The Miocene to Pliocene depositional environments of the Llanos Basin: from the decline of the Pebas System to the inception of the Amazon fluvial complex

Abstract

The Guayabo Formation in the Llanos Basin of Colombia is a critical stratigraphic unit for understanding the transition from the Miocene marine-influenced Pebas System to a fluvial-dominated depositional environment. This manuscript examines the sedimentological, mineralogical, and electrofacies characteristics of six intervals (G1 to G6) within the Guayabo Formation, offering insights into the paleoenvironmental evolution from the Middle Miocene to the Pliocene. The analysis reveals a dynamic progression of depositional environments. The G1 interval (Serravallian) exhibits funnel-shaped gamma-ray log patterns indicative of deltaic progradation, marking the transition from marine lagoonal facies to marginal deltaic settings. The G2/G3 interval (early-middle Tortonian) is unified due to similarities in sedimentological patterns, consisting of floodplains and swamp environments that predominated under reducing conditions. The G4 interval (middle-late Tortonian) reflects mixed depositional influences, including tidal flats and fluvial point bars, supported by glauconite pellets and siderite under low-oxygen conditions. The G5 interval (early-middle Messinian) indicates tide-dominated estuarine environments characterized by aggradational sandstone layers, highlighting the influence of marine transgressions and intertidal zones. The G6 interval (late Messinian-Pliocene) exhibits a progradational sequence transitioning from swamps to meandering fluvial channels, driven by climatic changes and sea-level fall. Mineralogical analyses emphasize variations in climatic conditions, depositional dynamics, and diagenetic processes across these intervals. Therefore, our findings suggest that the Pebas System's influence persisted in the Llanos Basin until the Messinian age (ca. 7.2–5.3 Ma), extending beyond its proposed termination in the late Tortonian.