argent. [online]. 2010, vol.67, n.2, pp.193-204. ISSN 0004-4822.

Sedimentary units of the Chubut Group in the proximity of the Telsen locality, in Chubut province affected by a telodiagenesis stage, display evidence of brittle deformation related to over-pressurizing mechanisms. These sub-planar deposits of Aptian age, constitute laterally-extended fluvial facies associations. A gradation of deformational sedimentary structures occurs entirely in the pelitic deposits of the flood plain facies association. Four intervals were recognized showing a continuous transformation: 1) fractured, 2) brecciated, 3) mud-rich breccias and 4) fully sheared. Potential escape conduits of diverse sizes develop in the mud-rich breccias interval and sometimes these structures reach the fully sheared interval. The overlying tuffaceous conglomeradic sandstones, would have prevented the escape of fluids and clearly favored the over pressuring mechanism that generated partial to complete liquefaction and fluidization of the fine-grained sedimentary rocks of the alluvial plain facies. Recognized normal faults in the area, that affected the Chubut Group, can constitute an element of judgment in order to state a seismic origin for these brecciated facies whose magnitude might have exceeded M>5 in modified Mercalli scale. Because of the brittle reologic characteristic of the material from which the deformational structures were originated, these structures constitute an exceptional natural example of brecciation from a fossilized mechanism of overpressuring.

Keywords : Overpressure; Brittle deformation; Liquefaction; Fluidization.

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