Pradip K. Bose

1.6 Ga U-Pb zircon age for the Chorhat Sandstone, lower Vindhyan, India: Possible implications for early evolution of animals

Bedding-plane markings in the Chorhat Sandstone (lower Vindhyan), central India, were recently interpreted as burrows produced by triploblastic animals. Because the rocks were thought to be older than 1000 Ma, these structures were regarded as the oldest fossil evidence for metazoan life. However, the biological origin of the markings has been questioned, as has their age. Current age estimates are based on K-Ar, Rb-Sr, and fission- track dates, though some contentious evidence suggests that the rocks may be only 540 Ma. Here we provide the first robust age data for the lower Vindhyan by using SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon geochronology to date silicified tuffs bounding the Chorhat Sandstone. Our results show that the sediments were deposited between 1628 ± 8 Ma and 1599 ± 8 Ma. If the Chorhat markings are burrows left by worm-like animals, then our data suggest that complex metazoans had evolved before 1600 Ma, 1 b.y. before the “Cambrian explosion” when animals rapidly diversified and became ecologically dominant. However, given the doubts expressed about the origin of the bedding-plane structures, as well as the surprisingly “old” age of the host rocks, further studies are urgently required to provide supportive evidence.

Overview of the meso- to neoproterozoic evolution of the Vindhyan basin, central India

Abstract Despite its origin in a stable intracratonic basin, the Vindhyan Supergroup, central India, embodies tectonic-driven depositional cycles of various orders. A marked change in sedimentation pattern was coupled with a transient plate-margin compression in the otherwise extensional regime. An unconformity laterally correlatable with a conformity, that divides the Supergroup in two sequences arose in consequence. Maximum flooding surfaces (MFS) within the sequences can generally be correlated with intrabasinal volcanism that drastically diminished through the initial rift to later sag stage of basin evolution. Concomitantly with the change in tectonic milieu, paleoseismic deformation structures and related deposits also became drastically reduced in scale. Nevertheless, metre- and decametre-scale depositional cycles correlate well with the subtle record of paleoseismicity. Intermittent tilting of the depositional substratum is demonstrable in systematic bed-dip changes in metre-scale fining upward depositional cycles in the rift stage. In the sag stage, it is reflected merely in consistent deflection of paleocurrent directions at the top of the decametre-scale parasequence sets. Towards the top of the Supergroup, progressive thickening and decreasing frequency of parasequences indicates a declining rate of basin subsidence. Progradation, despite punctuation, thus resulted in basin-filling.

A commentary on the tectono-sedimentary record of the pre-2.0 Ga continental growth of India vis-à-vis a possible pre-Gondwana Afro-Indian supercontinent

An integrated chronicle of major events leading to the growth of the pre-2.0 Ga Indian Craton, which is the aim of this paper, is an essential requirement to constrain the possibility of Neoarchaean unification between Africa and India. The primordial sialic crust that eventually developed into the early Indian Craton segregated from the mantle before 3.8 Ga. Intially there were two seperate Indian blocks, the northern (NIB) and the southern (SIB), and they possibly amalgamated before 2.5 Ga. Rapid and extensive crustal growth at ca 3.1, 2.5 and 2.0 Ga, in conjunction with a related rise in relative sea level due to ocean basin volume reduction, kept the continental freeboard at a moderate level. The 2.5 Ga event was the greatest in magnitude and is likely to have led to the formation of an Indian supercontinent. Four sedimentary basins, one in the NIB and three in the SIB, developed on the typical Archaean tonal ite-trondhjemite-granodiorite basement, through rifting induced by mantle upwelling. Continental freeboard was lowered as a consequence and transgressions generally followed. Rifting persisted in all the pre-2.0 Ga basins, except one (Bastar) in the SIB, which only underwent a Wilson cycle as the two blocks collided. All the SIB basins were closed by 2.0 Ga, while the basin in the NIB, which only developed at ca 2.5 Ga, still persisted. Neoarchaean continuity between the Central Indian Tectonic Zone and the Limpopo Belt appears likely from all major aspects, but for the deformation history, which still remains elusive.

