Depositional setting and facies of the late Holocene sediments at the Atrato River Delta, Colombia

The Atrato River Delta (ARD), situated in the Gulf of Urabá along the Colombian Caribbean coast, is shaped by both fluvial and marine influences. Characterized by high sediment discharge from a small, tectonically active, and rain-rich catchment, the ARD exhibits sedimentation rates comparable to other tropical and subtropical deltas worldwide. This study analyzes late Holocene and modern sedimentation patterns using six shallow sediment cores extracted from the Delta's swamps and bays. A multiproxy approach—incorporating grain size, mineralogy, organic matter content, geochemistry, and luminescence dating—reveals a suite of facies ranging from coarse sand to clay, indicative of mid-to high-energy transport, sediment reworking, and relative sea-level fluctuations. These findings challenge the conventional classification of the ARD as a fluvially dominated delta. The presence of organic matter throughout the sequences suggests a gradual transition from fluvial channel infill to estuarine and marine deposition. The fine sediments of the Atrato River have contributed to the development of an extensive deltaic floodplain. Three distinct phases of sedimentary dynamics are identified: (1) a marine-dominated phase during relative sea-level rise in the late Holocene (∼1680 ± 130 to 870 ± 70 a); (2) a transitional fluvial–tidal regime (∼740 ± 60 to 550 ± 50 a); and (3) a fluvial-dominated phase associated with relative sea-level fall (∼550 ± 70 a to present). These insights shed light the formative processes of this tropical “bird-foot” delta and offer new perspectives for paleoenvironmental and sedimentary research in the Caribbean region of South America.