A planície costeira holocênica de Campos Verdes (Laguna, SC): evolução sedimentar inferida a partir de georradar (GPR), granulometria e minerais pesados

Abstract

Campos Verdes region, in Laguna, mid-south coast of Santa Catarina, is characterized by sets of beach ridges alignments, oriented transversal to the present coastline and separated from each other by truncations with a concave-convex pattern. This strandplain is limited towards SW by a higher area (1m) without beach ridges, where four shell mounds are located. The alignments are superposed by several groups of inactive parabolic dunes, attributed to the aeolian generation 3 (post maximum Holocene flooding). The sedimentary growing began in the southwest (higher area), where the former beach ridges developed anchored on a sandy spit. This spit was formed according to the Zenkovitch’s models of lagoonal circulation; moreover, it would have worked as a “trap” for the accumulation of sediments that formed the strandplain. This hypothesis is highlighted by the GPR sections, with reflectors dipping towards N, indication of progradation towards this azimuth. The McLaren’s granulometric variation pattern of “fining, better sorting and more negative skewness” towards N can be interpreted as resulting from a progressive sedimentary reworking during progradation. In a longitudinal profile, the pattern of “coarsening, better sorting and more positive skewness” towards W, suggests transport by longshore currents towards this azimuth. The variation in the heavy minerals assemblage shows a higher concentration of unstable components in opposite to the decrease of ultrastable components towards N and W, with an increase in the relation tourmaline/zircon towards NW. This decreasing mineralogical maturity towards the sector formed in the end of the progradation can be attributed to three factors: 1. progressive concentration by hydraulic sorting; 2. increase in the sedimentary input of newer sediments, because of the stronger influence of Tubarão river deltaic front; and 3. least time for the action of pos-depositional solution in the newer beach ridges.