Septarian carbonate concretions in the Permian Rio do Rasto Formation: Birth, growth and implications for the early diagenetic history of southwestern Gondwana succession

摘要: Abstract Between the Late Carboniferous and Early Triassic, southwestern Gondwana supercontinent was characterized by development of a huge intracratonic basin. A large confined epeiric sea accumulation transgressive–regressive sequence were formed continuous subsidence related to tectonic effects caused Sanrafaelic Orogeny consequent generation accommodation space. The Permian Rio do Rasto Formation documents last progradational cycle complete continentalization this sea. basal member (Serrinha) is believed have been deposited in shallow epicontinental water body subjected storms influenced episodic deltaic incursions. One most remarkable characteristics Serrinha Member presence carbonate concretions hosted mudstones very fine sandstones. Here, we combine sedimentological petrographic descriptions coupled with geochemical stable carbon oxygen isotopic data elucidate nature these concretions. non-deformed internal structure, decreasing proportion cements relative detrital grains toward concretion edges, core-to-rim variations, perhaps importantly, preservation well-developed cardhouse fabric support an early diagenetic origin for structures at burial depths tens meters. Stable isotope analyses micritic calcite calcites filling septarian fractures reveal major negative excursions both δ 18 O 13 C values. Oxygen ratios obtained vary between − 12.1 − 2.6‰. also exhibit values (− 14.2 − 13.8‰), average − 14‰. range from − 5.0–0.2‰. calcite-filling are (− 4.4 − 3.3‰). signatures suggest that precipitated freshwater environment rather than marine setting. isotopes derived source slightly depleted C, supporting least partial organogenic contribution weak sulfate reduction rates typical systems. Sedimentological analysis shows which constantly supplied rivers meteoric waters, suggests enormous basin restricted connections Panthalassa Ocean once existed.