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MAGNETOSTRATIGRAPHIC PERSPECTIVES ON MIO-PLIOCENE SEDIMENTATION IN THE HIMALAYAN FORELAND AND BENGAL BASIN
Abstract
The Mio-Pliocene sedimentation in the Himalayan foreland basin (Hfb) and the adjoining Bengal basin (Bb) records heterogeneous basin evolutionary phases resulting from temporal changes in basin floor morphology, accommodation space, sediment transport mechanism; and the geometry and style of hinterland thrust sheet erosion. These two 'great basins' are subdivided into smaller sub basins partly explaining the lateral lithofacies variations. The resulting lithostratigraphic heterogeneity however put forward several challenges in the basin wide correlation demanding alternative approaches like magnetostratigraphy. The current knowledge on the evolution of Hfb is aided by magnetostratigraphy. On the contrary the Bb is poorly explored although the gradual facies transition under higher rates of sedimentation in marine and continental environments anticipates ideal conditions for higher order of completeness of magnetozones in this basin. Mio-Pliocene time, particularly in Hfb, records the transition from basin under-fill to over-fill condition followed by basin partitioning and upliftment with accelerated rates of intra-basinal shortening. The first order lithofacies changes like introduction of conglomerate-mudstone association, pedo-complexes, replacement of multistoried with piedmont drainage system depict the transition from lateral to vertical accretion as a result of wedge-top uplift in the Hfb also producing gaps in sedimentation visa -vis magnetozone records at various scales. The poor spread of available magnetostratigraphic data across the basin further limits any detailed estimates on the above processes. The contemporary changes in the Bengal basin are marked by gradual marine to continental transition by subsequent addition of characteristic lithofacies such as buff sandstones awaiting magnetostratigraphic controls. The available exposure based magnetostratiraphic data is scanty in both the basins in general and Bb in particular demanding better spacing with new sections to constrain the basin evolutionary phases within the Mio-Pliocene stratigraphic framework. Both these basins are routinely explored for hydrocarbon resources, therefore attempting 'magnetic' stratigraphy on sediment core samples (described here) will be of great help in developing a clear understanding on contemporaneous Mio-Pliocene evolution of the two basins. INTRODUCTION Evolution of Himalaya and Indo-Bermese ranges has played the most important role in establishing the tectonics-orography-climate ensemble in the southern Asia and China. The Himalayan foreland basin (Hfb) and Bengal basin (Bb) have witnessed the evolution of these two ranges and thus become the most important candidates to study the Cenozoic era in this region. Moreover, understanding the Mio-Pliocene records is globally significant in the study of paleobiology and paleoecology as the routes of modern representatives of flora and fauna, their geographic distribution and interchange goes back to this time (Jablonski et al., 1996). This time also marks some of the global events like the lowest eustatic sea level (Haq et al. 1987), formation of the Alpine-Himalayan orography, foundation to modern climates (Kutzbach,
... (Received : 5/11/2018;Revised accepted : 24/07/2019) https: //doi.org/10.18814/epiiugs/2020/020026 understand the variability in depositional setting (Tandon, 1976(Tandon, , 1991Kumar and Tandon, 1985;Burbank et al., 1996;Kumar et al., 2003a;2004a,b;Goswami and Deopa, 2018 and references therein), marine to fluvial transition (Srivastava and Casshyap, 1983;Singh, 1978;Najman;Bera et al., 2008; and references therein), biostratigraphy, faunal evolution and migration route (Pilgrim., 1913;Colbert., 1935;Agrawal et al., 1993;Nanda and Sehgal, 1993;Nanda, 2002;2015;Basu., 2004;Patnaik, 2013;Gilbert et al., 2017;Nanda et al., 2018 and references therein), magnetostratigraphy (Azzaroli, and Naponeone, 1982;Johnson et al., 1983;Ranga Rao, 1993;Ranga Rao et al., 1995, Sangode et al., 1996Chirouze et al., 2012;Sangode, 2014;Govin et al., 2018 and references therein), thrusting event (e.g. Meigs et al., 1995;Kumar et al., 1999;2003a,b;Ghosh and Kumar, 2000;Jain et al., 2000;Gavillot et al., 2018), exhumation history (e.g. ...
Abstract: An array of 172 oriented sites in the siltstone-mudstones of the overbank facies (including levee and crevasse
splay) across 1927 m thick Ranital-Kangra section, north of the Jawalamukhi Thrust (JT) in the Kangra Sub-basin (KSB)
of Himachal Himalaya is analyzed to reconstruct the magnetic polarity stratigraphy. Magnetic cleaning using thermal as
well as alternating field demagnetization techniques have shown a multi-component assembly of the remanent magnetic
vectors in these samples. Mineral magnetic studies suggested occurrence of high coercivity anti-ferromagnetic minerals
(goethite and hematite) in addition to the ferrimagnetic minerals (magnetite and maghemite). Therefore a demagnetization
strategy of; (I) initial thermal demagnetization up to 200oC to remove goethite and/or viscous/pigmentary (SP) hematite
components, (II) alternating field demagnetization till 60 mT to remove the soft ferrimagnetic components, and finally
(III) thermal demagnetization above 550oC to isolate the primary component was found most successful.
Total ten out of 27 reversals encountered were assigned ages based on iterative matching of the long normal event
[‘C5n’ (9.7 to 10.95 Ma) of Cande and Kent, 1995] besides lateral correlation from the earlier reported sections in the
hanging wall of JT. The long normal interval of RK section has been extended laterally to the 900m thick part of Kotla
section 35 km westward, using Anisotropy of Magnetic Susceptibility (AMS) over channel facies. A basin-wide correlation
of the magnetostratigraphic sections in the KSB suggest that the magnetic polarity events; (a) base of the long normal
(C5n) at 10.95 Ma, (b) base of C4n at 8.07Ma, (c) base of C3n at 5.23 Ma, and (d) the Gauss-Matuyama boundary (top of
C2An) at 2.58 Ma are more distinct. It is envisaged therefore that these events may be used as independent benchmark
levels provided a detail analogue of paleointensity, rock magnetism (including AMS), petrographic studies are made available
closely around these intervals. This would allow better stratigraphic correlation of numerous smaller discrete sections and
fossil bearing localities throughout the foreland.
The Himalayan foreland basin comprises a number of subbasins developed in various sedimentary environments during the
Plio-Pleistocene phases of Himalayan orogeny. This paper describes the evolutionary processes that led to the development of
the Himalayan foreland basin in the Dehra Dun and Subathu subbasins. The Dehra Dun subbasin essentially consists of
conglomerate facies whereas Subathu subbasin is primarily composed of sandstone–mudstone intercalated with conglomerate.
The two subbasins were sites of braided river systems. Evidence from chronostratigraphy, provenance and tectonic and climatic
conditions suggests that differential tectonic movement, sedimentation rate and basin subsidence, coupled with climatic
variation, were responsible for the contrasting fluvial facies in these subbasins.
