OSL chronology of fluvial deposits in the Peruvian Amazon: implications for landscape evolution during the Quaternary.

The Amazon River is the largest fluvial system on Earth, and its present configuration reflects long-term interactions among climate, tectonics, and fluvial processes. Hydroclimatic variability during the Quaternary period played a fundamental role in shaping the spatial organization of the Amazon River Basin, influencing sediment transport, deposition, and landscape evolution. Variations in precipitation patterns and river dynamics contributed to the development of the modern mosaic of upland areas and seasonally flooded lowlands.

This study investigates geological records from the proximal sector of the Amazon River Basin, focusing on sedimentary deposits and fluvial terraces along the Amazon River and its major tributaries, including the Marañón and Ucayali rivers, in the Iquitos region of northeastern Peru. Previous studies have interpreted this area as the remnant of an ancient topographic high, the Iquitos forebulge depozone, which acted as a structural boundary influencing the evolution of the Marañón River Basin.

Regional geomorphology was mapped using high-resolution digital elevation models derived from 10 m TanDEM-X data. Sedimentological characteristics were documented through field observations, and the stratigraphic units were chronologically constrained using quartz optically stimulated luminescence (OSL) dating. One active floodplain and three distinct fluvial terrace levels were identified based on elevation, sedimentary features, and age.

The floodplain deposits, located between 88 and 96 m above sea level, represent the youngest unit and consist of very fine- to fine-grained sands with planar-parallel and trough cross-stratification, interbedded with silt- and clay-rich layers. Their mineralogical composition includes quartz, feldspar, and heavy minerals, with OSL ages ranging from 12.66 ± 0.15 ka to 3.14 ± 0.34 ka. The first fluvial terrace (T1), occurring at approximately 125 m elevation, is composed of sands with variable grain sizes capped by silt- and clay-rich horizons and displays mineralogical variability expressed by color changes from yellow to yellowish red. OSL ages range from 112.4 ± 0.16 ka to 42.44 ± 0.11 ka. The second terrace level (T2), located at about 133 m above sea level, consists of well-sorted, medium- to coarse-grained sands with subrounded grains and a high degree of sedimentary maturity, yielding ages between 354.69 ± 0.10 ka and 228.58 ± 0.09 ka. The uppermost terrace (T3), found at 139 m elevation, is dominated by silt- and clay-sized sediments, overlain by ferruginous and sandy facies, with ages ranging from 172.6 ± 0.12 ka to 133.66 ± 0.08 ka.

Some of these units have been previously described in the literature, notably the T2 deposits, formerly referred to as the “White Sands” formation (Roddaz et al., 2006)  and assigned Miocene ages (~7 Ma). The new OSL data indicate substantially younger ages, requiring a reassessment of the regional stratigraphic framework. Overall, this study refines the spatial and temporal characterization of fluvial terraces in the Peruvian Amazon and provides new insights into the Quaternary landscape evolution of the Amazon River Basin.