Sequence Stratigraphic Analyses and Early Cambrian Fauna of the Lagoa Do Jacaré Formation: Implications for the Cambrian Radiation

The age and paleoenvironmental significance of the Bambuí Group, Brazil, have long been debated due to the absence of biostratigraphically diagnostic fossils, particularly in strata hosting the Middle Bambuí Isotope Excursion (MIBE), extreme positive δ¹³C anomaly. This study presents a high-resolution sedimentological, sequence-stratigraphic, and paleontological analysis of the early Cambrian Lagoa do Jacaré Formation (Bambuí Group, Brazil), reconstructing a carbonate barrier island system developed under greenhouse climatic conditions. Four stratigraphic sections were logged at a 1:20 scale encompassing more than 90 meters. Fossil occurrences were investigated through integrated petrographic observations, SEM–EDS analyses, and taxonomic identification. Facies stacking patterns and depositional geometries were evaluated within a modern systems-based sequence-stratigraphic framework, in which transgressive–regressive cycles are the fundamental stratigraphic unit. The integration of facies analysis, hierarchical stratigraphic surfaces, and paleocurrent measurements enabled a detailed reconstruction of the paleoenvironment and paleogeography of the barrier system. Regional correlation with the GMD quarry provided additional constraints on lateral facies migration and shoreline dynamics. This study identified twelve lithofacies and six facies associations: beach-barrier, back-barrier, lagoon, tidal channel, flood-tidal delta, and ebb-tidal delta. These facies associations record the influence of wave, tidal, storm surge, and longshore drift processes in a carbonate coastal setting. Sandy carbonates dominate barrier and proximal deposits, whereas heterolithic facies prevail in lagoonal environments. Paleocurrent data constrain barrier orientation, indicating longshore current directions of 165° and tidal-channel direction of 70°–240°. We report the first confirmed early Cambrian skeletal fauna from the Lagoa do Jacaré Formation. The assemblage includes gastropod mollusks, probable trilobite fragments, and other small shelly fossils fragments preserved in high-energy grainstones. The occurrence of oxygen-dependent biomineralizing metazoans, together with evidence of early phosphogenesis, glauconitization, and wave- to tide-influenced facies, supports the interpretation of a redox-stratified epicontinental basin characterized by pronounced spatial and temporal oxygen heterogeneity. These findings challenge the hypothesis that methanogenesis and marine restriction in the Bambuí Sea were the primary drivers of the isotopic anomaly recorded by the MIBE. Persistent sediment reworking along the barrier likely enhanced oxygenation of shallow-marine environments, promoting the development of stable ecological niches within a basin otherwise characterized by anoxic behavior. Paleogeographic reconstruction of the barrier system and regional correlation with the GMD quarry indicate a maximum landward shift of approximately 10 km, reflecting low-amplitude relative sea-level fluctuations typical of greenhouse periods. This system exhibited two dynamic behaviors: intensified reworking and enhanced oxygenation in environments associated with the carbonate barrier, and limited facies mobility, generating more stable oxic belts. Given the prevalence of carbonate ramps during greenhouse intervals, this type of coastal configuration was likely widespread globally. In the early Cambrian, such settings were particularly significant because persistent sediment reworking enhanced oxygenation at the sediment–water interface, creating localized conditions favorable for faunal establishment. Periods of increased oxygenation (oceanic oxygenation events, OOEs) expanded available habitats, whereas returns to more anoxic conditions likely promoted speciation through habitat restriction. Consequently, the Lagoa do Jacaré Formation highlights how coastal dynamics may have played a significant role in the Cambrian radiation.