Results from Downhole Logging in the ICDP Project Scopsco at Lake Ohrid (Macedonia, Albania)

Abstract:

Lake Ohrid is located at the border between Macedonia and Albania (40°70′ N, 20°42 E) and is assumed as the oldest lake in Europe. The lake with a surface area of 360 km² has trapped sediments and volcanic ashes and hence, contains essential information of major climatic and environmental change of the central northern Mediterranean region.

In the frame of the ICDP project SCOPSCO (Scientific Collaboration on Past Speciation Conditions in Lake Ohrid), several scientific questions are adressed: age and origin of the lake, paleoclimatic change during the Quaternary, tephrostratigraphy, and driving forces for the outstanding biodiversity.

Sediments of the lake have been targeted for a deep drilling campaign in spring 2013. Four sites (DEEP, CERAVA, GRADISTE, PESTANI) have been cored to a depth of 569 m below lake floor (blf).

High-quality continuous downhole logging data have been achieved at all drill sites by the use of the following tools: spectral gamma ray, magnetic susceptibility, resistivity, dipmeter, borehole televiewer and sonic. Additionally, vertical seismic profiling was conducted at the DEEP site. Seismic investigations indicate a sediment fill of the lake basin up to a thickness of 700 m. First results from cores revealed, that the bottom part, below 430 m blf is characterized by coarser grained deposits while the upper part yields fine grained pelagic sediments.

The borehole logging data at the DEEP site shows strong contrasts in all physical properties, in particular in spectral gamma ray (GR), magnetic susceptibility, resistivity and seismic velocity (vp). Furthermore, the pelagic facies show a cyclic alternation in these parameters as well as in the structural features from borehole televiewer.

By use of the current age estimate of the record of around 1.2 Ma, the GR data was tested for matching with the benthic δ¹⁸O record and shows high correlation (R² = 0.72), whereas high GR indicate glacial and low GR interglacial periods. Spectral analysis was applied and several emphasized wavelengths were identified. To investigate the variability of these high amplitudes, the sliding window technique was used and reveals a temporal evolution. Linking of the signals to Milanković cycles suggest orbitally driven sedimentation for 1.3 Ma and a mean sedimentation rate of 33 cm/ka.