Upper Jurassic Carbonates, Part I

From: "James Clarke" Subject: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Unsent: 1 X-MimeOLE: Produced By Microsoft MimeOLE V4.72.3110.3 Upper Jurassic Carbonates, Part I

In the south of Central Asia the Jurassic System consists of a lower carbonate complex and an upper evaporite complex.
The carbonate complex consists of sediments of the Kugitang Formation of Callovian-Oxfordian age. These are marine deposits which contain deep-water facies, barrier reefs, biohermal masses, and back-reef and lagoonal facies. The character of the overlying evaporites of the Gaurdak Formation is dependent on the character of the underlying carbonate complex. Five types of sections are recognized.
The Gaurdak type of section is found in the region of the southwest spurs of the Gissar Mountains, bounded on the south by the Repetek-Yerbent fault and on the west by the Bashkent downwarp. Near the base of this section is marker limestone horizon "R", the top of which is the boundary between a carbonate-sulfate member below and a sulfate member above. Above this is salt with beds of anhydrite and then salt with potassium salt beds. Total thickness of the Gaurdak Formation here is 625-850 m.
Above buried reefs in the underlying Kugitang Formation the lower part of the Gaurdak Formation is almost mono- mineralic anhydrite. Above the flanks of biohermal masses there is an inter-tonging of anhydrite and carbonate rock with lenses of limestone breccia. In the inter-reef and back- reef areas the lower part of the Gaurdak contains a higher content of carbonate rock.
The Chardzhou type of section has five members (upward): transitional, lower salt, inter-salt anhydrite, upper salt, and covering anhydrite.
The transitional member is carbonate-sulfate rock of Oxfordian and early Kimmeridgian age. It consists of various kinds of limestone. Above barrier and biohermal types of organic buildups the lower part of this member passes into 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 the biohermal masses and barrier reefs its thickness is greatly reduced (to 50 m) and clastic material is practically absent. Thick- ness of this member is 50-310 m.
The third member, the inter-salt anhydrite, is 10-50 m thick, greater values occurring above depression facies in the underlying Kugitang Formation and lesser above the bioherms and reefs.
The overlying upper salt has two sub-types: that of the northeast and that of the southwest. The northeast sub-type consists of salt with admixed clastic rock in the upper part and beds of anhydrite in the lower. The upper part contains in addition beds of potassium salts in paleogeographic lows. The southwest sub-type is salt with some admixed anhydrite. (Continued in next News Letter)
Taken from Khudaykuletev, 1986, one map and one section; digested in Petroleum Geology, Vol. 23, No. 7/8. Copyright 2003 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Eaelier News Letters are avaiable at: http://geocities.com/internetgeology/
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