Following the merger of the two subbasins at about 5.23 Ma, the conglomerate facies assemblage of the Dehra Dun subbasin
passes laterally into a sandstone–mudstone–conglomerate facies assemblage of the Subathu subbasin. Thickly bedded,
amalgamated sheet conglomerate of Dehra Dun subbasin has a high sedimentation rate (0.70 mm/year) between 5.23 and 4.8
Ma, with a slow rate of basin subsidence. In contrast, a slow sedimentation rate (0.39 mm/year) in coeval sediments of the
Subathu subbasin, with relatively fast subsidence, produced a succession of thick multistorey sandstone body with mature
paleosols. Petrographic and clast composition data from Dehra Dun subbasin suggest that the detritus was derived from the pre-
Tertiary Lesser Himalayan sequence lying in the north, during an active tectonic phase of the Main Boundary Thrust. Between
4.8 and 1.77 Ma, progradation of posttectonic conglomerate of the Dehra Dun subbasin continued during a quiescent phase of
the Main Boundary Thrust. However, as a consequence of partitioning of Subathu subbasin along the Intra-Foreland Thrust,
interfingering of buff (piedmont drainage) and grey (trunk drainage) sandstones took place. In contrast, the syntectonic
conglomerate of Subathu subbasin originated from the uplifted proximal part of the foreland basin (Tertiary belt) during the
active phase of Intra-Foreland Thrust around 1.77 Ma.
Abstract: A thick ( > 2000 m) sequence of the Upper Siwalik Boulder Conglomerate in the Mohand
area has been classified into sandy matrix to cIast supported conglomerate (Gm}, Gm2)facies, trough
cross-stratified conglomerate (Gt) facies, planar cross-statified conglomerate (Gp) facies, sandstone (St,
Sp) and mudstone (Fm, FI) facies. The conglomerate shows both coarsening and fining upward
stratigraphic sequence. The palaeocurrent data obtained from clast imbrication show palaeoflow
towards S-SW. The framework composition of the conglomerate is cobble to pebble size clasts of
quartzite, limestone, sandstone, shale and rarely granite-gneiss as well as basic rock.
Based on lithofacies association, palaeoflow distribution and framework composition, it has been
inferred that the Upper Siwalik Boulder Conglomerate was deposited in proximal-distal braided stream
to distal alluvial fan situation. It further suggests that the progradation of the alluvial fan was towards
southwest.
The Siwalik stratigraphic fill which dates back to 12.77-4.48Ma, east of Ravi River in the Himalayan Foreland Basin has three dominant facies associations: A - Trunk river; B - Piedmont, C - Alluvial fan. The repeated alternation of Facies Associations A and B represent contraction and expansion of accommodation space in response to pulsating tectonics. Both the Facies Associations are capped by Facies Association C. During creation of high accommodation space, Facies Association B was deposited along the basin margin. Consequently Facies Association A gradually shifted towards basin margin resulting into contraction of Facies Association B. The changeover from high to low accommodation space caused gradual migration of Facies Association A away from the basin margin and expansion of Facies Association B. The contraction and expansion took place in the order of 0.1-1.02Ma. This cyclicity in the stratigraphy is more pertinent to a linear elastic model with a range <105-106years. The striking difference in fluvial architecture of Facies Association A and B is ascribed to differences in stress relaxation following thrust loading, sediment load dispersal and discharge through the channels. These parameters are further related to a long term influence of basin tectonics. The contemporary climatic conditions reflect high fluctuation but do not show any significant impact on sedimentation pattern.
Neogene basin fills of the Middle and Upper Siwalik Subgroup in the central sector of Indian foreland basin record changes in fluvial architecture, dispersal pattern and provenance in response to deformation and uplift of the frontal Himalaya. The 2.4 km thick Neogene Siwalik succession in the Subathu sub-basin exposed in the vicinity of Nahan Thrust (intra-foreland thrust or IFT) shows fine-to medium-grained grey sandstones of multi-storey, sheet type deposited by transverse trunk rivers. These sandstones are gradually replaced by fine-grained buff sandstone of ribbon type deposited by relatively small piedmont rivers. A significant change in sediment dispersal from SE to SW is observed at 5.5 Ma for grey sandstone and to SE around 4.8 Ma for buff sandstone. Both the sandstones are lithic arenites with dominance of sedimentary rock fragments in the buff sandstone. The filling pattern and occurrence of soft-sediment deformation structures, particularly between 4.8 and 2.6 Ma, indicate tectonically controlled re-organisation of drainage pattern and gradual change in source area from Lesser Himalayan to Sub-Himalayan regions. These changes after 4.8 Ma are mainly governed by intra-foreland thrust activity, which resulted in partitioning of foreland basin. Further, a significant overlap of Tertiary clast-bearing conglomerate with that of pre-Tertiary clast-bearing conglomerate at around 1.77 Ma shows a major reactivation along IFT.
The Neogene Siwalik fluvial succession (12.77-4.48 Ma) of the NW Himalayan foreland basin, east of Ravi River, are deposited under varied tectono-climatic conditions. Petrography and geochemistry of the sediments were carried out to understand the relative roles of tectonics and climate in this basin. The sandstones are classified as sub-litharenites with subordinate arkosic-lithicwacke. The detrital components particularly the rock fragments are consistent with the inferred source area presently disposed towards the north of the depocenter and suggest that the Higher, Lesser and Lower Tertiary formations supplied detritus since 12.77 Ma. The Th/Sc vs Zr/Sc ratio indicates sediment recycling. The Cr and Ni, negative Eu anomaly, LREE enrichment and moderately flat HREE in the mudstones collectively suggest dominant contribution from felsic igneous rocks beside contribution from basic, sedimentary/metamorphic lithology. Near consistent nature of the detrital modes and geochemical parameters through time suggest unvarying source since 12.77 Ma. The present study also infers that the source areas uplift owing to tectonic activity of Chail Thrust at least by 12.77 Ma with a perceptible activity of MBT after 8 Ma. The δ18O variations in soil carbonates reveals ongoing intense monsoon system since 12.77 Ma followed by a phase of aridity at 9.1 Ma. Presence of fresh and weathered feldspar, limestone, basic volcanics and mica, both in humid and and phase indicate rapid deposition and preservation. Despite the climatic turnover from humid to arid, distinctive changes in the framework mineral compositions over time is not discernable. The petrographic and geochemical data reveals that the near similar source area, relatively rapid transport, moderate chemical weathering and sediment recycling controlled the composition of the sediments of the HFB in response to tectonics and climate.
