QUANTITATIVE INTERPRETATION OF FLUVIAL DUNE DEPOSITS: AN APPLICATION TO THE QUATERNARY LOIRE RIVER, FRANCE

Studies of the Loire River, France, from the Massif Central downstream to Orleans, have established a late Pleistocene to Holocene stratigraphic framework. Several distinct allostratigraphic units were identified in which trough cross-stratified sand and gravel (mostly due to dune migration) were dominant (glacial-age units) or intercalated with thick overbank fines (Holocene units). Here, we present preliminary results from studies aimed at using characteristics of preserved cross-strata to reconstruct the paleohydraulic responses of the Loire River to changes in floods regime during the Late Quaternary.

Methods for interpreting dune height and paleoflow depths from preserved cross-strata are based on a modified Paola-Borgman theory, and have recently been refined using experimental results. These methods analyze distributions of cross-set thickness, and require that cosets are homogeneous, with no spatial changes in stratal type or mean grain size, implying that the size distribution of formative dunes did not change significantly in time or space. Numerous cases within Loire deposits from the last glacial period satisfied these conditions. Preliminary results from upstream sites within the Massif Central predict a mean dune height ~0.15 m, and are consistent between sites of similar ages, whereas downstream, in the Orleans area, predicted mean dune heights range from ~0.15-0.3 m. Once dune height is estimated from cross-set thickness, it is possible to estimate paleoflow depths, given a number of assumptions. Estimated formative paleoflow depths for the glacial period Loire ranged from ~0.4-1 m in the Massif Central, and increased to ~1.2-2 m farther downstream in the Orleans area. Although these results are not comprehensive, they seem to indicate consistently shallow paleoflow depths for the late Pleistocene Loire River, significantly less than for the modern channel, suggesting that glacial period flood discharges were not comparable with those of the Holocene. Although the above methods have been used successfully on data from modern rivers, we do not yet know the exact effect of using partial distributions of cross-set thickness, which is typical of ancient deposits. We will continue to refine this method using data from Quaternary deposits of the Loire and elsewhere.