International Journal of Marine Science 2015, Vol.5, No.31, 1-6
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Figure 2 The sedimentary terrace of early Pleistocene in
GachBashi village in 6 km from north east of Urmia
Figure 3 The Urmia lake region in early Pleistocene about
40,000 years ago
2.2 The bio stratigraphy evidences in bed deposits
of Urmia Lake
For the first time, the thin sedimentary layers of
different parts of Urmia Lake bed were studied under
the attention of Iranian geological survey (Keltz and
Shahrabi, 1986). Then during the study phase of
ShahidKalantari plan in 2005, the geological and
geophysics investigations on the lake bed have been
conducted from east (Islami Island) to west (Zanbil
Mountain) by drilling four 100-m long observation
holes under the bed, and by high resolution seismic
profiling the sedimentary sequence of lake up to
800 m depth has been evaluated and studied (Tarh
No Andishan Company 2005). The obtained results
illustrate that the sedimentation sequence under the
lake bed isn’t uniform or homogeneous and the
deposition components are variable with the layers
thickness. Keltz et al (1986), have conducted a
study on Urmia lake bed and chosen a region in bed
near IsalmiIsland with 7.3 meters depth as a type
section. Interpreting of seismic waves, identified
two distinguished borders in 3.5 and 5 meters
depths. The sedimentary sequence of mentioned site
consists of 5 stratigraphy units, and two rock facies
have been identified in deposits: an aragonite pellet
mud (APM) facies and a playa lake mud (PL) (keltz
and Shahrabi 1986). The studies on sedimentary
facies equipped geologists with lots of valuable
evidences about past geographic and climatic
conditions of Urmia Lake. On the base of relative
age determination by
14
C of residual fossil’s lime
shells in these sediments, it is obvious that the
sedimentation rhythm of the lake is about 0.175 to
0.3 mm per year. As a result, the age of sediments in
deepest core is about 12,000 to 14,000 years old. So
the Urmia Lake has been completely dried up and
turned to a deserted environment 4,000 and 12,000
years ago. The lake environment hasn’t experienced
any serious changes from 4,000 years ago up to 90
,
s. now it is possible to estimate the duration of
drought periods of lake, considering the 3 and 5
deposition units sedimentation thicknesses which
are 56 and 48 cm respectively and the average rate
of sedimentation, which is about 0.2 mm per year.
The obtained results illustrated that the length of
wide rollback period of Urmia Lake was about 2400
to 2800 years.High resulation seismic profiling
records on Urmia Lakefloor determined four fluvial
and calcalerous sedimentary sequences (A, B, C, D)
that were overlaid on volcanic rocks in Holocene –
Pleistocene Periods (Tarh No Andishan Company
2005) (Figure 4).
