Identification of a fault zone beneath Moxa observatory (Central Germany): evidence from combining logging, rock physical measurements, and geophysical profiling

Valentin Kasburg�, Todor Valchev, Andreas Goepel, Cornelius O. Schwarze & Nina Kukowski

�valentin.kasburg@uni-jena.de

Friedrich-Schiller-Universität Jena, Institut für Geowissenschaften, AG Allgemeine Geophysik

Moxa Geodynamic Observatory is located about 30 km south of Jena in the Thuringian Slate Mountains (Paleozoic Saxo-Thuringian). Folding in the surrounding of the observatory shows SW-NE striking and SE vergence (Franke, 1984). The predominantly fine clastic lithologies are of Lower Carboniferous age. Kasch (2006) carried out a detailed mapping around the observatory and suspected a fault in the valley of the Silberleite creek, where the observatory is located.

To better characterize the subsurface and understand the underground structures, a 99.6 m deep borehole was drilled in 2013, core material was collected and borehole geophysical measurements were carried out. The borehole was developed and a temperature measurement system was installed.

Geological setting

N

Borehole

Observatory

100 m

DEM data from TLVermGeo, 2014; vertical exaggeration = 2

Geological formations after Kasch et al. 2013,

Jena

BerlinGermany

6° 8° 10° 12° 14°

48°

50°

52°

54°

Start- endpoint of thecross section (subsurface model)

interbedded-Rottersdorf-subformation

interbedded-lower-Ziegenrueck-subformation

Quarternary

Fault / suspected

Roettersdorf-banded-slates-subformation

A/B

open fractures (0,1 cm), dark pelitic layering approx.

identified fault: fine sandy layers, partly milky quartz layers with a dip of 60 ° superimpose almost horizontally stored silty greywacke (N4 - Medium Dark Gray)

(N9 - White)fine sandy greywacke

strongly broken

(N4 - Medium Dark Gray)silty greywacke

fine sandy greywacke(N7 - Light Gray)

silty greywacke(N4 - Medium Dark Gray)calcite precipitation in fracture silty greywacke

(5YR 6/1 - Light Brownish Gray),broken

(N6 - Medium Light Gray),lithoclasts of fine sandy graywacke with a diameter of 0.1 to 3 cm in a reddish clay matrix, calcite precipitaion

greywacke-breccia

hematite on fracture surface(N5 - Medium Gray),silty greywacke

74,5 m

46,5 m

The cores consist of silty graywackes corresponding to a sequence of the upper three layers of the Bouma sequence. The dip of the layers was measured with a spacing of 0.5 m on the cores. Fractures with several secondary minerals occure with a spacing of approximately 60 cm. At 46.5 m depth 6 cm thick quartz layers overlaid by 4 cm fault gouch were found. At 74.5 m depth a 60 cm thick fault breccia was identified.

Core sample information

Fracture, partially filled with calcite at 25,70 m

Fracture

Cleavage

37- 38 m; Scan: P. Methe

Kasburg (2020)

Using geoelectrical resistivity tomography (ERT) for resistivity profile measurements Valchev (2020) was able to demonstrate a low resistivity anomaly in the in the valley of the Silberleite where the observatory is located. Low resistivity anomalies also occur in profiles crossing the “Forest”-fault northeast of the observatory.

Surface geophysical surveys

Valchev (2020)

Kasburg (2020)

Thermal conductivity, thermal diffusivity, permeability and sonic velocities have been determined on the cores (Iwakiri (2015), Schwarze (2015), Kasburg (2020)). While the thermal conductivity is relatively constant, the thermal diffusivity shows small variations. Permeability is highly variable, which can be related to the width of the fractures and degree of refilling with secondary minerals. Sonic velocities have been measured in the xy-plane. Above the two identified faults the velocities in strike direction decrease. For the velocities in the dip direction, this only applies to the cores above 46.5 m.

