The heterogeneous Tibetan lithosphere has led to the formation of distinct geomorphic units on the plateau surface over the past ~250 Myr. One prominent topographical feature is the delineation of intervening sedimentary basins by high mountain belts. Unlike the high and rugged Kunlun, Gangdese and Himalaya Mountains, the Central Tibet Watershed Mountains (CTWM) in the Qiangtang terrane exhibit a relatively low relief of ~1 km or less compared with surrounding basins. They are important geological and geographical barriers with perspectives on the formation process that are subject to dispute. The outburst of detrital zircon geochronology data sets in the Qiangtang basin provides an opportunity to address this issue. The combination of inverse and forward modeling of 6197 detrital zircon U-Pb ages enables the establishment of provenance mapping, which averts tedious descriptions of individual age modes. Integrated with petrographic analysis and paleocurrents, the provenance of the Jurassic Qiangtang basin is quantitatively constrained. The CTWM remained consistently significant sources throughout the Jurassic time. Internal sources of Triassic and Jurassic magmatic rocks locally supplied younger zircon grains. The source proportion of the Hoh Xil- Songpan Ganze (HSG) terrane increased across the basin in the Middle Jurassic but decreased dramatically in the Southern Qiangtang in the Late Jurassic. Contextualized in geological details, an embryonic watershed that separates rivers flowing into the Pacific and Indian oceans formed in central Tibet during the Late Jurassic.

How to cite: Zhang, J., Li, Y., Zhang, H., and Tu, J.: Initiation of the Central Tibet Watershed Mountains in Qiangtang: Insights from Provenance Mapping of Detrital Zircon Data Set, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2975, https://doi.org/10.5194/egusphere-egu24-2975, 2024.

The establishment of modern drainage patterns of large rivers in eastern Tibet is thought to have resulted from drainage reorganization by serial river capture and reversal events, but the timing and driving mechanisms are still under debate. The capture that created the First Bend of the Yangtze River (YFB) is the most well-known event but also the most controversial. Here, sedimentary provenance of Late Miocene–Quaternary Dali basin strata south of the YFB demonstrates that a south-flowing Jinsha River briefly drained the Dali basin at ~7.4–6.4 Ma. This would require the occurrence of two fluvial diversions at the YFB, before 7.4 Ma and after 6.4 Ma, respectively. Together with landscape evolution modeling results, we infer that a river-blocking landslide downstream of the YFB and resulting lake overspill may have been responsible for this drainage reorganization process. Our results highlight for the first time that river-damming landslides may be a key mechanism for driving dynamic drainage reorganization in eastern Tibet.

How to cite: Zhao, X., Zhang, H., Li, Y., and Lease, R.: Dynamic drainage reorganization in the eastern Tibetan Plateau: A perspective from the First Bend of the Yangtze River, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4832, https://doi.org/10.5194/egusphere-egu24-4832, 2024.

Located in the western region of Bahia state, Brazil, the Fêmeas and Grande rivers are important tributaries of the São Francisco River. Despite the significance of these systems for the regional and national contexts, few studies have addressed the geomorphological processes and dynamics throughout the Holocene in these areas. Thus, through the characterization of grain size and morphoscopy of 100 quartz grains from deposits corresponding to an alluvial fan complex and low fluvial terrace on the left bank of the Fêmeas River, as well as two low fluvial terraces of the Grande River (one on each bank), we aim to identify discontinuities indicative of changes in morphodynamic processes throughout the Holocene. The results have shown that in the low fluvial terrace on the left bank of the Grande River, the deeper layers (45-90 cm) present values for the silt and clay fractions of 1.7% and 1.3%, respectively. On the other hand, in the more superficial layer (0-10 cm), the values for the silt and clay fractions were 5.5% and 6.7%, respectively. The values for the fine sand fraction in all layers averaged from 69.8% to 86.4%. No changes were observed in the morphoscopy of quartz grains, with rounded and sub-discoidal grains being predominant. On the right bank, the fluvial deposit predominantly showed silt and clay fractions with values of 24.8% and 29.9% at greater depths (40-60 cm), while on the surface (0-30 cm), the values for these fractions significantly decreased, reaching values of 6.3% and 12.7%. Morphoscopy analyses resulted in mostly sub-rounded and sub-discoidal grains. In the alluvial fan complex of the Fêmeas River basin, erosive-depositional discontinuities were identified in a deposit corresponding to the proximal facies of the complex. The results have shown differentiations in the clay, silt, fine sand, and coarse sand fractions at 0-70 cm, as well as an increase in the clay fraction in deeper layers, with values ranging between 36.9% and 40.8%. Morphoscopy analyses indicated 36.8% sub-rounded grains and 50% spherical-rounded grains. In another deposit with a thickness of 140 cm and corresponding to the distal facies of the alluvial fan complex, the results showed a decrease in coarse sand values to fine sand in deeper layers. Morphoscopy indicated predominantly sub-rounded grains in the more superficial layers (0-50 cm), predominantly rounded grains in the intermediate layer (50-70 cm), while the deeper layers (70-140 cm) presented sub-rounded and rounded grains. Overall, this deposit showed 13.2% sub-angular grains. Finally, our results indicated significant variations in silt and clay fractions, which, when analysed together with morphoscopy, suggest energetic variations in depositional processes throughout the Holocene. This research has been funded by the Bahia State Research Support Foundation (Grant 4341/2022).

