LIQUEFACTION STRUCTURES IN THE SANTO ANASTÁCIO RIVER VALLEY, WESTERN SÃO PAULO STATE, SOUTHEASTERN BRAZIL – ARE THEY QUATERNARY SEISMIC ACTIVITY SHOWINGS?

Abstract

Liquefaction structures, when originated by seismic vibration (seismites), are important tools in neotectonics investigation. Those structures occurre in meizoseismic zones associated with events higher than 5 in magnitude and their type, size and special distribution give indications to the paleoseism epicenter and intensity. Liquefaction structures in the Santo Anastacio River valley are presented and described for the first time in this paper. A comparison with those of the Rio do Peixe valley is also presented. Sandy intrusions (dikes, sills and breccias) and intrusions (sand volcano) are occurred in the Santo Anastacio River sand alluvial deposits. Liquefied sand material is massive, friable, saccharoidal and white with fragment of pre-existing sediment. Local geological and geomorphological context exclude the possibility of those structures are associated with artesian or mass movement processes. The age of paleoseism can be estimated from a black clay layer affected by the structures and dated 14C 32.340+/-320 B.P. This estimative is approximately the same obtained for the structures found in three events in the Peixe River valley (less than 33 ka B.P.). Future studies in the area could improve the delimitation of the meizoseismic zone and estimate the earthquake magnitude.