Estratigrafia e eventos da transição Neoarqueano-Paleoproterozoico da Bacia de Carajás, sudeste do Cráton Amazônico

Abstract

The Neoarchean-Paleoproterozoic transition is marked by a series of paleoenvironmental, paleoclimatic and tectonic changes that resulted in dramatic events, which imposed to the Earth novel conditions, some of them with irreversible characteristics. On the paleoenvironmental aspect, it is highlighted the rise of oxygen in the hydrosphere-atmosphere system, onset the Great Oxidation Event (GOE) at around ca. 2.45 Ga. The rise of this gas caused consequently the decrease of the greenhouse gases such as CH4, which promoted the emergence of glacial episodes at around ca. 2.45–2.22 Ga, generically termed the Huronian Glacial Event (HGE). Although several studies support the hypothesis that these glacial episodes represent the first global glaciation of the Earth's history (Paleoproterozoic snowball Earth), stratigraphic and geochronological contradictions impose doubt as to its global extension. Strangely, although this set of events is widely recognized in several cratonic areas around the globe, these events are still poorly understood and/or not yet reported in the Amazonian Craton. In this study, the stratigraphic, sedimentological and geochronological investigation of the volcano-sedimentary succession (ca. 5-km-thick) of the Carajás Basin, situated in the southeastern Amazonian Craton, northern Brazil, allowed the recognition and sequencing of some of these events in this basin. Two new units are being formally proposed to this basin: the Serra Sul and Azul formations. Glacial diamictite intervals of the Siderian–Rhyacian (ca. 2.58–2.06 Ga) occur stacked within the Serra Sul Formation, and are the first reported occurrence of glacial deposits of that age in South America. In paleogeographic terms, the occurrence of Paleoproterozoic glacial deposits in this part of the globe, expands the reach of these glaciations to the Amazonian Craton for the first time, although the Serra Sul diamictite may be correlated with any of the know Paleoproterozoic glaciations, or none of them. Well-preserved textures, such as glacial foliation and dropstone features, indicate that the deposition of the Serra Sul Formation occurred in a coastal subglacial setting, in which glaciogenic sediments were resedimented in submarine fan system, and through ice rafting process in distal waters of the marine environment. The Serra Sul glacial system was developed immediately above of pre-glacial strata represented by the Neoarchean banded iron formation and volcanic rock units, which not was the main substrate, but also was the main source of sediments to this glacial system. Additionally, the stratigraphic results indicate that the immediately above of the Serra Sul diamictite, rhythmite deposits of the Azul Formation, locally enriched in manganese, were deposited in a shallow marine environment (offshore and offshore transition/shoreface zones), as a result of the sea level rise during the deglaciation phase. The manganese-bearing strata were possibly deposited in association with black shale deposits—which allowed the formation of rhodochrosite during diagenesis—in deep zones of the marine basin. Petrographic and mineralogical evidences, supported by field observation, indicate that manganese oxides were secondarily remobilized through faults to zones with low strain and high permo-porosity within red bed strata of the Azul Formation, similarly to that observed in hydrocarbon migration. In stratigraphic terms, the Azul Formation represents the same interval previously arranged in the lower member of the Águas Claras Formation. This formation was redefined to designate exclusively sandstone, conglomerate and jasper conglomerate strata, deposited in a braided fluvial system, which occur in unconformably immediately above of the Azul Formation. Moreover, it is suggested that the Azul and Águas Claras formations are the stratigraphic record associated with a transgressive-regressive sequence (T-R). The dating (U-Pb) of detrital zircon grains separated from the Azul and Águas Claras formations indicate that Meso- to Neoarchean rocks, possibly of the Rio Maria and Carajás domains, were the main source of sediments. The 207Pb/206Pb Age distribution of the 76 concordant analysis of the Azul Formation indicate a youngest population at ca. 2.27 Ga, interpreted as the maximum deposition age of this unit. The occurrence of Rhyacian to Siderian zircon grains in this unit strongly suggest that the Bacajá Domain may have been a subordinated source of sediments, and in paleogeographic terms, suggest a possible connection between this domain and the Carajás Domain at that time period. The integration of the results obtained from this study, supported by previous data on the regional geology, allowed the proposition of a tectono-sedimentary evolutive model to the Paleoproterozoic succession of the Carajás Basin. It is envisaged that this basin evolved during the greater part of the Paleoproterozoic in a foreland style, as result of the collision of the Bacajá and the Carajás domains during the Transamazonian orogenetic cycle at ca. 2.2–2.0 Ga. The convergent movement of these blocks caused the gradual uplift of the Carajás protocontinent; the closure of the Azul Sea, and installation of a wide fluvial-alluvial system, in which the Águas Claras and Gorotire formations were deposited. This scenario of profound changes is directly related to the Columbia supercontinent assembly at the end of the Paleoproterozoic, that promoted the continentalization and amalgamation of the ancient landmasses that later formed the proto-Amazonian Craton at the end of Paleoproterozoic.