Deep-water Dolomites from the Proterozoic Penganga Group in the Pranhita-Godavari Valley, Andhra Pradesh, India

Abstract

ABSTRACT 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 µm and have planar-s to idiotopic-p fabrics. A dolomitic bed contains 40-45% dolomite. The dolomite content ommonly decreases from the base to the top of a bed with decline in frequency and size of coarse dolomite rhombs. The dolomite is nonstoichiometric (Ca0.54-0.56 Mg0.46-0.44CO3) and has cloudy core with inclusions of micritic calcite and dolomite. The d13CPDB values of limestone and dolomite vary between +2.0 to +3.4 and +3.8 to +4.3, respectively. The d18OPDB values of limestone and dolomite range from -6.0 to -7.6 and +0.4 to -8.7, 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 Mg2+ derived mainly from dissolution of precursor high-magnesium calcite. A mass-balance calculation suggests that 9 mole % MgCO3 in the precursor would provide sufficient Mg2+. Normal grading of dolomite rhombs suggests that upward movement of Mg-enriched pore water dolomitized a thin interval of lime tone wherever the Mg content reached the threshold for dolomitization.