Abstract
ABSTRACT The scarcity of stratigraphic data has hindered the demarcation of the outcropping area of Miocene deposits of the Amazon Basin, represented informally by the Novo Remanso Formation. Moreover, this unit is characterized by a sparse and irregular geographic distribution due to its sedimentological features and rare fossil content. Miocene deposits cropping out in central Amazonas Basin area were described in sedimentological terms and analyzed palynologically. All analyses were undertaken in samples collected at the Uatumã River banks (Itapiranga and São Sebastião do Uatumã cities). Lithostratigraphic data shows that Novo Remanso Formation consists of sandstones, with subordinate conglomerates and pelites, characteristic of a meandering fluvial paleosystem, with fluvial channel, point bar, floodplain and crevasse splay facies. The palynoflora retrieved from five samples consists exclusively of continental-origin palynomorphs dominated by angiosperms species. Trilete spores are well represented, while gymnosperms pollen grains are minor components. The presence of Psilastephanoporites tesseroporus, Syncolporites poricostatus, Jandufouria seamrogiformis and Polypodiaceoisporites potoniei ensure these deposits fits into the Grimsdalea magnaclavata palynozone (Regali et al. 1974a, b), and the Grimsdalea magnaclavata/Crassoretitriletes vanraadshooveni palynozones of Jaramillo et al. (2011) considered Middle Miocene age. This age is confirmed by the zonation of Jaramillo et al. (2011), based on the LADs of Bombacacidites baumfalki (11.57Ma) and Crototricolpites annemariae (12.91Ma); and the FAD of Psilastephanoporites tesseroporus (14.00Ma). With these new data presented herein, it is possible to assume that the Miocene strata represented by the Novo Remanso Formation covers a larger area in the basin than previously considered, and that it may be extended for about 300 km until the Manacapuru village, indicating a Miocene subsidence phase.
Highlights
The current stratigraphic framework of the Amazonas Basin, proposed by Cunha et al (2007), comprises two first order megasequences, one Paleozoic and the other MesozoicCenozoic
While the Paleozoic sequence has been object of numerous studies, due to its interest by the oil industry, the Mesozoic-Cenozoic (Javari Group) still lacks detailed lithostratigraphic/geochronological data aiming at the individualization of its constituent units
The Cenozoic sedimentary record, with a maximum thickness of 200 m (Cunha et al 2007), has been assigned to the strata of the Solimões and Marajó formations, occurring restrictedly nearby Purus and Gurupá arches, respectively. These formations belong to the stratigraphic framework of the Solimões and Marajó basins, and their occurrence in the Amazon Basin has been questioned
Summary
The current stratigraphic framework of the Amazonas Basin, proposed by Cunha et al (2007), comprises two first order megasequences, one Paleozoic and the other MesozoicCenozoic. While the Paleozoic sequence has been object of numerous studies, due to its interest by the oil industry, the Mesozoic-Cenozoic (Javari Group) still lacks detailed lithostratigraphic/geochronological data aiming at the individualization of its constituent units This sequence is represented mainly by the Cretaceous unit (Alter do Chão Formation), which extends uniformly throughout the basin and exhibits maximum thickness of 1,250 m. Since 2000, stratigraphic, sedimentological and sedimentary facies studies (Rozo 2004, Soares 2007, Soares et al 2010) allowed the individualization of post-Cretaceous sediments in outcrops along the Solimões-Amazonas river system in the area between Manacapuru and Itacoatiara, central portion of the Amazon Basin These deposits occur unconformably on top of the Alter do Chão Formation, being marked at the base and at the top by lateritic paleosols, and they have been informally termed Novo Remanso Formation, according to the homonymous locality east of Manaus. Additional geological and subsurface geophysical data (boreholes) confirmed the continued presence of this sedimentary cover for ca. 180 km between Manaus and Itacoatiara (Costa 2002, Andrade & Soares 2009)
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