The Siwalik Group extending east-west collinear to the Himalayan southern front records Himalayan... more The Siwalik Group extending east-west collinear to the Himalayan southern front records Himalayan denudation history. In this study, we present sandstone petrography, heavy mineral assemblages, Nd isotopes and chemical composition of detrital garnet from the Siwalik succession along the Muksar Khola section. This study reveals the Tibetan Tethys Himalaya, Higher Himalayan Crystalline and Lesser Himalaya Sequence as the source of sediments and records a provenance change from the middle Miocene to Pliocene/Pleistocene. This was observed as a shift from shallow to deeper parts of the Higher Himalayan Crystalline as a source area around 10.6 Ma. The Lesser Himalaya Sequence supplied a significant amount of sediments after 7.5 Ma, but the deeper part was only exposed after the late Pliocene to Pleistocene. Around 4.0 Ma an increase in the sediments from the Higher Himalayan Crystalline was observed. This provenance study reveals two stages of the exhumation of the eastern Nepal Himalaya. The first occurred between 11.0 and 5.5 Ma and the second after 4.0 Ma, due to the activation of an out-of-sequence thrust known as the Sun Koshi Thrust, and formation of the Lesser Himalayan duplex respectively.
The Siwalik Group extending east to west co-linear to the main Himalayan range is well exposed al... more The Siwalik Group extending east to west co-linear to the main Himalayan range is well exposed along the Muksar Khola section, Siraha-Udayapur district, eastern Nepal Himalaya. The Siwalik Group in the present study area is divided into the Lower, Middle, and Upper Siwaliks based on the grain size and the proportion of sandstone-mudstone.The Lower Siwaliks is characterized by very fine-to fine-grained, light grey sandstone interbedded with dark grey to olive black mudstone. The Middle Siwaliks, is characterized by the domination of fine-to coarse-grained sandstone, and based on the lithology and bed thickness it is divided into two members. The lower member is dominated by fine-to medium-grained "salt and pepper" sandstone with dark greenish to olive-grey mudstone while, the upper member is dominated by light grey to white medium-to coarse-grained sandstone with grey, dark grey to black mudstone. An increase in the grain size and thickness of sandstone beds, an increase in the proportion of mudstone, a decrease in induration of sandstone and a decrease in the proportion of biotite grain in sandstone makes the upper member different from the lower member of the Middle Siwaliks. The Upper Siwaliks is characterized by very thick beds of clast supported conglomerate associated with coarse-to very coarse-grained, very thick bedded sandstone and dull yellowish-grey to grey mudstone. The boundary between the Lower and the Middle Siwaliks, lower and upper members of the Middle Siwaliks, and the Upper Siwaliks are 10.0 Ma, 5.7 Ma, and 3.5 Ma, respectively. The present study records the presence of a large succession of intra-formational conglomerate succession in the Lower Siwaliks.
Bengal Fan Miocene sediments were collected during International Ocean Discovery
Program Expedit... more Bengal Fan Miocene sediments were collected during International Ocean Discovery
Program Expedition 354 and investigated using petrographic and detrital garnet
chemistry analyses. The Miocene Siwalik Group, which is composed of sediments
deposited in the Himalayan foreland basin, was also analyzed for comparison with
the Bengal Fan data for the provenance change during the Miocene. Our petrographic
analyses revealed that the Miocene sediments of the Bengal Fan and Siwalik
Group consist predominantly of Higher Himalayan Crystalline (HHC)-derived detritus
such as chloritoid, staurolite, sillimanite, and/or kyanite, which appear among the
accessory minerals. The chemistry of the detrital garnet varies across the stratigraphy;
most of the garnet is rich in almandine and poor in spessartine and pyrope.
However, pyrope-rich garnet, which is considered to originate from the HHC core
(granulite facies), was found in the lower to upper Miocene deposits. The deposition
of HHC-derived detrital garnet began before the Middle Miocene (15 Ma) and before
the Late Miocene (10–9 Ma) in the Siwalik Group. The Bengal Fan data, by contrast,
indicated that pyrope-rich garnet appeared in the Early Miocene (17.3 Ma) and Late
Miocene (8.5–6.5 Ma). We conclude that the Bengal Fan sediments record the erosion
of the HHC zone since the Early Miocene that appears in the Siwalik sediments.
Furthermore, we found that the HHC-derived inputs decreased from the late Middle
Miocene (12 Ma) to the early Middle Miocene (10 Ma) in both the Nepal Himalaya
foreland basin and the Bengal Fan. The disappearance of the HHC-derived detritus
is probably the result of dilution by Lesser Himalayan detritus, which suggests
that the Lesser Himalayan zone, which is composed of metamorphosed and
unmetamorphosed sedimentary rocks, was uplifted.
