Soumen Sarkar

Evidence of lacustrine sedimentation in the Upper Permian Bijori Formation, Satpura Gondwana basin : Palaeogeographic and tectonic implications

The Upper Permian Bijori Formation of the Satpura Gondwana basin comprising fineto coarse-grained sandstone, carbonaceous shale/mudstone and thin coal bands was previously interpreted as the deposits of meandering rivers. The present study documents abundance of wave ripples, hummocky and swaley cross-stratification and combined flow bedforms in the Bijori Formation, suggesting that a significant part of the formation was deposited in a wave-agitated environment. Evidence of near-emergent depositional conditions provided by repeated occurrence of rootlet beds and hydromorphic paleosols, local flooding surfaces denoting rapid fluctuation of water level, occurrences of temnospondyl vertebrate fossils, and absence of tidal signatures and marine fossils suggest a lacustrine rather than marine depositional regime. Five facies associations recognised within the Bijori Formation are inferred to represent fluvial channels and associated floodplains (FA1), lake shorelines (FA2), subaqueous distributary channels and associated levees (FA3), waveand storm-affected delta front (FA4), and open lacustrine/lower shoreface (FA5) deposits. The planoconcave fluvial channel-fill sandbodies with unidirectional cross-beds are clearly distinguishable from the delta front bars that show a convexo-plan or bi-convex sandbody geometry and dominance of wave and combined flow bedforms. Some of the distributary channels record interaction of fluvial and wave-dominated basinal processes. Major distributary sandbodies show a north to northwest flow direction while wave-affected delta front sandbodies show very complex flow patterns reflecting interaction between fluvial discharge and wave processes. Wave ripple crest trends show that the lake shoreline had an overall east-northeast to west-southwest orientation. The lack of documented contemporaneous lacustrine or marine sediments in the Satpura Gondwana basin posed a major problem of basin-scale palaeogeographic reconstruction. The existence of Bijori lake solves the problem and the lake is inferred to have acted as repository for the contemporaneous alluvial drainage. Development of the large Bijori lake body implies generation of accommodation space exceeding the rate of sediment supplied and thus represents locus of high tectonic subsidence. Transition of fluvial sediments with red mudstone and calcareous soil profile in the lower part of the succession to carbonaceous shale and coal-bearing lacustrine sediments in the upper part, denote a change from a warm semi-arid climate with seasonal rainfall to a more humid one.

Trace fossils in middle to late triassic fluvial redbeds, pranhita‐godavari valley, south India

Several morphological varieties of trace fossils abound in Middle and Late Triassic fluvial redbeds in the Pranhita‐Godavari Valley, south India, including Skolithos, Palaeophycus, Taenidium, escape burrows, and a type of trace very similar to ‘small stuffed burrows’ from the Triassic of Greenland. Burrow morphology was influenced by local hydrodynamic conditions. The distribution of burrows was facies controlled; some forms are restricted to channel deposits whereas others occur only in floodplains. Vertical dwelling burrows (Skolithos) occur in both channel and floodplain deposits. Horizontal structures representing deposit feeding (Taenidium) are confined to nondepositional surfaces within parallel‐laminated sandstones having parting lineations that represent catastrophically emplaced sand‐sheets in channels and proximal floodplains. Vertical escape burrows are confined to what were slowly but continually accreting parallel‐laminated sands of channel bars. Horizontal dwelling burrows (Palaeophycus) and...

Petrology of caliche-derived peloidal calcirudite/ calcarenite in the late triassic maleri formation of the pranhita-Godavari valley, South India

Abstract The Maleri Formation (Late Triassic) of the Pranhita-Godavari Valley, South India, is a continental red-bed sequence represented mainly by red clays with a few sheet-like bodies of channel-fill sandstone. Crudely bedded and cross-bedded peloidal calcirudite/calcarenite occurs at the base of fining-upward sequences in multistorey sand bodies and also as discrete solitary lenses within red clay. The calcirudite/calcarenite consists mainly of calcite-cemented spherical peloids which are made up of micrite and microspary calcite with microfabrics characteristic of caliche. Pure calcitic peloids are often reddened and those having a quartzose component are generally not red. The paucity of broken and abraded peloids, close association with intraformational materials and other evidence indicate a local source of the peloids. The microfabric of the peloids suggest their derivation from caliches. Available evidence favours a pedogenic origin for the peloids which mostly developed by displacive precipitation in a set-up of alternate wetting and drying. The lack of in-situ caliche profiles in the Maleri sequence and the rarity of compound grains imply that the peloids were derived from incipient caliche profiles which were localized at or near the surface and were completely stripped off by subsequent erosion. The ghost caliche profiles suggest the presence of periodically stable levels in the Maleri alluvial plain and low to moderate rates of alluviation. The residence intervals of channels were long enough to allow incipient pedogenesis in the temporarily stable inactive areas. In the present context, the predominance of smectite in fines, the poor floral content, the Maleri faunal assemblage and the paucity of evaporites point to a low seasonal rainfall in a semi-arid climatic environment.

