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Arbeitsbericht NAB 19-04

Quaternary Borehole QBO Marthalen-Oobist (QMOB) – Data Report

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In the context of the sectoral plan "Sachplan geologische Tiefenlager" (SGT) Nagra is currently investigating three sites (Jura Ost, Nördlich Lägern, Zürich Nordost) in northern Switzerland to potentially host repositories for radioactive waste. The field exploration project "Quartäruntersuchungen" (QAU) is dedicated to the characterisation of Quaternary deposits within and around these siting regions to further constrain scenarios of their geological long-term evolution. The investigations support the reconstruction of Quaternary landscape evolution.

The borehole (Quartärbohrung, QBO) at Marthalen-Oobist (QBO Marthalen-Oobist / QMOB, Tab. 1) is part of Nagra’s Quaternary investigation program (QAU; compare Nagra 2018a). The drill site was chosen to constrain the bedrock geometry and the valley fill of the Rudolfingen Trough together with borehole QBO Trüllikon-Rudolfingen (QTRU, Huber et al. 2020 in prep.) (Fig. 1, 2). The borehole QMOB was positioned at the southern margin of the Rudolfingen Trough, where the previous drilling AZ 97-11 indicated the possibility that at this site an older and a younger basin fill separated by till are preserved (Frey & Günther 1997, Graf 2009, Nagra 2018a, b). The locality of QMOB and QTRU was defined based on the findings of AZ 97-11 and on the recently acquired reflection seismic line 16-QAU-11 (Nagra 2018b).

The borehole QMOB was conducted between 29.10 and 16.11.2018 by a combination of Düsterloh Hammer drilling and wire-line drilling with a triple-tube core-barrel, i.e. with a plastic core liner.

This report covers the request for a documentation of the authorized work on-site (UVEK 2018, Dispositiv 5.29) and documents also the core description and first lab analyses (UVEK 2018, Dispositiv 5.30). The work has been carried out according to the work program (Arbeits-programm, Nagra 2018c) and this report corresponds to the data report announced there.

In this report, we present core images, sedimentological descriptions, and geophysical measurements of the cores, all of which were acquired at the University of Bern. Additionally, a report on geophysical borehole logging by terratec Geophysical Services GmbH & CO KG and L. Keller (roXplore gmbh; responsible for quality control) and a technical drilling report by Blétry AG edited by P. Hinterholzer-Reisegger (Nagra) are presented in appendices H and I, respectively. This report is the basis for planning further core analysis and later interpretation in the larger geological context.

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