Recent marine limestones from the shelf off tropical Brazil

Abstract

Samples of recent limestones and calcareous sandstones dredged from the continental shelf off northern and northeastern Brazil have been studied. Three types of rock can be distinguished: (1) calcarenites composed of many organisms and fragments, classified as biomicrites in thin section; (2) algal accretionary limestones, showing almost entirely algal structures, named algal biomicrites; (3) calcareous sandstones, lime-cemented shelf sands of fluvial origin. Besides these, one sample was collected from the continental slope, determined as a foraminiferal intrabiosparite. Study of fauna and mineralogical composition revealed a recent age for all types, confirmed by the faunal assemblage and the dominance of aragonite and high-Mg calcite. The lithification of calcarenites and calcareous sandstones occurred by cementation of the loose bottom sediment under favourable conditions, in a marine open shelf environment, up to depths of about 100 m. The algal limestones are growth structures of calcareous algae, chiefly Lithothamnium.