J. D. A. Clarke1 and S McGuirk1, 2
1Mars Society Australia, c/o 43 Michell St Monash, ACT 2904; 2Fenner School, Australian National University, ACT 0200
The Hunder Dunes are one of a series of small dune fields in the Shyok and Nubra valleys of the Ladakh region of north-western India. The dune fields are composed of mostly barchanoid and transverse dunes and are composed of sand reworked from the seasonally exposed beds of these rivers. Wind directions are strongly uni-modal in orientation and controlled by valley orientation (McGuirk 2017). The Hunder Dunes occur at an altitude of 3083 m in the Nubra valley and cover an area of about 1500 x 700 m. The area is a popular tourist location because of the visual impression of the dunes against the background of the Karakorum Range rising to over 6000 m, and the presence of a small herd of Bactrian camels. No scientific investigation of the dunes have been previously made. During a 2016 expedition to the area (Pandey et al. 2019) we noticed the presence of numerous inverted swale deposits. Inverted swales have not, to our knowledge, been described in the literature. We here present a brief description of them, and discuss their formation and possible significance.
The Hunder Dune field occurs in the Nubra valley and are centred at 77°29’47.60″E and 34°34’49.00″N. The dune fields are composed of mostly barchanoid at the present time, although the evidence from Google Earth imagery shows that they have been dominated by transverse bedforms in the past. The dunes lie on the south bank of the Nubra River The Hunder Dunes occur at an altitude of 3083 m in the Nubra valley and cover an area of about 1500 x 700 m
The climate of the Hunder Dunes is arid with warm summers and cold winters, temperatures falling below zero. The area corresponds with zone IV of Hewett (1989). Valley floors experience strong winds, the dominant wind vector responsible for the dunes is from the northwest. No rainfall data exists for Hunder, but the city of Leh in the valley of the Indus, which is the next valley to the south, has an annual rainfall of 104 mm (Climate data 2018).
Morphology and stratigraphy
The features interpreted as inverted swales consist of small (10-20m long, 5-10m wide, and up to 1.5m high), ellipsoidal mesas exposed in the swales between active dunes. Most of the mesas have a tiered, wedding cake appearance, due to the presence of stacked cycles of sediment (Figure 2). Individual cycles range from 40 to 50 cm in thickness, and are composed of a repeated succession of lithologies (Table 1), forming cycles. Not all units are present in each cycle, unit A is only present only at the base of the mesa, and is not always exposed. Only one of the rippled units B and C may be present, and rarely the muddy unit D is absent. Some swales are partly buried by dune migration, some others are being exposed by the same process.
As shown in Table 1, the cyclic units are interpreted as showing the flooding of dune swales by muddy river waters, followed by their desiccation. The convex-upward basal unit A represents the aeolian swale deposits. The convex upward geometry of the swale is propagated ward through each cycle. The asymmetric cross-laminated unit B represents the influx of river flood water carrying sand and mud. The symmetric cross-laminated unit C represents wind-driven wave sand deposition water flood recession as isolated the sales into ponds. Interference patterns between ripple sets are visible on the upper surfaces of the topmost cycles exposed in the mesas. The uppermost unit C represents settling out of suspended mud in the final phases of drying out of the pond in the swales. The muddy unit is often desiccation cracked and may preserve vertebrate footprints. Multiple cycles of flooding and desiccation form the swale deposits.
The deposits are very weakly indurated following deposition by precipitated carbonate. This allows them to withstand deflation to form the mesas. The muddy unit C is also slightly more resistant than the sands to erosion, so form overhanging caps. The mesas show small scale landforms showing wind erosion, including polished surfaces and ventifacts (Figure 5).
Several swales in the Hunder dunes were flooded at the time of our visit. These ponds lacked mud, however, so we interpret the water as being due to either a raised groundwater table, or rainfall runoff, or both. The sale flooding from the river would carry mud, as the suspended sediment load of the Nubra River is substantial.
While the dune swales can be flooded by the Nubra River we expect the deposits to accrete to the height of the river flood. When isolated from the river by dune migration, erosion of the swale deposits can begin, leading to formation of the concave upward wedding cake inverted mesas
Jonathan Clarke has more than 40 years professional experience in industry, government and university sectors He is an associate of the Australian Centre for Astrobiology, Adjunct faculty of the Amity Centre of Excellence in Astrobiology, and post-graduate instructor in astrobiology at Swinburne University