ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

ZECHUN LIU 1, YONGJIN WANG 1 & XUESONG LI 2
Changes of the palaeolake shoreline of the central Qaidam
Lacustrine Basin in the arid area ofwestern China
1 Department of Geography, Nanjing Normal Uni.,
Nanjing 210097, China
2 Department de Physique.Universite Catholique,
Chemin du cyclotron,&, B-1348, Louvain-La-Neuve, Belgique
The QaidamBasin, located on the northeastern Tibet Plateau
in western China, is a great arid lacustrine basin. The
lacustrin basin is about 2800 m above sea level and it co ..
vers an area of approximately 121,000 krn", with lot of dispersed
salt lakes and vast salt march and salt desert. Detailed
basin analysis has proved that the center of subsidence,
i.e. palaeolacustrine basin, in the basin was not sta ..
tionary during its entire history.It was initially in the west
of the basin during the Tertiary period, towards the end of
this period (Pliocene), this center of subsidence moved to
east, and during the Quaternary this center of subsidence
moved further eastwards to the Threee-lake area in the
middle of the basin where the thickness of materials accumulated
during this time is in the range of 2000-3000 m.
According to the results of boundary ages determined palaeomagnetically
and deposition rates of the typical facies
obtained by sedimentological analysis, the correlation, subdivision
and geochronological system of Quaternary deposits
with 30QOm thick in central Qaidam Basin has been
established on basis of the biostratigraphic correlation.
The Olduvai event and the M/G boundary are separately
at depth of 880-710 m in Se Zhong-6 BH(bore hole) and
the depth of 1460m in SeShen-1 BH.The yearly rate of accumulation
is obtained for the following typical facies: 1)
semi-deep lake facies, 0.45 mm/yr.; 2) shallow lake facies,
1.03mm/yr.; 3) lake shore facies, 1.28mm/yr.; 4) lake basin
bog facies, 0.94mm/yr.; and 5) delta facies, 0.90 mm/yr.
The analysis of climatical indicators, including lithology,
palynology, cr content, 5 18 0 and so on indicates that the
Qaidam Basin has been generally becoming drier and drier,
with climatical fluctuations obviously controlled by the periodic
changes of eccentricity of the earth orbit. In glacial,
the ancient Qaidam lake tended to expand and climate was
relatively humid. In interglacial, however, the lake tended
to contract and climate was relatively dry. However tectonics
is the critical factor controlling general developing
trend of the Qaidam lake basin. On the basis of analysis of
vertical facies of long cores, the paleolake's evolution of
Qaidam Basin can be divided into three stages as follow: a)
In the interval between 3.05 Ma B.P. and 1.95 Ma B.P.,the
transgression were developed gradually; b) Transgression
of lake happened widely from 1.95 Ma B.P.,especially after
1.35 Ma B.P.,the lake enlarge and deepen in water depth;
c) Beginning with 1.00Ma B.P.,the lake contracted remarkably.
Both the south shoreline and north shoreline
moved a distance of 25 km to center of lake.
Present geomorphy of the lacustrin basin of the central
Qaidam Basin has been gradully developed after the re-
gression of the palaeolake shoreline in the central basin.
The large lacustrine basin of the central Qaidam Basin has
been separated into several salt lakes by the neotectonic
movement and the underground deformation of structure
during the late Pleistocene and by the climate becoming
most drier during the Holocene.
ALEJANDRO LOMOSCHITZ 1
& JORDI COROMINAS 2
Analysis of the evolution of the landslides using
geomorphologic criteria. Applicability to the
Barranco de Tirajana basin (Gran Canaria, Spain)
1
Departamento de Ingenieria Civil, Universidad de Las Palmas de G.C.,
Campus de Tafira, 35017 Las Palmas, Spain
2 Departament d'Enginyeria del Terreny, E.T.S. Enginyers de Camins,
C y P., Universitat Politecnica de Catalunya, cl Gran Capita sin,
08034 Barcelona, Spain
The Barranco de Tirajana is located SE of Gran Canaria
island. The role of the mass movements in shaping this basin
has only been acknowledged very recently. Former
theories suggested either a volcanic origin or a tectonic
one. Since 1968 only the erosive genesis has been found
feasible.
Up to 28 large rotational and translational landslides were
identified whithin the basin, some of them over 1 km' (Lomoschitz
& Corominas, 1992). Three different stages have
been marked out in the landslide evolution (fig. 1). The
most active period triggering landslides corresponds to the
interval between 0.6 and 0.125 my ag0 (Middle Pleistocene)
and redates the ages of the landslides that were formerly
considered as starting 3.4 my ago (Itge, 1990). Most
ofthe landslides are inactive at present. Recent activity is
due to reactivation of the foot of a large ancient landslide
in the Rosiana area, where four historic events have been
recorded in 1879, 1921, 1923 and 1956 (Lomoschitz,
1995).
MAIN STAGES
I. ANCIENTLANDSLIDES
II. OLD LANDSLIDES
III. RECENTI PRESENTLANDSLIDES
Figure 1.
GEOMORPHOLOGIC CRITERIA
- Bedrockappears below.
- Hummockysurfacesand veryeroded zones.
- Largegrabens filledwith deposits.
• High weatheredrock masses, clay-siltrich matrix.
- Middle Pleistocene« 0.6 my.).
- Welldevelopedscarpmentsand tilted bodiesquite
unweathered.
• Grabenspartly coveredwith scree deposits.
- Obturationdeposits> 51.700 y. HP.
• Middle- upper?Pleistocene.
• Wellpreserved forms, present drainage network
invadedby landslides,clearlydefinedboundaries.
- Small landslideslocatedat the valleybottom,
,. The increaseofbed load causesbraided river channels.
- Activelandslidesin Rosianaarea. open scars, tilted
trees,etc.
253