The Vindhyan basin of centralpeninsularIndia contains strata ranging in age from 1400m.y. to 550 m.y. It is thus likely that the Precambrian-Cambrian boundary interval may be present in the upper part of the Vindhyan succession. In order to locate the boundary interval in the Vindhyan basin, carbon isotopic analyses of the carbonatehorizons of the successionhave been performed. The samples chosen for isotopic analysiswere carefully selected after petrographic, cathodoluminesence and microelement studies to get the originalisotopicsignal.The analysesrevealthat the carbonatesof the lowerpart of the succession(middleProterozoic)have lj13C ID B values close to zero (-1.1 to +0.9 permil). However, the carbon isotope profile of the uppermost part of the succession (Bhander and Sirbu Limestones) shows a positive shift of around 5 permilfollowedby a sharp drop in value to 2.7 permil. The isotope profile of this part of the Vindhyansuccessionis remarkablysimilarto those of well-established Precambrian-Cambrian boundarysections of the world. It thus seems that the Bhander and Sirbu Limestones may represent the Precambrian-Cambrian boundary interval in the Vindhyan basin. The carbon isotope chronostratigraphic marker for the Precambrian-Cambrian boundary interval has been identified in the Krol strata of the Himalayas, which were deposited in the late Precambrlan-early CambrianTethys seaway. Of special significanceto the present study is the fact that during this period the Vindhyanbasin was alsoconnected with the Tethys seaway.

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Petrographic study of selected thin sections from the Aptian dolomites in the Razzak Oil Field, Egypt reveals three dolomite textures. These textures have been classified based on crystal size, shape, uniformity, and trace element distribution. These textures are as follows: I) finely crystalline, planar-e dolomite, 2) medium to coarsely crystalline, non-planar dolomite, and 3) very coarsely crystalline, non-planar dolomite (saddle dolomite). Stable isotope analyses indicates that the Aptian dolomites formed in a marine environment and were later subjected to several diagenetic modifications that affected the original sediments. The finely crystalline, planar-e dolomite is penecontemporaneous in origin with 8 18 0 that ranges from -7.51 to -8.14%0VPDB and 8 lJ e that ranges from +0.39 to -1.27%0 VPDB. The medium to coarsely crystalline, non-planar dolomite is a replacement type with a meteoric origin. In this non-planar dolomite, the 1) 18 0 ranges from -7.99 to -8.82%0 VPDB and the o l3e ranges from -0.58 to -1.35%0VPDB. The void-filling (saddle dolomite) is the product of chemical compaction and is associated with the thermal sulfate reduction event. This saddle dolomite has 8 18 0 that ranges from -7.89 to -8.76%0 VPDB and 8 1lC that ranges from + 1.41to +2.42%0 VPDB. The diagenetic parasequence ofthe Aptian Dolomite is recorded by: I) deposition of the original sediments (lime mud); 2) dolomitization; 3) silicification, 4) pyritization, and 5) precipitation of saddle dolomites (pore-filling dolomite).