Deciphering the sedimentological imprint of paleoseismic events: an example from the Aptian Codo Formation, northern Brazil

A stratigraphic horizon bearing several styles of soft-sediment structures occurs sandwiched within entirely undisturbed deposits in the lacustrine Codo Formation (Aptian) located in the eastern part of the Grajau Basin, northern Brazil. These deposits show a variety of sedimentary features distinguished according to their deformation style as follows (from bottom to top): (a) Zone 1, consisting of widespread molar-tooth structures (i.e. fine-grained spar-filled cracks) that are interconnected with small-scale faults, fissures and stylolites inclined at a high angle to bedding; (b) Zone 2, represented by complex convolute folds with lateral gradation of smooth synclines and anticlines into isoclinic folds and internal layers that retain the normal bedding features, recumbent folds, nappe folds with associated thrust faults, pseudonodules, and mound-and-sag structures; (c) Zone 3, consisting of normal faults and fissures that are vertical to near vertical, show ragged morphology with small delicate peaks, taper both downward and upward after a few centimeters, and are associated with large (up to 2.5 m long) collapsed boulders; and (d) Zone 4, characterized by shales with irregular convolute folds. These deformation zones are interpreted to reflect sediment rheology and alternating periods of sediment accumulation and deformation under the influence of synsedimentary seismic events, as suggested mostly by: (1) occurrence between undeformed deposits; (2) lateral correlation for several kilometers of distance; (3) similarity with other features formed in areas affected by seismic activity; (4) presence of structures attesting to sediment deformation in an unconsolidated to semi-consolidated state; and (5) complex association of brittle and ductile deformation structures. Hence, Zone 1 records small scale cracking and faulting of early-cemented, cohesive deposits. Zone 2 is dominated by ductile deformation structures formed on water-saturated, unconsolidated sediments, probably as a reflex of compressional stress. Zone 3 records a change in tectonic style to dominantly distension, as evidenced by the horizon with normal faults. Finally, Zone 4 shows structures attributed to ductile deformation of water-saturated muds, which was probably formed during the same episode of deformation responsible for the faults and fissures in Zone 3. This succession of deformation events appears to be consistent with transtensional and compressional shear stresses during the sedimentary evolution of the Grajau Basin. (C) 2000 Elsevier Science B.V. All rights reserved.