The middle to upper Proterozoic Penganga Group of the Pranhita-Godavari valley, South India, contains a thick slope-to-basin limestone-shale succession. The limestone, the Chanda Limestone, is well bedded and micritic with several interbeds of slope-related autoclastic debris-flow lime-clast conglomerates in its lower part. The limestone near its base encloses a 30 m thick interval of rhythmically alternating centimeter- to decimeter-thick beds of limestone and dolomitic limestone. The autoclasts of debris-flow conglomerates within the dolomite-bearing sequence shows similar rhythmic repetition of dolomite and limestone. Dolomite crystals range in size from micrite to 225 mu m and have planar-s to idiotopic-p fabrics. A dolomitic bed contains 40-45% dolomite. The dolomite content commonly decreases from the base to the top of a bed with decline in frequency and size of coarse dolomite rhombs. The dolomite is nonstoichiometric (Ca (sub 0.54-0.56) Mg (sub 0.46-0.44) CO 3 ) and has cloudy core with inclusions of micritic calcite and dolomite. The delta 13 C PDB values of limestone and dolomite vary between +2.0 per thousand to +3.4 per thousand and +3.8 per thousand to +4.3 per thousand , respectively. The delta 18 O PDB values of limestone and dolomite range from -6.0 per thousand to -7.6 per thousand and +0.4 per thousand to -8.7 per thousand , respectively. Stratigraphic, petrographic, and geochemical studies suggest dolomitization in normal marine pore water during shallow burial diagenesis. Rhythmicity is attributed to recurrent episodes of dolomitization with Mg (super 2+) derived mainly from dissolution of precursor high-magnesium calcite. A mass-balance calculation suggests that 9 mole % MgCO 3 in the precursor would provide sufficient Mg (super 2+) . Normal grading of dolomite rhombs suggests that upward movement of Mg-enriched pore water dolomitized a thin interval of limestone wherever the Mg content reached the threshold for dolomitization.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.