Seismic-induced disturbances in late Cretaceous rocks of the São Luís Basin, Marnhão State, Brazil.

Abstract

Unusual deposits including faults of small displacements, fractures, soft sediment deformation and channel-like (i.e, concave-shaped) structures ocurring wilhin undisturbed estuarine strata of the uppermost Itapecurú Formation are attribuled to peneconlemporaneous sediment disturbances produced by water escape and slumping mechanisms. Such features are considered to be the result of externally-triggered processes associated with seismic shock episodes that affécted unconsolidated and/or semiconsolidated deposits. It is possible that seismically-induced moviments preferentially affect those areas of the stuary experiencing overloading due to constant sand influx (e.g., tidal deltas, bayhead deltas), which were naturally more susceptible to sediment failure.