How to cite: Santos, T. A. D. S., Machado, I. G. L., Bedendo, J. P., Brito, A. M., Silva, E. C. L., de Souza, J. J. L. L., and Souza, A. D. O.: Grain-size and morphoscopy analyses of surficial cover associated with an alluvial fan complex and low river terraces in the Grande and Fêmeas Rivers, western region of the state of Bahia, Brazil., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10220, https://doi.org/10.5194/egusphere-egu24-10220, 2024.

This work aimed to present partial results of chemical analyses conducted on surficial cover samples collected in alluvial fans located in the lower course of the Fêmeas River, in the Western region of Bahia, Brazil. This region is characterized by a sub-humid climate with two well-defined seasons: a rainy season (November to April) and a dry season (May to October). The hypothesis of this study considers that the deposits exhibit erosive-depositional discontinuities with different values of potassium (K) and sodium (Na), which could indicate the influence of complex morphodynamic processes throughout the Holocene. Therefore, the amounts of K and Na in different layers identified in the field and confirmed through laboratory textural analyses were measured. The analysis of these two cations allows for the possibility to infer a relative geochronology considering different exposures of the layers to weathering. The samples were collected at three sites on the left bank of the Rio das Fêmeas (LQME4; LQME5; LQME6). In LQME4, the results showed high potassium and sodium content at 20-30 cm (K=358 mg/dm³, Na=36.73 mg/dm³), 30-50 cm (K=347 mg/dm³, Na=36.73 mg/dm³), and 50-70 cm (K=307 mg/dm³, Na=53.38), except for the more superficial layer at 0-20 cm (K=8 mg/dm³, Na=16.08). This result suggests that the deeper layers underwent less weathering, while the low values of K and Na in the surface layer are possibly due to leaching from the runoff. With two layers, LQME5 presented high K and low Na values in the deeper layer of 20-60 cm (K=228 mg/dm³, Na=4.95 mg/dm³) contrasting with the more superficial layer at 0-20 cm (K=111 mg/dm³, Na=13.88 mg/dm³), where the K value decreases considerably while Na increases. The values in the most superficial layer are correlated with the modern sub-humid period that has resulted in lower runoff activity and, therefore, influencing mineral dissolution as indicated by the low Na value. The deeper layer suggests more humid periods characterized by lower K and higher Na values, indicating a more humid period resulting in more efficient mineral dissolution. In LQME6, six layers were analysed: 0-10 cm (K=195 mg/dm³, Na=11.07), 10-50 cm (K=147 mg/dm³, Na=11.49), 50-60 cm (K=69 mg/dm³, Na=13.72 mg/dm³), 60-75 cm (K=42 mg/dm³, Na=13.19), 75-85 cm (K=85 mg/dm³, Na=12.90 mg/dm³), 85-140 cm (K=87 mg/dm³, Na=16.30). The first two layers presented the highest K value in contrast to the other layers showing low values. LQME6 exhibits several discontinuities in K values, indicating that the upper layers underwent less weathering than the deepest ones. This research is being funded by the Bahia State Research Support Foundation (Grant 4341/2022).

How to cite: Brito, A. M., Santos, T. A. D. S., Machado, I. G. L., Bedendo, J. P., Silva, E. C. L., Souza, J. J. L. L., and Souza, A. D. O.: Chemical analyses of surficial covers associated with alluvial fans in the western state of Bahia, Brazil, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11979, https://doi.org/10.5194/egusphere-egu24-11979, 2024.

