"Morfologia e sedimentologia ao longo do sistema praia-duna frontal de Ilha Comprida, SP" / "Morphology and sedimentology along the beach-foredune system of Ilha Comprida, SP".

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

Ilha Comprida is a Holocene sandy barrier-island located on the south coast of São Paulo State, 63 km long and 2 to 5 km wide. Its transverse boundaries are Cananéia and Icapara inlets, at the SW and NE, respectively. The beach-foredune system of Ilha Comprida have experienced intense morphological variation in time, as showed by the successive truncations of littoral ridges. In a more restricted time-scale (at least 200 years) as covered by historical and cartographic records and aerial photographs, the most relevant changes have occurred on the northeast extremity of the island. These changes include: i) variation in width at the present most narrow region; ii) formation of a little transgressive dunefield (2 km2) in the same region; iii) initiation (in 1943) of a projection in the coastline, actually in advanced erosional state; iv) intermittent and successive appearing of lagoonal embayments at the neighboring sector of the Iguape island. The opening of the Valo Grande artificial channel, in 1852, acted either directly and indirectly on these changes. The increase of sedimentary bed-load supply induced the appearance of sand islands and submerged sand patches along the lagoonal channel. Beside this, the increasing meandering of Icapara inlet forced its rapid migration to the NE, and the longitudinal growing of the barrier. The beach-foredune system also shows great variability alongshore, observed on the morphologic alternance of depositional and erosional sectors. The tips of the barrier are characterized by the marked occurrence of small nebkha fields on foredune terraces (up to 0.5 m high). These aeolian morphological features can be related with a continuous progradation nearly Icapara and Cananéia inlets. The most-evidenced erosional zone stays at the mid-southwestern portion of the beach, as observed on the truncation of former beach and foredune ridges by the present coastline, and on the reduction in size of active incipient and established foredunes (that often appears with active cliffs). In the same region, the coastal morphodynamics changes from dissipative to intermediary beach, with presence of swash cusps and berm. Moreover, this site concentrates the highest variability and complexity of dune morphology (e.g. obliquely amalgamated incipient and established foredune ridges), beyond a increasing occurrence of ramp foredunes. In the remains of the beach-dune system, large shorefaces of low declivity (0.66o in mean), with tendency to more stable shapes of foredunes (terraces and ridges), are dominant. Statistical measures on the distribuition of granulometric frequencies (mean diameter, standard deviation, skewness), calculated through the Pearson?s moments method, indicate according to the McLaren rule two main longshore drift cells. The northeastern cell, oriented towards the NE, comprises about 53 km. The southwestern cell, comprising the remaining 10 km, is oriented towards the SW. The two depositional poles of longshore drift cells are characterized by wide and flat progradational, incipient foredune terraces. The divergence zone among these longshore cells drift coincides with the zone of the most intense coastal erosion. The mass distribution of heavy minerals is in agree with the deduced directions of longshore transport. This fact is confirmed by the tendency of increasing concentration of minerals with minor hydraulic equivalence (usually, more unstable minerals) in despite of reduction of minerals with high hydraulic equivalence (usually, more stable minerals) along the net longshore drift direction. The integration of data obtained by morphologic analysis (interpretation of aerial photographs and field features) and sedimentology (grain size, heavy minerals), allowed to postulate a morphodynamic model for the recent beach-foredune system of Ilha Comprida. According to this model, the late phase of barrier evolution is related both to the surpassing an obstacle to longitudinal growth (Icapara hill), and to the increasing sedimentary input at the Icapara inlet, favored by the opening of the Valo Grande channel.

ASSUNTO(S)

coastal quaternary beach-dune system. ilha-barreira sistema praia-duna. quaternário costeiro barrier-island

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