The Himalaya has a major influence on global and regional climate, in particular on the Asian monsoon system. The foreland basin of the Himalaya contains a record of tectonics and paleoclimate since ...the Miocene. Previous work on the evolution of vegetation and climate has focused on the central and western Himalaya, where a shift from C3 to C4 vegetation has been observed at ∼7 Ma and linked to increased seasonality, but the climatic evolution of the eastern part of the orogen is less well understood. In order to track vegetation as a marker of monsoon intensity and seasonality, we analyzed δ13C and δ18O values of soil carbonate and associated δ13C values of bulk organic carbon from previously dated sedimentary sections exposing the syn-orogenic detrital Dharamsala and Siwalik Groups in the west, and, for the first time, the Siwalik Group in the east of the Himalayan foreland basin. Sedimentary records span from 20 to 1 Myr in the west (Joginder Nagar, Jawalamukhi, and Haripur Kolar sections) and from 13 to 1 Myr in the east (Kameng section), respectively. The presence of soil carbonate in the west and its absence in the east is a first indication of long-term lateral climatic variation, as soil carbonate requires seasonally arid conditions to develop. δ13C values in soil carbonate show a shift from around −10‰ to −2‰ at ∼7 Ma in the west, which is confirmed by δ13C analyses on bulk organic carbon that show a shift from around −23‰ to −19‰ at the same time. Such a shift in isotopic values is likely to be associated with a change from C3 to C4 vegetation. In contrast, δ13C values of bulk organic carbon remain at ∼−23‰ in the east. Thus, our data show that the current east–west variation in climate was established at 7 Ma. We propose that the regional change towards a more seasonal climate in the west is linked to a decrease of the influence of the Westerlies, delivering less winter precipitation to the western Himalaya, while the east remained annually humid due to its proximity to the monsoonal moisture source.
•Different lateral evolution of vegetation along the Himalayan mountain belt.•Climate record from stable carbon isotopes in the poorly studied eastern Himalaya.•Development of more seasonal climate at 7 Ma in the western Himalayan region.•Climatic difference from western to eastern Himalaya since at least mid-Miocene.
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•The transition from the Lower to the Middle Siwalik took place around 7.5 Ma.•The Lower Siwalik of this area is a shallow marine/deltaic deposit.•A large marine embayment existed at ...least till ~7 Ma in the eastern Himalaya.•The Middle Siwalik of this area is an axial braided fluvial deposit.•Sea level fall and Shillong Plateau uplift reduced the marine accommodation space.•Reduced accommodation and additional sediment load forced marine-fluvial transition
An understanding of the depositional environment and paleogeography of the Siwalik foreland basin are crucial in interpreting the basin configuration, sediment transport pathways and its evolutionary history. This study examines the sedimentology of the Siwalik succession of the Kameng River valley, Arunachal Himalaya, northeastern India. The facies characteristics of the fine-grained, well-sorted sediments of the Dafla Formation and its complex, polymodal paleocurrent pattern in this section, reveals deposition in a variety of open marine to deltaic environment. The overlying Subansiri Formation, characterized by coarse-grained, thick, multistoried sandstone, and showing more consistent SW-ward paleocurrent, indicate deposition from a large, axial braided river system. The proposed redefinition of the boundary between the Lower Siwalik Dafla and the Middle Siwalik Subansiri formations implies their transition at around 7.5 Ma, instead of 10.5 Ma, suggested earlier. The revised age of the transition is consistent with the age of arrival of the Transhimalayan sediments at 7 Ma and also denotes the time of marine to fluvial transition in this area. Presence of marine sediments in the Kameng section, with similar records further west, indicates the existence of an extensive seaway in the eastern Himalaya during the lower Siwalik time. The extant paleodrainage reconstructions have been recast on the basis of new data on the sedimentology and paleocurrent from this section. It is inferred that the changing sea level, uplifting Shillong Plateau and drainage evolution in the eastern Himalayan foreland during the middle Miocene time controlled the marine to fluvial transition in the basin.
