Multi-scale tectonic controls on fluvial terrace formation in a glacioeustatically-dominated river system: inference from the lower Min¿o terrace record

The general aim of this thesis is to untangle the interacting effects of climate, glacioeustacy, and regional, and local tectonics on fluvial terrace formation. The NW Iberian lower Miño River valley was chosen as a study site, because for this region, a very detailed, long-term, climate record is available. The lower Miño is situated near the Atlantic Ocean, which ensures that the influence of changing past sea levels was registered in the terrace record. Then, there is controversy about the presence or absence of tectonic activity, although a well-developed network of pre-existing faults and seismic activity in the region suggest that tectonic activity is present. Lastly, a completely preserved terrace sequence makes it possible to study the evolution of the area in detail. These, and more details, are found in Chapter 1. In Chapter 2, a regional assessment of recent tectonic activity is made. Studies on faulted terrace deposits and the recognition of small, fault-bounded tectonic basins indicate the presence of neo-tectonic activity. Further evidence is gathered from a tectono-geomorphic analysis, whereby deeply incised valleys, as well as asymmetrically-developed tributary catchments, and the presence of knick points in river profiles that coincide with the presence of structural lineaments, show that the eastern part of the study area experiences tectonic deformation. It is proposed that due to the non-optimal angle between the orientation of the pre-existing faults, and the current horizontal stress orientation, these older faults are re-activated, resulting in strain transfer from one fault segment to another. This results in differential block movements leading to local extension and basin subsidence. Alternatively, strike-slip activity may have caused the tectonic basins, but for this mechanism no evidence was found. The focus of Chapter 3 is on a local terrace staircase near the village of Vila Meã. First, the terrace staircase and associated fluvial deposits are described in detail. Then, an age model for the Vila Meã terraces is presented on basis of thermoluminescence and Cosmogenic Ray Exposure (CRE) dating. Minimum ages of up to 650 ka are calculated. On basis of these ages, and terrace surface altitudes, maximum incision rates of 0.07 to 0.09 m ka-1 are reconstructed. It is then discussed that these rates can be used as proxies for regional, vertical tectonic uplift. In the final part of the Chapter, new ideas are presented on the evolution of the lower Miño fluvial terraces. Based on observations made from the terrace deposits, and the proximity of a narrow, steep continental shelf, it is suggested that the fluvial terraces were formed during the initial period of sea level fall, and subsequently incised. Vertical uplift would then have occurred to preserve the terraces above the current river bed. In Chapter 4, the focus shifts from a local terrace staircase to the regional terrace record. The entire 55-km long terrace section of the lower Miño is investigated, and 4 selected terrace transects are discussed in terms of number of terraces and sedimentology. Because there is disagreement on the exact number of terraces and their correlations, a new long-distance terrace correlation scheme is presented. The new scheme is based on studies of weathered quartzite gravels in the 4 selected transects. Observed similarities in weathering rate between the transects leads to a proposed terrace correlation gradient of 1 m km-1. The often used correlation model that the terraces tread parallel to the current river bed (gradient 0 m km-1) is then rejected. The second half of the Chapter focuses on a longitudinal profile modelling experiment with the FLUVER 2 model. The evolution of the entire Miño-Sil system is modelled over a time period of 450 ka. The outcomes show that a regional uplift rate of 0.08 m ka-1 in combination with glacioeustatic movements seem to be responsible for terrace formation in the lower Miño valley, and thus confirm the earlier hypotheses in Chapter 3. Climate-induces variations in discharge intensity or timing do not have a dominant effect on terrace formation. The outcomes furthermore indicate that the CRE ages presented in Chapter 3, appear to be very close to exact timing of terrace abandonment. The results of the foregoing Chapters are integrated and implemented in Chapter 5, resulting in a new, detailed, fluvial terrace map of the entire 67-km reach of the lower Miño River. Both the Spanish and Portuguese part is incorporated. The map is derived from detailed mapping from a 5-m Digital Elevation Model (DEM) and over 1500 hours of fieldwork. The map shows the regional distribution of 10 terrace levels and one floodplain level, as well as 9 tectonic basins. A layer with fault elements gives a structural tectonic context to the map. Additional layers give information about more than 400 sites with mapped terrace sediment thicknesses and palaeoflow directions. Results from this mapping exercise show the highly fragmented nature of terrace and basin distribution, which is controlled by N-S, E-W and NW-SE trending faults. The map also suggests the presence of unpaired terraces along the river, which may be caused by localised differential movements of tectonic blocks. These localised movements are the topic of Chapter 6. Here, the interactions between regional vertical uplift, local basin subsidence, and unequal uplift on both sides of the Miño River on terrace formation are investigated by means of a forward modelling exercise with the TERRACE model. The model simulations that match best with mapped terraces and fluvial sediment thicknesses are the ones that incorporate all three effects of vertical uplift, basin subsidence, and unequal uplift. This shows that terrace preservation is the complex end result of three, interacting, tectonic processes. A regional uplift rate of 0.10 m ka-1 gave the best results, which is slightly higher than the rate of 0.08 m ka-1 presented in Chapter 3. This confirms that regional uplift increases from the coast towards the east, which is in agreement with the findings of Chapter 2. Another important result is that the interacting effect of the three aforementioned tectonic processes can lead to fill terraces one valley side, and strath terraces at the other. In Chapter 7, all findings of the previous Chapters are combined. The separate effects of climate change, glacioeustacy, and regional and local tectonic movements on fluvial terrace formation are discussed. This shows that in many published terrace correlation schemes for tectonically active regions, the effects of multi-scale tectonics are insufficiently incorporated or considered. The same applies for the possible effects of variable uplift pulses over middle to late Quaternary timescales. This leads for instance to the separation of fill and strath terraces in a chronological context, because they are still thought to be the resultant of climate-triggered changes in discharge and sediment load of the river. But this thesis shows that they can form at the same time due to localised tectonic movements. The Chapter concludes with a number of recommendations on how to incorporate tectono-geomorphic analysis in fluvial terrace research, which will lead to a better understanding of tectonic control on fluvial terrace formation world-wide.</p

