Carbon isotope chemostratigraphy, detrital zircon geochronology, and microfossils from the Rio Pardo Group, NE Brazil: Implications for the Cryogenian interglacial dynamics

Abstract

The Sturtian (ca. 720–661 Ma) and Marinoan (ca. 650–635 Ma) Snowball Earth glaciations caused major disturbances to Earth's biosphere and biogeochemical cycles during the Cryogenian. The nonglacial interval in-between these glaciations records the putative emergence of macrofossils, increased biotic complexity, and significant perturbations to the global carbon cycle, making it an optimal archive for investigating Cryogenian ecosystems and enabling global correlation of sedimentary successions through chemostratigraphy and paleontological data. Despite this clear significance, some Cryogenian basins remain poorly investigated. Here we present field data, detrital zircon geochronology, carbon isotope chemostratigraphy, and new microfossils from the Rio Pardo Group, northeastern Brazil. This group includes, from base to top, the Panelinha Formation, the Itaimbé Subgroup (with the lateral equivalents Camacã, Água Preta, Serra do Paraíso, and Santa Maria Eterna formations), and the Salobro Formation. The basal diamictite of the Panelinha Formation rests unconformably on the crystalline basement and is here associated with the Sturtian Glaciation. The Serra do Paraíso Formation, stratigraphically above, comprises dolostones interbedded with pelites deposited in a mixed carbonate-siliciclastic marine platform during the nonglacial interval. These dolostones display positively fractionated δ¹³C_carb values (up to +8 ‰), typical of the glacial interlude δ¹³C_carb plateau. The overlying Salobro Formation contains diamictites, correlated with the Marinoan Glaciation. Detrital zircon data from the Panelinha and Salobro formations indicates that ca. 2.1 Gyrs-old basement rocks were the primary sediment sources for these units. Carbonaceous compressions microfossils (CCMs) and agglutinated microfossils (AMs) were found in microbialites and rhytmites of the Serra do Paraíso Formation. Raman spectroscopy and EDS analyses reveal an association of organic matter and silica in the CCMs, as well as phosphate. AMs, in turn, present agglutination of clay minerals. This study highlights the Rio Pardo Group as a key unit in Brazil and expands the record of Cryogenian microfossil-bearing successions worldwide. If the CCMs presented here are indeed associated with vase-shaped microfossils (VSMs), this would represent the second report of VSMs in Cryogenian strata to date.