Faciologia e evolução dos depósitos eólicos costeiros do oeste do Ceará (Brasil) no Holoceno tardio

Abstract

This study aimed to characterize the morphology and evolution of coastal deposits in western Ceará state based on the variation of the sea level in the Late Holocene, thus reconstructing the paleogeography of the area. Based on the petrography and facies analysis, fi ve sedimentary facies and their original depositional forms (facies associations) have been identifi ed as follows: coarse-grained sandstone with layers of conglomerate and inclined planar stratifi cation (foreshore); medium sandstone with low-angle cross-stratifi cation (foredunes), sometimes obliterated by rhizoliths (vegetated foredunes); medium sandstone with trough cross-stratifi cation (the base of barchanoid dunes); medium sandstone with crossstratifi cation; and Z-shaped truncation surfaces (linear extensions associated with barchanoid dunes). Coarse-grained sandstones correspond to beachrocks, which were overlaid by eolianites (remaining facies), in the form of yardangs subparallel to the direction of present eff ective wind (NE-SW). The key features are contact between beachrocks and eolianites (a former foreshore/backshore limit) about 2.0 m above the maximum of present high tide; inland outcrops of beachrock; succession of phreatic to vadose marine cementation in the beachrocks; and eolianite/beachrock incised valleys. All this evidence indicates that the eolian deposits were generated during a relative rise in the sea level, when there was greater availability of sediment to supply the dune fi elds, being cemented and locally eroded by streams during the subsequent relative fall.