INCORPORATION OF SOIL STRATIGRAPHY AND GEOCHRONOLOGY OF REGIONAL TERRACES IN THE EBRO BASIN, SPAIN, FOR DIFFERENTIATING CLIMATIC FROM TECTONIC CONTROLS ON LANDSCAPE EVOLUTION

Quantifying landscape and lithospheric responses to climatic and tectonic forcing is enhanced through the analysis of regional deformation of fluvial terraces. Landscape position often provides the primary basis for correlation of terraces; however, development of a regional stratigraphic framework of fluvial deposits and strath terraces is required to adequately elucidate climate and tectonic controls on landscape history. We integrate detailed geomorphic mapping, soil stratigraphy, and radiometric dating of fluvial deposits to establish regional correlations among stepped sequences of fluvial terraces along two principal rivers (Rio Cinca and Rio Gallego) in the central Pyrenees and adjacent Ebro basin of Spain.

Our analysis focuses primarily on the regional distribution of two predominant Pleistocene strath terraces (Qt7 and Qt3). The large vertical separation between the Qt7 and Qt3 straths (~75-100 m), and their great longitudinal and lateral extent, facilitate their use as markers of rock uplift and associated fluvial incision. Soils show strong time-related trends related to pedogenic accumulation of carbonate, iron oxides, and soil morphology. Regional terrace correlations are confirmed using radiometric dating integrated with soil chronofunctions based on progressive changes in soil morphology, iron oxide, and carbonate accumulation.

Dating by optically stimulated luminescence (OSL) of sand lenses within fluvial gravels of the Qt7 give weighted mean ages of 64 ± 4 ka (R. Cinca) and 67 ± 7 ka (R. Gallego). The degree of soil formation for soils on the Qt7 surfaces supports these age estimates based on comparison with soil studies in similar parent materials and climate. Preliminary terrestrial cosmogenic dating of soil profiles, magnetostratigraphy, and the degree of soil development indicate that the Qt3 is between 400-800 kyr. Moreover, soil stratigraphy is used to link dated and undated terrace remnants to develop robust regional stratigraphic framework of fluvial terraces. It is the analysis of this framework that allows recognition that strath surfaces diverge from the active channel towards the rivers’ headwaters. We attribute upstream divergence to erosional denudation and isostatic rebound of the Pyrenees and adjacent basin margin.