Timing of Lithofacies Development in Rift Sediments of Lake Baikal (Siberia, Russia)

This work has been based on samples collected from, so far, the deepest borehole (core BDP-98, 600 m deep) penetrating sediment at the Academician Ridge (53 deg 4' N and 108 deg 2' E, water depth of 333 m) in Lake Baikal (BDP-Members, 2001). The Academician Ridge, structural and bathymetrical high between central and south Baikal basins with limited influence of turbidity flows, represents a suitable site for retrieval of paleoclimatic and tectonic records. The scope of the investigation includes a survey of the sedimentological, mineralogical and geochemical variability in the sediment section. Studied samples, initially aimed for Be-10 dating, have been chosen to represent different lithofacies and subjected to examination by various analytical methods including grain size analyses (laser based grain size analyzer), mineralogy (X-ray diffractometry and light and electron scanning microscopy), chemistry (ICP MS). The studied samples are dominated by silt-size particles with variable content of diatoms. The inorganic particles are composed of quartz, plagioclase feldspar, K-feldspar, clay minerals (micas, chlorites, and kaolinite), carbonates (calcite and siderite), Fe-minerals (oxides and sulfides) and Ti-minerals. We observe no specific variability in mineralogy with respect to the three major facies that spread from the deltaic one at the bottom of the section (dated at 8-7 Myr) through the hemipelagic (dated at 5.5-3 Myr) to the glacial (<3 Myr). The chemical data show a generally down core decrease in silica and increase in alumina and loss on ignition. The explanation for this likely to be a higher diatom content in the hemipelagic and Quaternary facies compared to the deltaic one. Apart from strong fluctuations in each facies, the general trends of trace elements are fairly constant down core. Samples enriched in vivianite (Fe3[PO4]2 x 8H2O), concretionary siderite (FeCO3) and occasionally nests of framboidal pyrite (FeS)occur throughout the facies. There is apparent association of Fe, Mn and P in the sediments that is related to these minerals. This association is produced probably by rift-related pulses of hydrothermal activity that took place at the end of the major volcanic episodes in this region at about 8 Myr ago. The expected origin of siderite microconcretions is probably diagenetic alteration of organic material (e.g. fecal pellets) by Fe-rich hydrothermal fluids. These alterations will be a target of detailed diagenetic investigations in future. References BDP-Members, 2001. The new BDP-98 600-m drill core from Lake Baikal: a key late Cenozoic sedimentary section in continental Asia. Quaternary International, 80-81, p. 19-36.