O sistema desértico paleoproterozóico da formação Urupi, região do Rio Pitinga, Amazonas

Abstract

The terminal Paleoproterozoic (1.8 1.6 Ga) was punctuated by severe climatic changes that culminated in the worldwide desertification which affected the Columbia supercontinent. The records of this phase in the Amazonia, formely a piece of this supercontinent, were found in the Pitinga braquisyncline, northeastern State of Amazonas, enclosured in the siliciclastic rocks of Urupi Formation. The outcrop-based facies analysis carried out in the Urupi Formation allowed the identification of 9 sedimentary facies grouped in 2 facies associations named 1) field dune interdune and 2) sandy sheet/wadi, separated by 2nd order eolian surface. The field dune association consists mainly of well-sorted, fine to medium-grained sandstone, with rounded grains and medium to large scale cross bedding exhibiting foresets dips of 28º up to 34º and inversely grading lamination generated by eolian ripple marks migration. This eolian dune deposits are interlayered to interdune laminated pelites. The sandy sheet/wady association include well-sorted fine to medium sandstone, with rounded grains, comprises a dozens of meters laterally continuous beds succession, which predominate the low-angle, concave and planar cross bedding related to grainfall and grainflow deposits. These sandstones show complex foresets and occur alternated with subcritically climbing translatent cross-laminated sandstones. Strata generated by eolian action, constituted of subcritically climbing translatent cross-laminated sandstones alternate with wadi deposits, characterized by fine-grained sandstones embrace disseminated pelite pebbles, laminated pelite and sandstones with adhesion, warts and subaquous ripple marks and rill marks. Coarsening upward cycles formed by laminated pelites, wavy bedded rhytmite and sandstone with sigmoidal cross bedding and supercritically climbing cross lamination suggest suspension lobes in desert lakes. These deposits reached the late diagenesis to anquimetamophism, as indicated by abundant generation of sericite. The identification of a part of a Paleoproterozoic desert in the Amazônia suggests a basin with limits that traspass the exposed area of Urupi Formation. This hypothesis yield a possible correlation of studied unit with eolian deposits of Arai Formation of Roraima Supergroup, both included in the onset 1.8 Ga Proterozoic desertification event.