Upper Aptian mixed carbonate-siliciclastic sequences from Tucano Basin, Northeastern Brazil: Implications for paleogeographic reconstructions following Gondwana break-up

Abstract

The evolution of the Cretaceous basins of the Brazilian northeastern hinterland was associated with the Gondwana rifting and opening of the South Atlantic Ocean. The first marine ingression in northeastern Brazil occurred in the late Aptian and was recorded as the Santana Group of the Araripe Basin, which is currently an isolated basin, located hundreds of kilometers away from the Brazilian marginal basins. Bellow the first upper Aptian marine deposits, an important section of fossiliferous limestone (Lagerstätte) was deposited and preserved in the Crato Formation transitioning upward into evaporites of the Ipubi Formation. The direction of the marine ingression is controversial, with several possibilities being suggested, mainly due to the absence of other areas of upper Aptian marine sections within the hinterland. Serra do Tonã is a sedimentary mesa with scarped edges where the upper part of the Marizal Formation crops out, displaying laminated limestones, litho- and chrono-correlated with those of the Crato Formation, is preserved. Therefore, this mixed upper Aptian section, at the North Tucano Basin (Serra do Tonã), is a unique occurrence of utmost importance to the definition of sedimentary events and paleogeographical reconstruction of northeastern Brazil during the late Aptian. A detailed stratigraphic analysis allowed the definition and characterization of two upper Aptian depositional sequences bounded by regional disconformities. Both sequences are dominantly transgressive and carbonate-siliciclastic in composition. The lower sequence comprises the basal portion of the Marizal Formation and consists of a succession of fluvial sandstones, ending on a laterally continuous thin interval (<15 m) of interbedded shales and limestones bearing exposure features and paleosols on the top. The limestones show a diversity of microfacies, including microbialites, organized in high-frequency deepening-upward cycles. The recognized sequence stratigraphic architecture resembles the lower part of the Barbalha Formation in the Araripe Basin, positioned in the same palynological zone, suggesting the correlation of the shale-carbonate interval in the Serra Tonã with the Batateira Beds in the Araripe Basin. The upper sequence also exhibits a fining upward pattern, with a vertical succession starting with sandstones and shales deposited in fluvial and deltaic environments, culminating upward in laminated limestones and lacustrine shales. The stratigraphic succession is very similar to the upper portion of the Barbalha Formation in the Araripe Basin, and the laminated limestones are lithostratigraphically classified as the Crato Formation. These limestones also comprise several microfacies, organized in a transgressive-regressive cycle with the maximum flooding surface positioned on relatively deep-water carbonates. Fluvial paleocurrent directions, which are similar to those of the Araripe Basin, suggest that both basins were part of the same continental paleodrainage, flowing to the south, where the South Atlantic proto-ocean was located. Fish fossils found in shales of the Marizal Formation, further south in the Central Tucano Basin and in the same stratigraphic interval of those of the lower sequence, were interpreted as marine forms. Indeed, some of them were considered to have Tethyan affinity, probably coming from an incipient Equatorial Atlantic gateway, supporting the interpretation based on the paleocurrents. The limestones at the top of the Serra do Tonã, which are also found in inselbergs in the Jatobá Basin, are relicts of a once extensive cover of Aptian carbonate deposits, now restricted because of uplifting and erosion events from the Late Cretaceous to the Cenozoic.