DEEP-WATER DOLOMITES FROM THE PROTEROZOIC PENGANGA GROUP IN THE PRANHITA-GODAVARI VALLEY, ANDHRA-PRADESH, INDIA

The middle to upper Proterozoic Penganga Group of the Pranhita-Godavar i valley, South India, contains a thick slope-to-basin limestone-shale succession. The limestone, the Chanda Limestone, is well bedded and m icritic with several interbeds of slope-related auto-clastic debris-fl ow lime-clast conglomerates in its lower part. The limestone near its base encloses a 30 m thick interval of rhythmically alternating centim eter- to decimeter-thick beds of limestone and dolomitic limestone. Th e 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 plan ar-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-0.54-0.56 Mg0.46-0.44CO3) and has c loudy core with inclusions of micritic calcite and dolomite. The delta (13)C(PDB) values of limestone and dolomite vary between +2.0 parts pe r thousand to +3.4 parts per thousand and +3.8 parts per thousand to 4.3 parts per thousand, respectively. The delta(18)O(PDB) values of li mestone and dolomite range from -6.0 parts per thousand to -7.6 parts per thousand and +0.4 parts per thousand to -8.7 parts per thousand, r espectively. Stratigraphic, petrographic, and geochemical studies sugg est dolomitization in normal marine pore mater during shallow burial d iagenesis. Rhythmicity is attributed to recurrent episodes of dolomiti zation with Mg2+ derived mainly from dissolution of precursor high-mag nesium calcite. A mass-balance calculation suggests that 9 mole % MgCO 3 in the precursor would provide sufficient Mg2+. Normal grading of do lomite rhombs suggests that upward movement of Mg-enriched pore water dolomitized a thin interval of limestone wherever the Mg content reach ed the threshold for dolomitization.