Tidal sandwaves and related storm deposits in the transgressive Protoproterozoic Chaibasa Formation, India

Abstract Despite deformation and metamorphism the 2.3 Ga fine-grained siliciclastic Chaibasa Formation of eastern India locally retains an excellent record of sandwave migration under the influence of tides and storms within a precursor sandstone facies in a largely subtidal setting and provides a rare opportunity to detect tidal periodicities. In the finer facies locally preserved, minor features provide glimpses of the depositional mechanisms and settings. The silt-dominated precursor heterolithic (silt-mud) facies contain numerous slides and slumps and was formed on a relatively steep slope between the fairweather and storm wave bases. The shale facies formed in a deeper setting beneath the wave base. Earthquakes occasionally have interrupted normal sedimentation and massflow products are present in all the facies, but with increasing frequency within the finer facies. The sandstone facies units, which were emplaced during repeated lowstands, were first subjected to progradation and subsequently drowned, in an overall transgression trend.

Slope-controlled seismic deformation and tectonic framework of deposition: Koldaha Shale, India

Abstract Penecontemporaneous deformations with remarkable lateral continuity at selected stratigraphic levels record frequent seismicity in the Meso–Proterozoic Koldaha Shale, Vindhyan Supergroup, central India. Their consistent bipolar orientation reflects the master control of basinal tectonics. Boudinage, small-scale conjugate faults and bidirectional joint planes indicate east-northeast–west-southwest basinal extension. Detailed study of a 27-km strike-parallel stretch between Chorhat and Shikarganj against a broader perspective reveals existence of multiple NW–SE-elongated subbasins. Stratigraphic and sedimentologic criteria point to inheritance and subsequent sustenance of steep western and gentler eastern flanks of the sub-basins. It is also indicated that the perennial supply of sediment took place from the east, while the western slope contributed only during tectonic seiches. The relatively large-scale faults are correlatable with the steeper and the smaller deformation structures to the gentler flank. The NNW-oriented half graben structures that were reactivated time and again, is elicited. Previous geophysical studies inferred roughly E–W-elongated rift valley in the granitic basement and the present observations indicate that the rifting continued at least up to the time of Koldaha Shale deposition. The half graben could then be guided by cross-faults resulted from dextral shear. It is imperative then, the basin-interior sedimentation and deformation patterns were more closely controlled by the cross-faults, rather than by the basin-margin faults. Only a set of slump folds in a 10-m-thick interval indicates that the northern margin of the main basin was relatively steeper. This is further corroborated by concentration of large fans in mineralogically immature sediments and southerly paleocurrent at the northern fringe of the basin. In contrast, the Vindhyan sediments commonly belong to orthoquartzite–carbonate association. A regional northerly slope of the Vindhyan basin floor towards the relatively steeper flank provides an explanation for the general northwesterly paleocurrent, even though the eastern flank of the sub-basins sloped WSW. This integrated study of the physical aspects of the sedimentary pile against the background information of basement tectonics recognize rifting with a dextral shear component as the overriding factor controlling the early part of evolution of the Vindhyan basin.

Marine to fluvial transition: Proterozoic Upper Rewa Sandstone, Maihar, India

Abstract The Proterozoic Upper Rewa Sandstone in central India, is generally interpreted as entirely marine. A detailed study in Maihar, Madhya Pradesh, however, reveals upward transition from marine to fluvial through a mixed facies association also bearing eolian imprints. Facies associations differ in stratal geometry and arrangement, palaeocurrent direction and pattern as well as in grain size and sorting. The fine-grained marine association is dominated by tidal sheet deposits with characteristic rhythmic changes in thickness and style of cross-stratification and their packages, as well as local evidence of current reversals. Subordinate beach deposits are also present. The major palaeocurrent direction is highly consistent and westward. The coarse-grained, often granule-rich, sandstones of the fluvial association are embodied by four facies correlatable with flow stages and relative bed shear. These braided stream deposits are preserved in vertical stacks of tabular sandbodies in response mainly to basin subsidence. The palaeocurrent pattern is unimodal and northwestward, but its dispersion covers 180°. The rocks are poorly sorted and at places incorporate sand clasts, although they are virtually mud-free and mineralogically mature as in two other associations. The medium- to fine-grained sandstones at the intermediate interval between the marine and fluvial associations are of distinct lensoid geometry. They incorporate representatives from the previous two associations as well as translatent strata, interdune erg deposits and possibly aeolian cross-strata. Consequently the palaeocurrent pattern is polymodal with a spread over 270°. An overall coarsening-up trend in the fluvial part and thinning-up trend in the tidal part imply progradation in spite of evidence of basin subsidence. A high rate of sediment discharge from a perennial river system in a humid climate is suggested.

