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thology and the sedimentary environments
f the Lake Baikal deep drilling sites
DP-93, Buguldeika Saddle
he first BDP (Leg I) drilling took place at the Bugul-
eika Saddle, approximately 6 km to the southeast
om the Buguldeika River mouth (Fig. 1), at the loca-
on 52731b05n N and 106709b11n E in a water depth of
354 m. During the drilling season two boreholes were
drilled at this site: BDP-93-1 (98 m) and BDP-93-2
(102 m; BDP Members 1995, 1997a).
Lithologically, the sediments from the BDP-93 cores
consist primarily of silty clay with sand lenses and
diatom mud. The amount of coarser material in the
boreholes increases with depth. The textures in general
are laminated. Some intervals are characterised by
finely laminated and lens textures. Numerous intervals
clearly reveal graded bedding with coarse laminae at
the bottom, the structure typical of turbidites. Many of
those layers have uneven lower boundaries, which indi-
cate erosion of underlying sediments. These turbidite
beds are most likely related to the river freshets and the
highest number of these events corresponds to deepest
parts of the cores, at the interval of 50–102 m depth
(Fig. 2).
Two sedimentary units are clearly distinguished in
the BDP-93 section. The sediments above 50 m core
depth (unit I) are fine-grained and contain a significant
amount of diatom material and demonstrate rhythmic
structure. Each sedimentary rhythm consists of two
parts. One of them is clayey silt with diatom content up
to 25–30% by volume. Another part consists primarily
of silty clay with diatom content below 3%. Such
rhythms are normally 1–2 m thick and in the upper
20 m of the core they are the most distinct (Fig. 2). The
lower sedimentary section (unit II) of BDP-93 (below
50 m core depth) consists of coarser sediment and
contains a large number of sand lenses and turbidite
beds (Fig. 2).
The watergun seismic-reflection profiles VERE-92-2
Line 7 and 12 (Colman et al. 1996a; BDP Members
1995, 1997a) reveal the two-unit structure of the drilled
sedimentary strata (Fig. 3). The upper unit layers
(above 50 m) are sub-parallel with a slight tilt to the
southeast towards the axial part of the basin. Lower
unit layers are tilted to the northwest towards the
Buguldeika River mouth. The reflection characteristics
also change at the depth of approximately 40 m. Above
this depth, seismic signals are weak and discontinuous,
whereas below 40 m the signals are strong, continuous
and closely spaced. This change is probably caused by
the distinct lithological differences observed in the two
sections in the BDP-93 cores. The upper part of the
section (unit I) is characterised by fine-grained hemipe-
lagic lacustrine sediments, and by few distal turbidite
laminae. In the lower part (unit II) turbidite beds are
abundant. The primary production and terrigenous
material of the Selenga River played a major role in the
accumulation of sediments during warm interglacial
stages of the upper unit. During glacial stages the
contribution of primary production was very low, but
ice-rafted transportation played a significant role. In
contrast, the accumulation of the lower sedimentary
unit II was dominated by terrigenous material from the
Buguldeika River with a significant contribution of
sand turbidites.
The deposition of this lower unit II probably corre-
sponds to the period of initial development of the
Buguldeika River drainage to Lake Baikal. Prior to
that, a Palaeo-Buguldeika River was flowing to the
northwest into the Lena River, but the rapid tectonic
movements of the Primorsky Range on the northwes-
tern shore of Lake Baikal diverted its flow into the lake
(0.8 Ma according to Mats 1993). To the west of the
BDP-93 drilling site the deposits of the modern sub-
lacustrine fan of the Buguldeika River can be distin-
guished on Line 12 (Fig. 3). This sub-lacustrine fan is a
major source of turbidite flows in the Buguldeika-
Uplift area.