ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

VALENTINA A. DROUCHITS
Formation of the Issyk-Kul Lake bottom relief
Geological Institute RAS, Pyzhevskiy lane 7, 109017 Moscou, Russia
From the geological point of view the Issyk-Kullake depression
can be divided into four parts (west, north, east
and south). These distinctions are inherited by lake geomorfology.
All these parts have general features of relief
and differences have local character. On the base of bathymetric
map, serie of seismogeological sections and coastal
drilling materials the geomorphological scheme for Issyk­
Kul lake depression was compiled. It was distinguished
four major stages for relief formation:
1. stage. Eopleistocene - Early Pleistocene
a) Formation of bottom and slope (700-300m), Eopleistocene
b) Formation of ancient submerged lacustrine plain (300­
200m), Eopleistocene-Early Pleistocene.
c) Formation of deepwater wedge and canyons, Eopleistocene-
Early Pleistocene.
2. stage. Middle Pleistocene-Late Pleistocene (first part).
Formation of gently sloping surfase (200-100 m)
3. stage. Second part of Late Pleistocene
a) Formation of submerged alluvial-lacustrine plain (100 m
- recent level)
b) Formation of submarine valleys (recent level- 100 m),
4. stage. Holocene
a) Formation of coastal area relief (+ 15 - -20m)
b) Formation of bottom microrelief
The first stage of relief evolution is caused by global event
such as N aryn phase of Late Alpine orogenic epoch. The
glacial epoch is respondent for the second stage genesis.
The third is associated with regional falls of lake levels in
arid zone in the North Hemisphere. The last stage is conditioned
by complex of local processes.
At the base of seismoacoustic profiles it is possible to determine
the relief genesis for Late Pleistocene. It was not
revealed clear features of glacial relief. Probably the present
lacustrine shelf was aggradation plain. Its west part
developed under of subsidence conditions, but east part
tested intensive raising. Western cutting valley system is
rather shallow, there are burried valleys. The facial replacement
could be to observe from thin to coarse ones from
submerged valley to coastal area on seismogeological sections.
At present on satellite photos the block structure of lake
depression is very good displaied, but on the seismic profiles
only the large blocks can be mark out. The Holocene
stage also can be characterized by different types of microrelief.
In the north and east parts hemogenic microrelief is
predominated, while the central shelf area is recognized by
biogenic and hydrogenic microrelief. For the north slope
and whole south part the slumps are typical forms.
There are clear changes between all environment stages
despite of palimpsest character for some ancient surfase relief.
For Late Pleistocene and Holocene in amplitude and
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distribution of relief and also in correponding deposit
thickness particularly it is noticeable. Naturally the formation
of meso-and microrelief during all these stages depended
on local endogenius and exogenic processes, but
general features of lake bottom relief are related to global
changes.
RUSSELL DRYSDALE
The Biogeomorphology of meteogene travertines from a
tropical karst: the role of hydrodynamics and some
implications for interpreting travertine fabrics
Geomorphology and Quaternary Science Research Unit,
Department of Geography, The University of Newcastle,
Callaghan, NSW 2308, Australia
Louie Creek is a karst spring-fed creek which drains the
northeastern Barkly karst, tropical northern Australia. The
stream deposits travertine along a reach of approximately
1.5 km. The travertines occur primarily as a series of barrages;
waterholes of up to several hundred metres in length
separate sets of barrages, act as sediment traps and host
stromatolitic travertines. All travertines are composed primarily
of calcite with traces of aragonite and insoluble material.
Carbonate precipitation commences once the source
waters reach approximately 3 to 5 times supersaturation
with respect to calcite.
Although physico-chemical processes dominate the downstream
evolution of the bulk solution, biota play both a direct
and an indirect role in travertine deposition at the microenvironment
level. The two most significant biotic groups'
at Louie Creek are aquatic insect larvae and microbes.
By far the most important insect order is Trichoptera (caddis
flies), which is dominated by genera of the superfamily
Hydropsychoidea. Several hydropsychid genera construct
cylindrical cases and silk nets on the travertine surfaces.
The cases consist of local materials, including travertine
quarried from the stream bed. The nets are usually erected
between cases or constructed over the case opening and
serve to trap food carried downstream in suspension. Nets
and retreats are usually aligned -mormal to stream flow in
successive linear arrays, which protrude up to 10 mm above
the travertine substrate. Microrelief of this magnitude
probably enhances turbulence, giving rise to increased carbon
dioxide outgassing; such a phenomenon may explain
why the nets and cases become heavily encrusted with calcite,
even where 'background' rates of travertine deposition
are low. Larval activity may also be used as a palaeoenvironmental
indicator: many larvae at Louie Creek, such as
hydropsychids, tolerate a relatively narrow range of hydrodynamic
conditions, and their activities (such as casebuilding)
produce distinctive sedimentary fabrics.
As with meteogene travertines elsewhere in the world, the
dominant microbes associated with the Louie Creek tra-