Papers by Jean-Louis Grimaud
Sediment partitioning related to a deep structure in Northwestern Bengal during the Late Quaternary
International audienceSubsurface deformation is a driver for surface dynamics-such as the path se... more International audienceSubsurface deformation is a driver for surface dynamics-such as the path selection of rivers-when deformation rates can outpace the autogenic mobility rate of rivers. Here we study the Late Quaternary Northwest Bengal-where the Ganges and Brahmaputra are entering the Ganges Brahmaputra Meghna (GBM) delta-by combining analyses of geophysical, geomorphic and borehole data. We show that the area between these two major rivers may be divided into two main geomorphic domains, namely the Tista megafan and Barind Tract. Analysis of soils and sediment facies distributions indicates that the former is a subsiding area filled with alluvial sediments by the Tista river system while the latter is characterized by a very low sedimentation rate and the development of mature soils at its surface. Gravity data analyses are in agreement with the Tista fan area subsiding and the Barind Tract being relatively uplifted. Provenance analysis based on bulk strontium concentration sho...
Infill dynamics and depositional patterns in gravel-bed chute cutoffs channels: the Ain River, France
International audienc
Preservation and completeness of meandering rivers deposits: insights from numerical simulations
International audienc
PIREN Seine, Dec 20, 2020
Les chenaux abandonnés constituent des entités sédimentaires de première importance sur le territ... more Les chenaux abandonnés constituent des entités sédimentaires de première importance sur le territoire de la Petite-Seine. Leur présence témoigne de l'évolution morphologique de la plaine alluviale de la Bassée en lien avec les forçages environnementaux et anthropiques. Les hétérogénéités physiques de leur remplissage influencent les écoulements souterrains. Une analyse combinant géomorphologie, sédimentologie et archéologie s'intéresse au contexte d'abandon des chenaux de la plaine alluviale de la Bassée à différentes échelles de temps. Deux exemples sont présentés. Points clefs ✓ Les chenaux abandonnés sont des archives géomorphologiques qui permettent de compléter les études historiques afin de mieux comprendre le fonctionnement passé de la Petite-Seine et son devenir. ✓ Les hétérogénéités sédimentaires de ces chenaux doivent être prises en compte pour améliorer les modèles d'écoulements souterrains.
Relations entre sociétés et environnement en Petite Seine du Mésolithique à la fin du Moyen Âge : nouvelles problématiques et résultats récents d’archéologie environnementale
Actes du colloque ArkéaubeInternational audienc
Braiding Mechanisms and Bar Geometries in Rivers and Submarine Density Current Channels
Geological Society of America Abstracts with Programs, 2017
Sediment budget of cratons: insights from West Africa over the Cenozoic
The EGU General Assembly, 2019
Abandoned channels are major features of fluvial systems as their distribution and sedimentary fi... more Abandoned channels are major features of fluvial systems as their distribution and sedimentary fills control the architecture of alluvial plains and the connectivity between fluvial reservoirs. The extent and geometry of the bodies formed by bedload deposition in abandoned channel, e.g., sand-plugs and sand-bars, depend on a number of factors, one of which is the geometry at the bifurcation point. In this study we build on existing modeling experiments, which focused on the global repartition of water or sediment between two active channels. We present a novel series of experiments designed to test the conditions for channel abandonment by modifying the bifurcation angle between channels, the flow incidence angle and differential slope. The surface evolution of disconnecting channels was monitored to constrain construction processes and geometry of sedimentary bodies. For a given bifurcation geometry, three scenarios were tested: a free evolution, a simulated levee breach-i.e. openi...
Flexural deformation controls on Late Quaternary sediment dispersal in the Garo‐Rajmahal Gap, NW Bengal Basin
Basin Research, 2019
Subsurface deformation is a driver for river path selection when deformation rates become compara... more Subsurface deformation is a driver for river path selection when deformation rates become comparable to the autogenic mobility rate of rivers. Here we combine geomorphology, soil and sediment facies analyses, and geophysical data of the Late Quaternary sediments of the central Garo‐Rajmahal Gap in Northwest Bengal to link subsurface deformation with surface processes. We show variable sedimentation characteristics, from slow rates (<0.8 mm/year) in the Tista megafan at the foot of the Himalaya to nondeposition at the exposed surface of the Barind Tract to the south, enabling the development of mature soils. Combined subsidence in the Tista fan and uplift of the Barind Tract are consistent with a N‐S flexural response of the Indian plate to loading of the Himalaya Mountains given a low value of elastic thickness (15–25 km). Provenance analysis based on bulk strontium concentration suggests a dispersal of sediment consistent with this flexural deformation—in particular the abandonment of the Barind Tract by a Pleistocene Brahmaputra River and the current extents of the Tista megafan lobes. Overall, these results highlight the control by deeply rooted deformation patterns on the routing of sediment by large rivers in foreland settings.
