A sedimentação proterozóica (esteniana-toniana) na borda oeste do Craton Amazônico

Abstract

The Serra dos Pacaas Novos, located in the part of Rondonia, near the town of Guajara Mirim, and inserted in the Amazonian Craton, is built up mainly by conglomerates and sandstones of the Upper Proterozoic Palmeiral Formation. The facies analysis allows to distinguish six lithofacies: (1) massive or crudely bedded ortoconglomerate (Gm); (2) sandstone with horizontal bedding (Sh); (3) trough cross-bedded sandstone (St); (4) planar crossbedded sandstones (Sp); (5) sigmoidal cross-bedded (Ss) and (6) massive sandstones (Sm). The conglomerate facies was interpreted as longitudinal bars or stacked sheets formed during events of high discharge. The interbedded lenses of sandstone, on the other hand, was deposited during low water in minor channels. This interpretation points to a fluvial style with a braided channel patern similar to the Scott type. However, it is uncertain if this depositional pattern may be related to a proximal fluvial system or to a alluvial fan.The provenance of the Palmeiral deposits is shown by their different types of lithic fragments including quartz, quartzite, sandstone and volcanic rocks, which only in part are derived from the crystalline basement. Rock fragments from the nearby Jamari Complex (basement) were not found.The Palmeiral Formation went through several diagenetic processes which include quartz overgrowths, formation of kaolinite, illitization of kaolinite and pressure solution well documented by cavities in gravels. In the Serra of Pacaas Novos, the Palmeiral Formation is confined to a graben structure which, possibly during Cenozoic, suffered relief inversion leading to the present morphological configuration of the graben. The structural analysis resulted in the separation of three strutural sectors.