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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 66 (1996)No.
1. (January), Pages 223-230
Deep-water Dolomites from the Proterozoic Penganga Group in the Pranhita-Godavari
Valley, Andhra Pradesh, India
Joydip Mukhopadhyay (1), S. K. Chanda (2), M. Fukuoka
(3), Asru K. Chaudhuri (4)(*)
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.
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