THE DEPOSITIONAL ENVIRONMENT OF THE BOA VISTA FORMATION OF RORAIMA, NORTHERNMOST BRAZIL

The Boa Vista Formation of northernmost Brazil comprises sediments of late Cenozoic age that occur in a sedimentary basin with gentle relief and under-developed drainage systems marked by riparian vegetation in a savanna environment. The formation is composed of sandstone and local conglomeratic deposits. It crops out in the drainage basins of the Urariquera, Tacutu, Maú, Cauamame, Parimé, Surumu and Amajari rivers, as well as the northern area of the Rio Branco basin. The lower contact of the deposits is laid on Precambrian granitic rocks cut by several intrusive suits of Cretaceous age.

The Boa Vista Formation is not well known, and as a consequence there is debate on several questions regarding its age, depositional environment and regional correlation, although the formation has been interpreted as paleodunefield deposits. The region is of great importance to the understanding of the paleogeographic evolution of the western Amazon in the Neogene, possibly recording a large-scale drainage reversal, subsequent to a major river system that ran to the north, toward the central Atlantic Ocean.

Recent field work was undertaken in the state of Roraima in order to evaluate the hypotheses of aeolian or fluvial origin for the unit. Field descriptions and petrographic data reveal a preponderance of poorly-sorted coarse to medium-grained quartzose sandstone, interbeded with whitish to grayish claystone and siltstone. They are characterized by 4-5 m thick beds of rounded pebble to boulder-grade conglomerate unconformably overlying an irregular regional erosional surface. The composition of the clasts is dominated by vein quartz, but lateritic clasts are dominant locally. The textural characteristics of the unit point to a fluvial origin for the formation, in contrast to prior interpretations of an aeolian origin, but because of limited exposures and the conditions, not enough paleocurrent data could be acquired to evalute the hypothesis of a paleoflow to the north.