Sedimentation processes and new age constraints on rifling stages in Lake Baikal: results of deep-water drilling

With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The sedimentary environments represented by the BDP borehole s are markedly different and possess characteristic lithological features. The results of the deep drilling provide the essential means for testing nu merous age models used in geological reconstructions of the Lake Baikal rif ting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted change from shallow-water to deep-water facies a t the boundary of the seismic stratigraphic complexes were found in the BDP -96 boreholes on Academician Ridge. Also, lithology does not support the pr oposed reconstructions of intense lake level fluctuations and transgression s during the Pliocene at Academician Ridge. The continuous deep-water hemip elagic sedimentation at Academician Ridge has existed for the past 5 Ma. Th e beginning of an intense rifting phase of the Neobaikalian sub-stage and r elated drastic changes in sedimentation processes were interpreted on seism ic sections as the basin-wide unconformity B10. Different age estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene bound ary. As shown by BDP-96 borehole, B10 is associated with a lithological cha nge from diatomaceous ooze to dense silty clay and not with an erosional co ntact. The new age for this boundary in BDP-96 is approximately 2.5 Ma. Thi s new age constraint suggests that the upper sedimentary strata of Northern Baikal (1.5-1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60-70 cm/ka. The BDP-93 boreholes at Buguldeika sugg est that uplift in Primorsky Range took place prior to 1.07-1.31 Ma, a date which exceeds the age of previous geological models.