The Siwalik Group, ranging from the Early Miocene to Pleistocene, is believed to be deposited in ... more The Siwalik Group, ranging from the Early Miocene to Pleistocene, is believed to be deposited in the fluvial environment and controlled by contemporary Himalayan tectonics and climate. In this study, we established the fluvial environment and its controlling factors responsible for the deposition of the Siwalik succession along the Muksar Khola section in the eastern Nepal Himalaya. Five sedimentary facies associations are identified; these are interpreted as the deposits of flood plain-dominated fine-grained meandering river (FA1), flood-dominated overbank environment (FA2), sandy meandering river (FA3), anastomosing river (FA4), and debris flow-dominated gravelly braided river (FA5). These changes in the fluvial system occurred around 10.5 Ma, 10.0 Ma, 5.9 Ma and 3.5 Ma, defined by existing magnetostratigraphy constraints, due to the effects of hinterland tectonics, climate and sea-level change and continuous drifting of the foreland basin towards the hinterland concerning deposit...
The Dharan–Mulghat area of the eastern Nepal can be divided into three tectonic units: the Higher... more The Dharan–Mulghat area of the eastern Nepal can be divided into three tectonic units: the Higher Himalayan Crystallines, the Lesser Himalayan Sequence and the Siwaliks from north to south separated by the Main Central Thrust (MCT) and Main Boundary Thrust (MBT), respectively. The Lesser Himalayan Sequence is divided into two groups separated by Chimra Thrust: the Bhedetar Group and the Dada Bajar Group. The Bhedetar Group includes the Raguwa Formation, the Phalametar Quartzite, the Churibas Formation, the Sangure Quartzite, and the Karkichhap Formation from the bottom to top, respectively; over-thrusted by the Dada Bajar Group consisting: the Ukhudanda Formation, the Mulghat Formation, the Okhre Formation, and the Patigau Formation, from lower to upper sections, respectively along the Chimra Thrust and the Bhorleni Formation as an individual formation overthrusted by Bhedetar Group along the Chhotimorang Thrust. The Main Central Thrust, the Main Boundary Thrust, the Chimra Thrust and the Chhotimorang Thrust are the major faults in Dharan–Mulghat area. The Leutiphedi Anticline and the Malbase Syncline are the major folds in the study area plunging towards east. The trend/plunge of anticline and syncline are 131 o /24 o and 096 o /09 o respectively. The microstructural study in the quartz grains reveals a sharp difference in the history across the MCT; dynamic in the rocks of the Lesser Himalayan Sequences and static in the rocks of the Higher Himalayan Crystallines.
The Siwalik Group extending east-west collinear to the Himalayan southern front records Himalayan... more The Siwalik Group extending east-west collinear to the Himalayan southern front records Himalayan denudation history. In this study, we present sandstone petrography, heavy mineral assemblages, Nd isotopes and chemical composition of detrital garnet from the Siwalik succession along the Muksar Khola section. This study reveals the Tibetan Tethys Himalaya, Higher Himalayan Crystalline and Lesser Himalaya Sequence as the source of sediments and records a provenance change from the middle Miocene to Pliocene/Pleistocene. This was observed as a shift from shallow to deeper parts of the Higher Himalayan Crystalline as a source area around 10.6 Ma. The Lesser Himalaya Sequence supplied a significant amount of sediments after 7.5 Ma, but the deeper part was only exposed after the late Pliocene to Pleistocene. Around 4.0 Ma an increase in the sediments from the Higher Himalayan Crystalline was observed. This provenance study reveals two stages of the exhumation of the eastern Nepal Himalaya. The first occurred between 11.0 and 5.5 Ma and the second after 4.0 Ma, due to the activation of an out-of-sequence thrust known as the Sun Koshi Thrust, and formation of the Lesser Himalayan duplex respectively.
