Reconstruction of Miocene geodynamics in the Central Alps using detrital garnet geochemistry in sandstones of the Swiss foreland basin
TU Darmstadt, Germany
The geodynamics in the Central Alps during the Miocene were majorly characterized by the exhumation of crystalline basement, the so-called external crystalline massifs. Their exhumation had a major impact on the evolution of relief, distribution of drainage networks and sediment-generation. The timing of their first surficial exposure and their total thickness still remains debated. The Swiss Molasse Basin contains ~35 to 13 Ma old clastic deposits that were mostly derived from the immediate hinterland and contain valuable information about its tectonic and climatic evolution.
For this thesis, this sedimentary archive is used to trace the exhumation of the external crystalline massifs. In particular detrital garnet geochemistry is used to investigate 21.8 to 15 Ma old alluvial fan deposits of three different fan systems of the Swiss Molasse Basin. In this context, unusually grossular- and spessartine-rich garnets supplied from greenschist-facies metamorphic granitoid rocks serve as a unique proxy for the sediment supply from the external crystalline massifs. Their first appearance in a 15 Ma old sample in the Napf Fan indicates an at least partial exposure to the surface at that time. These garnets can neither be found in the alluvial fans to the east nor to the west, suggesting a relatively stable N/NW-directed drainage system since Miocene times. Another species of extremely grossular-rich garnets of so far unknown provenance has been found in 16.5-17 Ma sandstone of the Napf Fan, indicating a highly dynamic hinterland evolution during the Miocene, which is contrasting previous findings based on more traditional provenance proxies.
The temporal variability of sediment composition in modern rivers: provenance or grain size signal?
TU Darmstadt, Germany
The first step in any provenance study is sampling the sediment of interest, which is subject to many potential sources of error. It is nevertheless commonly assumed that one sample of sediment is enough to provide a somewhat “basin-averaged” compositional signal. Spatial or temporal variability of this signal is often not considered.
In this study, we test the temporal variability of sediment composition in modern fluvial deposits in four German rivers: the Gersprenz, Modau and Mümling in the Odenwald, and the Neckar close to the city of Tübingen. We revisited the same locations 12 times in the course of one year to take a sample. The samples were analyzed for grain size distribution (GSD) and geochemistry via XRF measurement.
The results show that
(1) the four river sediments have overall different GSD, which could be related to their different source lithologies;
(2) there are variabilities in the GSD of samples taken in different months, and these can be correlated with flood events;
(3) the bulk geochemistry changes significantly between months;
(4) within narrow grain size windows (1Φ-steps), the chemical composition varies less than the bulk geochemistry.
We conclude that bulk geochemistry of fluvial sediment varies mostly as a result of varying GSD, and not due to actual provenance changes throughout the year. On the one hand, this is promising for studies that assume fluvial sediment to faithfully reflect a “basin-averaged” provenance signal. On the other hand, it shows that geochemical data should always be interpreted in tandem with GSD.
Provenance shift at the northern margin of Gondwana during the Ordovician and Silurian recorded by detrital U-Pb zircon dating from the Eastern Alps
1Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Schnittspahnstraße 9, 64287 Darmstadt, Germany; 2Kommission für Geowissenschaften, Österreichische Akademie der Wissenschaften, Dr.-Ignaz-Seipal-Platz 2, 1010 Wien, Austria; 3Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstraße 24, 48149 Münster, Germany
The paleogeographic position of the Alpine terrane with respect to (peri-) Gondwana during the Paleozoic is still a matter of debate. In this study we use a multi-proxy approach to analyze the provenance of siliciclastic sedimentary rocks from the Carnic Alps and the Northern Greywacke Zone, which include detrital U-Pb zircon dating, petrology and bulk geochemistry. The biostratigraphically well constrained deposits from the Carnic Alps have been used as a reference profile, and these results were compared to samples from the monotonous sediment record of the Northern Greywacke Zone. The stratigraphic interval ranges from the Middle Ordovician to the Silurian.