Sedimentary records in peripheral basins of mountain belts record changes in erosion dynamics and drainage-network reorganization, but it is often difficult to discriminate between these different ...controls. Geochemical provenance data on paleo-Indus deposits from the western Himalayan foreland provide constraints on the possible variation of the position of the drainage divide between the Indus and Ganges river systems. Here we present geochemical (trace element and Hf-Nd isotopic) and thermochronological (detrital zircon fission-track DZFT) analyses of modern Indus and Miocene Siwalik sediments from northern Pakistan and compare these with published data on the Indus Fan. Available bedrock isotopic data are used to define three end-member sediment sources (Himalaya, Karakorum, and the Kohistan-Ladakh arc) and to calculate the contribution of each of these sources to the foreland basin and Indus Fan. Our results indicate that since the Miocene the contribution of the Himalayan rivers reaching the Indus in the foreland remained constant, whereas the contributions of sediment sources of the upper Indus catchment changed: those of the Kohistan-Ladakh arc diminished strongly in favor of Karakorum and Himalayan sources. Analysis of the DZFT data from the Miocene foreland basin and sediments of the modern upper Indus reach suggests that the exhumation pattern changed due to an increase in exhumation rate of the Karakorum and Himalayan units of the syntaxis since Miocene times, whereas that in the Kohistan-Ladakh arc remained relatively stable. These results imply that the Indus sediments record changing relative erosion rates in the different source regions rather than widespread drainage rearrangement, as suggested previously.
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Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
ABSTRACT
Thermochronological analysis of detrital sediments derived from the erosion of mountain belts and contained in the sedimentary basins surrounding them allows reconstructing the long‐term ...exhumation history of the sediment source areas. The effective closure temperature of the thermochronological system analysed determines the spatial and temporal resolution of the analysis through the duration of the lag time between closure of the system during exhumation and its deposition in the sedimentary basin. Here, we report apatite fission‐track (AFT) data from 31 detrital samples collected from Miocene to Pliocene stratigraphic sections of the Siwalik Group in western and central Nepal, as well as three samples from modern river sediments from the same area, that complement detrital zircon fission‐track (ZFT) and U–Pb data from the same samples presented in a companion paper. Samples from the upper part of the stratigraphic sections are unreset and retain a signal of source‐area exhumation; they show spatial variations in source‐area exhumation rates that are not picked up by the higher‐temperature systems. More deeply buried samples have been partially reset within the Siwalik basin and provide constraints on the thermal and kinematic history of the fold‐and‐thrust belt itself. The results suggest that peak source‐area exhumation rates have been constant at ∼1.8 km Myr−1 over the last ∼7 Ma in central Nepal, whereas they ranged between 1 and ∼1.5 km Myr−1 in western Nepal over the same time interval; these spatial variations may be explained by either a tectonic or climatic control on exhumation rates, or possibly a combination of the two. Increasing lag times within the uppermost part of the sections suggest an increasing component of apatites that have been recycled within the Siwalik belt and are corroborated by AFT ages of modern river sediment downstream as well as the record of the distal Bengal Fan. The most deeply buried and most strongly annealed samples record onset of exhumation of the frontal Siwaliks along the Himalayan frontal thrust at ∼2 Ma and continuous shortening at rates comparable with the present‐day shortening rates from at least 0.3 Ma onward.
