Paleoenvironment, chemostratigraphy, and age of Pennsylvanian carbonate platform succession of the Amazonas basin, northern Brazil, Uruara region

摘要

Extensive Pennsylvanian carbonate platforms are the main transgressive successions recorded on Laurasia and Gondwana continents. The western Gondwana was one of the important sites of these transgressive carbonate accumulation exposed on several basins in Amazonia. Carbonate platform successions are exceptionally preserved in the southern border of the Amazonas Basin, northern Brazil, represented by the Pennsylvanian Itaituba Formation, previously interpreted as tidal flat, lagoon, and carbonate subtidal deposits. Microfacies and petrographic analysis of similar to 32 m-thick succession of core F5 allowed the definition of nine microfacies grouped in three facies associations: lagoon (FA1), bioclastic bars (FA2) and open carbonate platform (FA3). The FA1 is composed by massive lime-mudstones and dolostones with silt to sand-size terrigenous grains, brachiopods, echinoderms, and rare foraminifers. The lagoon deposits represent a low energy environment with intense micrite precipitation, low diversity of fossils, but that indicates a marine connection, mainly because of the presence of echinoderm and brachiopods. The FA2 is composed of massive grainstone with ooids, intraclasts, brachiopods, echinoderms, corals, bivalves, and foraminifers cemented by granular calcite. The bioclastic bars are affected by wave actions and currents that reworked the carbonate substrate. The FA3 is composed of wackestone and lime-mudstone with micritic matrix, terrigenous, pyrite, brachiopods, bivalves, echinoderms, foraminifers, and ostracods. The Itaituba carbonate platform deposits indicate sedimentation under warm climate, normal salinity and were directly connected with the ocean by the record of benthic organisms. delta C-13 and delta O-18, were obtained for these carbonate rocks and the determination of alteration degree was assisted by Rb/Sr, Fe/Sr, Ca/Mg, and Fe/Sr ratios assisted by digenetic indicators. The Ca and Mg become constant between 30 and 40% in the succession while the Si, Al, and Fe contents decrease upward coincident with the transgressive trend of the succession revealing low continental inflow. Diagenetic processes are represented mainly by stylolitization, cementation, micritization, dolomitization, fracturing, and pyritization that partially modify the framework of carbonate rocks, attesting low diagenetic alteration. The delta C-13 and delta O-18 range from +1.60 parts per thousand to +5.42 parts per thousand, and -8.73 parts per thousand to +0.80 parts per thousand respectively. These values show wide dispersion (non-covariance), suggesting the circulation of diagenetic fluids through these rocks was not able to modify the original composition of d13C. The carbon and oxygen data coincide with values obtained for carboniferous marine deposits, and the positive data of d13C in FA2 and FA3 indicates intense organic productivity, similar to that recorded in transgressive deposits associated with the Panthalassa Ocean. The strontium ratios values vary from 0. 708466 +/- 0. 000011 to 0. 708417 +/- 0. 000009 with 30-299 mu g/g in the carbonate platform and presents three age intervals of deposition with a range varying between 293 and 303 Ma (Bashkirian to Kasimovian stages). These data are analogous to the palynomorphs and conodonts ages from the Pennsylvanian of the Amazonas Basin positioned the basal part of the Pennsylvanian succession in the Bashkirian stage.