A Spatial-Temporal Reconstruction of a Huge Early Cambrian Carbonate Barrier Island System of Lagoa do Jacaré Formation, Bambuí Group, Brazil

Modern carbonate coastal barrier systems are rare compared to their siliciclastic counterparts. Furthermore, most existing modern carbonate barrier systems are R-type (shoals and emergent barriers that occur at the margins of rimmed platforms; eg., the Great Bahama Bank and the Great Pearl Bank), while C-type systems (barrier systems that form in shallow near-coast settings) remain poorly represented. As a result, C-type carbonate barrier facies models are still poorly defined. This study presents a high-resolution sedimentological and sequence-stratigraphic analysis of the Lagoa do Jacaré Formation (Bambuí Group, Brazil). A detailed facies analysis, conducted at a 1:20 scale across four vertical sections, identified twelve lithofacies grouped into six depositional associations: beach-barrier, back-barrier, lagoon, tidal channel, flood-tidal delta, and ebb-tidal delta. These associations illustrate the influence of wave, tidal, and storm surge processes on a barrier system. Sandy carbonates dominate the barrier and proximal deposits, while heterolithic facies are prevalent within the lagoon. Additionally, paleocurrent analysis enabled the identification of barrier orientation, as well as the directions of longshore currents (165°) and tidal channels that cross the barrier system (bidirectional at 70° and 240°). To reconstruct the spatial and temporal evolution of the depositional system, sequence-stratigraphic analysis at different hierarchical orders was essential. Regressive and transgressive cycles controlled changes in barrier morphology, with progradational patterns linked to barrier thickening and retrogradational patterns associated with barrier thinning. Two maximum regression surfaces were correlated with the GMD quarry, which increased the robustness of the depositional model and supported lateral facies migration of up to 10 km. Such migration is considered robust under the extreme greenhouse conditions of the period, when polar ice caps were absent and epicontinental seas, such as the subtropical Bambuí Sea, exhibited characteristics comparable to tropical marine systems. Barrier-island coastal configurations were more prevalent during greenhouse intervals. In the Early Cambrian, this setting was particularly important because persistent sediment reworking enhanced oxygenation at the sediment–water interface, creating localized pockets of favorable conditions for fauna to establish. Periods of increased oxygenation (the oceanic oxygenation events - OOEs) expanded available habitats, while returns to more anoxic conditions, when habitats became restricted again, likely promoted speciation processes. Consequently, the Lagoa do Jacaré Formation represents an exceptionally well-preserved example of a C-type carbonate barrier island system. It closely resembles siliciclastic barrier systems regardless of the sedimentary process and facies mobility. Its high preservation level makes it a valuable analog for improving carbonate shoreline facies models and for understanding how coastal configurations may have influenced the Cambrian radiation.