ABSTRACT
Fission‐track (FT) analysis of detrital zircon from synorogenic sediment is a well‐established tool to examine the cooling and exhumation history of convergent mountain belts, but has so far ...not been used to determine the long‐term evolution of the central Himalaya. This study presents FT analysis of detrital zircon from 22 sandstone and modern sediment samples that were collected along three stratigraphic sections within the Miocene to Pliocene Siwalik Group, and from modern rivers, in western and central Nepal. The results provide evidence for widespread cooling in the Nepalese Himalaya at about 16.0±1.4 Ma, and continuous exhumation at a rate of about 1.4±0.2 km Myr−1 thereafter. The ∼16 Ma cooling is likely related to a combination of tectonic and erosional activity, including movement on the Main Central thrust and Southern Tibetan Detachment system, as well as emplacement of the Ramgarh thrust on Lesser Himalayan sedimentary and meta‐sedimentary units. The continuous exhumation signal following the ∼16 Ma cooling event is seen in connection with ongoing tectonic uplift, river incision and erosion of lower Lesser Himalayan rocks exposed below the MCT and Higher Himalayan rocks in the hanging wall of the MCT, controlled by orographic precipitation and crustal extrusion. Provenance analysis, to distinguish between Higher Himalayan and Lesser Himalayan zircon sources, is based on double dating of individual zircons with the FT and U/Pb methods. Zircons with pre‐Himalayan FT cooling ages may be derived from either nonmetamorphic parts of the Tethyan sedimentary succession or Higher Himalayan protolith that formerly covered the Dadeldhura and Ramgarh thrust sheets, but that have been removed by erosion. Both the Higher and Lesser Himalaya appear to be sources for the zircons that record either ∼16 Ma cooling or the continuous exhumation afterwards.
► One fast moving peak in zircon fission-track data. ► Two static peaks in zircon fission-track data. ► Partial resetting of apatite fission-track ages in Siwalik sediments. ► Slow exhumation rate ...before 4 Ma.
Sediment derived from erosion of the Himalaya during the Miocene–Pliocene was deposited in the Himalayan foreland basin and is now exposed in the Siwalik Formation between the Main Boundary Thrust and Main Frontal Thrust. These sediments hold important information on orogenic exhumation during this time. In this preliminary study we sampled the middle Siwalik Formation in eastern Nepal along the Muksar Khola section for thermochronologic and sediment petrologic analysis. Detrital zircon fission-track thermochronology shows that the age signal of the middle Siwalik Formation of eastern Nepal has some similarities but also differences with published data for western-central Nepal. The Muksar Khola section samples show two static peaks at about 14 and >140
Ma, which is similar to the Siwalik Formation of western-central Nepal. In contrast, a signal of consistently fast exhumation was not observed in the upper middle Siwaliks in eastern Nepal. This could be related to a position of the Main Central Thrust further south, to overall less erosional exhumation of Higher Himalayan rocks, or to a reduced exposure of the underlying Lesser Himalayan units in comparison with western-central Nepal. Based on our apatite fission-track data we propose that above the section dated by
Ojha et al. (2009) with magnetostratigraphy about 1000
m of overlying upper Siwalik Formation exist.
ABSTRACT
Clay mineral assemblages of the Neogene Himalayan foreland basin are studied to decipher their significance with respect to tectonic and climate processes. Fluvial deposits of the Siwalik ...Group (west‐central Nepal), and sediment of the Ganga River drainage system were analysed for clay mineralogy. The observed clay mineral assemblages are mainly composed of illite (dominant), chlorite, smectite and kaolinite. Illite and chlorite are chiefly of detrital origin, derived from Himalayan sources. Kaolinite and smectite are authigenic, and mainly developed within pore space and as coating of detrital particles. With increasing burial, diagenetic processes affected the original clay mineral signature. Illitisation of smectite and kaolinite occurred below 2500 and 3500 m depth, respectively. Therefore, illite in the lower parts of the Siwalik Group consists of a mixture of inherited illite and illitised smectite and kaolinite, as suggested by illite crystallinity. Detrital grains that make up the framework of the Siwalik Group sandstones mainly consist of quartz, feldspar and lithic fragments, which are principally of sedimentary and metamorphic origin. Lithoclast content increases over time at the expense of quartz and K‐feldspar in response to uplift and erosion of the Lesser Himalaya Series since about 11–10 Ma. Despite mainly felsic source rocks, dominantly physical erosion processes in the Himalayan belt, and high‐energy fluvial depositional systems, smectite is abundant in the <7 Ma Siwalik Group deposits. Analyses of the Siwalik deposits and comparison with the clay mineralogy of the modern drainage system suggest that smectite preferentially formed in floodplains and intermontane valleys during early diagenesis because of downward percolating fluids rich in cations from weathering and soil development. In general, increasing seasonality and aridity linked to variability of the Asian monsoon from about 8 Ma enhanced clay mineral formation and development of authigenic smectite in paleo‐plains on the southern side of the Himalaya.