The Siwalik Group extending east to west co-linear to the main Himalayan range is well exposed al... more The Siwalik Group extending east to west co-linear to the main Himalayan range is well exposed along the Muksar Khola section, Siraha-Udayapur district, eastern Nepal Himalaya. The Siwalik Group in the present study area is divided into the Lower, Middle, and Upper Siwaliks based on the grain size and the proportion of sandstone-mudstone.The Lower Siwaliks is characterized by very fine-to fine-grained, light grey sandstone interbedded with dark grey to olive black mudstone. The Middle Siwaliks, is characterized by the domination of fine-to coarse-grained sandstone, and based on the lithology and bed thickness it is divided into two members. The lower member is dominated by fine-to medium-grained "salt and pepper" sandstone with dark greenish to olive-grey mudstone while, the upper member is dominated by light grey to white medium-to coarse-grained sandstone with grey, dark grey to black mudstone. An increase in the grain size and thickness of sandstone beds, an increase in the proportion of mudstone, a decrease in induration of sandstone and a decrease in the proportion of biotite grain in sandstone makes the upper member different from the lower member of the Middle Siwaliks. The Upper Siwaliks is characterized by very thick beds of clast supported conglomerate associated with coarse-to very coarse-grained, very thick bedded sandstone and dull yellowish-grey to grey mudstone. The boundary between the Lower and the Middle Siwaliks, lower and upper members of the Middle Siwaliks, and the Upper Siwaliks are 10.0 Ma, 5.7 Ma, and 3.5 Ma, respectively. The present study records the presence of a large succession of intra-formational conglomerate succession in the Lower Siwaliks.
Bengal Fan Miocene sediments were collected during International Ocean Discovery
Program Expedit... more Bengal Fan Miocene sediments were collected during International Ocean Discovery
Program Expedition 354 and investigated using petrographic and detrital garnet
chemistry analyses. The Miocene Siwalik Group, which is composed of sediments
deposited in the Himalayan foreland basin, was also analyzed for comparison with
the Bengal Fan data for the provenance change during the Miocene. Our petrographic
analyses revealed that the Miocene sediments of the Bengal Fan and Siwalik
Group consist predominantly of Higher Himalayan Crystalline (HHC)-derived detritus
such as chloritoid, staurolite, sillimanite, and/or kyanite, which appear among the
accessory minerals. The chemistry of the detrital garnet varies across the stratigraphy;
most of the garnet is rich in almandine and poor in spessartine and pyrope.
However, pyrope-rich garnet, which is considered to originate from the HHC core
(granulite facies), was found in the lower to upper Miocene deposits. The deposition
of HHC-derived detrital garnet began before the Middle Miocene (15 Ma) and before
the Late Miocene (10–9 Ma) in the Siwalik Group. The Bengal Fan data, by contrast,
indicated that pyrope-rich garnet appeared in the Early Miocene (17.3 Ma) and Late
Miocene (8.5–6.5 Ma). We conclude that the Bengal Fan sediments record the erosion
of the HHC zone since the Early Miocene that appears in the Siwalik sediments.
Furthermore, we found that the HHC-derived inputs decreased from the late Middle
Miocene (12 Ma) to the early Middle Miocene (10 Ma) in both the Nepal Himalaya
foreland basin and the Bengal Fan. The disappearance of the HHC-derived detritus
is probably the result of dilution by Lesser Himalayan detritus, which suggests
that the Lesser Himalayan zone, which is composed of metamorphosed and
unmetamorphosed sedimentary rocks, was uplifted.
The Siwalik Group, ranging from the Early Miocene to Pleistocene, is believed to be deposited in ... more The Siwalik Group, ranging from the Early Miocene to Pleistocene, is believed to be deposited in the fluvial environment and controlled by contemporary Himalayan tectonics and climate. In this study, we established the fluvial environment and its controlling factors responsible for the deposition of the Siwalik succession along the Muksar Khola section in the eastern Nepal Himalaya. Five sedimentary facies associations are identified; these are interpreted as the deposits of flood plain-dominated fine-grained meandering river (FA1), flood-dominated overbank environment (FA2), sandy meandering river (FA3), anastomosing river (FA4), and debris flow-dominated gravelly braided river (FA5). These changes in the fluvial system occurred around 10.5 Ma, 10.0 Ma, 5.9 Ma and 3.5 Ma, defined by existing magnetostratigraphy constraints, due to the effects of hinterland tectonics, climate and sea-level change and continuous drifting of the foreland basin towards the hinterland concerning deposit...
