Tapan Chakraborty

Tectonics, exhumation, and drainage evolution of the eastern Himalaya since 13 Ma from detrital geochemistry and thermochronology, Kameng River Section, Arunachal Pradesh

The exhumation history of the central Himalaya is well documented, but lateral variations in exhumation remain poorly constrained. In this study, we identify sediment source areas and examine the late Neogene exhumation history of the eastern Himalaya from the synorogenic sedimentary record of its foreland basin. We present Nd and Hf isotopic data as well as apatite and zircon fission-track analyses from the Miocene–Pliocene Siwalik Group along the recently dated Kameng River section in Arunachal Pradesh, northeastern India. Our isotopic data show that Siwalik Group sediments deposited between 13–7 and <2.6 Ma in Arunachal Pradesh were mainly derived from Higher Himalayan source rocks. In contrast, sediments deposited between ca. 7 and 3 Ma have far less negative eNd and eHf values that require involvement of the Gangdese Batholith and Yarlung suture zone source areas via the Brahmaputra River system. Consequently, these sediments should also record incision of the Namche Barwa massif by this river. Source-area exhumation rates of Himalayan-derived sediments, determined from detrital zircon fission-track data, were on the order of 1.8 km/m.y. in the fastest-exhuming areas. These rates are very similar to those calculated for the central Himalaya and have been relatively constant since ca. 13 Ma. Our results do not support the hypothesis of a major change in exhumation rate linked to either local or regional climate change or to Shillong Plateau uplift during the Miocene, as reported elsewhere. The zircon fission-track data further suggest that exhumation of the Namche Barwa massif between 7 and 3 Ma was much slower than the ∼10 km/m.y. rate recorded in the recent past. Detrital apatite fission-track data indicate deformation of the Siwaliks due to forward propagation of the frontal thrust since around 1 Ma.

Lower Gondwana fluvial succession of the Pench–Kanhan valley, India: stratigraphic architecture and depositional controls

Abstract The Lower Permian Barakar and overlying Motur Formations, in the southeastern part of the Satpura Gondwana basin, India, reveal contrasting lithology and alluvial architecture. Barakar Formation (∼225 m thick) consists of laterally extensive 5- to 20-m-thick multistoreyed, multilateral coarse-grained sandstone bodies. In the upper part of the formation, a few 1.5- to 11-m-thick coal–carbonaceous shale units alternate with the thick sandstone bodies. In contrast, the Motur Formation is characterised by a thick (∼500 m thick) succession of red mudstone with usually isolated, comparatively thinner (1–15 m thick) sandstone bodies. The multistoreyed Barakar sandstone bodies are inferred to represent deposition in sandy braided streams. Earlier workers inferred development of the associated coal/carbonaceous shale units in contemporaneous floodplains. The present study, on the other hand, indicates that the coal–carbonaceous shale units accumulated in an extensive vegetated marshland with small channels and lakes, and were temporally and spatially unrelated to the Barakar braided rivers. Sedimentologic and stratigraphic data suggest that during periods of active subsidence of the basin floor, the braided alluvial plain was transformed to an extensive, low-gradient wetland, and at times of tectonic quiescence, elevated source regions forced the braided system to prograde over the coal-forming marshland. Thicker (1–15 m) sandstone bodies embedded in the red mudstones of the Motur Formation are inferred as channel fills. Whereas the thinner (0.2–2.0 m) sandstone sheets, at places occurring as wings of the channel sandstones, represent levee–splay complexes of the Motur channels. The red mudstone intervals perhaps represent the alluvial floodplain environment. Abundant calcareous nodules within mudstones are inferred to record development of calcareous paleosols on the floodplain deposits. Dominance of mudrocks, the smaller dimension as well as isolated nature of the channel fills and well-developed levee deposits in the Motur Formation, are suggestive of deposition in an anastomosed fluvial system characterised by multiple, laterally stable channel–levee complexes flanked by extensive floodplains. Occurrence of coal in the Barakar Formation and red mudstone with calcareous paleosols in the Motur Formation indicates a change of paleoclimate from humid (in Barakar) to semi-arid type (in Motur) during the Lower Permian time in the Satpura Gondwana basin. There is no independent evidence of major tectonic reorganisation (stratigraphic discordance, change of paleocurrent pattern) of the basin during the transition from Barakar to Motur Formation. It is inferred that the change from the thick multistoreyed, multilateral sandstone sheets of Barakar Formation to that of the isolated, thinner sandstone bodies within thick mudstones of the Motur Formation reflects response of the alluvial system to increasing climatic aridity rather than to increasing rate of basin subsidence.