The aim of this paper is to unfold the relationship between the O and C isotope compositions of modern fresh-water mollusc shells and water in order to refine the basis of interpretation for ...paleoenvironnemental reconstruction in the sub-Himalayan river basins. Large number of mollusc shells and associated host water from both running water and closed body of water were analysed including intra-shell variability in a few cases.
The O isotopic compositions of river waters in the Himalaya and Ganga plain have a large range, from −
18‰ in the north of the high range up to −
8‰ to −
4‰ in the Ganga plain. δ
18O of rivers are also seasonally variable, especially in foothills rivers whereas the seasonal contrast is smoothed out for the Main Himalayan rivers having large catchments. O isotopic compositions of bulk shells (Aragonite) vary between −
15‰ and −
5‰. Average δ
18O
Ara values are consistent with precipitation at equilibrium with host waters at a temperature range of 20–25 °C suggesting that shell growth may be favoured during non-monsoon conditions. Shells collected along the Main Himalayan rivers have δ
18O values uniformly distributed within −
11‰ and −
6‰ reflecting the minimal seasonal contrast shown by these rivers. In contrast, O isotopic compositions of shells from foothills rivers vary only by 4‰. This shows that, depending on the type of river where the molluscs grow, the information in term of δ
18O amplitude will be different for identical climatic conditions. In closed or pond water bodies significant enrichment in
18O due to evaporation is observed.
The C isotopic compositions of river dissolved inorganic carbon (DIC) decrease downstream from 0‰ to −
10‰ reflecting input of soil derived alkalinity and plant productivity in the river. δ
13C of shells are systematically lower than compositions calculated for equilibrium with river DIC indicating that in addition to DIC, a significant fraction of carbon is derived from metabolic sources. Intra-shell δ
13C are stable compared to the seasonal variability of DIC suggesting that the pool of organic carbon changes throughout year.
► Feldspar alteration during early and burial diagenesis of Siwaliks sandstone. ► Slight impact on original sediment composition. ► Nonetheless feldspar may be unrepresented in point-count analyses. ...► Polyquartz type analysis supports standard point-counting analysis.
The petrologic composition of synorogenic sediments provides information on source rock lithologies and can be used for reconstructing the erosional history of mountain belts. However, diagenetic processes such as compaction, dissolution of unstable framework grains, and formation of authigenic minerals may significantly alter the original sediment composition. The question is how important are the diagenetic alterations? To address this question, we review published and new petrologic and diagenetic data of fluvial sandstones from the Miocene–Pliocene Siwalik Group of the Himalayan foreland basin in western-central Nepal. The new data include petrographic analyses of 18 samples collected along the Tinau Khola and Surai Khola sections. In these samples evidence for early, burial and late diagenesis have been recognized. Early and burial diagenetic calcite cement is the most important authigenic phase which, in addition to moderate compaction and authigenic clay minerals, occludes primary intergranular porosity. Patchy secondary porosity probably formed during late diagenesis related to exhumation. Standard point-counting and polyquartz-type analysis of detrital grains support previously published petrologic data. High-grade metamorphic lithic grains have a Higher Himalayan provenance, and low-grade metamorphic and sedimentary lithic grains being derived from Lesser Himalayan sources, which became more important during the Late Miocene. In the case of the Himalayan foreland basin it seems that diagenetic alteration of the original sediment composition is limited to feldspars and certain sedimentary lithic grains, which as a consequence may be under-represented in point counting analyses because of replacement, dissolution and compaction.
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