SOFT-SEDIMENT DEFORMATION STRUCTURES INTERPRETED AS SEISMITES IN THE UPPERMOST APTIAN TO LOWERMOST ALBIAN TRANSGRESSIVE DEPOSITS OF THE CHIHUAHUA BASIN (MEXICO)

Several levels of soft-sediment deformation structures (s.-s.d.s.) cut by synsedimentary normal faults have been observed in the transition beds between the ''Las Vigas'' and ''La Virgen'' formations (Cretaceou s) in the northeastern part of the Chihuahua basin in Mexico. These st ructures consisted of four kinds of motifs (floating breccias, flame-l ike structures, large pillow structures, and wavy structures). They ar e restricted to five ''stratigraphic'' levels (Sigma 1-Sigma 5) and su rrounded by undeformed beds in fluvio-lacustrine and tidal deposits an d can be traced over a distance of several hundred meters. This deform ation is interpreted to have resulted from the combined effects of liq uidization and shear stress in soft-sediments due to local earthquakes in the area which could have been generated during the rifting stage of the Chihuahua basin. New constraints placed on the age of the ''Las Vigas'' Formation (bracketed by Late Aptian charophytes at the bottom and colomiellids of late Aptian to earliest Albian age at the top) su ggest that this synrift tectonism lasted at least until the end of the Aptian.