S. K. Chanda

The Neoproterozoic Cratonic Successions of Peninsular India

Abstract The Peninsular India hosts extensive record of Mesoproterozoic, and Neoproterozoic successions in several mobile belts, and cratonic basins. The successions provide excellent opportunities for chronostratigraphic classification, in tune with the chronometric classification adopted by IUGS for inter-regional correlation on a global scale. Major tectono-thermal events at 1000–950 Ma in the mobile belts, correlatable with the Grenville orogeny may be considered as the datum for Meso-Neoproterozoic classification in India. Principles of chronostratigraphic classification, however, can not be applied yet to the cratonic successions of India because of inadequate radiometric data, paucity of biostratigraphic studies, and lack of regionally correlatable stratigraphic or palaeoclimatic datum. The kimberlite magmatism which affected the Peninsular India on a continental scale at about 1100 Ma, holds the key to the identification of Neoproterozoic successions of the cratonic basins. Thus, the stratigraphically confined diamond-bearing conglomerates and/or the tuffs associated with kimberlites, may be considered as the datum to define the base of the Neoproterozoic, fixed at about 1000 Ma. Accordingly, the Rewa, and Bhander Groups in the Vindhyan basin, the Kurnool Group in the Cuddapah basin, the Jagdalpur Formation in the Indravati basin, and the Sullavai Group in the Pranhita-Godavari basin are taken to represent the Neoproterozoic successions in the Peninsular India. The Chattisgarh Group in the central India, the lower part of the Marwar Supergroup in western Rajasthan, the Badami Group in the Kaladgi basin, and the Bhima Group are the other “possible Neoproterozoics” in the Peninsula. The closing phase of the Mesoproterozoic in all these basins are characterised by stable shelf lithologic associations attesting to high crustal stability. The Neoproterozoic basins, by contrast, mark a new phase of rifting, and extension, and the basin fills exhibit signatures of initial instability which evolved with time into a more stable platformal condition. A major episode of sea level rise has been recorded in most of the basins. The riftogenic origin, and evolution of the basins are comparable with the history of Neoproterozoic basins of Australia though there is no unequivocal record of glaciation in the Indian formations.

Storm deposits in the late Proterozoic lower Bhander Sandstone of Vindhyan Supergroup around Maihar, Satna District, Madhya Pradesh, India

ABSTRACT Intertidal late Proterozoic Lower Bhander Sandstone around Maihar, Satna district, Madhya Pradesh, characteristically displays three orders of sandstone/shale alternations. The lowest order millimeter-thick alternations can be explained as related to tidal rhythms. The second-order alternations in the range of centimeters appear to be related to processes with periodicities greater than tidal rhythms-storm/calm condition. That these centimeter-thick sandstones are possible product of suspension-laden storm surge ebb current is supported by 1) the presence of various types of tool marks, scour marks, and load structures, 2) the presence of sharp, erosional bases and diffused tops, 3) gradual transition from parallel lamination to ripple lamination up the sequence, 4) sporadic preservat on of larger dune bedforms, and finally by 5) consistent seaward directed paleocurrent. This contention is further strengthened by the fact that the vertical lithofacies change is nonmarkovian in character. The uncommon overthickened first order sandstones in the range of decimeters are composite in character and appear to be product of storms coming at close intervals or of complete erosion of mud interlayers.

Early Diagenetic Chert Nodules in Bhander Limestone, Maihar, Satna District, Madhya Pradesh, India

Differential compaction of oolites in the limestones around and lack of compression of the same within most of the chert nodules in the upper part of the Bhander Limestone point to emplacement of silica at early stages of dewatering-compaction. As precipitated, silica was possibly a plastic material and its time of hardening varied within and between the nodules. The degree of deformation of the oolites varied according to the stage of hardening of the silica within the nodules at the time of compaction. Replacement of carbonate sediments took place due to addition of carbon dioxide derived from decomposition of organic matter, preferentially concentrated in the burrows formed by deposit feeders. Silica, on the other hand, was immobilized initially in the burrows by adsorption on organic matter. In all probability, silica was derived by post-mortem dissolution of siliceous organisms trapped in the sediment. Contemporaneous with silicification, dissolved carbonates released from areas enriched in organic matter were reprecipitated in zones lacking organic matter as calcito veinlets, that were subsequently deformed during compaction. Circumstantial evidences are, therefore, consistent with the hypothesis postulated by Siever for the origin of early diagenetic chert nodules in limestones.

Deep-water manganese deposits in the mid- to late Proterozoic Penganga Group of the Pranhita-Godavari Valley, South India

Abstract Facies analysis of the unmetamorphosed sediments enclosing the stratiform manganese oxide deposits of the mid- to late Proterozoic Penganga Group identifies the base of slope of a distally steepened deep-water ramp as their site of accumulation. The interpretation is based on their close association with a variety of mass flow deposits ranging from limestone conglomerates to calcarenites and deep-water, plane-bedded micritic limestone devoid of current- or wave-generated structures as well as detritus coarser than fine silt. These deposits occur within a major transgressive succession. The base of slope origin of stratiform manganese deposits is uncommon in the rock record and their origin is to be constrained against the background of base of slope depositional setting.