Ferricretes, or iron duricrusts, are hard iron layers, which predominantly develop in tropical and subtropical environments. They commonly cap elevated topographical features, potentially safeguarding old landscapes. The genesis of duricrusts is intricately tied to climatic conditions, particularly relying on intense seasonal precipitation cycles.

Two hypotheses for iron duricrust formation exist: the hydrological or horizontal hypothesis and the laterisation or vertical hypothesis. In the first case, elements forming the duricrust are transported from distant areas and concentrated by hydrological processes. In the second case, the protolith is the underlying basement, and ferricretes form through leaching of soluble elements and compaction of less soluble ones.

As no numerical model has been proposed for ferricrete formation until recently, we incorporated both formation hypotheses in a previously described numerical model for regolith formation (Braun et al., 2016).  The hydrological model was profusely described last year (Fenske et al. at EGU23), thus, we will concentrate on the laterisation model. In accordance with the second hypothesis, ferricrete formation follows laterisation of the regolith. During laterisation, the most soluble elements gradually dissolve and leach, resulting in the enrichment of non-soluble elements like iron and compaction. The model is characterized by two parameters: the time scales for iron enrichment τ_l and compaction τ_c, respectively. Various numerical scenarios were performed under diverse tectonic and climatic. The threshold Ω_min was determined to state formation or not of ferricretes.

To calibrate the model, a case study has been defined in the southeastern part of Brazil, the Quadrilátero Ferrífero (QF) region. Ranging almost 7 000 km², this region is known for its abundance in iron ores and distinct topography with escarpments and high plateaus commonly topped by cangas. Cangas are a type of ferricrete which form from the weathering of BIFs (Banded Iron Formations). The oldest registered formations are up to 70 Ma old and seem to protect some of the mountain peaks for extended periods of time. Multiple scenarios are proposed to describe today’s landscape, with different climatic and tectonic parameters in play. With the help of the laterisation model, it is possible to model different scenarios to attempt to depict the formation of the QF.

How to cite: Fenske, C., Braun, J., Guillocheau, F., and Robin, C.: Modelling Solutions for Ferricrete Formation and their Impacts on Topography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11556, https://doi.org/10.5194/egusphere-egu24-11556, 2024.

A fundamental shift in fluvial architecture is often argued to coincide with the evolution of terrestrial plants in the Silurian period. This shift away from “sheet-braided” conditions has traditionally been attributed to the influence of vegetation on river geometry and planform, resulting in Earth’s first single-threaded rivers. However, recent paleohydraulic reconstructions of Proterozoic rivers suggest that rivers of this time commonly attained aspect ratios and slopes similar to modern meandering and anastomosing rivers. At the same time, a wider range of channel planforms is increasingly recognised for pre-Silurian strata, although these interpretations have resulted from varied methodologies. Thus, a consistent and multi-faceted approach, applied to a series of fluvial successions, is needed to develop a unified model of pre-vegetation fluvial morphodynamics. Here, we present field observations and paleohydraulic reconstructions of fluvial strata from the exceptionally well-preserved Mesoproterozoic (1.2-1 Ga) Stoer Group located in NW Scotland. These strata host a range of fluvial architectures, from low aspect-ratio channel bodies isolated within muddy floodplain sediments, to amalgamated channel facies forming apparent “sheet-braided” successions with high sand/mud ratios. Reconstructions and bar-dune orientations from the Clachtoll, Bay of Stoer, and Meall Dearg Formations of the Stoer Group reveal a range of channel morphologies, including meandering, wandering, and braided planforms. Furthermore, we show that mud, even in relatively low volumes, was capable of providing sufficient cohesion to foster single-threaded planforms. We propose that evolving channel kinematics post the greening of the continents was as important as changing planform for determining the fluvial architectures we see preserved in the rock record.

How to cite: Whittaker, A., Valenza, J., Ganti, V., McLeod, J., and Wild, A.: Reconstructing Proterozoic Planforms and Channel Dynamics: New insights from the 1.2 Ga Stoer Group, NW Scotland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11612, https://doi.org/10.5194/egusphere-egu24-11612, 2024.

