ALLUVIAL SOILS, PALEOSOLS AND STRATIGRAPHY OF THE LOIRE VALLEY, ORLEANS, FRANCE

Large alluvial terraces, associated sediments, and soils/paleosols along the Loire River, near Orleans, France, record changing fluvial dynamics over the late Quaternary. While terrace morphology and distribution has been previously documented and mapped, details concerning the internal alluvial architecture and depositional history have been lacking.

Terraces comprised of trough cross-stratified sand and gravel formed during glacial episodes in braided stream environments. Weakly developed paleosols and associated cryogenic features separate discrete depositional units, resulting in thick, aggradational packages. These terrace surfaces have thick soils with poor structural development and horizonation, and weak clay accumulation in B horizons.

Meandering stream facies, typical of Holocene units, include coarse channel gravel sheets overlain by overbank sand and silt. Moderately well developed paleosols are preserved where more recent, sandy channel facies have buried formerly stable floodplain surfaces. Composite soils are developed where overbank deposits overlap, and show distinctive clay accumulations and B horizons with strong structural development.

Differences in the textural characteristics of parent materials account for the major differences in clay accumulation in B horizons. The coarse sand and gravel of braided stream facies apparently permit a much greater depth of infiltration of pedogenic clays than do finer overbank sediments, typical of the meandering stream facies. Clay mineralogy also varies between terraces as a function of age. Kaolinite generally increases in Bt horizons with age, while mixed layer and expandable clay contents decrease with age.

The ages of terraces near Orleans are poorly constrained, however the soils and paleosols developed within them provide a means by which to asses their relative ages, through correlation with soils developed in terraces of known age both upstream and downstream. Paleosols help to define the alluvial architecture within units, clearly demarcating episodes of landscape stability between aggradational episodes.