eLearning

The third drilling site (Leg III) BDP-97 was in the
Southern Basin of Lake Baikal in a water depth of
1428 m at 51747b51n N and 105729b14n E (Fig. 1). The
great water depth did not allow the riser (the borehole
reinforcing column) to be used and therefore the
drilling column had to be lifted several times during
drilling. This mode of operation did not allow obtain-
ment of long continuous cores (such as BDP-93 or
BDP-96 cores). The longest continuous section recov-
ered at this site was 30 m of sediment (Fig. 6). The sedi-
ments of BDP-97 core are characterised by abundant
turbidites 0.05–1.5 m thick. These deposits have uneven
erosional lower boundaries and pronounced graded
bedding. The lower parts of the turbidites contain
coarse-grained sand and gravel, and the upper parts
consist of silt and clay typical for pelagic areas of Lake
Baikal. Turbidite flows were eroding not only fine
lacustrine sediments but also reworking previously
deposited turbidites. The fine, undisturbed pelagic
facies at this site comprises not more than 25–30% of
the section, and the rest of the section is turbidite mate-
rial. This material is enriched in terrestrial plant debris.
The high content of organic material facilitates forma-
tion of organic gas. An important finding of the BDP-
97 drilling became the recovery of sediments containing
gas hydrates from depths of 121 and 161 m in two BDP-
97 boreholes. The gas hydrates consist of biogenic
methane (CH46H2O) as suggested by the isotopic
studies of the gas and by the absence of high molecular
weight hydrocarbons in the Baikal gas hydrates
(Kuzmin et al. 1998).
Seismic studies of the BDP-97 area demonstrated
the highly continuous sub-parallel reflections that are
generally unfaulted along the lake floor. The individual
layers in Southern Basin tend to have greater thickness
than individual layers at the Buguldeika Saddle or at
Academician Ridge (Fig. 7). This is explained by the
high content of coarse sand and gravel in the turbidite
beds. Comparison of the seismic data suggests that the
recovered BDP-97 sediments belong to the upper
seismic stratigraphic complex (Hutchinson et al. 1993)
represented in other areas by BDP-93 and BDP-96
sections. At a depth of approximately 300 m, a bottom-
simulating reflector (BSR) is clearly visible on the
profile (Fig. 7). This BSR, which crosses in many cases
the sediment bedding, is interpreted as the geophysical
expression of the lower boundary of the gas-hydrate
layer (Scholz et al. 1993).