A magnetostratigraphic study on four sections near Pinjor showed that the Tatrot stage falls in the Gauss magnetic epoch and the Pinjor corresponds to most of the Matuyama epoch. A significant faunal change, already observed by other authors in Pakistan, marks the Tatrot-Pinjor boundary and approximately coincides with the Gauss-Matuyama transition. Sedimentation becomes entirely conglomeratic in a positive episode tentatively interpreted as Jaramillo. The time interval represented by this event witnessed a deep climatic change in Europe and an extensive faunal turnover in all Eurasia, and seems also to correspond to the onset of a strong tectonic activity in the Himalayan belt.-Authors
A 260 m thick Paleogene sedimentary succession encompassing Subathu and Dagshai formations, shows variations in genetically linked lithofacies, detrital modes and rock magnetic parameters. The Subathu Formation comprising grey to dark grey splintery shales with minor inter-layered lenticular calcareous sandstone and Nummulites bearing limestone having typical ferrimagnetic assemblage reflects the shallow marine setup. It is followed by mudstone with calcareous concretion, trace fossils and mixed ferri- and anti-ferromagnetic mineralogy of 'passage beds' and is capped by interbedded greyish white medium- to coarse- grained sharp based sandstone (siliciclastic) and red to purple mudstone with profuse concretions. Subsequently this is overlain by sandstone - mudstone alternations of the Dagshai Formation representing the interfluve domain of the coastal plain set up. Two maximum flooding surfaces within the Subathu Formation and sequence boundary at the sharp based sandstone are recognised. The withdrawal of the Subathu sea and contemporary sea level changes are largely controlled by the Himalayan embryonic setup during middle Lutetian. The coarsening-up Dagshai fluvial succession suggests basin-ward facies migration with a significant change in basin morphology owing to the availability of large accommodation space. Variations in architectural elements, detrital modes and rock magnetic characteristics across the marine to fluvial transition establish the sequence boundary, which is contemporaneous with the accelerated Himalayan uplift coeval to India-Asia collision.
Abstract : Mio-Pleistocene coarse clastic facies (conglomerate) of the Kangra re-entrant in Himalayan Foreland Basin
(HFB) have two distinct occurrences – (i) along the basin axis between 10 and 7 Ma and (ii) along basin margin 6 Ma
onward as upper age limit not constrained. These coarse clastic deposits are broadly classified into three facies
associations A, B and C. The facies association A, constrained between 10 and 7 Ma belong to Middle Siwalik. The
facies association B passes gradationally into facies association C that belongs to Upper Siwalik conglomerate (6 Ma
onwards). Facies association A was deposited by confined braided stream with well developed floodplains. Facies
association B and C was deposited by unconfined, laterally migrating streams punctuated by hyperconcentrated flood
events as a longitudinal alluvial fan. The clast composition of all the facies associations resembles with the present day
clast types in the Beas River that lies north of the Main Boundary Thrust and have consistent southwestward palaeoflow
with variance of ± 40o. Composition and palaeocurrent data indicate that both the conglomerate was product of uplift
and subsequent unroofing of Proterozoic – Lower Palaeozoic lithounits located on two thrust sheets to the north and
northeast. Based on present observation it is inferred that coarse clastic facies was deposited by SW draining protoBeas
River which entered into the foreland basin around 10 Ma.
A critical review of the ideas regarding the basin dynamics of the Messinian salinity crisis in the Central Mediterranean. Highlighted with the most detailed sequence stratigraphic framework ever presented for the area and for this geological epoch. Presentation of a number of important constrains for future models.
This is the result of these unique and rare occasions where extensive experience in outcrop geology can be combined with industrial information on exploration and production wells and seismic (2d and 3d) data. Thanks to the kind authorization of Eni management, the paper contains in fact a few of the only published 3d seismic sections of the Central Mediterranean. The tectonic style of the fore-arc region as deduced from geological mapping and previously published can clearly be recognized: transpressional strike slip tectonics, low angle thrusting with basal detachment in the salt, inversion tectonics, and high tectonic mobility up to recent time. A flavor of the enormous amount of information the author has been so lucky to have studied during these years.
Pliocene-Pleistocene evolution of the Himalayan foreland (Siwalik) basin is documented from a 2.4-km-thick fluvial succession in the Haripur sub-basin, Himachal Pradesh, India. Variations in deposit architecture, paleoflow orientations, and mineralogy observed within the succession are related to tectonic activity along Main Boundary thrust and Intra-Foreland thrust. The Dhok Pathan Formation (ca. 6 to 5.5 Ma) of the Middle Siwalik sub-group is at the base of the succession. It contains thick multistoried gray sheet sandstones and minor mudstones interpreted to be deposits of a major trunk drainage system. The gradual upsection addition of buff ribbon sandstone and a substantial increase in overbank mudstones in the overlying Tatrot Formation of the Upper Siwalik sub-group, starting at ca. 4.8 Ma, marks the inception of piedmont drainage in this area. Pre-Tertiary-clast-bearing conglomerate, derived mainly from the Lesser Himalayan zone formations, first occurs at 1,375 m (3.36 Ma) in the succession and substantially increases in abundance at 1,685 m (2.6 Ma), at the base of the Pinjor Formation. Tertiary-clast-bearing syntectonic conglomerates from the Sub-Himalayan Tertiary belt first appear in the succession at 2,100 m (1.77 Ma) and persist through the top of the section at about 0.5 Ma. Differences in sandstone framework composition and paleoflow orientation indicate the presence of three distinct drainage systems at various times during basin filling. The southeasterly flowing axial trunk drainage was joined by a southwesterly flowing transverse trunk drainage at 5.5 Ma and was completely displaced by largely south flowing piedmont drainage in the study area at about 1.77 Ma. Upsection variations in the fluvial architecture are interpreted to record: (1) movement of the Main Boundary thrust at about 5.5 Ma and tilting of the basin to the southwest; (2) inception of piedmont drainage around 4.8 Ma during initiation of the Intra-Foreland thrust; (3) enhanced activity along the Intra-Foreland thrust at 1.77 Ma, forming a small depression that filled with syntectonic Tertiary-clast-bearing conglomerate; and (4) southward tilting of the basin and displacement of the transverse trunk drainage by piedmont drainage at about 1.77 Ma.
Magnetic polarity stratigraphy of Upper Siwalik Subgroup, Jammu Hills was worked out based on partially demagnetized data from 208 sites in three sections: Parmandal-Utterbeni, Nagrota-Jammu and Balli. One/two bentonitized tuff horizons occur close to the base of a long reversed polarity zone associated with Pinjor age fossils. Fisson track dates of 2.8±0.56 m.y. and 2.31±0.54 m.y. have been obtained on the zircon phenocrysts from these tuffs. These data permit correlation of our magnetic stratigraphy with the standard magnetic polarity time scale. The longest sampled section spans the interval from 4.92 to 0.22 m.y. This dating places the lower part of Upper Siwalik in the early Pliocene.