Petrophysical measurements

The combined evaluation of logging, rock physical measurements and geophysical profiling allows a better understanding of the subsurface structure:

permability approx. 75 ° dipping fractures occur frequently and serve as minor aquifers with variable

several faults were identified in the cores, but only sections at 46.5 m and 74.5 m are relevant

major horizontal aquifers are at depths of 3 – 6 m, 10 – 11 m and 22 – 23 m

for a major fault

Conclusions

interbedded-Roettersdorf-subformation

Roettersdorf-banded-slates-subformation

Fractures

Fault / suspected

Heig

ht [m

]

ground surface

Borehole

Obsveratory

0 50 100 150 200 250 300 350 400

500

480

460

440

420

400

380

360

A B

Cluster analysis was performed for Gaussian mixed models using the expectation maximization algorithm. In the depth representation of selected measurements, class changes in the encountered fault zones and aquifers become visible.

Cluster Analysis

Valchev, T. 2020: Aquisition und Inversion 2D/3D-geoelektrischer Daten: Ableitung eines

Thesis, FSU Jena. tektonischen Untergrundmodells des Geodynamischen Observatoriums Moxa. Masters

Schwarze, C. 2015: Bestimmung der thermischen Gesteinseigenschaften imUntergrund durch Labor- und Feldversuche an den Bohrungen KB-Moxa 1/13 und FB-Moxa 1/13. Diploma Thesis, FSU Jena.

102. 10.1007/s00531-012-0859-z. the Global Geodynamics Project (GGP) station Moxa, Germany. International Journal of Earth Sciences.

Kasch, N., Naujoks, M., Kley, J. and Jahr, T. (2013). Combined geological and gravimetric mapping and modelling for an improved understanding of observed high-resolution gravity variations: A case study for

Kasburg, V., 2020: Kernbohrung Moxa - Bericht zur Untersuchung des Bohrkerns unter Einbeziehung der bohrlochgeophysikalischen, geologischen, als auch der im Labor erhobenen petrophysikalischen Daten. Research report, FSU Jena.

Iwakiri, S., 2015: Gesteinsphysikalische Untersuchungen an Kernmaterialien aus der Bohrung Moxa 1/2013.

nordwestlichen Teilabschnitt des Ziegenrück-Synklinoriums bei Moxa. Diploma Thesis, FSU Jena.

Bachelors Thesis, FSU Jena.Kasch, N., 2006: Diplomkartierung & Diplomarbeit - Strukturgeologische Betrachtungen im Paläozoikum. Geotektonische Forschungen, W. Zeil, V. 68, E. Schweizerbart‘sche

Verlagsbuchhandlung.

Franke, W., 1984: Variszischer Deckenbau im Raume der Münchberger Gneismasse - abgeleitet aus der Fazies, Deckenbauinformation und Metamorphose im umgebenden References

Borehole measurements

The borehole measurements were taken directly after drilling and may have been influenced by the drilling fluid taken from Silberleite creek. Measured temperatures from the log in comparsion with the temperatures measured three years later via the distributed temperature sensing (DTS) glass fiber show no influence of the drilling fluid down to a depth of 73 m. From there the trend of pH changes and a local minimum of oxygen saturation is evident. Two major aquifers can be identified in conjunction with the weathered and strongly broken cores at 10 – 11 m and 22 – 23 m. Calculated dipping shows similar dip angles of the layering measured at the cores. Higher dip values fit well with the dipping of fractures measured by Kasch (2006) at surrounding outcrops.

5 cm

Acknowledgements Thanks to the Honours Program of the FSU Jena, in the framework of which this poster was created.

Forest-Fault

5 cm

Due to the greater thickness of the fault breccia and the deviating temperatures of the DTS measurement in the same depth it can be considered as the main fault. During construction in 1964, the entrance to the gallery had to be shifted backwards by several meters because increased moisture was encountered there along with a f r a c t u r e d a n d u n s t a b l e lithology. We interpret this as fault breccia and relate it to the faulted zone observed in the drill core.

Silberleite valley