Faciologia e evolução quaternária dos eolianitos da faixa costeira entre os Municípios de Amontada e Acaraú, Litoral Oeste do Ceará

Abstract

The Ceará coast area situated between the counties of Amontada and Acaraú is marked by a number of wind foinied structures from a Holocene age, geomorphologically named by yardangs. These structures are casually called eolianite. Such rocks are of a siliciclastic type with bioclasts percentages among 10-25% and weakly cemented by calcium carbonate. This forms of deposits represents the phenoms of shoreline retreat, presenting shore facies beneath continental facies, recorded in the stratigraphic succession of the area. Six facies units were described, which represents several depositional systems wrapped in the quatemary evolution of the area, as of in columnar sections generated in field, these related to the current levei of the sea. The described facies have an intimate relation with the evolutionary model of the current foredunes. Hence, based on the principie of actualism, we can consider that this model can be applied to a recent past. It was considered by then the internai structure and the biogenic content in this interpretation, thus identifying four stages of the foredunes, where the first two stages are usually represented with stratification of iow dip angles (<15°) and the others with stratification of high angle (>15') and the presence of icnofossils. As regards the evolutionary model of eolianite, from existing ones, can be considered the seven stages in the evolution of stratigraphic succession area. These are: 1 - Accumulation of plataformal biogenic carbonate; 2 - Relative lowering of sea levei; 3 - Formation of protodunes; 4 - Foiniation of foredunes in stage 1 and 2 by the accommodation of fixing vegetation; 5 - Formation of dunes in stage 3 and 4 and icnofossils; 6 - Formation of beach rocks at the base of the shore deposits events from different cementing and 7 - Erosion and deflation.