Idades de deposição dos sedimentos da falésia de Itaguaré (Bertioga-SP) determinadas por luminescência opticamente estimulada

Abstract

The Itaguare River mouth region, located in "Parque Estadual da Restinga de Bertioga-SP", is frequently used for didactic classes in sedimentary geology and related areas. It presents a miscellany of sedimentary processes and bed forms generated by the action of tides, waves and the river. Quaternary sedimentary deposits outcropping along a cliff with tens of meters of extension and up to 4m in height formed by beach processes, similar to those observed at the beach nearby. The deposits are composed of fine, moderately to well-selected sands with plane-parallel stratification, presenting in their lower portion (~ 0.5m) Ophiomorfa nodosa (Callichirus sp. galleries) ichnofossils. This sedimentary succession is indicative of deposition in the swash zone, in a regressive condition, and with a relative sea level around 1.4±1.0 m above the present, estimated by comparing the maximum level of O. nodosa occurrence with the top of the living Callichirus biozone. Two sediment samples (Ber-01, basal, and Ber-02, top) were dated by optically stimulated luminescence (OSL) using the Single Aliquot Regeneration-dose (OSL-SAR) protocol. Luminescence geochronological methods indicates, on the premise, the age of last exposure to sunlight, which, in this case, is the age of sediment deposition. Equivalent dose measurements obtained at 18 (Ber-01) and 20 (Ber-02) aliquots presented low values of dispersion (7.8% and 7.5%, respectively), calculated by the overdispersion parameter of data distribution. This low dispersion values is the best indicator that samples were bleached prior to deposition and buried with near zero residual OSL signal, and, also, the post-depositional mixing processes are negligible for the interpretation of the results. The ages (Ber-01: 4,4±0,3 ky and Ber-02: 3,6±0,2 ky) are stratigraphically coherent and, in addition, compatible with the most accepted regional relative sea-level curves, which increases the reliability of the results.