Swaley cross-stratification in medium to coarse sandstone produced by oscillatory and combined flows: examples from the Proterozoic Kansapathar Formation, Chhattisgarh Basin, M.P., India

Abstract The Proterozoic Kansapathar Formation is the topmost unit of the siliciclastic Chandarpur Group of rocks which records a fluvial to marine transition and comprises a fining-upwards transgressive succession overlain by a coarsening-upwards prograding succession. The Kansapathar Formation around Panduka, M.P., is composed of medium- to coarse-grained, well sorted quartzarenite and is replete with swaley cross-strata (SCS), trough cross-strata and other structures of intermediate morphology. In laterally continuous exposures SCS are more common in the northeastern part of the study area, whereas trough cross-strata are more dominant in the southwestern part. We have recognised the SCS on the basis of the stratal geometry, originally described by Leckie and Walker (1982) . SCS occur as nested sets of elliptical swales which are commonly symmetrically filled. The trough cross-strata are variable in scales and dominantly SW-directed. The major axes of swales show a strong (NNW–SSE) preferred orientation orthogonal to the orientation of trough cross-strata and at a low angle to the shoreline orientation determined from the megaripple orientation in the immediately underlying unit. SCS described herein, in contrast to many examples reported from elsewhere, formed in medium to coarse sand and the maximum inclinations of their lower bounding surfaces and of the infilling laminae are greater than those of fine-grained hummocky cross-strata (HCS) and SCS. This is explicable since medium- to coarse-grained sediments did not experience significant suspension transport and, thus, the development of low-angle lamination was suppressed. The symmetrically-filled swales do not show any evidence of lateral migration and might have been formed by bidirectional oscillatory flow. The trough cross-strata could have been formed by translatory current-dominated combined flow. The spectrum of structures may owe their origin to symmetrical oscillatory flow transforming into asymmetric flow during onshore propagation of waves. This is expected to be common in shoreface environment. The spatial variation in relative frequency of occurrence of various types of cross-strata is also consistent with the palaeogeographic reconstruction of the study area. The trough cross-strata seem to be more common towards the constantly shifting sands of nearshore. Thus the relative frequency of trough cross-strata and SCS has been used as a clue to determine relative proximity to shoreline.

Paleogeographic and tectonic evolution of the Pranhita-Godavari Valley, Central India: a Stratigraphic perspective

The Pranhita-Godavari Valley of central India preserves records of repeated opening and closing of Proterozoic and Gondwana rifts along the zone of NW-SE trending Neoarchean suture between the Dharwar and Bastar cratonic nuclei. The Proterozoic succession in the Valley comprises several major unconformity-bound sequences, each with a distinctive set of litho-assemblage, deposited under diverse tectonic environments with highly variable modes and tempos of sedimentation. Analysis of sedimentary attributes of unconformity-bound sequences indicates that Purana sedimentation in the Valley started with the deposition of stable platformal assemblage of carbonate-quartzarenite in an extensional sag basin. The basin opened at c. 1700 Ma. The stability of the craton was disrupted when the first major cratonic rift nucleated at c. 1620 Ma, and shortly evolved into an oceanic basin along the northeastern margin of the Valley with consanguineous deposition of a thick wedge of conglomerates, tidal sandstones and shales in the Mulug shelf, and a succession of graywacke, carbonate, siltstone-mudstone-black shale and tuff turbidites of the Somanpalli flysch in two parallel adjoining belts. The Somanpalli flysch is bounded by two shallow marine successions, analogous to several geosynclinal deposits preserved in orogenic belts, and was deposited in a deep oceanic basin along a rifted continental margin. Paleocurrent analysis reveals that siliciclastics in the flysch succession were derived primarily from a continental provenance to the southwest of the Mulug shelf, and were transported to the flysch basin across the margin. Coarse sands were deposited in large submarine fan complexes of graywackes and mixed carbonate-siliciclastics at several points. The voluminous carbonate turbidites in the flysch were also derived from the adjoining Mulug shelf. High quartz content of graywackes, their close association with tidally deposited arenites, and ocean-ward transport of sediments from cratonic hinterlands collectively attest to the deposition of the flysch in a trailing margin basin. Opening of the oceanic basin and deposition of the flysch were accompanied by extensive granulite facies metamorphism at deep crustal level indicating a genetic linkage between them, and the litho-tectonic elements were deformed into an orogenic belt during re-assembly of Dharwar and Bastar blocks. The orogenic belt girdled the northeastern margin of the PG Valley, wrapped around its southeastern corner making a syntaxial bend, and continued into the Ongole-NKSB orogenic belt which also had its peak metamorphism at c. 1600 Ma. The age of the orogeny is not well constrained, but was followed by a Neoproterozoic episode of extension, and formation of cratonic rift basin with the deposition of several thousand meters of quartz-rich sandstones of the Sullavai, Albaka and Usur sequences.

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.

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.

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.

Tracking the migration of the Indian continent using the carbonate clumped isotope technique on Phanerozoic soil carbonates.

Approximately 140 million years ago, the Indian plate separated from Gondwana and migrated by almost 90° latitude to its current location, forming the Himalayan-Tibetan system. Large discrepancies exist in the rate of migration of Indian plate during Phanerozoic. Here we describe a new approach to paleo-latitudinal reconstruction based on simultaneous determination of carbonate formation temperature and δ18O of soil carbonates, constrained by the abundances of 13C-18O bonds in palaeosol carbonates. Assuming that the palaeosol carbonates have a strong relationship with the composition of the meteoric water, δ18O carbonate of palaeosol can constrain paleo-latitudinal position. Weighted mean annual rainfall δ18O water values measured at several stations across the southern latitudes are used to derive a polynomial equation: δ18Ow = −0.006 × (LAT)2 − 0.294 × (LAT) − 5.29 which is used for latitudinal reconstruction. We use this approach to show the northward migration of the Indian plate from 46.8 ± 5.8°S during the Permian (269 M.y.) to 30 ± 11°S during the Triassic (248 M.y.), 14.7 ± 8.7°S during the early Cretaceous (135 M.y.), and 28 ± 8.8°S during the late Cretaceous (68 M.y.). Soil carbonate δ18O provides an alternative method for tracing the latitudinal position of Indian plate in the past and the estimates are consistent with the paleo-magnetic records which document the position of Indian plate prior to 135 ± 3 M.y.