Last glacial to Holocene fluvial aggradation and incision in the southern Upper Rhine graben: climatic and neotectonic controls. Dépôt et incision fluviatiles dans le sud du fossé du Rhin supérieur du dernier glaciaire à l´Holocène : contrôles climatiques et néotectoniques

We present a new model of landscape evolution for the southern Upper Rhine Graben based on analysis of Digital Elevation Models, new OSL and 14C dating, and a sediment budget. According to these data a three step scenario was developed (i) between ca. 60 to 16 ka BP accumulation of an alluvial fan in the southern Upper Rhine Graben, here called Upper Rhine Fan, downstream of the mouth of the Hochrhein valley. This fan is mostly composed of coarse-grained meltwater deposits. (ii) 16 to 10 ka BP incision of the Upper Rhine Fan and accumulation of a successive fan further down-stream by redepositing the eroded sediments (iii) 10 ka BP to present incision of the Rhine river into the younger fan leading to the present situation. Only phase (i) has experienced a significant sediment supply from the Alps and/or Alpine foreland. During the middle Würmian, fluvial aggradation is proved by several ages from sand lenses between ca. 60 and 27.5 ka BP. The period between ca. 27.5 and 16 ka BP is represented by a single or few layers of coarse-grained cobble and boulder-rich gravels and blocks. Because sand lenses are lacking they cannot be dated directly, but most presumably their deposition was related to the Late glacial meltdown. The repeated incision of the two fan surfaces after the Last Glacial Maximum and the Younger Dryas may be caused by high inputs of sediment-poor meltwater at this time. Weak Holocene aggradations may be linked with periods of climatic deterioration, tectonic pulses, and direct human impacts.Le nouveau modèle évolutif présenté pour le Sud du fossé du Rhin supérieur a été obtenu à partir de l´analyse des isohypses, des nouvelles datations OSL et 14C et du budget sédimentaire.Cette évolution s’est déroulée en trois phases: (i) de 60 à 30 ka B.P., un cône alluvial, appelé ici «Cône du Rhin supérieur» s’est constitué dans le Sud du fossé du Rhin supérieur, immédiatement en aval de la vallée du Haut-Rhin; ce cône est constitué essentiellement de dépôts grossiers mis en place par des eaux de fonte; (ii) de 16 à 10 ka B.P., l’incision du «Cône du Rhin supérieur» a pour conséquence, plus loin en aval, la construction d’un second cône à partir des sédiments érodés et (iii) de 10 ka B.P. à l`actuel, l’incision du Rhin dans le cône le plus récent est responsable de la morphologie actuelle.Les volumes considérables de sédiments provenant des Alpes ou de leur Avant-pays ont été transportés uniquement pendant la phase (i). L’aggradation fluviatile du milieu du Würm est démontrée par les datations obtenues sur les lentilles de sable qui ont livré des âges de 60 à 27,5 ka B.P. La période comprise entre 27,5 et 16 ka B.P. est représentée par une simple ou plusieurs séquences de gravières grossiers. En l’absence de datations absolues, on suppose que ces derniers sont corrélés à la fonte des glaciers à la fin du Würm. Les incisions des deux cônes après le dernier maximum glaciaire et le Dryas récent seraient à mettre en relation avec l’apport de grandes quantités d`eaux froides pauvre en sédiments. Les dépôts peu volumineux de l’Holocène peuvent être reliés aux périodes de détérioration climatique, aux mouvements tectoniques ou aux impacts dus à l’Homme

Last Glacial to Holocene fluvial aggradation and incision in the southern Upper Rhine Graben: climatic and neotectonic controls

We present a new model of landscape evolution for the southern Upper Rhine Graben based on analysis of Digital Elevation Models, new OSL and 14C dating, and a sediment budget. According to these data a three step scenario was developed (i) between ca. 60 to 16 ka BP accumulation of an alluvial fan in the southern Upper Rhine Graben, here called Upper Rhine Fan, downstream of the mouth of the Hochrhein valley. This fan is mostly composed of coarse-grained meltwater deposits. (ii) 16 to 10 ka BP incision of the Upper Rhine Fan and accumulation of a successive fan further downstream by redepositing the eroded sediments (iii) 10 ka BP to present incision of the Rhine river into the younger fan leading to the present situation. Only phase (i) has experienced a significant sediment supply from the Alps and/or Alpine foreland. During the middle Würmian, fluvial aggradation is proved by several ages from sand lenses between ca. 60 and 27.5 ka BP. The period between ca. 27.5 and 16 ka BP is represented by a single or few layers of coarse-grained cobble and boulder-rich gravels and blocks. Because sand lenses are lacking they cannot be dated directly, but most presumably their deposition was related to the Late glacial meltdown. The repeated incision of the two fan surfaces after the Last Glacial Maximum and the Younger Dryas may be caused by high inputs of sediment-poor meltwater at this time.Weak Holocene aggradations may be linked with periods of climatic deterioration, tectonic pulses, and direct human impacts

Late pleistocene fluvial dynamics in the Upper Rhine Graben : chronological frame

During the Pleistocene, the Rhine glacier system acted as a major south–north erosion and transport medium from the Swiss Alps into the Upper Rhine Graben, which has been the main sediment sink forming low angle debris fans. Only some aggradation resulted in the formation of terraces. Optically stimulated luminescence (OSL) and radiocarbon dating have been applied to set up a more reliable chronological frame of Late Pleistocene and Holocene fluvial activity in the western Hochrhein Valley and in the southern part of the Upper Rhine Graben. The stratigraphically oldest deposits exposed, a braided-river facies, yielded OSL age estimates ranging from 59.6 ± 6.2 to 33.1 ± 3.0 ka. The data set does not enable to distinguish between a linear age increase triggered by a continuous autocyclical aggradation or two (or more) age clusters, for example around 35 ka and around 55 ka, triggered by climate change, including stadial and interstadial periods (sensu Dansgaard–Oeschger cycles). The braided river facies is discontinuously (hiatus) covered by coarse-grained gravel-rich sediments deposited most likely during a single event or short-time period of major melt water discharge postdating the Last Glacial Maximum. OSL age estimates of fluvial and aeolian sediments from the above coarse-grained sediment layer are between 16.4 ± 0.8 and 10.6 ± 0.5 ka, and make a correlation with the Late Glacial period very likely. The youngest fluvial aggradation period correlates to the beginning of the Little Ice Age, as confirmed by OSL and radiocarbon ages