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As sucessões cambriana-siluriana da Bacia do Parnaíba e da Província Mineral de Carajás: paleoambiente, proveniência e extensão da glaciação siluriana no Gondwana Oeste.
(Universidade Federal do Pará, 2024-01-12) ROMERO BARRERA, Ivan Alfredo; NOGUEIRA, Afonso Cesar Rodrigues; http://lattes.cnpq.br/8867836268820998
The Lower Paleozoic sedimentary record corresponding to the northern and northeastern Brazilian sedimentary basins is well exposed in outcrops within the Parnaíba Basin and locally in the Carajás Mineral Province (CMP). These deposits, considered Cambrian/OrdovicianSilurian age, based on the Silurian glacial strata identification maximum depositional age (UPb in detrital zircon), represent a unique opportunity to understand the paleoenvironmental and paleogeographic evolution of Western Gondwana. The records analyzed in this study include siliciclastic sequences with thicknesses ranging between 300–400m and lateral continuity that exceeds 300,000 km², representing alluvial, glacio-marine, and deltaic deposits unconformably overlying crystalline basement rocks. The basal succession comprises coarse-grained lithoarenites and fluvial conglomerates exposed in intracratonic basins and isolated grabens in northeastern Brazil and central Africa. In Brazil, these units correspond to the Ipu, Cariri, and Tacaratu formations, while in Africa, they are represented by the Inkisi, Banalia, and Biano groups. The conglomerates and sandstones of the Ipu Formation, at the base of the Serra Grande Group in the Parnaíba Basin, are here correlated with the corresponding alluvial deposits of the Gorotire Formation in the Paredão Group within isolated grabens in the CMP. These deposits are sharply overlain by massive to stratified diamictites, carbonaceous shales with dumpstone and dropstone structures, and sandstones with sigmoidal cross-stratification belonging to the upper member of the Paredão Group in CMP and the Tianguá Formation in Parnaíba Basin. During the amalgamation of the Gondwana supercontinent in the Neoproterozoic-Cambrian periods transition, the coalescence of continental masses was controlled by the development of major orogenic systems, accompanied by the deposition of the first sedimentary sequences in rift-type basins. These extensive subsiding geotectonic compartments coincided with the reorganization of regional drainage patterns and the generation of large sediment volumes transported by transcontinental fluvial systems. At the Cambrian-Ordovician period boundary, these “Big Rivers” extended across vast peneplains for hundreds of kilometers, overcoming the current intracratonic basin limits along the northwestern margin of the supercontinent and fed by diverse source areas. The fluvial architecture of these successions consists of massive to stratified tabular beds of conglomerates and sandstones, laterally continuous over kilometers and sometimes filling concave-channel geometries. These deposits are arranged in metric-scale fining-upward cycles, reflecting the dominance of sheet flow processes with sporadic channel incisions filled by small- to large-scale two- and three-dimensional bedforms. Alluvial fan facies for the Ipu Formation were not observed, suggesting basinal edges erosion. In addition, unidirectional paleocurrent data predominantly oriented NNW extend beyond the Parnaíba Basin current limits, indicating that the depositional area was more extensive than presently preserved. During the transition between the Ordovician and Silurian periods, the western portion of the Gondwana Supercontinent experienced an extensive glacial period, climaxing in the Hirnantian (~445 Ma), as recorded in rocks from northern African basins. Thus, the glacial event that began in the Middle Ordovician likely froze continental areas representing the headboard of large alluvial systems in Africa and Brazil, initiating the decline of these transcontinental drainages. The migration of the supercontinent toward the South Pole coincided with astronomical factors such as Earth rotational changes and decreased solar radiation, promoting ice sheet growth. The end of the glacial event was marked by the largest eustatic sea-level rise in Earth history, triggering significant marine transgressions starting in the Llandovery (~443 Ma) and ending in the Ludlow (~423 Ma). The melting phase promotes the development of proglacial glacio-marine systems with widespread icebergs, ablation processes, and an expansion of anoxic zones in seas, allowing extensive global organic matter preservation. The ice-melt deltaic systems progradation marks the end of the post-glacial sequence deposition. These events records are well represented in Paredão Group upper member and Tianguá Formation successions. Through the investigation of Ordovician sedimentary records from northeastern Brazil and central Africa, complemented with detailed analyses of fluvial architecture and U-Pb detrital zircon ages, a compelling narrative emerges of immense transcontinental drainage systems that shaped the landscape after the Gondwana Supercontinent amalgamation. Additionally, Paleozoic records are described for the first time in the CMP, providing a critical key for improved stratigraphic understanding and enhancing paleoenvironmental, paleogeographic, and paleoclimatic reconstructions of the Gondwana supercontinent during the Lower Paleozoic.