TECTONICS AND ENVIRONMENT AT THE WESTERN END OF THE BAIKAL RIFT: PALEOLAKE SEDIMENT RECORD FROM DARHAD BASIN, NORTHERN MONGOLIA

Darhad is a north-south extensional basin at the western end of the Baikal rift, just south of the border of Mongolia and Siberia near 99.5°E longitude. It hosted large (≤809 km³) ice-dammed ephemeral lakes during late-Pleistocene glaciations. Today, the basin is filled with glaciogenic silt and hosts a through-flowing river. A 92-m-long sediment core was extracted in 2004 from the center of the basin at 1547 m to determine timing of the ice-dammed interval during the LGM (MIS 2) (Gillespie et al., 2008; Batbaatar et al., 2009). Here we report a new OSL date from deep in the core and discuss the inferred environmental and depositional setting.

Most of the core comprises silt, but there is a 1-m-thick calcareous layer with micritic texture, unique in the core, at 79-m depth. We dated silt from 76.9 m, ~2.1 m above the calcareous layer, using IRSL SAR. Quartz gave an age of 123.6±9.9 (1σ) ka (n=6); feldspar gave an overlapping age of 135.0±10.6 (1σ) ka (n=7). The calcareous layer was deposited in a lake that contained mollusk shells plus fresh-water diatoms and sponge spicules. Chrysophyte statospores, also present, indicate summer desiccation of ~ 5–15°C shallow lakes. Together, the evidence suggests an interval when the lake was shallow and mildly alkaline in a restricted environment in a closed basin. Closure occurred when sediment influx failed to keep pace with tectonic subsidence. The lack of clasts in the calcareous layer corresponds to minimal erosion and a dry climate. The inferred low clastic deposition rate at ~125 ka, near the beginning the MIS-5 interglaciation, argues for a non-glacial interval of low precipitation then.

Normal fault scarps from multiple earthquakes have been reported on the NE side of the basin (Bacon et al., 2003) and suggest an average relative subsidence rate there of ~0.44±0.04 mm/yr since ~18 ka (¹⁰Be). The basin lacks prominent scarps on the west and appears to be hinged there, rotating down to the east. The age and depth of the calcareous layer relative to the modern sill (1505 m) of the basin gives a maximum limit to the subsidence rate of ~0.28±0.05 mm/yr for the core since ~124 ka, consistent with the rate calculated from faulting of 0.22±0.02 mm/yr. This may be evidence that the subsidence rate on the 10⁵ yr time scale is ~same as on the 10⁴ yr time scale in Darhad Basin.