Paleohydrology: A tool for restitution of catchment area and climatic fluctuations. Case study from the Oligocene of South-eastern France.
Résumé
In fluvial successions, channel-fill geometry and internal structures are commonly used to define the fluvial style. On modern systems, the hydrological parameters are most often defined from the channel bankfull width and depth in association to sediment distribution. Paleohydrological parameters such as discharge, catchment area are also reconstructed from fluvial bar geometry and grain-size. Such results are mostly used to characterize paleoclimatic conditions during sedimentation. We present here the results from the Oligocene fluvial meandering succession located in the western alpine foreland basin (south-eastern France): Barrême and Digne area. We investigate on a regional scale the drainage pattern and the climatic fluctuations over a 2Ma period. The height of the point bars is considered as the bankfull depth. The channel bankfull width is then calculated using classic equations linking channel bankfull width and depth in fluvial meandering systems. Paleohydrological reconstruction is based on channel-point bar thicknesses that range from 4 to 9m. Estimated channel bankfull widths vary at most of 30%, depending on the equation. These results will greatly influence the other parameters such as discharge, drainage area, stream length. Data from the Barrême basin enable us to define the best set of equations as the source area is precisely known based on sandstone mineralogy. The set is then being used for the other basins, thus reducing the uncertainties on the hydrological parameters. Drainage area of the Barrême and Digne paleomeandering systems are estimated around 1200 km2 and 2600 km2, respectively. Stream length was in the range of 100 km for Barrême with a source in the Embrun Ubaye area and in the range of 200 km for the Digne system with a source in the internal alps to be precised. Paleoclimatic study is conducted based on the assumption that there was no major paleogeographic rearrangement during the upper Oligocene-lower Miocene. Considering a given catchment area, changes in the discharge are then related to climatic fluctuations, pointing to humid and semi-arid periods. The Digne system (late Chattian) deposited during a relatively humid period and did not record major climatic fluctuations, whereas the Barrême system (Late Chattian to early Aquitanian) recorded significant climatic fluctuations from humid to semi-arid conditions. These results are in agreement with those from other western European continental climatic reconstruction. These first results, show the large potential of paleohydrology on paleogeographical and paleoclimatic reconstitutions when few outcrops of former fluvial systems are available.