La structure géomorphologique et géologique des fonds de vallées dans les domaines subalpin et hercynien d'Allemagne

Abstract

The morphologie and géologie différenciation of some valley floors in the German Alpine foreland and the Central Uplands. — In the larger valley floors of the German Alpine foreland like the Isar and Danube and also in valleys of the Central Uplands as shown by the rivers Weser, Main and Rhine, the same sequence of terraces has been formed by fluvial activity since the maximum of the Würmian glaciation (fig. 8). Stratigraphically they can be divided into the Wûrmian « Niederterrassen » (lower terraces) and the Holocene « Auenterrassen » (floodplain terraces). The Niederterrassen (NT) are composed by a terracing of three single terraces : from the highest NT1 (Wùrm/Weichsel Pleniglacial), over the NT2 (post-Pleniglacial — pre-Bölling) to the lowest NT3 (Bölling - pre-Holocene). In many valley floors (Isar, Danube, Hier, Weser, Main, Enz) exists another higher « Niederterrasse », which is covered by lœss. Morphologically seen, this terrace forms the transition from the valley bottom to the higher level of the Middle Pleistocene terraces. Because of this it is named « Ûbergangsterrasse » (transitional terrace, Schellmann, 1988). It is older than the Würm Pleniglacial, perhaps of Middle or Early Würmian âge, but a Riss âge may also be possible. The greater parts of the valley floor are mostly built up by the Würm Pleniglacial and/or Lateglacial Niederterrassen. In contrast to them the Holocene floodplain terraces follow the present river course in a mosaic of small dimensioned terrace surfaces. They are composed of up to seven single terraces (Hl to H7 ; H = Holocene). Three of them were deposited during the Prebo- real/Boreal, Atlantic and Subboreal, but four of them during the Subat- lantic (fig. 8). In ail valleys the Holocene terraces were formed by latéral accretion of a meandering river. The oldest Wûrm Pleniglacial Niederterrasse originated from vertical sédiment aggradation. The change from a braided to a meandering channel pattern occurred in the Isar valley near Munich, in the Weser and Main valleys during the early Lateglacial period (pre-Bôlling/Allerôd) and in the lower Isar and Danube valley at the transition from Younger Dryas to Holocene. Therefore this change is strongly influenced by local factors. As shown for the Isar, Danube, Weser, Main and Rhine, valleys this Wùrmian and Holocene terrace sequence is not of local or régional nature. Because of this, climatic changes and fluctuations seem to control the fluvial activity. But local and régional factors, and since the Subatlantic period, also the human activities strongly influence the individual features of local terraces. That is the reason why each valley floor has its spécifie morphological, pedological and geological individuality.