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(OSL- OJRI- 1). These deposits rest over bedrock at a height of ~125 m from present
day Yamuna River bed. Whereas In sector-II, equivalent age of 87 ± 4 ka (LKT-1) is
obtained from around 30 m below the exposed upper part of the topmost terrace
section. The basal part of deposit is exposed at road cutting around 160 m below top
part. An OSL age from sandy lens from this exposed section gave an OSL age of
105 ± 9 ka (LKT-2). In Sector- I, another OSL age of 47 ± 3 ka (OSL-Y1) is recorded
at Yamunotri from a sandy layer in ~50 m fluvial deposit along left bank of river.
The deposit is well defined terrace surface. At Kisala village, OSL age of 4 ± 3 ka
(OSL- Kisala) is obtained from the basal part of same terrace deposit at height of
~12 m from present day Yamuna channel. From the topmost part of fluvial sequence
representing same terrace deposit at Phoolchatti and Yamunotri gave OSL ages of
38 ± 2 ka (OSL-PC-1) and 37 ± 2 ka (OSL-Y-2) respectively. Whereas, an equivalent OSL age of 40 ± 2 ka (OSL-PC-2) is obtained from Phoolchatti terrace deposit.
In Sector-II (south of MCT zone), several equivalent OSL ages of 43 ± 3 ka (OSLBAR- 2-new), 50 ± 3 ka (OSL-Barkot-1), 41 ± 3 ka (LM-2) were obtained from fluvial sandy horizons on both left and right bank of Yamuna River near Chatanga and
Lakhamandal area. In the same area, OSL ages viz. 24 ± 1 ka (BAR-1), 25 ± 3 ka
(BAR-2), 28 ± 2 ka (BAR-5), 35 ± 4 ka (BAR-4) were obtained from top part of
well preserved and extensive terrace deposit in Chatanga-Barkot section. A sand
lens at the basal part of the terrace deposit near Phaunti area (near bridge) gave an
OSL age of 35 ± 2 ka (OSL-PHTP-1). Equivalent age of 23 ± 1 ka (LM-1) is
obtained from sandy horizon from top part of terrace deposit at Lakhamandal.
Another set of OSL ages were obtained from fluvial terrace deposit confirming
downstream continuity of aggradation near Barkot and Lakhamandal area viz.
19 ± 1 ka (OSL-B-1), 18 ± 1 ka (OSL-LM-10).
5 Role of Monsoon (ISM) and Glacial-Interglacial Transition
in Aggradation and Incision
Landscape evolution is defined as a resultant product of climate change, rock erosion and tectonics. The orogenic processes result in upliftment of land mass and
thereby developing orographic controlled monsoon circulation (Bookhagen et al.
2005). These processes catalyze the physical and chemical weathering (Raymo and
Ruddiman 1992) through enhanced precipitation promoting the driving mechanism
of river systems. Hence, precipitation controlled rock erosion is characteristic of
river development and it plays a key role in peneplanation through surficial processes. These processes mainly operate at both shorter (10
3
years) as well as longer
temporal scales (10
6
years) over a region with variable rates leading to geomorphic
development of a river valley leaving behind remnant landforms like alluvial fans,
fluvial terraces etc. Moreover, the local base level uplift may respond to bedrock
incision points towards tectonic activity and/or change in river gradient (Kumar and
Srivastava 2017). Therefore, for appreciation of Late Quaternary Palaeoclimatic
Responses of Indian Summer Monsoon Dynamics and Late Quaternary Fluvial…
(OSL- OJRI- 1). These deposits rest over bedrock at a height of ~125 m from present
day Yamuna River bed. Whereas In sector-II, equivalent age of 87 ± 4 ka (LKT-1) is
obtained from around 30 m below the exposed upper part of the topmost terrace
section. The basal part of deposit is exposed at road cutting around 160 m below top
part. An OSL age from sandy lens from this exposed section gave an OSL age of
105 ± 9 ka (LKT-2). In Sector- I, another OSL age of 47 ± 3 ka (OSL-Y1) is recorded
at Yamunotri from a sandy layer in ~50 m fluvial deposit along left bank of river.
The deposit is well defined terrace surface. At Kisala village, OSL age of 4 ± 3 ka
(OSL- Kisala) is obtained from the basal part of same terrace deposit at height of
~12 m from present day Yamuna channel. From the topmost part of fluvial sequence
representing same terrace deposit at Phoolchatti and Yamunotri gave OSL ages of
38 ± 2 ka (OSL-PC-1) and 37 ± 2 ka (OSL-Y-2) respectively. Whereas, an equivalent OSL age of 40 ± 2 ka (OSL-PC-2) is obtained from Phoolchatti terrace deposit.
In Sector-II (south of MCT zone), several equivalent OSL ages of 43 ± 3 ka (OSLBAR- 2-new), 50 ± 3 ka (OSL-Barkot-1), 41 ± 3 ka (LM-2) were obtained from fluvial sandy horizons on both left and right bank of Yamuna River near Chatanga and
Lakhamandal area. In the same area, OSL ages viz. 24 ± 1 ka (BAR-1), 25 ± 3 ka
(BAR-2), 28 ± 2 ka (BAR-5), 35 ± 4 ka (BAR-4) were obtained from top part of
well preserved and extensive terrace deposit in Chatanga-Barkot section. A sand
lens at the basal part of the terrace deposit near Phaunti area (near bridge) gave an
OSL age of 35 ± 2 ka (OSL-PHTP-1). Equivalent age of 23 ± 1 ka (LM-1) is
obtained from sandy horizon from top part of terrace deposit at Lakhamandal.
Another set of OSL ages were obtained from fluvial terrace deposit confirming
downstream continuity of aggradation near Barkot and Lakhamandal area viz.
19 ± 1 ka (OSL-B-1), 18 ± 1 ka (OSL-LM-10).
5 Role of Monsoon (ISM) and Glacial-Interglacial Transition
in Aggradation and Incision
Landscape evolution is defined as a resultant product of climate change, rock erosion and tectonics. The orogenic processes result in upliftment of land mass and
thereby developing orographic controlled monsoon circulation (Bookhagen et al.
2005). These processes catalyze the physical and chemical weathering (Raymo and
Ruddiman 1992) through enhanced precipitation promoting the driving mechanism
of river systems. Hence, precipitation controlled rock erosion is characteristic of
river development and it plays a key role in peneplanation through surficial processes. These processes mainly operate at both shorter (10
3
years) as well as longer
temporal scales (10
6
years) over a region with variable rates leading to geomorphic
development of a river valley leaving behind remnant landforms like alluvial fans,
fluvial terraces etc. Moreover, the local base level uplift may respond to bedrock
incision points towards tectonic activity and/or change in river gradient (Kumar and
Srivastava 2017). Therefore, for appreciation of Late Quaternary Palaeoclimatic
Responses of Indian Summer Monsoon Dynamics and Late Quaternary Fluvial…