extended abstracts - Geomorphic Processes and Geoarchaeology

Geomorphic processes and geoarchaeology
drained by a Saldam River – left tributary of one of the sources of the Yenisei River.
Water levels are therefore limited in their seasonal and long-term variations. Under the
lake there is a talik (melted ground), but severe winters (mean January temperature -
29º C) promote the lake freeze through in most shallow places. Lake banks and
surrounding plain are permanently frozen down to 150-170 m [5]. This permafrost is
probably of Pleistocene age. On lake islands permafrost is much thinner – 15-20 m
only [5], which may imply the Holocene age of permafrost accreted on previously melted
ground (under-lake talik) and therefore a post-lake formation of islands.
To estimate absolute age of the lake and Island Por-Bajin we made two coring
profiles between the island and lake banks (west and south from the island), a number of
pits on lake banks and surrounding plain and several cores into permafrost in Island Por-
Bajin. Sediment chronology is established by about 70 14 C dates. At four sections
incremental sampling was made and a set of laboratory analyses processed (grain size
distribution, loss on ignition, bulk chemical compositioin, composition of biological
remains), which provided assessment of lacustrine environment changes through time.
Study of sedimentary record gave the following results.
1. Surrounding alluvial plain. A set of pits opened loams and sandy loams
interpreted as overbank alluvium of deltaic alluvial fans. Coarse-grained channel
alluvium lies at depth of 1.5-2 m. Overbank loams contain one or two buried soils dated
to 10.5-12.0 ka BP (cal). By ca 10.5 ka BP (cal) overbank sedimentation had finalized
because of channel incision into the fans. This incision was initiated by thermokarstic
subsidence of river lower courses occurred shortly after the formation of the lake
(see below).
2. Lake sediments. In sedimentary cores taken from the deepest (1.0-1.2 m) part of
the lake west from the Por-Bajin Island overbank alluvial loams change into highly
calcareous lacustrine silts and gyttja ca 11 ka BP. This date is taken as the age of the
Tere-Khol Lake. After the lake formation no sedimentation breaks has been found in this
part of the lake. Total thickness of chemo-biogenic highly carbonate lacustrine silts
accumulated during 11,000 years varies between 2.5-3.5 m. Sediments synchronous to
the Por-Bajin Fortress construction lie at depth of 35-40 cm. Oldest dates on lacustrine
silts south from the island are about 8.5 ka BP (cal). In spite of younger age, their total
thickness is almost the same as west form the island due to higher sedimentation rates
promoted by shorter distance to modern river outlet. As the base of lacustrine sediments
lie a little below the dated layers, lake occupation of this area is dated to an earlier time –
9.5 ka BP (cal), which is obtained from a core in the Por-Bajin Island.
3. Composition of the Por-Bajin Island. The island banks 1-1.5 m high are
composed totally from lacustrine carbonate silts that contain some distinguishable layers
identified in the cores in the bottom of the lake. A core in the south-central part of the
island revealed two lacustrine units divided and underlain by alluvial sediments.
Permanently frozen sediments below the active layer (1.8 m) are characterized by high
total ice content, presence of numerous ice lenses up to 20 cm thick, which cause
considerable rise of initial sediment thickness. At depth 6.95 m roof of gravelly coarse
sands was found that was interpreted as active channel facies of alluvial fans same as
lying under overbank loams outside the lake. It was dated 12.2 (12.0-12.5) ka BP (cal).
This date is accepted as the end moment of active formation of alluvial fans. It was
succeeded by accumulation of overbank alluvium – sandy loams at depths 6.95-5.80 m.
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