Synsedimentary seismic activity in an immature passive margin basin (Lower Member of the Katrol Formation, Upper Jurassic, Kutch, India)

Abstract Seismites are difficult to identify in a sedimentary sequence that has been exposed to a number of other synsedimentary disturbances. The paper identifies some possible signatures of earthquakes in a lower submarine fan complex that formed in an extensional basin at the northwestern margin of the Indian plate. Dying growth faults, preferential basal brecciation of shale beds, frequent local intraformational unconformities, reverse grading attributed to mechanical sieving and intrabasinal mass flows through sand volcanoes bear the distinctive records of seismicity. Abundant synsedimentary grabens and horsts and profuse signs of liquifaction are suggestive. In this association, the thinning and fining up channel-fill sequences with evidence of gradually declining mass flow regimes also corroborate.

Shelf storm flow dynamics: insight from the Mesoproterozoic Rampur Shale, central India

Abstract The intracratonic rift-related Mesoproterozoic Rampur Shale in Kudri, India formed in the marine mid- and outer shelf domain. It bears a dominant imprint of shore-parallel, storm-driven flow, as predicted by oceanographers for modern shelves, and thus differs from other ancient storm deposits in the geological record. In the Rampur Shale, gutters are shore-parallel, while tool marks of various kinds are both shore-parallel and shore-normal, and show bipolarity in both directions. Hydrodynamic analysis and inter-relationships between the flow features suggest that the storm-driven flow was shore-parallel, dominantly unidirectional and steady initially, and that it became increasingly unsteady thereafter, while also becoming dominantly shore-normal during peak oscillation. As storm conditions waned, the gutters were filled by sediments under dominant wave influence. These Rampur data are consistent with observations made in modern seas and suggest that storm-driven flows, in general, have evolved in the same way over time, although their geological records may vary in details because of local constraints.

Seismic and non-seismic soft-sediment deformation structures in the Proterozoic Bhander Limestone, central India

Abstract Numerous soft-sediment deformation structures occur within the Proterozoic Bhander Limestone of an intracratonic sag basin in a 750 m long section along the Thomas River, near Maihar, central India. Part of these deformation structures have most probably a non-seismic origin, but other structures are interpreted as resulting from earthquake-induced shocks. These seismic structures are concentrated in a 60 cm thick interval, which is interpreted as three stacked seismi-tes. These three seismites are traceable over the entire length of the section. They divide the sedimentary succession in a lower part (including the seismites) deposited in a hypersaline lagoon, and an upper open-marine (shelf) part. Most of the soft-sediment deformations outside the seismite interval occur in a lagoonal intraclastic and muddy facies association. The SSDS within the seismite interval show a lateral continuity. They record simultaneous fluidisation and liquefaction. The bases of each of the three composing seismite bands are defined by small-scale shear folds, probably recording an earthquake and aftershocks. The presence of the three seismite bands at the boundary between the lagoonal and the overlying open-marine oolitic facies association suggests that the seismic event also triggered basin subsidence.

Facies, flow and bedform patterns across a storm-dominated inner continental shelf: Proterozoic Kaimur Formation, Rajasthan, India

Abstract A comprehensive sedimentation model for an epeiric sea, useful for further comparison and prediction, is derived crom the Proterozoic Kaimur Formation, Rajasthan, India. Facies analysis of a steadily regressive shale-quartz-arenite sequence documents the patterns of flow, sediment transport and deposition from beach to offshore across a storm-dominated gently sloping (