The Anthropocene describes a potential new chronostratigraphic unit of the Geological Time Scale of intensified anthropogenic influence on environmental and geological processes, leaving traces in geological archives. Even though this human impact can be seen on a global scale, regional studies characterizing the scope and growth of anthropogenic influence, are scarce, especially for urban or peri-urban environments.

In this study, we investigate the anthropogenic impact of the metropolis Vienna on its peri-urban environment and the potential base of the Anthropocene epoch in the 1950s CE by applying sedimentological and geochemical methods.

The human influence in urban sedimentary archives of Vienna has already been detected in previous studies by Wagreich et al. (2022) using artificial isotopes and trace metals as Anthropocene stratigraphic markers on urban coarse artificial ground. The study area is set downstream of Vienna, in the National Park Donau-Auen, where direct human intervention into the archived Danube river sediments is currently nil. These river sediments represent an ‘Urban Anthropocene Field Lab’ to trace and quantify the human stratigraphic fingerprint and to search for potential markers and correlations to proposed GSSP Golden Spikes of the Anthropocene.

Within the proximal flood plain sediments of the Danube, i.e. erosional profiles and sediment cores, sedimentological, geochronological and chemostratigraphic markers are applied to characterize and date the anthropogenic strata in this area. First observations indicate three periods of distinct sedimentation patterns, potentially corresponding to the natural state prior to significant human intervention, the river system’s reaction to the first extensive river channelization in the 1870s CE, and it’s following response to the construction of hydropower stations (1956-1998 CE) and second river regulation (1990s). The lowermost section is characterised by clay and organic rich thin layers (few cm to mm) being suddenly replaced by alternating silt and sand packages of 5 to 20 cm beds. The uppermost silt to fine-sand dominated section is massive and shows almost not sediment structures, unlike the other sections, and exhibits a uniform light grey colour distinct from the light beige and dark brown colour of the underlying deposits.

The archive of natural Danube deposits is further analysed for artificial radiogenic isotopes, trace metals, and (micro-)plastics with the aim (i) to disentangle the anthropogenic fingerprint of Vienna from the sediment and characterise the interplay between upstream human interventions and local river dynamics, (ii) to identify and evaluate the proposed Holocene-Anthropocene geological boundary around 1950 CE, and (iii) to evaluate markers for the Anthropocene and a potential correlative stratigraphic reference section downstream of Vienna.

Reference:  
Wagreich, M., et al. 2022. The Anthropocene Review 10, 316–329.

How to cite: Hatzenbühler, D., Weißl, M., Baumgartner, C., Hain, K., Hubmer, A., Lang, A., and Wagreich, M.: Anthropogenic stratigraphic signals downstream a metropolis:Extracting Vienna’s signature from Danube river plain archives , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12448, https://doi.org/10.5194/egusphere-egu24-12448, 2024.

The Godavari River Basin (GRB), the biggest Peninsular Indian basin, has experienced a major decline in both streamflow and sediment discharge since 1965, with a particularly notable loss occurring after around 1990. Previous studies reported an overall decrease of suspended sediment load around 123 Mt/year in the GRB, which places it as the third highest among all major river basins worldwide. However, there is a lack of adequate understanding of the consequences of reservoir operations, variations in flow, and the broader dynamics of sediment.  Here, we employed a dataset provided by the Central Water Commission (CWC), India, to better comprehend the fluctuations in suspended sediment load and discharge throughout different regions of the GRB. Our research focuses on analyzing the relationship between the variability of suspended sediment load and its response to factors such as dam constructions and discharge fluctuations. Our research findings indicate that a significant number of gauging stations had a decline of more than 50% in suspended sediment load after 1990. Further investigation clearly demonstrates a substantial decrease in suspended sediment load after 1990 due to the entrapment of suspended sediment load induced by the installation of large-scale dams. The temporal change in suspended sediment load in the Godavari and its main tributaries is associated with the rise in human activities observed in recent decades. The findings of this study have important significance for recognizing the complex relationships between land use land cover, suspended sediment loads, soil erosion, and reservoir management in the GRB.  In addition, this study can provide valuable information for policymakers to adopt more effective reservoir management, soil erosion control, and soil-water conservation measures in the GRB.

How to cite: Singh, A., Swarnkar, S., Kundu, S., and Prakash, S.: Assessment of Suspended Sediment Dynamics in the Largest Peninsular Basin of India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-617, https://doi.org/10.5194/egusphere-egu24-617, 2024.