Cross-comparison of last glacial radiocarbon and OSL ages using periglacial fan deposits

Two cores from a Weichselian periglacial alluvial fan were dated using 14C and OSL, to verify the reliability of both methods and check the upper dating limit of the 14C method. Both dating methods yielded a similar chronology for core Eerbeek-I, with infinite 14C dates for the lower part where OSL dates indicated ages of over 45 ka. Finite 14C dates were obtained throughout the core for Eerbeek-II, despite stratigraphic and OSL evidence suggesting ages beyond 14C limits. Apparently, additional chemical pre-treatment to remove younger carbon fractions did not work adequately for samples from this core. We hypothesize that this may be related to a larger influence of younger-age humin fractions in the mainly sandy Eerbeek-II deposits compared to those buffered by a thick peat layer of Eerbeek-I. We suggest that (local) stratigraphy, percolation and humification processes may impact 14C ages of organic deposits more than commonly assumed, and should receive more attention. In addition, we introduce a new method to assess robustness and validity of OSL dates and demonstrate the applicability of OSL dating methods in this setting. Our results highlight that the 14C method requires additional verification methods, such as OSL, for deposits older than 30 ka

Two decades of numerical modelling to understand long term fluvial archives: Advances and future perspectives

The development and application of numerical models to investigate fluvial sedimentary archives has increased during the last decades resulting in a sustained growth in the number of scientific publications with keywords, 'fluvial models', 'fluvial process models' and 'fluvial numerical models'. In this context we compile and review the current contributions of numerical modelling to the understanding of fluvial archives. In particular, recent advances, current limitations, previous unexpected results and future perspectives are all discussed. Numerical modelling efforts have demonstrated that fluvial systems can display non-linear behaviour with often unexpected dynamics causing significant delay, amplification, attenuation or blurring of externally controlled signals in their simulated record. Numerical simulations have also demonstrated that fluvial records can be generated by intrinsic dynamics without any change in external controls. Many other model applications demonstrate that fluvial archives, specifically of large fluvial systems, can be convincingly simulated as a function of the interplay of (palaeo) landscape properties and extrinsic climate, base level and crustal controls. All discussed models can, after some calibration, produce believable matches with real world systems suggesting that equifinality - where a given end state can be reached through many different pathways starting from different initial conditions and physical assumptions - plays an important role in fluvial records and their modelling. The overall future challenge lies in the development of new methodologies for a more independent validation of system dynamics and research strategies that allow the separation of intrinsic and extrinsic record signals using combined fieldwork and modelling

