Geological Observations from a Palaeolake Basin, Lamayuru, Ladakh, Northwestern Himalaya

A well known Buddhist monastery of Lamayuru is located in a village about 128 Km West of Leh. It is situated on more than 100 m thick Late Quaternary palaeolake deposits which are surrounded by rocks of Lamayuru Formation. Geologically, the Lamayuru Formation includes the Lamayuru and Namikala flysch deposits of Triassic-Jurassic age. This Formation is composed of shales, schist and phyllites. This Lamayuru Formation forms the base and source of palaeolake deposits. In Late-Pleistocene (35 ka B.P.) the Lamayuru River was dammed due to tectonically triggered landslide and the Lamayuru palaeolake came into existence. The sedimentation in the palaeolake basin commenced at 35 ka B.P. and culminated at 1 ka B.P. The deposits of palaeolake consist of carbonaceous mud, sand, silty clay and matrix supported breccia. The palaeolake deposits are a product of complex interplay of lacustrine, fluvio-deltaic to colluvial processes. The research study shows the prevalence of glacio-lacustrine conditions during the major part of depositional history as evidenced by the dominance of varves in these deposits.


Open Journal of Geology
Himalayas of Zanskar in the south [2]. Developed in the main valley along the Indus Suture Zone in the Trans Himalaya, the basin consists of lacustrine deposits ranging in age from late Pleistocene to Holocene [3]. The Lamayuru palaeolake was developed due to damming of Lamayuru River by tectonically triggered landslide [3]. The sediments of Lamayuru palaeolake are more than 100 m thick and consists of carbonaceous mud, sand, silty clay and matrix supported breccia [4] [5] [6]. The sharp contact between Lamayuru flysch consisting breccia dominated limestone and overlying lacustrine silts indicates that the lake was massive and resistant enough to cause an immediate flooding of the valley and to initiate lacustrine sedimentation. The present discharge of northeast flowing Lamayuru River is very low. During the Lamayuru palaeolake event, the discharge might have been higher with more humid periglacial conditions [3]. The basal lacustrine deposits are represented by white chalky beds that crop out strikingly along the valley walls. These beds are composed of mud, silt and fine sand and are horizontally stratified and are rich in organic debris. The occurrence of gastropods and ostracods in the centre of the valley suggests the shallower depth of the lake.

Geological Setup
Lamayuru basin (N34˚17'05" lat. and E76˚47'00" long.) ( Figure 1) is located at an altitude of 3600 m in the Ladakh region and is very well connected by metalled

Previous Work
For more than a century, travellers following the ancient trail joining Srinagar to Leh have remarked on the spectacular lacustrine deposits of Lamayuru. In 1848, Cunningham recognized the sediments as the undoubted evidence for a huge lake that existed at a relatively recent epoch. According to the legend reported by Cunningham [7], the lake was drained out by the lama Naropa when he founded the first Buddhist monastery at Lamayuru. Drew [8] was the first to point out the abundance of alluvium interfingered with lacustrine silts. Dainelli [9] suggested the lake was formed during a "fairly old glacial period" at a time when a glacier tongue descended the adjacent Yapola valley and dammed the small Lamayuru tributary before converging with the Indus River. Pandey [10] mentioned again the legend specifying that the Lamayuru Lake had clear waters which were rapidly drained.
Burgisser et al. [11] suggested that this sudden draining probably caused catastrophic event downstream and that caused the erosion of the lacustrine sediments from the centre of the valley. Bagati et al. [12], Fort et al. [3] and Shukla et al. [6] studied the sedimentation pattern in the basin and gave different ages for initiation of lacustrine sedimentation in the basin. Kotlia et al. [13] studied the palaeomagnetic properties of the lake sediments. Kotlia et al. [5] recorded the minor reversal polarity event at Lamayuru dated 35.5 ± 0.6 ka B.P.
Kachroo et al. [14], Malik and Shah [15] and Mathur and Kotlia [16] worked over the microinvertebrates including molluscs and ostracods deposited in the lake sediments. Kachroo et al. [14] gave the brief paleoecological implication of fauna of Lamayuru lake sediments and showed that the ostracod assemblages from Lamayuru have close affinities with the similar ostracods described from the upper Karewa Formation of Kashmir. Malik and Shah [15] reported various ostracod taxa from Lamayuru especially the Parastenocypris cf. simils species, a central Asian form that is reported for the first time in India. Mathur and Kotlia [16] reported molluscs (bivalves and gastropods) and ostracodes from the late Quaternary fluvio-lacustrine sediments of Lamayuru and showed that the abundance of Pisidium sp. (bivalves) in the lowermost horizon of lake sediments indicates typical lacustrine environment. Ranhotra et al. [17] studied the palynological analysis of the paleaolake sediments and described that the prevailing semi-arid climate of this region has been continuing at least from prior to 35 ka B.P. and before it climate was comparatively less arid.

Observations
During the field investigations, the Lamayuru basin was studied for the deposition of varvites at different locations. Ten localities of varvites were visited and their geographical coordinates recorded ( Figure 2, Table 1  are also found near the top of the palaeolake deposits e.g., Location 10 at 11,000 ft. In between from base to the top there are also thick varve deposits as recorded in Location 1 to Location 10. The lake deposits also consist of carbonaceous mud, silty clay, sand with peb- 2) Mud-cracks (Figure 3(b)) are also seen on the top of the lake sediments which show the rapid drain out of the water during lacustrine conditions.
4) Lake sediments comprising sand, clay and mud can be seen from a distance on account of their colour contrast with grey Triassic-Jurassic age bedrock ( Figure 3(d)).    shows the prevalence of glacio-lacustrine conditions during the major part of depositional history as evidenced by the dominance of varves in these deposits.