Petrografia e gênese de alguns calcários cenozóicos não marinhos do Nordeste

Abstract

Cenozoic limestones found in non-marine environments (caves, springs and lakes) of northeastern Brazil have been studied on their petropraphy and origin. Two main petrograpric and genetic types are recognized: chemical, sparitic in composition, and biochemical, mainly micritic. Spelic limestones are proved to be both chemical and biochemical; the generic denomination has more an environmental than petrographical significance. Purely chemical limestones were found only in caves, being absent in springs and lakes. Algal contribution is a maior factor in the precipitation of calcium carbonate; structures such us oolites, stromatolites and filaments are common in travertines and tuffas. Algal colonies were found in stalactites, with typical stromatolitic structure. Diagenetic changes are of low grade, showing the predominance of open-space filling and, in some rare cases, grain growth under rosette forms. There is no possibility for a precise datation of the limestones; evidences point to Pleistocene to Subrecent age