International Journal of Marine Science 2015, Vol.5, No.31, 1-6
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the sedimentary terraces surrounding the lake.
Because during the evolution of lake, the sediments
have been remained in lake bed and edge, which
physicochemical and biological conditions of lake in
previous periods can be identified through studying
these sediments depositories. As a result, the previous
fluctuations of Urmia Lake can be reconstructed by
evaluating the results of stratigraphy studies on
sedimentary layers of terraces around the lake and the
sedimentation sequences under its bed.
2.1 The bio stratigraphy evidences of sedimentary
terraces
For the first time,Bobek (1934) detected sedimentary
terraces around the lake in 45 and 55 meters
elevations from the lake water level, after him
Schreiber (1975) found 4 other sedimentary terraces
around the lake in 30, 65, 85 and 115 meters
elevations (Keltz and Shahrabi 1986). Results from
age surveying by
14
C on limestone mollusk fossils
have illustrated that these sediments belong to 35,000
years ago. The sedimentary terraces with sandstone,
conglomerate and lime components in the eastern and
western edges of lake in Bonab, Bandar Golmankhane,
Heydarabad, Shabestar, Gamichi village, the north
part of Sharafkhane port and Malekkandi regions have
illustrated the existence of lake and wetland
sedimentations combined with brackish water in
Pleistocene period.The study on sedimentary terraces
which is located in North- eastern part of Urmia city
near to GachBashi County showed that the thickness
of sedimentary sequenceis about 15 meters and they
contain frequent mollusks and Ostracods fossils which
belonge to brackish and fresh water environments.
The choronostratigraphy results determined
approximately 46000 years age for this formation
(Sabouri 2009). On the base of conducted field studies
on the GachBashi village sedimentary terrace and
evaluating the sedimentary sequence of repeating
colloidal – clastic and carbonate sediments it was
identified that the river-alluvial, flood plain,
intermediate and lake sedimentary environments were
dominant sequentially in Urmia Lake watershed
(Figure 2). Presently, the above mentioned terrace is
located at the elevation of 1311 meters from sea level
and is higher about 40 meters than current water level
of lake. It means that less than 46,000 years ago the
water level of the lake was located about 40 meters
higher than current position, and because of abundant
fresh water infiltration in the region, the salinity of lake
water was decreased and wetland species of brackish to
fresh water were living in the lake. The old coastal
line with mentioned elevation code on the map of lake
surroundings, clarifies that the large parts of eastern
and southern cities such as: Bonab, Maraghe, and
Myandoab were under water at that time, and it means
that the area of the lake was twice of current area
(Figure 3). This issue nicely shows the impact of
climatic factors on physiographic transformation of
Urmia Lake watershed. The cross bedding
sedimentation structure in sandy and fine gravel
sediments is an identifier of powerful river flows
with often changing flow directions and on the other
hand graded bedding sedimentation structure
illustrates the gradual reduction of watershed energy
level and increment of the lake water level and
formation of the lake environment which is
detectable with calcareous marlstone sediments
(Figure 2). The start of sedimentation with
well-rounded gravel sediments, is an evidence of
affecting rocks by mechanical weathering in flow
paths and is a marker of erosive performance of
coastal flows and previous lake waves, which mostly
entered the coastal line because of flash floods
(Figure 2). After the mentioned sedimentation type,
the coarse sandy materials along with well-rounded
pebble and finally calcareous clay and marl
sediments are deposited which are full of brackish
water fossils and special Paratethys period
invertebrates, which illustrates the dominance of old
lake in the region (figure 2). On the base of
biostratigraphy evidences in mentioned terrace, three
complete swinging periods with specific recessions
and advances during upper Pleistocene, have been
identified (Figure 2). The interesting point is that
there didn’t exist any Artemia fecal pellets in
mentioned sedimentary structure, while in
simultaneous deposits in lake bed, the Aragonitic
pellets were recorded (Djamali, 2008). This issue is a
reminder of structural differences between marginal
basins of lake and its central parts. It is the same as
ongoing procedure in south west coastal parts of lake
near a pond in northern part of Mahabad Rood river
delta, at the presence of brackish to fresh water ponds.
The deserted ochre muds without any fossils, in the
upper layers of mentioned terrace,shows that the
watershed was out of water because of vast warming
and disappearance of brackish to fresh water lake at
early Holocene about 10,000 years ago (Figure 2).
