Süss, M. P.1; Schrader, S.2;
Fischer, K.3; Schäfer, A.1; Ricken,
W.2; Jentzsch, G.3; Jahr, T.3
& Drozdzewski, G.4:
1 Geological Institute University of
Bonn;2 Geological Institute University of
Cologne;3 Institute of Geosciences University of
Jena;4 Geological Survey of NRW, Krefeld
The history of the Variscan Rhenohercynian foreland-basin from Visean to Stephanian was subject of several studies by the authors in the last years. These studies raised various new questions, as they provided new constraints on the crustal dynamics of the Variscan lithosphere. Simulations of the lithospheric flexure and the sedimentation in the basins allowed to test the various parameters of the resulting geological models.
Starting in the Upper Visean, more than 2000 m of a turbiditic succession was deposited in the asymmetric Rhenohercynian flysch-basin. This flyschoid sedimentation apparently continued until the Namurian B. At this time, the Ruhr Basin developed on the northern border of the Rhenish Basin. This subsiding basin was rapidly filled by molassoid deltaic sediments.
The flyschoid successions consist of conglomerates and sandstones with intervening mudstones. Subsequently, carbonatic turbidites were deposited. In the sub-basins, the sedimentary processes were controlled by accommodation cycles, sedimentary input, and the basin geometry. The start of 3d-order turbiditic sequences is characterized by a thickening-upward trend, that is modulated by subsequent, autocyclic thinning-upward successions. These large-scale changes in accommodation space probably resulted from a combination of tectonic activity of the Variscan orogen and eustatic sea-level changes. Interlaced, dislocated fragments of epeiric carbonates indicate times of rising and high sea-level, with reduced siliciclastic input, allowing build-ups to develop on the basin margins.
Sea-level changes controlled the sedimentation throughout the Late Carboniferous. The transition from flyschoid to deltaic sedimentation took place in the Namurian B. The remaining flyschoid sedimentation in Namurian B might have been caused by the principal geometry of the basin as well as the sedimentary environment of a heavily overloaded flyschoid delta. The transition from flyschoid to delta plain sedimentation was not a unique event but was repeated several times, as can be shown in the early occurrence of seat-earth and small coal seams in the Early Namurian B. Starting with the Sprockhövel Formation in the Late Namurian B, deltaic sedimentation became dominant. Sea-level highstands forced heavily loaded sedimentary systems to prograde rapidly northward into the basin. Sea-level changes with large amplitudes easily flooded the flat delta plains, giving space for extended flooding surfaces. During the Late Westphalian, in the Ruhr-Basin and the Aachen-Erkelenz Basin subsidence decreased, forcing the sedimentary environment to be controlled more by alluvial processes. Starting in Westphalian D, alluvial sediments are preserved only north of the Ruhr Basin.
Sections crossing the eastern Rhenish Massive draw a detailed picture of the thicknesses and spatial relationships of the sedimentary bodies from the Visean to the Stephanian.
In the Lower Carboniferous, a general thinning of the successions of the foreland in the NW is observed. An additional thinning to the SW yields a complex prism-like shape of the sedimentary basin. The orientation of stream marks and the decrease in size of the components in the channel-fill-conglomerates indicate a sedimentary source in the present hessian low. The sediments must have been transported into the basin through a central channel by turbiditic currents from the Mid-German Crystalline Rise. Parallel to the probable southern border of the basin, single elongated flyschoid fans developed. These fans were not able to prograde to the North, but were forced to east-western directions by small basinal heights. Five or six sub-basins are distinguished by the maxima and the wedging of the sedimentary bodies. Comparison of the position of these basinal heights with the DEKORP-2N seismic profile shows that they are situated next to main fault- and thrust-zones. This spatial association is a strong indication of the tectonic origin of these early Carboniferous highlands in the sedimentary basin. The existence of a foreland bulge is not related to these heights. The geometry of the profiles suggest, that the strata systematically lap on a bulge to the North. Thus, it is more likely that this bulge was a stable feature.
In the Ruhr Basin, a general change of the basin geometry can be demonstrated throughout the Late Carboniferous. Subsidence increased to the North of the basin in the Namurian C. In the late Westphalian C, the remaining subsidence clearly increased to the South. This change can be related to a constantly moving subsidence low in the basin, as shown by Drozdzewski.
In the Ruhr Basin, no indication of local synsedimentary tectonics was observed. Whereas in the Aachen-Erkelenz Basin, synsedimentary faults were proven to be active. In the foreland, major palaeo-currents from East to West still dominated the sedimentation until the Late Westphalian. Sedimentary input from the adjacent Rhenish Massive can only be related to extensive lows of sea-level.
We conclude that the flysch and the molasse basins differ not only in their sedimentary fill. They also show remarkable differences in the basin due to tectonic processes. The flysch basins were generated while active tectonic processes beneath were ongoing, producing multiple piggy-back style sub-basins. These piggy-back-style basins might have provided accommodation space until the late Westphalian. At that time, they prevented direct sedimentary input into the strongly subsiding foreland. The change of the basin type was coincident with a change in tectonic style and shortening ratio. In the Rhenish Massive, large scale thrust-zones and fault-zones provided high shortening rates. In the foreland, only internal faults and northward diminishing folds are present, indicating a reduced shortening.
Two- and three-dimensional numerical simulations are useful tools to understand the deformation of the lithosphere under a dynamically changing load. The two-dimensional simulation included additional sedimentary processes, that contribute to the change in the overall load through erosion and basinal sedimentation. From the two-dimensional model we conclude that the subsidence of the Variscan foreland resulted not only from the loading of the orogenic wedge or the sediments that were deposited in the foreland. The moving subsidence low can be correlated to a foreland bulge on a continental lithosphere. Under such conditions, a basin of a size of about 150 km of width was generated.
The three-dimensional model currently under construction demonstrates that the geometric properties of the different tectonic units strongly influence the local deformation of the lithosphere. There is indication that under compressional conditions, intra-plate stresses might contribute also to basin subsidence in areas which are not directly adjacent to the orogenic front.
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