Cenozoic drainage evolution of the West African transform marginal upwarp
ABSTRACT We explore the large-scale relief and drainage evolution of the West African marginal up... more ABSTRACT We explore the large-scale relief and drainage evolution of the West African marginal upwarp by a spatial analysis of lateritic relict landscapes recording successive incision stages of a low relief, Early Cenozoic bauxitic envelope topography called the African surface. Four generations of stepped ironduricrust-capped paleolandsurfaces have been formed and abandoned on the slopes of interfluves below bauxitic relicts. Incision chronology is constrained by stratigraphic dating of the bauxites and Ar-Ar geochronology of Mn oxy-hydroxides produced in the weathering mantle of each paleolandsurface from the type locality of Tambao, in Northern Burkina Faso [1]. The Bauxites of the African surface result from intense rock chemical weathering that ended in the Middle Eocene (ca. 45 Ma). The so-called Intermediate paleolandsurface developed until the Oligocene-Miocene transition (ca. 24 Ma). Three generations of pediment (glacis from the French literature) emplaced afterwards. The so-called High glacis was shaped and weathered until ca. 11 Ma. The Middle Glacis settled by the end of the Pliocene (ca. 7-6 Ma) and the Low Glacis, which is mostly connected to the modern base level, dates from the end of the Pliocene. The regional study reasonably assumes the broad synchronicity of the lateritic levels at the scale of West Africa. We have produced elevation maps of the first three erosion levels corresponding to the topography of the marginal upwarp at ca. 45, 24 and 11 Ma. They show the successive positions of the main drainage divides and thus drainage reorganisation since the Eocene. The elevation of paleolandsurface relicts along the main drains allowed reconstructing paleo-river long profiles at ca. 45, 24, 11 and 6 Ma to be compared with the modern long profiles. The modern drainage of West Africa was established before the Oligocene-Miocene transition as a conse-quence of the inland growth of coastal catchments that have cut through the Eocene marginal upwarp. At this time West Africa was already entirely externally drained, i.e. the capture of the Niger internal delta by an Atlantic drain had already occurred. Drainage reorganization coincides with the onset of the Niger delta progradation and increased clastic fluxes along the margin. The overall shape of large river long profiles remained nearly stationary after drainage reorganization. Distributed low incision rates (&lt; 8-10 m/my) attest to dominantly diffuse lowering of base levels controlled by the stability of major lithologically-controlled knickzones. Knickpoint retreat may have been locally efficient where incision rate exceeded 8-10 m/my. Upward-convex modern river segments delineate a large-scale active swell across the upwarp. The imaged swell is in agreement with epeirogenic patterns predicted from models of mantle dynamics and may tentatively by related to the Hoggar uplift.
Quantifying denudation of the West African passive-transform margin: implications for Cenozoic erosion budget of cratons and source-to-sink systems
ABSTRACT We develop an approach based on the differential elevation of dated successive topograph... more ABSTRACT We develop an approach based on the differential elevation of dated successive topographies of the onshore part of the West African margin to calibrate in-situ volumetric denudation over a 3.9 million km2 cratonic surface for the past 45 Ma. We obtain a regionally averaged volumetric erosion rate of 5 x 10-3 km3/km2/m.y. corresponding to a total average denudation of 300 m and a denudation rate of 6 m/m.y., which remained nearly constant over the three time spans (45-24, 24-11 and 11-0 Ma) despite spatial variations related to epeirogenic movements. Denudation is converted into a minimum yield of 12 +/-2 t/km2/yr with a minimum solute component of 4 +/-2 t/km2/yr accounting for the porosity of the eroded regoliths. Our results would imply a minimum contribution of 1.6 +/-0.4 Gt/yr of the non-orogenic landmass to the global continental yield since the last peak greenhouse. Reconstruction of two incision stages of West Africa landscape from the reconstructed topographies combined with paleogeographic data shows that the current river catchments of the sub region have acquired their current configuration by the end of the Oligocene at the latest (24 Ma ago). The fairly steady geometry of the West African drainage since then offers the opportunity to effectively link the inland geomorphic record to offshore sedimentation. Volumetric denudation analysis applied to West African sub-drainage areas attests to the role of drainage reorganization and epeirogenic mouvements (flexural growth of the marginal upwarp and amplification of the Hoggar intraplate swell) on the spatial and temporal distribution of continental denudation and yield. Onshore denudation and clastic sediments accumulation in the post-24 Ma Niger catchment -delta system are within the same order of magnitude. These results suggest that cratonic-type erosion fluxes estimated from the West African margin may be used to estimate the size of drainage basins from the fossil sedimentary record.
