In the Amu-Dar'ya basin of Turkmenistan and Uzbekistan the
Jurassic System consists of a carbonate complex overlain by an
evaporite complex. The carbonate complex consists of sediments of
the Kugitang Formation of Callovian-Oxfordian age. These are marine
deposits that contain deep-water facies, barrier reefs, solitary reefs,
biohermal masses, and back-reef and lagoonal facies. The character of
the overlying evaporites of the Upper Jurassic Gaurdak Formation is
dependent on the character of the underlying carbonates. This
relationship can be used in exploration for reefs, with which are
associated oil fields of the region.
Five different types of evaporite section are recognized in the
Amu-Dar'ya basin: Gaurdak, Chardzhou, Beurdeshik-Kirpichli,
Bayram-Ali, and Sandykachi.
The Gaurdak type of section occurs in the northeast of the basin.
Near the base of this evaporite section is a limestone marker horizon,
the top of which is the boundary between a carbonate-sulfate member
below and a sulfate member above. Above this is salt and then salt with
potassium-salt beds. Above buried reefs in the underlying Kugitang
the lower part of the Gaurdak is almost monomineralic anhydrite. Above
the flanks of biohermal masses there is inter-tonguing of anhydrite and
carbonate rock with lenses of limestone breccia. In the inter-reef and
back-reef areas the Gaurdak contains more carbonate rock.
The Chardzhou type of section borders the Gaurdak type on the
southwest, the two comprising the northeast fourth of the Amu-Dar'ya
basin. It consists of five members (upward in the section): carbonate-
sulfate, lower salt, middle anhydrite, upper salt, and upper anhydrite.
Various types of limestone are present in the basal member. Above
barrier and biohermal organic buildups it consists of organo-clastic,
biomorphic, and breccia limestone. Thickness of this member is
80-150 m. The lower salt member is salt with admixed clayey rock
above depression zones. Directly above biohermal masses and barrier
reefs its thickness is greatly reduced, and practically no clastic material
is present. Thickness of this member ranges from 50 to 310 m. The
middle anhydrite is 10-50 m thick, greater values above depression
facies in the underlying Kugitang and lesser above bioherms and reefs.
The Chardzhou type of section is so different from the Gaurdak that
it can be recognized as an independent formation - the Sakar. Toward
the west the Sakar passes gradually into the Beurdeshik-Kirpichli type of section, occuping the northwest quarter of Amu-Dar'ya basin. There the
basal member consists of alternating limestone and anhydrite with some
siltstone and clay beds. The composition is more unifrom above reef
buildups. Where the underlying rocks are near-shore facies, thickness
of the evaporites is greatly reduced on to complete pinchout.
The Bayram-Ali type of section occupies the central part of Amu-
Dar'ya basin. At its base is an anhydrite member 110-140 m thick.
Next is a thick salt, followed by the middle anhydrite, upper salt, and
finally the upper anhydrite.
The Sandykachi type of section extends along the southern part of
Amu-Dar'ya basin. It consists largely of anhydrite, but contains also
subordinate beds of limestone and dolomite.
The main criteria for predicting reefs in the Kugitang Formation is
uniformity of composition and reduced thickness of the overlying
evaporites. Abrupt facies change in the evaporites indicates close
proximity of barrier reefs and bioherms in the underlying Kugitang.
Extensive presence of potassium salts in an area indicates absence of
any underlying reef complexes. (Taken from Khudaykuleyev, 1986;
digested in Petroleum Geology, vol. 23, no. 7/8, one litho-facies map
and several lithologic columns)
Copyright 2000 James Clarke. You are encouraged to print out this
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