In the sampled parts of the sections, the magnetic polarity zonation establishes 16 magnetozones in Parmandal-Utterbeni, 13 in Nagrota and one in Balli. Individual magnetic transitions can be correlated over a lateral distance of 62 km and these provide time lines for correlation of lithology from one section to the other.
Estimated rates of sedimentation indicate rates varying from 0-45 m to 0.71 m/l000 years for Gilbert and Gauss epochs, while a decreased rate varying from 0.21 m to 0.37 m/l000 years is indicated during the Matuyama epoch.
The ]ast folding event of Suruin-Mastgarh anticline is not older than 0.22 m.y. However, the decrease in the rate of sedimentation during post Gauss time could possibly be due to an earlier period of deformation.
A significant change in fauna near the Gauss-Matuyama boundary is observed by the appearance of Equus and cervids with antlers and by the absence of Hipparion. We correlate this boundary with the Tatrot-Pinjor faunal boundary, No change either in lithology or in the vertebrate fauna is observed at the Pliocene-Pleistocene boundary if it is to be placed at the top of the Olduvai Normal Subchron.
A 2600 m thick succession, exposed between Kathgodam and Ranibagh in the Kumaun Himalaya and belonging to the Lower and Middle Siwalik, was subjected to lithological and palaeomagnetic studies. Lithologically, four units are recognized in the sequence. Units 1-2 are correlated with the upper part of the Lower Siwalik, whereas the Units 3-4 are correlated with the Middle Siwalik. In the absence of the vertebrate fossils and absolute chronology, the lithological boundaries are used to explain the magnetic polarity stratigraphy. We suggest that the lithological boundary between the Lower and Middle Siwalik (e.g., Chinji-Nagri) in the Kathgodam-Ranibagh section lies at ca. 850 m level and is palaeomagnetically estimated as ca. 9.8 Ma. If such a correlation is correct, the section ranges in age from Chron 5 An to Chron 2 Ar (ca. 12.0 to 4.0 Ma).
We present here comprehensive petromineralogic, mineral magnetic and lithologic observations from five stratigraphic sections representing the Barail Group, Middle and Upper units of Bhuban Formation, Bokabil Formation and Tipam Group in the Mizoram area. These stratigraphic units mainly display interplay of the grey and buff colored sandstones of the clast compositions varying from sub-lithic to lithic arenites (Q79F4L17 to Q55F3L42) including sedimentary, meta-sedimentary and subordinate igneous rock fragments. The buff sandstones with higher lithic fragments [Q69F4L27 (Ls61Lm38Lv1)] are dominated by recycled components with higher clast angularity (VA1A15SA51SR24R8WR0.4) relative to the grey sandstones. Mineral magnetic studies decipher bimodal (ferri- and antiferromagnetic) mineralogy with higher concentration in buff sandstones relative to the unimodal ferrimagnetic nature of the grey sandstones.
The study infers that the buff sandstones mark the regressive phases driven by hinterland uplifts; whereas the growth of the grey sandstone facies is marked by transgressive basinal processes. Gradual increase in the frequency and appearance of the buff sandstones in the Surma stratigraphy, therefore, can be related to the evolution of the Indo-Burmese ranges.
We report here an account of sedimentary magnetic fabrics in 44 channel sand-bodies of a Siwalik fluvial sequence (6 to 0.5 Ma) near Nahan, Himachal Pradesh. These sandstones in distal alluvial fan-setting record oblate fabrics (Tmean = 0.51) and relatively higher degree of anisotropy (Pjmean= 1.05). The minimum axis (K3) of the anisotropy ellipsoid is aligned parallel to the palaeoflow direction by traction carpet mechanism. Variations with time of latitudes derived from azimuth of principle ellipsoid axes show a greater sensitivity to the basin tectonic impulses with smaller response time than conventional methods. This signifies the scope of magnetic fabric techniques for palaeo-hydrodynamic and basin tectonic studies in the Indo-Gangetic foreland of Himalaya.
In this study we have compiled and synthesized the published magnetic polarity data on Late Cenozoic sequences in the Indian sector, and discuss their util - ity for time stratigraphic inferences useful to con - strain tectonic and climatic changes. We also discuss the use of magnetic fabrics over channel sandstones and rock magnetic ratios of the pedogenic horizons as correlation tools. It is observed that the magnetic po - larity events (a) the base of long normal (C5 n) at 10.95 Ma, (b) the base of C4n at 8.07 Ma, (c) the base of C3n at 5.23 Ma, (d) the Gauss-Matuyama boundary (top of C2An) at 2.58 Ma, and (e) the Brunhes- Matuyama boundary (base of C1 n) at 0.78 Ma, are the most characteristic events so far reported in the Late Cenozoic Siwalik sequence. It is, therefore, necessary to make more detailed studies on palaeo - intensity, in situ rock magnetic signals, and magnetic fabrics closely around these intervals to strengthen their application to basin -wide correlation. New data in the eastern part, and a more judicious density of magnetic polarity data in the central part of the Himalayan Foreland Basin (HFB) are required for a robust correlation of the Late Cenozoic stratigraphic units from Pakistan, NW India, through Nepal to the northeastern Indian sector. The sediment accumu- lation rates derived from a total of 56 sections throughout the Himalayan foreland indicate a notable rise during 10.8 to 9.5 Ma (deposition of Nagri sandstone) and records the largest fluctuations after 1.7 Ma (widespread occurrence of boulder conglo- merates). Magnetic fabrics provide high resolution information on hydrodynamic changes suitable for correlating the association of major channel sand- stones across the basin. The rock magnetic ratios contain information on dynamic pedogenic trans- formation of magnetic minerals with time, and are envisaged as a high resolution tool of basin -wide correlation for the pedofacies. Further, a use of mag - netic susceptibility for basin-source modeling has been demonstrated.
The use of gravels as syntectonic indicators of thrusting has recently been questioned by foreland-basin models that assign gravels to a post-thrusting interval of progradation, except in very proximal areas. On the basis of precise temporal control provided by magnetostratigraphically dated sections, the history of gravel progradation after a major thrusting and uplift event in the northwestern Himalaya is shown to be a virtually syntectonic phenomenon. Despite considerable crustal subsidence driven by a thick-skinned thrust, gravels prograded ˜70 km during a 1.5-m.y.-long thrusting event. By 3 m.y. after the start off thrusting, gravels extended more than 110 km into the basin. Although delayed gravel progradation appears appropriate for many Rocky Mountain foreland basins, it is clearly not valid for the Himalaya. We attribute the difference in depositional response between these basins to differences in the quantity of sediment supplied to them (sediment starved vs. overfilled), the availability of resistates in the source area, and the size of the antecedent drainage.
