STRATIGRAPHIC SIGNIFICANCE OF HEAVY MINERALS IN SEDIMENTS OF THE NORTHEASTERN BRAZIL

Abstract

Sediments from the Upper Cretaceous post-rift section of Potiguar Basin and their Cenozoic continental covers show that their heavy mineral associations can distinguish stratigraphic units. The Açu River flows along the axis of the basin trough, and its deltaic plain is developed in the coastal region of Macau within the main axis of the basin. The oldest outcropping units are the siliciclastic rocks of the Açu Formation (Albian-Cenomanian) and carbonates from the Jandaira Formation (Turonian-Early Campanian). Oligocene to Miocene volcanism constitutes the Macau Formation. Miocene fluvial and alluvial fan deposits of the Barreiras Formation and Quaternary fluvial and marine deposits synthesize the continental sedimentation in the basin. Different assemblages have been observed for each siliciclastic unit, with the heavy mineral suites become increasingly complex from the oldest (AÇU Formation) to the youngest one, probably reflecting the tectonic events and the erosion history of the source terranes: i) the assemblage from Açu Formation shows almost no transparent minerals (average of 10%) and the texture of opaque grains is intensely corroded; ii) Barreiras Formation are rich in zircon (up to 40%), both prismatic and rounded, and small amounts of epidote, rutile, tourmaline and garnet; iii) gravel deposits are rich in epidote with subordinate amounts of zircon, tourmaline and hornblende; iv) recent alluvial deposits show an increase in the amount of unstable heavy minerals, chiefly amphibole, reflecting a continues sediment transport from the basement source. The difference in heavy mineral suites from Barreiras Formation and the gravel deposits provides a mean to separate one unit from the other.