The Dharan–Mulghat area of the eastern Nepal can be divided into three tectonic units: the Higher... more The Dharan–Mulghat area of the eastern Nepal can be divided into three tectonic units: the Higher Himalayan Crystallines, the Lesser Himalayan Sequence and the Siwaliks from north to south separated by the Main Central Thrust (MCT) and Main Boundary Thrust (MBT), respectively. The Lesser Himalayan Sequence is divided into two groups separated by Chimra Thrust: the Bhedetar Group and the Dada Bajar Group. The Bhedetar Group includes the Raguwa Formation, the Phalametar Quartzite, the Churibas Formation, the Sangure Quartzite, and the Karkichhap Formation from the bottom to top, respectively; over-thrusted by the Dada Bajar Group consisting: the Ukhudanda Formation, the Mulghat Formation, the Okhre Formation, and the Patigau Formation, from lower to upper sections, respectively along the Chimra Thrust and the Bhorleni Formation as an individual formation overthrusted by Bhedetar Group along the Chhotimorang Thrust. The Main Central Thrust, the Main Boundary Thrust, the Chimra Thrust and the Chhotimorang Thrust are the major faults in Dharan–Mulghat area. The Leutiphedi Anticline and the Malbase Syncline are the major folds in the study area plunging towards east. The trend/plunge of anticline and syncline are 131 o /24 o and 096 o /09 o respectively. The microstructural study in the quartz grains reveals a sharp difference in the history across the MCT; dynamic in the rocks of the Lesser Himalayan Sequences and static in the rocks of the Higher Himalayan Crystallines.
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Papers by Lalit Rai
Program Expedition 354 and investigated using petrographic and detrital garnet
chemistry analyses. The Miocene Siwalik Group, which is composed of sediments
deposited in the Himalayan foreland basin, was also analyzed for comparison with
the Bengal Fan data for the provenance change during the Miocene. Our petrographic
analyses revealed that the Miocene sediments of the Bengal Fan and Siwalik
Group consist predominantly of Higher Himalayan Crystalline (HHC)-derived detritus
such as chloritoid, staurolite, sillimanite, and/or kyanite, which appear among the
accessory minerals. The chemistry of the detrital garnet varies across the stratigraphy;
most of the garnet is rich in almandine and poor in spessartine and pyrope.
However, pyrope-rich garnet, which is considered to originate from the HHC core
(granulite facies), was found in the lower to upper Miocene deposits. The deposition
of HHC-derived detrital garnet began before the Middle Miocene (15 Ma) and before
the Late Miocene (10–9 Ma) in the Siwalik Group. The Bengal Fan data, by contrast,
indicated that pyrope-rich garnet appeared in the Early Miocene (17.3 Ma) and Late
Miocene (8.5–6.5 Ma). We conclude that the Bengal Fan sediments record the erosion
of the HHC zone since the Early Miocene that appears in the Siwalik sediments.
Furthermore, we found that the HHC-derived inputs decreased from the late Middle
Miocene (12 Ma) to the early Middle Miocene (10 Ma) in both the Nepal Himalaya
foreland basin and the Bengal Fan. The disappearance of the HHC-derived detritus
is probably the result of dilution by Lesser Himalayan detritus, which suggests
that the Lesser Himalayan zone, which is composed of metamorphosed and
unmetamorphosed sedimentary rocks, was uplifted.
Program Expedition 354 and investigated using petrographic and detrital garnet
chemistry analyses. The Miocene Siwalik Group, which is composed of sediments
deposited in the Himalayan foreland basin, was also analyzed for comparison with
the Bengal Fan data for the provenance change during the Miocene. Our petrographic
analyses revealed that the Miocene sediments of the Bengal Fan and Siwalik
Group consist predominantly of Higher Himalayan Crystalline (HHC)-derived detritus
such as chloritoid, staurolite, sillimanite, and/or kyanite, which appear among the
accessory minerals. The chemistry of the detrital garnet varies across the stratigraphy;
most of the garnet is rich in almandine and poor in spessartine and pyrope.
However, pyrope-rich garnet, which is considered to originate from the HHC core
(granulite facies), was found in the lower to upper Miocene deposits. The deposition
of HHC-derived detrital garnet began before the Middle Miocene (15 Ma) and before
the Late Miocene (10–9 Ma) in the Siwalik Group. The Bengal Fan data, by contrast,
indicated that pyrope-rich garnet appeared in the Early Miocene (17.3 Ma) and Late
Miocene (8.5–6.5 Ma). We conclude that the Bengal Fan sediments record the erosion
of the HHC zone since the Early Miocene that appears in the Siwalik sediments.
Furthermore, we found that the HHC-derived inputs decreased from the late Middle
Miocene (12 Ma) to the early Middle Miocene (10 Ma) in both the Nepal Himalaya
foreland basin and the Bengal Fan. The disappearance of the HHC-derived detritus
is probably the result of dilution by Lesser Himalayan detritus, which suggests
that the Lesser Himalayan zone, which is composed of metamorphosed and
unmetamorphosed sedimentary rocks, was uplifted.