Fluvial sequences of the Maas: a 10 Ma record of neotectonics and climate change at various time-scales.

Around 10 million years ago, the interplay of tectonics, climate and sea level changed markedly in the southern North Sea Basin. One of the results was the start of rapid progradation of the Rhine/Meuse delta. The sediments of this basin-filling complex have been preserved in a number of depocentres in an en-echelon arrangement which form the backbone of the North Sea Basin. These depocentres were located within the west-central part of the European intra-plate rift system. The southernmost part of this rift system, the Roer valley graben (part of the lower Rhine Embayment), was the first depocentre to be filled by sediments supplied by the rising hinterland in the south. The sediment- source area expanded gradually to include the Alpine collision front, which was first tapped by the river Rhine around three million years ago. Therefore, the sediments that filled the Roer valley graben provide information for unravelling the geological history of the north-west European plate.Using high-resolution, sequence-stratigraphic techniques, this thesis focuses on the stratigraphic, morphologic and tectonic aspects of the upper-delta and fluvial sediments, laid down by the Rhine/Maas (Meuse) fluvial system in the present-day Netherlands. The present study benefited greatly form the large database, compiled during the past decades by the Geological Survey of the Netherlands and the Winand Staring Centre-DLO Institute. Vital data sources were Zagwijn's pollen- based regional palaeo-climatic interpretations as a well as climatic data derived from deep-sea cores and ice cores. In addition, correlation of deep-sea geological records to astronomical parameters, proved of great value as it enabled conversion of the relative time scales based on palaeontological data to a linear one.In general, precise dating of fluvial deposits is difficult. As a consequence, interpretation largely depends on circumstantial evidence, such as the fractal-type hierarchical structure of the climatically controlled fluvial systems.A total of 57 stacked units (around 5-20 in in thickness) have been identified by studying terraces and fining upward sequences in borehole records from the Roer valley graben. In both types of exposure, the bounding surfaces of these stacked sequences generally reflect long-term, basin-wide episodes of fluvial deposition and erosion. Dating and modelling support the interpretation that the sequences represent a fourth-order cyclicity in the hierarchy of environmental changes that affected fluviatile processes and caused river reactivation (at fifth-order level in Miall's classification). For this entire period of 10 million years, a strong correlation exists between the number of (buried) surfaces reflecting river reactivation and major climate cycles with a duration varying between approximately 400 to 50 thousand years. Measured in time-steps at million years scale (1-2 Ma), the dominant average duration shifts from 200 thousand years for the period from 10-2 Ma to 100 thousand years for the last 2 million years. Such climatically-controlled cyclicity is well-known from deep-sea cores. This correlation shows that climatic change is an important control on cyclicity in fluvial sequences as well. Moreover, it demonstrates that it is possible to meaningfully correlate the oceanic record with the continental fluvial record. Consequently, a continuous series of fifth-order fluvial sequences can be used to develop a high-resolution time frame, which will be of great value in the study of basin dynamics. Fluvial sequences, which reflect the two extremes of climatic cycles, are a welcome addition to palynological records, which are restricted to warm episodes.Zooming in on the components of fifth-order fluvial sequences has enabled us to demonstrate a fractal-type hierarchy within the coupling between sedimentary units and climate cycles. Fourth-, fifth- and sixth-order climate cycles equally reflect fifth-, fourth-, and probably third-order sedimentary cycles.One of the problems involved in direct correlation of climate changes and changes in fluvial dynamics is accounting for the effects of the vegetation cover. Regeneration of a particular vegetation cover takes a few hundred years at the most. This introduces a time-lag factor in the process-response relationship. Obviously, this time-lag factor is particularly relevant in the case of sixth-order (millennial-scale) climate changes.Preservation of sedimentary sequences depends greatly on regional tectonics. The present study shows that subsidence of the graben started much earlier than uplift of the south flank of the Roer valley rift. Consequently, the two processes must have different control mechanisms. Subsidence is fault-controlled, caused by deep-seated extension, governed by the dynamics of the European plate. Uplift, possibly controlled by underthrusting of the Ardennes/Rhenish Shield, resulted in overall shortening of the graben owing to foreland compression. This interaction between extension and compression is reflected in a right-lateral strike-slip movement along the principal displacement zones of the graben and in the formation of drainage divides perpendicular to the length axis of the graben. The faults are extensional in character and show predominantly normal displacements. The strike-slip dynamics are reflected in the regional morphology and the changing patterns of the palaeoriver systems. During episodes in which horizontal movement along the boundary faults prevails, a river can change course and cross major faults to start flowing to other depocentres in the same rift zone. This highlights the importance of hiatuses in the sedimentary record for the interpretation of basin tectonics.An extensive, long-term geodetic benchmark data set, compiled over 117 years, corroborates the direction of relative displacement as inferred from geomorphology. The combination of this geodetic data set and the uplift history as recorded by the series of river terraces, is crucial for the analysis of vertical movements observed in other parts of the Netherlands. In addition, it helps to establish links between regions with different subsurface characteristics.At the resolution scale of the fourth-order climate cycles, both uplift and subsidence show variations in pace: acceleration and deceleration. Three important episodes showed up: (1) the onset of the hinterland uplift occurred approximately around ten millions years ago simultaneously with the, type one, rapid eustatic sea-level fall known as the boundary between the supercycles TBl-TB2 from Haq's cycle chart. (2) The apparently simultaneous onset of Quaternary-type climate dynamics and the transition from relatively slow to rapid vertical crustal dynamics, which occurred approximately three millions years ago. (3) Another, albeit less dramatic indication of a geodynamic effect is the temporary cessation of uplift which occurred between 1.5 and 1.1 million years ago. An event which seems to coincide with episodes of major plate-tectonic physiographic changes.The apparent synchronism between crustal dynamics and changes in climate system brings up an intriguing topic for future research: would it be possible to link the changing frequency of orbital forcing and its effect on climatic and geodynamic changes? The events mentioned above suggest that such a link may exist. Similarly detailed data on other basins could provide answers. Including fluvial sequence stratigraphy in general stratigraphic research may greatly improve our knowledge of basin dynamics