Impacts of sediment connectivity on Holocene alluvial records across a Mediterranean basin (Guadalentín River, SE-Spain)

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Two contiguous alluvial valleys (Maria and Guadalupe sectors) of the upper Guadalentín River (SE Spain) show distinct Holocene alluvial architecture (cut-and-fill, nested and stepped fill terraces), revealing a strong control of valley morphology and bedrock barriers on sediment delivery, storage and preservation. The headwater valley (Maria sector) preserves four morphosedimentary alluvial units corresponding to two palaeogeographic stages. Morphosedimentary unit (MSU1), the most extensive, is a major depositional valley infill dominated by sand and silt texture, showing three stratigraphic sets dated 9000–8500, 5300–4800, 4000–3000 cal BP. This vertically stacked alluvial accumulation occurs upstream of a narrow bedrock channel, that limited sediment connectivity with the downstream Guadalupe sector. Between 3000 and 2350 cal BP, a 15-m deep incision on MSU1 occurred, and subsequent cut-and-fill cycles generated three alluvial terraces dated at 2350–1900 cal BP (MSU2), 1800–700 cal BP (MSU3), and post-400 years (MSU4). In the downstream valley (Guadalupe sector), six flights of alluvial terraces (MSUg1 to MSUg6) were formed by episodic aggradation and quasi-continuous incision, in response to baselevel changes of the lower Guadalentín River during the Holocene. Alluvial chronologies in these two contiguous valleys show out-of-phase sedimentation periods under low connectivity conditions (9–3 ka), and in-phase cut-and-fill cycles after valley re-connection (post-3.0 ka). The late Holocene alluvial activity periods also coincide with morpho-stratigraphic data from the lower Guadalentín, indicating that fluvial connectivity throughout the catchment was only completed in the late Holocene.