Eolianitos Quaternários do Nordeste do Brasil

Abstract

The coast of Northeast Brazil is under the influence of the winds that blow towards the Intertropical Convergence Zone (ITCZ) and that have favored the formation of eolian depositional systems in the late Quaternary. In areas with bioclastic contribution, such as Piauí and part of Ceará and Rio Grande do Norte, on the coast of the continental portion, and the Fernando de Noronha Archipelago, in the insular portion, these systems include eolianites, which were characterized here as for: paleowinds, depositional facies, granulometry, petrography, SEM, carbonate content, heavy minerals and chronology via 14 C and OSL. The continental eolianites are associated with an inner shelf in mixed ramp, with a smooth inclination (<1°), which extends in the emerged area, generating large space for eolian accumulation, and favors the development of deflation plains with kilometers of extension; the outcrops are extensive, a few meters thick, dominated by facies with low angle cross stratifications, attributed mainly to trailing ridges. Island eolianites are associated with a narrow isolated carbonate shelf (less than 10 km), with reef edges and sharp slopes; because the islands are supported by volcanic-pyroclastic rocks of Quaternary age, with rugged relief, the area of eolian accumulation is limited, which means that the outcrops are less extensive and of greater thickness, with predominance of high angle cross stratifications, sometimes with preservation of bedform crests. Continental and island eolianites have in common the presence of red algae as the dominant intraclast grain and of early calcitic cement, formed in meteoric vadose conditions in Holocene deposits and phreatic conditions in the Pleistocene ones. Mainly in Pleistocene eolianites, inversions of 14 C ages between bioclast and cement, associated with the presence of bioclast pseudomorphs formed by moldic filling, indicate that the diagenetic history had at least three phases: early interparticle micritic cementation, under conditions of relative aridity; moldic dissolution in the phase of increase and/or peak of humidity; and filling of pores with spar or microspar at the beginning of the new cycle of falling humidity. Among continental eolianites, ages vary between 7.0 ka and 14 years, in less consolidated outcrops, with a preserved ridge shape; and between 86.2 to 40.7 ka, compatible with MIS 5 and MIS 3, in the strongly consolidated outcrops, which occur on coastal cliffs. In insular eolianites, two 14 C age ranges were found (between 46.1 and 21.7 ka cal AP and from 18.7 to 5.3 ka cal AP), the first of them containing bioclasts without age inversion with the cement, which allows suggesting deposition in the MIS 3 event.