Spatial variation in specific sediment yield along the Peruvian western Andes

The tropical Andes has been less studied in terms of erosion processes in comparison to other major mountain ranges in the world. Environmental gradients are steepest along the western flank of the Andes that is characterized by marked differences in vegetation, rough topography with deeply incised canyons, and highly variable and extreme precipitation patterns. Previous efforts mostly focused on sediment fluxes in large rivers draining to e.g. the Amazon basin while small to medium-sized rivers such as the ones flowing towards the Pacific Ocean have been relegated. They highlighted the link between sediment yield, anthropogenic and natural factors, e.g. climate, topography, river runoff, lithology and vegetation cover. In this study, we identified the spatial patterns of specific sediment yield along the western slopes of the Peruvian Andes between 3° and 13° S latitude for 21 catchments. We collected and analysed data from 22 environmental factors to elucidate their importance on spatially varying sediment yield. The sediment load was derived from gauging stations, reservoir sedimentation and water turbidity over a 30-yr period. The specific sediment yield varies strongly along the Peruvian western Andes as a consequence of the spatial variation in climate, topography and land cover controlling sediment production and transport. We reported higher-than-average specific sediment yields for the central part (6°-11°S) with values of 2130 and 2300 t km−2 yr−1 and low and uniform yields of 39 to 551 t km−2 yr−1 in the southern part (11° − 14.5° S). Given the scarcity of data on sediment yield, we included an uncertainty assessment based on bootstrapping approaches as to get a better grasp on the potential range of specific sediment yields in the study region. Using statistical techniques including Spearman correlation rank, univariate and multivariate regression analyses, we were able to determine the importance of the 22 environmental variables on the specific sediment yield. About 55 % of the observed variance can be explained by river discharge (Q90) and river steepness index (ks50). By adding an anthropogenic variable based on land cover, the explained variance in SSY increases up to 63 %, however, the effects of land cover on specific sediment yield are not clear because of spurious correlation between land cover, river discharge and topography. Our study therefore provides important new insights in the ongoing scientific debate on sediment yield variability in the western Andes. © 2022 Elsevier B.V

Sedimenthuishouding in het stroomgebied van de Nederlands-Duitse Vecht : potenties voor herstel van natuurlijke rivierdynamiek

In opdracht van LNV is onderzoek gedaan naar de sedimenthuishouding van de Nederlands-Duitse Vecht. Het accent lag hierbij op het verkrijgen van een beter inzicht in erosie- en sedimentatieprocessen op stroomgebiedsniveau. Vanwege deze reden is er een samenwerking opgezet met Duitse partners. Hierdoor kon er een grensoverschrijdende analyse gemaakt worden van het Vechtsysteem vanaf de bovenlopen in Duitsland tot de monding bij het Zwarte Water in Nederland. Door gebruik te maken van het landschapsproces-model LAPSUS, ontwikkeld door onderzoekers van de leerstoelgroep Land Dynamics van Wageningen Universiteit, kon er een analyse gemaakt worden van waar zich in het gebied de belangrijkste bronnen van sediment bevinden en hoe verschillende delen van het Vechtsysteem reageren op veranderingen in landgebruik en op aanpassingen aan de benedenstroomse loop van de Vecht. Op basis van drie scenario’s is dit gedrag nader onder de loep genomen

Modelling the impact of regional uplift and local tectonics on fluvial terrace preservation.

A terrace formation model (TERRACE) combined with a longitudinal river profile model (FLUVER) was used to simulate fluvial terrace formation and preservation in the northwest Iberian lower Miño River basin under the influence of three tectonic conditions; namely regional vertical uplift, local basin subsidence, and localised differential uplift. The simulation results were compared against mapped terrace altitudes and deposit thicknesses. The best results were achieved by combining all three tectonic factors, indicating that specific terrace formation is a complex interplay of regional and local tectonics. The best fit regional uplift rate of 0.10 m ka- 1 over the past 600 ka is higher than the 0.08 m ka- 1 previously estimated for a section farther to the west, which can be attributed to an increase in tectonic uplift from the NW Iberian Atlantic margin toward the east. Local relative subsidence causes sediment accumulation in the local basin and less sedimentation in the fluvial terraces on the surrounding uplifting blocks. Different uplift rates on both sides of the valley caused preservation of unpaired terraces, which are fill terraces on one side of the valley and strath terraces on the other side. Usually, the formation of fill or strath terraces is considered to be only climate-dependent. Our results indicate that local tectonics can be important in the terrace formation and preservation. This suggests that terrace correlations in other river systems, based on deposit thicknesses only, might be over-simplified