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The results of the BDP-96 drilling provide the age
estimates to test the aforementioned models for at least
the upper two fast rifting phases. The section recovered
at Academician Ridge spans the Holocene, the Pleisto-
cene and almost the entire Pliocene. Therefore, the age
of 2.5 Ma for the upper 900 m of sediments at Acade-
mician Ridge (Kazmin et al. 1995) overestimates the
sedimentation rates by at least nine times. The uniform
lithology and almost linear sedimentation rate of
approximately 4 cm/ka indicated by the age model
based on 13 magnetic event/reversal boundaries suggest
that the recovered section is continuous (Williams et al.
1997; BDP Members 1997b). The whole section of
BDP-96 consists of fine sediments. Lithology does not
support the idea that a channel of Barguzin delta was
the major sediment source for Academician Ridge
during the late Cenozoic (Kazmin et al. 1995). There is
also no evidence for the mid-Pliocene changes in facies
or sources of sediment supply at Academician Ridge as
proposed by Moore et al. (1997), because, as discussed
previously, the continuous 5 Ma section of BDP-96
(200 m long) does not exhibit facial changes.
The palaeomagnetic age model for BDP-96 cores
suggests that the B10 basin-wide mid-Pliocene uncon-
formity (Moore et al. 1997) or the middle-late Pleisto-
cene unconformity according to Kazmin et al. (1995) is
penetrated by the BDP-96 boreholes. However, there is
no significant lithological boundary in the recovered
section, which could be attributed to this event. We
calculate the position of the B10 boundary in BDP-96
cores using seismic velocities for Academician Ridge,
estimated by the Nedra seismic team for the BDP-98
borehole site located at 6.7 km to the northeast of the
BDP-96 site (“Nedra” open report 1998).
Seismic velocities were estimated taking into
account the directly measured seismic velocity in Lake
Baikal water (1430 m/s). The results indicate, for
instance, that seismic velocities increase from 1480 m/s
at approximately 100 m depth in the section to 1525 m/s
at the depth of approximately 300 m in the sediments
(“Nedra” open report 1998). These average values
appear low because they also include travel in 320 m
layer of water between the reflectors and the receiver
on board. The velocities in sediment for this interval
are around 1630 m/s.
The seismic profile 92-15 (Fig. 5) crosses the BDP-96
drill site location. The boundary B10 at the drill site is
located between 0.58 and 0.60 s two-way travel time
(Fig. 5). Using the range of seismic velocities of
1475–1480 m/s, the position of this boundary may be
estimated as 104B9 m core depth. The BDP-96 bore-
hole apparently does not penetrate the other wide-
spread boundary B6, which marks the top of the
Palaeo-Barguzin deltaic sequences on Academician
Ridge (Moore et al. 1997).
The level of 104 m core depth in BDP-96 roughly
corresponds to the age of 2.5 Ma as suggested by the
Matuyama/Gauss palaeomagnetic reversal at 104.77 m.
As mentioned previously, no evidence of erosion is
found around this interval. However, lithological
composition demonstrates dramatic changes at this
interval: diatom abundance drops drastically and few
layers of diatom-barren silty clay 40–70 cm thick are
present (101.95–102.35 m; 105.39–106.09 m; 106.28–
106.85 m). Apparently, such sharp changes in lithology
and sediment density could produce the pronounced
B10 seismic-reflection signal at Academician Ridge.
The seismic stratigraphy suggests that the sequence
boundaries of the upper two stratigraphic complexes
are traceable from Northern Basin across Academician
Ridge into Southern Basin (Kazmin et al. 1995; Moore
et al. 1997), thus allowing consideration of the rifting
stages in Lake Baikal to be synchronous. In light of this
correlation, the BDP-96 results from Academician
Ridge provide a new age constraint on the timing of the
upper rifting phase, placing it around 2.5 Ma.
Primorsky Range uplift and the age of Buguldeika
sedimentary strata
Synthesis of the geological data concerning the recent
evolution of the Lake Baikal rift suggests that the rapid
uplift of the western rift shoulder, which started with
the beginning of the middle event (according to Mats
1993) of the Neobaikalian sub-stage (0.7–0.8 Ma), has
cut off the Pra-Manzurka outflow from Lake Baikal
(Mats 1993). The former outflow was located in the
Goloustnaya River area; therefore, the BDP-93 section
is well situated to constrain the age of this tectonic
event. The lower unit of coarse sediments in BDP-93
boreholes represents a phase of intense sediment
supply from the Buguldeika River (BDP Members
1995, 1997a). The onset of hemipelagic sedimentation
at the Buguldeika Saddle marks the end of the rapid
uplift phase and the establishment of a balance
between the rates of uplift and erosion.
The current age model of BDP-93 cores is based on
magnetic excursion data (J. Peck, pers. commun.) and
the correlation of the pollen concentration curve with
BDP-96 biogenic silica data (Karabanov 1997; Kara-
banov et al., in press). It suggests the age of 660 ka B.P.
for the bottom of the section. According to the BDP-93
lithology, the intense accumulation of coarse terrige-
nous material from Buguldeika River started prior to
600–700 ka B.P. and ended at approximately 300 ka
B.P. with the transition to fine hemipelagic sedimenta-
tion.