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.

Depositional Record of Tidal-Flat Sedimentation in the Permian Coal Measures of Central India: Barakar Formation, Mohpani Coalfield, Satpura Gondwana Basin

Abstract The Permian Barakar Formation in the Mohpani coalfield, Satpura Gondwana basin, is composed of three broad lithologies that occur repetitively and are iterdigitated: (1) several metres thick coarse- to medium-grained sandstone bodies with scoured bases, (2) 5-20 m thick medium- to fine-grained sandstone bodies and (3) 5-20 m thick mudstone-dominated packages with variable proportions of centimetre- to decimetre-scale, fine- to medium-grained sandstone, carbonaceous shale and coal. The Barakar strata were previously interpreted as deposits of braided rivers and associated inter-channel flood basin in a continental setting. However, this study recognizes signatures of tidal current from the mudstone-dominated packages implying marine influence during Barakar sedimentation. The mudstone-dominated sediment bodies are the focus of this paper and comprise of three lithofacies that bear imprints of tidal processes during Barakar sedimentation: (1) heterolith, (2) sandstone, and (3) coal-carbonaceous shale, which alternate with one another within individual bodies. The heterolithic facies show interlayering of sandstone and claystone resembling flaser, wavy and lenticular bedding, as well as pinstripe stratification. Successive sandstone-mudstone couplets indicate periodic waxing and waning of flows. Within individual heterolithic packages, the sandstone:claystone ratio along with the bedding style, varies cyclically upwards giving rise to alternate sandstone-dominated and claystone-dominated intervals suggesting tidal velocity fluctuation reflective of spring-neap lunar cycle. Thickness plots of successive sand-mud couplets also reveal cyclic variation with a conspicuous periodicity of around 12 couplets per cycle, which corroborates the spring-neap-spring (or neap-spring-neap) lunar cycle. Presence of abundant desiccation cracks indicates periodic emergence and points towards an intertidal setting. The sandstone facies is characterized by a variety of wave-generated features such as bundled and chevron upbuilding of lamina, bi-directional foreset orientations, offshooting and draping laminae, scour-and-drape feature, swollen lens-like geometries suggesting their emplacement under storm-induced combined-flow on the tidal-flat. The coal-carbonaceous shale facies represent supratidal marsh environment.

Eolian-Aqueous Interactions in the Development of a Proterozoic Sand Sheet: Shikaoda Formation, Hosangabad, India

ABSTRACT A 40-m-thick eolian sand sheet deposit characterizes the upper part of the Proterozoic Shikaoda Formation near Hosangabad. It sharply overlies shoreface deposits and comprises wind-ripple strata ( 50%), adhesion strata ( 17%) and subaqueous strata ( 33%). Each stratification type defines stratal packages tens of centimeters thick and few meters wide that are superposed upon one another in a nearly random fashion. The eolian facies is inferred to have been deposited in a low-gradient, sandy supratidal setting. The subaqueous deposits of the sand sheet also reflect a tide-affected, westward-opening coastal setting with an approximately north-south shoreline. Abundance of aqueous and adhesion strata coupled with the absence of granule-rich coarse-grained layers, corrugated erosion surfaces and evaporites are the typical features of the Shikaoda eolian sand sheet. These features indicate that in spite of abundant sand supply from the coastal sources and a net aggradational setting, repeated flooding and high surface moisture were the principal factors that inhibited dune-building processes and favored the growth of a flat-bedded eolian sand sheet in the Shikaoda Sandstone. Numerous subhorizontal, nearly flat bounding surfaces split the sand sheet succession into tabular sediment bodies 50-100 cm thick. Each of the bounding surfaces can be traced for a few tens of meters and can be correlated to an event of aqueous flooding. Vertical stacking of tabular sandstone bodies implies that long-term sediment aggradation rate in the low-lying supratidal region kept pace with that of basin subsidence. The sedimentological features of the Shikaoda sand sheet facies when compared with known modern and ancient sand-sheet deposits suggest that the Shikaoda sand sheet developed independent of an erg in a comparatively wet climatic setting. The sub-humid coastal eolian sedimentary system of Padre Island, Texas is probably the closest modern analogue of the Shikaoda sand-sheet facies.

