Quaternary sedimentary evolution of Ilha Comprida, São Paulo state

The Ilha Comprida island is the largest sandy barrier of São Paulo Coast. The island is 63 km long and 500 m to 5 km wide. It extends following the coast (approximately NE-SW direction) from the Icapara inlet to the Cananéia inlet. This barrier was studied using aerial photographs, grain-size parameters, heavy minerals analysis and optically stimulated luminescence dating (OSL). Five evolutionary phases could be determined. The initial phase corresponds to the transgressive barrier at approximately 6000 years B.P.. In the next four phases, two growth components, longitudinal and transversal, occur with variable importance. From the initial phase to 5000 years B.P. the island had accelerated growth, both longitudinal (10.2 to 22.8 m / year) and transverse (2.6 m / year). During this period the identified coastal ridges have beach origin. From 5000 to 2000 years B.P. the transverse component of growth fell to almost zero while the longitudinal component was relatively low (5.2 to 6.9 m / year). From 2000 to 200 years B.P. the hills near Iguape blocked the longitudinal component and forced intense progradation (up to 0.5 m / year) for the most island length. In this phase there was a predominance of beach ridges formation before the Little Ice Age (AD 1450 to 1850) and of foredune ridges formation after this period. From 200 years B.P. until the present the blockade imposed by the Iguape hills ceased and the island reassumed the longitudinal component, accelerated by the opening of a artificial channel (Valo Grande) in the Ribeira de Iguape River. This period has variable progradational rates through the island, with regression in the extremities and transgression in mid-south. The marine facies succession that outcrops besides lagoon margin is predominantly regressive. Possibilities of transgressive facies succession were found only in the southern half of the barrier. Geochronology evidences reinforce the hypothesis of transgressive barrier for the initial phase of the island. The aeolian facies succession confirms the onshore advance of the blowouts onto the inner and older beach ridges. The ages for the most blowout activity are correlated with the Little Ice Age suggesting intensification of cold fronts incursions. The contrast between ridges types, beach ridges formed before the Little Ice Age versus foredune ridges formed after this age, is indicative that the wind conditions are intensified since then. The use of heavy-minerals paired indices allowed the identification of factors and processes that control their assemblage. The RZi (rutile and zircon) and THi (tourmaline and hornblende) show the contribution of the Ribeira de Iguape River sediment supplied in northeastern of the Ilha Comprida. The TZi (tourmaline and zircon) responds to the hydraulic sorting process showing. It indicates a sediment transport pattern with divergence of drift cells in the mid-south region, which is consistent with grain-size and field data. The sediment supply from Ribeira de Iguape River is interpreted both as a direct source and as an indirect source by inner continental shelf and alongshore drifts. The combination of grain-size and heavy-mineral analysis demonstrate to be reliable in determining sediment transport patterns, with indications of their control by hydraulic and aerodynamic sorting. The Single-Aliquot Regenerative-Dose procedure (SAR) used for OSL dating demonstrates to apply to the Ilha Comprida sediments, both wind and shallow marine ones. The ages are consistent with the more acceptable relative sea level curves and with the coastal ridges and stratigraphic succession. (AU)