Multi-storey calcrete profiles developed during the initial stages of the configuration of the Ebro Basin's exorrheic fluvial network

Abstract

Multi-storey calcrete profiles developed in the Quaternary on strath terraces of the Cinca and Alcanadre rivers, tributaries of the Ebro River in NE Spain. Two calcrete profiles (Tor 1 and Tor 2) near the village of El Tormillo show horizons with an arrangement that differs from that of commonly described calcrete profiles. Significant lateral changes occur in these profiles within a distance of less than 200 m, reflecting their pedofacies relationship. The Tor 1 profile on terrace Qt1 (the highest and oldest) consists of six horizons (from bottom to top): 1) coarse fluvial gravels; 2) mudstones with carbonate nodules; 3) a chalky horizon; 4) laminar horizons, including one peloidal horizon; 5) a multi-storey horizon formed of at least six minor sequences, each of which includes a lower detrital layer, a pisolithic horizon, and a thin discontinuous laminar horizon (these sequences indicate several cycles of brecciation and/or reworking); and 6) a topmost laminar and brecciated horizon also including reworked pisoliths. Some 200 m to the north of Tor 1, horizon 5 undergoes a lateral change to channel fill-deposits. The infill of the channels shows a fining-upwards sequence ranging from clasts of about 10 cm in diameter to red silts with sparse pebbles. All the clasts come from the underlying calcrete horizons. Laminar horizons are interbedded with the clastic channel deposits. The youngest calcrete profiles developed on terrace Qt3 of the Cinca River and on the Qp4 and Qp6 mantled pediment levels. All show relatively simple profiles composed mostly of lower horizons of coated gravels, with thin laminar horizons at the top. Most of the horizons, especially the laminar ones, show biogenic features such as alveolar septal structures, calcified filaments, biofilms, spherulites, micropores and needle-like calcite crystals. These features indicate the important role of vegetation in the formation of all the above profiles. The interbedding of clastic sediments and pisolithic horizons within the Tor 2 profile indicates several stages of stabilisation during profile formation. These sequences are an indication of the sedimentation, soil formation and reworking processes operating on the soil surface. The alternation of these processes is interpreted as the result of climate–vegetation changes. The channel-fills of Tor 2 indicate erosion and reworking of the hard laminar calcrete horizon. Both Tor 1 and Tor 2 are multi-storey profiles reflecting the complex sedimentation–erosion–pedogenesis relationships at the final stages of the development of its corresponding fluvial terrace. The study of these calcretes shows that these supposedly abandoned terraces continue to be active even though the fluvial network is entrenched. Both the pedofacies relationships and the complexity shown by Tor 1 and Tor 2 reflect the complex and unstable geomorphic setting in which these profiles developed. After the establishment of the exorrheic network, less complex calcrete profiles were produced in the lower terraces.