Structural pattern in the Precambrian rocks of Sonua-Lotapahar region, North Singhbhum, eastern India

In the western part of the North Singhbhum fold belt near Lotapahar and Sonua the remobilized basement block of Chakradharpur Gneiss is overlain by a metasedimentary assemblage consisting of quartz arenite, conglomerate, slate-phyllite, greywacke with volcanogenic material, volcaniclastic rocks and chert. The rock assemblage suggests an association of volcanism, turbidite deposition and debris flow in the basin. The grade of metamorphism is very low, the common metamorphic minerals being muscovite, chlorite, biotite and stilpnomelane. Three phases of deformation have affected the rocks. The principal D1 structure is a penetrative planar fabric, parallel to or at low angle to bedding. No D1 major fold is observed and the regional importance of this deformation is uncertain. The D2 deformation has given rise to a number of northerly plunging major folds on E-W axial planes. These have nearly reclined geometry and theL2lineation is mostly downdip on theS2surface, though some variation in pitch is observed. The morphology of D2 planar fabric varies from slaty cleavage/schistosity to crenulation cleavage and solution cleavage. D3 deformation is weak and has given rise to puckers and broad warps on schistosity and bedding. The D2 major folds south of Lotapahar are second order folds in the core of the Ongarbira syncline whose easterly closure is exposed east of the mapped area. Photogeological study suggests that the easterly and westerly closing folds together form a large synclinal sheath fold. There is a continuity of structures from north to south and no mylonite belt is present, though there is attenuation and disruption along the fold limbs. Therefore, the Singhbhum shear zone cannot be extended westwards in the present area. There is no evidence that in this area a discontinuity surface separates two orogenic belts of Archaean and Proterozoic age.

TILTVEC: a FORTRAN-77 program for the tilt correction of paleocurrent data with resolution of incongruities

Abstract TILTVEC is a FORTRAN-77 program for tilt corrections of paleocurrent data, which may be measurements on planar crossbeds or trough crossbeds. Tilt correction is effected by rotation of the dipping formation to its assumed initial horizontal position around present formation strike. The program borrows partly from a rotation of poles algorithm based on the transformation of the axes of the reference coordinate system. But new program modules take care of fallacies arising out of the distinction between axial data (poles) and vector data (paleocurrent directions).

AMBER EMBALMS ESSENTIAL OILS: A RARE PRESERVATION OF MONOTERPENOIDS IN FOSSIL RESINS FROM EASTERN HIMALAYA

Abstract Terpenoids are hydrocarbons, comprising isoprene as their fundamental building blocks, many of which serve defensive functions in plants and protect them from potential enemies in the environment. Preservation of volatile terpenoids in amber is unusual and rarely detected in fossilized remains. In the present study, a remarkable preservation of monoterpenoid constituents are detected in Pliocene–Pleistocene ambers collected from the Siwalik strata of eastern Himalaya. Amongst the monoterpenoids, eucalyptol, borneol, α-pinene and p-cymene are detected in significant abundance. The sesquiterpenoid fraction comprises both biomolecules and geomolecules. Biological compounds include copaene, selinene, OH-bearing compounds such as spathulenol, globulol and α-cadinol as well as isomers of elemene and muurolene. Calamenene and dihydro-ar-curcumene are the diagenetically altered sesquiterpenoids recorded in the samples. High abundances of biotriterpenoids like β-amyrin and α-amyrin as well as moderate concentrations of altered products such as double-bonded ketones and alcohols generated from the parent triterpenoids are detected in the ambers. The results demonstrate that the monoterpenoids, which are particularly susceptible to degradation, could survive exhaustive diagenesis and alteration over a long span of time on rare occasions. The biosynthetic pathways of the secondary metabolite terpenoids, particularly the monoterpenoids, evolved in plants long back in time and are present in the angiosperm clades that evolved much later in Earth history. This indicates the unique efficiency of these volatile terpenoid compounds as defense tools in more evolved and complex biota.

Record of Leaf and Pollen cf. Sloanea (Elaeocarpaceae) from the Middle Siwalik of Darjeeling sub-Himalaya, India and its Palaeobiogeographic Implications

Fossil leaf impressions and pollen grains comparable to modern Sloanea sp. of Elaeocarpaceae collected from the middle part of the Siwalik sediments (Geabdat Sandstone Formation; Pliocene) in Darjeeling foothills of eastern Himalaya are reported in the present communication. On the basis of macro morphological features, leaf remains are described as a new species Sloanea siwalika sp. nov. This is the first authentic record of the occurrence of leaf and pollen grains comparable to the genus Sloanea L. from the Cenozoic sediments of India and Asia as well. The recovery of this species and other earlier-described evergreen taxa from the same formation, suggests the existence of a tropical, warm and humid climatic conditions during the depositional period. The present study further suggests that after Pliocene the taxon might have shifted from Darjeeling Himalayan region to the adjoining southeast Asian land masses, due to possible climate change caused by post-Pliocene orogenic movement of the Himalaya.