A spur of ancient rocks partly covered by gently-dipping Tertiary beds extends from the Shillong Plateau and Mikir Hills north-eastwards beneath the alluvium of Upper Assam. Over this spur the Eastern Himalaya have been thrust southwards and the Naga Hills have been thrust north-westwards. The amount of movement of the overthrust masses cannot be determined but it is suggested that in each case the total displacement may be 150-300 kilometres or even more. The Shillong Plateau is separated from the Surma Valley by a faulted monocline with southerly dips. This fault, the Dauki tear-fault, is now shown to have a probable horizontal displacement of about 250 kilometres, and thus to be a major feature of the tectonic pattern of the Indian sub-continent. The horizontal movement along the Dauki tear-fault detached the Shillong Plateau from the main mass of the Indian Shield. The principal movements occurred late in the Tertiary, mostly in the Pliocene.
The Middle Siwalik Subgroup of Mohand area (1800 m thick) is divided into three units: (1) sandstone-mudstone interval (340 m), (2) thickly bedded multi storied sandstone (1060 m), and (3) conglomerate-sandstone-mudstone interval (400 m). Three major facies are recognised, namely: conglomerate, sandstone, and mudstone facies. Sandstone is the dominant litho-facies and displays frequent erosional surfaces along which mud-clasts are present. The common sedimentary structure is trough and planar cross-stratification.
Palaeo-flow measurements of trough cross-stratification show prominent modes in SE and NW directions. However, planar cross-stratification shows a high order of deviation in mean vector azimuth from trough cross-stratification. The fining-up in grain-size from erosional surface (Se facies) to mudstone (Fm facies) through trough and planar cross-stratified sandstone (St-Sp facies) and ripple drift laminated sandstone (Sr facies) has been recognised.
The mudstone intervals are almost negligible, while mud-clasts and suspension load in the sandstone (channel facies) are high (to to 25%). The absence of mudstone unit in multi storied sandstone complex may be attributed to the extensive and frequent avulsion on the large braided alluvial fan. This study reveals that sedimentation pattern of multi storied complex is similar to that of modern radial fan of Kosi river.
Abstract: Southward orogenic growth of the Himalaya, since the Eocene led to variable crustal loading, flexural subsidence and foreland basin sedimentation. Structural adjustments provide the mechanism of transferring the zones of deposition, subsidence and uplift causing the basin-wide spatio-temporal variation. These tectonic events are reconstructed from sediment architectural analysis, facies changes, petro-mineralogic studies, stable isotope geochemistry and depositional styles. More systematic and continuous records are available for the Late Cenozoic sequence of Himalayan foreland basin which consists of three distinct stratigraphic-sedimentologic successions. The ~13 to 10 Ma period of Lower Siwalik (succession I) consists of sandstone-mudstone couplets with more than 50% mudstone derived from northerly source represents a significant time interval of low sediment accumulation with larger accommodation space available. The ~10 to 6 Ma Middle Siwalik (succession II) is sandstone dominant deposited by large braided streams with low accommodation space. Observations reveals that major re-activation of Main Central Thrust (MCT) causing uplift and basin-ward thrust sheet migration resulted in altered foreland Basin slope, accommodation space and sediment supply (BAS) equation, characteristic of succession II. This was also responsible for the orographic barrier that accelerated the monsoon rain system to enhance the seasonal sediment supply and hence accelerated the hinterland exhumation. The ~6 to 0.5Ma conglomerate dominant Upper Siwalik (succession III) is in tandem with the intensified monsoon, hinterland uplift and development of intra-foreland thrust system causing another major change in the BAS equation. This succession is characteristic of relatively high accumulation and sediment supply concomitant with the onset of deposition of Lesser and Sub-Himalayan derived sediments in the proximal foredeep depo-centre by intense deformation along Main Boundary Thrust (MBT). Sudden influx of coarse detritus between ~6 and 5 Ma, represents frequent hyper-concentrated floods which transport boulder size clasts in to the basin. Apart from the above temporal variations, the lateral lithofaciesvariations are discernible from the sub-basin – sub-surface configurations and are also influenced by large scale lateral variations in thrust sheet geometry and composition. The sedimentologic work so far from the northwestern Himalayan foreland basin is unable to relate the lateral lithofacies variations at sub-basin scale with the hinterland configuration and enduring monsoonal climatic variations. Similar understanding should be developed using chronologically constrained regional data which is under demand to establish the relation of foreland sedimentation with Ganga basin and the Bay of Bengal sedimentation. This paper makes an overview of the studies in the above context and produces the guidelines for the future work.
Keywords: Himalaya; Foreland basin; Siwalik Group; Magnetostratigraphy; Sedimentology; Tectonic and Climate.
The magnetostratigraphy supplemented by radiometric dates and palaeontological data from Upper Siwalik Subgroup in six sections across the foothills of northwestern Himalaya provides absolute ages for the subdivisions of the Upper Siwalik, and a time framework to interpret the lithologicat information. The formational boundaries drawn on the basis of lithology are strongly diachronous with age deviations ranging from 3 x 10 5 to 2 x 10 6 years, and the age of the Upper Siwalik Subgroup extends from 5.6 to 0.22 m.y. (late Miocene to middle Pleistocene). The rate of sediment accumulation ranges from 0.27 to 0.71 m/1000 yr. The decreased sedimentation rates during the Matuyama chron in the northern part, and the accelerated rates for the same interval in the southern parts, as well as the presence of pebbles in the Upper Siwalik in the southern areas derived from the older lordeep-scdiments indicate a foreland-directed migration of the depositional centres of the foredeep.
An approximately 900m thick succession of pre-Pinjor (Tatrot) and Pinjor beds of the Upper Siwalik Subgroup is exposed in the Khetpurali section which lies to the east of Chandigarh. Samples collected from the mudstone interval from forty-nine fluvial cyclic units exhibit a stable detrital remanent magnetisation. The study of magnetic polarity of these samples reveals the presence of six normal and six reverse magnetozones. The Tatrot-Pinjor faunal boundary has been interpreted to approximately lie at the Gauss-Matuyama boundary. Based on this interpretation, the Olduvai Event in Matuyama reversed polarity epoch spans 160m of the middle part of the Pinjor Formation. The local Pliocene/Pleistocene boundary in this section is interpreted to lie 170m higher in the sequence with reference to the Tatrot-Pinjor faunal boundary.
Changes in the fluvial domain in the post-Gauss Epoch are interpreted to reflect changes from 'meander belt' deposits to 'braided stream' deposits. The first appearance of conglomerates in the section has been interpreted to occur in the post-Olduvai time.
A 560 m thick rock succession of Middle Bhuban Formation (Surma Group) exposed between Bawngkawn and Durtlang, Aizawl, Mizoram has been studied for its magnetostratigraphic attributes. A total of 7 normal and 7 reverse magneto-zones have been delineated in this section. The GPTS correlated ages of this section lie between ∼21.77 Ma (at the base) to ∼15.16 Ma (at the top) with a total duration of ∼6.6 Ma. The GPTS event C6n occurring at the stratigraphic level between 146m to 266 m may be considered for basin wide correlation as it is the longest normal event that has been recorded with greater confidence i. e. better alpha- 95. The average sediment accumulation rate (SAR) estimated for this section is 8.48 cm/Ka. Overall the SAR is higher in the lower part of the section with a spike of 26.8 cm/Ka at <21 Ma. The decrease in SAR to 2.1 cm/Ka at around 18 Ma in the upper part of the section may be investigated for possible hiatus.
