IP 3 - Project

High resolution 3D architectural analysis and chronology of the Illgraben fan.

Aims and Objectives

Alluvial fans represent sediment sinks directly at the outlet of source areas in mountain landscapes. They contain multiple information on short-term as well as on long-term changes of sediment supply and of environmental parameters such as climate and vegetation. The use of fan deposits for time series analysis of these parameters, however, is hampered by their strong heterogeneity and the lack of natural outcrops exposing also deeper deposits. Thus, most studies on alluvial fans are restricted to surface analysis and almost no studies exist which aim to clear the subsurface geometry of a fan in total. We aim to carry out such a case study of 3D architectural analysis for a prominent fan in the Swiss Alps using a combination of quantitative outcrop analysis and ground penetrating radar measurements. The Illgraben fan with a surface of ca. 7.5 km2 offers the unique opportunity to combine both methods because at the moment a highway line is cutted through the fan exposing the fan deposits up to 10 m depth. In addition, good accessibility, few settlements and relatively dense road networks allow for multiple GPR lines covering the fan with sufficient subsurface information. We assume a relatively high record rate for depositional events of the Illgraben fan because the Rhone river which controls its base level was blocked by a large postglacial rock slide little downstream. However, we are not able to give the time frame of the deposits we can detect so far, because any datings are completely lacking. Therefore, we have included a relatively extensive 14C dating programme to this proposal. In a similar case study of a debris flow cone in eastern Switzerland (Engadin) we got a time frame of 8500 years BP for a sediment thickness of about 10 m (Hornung et al. 2009). Considering the larger catchment area and the much stronger activity of the contributing channel of the Illgraben compared to the Engadin case study we suppose a shorter time span possibly in the order of hundreds to one or two thousands of years. In the Engadin case study we got excellent results with ground penetrating radar measurements showing both, depositional architecture incl. palaeo-channels as well as fan-wide depositional discontinuities that have been interpreted as palaeo-surfaces and quiescence phases. Similar catchment lithologies (crystalline rocks) and depositional processes guarantee a successful application of GPR methods also for the Illgraben fan.

Within the framework of SedyMONT we propose to:
  • Develop a history of the sediment supply and transport modes from the Illgraben catchment. This will be compared with the results of projects IP1, IP5 and AP2 that aim at analysing modern processes rates.
  • Map paleo-channels and paleo-lobes.
  • Determine frequencies of short-term events and their change with time.
  • Correlate major trends or discontinuities within the fan deposits which reflect long-term changes of fan aggradation, quiescence, or dissection.
  • Quantify sediment volumes of time periods between bounding surfaces. Under specific conditions we may be able to quantify also large individual debris flow events.
  • Esimate the sediment bypass through the fan directly to the Rhone river (trap efficiency of the fan) based on grain size spectra and proximal-distal trends of the fan deposits.
  • Elucidate the controlling factors of the fan evolution by comparing depositional trends with climate and environmental changes of the region in order to assess the vulnerability of the Illgraben catchment to these changes.

Research methods

Ground penetrating radar (GPR):
Ground penetrating radar uses the reflectance of electromagnetic pulses at geologic bounding surfaces. The benefit are two-dimensional and laterally continuous structural images of the subsurface stratigraphic patterns (similar to seismic sections). The resolution and penetration depth depends mainly on the used center frequency. According to our extensive GPR-experience especially on an alluvial fan in the Engadin region (Hornung et al. 2007) we expect for this lithological composition (almost free of clay) GPR penetration depths of approx. 10-15m with the 200MHz antenna (resolution 0,2m) and up to 40-50m with the 15 MHz antenna (resolution 2-3m). Hence, georadar measurements can be closely adopted to the requirements of the structure and size of the geobodies being mapped.
For the GPR-investigation two different observation scales are used. In an overall scale an arrangement of 6 GPR-lines will be established forming a half-spidernet shape which covers the whole Illgraben fan area. The total length of these overview lines will be approx. 16km for which we can use plenty of existing trails and small roads. Three overview lines (approx. 2km length each) will be shot downdip from proximal to distal areas while the 3 other sections (2,5 to 4 km in length) follow isopachs. Because of the good accessibility of the fan surface it is possible to equally space the overview-sections over the fan area. Along these sections a maximum penetration depth should be gathered but with respect to clearly resolve the size of the relevant geobodies. This could be achieved by using low frequencies of about 100MHz to 15 MHz.
On the detailed scale 5 areas up to 100m x 200m are chosen along the overview-lines: 3 of them. in a proximal, medial and a distal position to the apex along the fan center line. 2 other areas are arranged laterally along the medial isopach-overview line. These areas will the investigated by a three-dimensional orthogonal GPR raster with line spacings of 5m using frequencies up to 100 MHz. This requires about another 20km of radar lines in total. Depending on the results inside this raster an up to 40m x 80m raster is chosen for high resolution sedimentological studies with the 200 MHz antenna and a line spacing of 1m summing up to additional 30 km radar lines.
Sedimentological analysis:
Quantitative sedimentological analysis is essential to translate radar facies patterns into sedimentary units, to identify reflectors and their characteristics, and to investigate depositional processes. This will be done at outcrops and drillings. We will have the unique opportunity to get large scale outcrops because right now a new highway is under contruction cutting through the central part (A9, see figure) of the fan. We have the acceptance to monitor outcrop faces of trenches and in a tunnel during construction. The outcrop faces will be mapped for and analyzed for sedimentary architectural elements and documented by precisely scaled photo panels (Hornung and Aigner, 1999, 2002 and 2004). Detailed information about lithofacies will be gained from sediment logs of boreholes that will be drilled during A9 construction. We apply for 6 drillings of 50m depth with liner technology to optimize core recovery. The boreholes will quantitatively reveal grain sizes, rock compositions and weathering proxies. Together with the results of related projects within SedyMONT, we will establish the basis for a fan-wide time-stratigraphic framework with respect to the depositional styles and its controls. Based on such a sequence stratigraphic concept, the overall fan evolution can be interpreted in terms of up- or downdip migration of the equilibrium point between aggradation and degradation, i.e. the variation of the base level as a result of changing the rate of sediment supply.
Dating and palynology:
To receive a sound and fan-wide chronostratigraphic frame we will carry out 50 AMS 14C datings. Standard pollen and spore analysis will be carried out if suitable horizons are exposed. Recently, we could recover well-preserved pollen and spores from the matrix of debris flows in a volcanic setting (Lenhardt et al, 2005 and 2006). This encourage us to try palynological preparation also in this case study. In case of success, we may get snapshots of vegetational changes in the catchment (debris flows) as well as in the fan area (paleosols, fine-grained layer during depositional quiesience). Preliminary investigations revealed several layers with Corg especially at the lateral border of the fan. The A9 will cut into these deposits.
3D modelling:
Visualization, quantitative analysis of the fan sedimentary architecture (e.g. sediment volumes) and data interpolation will be done with the ‘gOcad’-computer software. This programme works in fully 3D and is able to deal with spatially distributed data of any kind, so DEM-models, radar sections, well logs, drill core and outcrop lithologs and the outrop wall panels can be imported and directly interpreted producing e.g. spatial geobody maps, stratigraphic horizons or lithological domains. Inbetween dated stratigraphic horizons the sediment fluxes can be calculated.
 
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