Ajit Bhattacharyya

Proterozoic braided ephemeral fluvial deposits: an example from the Dhandraul Sandstone Formation of the Kaimur Group, Son Valley, central India

Abstract The Dhandraul Sandstone Formation of the Middle Proterozoic Kaimur Group, the Vindhyan Supergroup, Son Valley, India, has been reinterpreted to represent a product of braided ephemeral fluvial environment. The previous interpretation was shallow-marine barrier bar. Evidences in favour of braided-river deposition of the Dhandraul Sandstone are: (a) cosets of low-angle planar cross-bed sets are closely associated with cosets of parallel-laminated sets with particle streaming on bedding plane surfaces; (b) vertical recurrence of planar, laterally impersistent erosional surfaces that are invariably carpeted by red-coloured intraformational shale pebble conglomerate; (c) profuse development of ripple marks; (d) sedimentary structures indicative of critical to supercritical flow conditions; and (e) consistent palaeocurrent patterns.

Deformation of ooids by compaction in the Precambrian Bhander Limestone, India: Implications for lithification

Nodules of silicified oomicrite (chert) occur in the upper part of the shallow-water Precambrian Bhander Limestone of central India. The nodules characteristically are surrounded by aureoles, elongated parallel to bedding, of plastically deformed ooids. Both the intensity of deformation and packing density of the ooids in the limestone decrease away from the rigid nodules until a point is reached where there is neither detectable deformation nor condensation (intensification of “closeness” of grains). Lack of pervasive deformation in the limestone is interpreted to suggest that although in places compaction-deformation began early in diagenesis, cementation also began almost simultaneously, preventing the process from affecting the sediment beyond the limits of the aureoles. Deformation of allochems, particularly nonductile ones, appears to be possible only under unusually intense overburden pressure (or shear stress). Anomalously high stress around rigid nodules of precompaction origin may lead, however, to the deformation of allochems at much lower overburden pressures, and therefore earlier than normally possible. Whereas the presence of deformed allochems proves compaction, the converse may not be true. Sediments do compact, but overburden pressure probably does not normally exceed the load-bearing capacity of the allochems; in such cases, allochems would not be visibly deformed. Within the normal range of lithostatic pressure likely in compaction, sutured micrite fabric and condensation of allochems are therefore more likely than deformed allochems. Cementation may commonly intervene before sufficient overburden pressure is built up to deform allochems. Overemphasis on the presence of deformed allochems as a sign of compaction appears to have overshadowed the importance of compaction in the diagenetic evolution of limestones.

Diagenesis of quartz in the Upper Proterozoic Kaimur Sandstones, Son Valley, central India

Abstract The Upper Proterozoic Kaimur Sandstones in central India are quartz-, sublithic- and lithic-arenites cemented by quartz, illite and hematite. Diagenetic quartz occurs in five modes: syntaxial overgrowths, fracture healings, aggregates of small euhedral crystals, quartz resulting from the alteration of detrital silicates and from the recrystallization of quartz. Intergranular pressure dissolution is suggested as the main source of silica with smaller contribution from other sources, such as silica dissolved in meteoric waters, stylolitization, clay-mineral diagenesis, and the alteration of detrital silicates. Studies on the fluid inclusions and oxygen isotopes of diagenetic quartz suggest that meteoric water modified by diagenetic reactions has mediated the quartz cementation.

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.

Ripple-Drift Cross-Lamination in Tidal Deposits: Examples from the Precambrian Bhander Formation of Maihar, Satna District, Madhya Pradesh, India

Ripple-drift cross-laminations are described from tidal deposits of the Precambrian Bhander Formation, which belongs to the Vindhyan Supergroup of central India. Considering the transport processes operative in modern midflats, ripple-drift cross-laminations should be common in ancient counterparts. The generation of ripple-drift cross-laminations is, however, usually hindered by the absence of persistent, unidirectional current flow. These structures can only develop in areas of the midflat where there is persistent, unidirectional current. Because of periodic velocity changes of tidal currents, rhythmically size-differentiated foreset laminae appear to be a diagnostic feature of tidal ripple-drift cross-laminations. Size differentiation is normally absent in the ripple-drift cross-laminations of turbidites and fluvial sequences, for in the environments responsible, there exists no cyclic pattern of water movement (that is, currents that alternate with periods of slack water) as is found in the tidal-flat environment.

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.

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.

Experimental Compaction of Ooids under Deep-Burial Diagenetic Temperatures and Pressures: ABSTRACT

Modern ooid sand particles were experimentally compacted at temperatures of 150°and 200°C (302°and 392°F) and at pressures varying between 824 and 1,565 kg/cm2 (11,700-22,250 psi) consistent with pressures caused by 3.5-6.5 km (11,000-21,000 ft) of overburden. Bulk volume reductions of 21-26% under the above experimental conditions were more substantial than usually considered reasonable. Particle breakage and deformed particle contacts developed that are comparable to those reported for similar lithologic characteristics from the rock record. Pressure solution can be produced successfully on compaction in ooid sand particles. This demonstrates that initial pore-volume reduction through mechanical grain adjustments and ultimate pressure solution are the major processes in the diagenetic evolution of limestones. This appears to solve the problem of mass balance. Additionally, ooids were noted to have been plastically deformed, giving rise to longitudinal and concavo-convex contacts. End_of_Article - Last_Page 454------------