Precise determination of the timing and nature of deformational events in foreland basins is critically dependent on the interpretation of stratigraphic indicators of tectonic activity within a detailed chronologic framework. Frequently data from numerous stratigraphic sections must be synthesized in order to generate a clear definition of a structural event and its response within a sedimentary sequence. In terrestrial deposits, magnetostratigraphic studies can provide the temporal control necessary to establish a reliable and coherent synthesis of tectonism that is based on an amalgamation of the diverse responses to, and indicators of, tectonic activity found in geographically separated localities. Useful indicators in this type of analysis include: 1) classical indicators of tectonism, such as the timing of changes in paleocurrents, provenance, and facies, and the ages of unconformities; and 2) indicators based on magnetic data or precise chronologic control, such as the history of syn- and post-depositional tectonic rotation and changes in the rate of sediment accumulation. Three examples based on the interpretation of such stratigraphic data from the northwestern Himalayan foreland basin are used to illustrate diverse thrusting events during the past 5 million years, including initial thrust propagation, later episodes of reactivation, and large-scale out-of-sequence thrusting. These examples illustrate the high degree to which reliable specification of the sequence of thrust events that are closely spaced in time but are distributed across a broad region is critically dependent on the availability of detailed, time-controlled stratigraphic records.
The theory of plate tectonics offers a fresh opportunity to interpret the evolution of sedimentary basins in terms of changing plate interactions and shifting plate junctures. Although plate-tectonic theory lays primary emphasis on horizontal movements of the lithosphere, large vertical movements are also implied in response to changes in the thickness of crust, in the thermal condition of lithosphere, and in the isostatic balance of lithosphere over asthenosphere. As thick sedimentation requires either an initial depression or progressive subsidence to proceed, the auxiliary vertical movements largely control the evolution of sedimentary basins. Ancillary geographic changes related to the governing horizontal movements also affect patterns of sedimentation strongly.
The geosynclinal terminology used prior to the advent of plate tectonics is inadequate to describe fully the plate-tectonic settings of sedimentary basins. Basins can be described instead in terms of the type of substratum beneath the basin, the proximity of the basin to a plate margin, and the type of plate juncture nearest to the basin. Intraplate settings of oceanic or continental character contrast with zones of plate interaction, which include those of divergent, convergent, and transform motions and within each of which the underlying crustal structure is or may be complex. The evolution of a sedimentary basin thus can be viewed as the result of a succession of discrete plate-tectonic settings and plate interactions whose effects blend into a continuum of development.
Oceanic basins contain an assemblage of diachronous facies whose relations are controlled by thermal subsidence of the lithosphere as it moves away from midoceanic rises. Rifted continental margins undergo successive stages of structural evolution as the following features are formed: prerift arch, rift valley, proto-oceanic gulf, narrow ocean, and open ocean. Sedimentary phases related to each stage are components of the rifted-margin prism of strata that masks the continent-ocean interface beneath a continental terrace-slope-rise association or a progradational continental embankment. Marginal fracture ridges along marginal offsets and aulacogens along failed arms of triple junctions locally break the continuity of rifted-margin prisms. Sedimentary basins associated with arc-trench systems where oceanic lithosphere is consumed include trenches beyond the subduction complex beneath the trench slope break, forearc basins in the arc-trench gap, intra-arc basins within the magmatic arc, and interarc basins or retroarc basins in the backarc area. Interarc basins are oceanic basins between a migratory intraoceanic arc and a remnant arc, whereas retroarc basins rest on continental basement adjacent to a foreland fold-thrust belt behind a continental margin arc. Peripheral basins adjacent to suture belts formed by crustal collision occur in an analogous foreland setting between orogen and craton, but in front of a colliding magmatic arc. Retroarc basins and peripheral basins both imply partial subduction of continental lithosphere. Intracontinental basins include infra-continental types, beneath which incipient continental separation gave rise to crust of transitional thickness, as well as supracontinental types.
Siwalik rocks of Pakistan are a virtually continuous, continental sedimentary sequence, extending in age from 18 to 1 ma b.p. This paper describes taphonomic features of late Miocene mammalian assemblages from a highly fossiliferous interval about 400 m thick, based on field documentation of sedimentary environments at 42 fossil localities and systematic fossil collection of 21 localities.
Within a broadly fluvial system, I recognize four sedimentary environments of bone accumulation, distinguished by lithology, unit-thickness, unit-geometry, contacts, sedimentary structures, and relationship to adjacent units. Each environment corresponds to an association of lithofacies. Facies Association I is interpreted as the persistent, major channel bodies of a meandering fluvial system; Facies Association II as coarse-grained flood deposits, such as crevasse splays, deposited beyond the main channels; Facies Association III as channel margins, including levees and swales; and Facies Association IV as predominantly subaerial floodplains.
Taphonomic features of bone assemblages from each facies association include skeletal-element composition, surface distribution of specimens, degree of articulation, hydraulic equivalence between organic and inorganic sedimentary particles, frequency of juvenile remains, size distribution of fauna, and an estimate of duration of accumulation of individual fossil localities. The distribution of these features among the four facies associations suggests that bone assemblages in Facies Associations I and II accumulated by the action of currents in river channels or floods, whereas bone assemblages in Facies Associations III and IV accumulated through concentration by biological agents and/or attrition at a repeatedly used site of predation.
Inclusion in fluvial accumulations depends on initial availability of skeletal remains and hydraulic characteristics of individual skeletal elements, but not taxonomic identity per se. For biological accumulations, however, taxonomic composition reflects the preferences of the individual agents of accumulation. The probability of preservation of taxa in fluvial accumulations is probably mainly a function of body size, as reflected in the sizes of isolated skeletal elements. Thus, in this Siwalik system, bone assemblages that experienced fluvial transport are better representations of original community composition than bone assemblages created by biological agents or passive accumulation.
A 2375m thick sequence consisting of sandstone-mudstone alternations was sampled for magnetic polarity stratigraphy in Somb Nadi and its tributary, Jamni-Khol in the Haripur area of Himachal Pradesh. The Tatrot/Pinjor faunal event is recognised on the local magnetic polarity time scale at 2.6 Ma and the Middle to upper Siwalik transition (Dhok Pathan/Tatrot boundary) at 5.26 Ma approximately. Predominance of piedmont depositional system at around 3.2 Ma suggests uplifted and denudation of the Outer Lesser Himalaya during this period. First record of the Pre-Tertiary clast-dominated conglomerate is observed at 2.6 Ma and that of Tertiary clast-dominated conglomerate at 1.76 Ma. An average rate of sedimentation of 45 cm/1000 years was observed till 2.6 Ma.