Recognition of Hardgrounds and Emersion Surfaces: A New Criterion

ABSTRACT Prelithification overburden stress does not normally exceed the load-bearing capacity of allochems (fossils or such other elements); therefore, compaction-deformation is an exception in limestones. In the presence of rigid bodies or surfaces, overburden stresses may be locally amplified to exceed the limit required to cause deformation of allochems earlier than usually possible. Consequently, deformed allochems may serve as signals for the presence of hardgrounds or emersion surfaces, otherwise unobstrusive in most limestones other than chalk. This idea is an outgrowth of our earlier study (Chanda et al., 1977) and is supported by the study of an oolite horizon in the Precambrian Bhander Limestone, India and relevant published data on hardgrounds.

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

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.

Soft-sediment Deformation Fabric in the Precambrian Bhander Oolite, Central India

ABSTRACT Chertified oolite in the lower part of the Precambrian Bhander Limestones, around Maihar, Madhya Pradesh, India, shows extensive compaction-deformation of ooids into oblate bodies, lying preferably parallel to bedding. Chertification, though begun locally when compaction was only slight, continued through and outlasted compaction-deformation in most parts. Unusual deformation of ooids is caused by overpressurization of ooids against a preburial rigid emersion surface below the chertified oolite. Unlike common fabrics of tectonically deformed rocks, compaction-deformation fabrics are uneven with respect to both shape and orientation of ooids. Though there is a strong tendency for preferred orientation of the deformed ooids parallel to bedding, directional variability is not uncommon. T is difference of soft-sediment deformation fabric from that of tectonically deformed rocks mainly stems from the fact that overburden stress is transmitted heterogeneously from grain to grain in unconsolidated sediments. Lithification, on the other hand, homogenizes stress distribution leading to greater uniformity of shape and orientation of fabric elements in tectonically deformed rocks. In addition, the soft-sediment deformation fabric lacks relationship with the external geometry of the rocks.

Proterozoic coastal sabkha halite pans: an example from the pranhita-godavari valley, South India

Abstract Extensive occurrences of pseudomorphs and casts of halite in association with wave-formed shallow water structures and evidence of emergence in quartzarenites at the base of the middle Proterozoic Pakhal Group (c. 1276 ± 20 Ma) in South India point to the development of sabkha environments in arid to semi-arid elimatic conditions. Periodic inundation of the sabkha during storm and subsequent desiccation led to displacive growth of the halite and its dissolution within the sediment just below the surface. The brine pools within the sabkha seem to have developed in fault controlled topographic depressions formed at the embryonic stage of an intra-continental rift ocean or graben system.

Petrogenesis of the Calcareous Constituents of the Lameta Group Around Jabalpur, M.P., India

ABSTRACT The calcareous constituents of the Lower Limestone and the Upper Sandy Limestone of the Lameta Group of Central India are largely of algal origin. These rocks were deposited under intertidal to inner-neritic marine waters. Tidal recession at times exposed the intertidal mudflats to atmosphere and desiccation and is responsible for formation of oxidized limeclasts. The algal limestones overlying the continental fresh-water deposits of the Jabalpur Group (Gondwana Supergroup) indicate a persistence of the depositional interface and in turn suggest that pre-Lameta tectonism did not result in a great change of elevation of the area. Recrystallization of intraclasts has been demonstrated to be a penecontemporaneous phenomenon. Discretely recrystallized carbonate rock fragments in an unrecr stallized host is thus an untrustworthy criterion of their extraenvironmental origin. Occurrence of extraformational and yet intrabasinal limeclasts in these rocks confirms the desirability of the genetic distinction of extraformational and extrabasinal limeclasts. On the basis of independent evidence, it has been demonstrated that open-space structures are good shallow-water indicators and helpful in delineating ancient shorelines. These data substantiate some of the observations by Wolf (1965b).

Dolomitized glauconite granules; a new kind of peloid from Proterozoic strata of central India

ABSTRACT Peloids, made up of homogeneous, structureless dolomicrite ranging in size from -3.0 to 1.5 , with the modal class between -1 to -0.5 , occur in the basal dolomite (Lower Proterozoic) of the Vindhyan Supergroup in central India. The peloids are discrete sedimentary grains, and morphologically they may be polylobate, tabular, sickle-shaped, or rounded/ovoidal with deep V-shaped cracks occurring along their periphery and converging inward. The rare presence of islands of glauconite within the peloids, together with other features, suggest that these dolomitized peloids are products of near-complete replacement of granule-size articles of glauconite.