The petrological and geochemical data indicate a high compositional maturity, which is supported by ZTR-dominated heavy mineral spectra. The results of U-Pb zircon dating (in total 957 concordant ages) show signatures typical for sedimentary rocks from the margin of Northern Gondwana and can be assigned to the East African-Arabian province. Most of the zircons show Pan-African ages, but presence of Meso- to Palaeoproterozoic and few Archean ages point to contributions from central Gondwana. Cambro-Ordovician zircon ages are linked to local volcanism at the northern margin of Gondwana. A stratigraphic trend both in the Carnic Alps and the Greywacke Zone is characterized by an increase of zircons with Tonian ages (900-700 Ma) and a decrease of zircons with Pan-African ages (700-550 Ma).
We interpret the shift in detrital zircon ages by enhanced sediment input from the center of Gondwana mainland and reduced input by detritus from Cadomian crust of Northern Gondwana.
Thrust and strike-slip fault control, in the late Eocene to Miocene, of Pindos foreland basin evolution: SE Aitoloakarnania area, western Greece.
1Department of Geology, University of Patras, Rion 26504, Greece; 2Department of Geology, Aristotle University of Thessaloniki, Thessaloniki, 54124, Greece
Evolutionary stages, from late Eocene to Miocene, of Pindos foreland, mark the transition from Pindos oceanic basin, in the east, to Gavrovo carbonate platform sedimentation in the west. Pindos Thrust as a crustal-scale structural element resulted in the formation of the Pindos foreland. Current research, which is based on detailed field campaign and on cross-sections, correlates and interprets the tectonic evolution of the area. The study area is part of the external Hellenides and belongs to the eastern Pindos foreland (SE Aitoloakarnania region). Pindos foreland consists of a syn-orogenic sedimentary succession with thick clastic deposits where compressional deformation acted synchronously to sedimentation. The activation of NNW elongated Gavrovo and Ionian Thrusts modified and controlled the syn-orogenic evolution of the area. In particular, the study area is characterized by the formation of fault-related fold structures of Gavrovo Thrust, such as the Varasova and Klokova anticlines and the Vassiliki syncline. Westwards propagating thrusting deformation has been partitioned by dextral strike-slip faults like Evinos Fault. This dextral strike-slip fault acts as a pathway to Evinos River flow. Finally, the syn- to late-orogenic evolution of the area is modified by normal faulting, forming the current geotectonic framework of the area.
This work was funded by the H.F.R.I. (Hellenic Foundation for Research and Innovation) and GSRI (General Secretarial for Research and Innovation) through the research project "Global climate and sea-level changes across the Latest Eocene-Early Oligocene, as reflected in the sedimentary record of Pindos foreland and Thrace basin, Greece, 80591".
Turbidity current sediment modeling in a rift basin
1Prof. Burmeier Ingenieurgesellschaft mbH, Germany; 2Federal do Rio Grande do Sul University; 3Alberta University
Turbidites are the main reservoir rocks in many sedimentary basins. Through well log profiles and using stratigraphy of high-resolution sequences, 6 stratigraphic surfaces of 3 and 4 order were interpreted. These surfaces were defined trough facies association (cores) and petrofacies (thin sections). Each surface has an electrical signature and a seismic amplitude. Correlating wells and mapping the seismic lines, the 2D and 3D stratigraphic surfaces are then obtained. With Schlumberger's Petrel a seismic and stratigraphic verification points were defined. Then the 3D structural model was built by the faults and zones of the top and bottom layers. For this area, a Grid with 50X50m cell size (horizontal limit) was defined. Following this, the pillar gridding and the layering are generated. For facies modeling, a facies profile was obtained in the stratigraphic sections trough the correlation between logs and rock. A good log and seismic signature can separate the reservoirs (sandstones), from the source rock (shale) and carbonates. Histograms are necessary to define the accuracy and representativeness of the model, defining the vertical modeling limit. To complete the model, it is necessary after the study with histograms, the scale up of the model and if necessary, insert a trend (a paleocurrent for example). The most representative interpolation method for this model was SIS Modeling (Sequential Indicator Simulation). The result was considered satisfactory and correlated with the profiles of existing wells, indicating a methodology to be used constantly by the industry. This serves the O&G sector, geothermal energy and groundwater exploration.