Preservation and Completeness of Fluvial Meandering Deposits Influenced by Channel Motions and Overbank Sedimentation
Journal of Geophysical Research: Earth Surface
Late Quaternary in the Paris Basin: 3D restitution of alluvium geometry in the bottom of major valleys in the Seine catchment
International audienc
Les plaines alluviales sont des entités dynamiques du point de vue géomorphologique. Elles se con... more Les plaines alluviales sont des entités dynamiques du point de vue géomorphologique. Elles se construisent par sédimentation lors des crues, générant aussi les déplacements et abandons des réseaux de rivières. On retrouve donc à la surface des plaines alluviales des chenaux abandonnés remplis de sédiments fins qui constituent non seulement des témoins des paléo-tracés fluviatiles mais aussi des archives paléo-environnementales. Dans cette étude, nous investiguons les paléo-chenaux
Knowledge of the dynamics of evolving landscapes in terms of their geomorphic and topologic reorg... more Knowledge of the dynamics of evolving landscapes in terms of their geomorphic and topologic reorganization in response to changing climatic or tectonic forcing is of scientific and practical interest. Although several studies have addressed the large-scale response (e.g., change in mean relief), studies on the smaller-scale drainage pattern reorganization and quantification of landscape vulnerability to the timing, magnitude, and frequency of changing forcing are lacking. The reason is the absence of data for such an analysis. To that goal, a series of controlled laboratory experiments were conducted at the St. Anthony Falls laboratory of the University of Minnesota to study the effect of space-time variable and changing precipitation patterns on landscape evolution at the short and long-time scales. High resolution digital elevation (DEM) both in space and time were measured for a range of rainfall patterns and uplift rates. Results from our study show a distinct signature of the p...
The objective of the Transform Source to Sink Project (TS2P) is to link the dynamics of the erosi... more The objective of the Transform Source to Sink Project (TS2P) is to link the dynamics of the erosion of the West African Craton to the offshore sedimentary basins of the African margin of the Equatorial Atlantic at geological time scales. This margin, alternating transform and oblique segments from Guinea to Nigeria, shows a strong structural variability in the margin width, continental geology and relief, drainage networks and subsidence/ accumulation patterns. We analyzed this system combining onshore geology and geomorphology as well as offshore sub-surface data. Mapping and regional correlation of dated lateritic paleo-landscape remnants allows us to reconstruct two physiographic configurations ofWest Africa during the Cenozoic.We corrected those reconstitutions from flexural isostasy related to the subsequent erosion. These geometries show that the present-day drainage organization stabilized by at least 29 Myrs ago (probably by 34 Myr) revealing the antiquity of the Senegambia,...
Experimental evidence of landscape reorganization under changing external forcing: implications to climate-driven knickpoints
The EGU General Assembly, 2017
Understanding and quantifying geomorphic and topologic reorganization of landscape in response to... more Understanding and quantifying geomorphic and topologic reorganization of landscape in response to changing climatic or tectonic forcing is of scientific and practical interest. Although several studies have addressed the large-scale response (e.g., change in mean relief), studies on the smaller-scale drainage pattern reorganization and quantification of landscape vulnerability to the timing, magnitude, and frequency of changing forcing are lacking. To that goal, a series of controlled laboratory experiments were conducted to study the effect of changing precipitation patterns on landscape evolution at the short and long-time scales. High resolution digital elevation (DEM) both in space and time were measured for a range of rainfall patterns and uplift rates. Results from our study show a distinct signature of the precipitation increase on the probabilistic and geometrical structure of landscape features, evident in widening and deepening of channels and valleys, change in drainage p...
River path selection in response to uplift and interaction with alluvial fans
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Papers by Jean-Louis Grimaud