Counter-intuitive links between cave genesis and subaerial stream dynamics in the Têt valley–Lachambre network, eastern Pyrenees, France

The Lachambre cave network (Eastern Pyrenees, France) is well suited to studying the interaction between vertical successions of low-gradient cave passages in the limestone and chronosequences of fluvial terraces in the adjacent valley. Investigations here focus on cave passages striking parallel to the Têt River, and on their topographic, geomorphological, sedimentological and geochronological relationship with the two youngest generations of Pleistocene fluvial terrace, T2 and T1. Results reveal that the longitudinal profiles of the terraces and modern thalweg (1.5–3%) are ten times steeper than the profiles of the subterranean passages (0.1–0.3%), which display typical characteristics of water table caves. The respective cave and fluvial terrace profiles consequently diverge scissor-like on either side of a point of intersection, with the elevation of cave levels upstream occurring below the elevation of comparatively younger terrace treads, and even below the modern thalweg. U/Th ages obtained from speleothems and previously published 26Al/10Be burial ages of quartz-rich sediment indicate that the downstream segment of the cave passage (i) formed during Marine Isotope Stage 6, (ii) was invaded soon after by a subterranean debris cone (at times of fluvial aggradation in the Têt catchment, cones of river bedload entered the caves through valleyside sinkholes), (iii) was partially filled by an influx of gravel from upstream during MIS 4, (iv) experienced speleothem growth during MIS 3, and (v) underwent further geomorphic changes during MIS 2. Such complicated interactions between subaerial and subterranean dynamics emphasize the necessity for caution when using caves as tools for quantifying valley incision by rivers – particularly when using them as substitutes for fluvial terraces in widespread situations where none are available. Cave altimetry, speleogen inventories, cross-cutting relations in cave sediment stratigraphy and age-bracketing of fluvial deposits and speleothems all contribute to elaborating a more accurate understanding of how cave development interrelates with Quaternary alluvial cycles.