Just beyond the western boundary of West Bengal, the great Indian shield disappears below a blanket of alluvium. The exposed part of the shield bordering the Bengal basin is marked by a row of intracratonic Gondwana basins, a series of thrust zones in Singhbhum, and extensive exposure of basic volcanics in the Rajmahal Hills. Intensive geophysical surveys and deep drilling in the alluvium-covered plains of West Bengal have revealed a thick section of Cretaceous and Tertiary sediments lying on a basement of basalt lava flows, presumably of the same age as the Rajmahal Group volcanics. An extension of the easternmost Gondwana basin farther east, below the Bengal alluvium, also is suggested. A series of buried basement ridges, marking the western margin of the Bengal basin, resumably kept the Gondwana continental basins isolated from the main Bengal basin through most of Tertiary time. Locally, during the late Tertiary, the sea transgressed over these basement ridges and onlapped parts of the Indian shield. Flanking the eastern margin of the buried ridges is a row of basin-margin en echelon faults and scarps, possibly the shallower expressions of some deep-seated movements in the basement. East of this marginal fault zone lies the stable shelf of West Bengal with a homoclinal dip toward the southeast. Seven seismic reflectors mapped in the Mesozoic and Tertiary sediments of the shelf indicate uniform increase of thickness (3,000 to 27,000 feet, approximately) of these sediments toward the southeast. Except for a few normal faults, the area is practically undisturbed structurally. An extensive unconformity between the Miocene and Pliocene has been recognized. Locally, weak evidence of another depositional break, at the top of the Oligocene, is present. Around Calcutta, the Eocene key horizon (Sylhet Limestone) shows a conspicuous basinward flexure (the "hinge zone") at a depth of about 15,500 feet. East of this "hinge," which traverses the whole Bengal basin, lies the deeper part of the basin with a greater rate of subsidence and a different lithofacies. Seismic interpretation suggests a sharp lithofacies change at this zone, from the Eocene nummulitic limestone of the stable shelf to a thick sequence of clay and shale in the deeper part of the basin. In the younger Tertiary sediments is a similar change of facies from the arenaceous sediments of the stable shelf to the dominantly argillaceous sediments downdip. Marine transgression on the West Bengal shelf occurred during the Late Cretaceous (locally), late Eocene (extensively), and Miocene (in the eastern parts only). Except for these periods of marine transgression, sedimentation took place under fresh-water, estuarine, or deltaic conditions. A summary of the tectonic and depositional history of the whole region, from the eastern margin of the Indian shield to the folded belt in Assam, is given in conclusion. This integrates the work done in West Bengal by the Indo-Stanvac Petroleum Project with that done in Assam by the Burmah Oil Company and its affiliates.
Significant data on the structure, tectonics and stratigraphy of Ganga basin have been obtained from aeromagnetic, ground magnetic, gravity and seismic surveys and the deep drilling conducted in the basin during the last fifteen years, Based on these data, the Ganga basin has been defined as a major platform depression and classified into seven tectonic zones, viz., Monghyr-Saharsa ridge, East Uttar Pradesh shelf, Gandak depression, Faizabad ridge, West Uttar Pradesh shelf, Sarda depression and Delhi-Hardwar ridge. This classification is based on the continuation of major tectonic trends from the Peninsular shield into the Ganga basin, the variations in the total thickness of the sedimentary cover, and the basement configuration as deduced from different surveys.
The sedimentary cover over most part of the Ganga basin is essentially composed of two main stratigraphic cum structural sequences representing the two main sedimentary stages in the geological evolution of the basin. The oldest, probably corresponding to the Vindhyans, is represented by stable to unstable shelf sediments composed of quartz-arenite-limestone-shale alternations. The younger sequence, unconformably overlying the Vindhyans, corresponds to the Neogene terrigenous clastics (Siwaliks). The structural and stratigraphic data of these sediments have been discussed. The presence of a profound unconformity between these two groups of sediments representing a considerable time gap ranging from (?) late Palaeozoic to Paleogene is an important factor in deciphering the tectonic evolution of the Himalaya. However, towards the northernmost depressed parts of the Ganga Basin, the age of the additional thickness of sediments intervening between the two above mentioned groups remains uncertain.
In order to help delineate the succession of late Cenozoic tectonic and stratigraphic events in the NW Himalayan foredeep and adjacent ranges, a geological transect is described which extends from the NE margin of the Kashmir Basin to the axis of the Jhelum Re-entrant along the E boundary of the Potwar Plateau. When combined with previous bedrock mapping, chronologic and stratigraphic studies of 9 sections in the intermontane basin and the bounding foredeep define 3 primary pulses of late Cenozoic uplift affecting the Pir Panjal Range (5-4, 1.9-1.5, and 0.4-0 Ma). Many of the changes in facies, provenance, and palaeocurrents observed in the sedimentary rocks along the transect can be related to these deformational episodes. -from Authors
Discusses various aspects of stratigraphic correlation of the surface and subsurface sections of the Cenozoic sediments in the region on the basis of time-related energy-sequence and heavy mineral stability order in the sediments. Reviews available geological and geophysical data on the structure and tectonics and presents a new scheme of structural classification of the area. The evolution of the basin in time and the relationship between tectonics sedimentation and their role in hydrocarbon prospects is also discussed. -after Authors
Upper Eocene to Neogene fill of the Bengal basin provides an earlier unroofing history of the eastern Himalaya and Indo-Burman ranges than that provided by drilling of the Bengal fan, and analysis of heavy minerals in these sequences provides useful provenance constraints. Quartzose sandstones of the Eocene Kopili and Oligocène Barail Formations contain only 0.2% heavy minerals, comprising abundant opaque minerals and stable minerals such as tourmaline, garnet, rutile, and zircon. This assemblage suggests sediment sources dominated by low- to intermediate-grade metamorphic, silicic igneous, and metasedimentary rocks; the low heavy-mineral content and abundance of stable minerals suggest intense chemical weathering. These sediments may have been derived in part from incipient uplifts of the proto-Himalaya and/or the Indo-Burman ranges, but a more likely source is the Indian craton immediately to the west. Miocene sandstones of the Surma Group contain more abundant and more diverse heavy-mineral assemblages than Oligocène sandstones. These include abundant opaque minerals and garnet, mostly almandine, and moderate to minor amounts of tourmaline, kyanite, zircon, calcic amphibole, rutile, chlorite, zoisite, staurolite, epidote, sillimanite, and clinopyroxene, indicating a broad range of mostly metamorphic source rocks. Upper strata of the Surma Group also contain abundant blue-green amphibole, orthopyroxene, and sparse chromite, suggesting exhumation of arc and ophiolitic rocks. Sands and sandstones from the upper Miocene to Pliocene Tipam Group and the Pliocene to Pleistocene Dupi Tila Formation contain assemblages similar to those of the underlying Miocene sandstones, along with more abundant orthopyroxene and sillimanite in Dupi Tila sands. Temporal variations in heavy-mineral assemblages in the Bengal basin suggest that orogenic detritus first appears in the eastern Himalayan foredeep much later (early Miocene) than in the western part of the foredeep (Eocene). The appearance of high-pressure phases in upper Miocene strata reflects continued unroofing of deeper-crustal metamorphic and silicic to ultramafic plutonic rocks.
