Detrital zircon U–Pb analysis indicate a provenance shift on the Neogene Barreiras formation, atlantic coast of Brazil

The Neogene Barreiras Formation is composed of continental to shallow marine sedimentary rocks that fringe the Brazilian Atlantic coast. Due to the large outcropping extent, various open questions about the evolution of this unit are still under debate. We present new U–Pb detrital zircon age spectra of four samples from the southern Espírito Santo state (Marataízes section) and two samples from the southern Bahia state (Arraial section). In both of the studied outcrops, a basal succession composed of coarse-grained cross-bedded sandstone, conglomerate and arkose with mudstone and claystone intercalations is superposed by an upper succession composed of massive sandstone, conglomerate and wacke. The U–Pb detrital zircon age spectra of all samples are dominated by Cambrian and Neoproterozoic grains. However, while the lower portion of both sections is dominated by Ediacaran-Cambrian grains, their upper portions also present significant Cryogenian-Tonian and sparse Paleoproterozoic (mainly Rhyacian and Statherian) and Neoarchean grains. This marks an important provenance shift within the Barreiras Formation, with a simple, local provenance sourced from the G1 to G5 granite supersuites of the Brasiliano Araçuaí Orogen for the basal layers contrasting with exposure of older basement rocks and Neoproterozoic anorogenic suites during deposition of the superior layers. We interpret the lower coarse sandstone and arkose-rich succession as deposited in an alluvial setting, with mud/claystones related to local flooding plains. Tectonic movements related to the evolution of the Atlantic passive margin, as described in previous works, caused a renewed uplift of the basement areas, exposing and denuding older rocks. Then, the upper layers deposited through debris flows covering the first succession. Further geochronological data is needed to test if the erosional event responsible for exposure of older basement rocks and sourcing of the upper layers would correspond to the Tortonian Sea level fall, related to growth of the Antarctic ice sheet to its present size. In this case, deposition of the lower succession would have occurred during the early Miocene, and the upper succession during the Pliocene. Our results shed light in the complex evolution of the sedimentary rocks generally grouped under the Barreiras Formation, and suggest important tectonic and climatic shifts during the Neogene evolution of the Brazilian Atlantic coast.