Analysis of a deltaic system reorganisation during regression and subordinate transgression: Insights from the Lower Cretaceous Pilmatué Member

ABSTRACT Deltaic systems are dynamic environments that can experience repeated spatial and temporal reorganisation; however, the stratigraphic expression of transitional states and their preservation in the rock record, specifically how fluvial‐dominated deltaic systems change through transgressive events, remains poorly understood. To address this gap this study focuses on a ~100 m thick interval of the Pilmatué Member (Lower Cretaceous, Neuquén Basin, Argentina), spanning a transgressive surface between two depositional sequences of this unit. Analysis of 228 m of cores from three wells revealed seventeen facies grouped into eight associations, namely: distributary channels (FA1), interdistributary plain (FA2), terminal distributary channels (FA3), proximal mouth bars (FA4), distal mouth bars (FA5), prodelta (FA6) and proximal and distal ‘estuary’ (FA7 to FA8). FA1 to 5 comprise predominantly sandstone to mixed sandstone–mudstone deposits with abundant carbonaceous material, soft‐sediment deformation and limited bioturbation, reflecting fluvial‐dominated deltaic conditions that grade laterally into the mudstone‐dominated prodelta deposits of FA6. FA7 to FA8 record heterolithic sandstone–mudstone intercalations with abundant bioclasts, carbonate cement, marine trace fossils and dewatering structures within a distinct fining‐upward trend. This evidences a shift towards more marine‐dominated settings, in contrast to the coarsening‐upward trend of the underlying progradational facies (FA1 to FA6). The stacking patterns allowed the recognition of five units (U1 to U5). Basal U1 to U3 comprise regressive deltaic deposits (FA1 to FA6), whereas U4 records a short‐lived transgressive episode with atypical facies resembling ‘estuarine’ deposits (FA7 to FA8), interpreted as a drowned‐delta stage. Overlying, U5 marks the return to progradational deltaic deposition (FA3 to FA6), with prodelta sediments grading into mouth‐bar deposits. This succession thus illustrates a temporary reorganisation of a fluvial‐dominated delta during a disequilibrium stage driven by relative sea‐level rise and/or reduced sediment supply, followed by a recovery of its progradational nature. The studied interval of the Pilmatué Member serves as an ancient analogue of delta response to environmental perturbations. This rarely recorded reorganisation highlights the significance of out‐of‐equilibrium conditions while contributing to more integrated models of dynamic fluvio‐marine systems.