The Sylhet trough, a sub-basin of the Bengal Basin in northeastern Bangladesh, contains a thick fill (12 to 16 km) of late Mesozoic and Cenozoic strata that record its tectonic evolution. Stratigraphic, sedimentologic, and petrographic data collected from outcrops, cores, well logs, and seismic lines are used to reconstruct the history of this trough. -from Authors
The Tibetan and ‘Sibumasu’ continental blocks rifted apart from the northern margin of Gondwanan Indo-Australia during the Permo-Carboniferous whereas, the IndoBurma-Andamans (IBA), Sikuleh, Lolotoi micro-continents did so during the Late Jurassic. This continental margin experienced glacial or cool conditions during the Permo-Carboniferous. The Tibetan and Sibumasu blocks drifted northward during mid-late Permian initiating opening of the Neo-Tethys. The arm of the Palaeo-Tethys located to the north of these blocks closed as these blocks were accreted to the South China and Indochina blocks which had separated from the same Gondwanan margin during early Palaeozoic. All these blocks were amalgamated to form the Cathaysialand. The Sibumasu block was accreted to the Indochina block, and the Tibetan Changtang block to Eurasia during the late Permian-mid Triassic. Contemporaneously Mesozoic Neo-Tethys expanded between the Indian, Lhasa and Changtang blocks. The Indian and Australian continents separated during the Cretaceous leading to the opening up of the Indian Ocean and closing of the Tethyan ocean. The Palaeo- and Neo-Tethyan sutures in Tibet, Yunnan, Myanmar, Laos-Thailand and Vietnam reveal the complex opening and closing history of the Tethys. IBA rotated clockwise from its earlier E-W orientation, because of dextral transcurrent fault movements which ensured faster northward movement of the Indian plate relative to Australia during late Cretaceous-early Eocene. Contemporaneous to India-Tibet terminal collision during early-mid Eocene there was thloeiitic-alkalic foreland volcanism (Abor and equivalents) at the leading edge of the Indian continent. Sustained post-collisional movement of the Indian plate, caused southward propagation of the Himalayan crystalline and frontal foreland thrust sheets. It also produced E-W trending folds and thrusts even in distal Central Indian Ocean areas as well as a clockwise rotation of the amalgamated SE Asian Cathaysian composite block. Dislocations of the 90 °E Ridge, indicate that the main compression occurred during early Eocene which was followed by movement since late Miocene.The ophiolite trail on IBA does not represent the eastern suture of the Indian continent. As a result of Late Oligocene oblique terminal collision between Sibumasu and 1BA, Mesozoic early Eocene ophiolites. their mid-Eocene cover and trench sediments occur as klippen on IBA. Oblique convergence between IBA and the Indian plate in turn produced an active subduction regime along the western margin of the Indo-Burma mobile belt and the Andaman-Java trench. The activity produced Neogcne-Quaternary volcanism. dextral strike-slip movement in Central Burma-Andaman-Sumatran region, and opened up the Andaman sea. Northward, the Shillong massif, representing NE prolongation of the Indian sub-continent, is technically juxtaposed against the northern end of IBA. The collision of these blocks might have uplifted the Shillong massif during Mio-Pliocene. Far eastward, the convergence was orthogonal between the Australian continent and the Indonesian Arc resulting emplacement of the Lolotoi continental rocks. Maubissi exotic blocks and ophiolitic rocks as nappe over the Timor shelf which possibly remained attached to the Australian continent.
The Siwalik Group which forms the southern zone of the Himalayan orogen, constitutes the deformed part of the Neogene foreland basin situated above the down ̄exed Indian lithosphere. It forms the outer part of the thin-skinned thrust belt of the Himalaya, a belt where the faults branch o a major de collement (MD) that is the external part of the basal detachment of Himalayan thrust belt. This deÂcollement is located beneath 13 Ma sediments in far-western Nepal, and beneath 14.6 Ma sediments in mid-western Nepal, i.e., above the base of the Siwalik Group. Unconformities have been observed in the upper Siwalik member of western Nepal both on satellite images and in the ®eld, and suggest that tectonics has aected the frontal part of the outer belt since more than 1.8 Ma. Several north dipping thrusts delineate tectonic boundaries in the Siwalik Group of western Nepal. The Main Dun Thrust (MDT) is formed by a succession of 4 laterally relayed thrusts, and the Main Frontal Thrust (MFT) is formed by three segments that die out laterally in propagating folds or branch and relay faults along lateral transfer zones. One of the major transfer zones is the West Dang Transfer Zone (WDTZ), which has a north-northeast strike and is formed by strike-slip faults, sigmoid folds and sigmoid reverse faults. The width of the outer belt of the Himalaya varies from 25 km west of the WDTZ to 40 km east of the WDTZ. The WDTZ is probably related to an underlying fault that induces: (a) a change of the stratigraphic thickness of the Siwalik members involved in the thin-skinned thrust belt, and particularly of the middle Siwalik member; (b) an increase, from west to east, of the depth of the de collement level; and (c) a lateral ramp that transfers displacement from one thrust to another. Large wedge-top basins (Duns) of western Nepal have developed east of the WDTZ. The superposition of two de collement levels in the lower Siwalik member is clear in a large portion of the Siwalik group of western Nepal where it induces duplexes development. The duplexes are formed either by far- travelled horses that crop out at the hangingwall of the Internal De collement Thrust (ID) to the south of the Main Boundary Thrust, or by horses that remain hidden below the middle Siwaliks or Lesser Himalayan rocks. Most of the thrusts sheets of the outer belt of western Nepal have moved toward the S±SW and balanced cross-sections show at least 40 km shortening through the outer belt. This value probably under-estimates the shortening because erosion has removed the hangingwall cut-o of the Siwalik series. The mean shortening rate has been 17 mm/yr in the outer belt for the last 2.3 Ma.
Chronologies for the Siwalik molasse and intermontane basins along the southern margin of the Himalaya and Hindu Kush Ranges constrain the timing and pattern of facies migration and structural disruption of the Indo-Gangetic foredeep. This synthesis indicates that quiescent intervals are punctuated by pulses of rapid deformation as thrusting migrates in a stepwise fashion across the foredeep.