We performed micro-fossil analysis by means of microscopic observation of smear slide of clayey sediments, taken from three drilled cores of the Kalimati Formation at Rabibawan (R), Tri-Chandra campus (T), and Pulchok (D) at 1 m interval (Fig. 2). Number of sponge spicules, phytoliths, pollen including spore, plant fragments, excepting diatom, were counted until a total of 200 pieces were reached under a magnification of 400, using Nikon ECLIPSE 50i POL, and each ratio was calculated.
In order to determine depositional age, 5–15 g homogeneous clay with very fine grained carbonaceous fragments were collected from the cores (Fig. 3) and exposures. Accelerator mass spectrometer (AMS) radio carbon ages were measured by Paleolabo Co. Ltd., Japan, and the obtained Libby ages were calibrated to calendar year using a calibration program of CALIB 7 (Stuiver et al. 2013).
Changes from lacustrine Kalimati Formation to deltaic Sunakothi Formation
The Sunakothi Formation (Sawamura 1994; Sakai 2001b) is distributed in the southern Kathmandu basin, forming lacustrine terraces (Fig. 4a). They are gently inclined toward the center of valley starting from an altitude of 1395 m in the south to 1302 m in the north (Fig. 3a). The total thickness of the formation decreases toward the north from 35 m at Jorkhu to 15 m at Ekantakuna (Fig. 3b). The northern limit of the Sunakothi Formation is along the Manohara and Hanumante rivers flowing from E to W in the center of the basin (Fig. 2a).
The top of the Kalimati Formation was eroded before the deposition of Sunakothi Formation, and the erosion surface is marked by the presence of lag deposits of meta-sandstone granule and carbonaceous wood fragments (Fig. 4b). Terrace gravel bed of a few meter thick is unconformably lying on the top of the uppermost bed of the Sunakothi Formation (Fig. 3b).
The Sunakothi Formation is divided into three parts: lower prodeltaic part, middle delta front part, and upper prodeltaic part (Fig. 3b). The lower part at Sunakothi Formation is characterized by thin interlayered bed of fine sand and carbonaceous silty mud (Fig. 4d), showing longitudinal cross-bedding, and wave- and current-ripple bedding. Those sedimentary structures are ubiquitously destroyed by bioturbation. The middle delta front part is characterized by large-scale cross-stratified thick sand bed and convoluted bed of slump origin (Fig. 4c). Large-scale planar cross-stratification shows northward paleo-current directions (Fig. 4e). The upper prodeltaic part comprised rhythmic sequence of thin lenticular and cross-laminated sand bed and carbonaceous mud bed. The lower half is sand dominant and upper half is mud dominant and has rhythmic sequence (Fig. 4f) with destructive bioturbation.
In the northern margin of the southern Kathmandu basin at Ekantakuna, there lacks the lower and middle parts of the Sunakothi Formation (Fig. 3b). The Kalimati Formation gradually changes into the mud-dominant rhythmite beds of the upper prodeltaic sediments of the Sunakothi Formation with 1-m-thick transition zone (Fig. 3b).
A coarsening upward sequence from the Kalimati to the Sunakothi Formation represents an environmental change from lacustrine to prodelta and delta front, indicating a progradation of lacustrine delta after erosion of the Kalimati Formation.
The beginning of deposition of Sunakothi Formation
In order to determine the age of erosion of the top of Kalimati Formation and beginning of deposition of the Sunakothi Formation, we preformed AMS14C dating for different samples taken from four localities at Jorkhu (J), Chhampi (CP), Chhyasikot (CK), and Ekantakuna (E) (Figs. 2a, 3a).
A carbonaceous mud sample taken from 5 cm above the base of the Sunakothi Formation at Jorkhu (Fig. 4b) yielded 44,120 ± 460 yr. BP (46,729–47,904 cal yr. BP), and that from the Kalimati Formation 3 m below the base of the Sunakothi Formation yielded 42,190 ± 370 yr. BP (45,149–45,829 cal yr. BP). A sample collected from the Kalimati Formation at 6 m below the base of the Sunakothi Formation in a drilled core at Chhampi yielded 45,260 ± 710 yr. BP (47,870–49,491 cal yr. BP). At Chhayasikot, one lacustrine clay sample was collected at 50 cm below the base of the Sunakothi Formation, and this sample yielded 44,700 ± 650 yr. BP (47,184–48,804 cal yr. BP). These data indicate that the depositional age of sediments at the base of the Sunakothi Formation in the southern area of the Kathmandu Valley is ca. 48 ka.
On the other hand, AMS14C age of the basal part of the Sunakothi Formation at Ekantakuna near the center of the basin shows 33,300 ± 160 yr. BP (37,187–37,922 cal yr. BP) and that of the uppermost part of the Kalimati Formation shows 35,970 ± 220 yr. BP (40,321–40,902 cal yr. BP). It suggests that deposition of the lacustrine clay has continued at least till ca. 40 ka in the central part of the basin.
When the Paleo-Kathmandu lake completely drained out?
As the answer to the above question lies in knowing the age of the youngest Kalimati Formation, we collected a carbonaceous clay sample of this formation from 1 km to the south of southern edge of the Tribhuvan International Airport of Kathmandu (Fig. 2a) at an altitude of 1296 m, and lying near the bank of the Manohara river in the central part of the basin to perform AMS14C dating. The dated sample gave the youngest age of 10,485 ± 40 yr. BP (12,405–12,531 cal yr. BP). This data narrowly constrain the date of the final drying out of the Paleo-Kathmandu Lake to ca.12 ka and also indicates that the deposition of the Kalimati Formation in the central part of the basin continued till this date.
Changes in micro-fossils assemblage in lacustrine sediments
Microscopic observation and counting of ratio of four proxy [phytoliths, sponge spicule (Fig. 5b), plant fragment, pollen] and number of Pediastrum (Fig. 5c) were performed (Fig. 6), in order to reveal environmental changes in and around the Paleo-Kathmandu Lake. In addition, charcoal grain (Fig. 5a) analysis was carried out to clarify the paleoclimatic changes during the late Pleistocene. A 60-m-long core between 9.4 and 61.4 m in depth of drilled cores recovered at Rabibhawan was used for the study. Additional study was carried out for two drilled cores at Tri-Chandra campus (TC core) and at Pulchok (DPTC core).
Phytoliths of Bambusoideae (Fig. 5d) abruptly increased its number at depth from 37.4 to 31.4 m and that from 26.4 to 24.4 m, and their peaks are at ca. 48 and 38 ka, respectively (Fig. 6). The ratio in four proxy drastically increased more than 90 % and attained 98 % at maximum, though it is usually less than 10 %.
At the same depth, number of sponge spicule abruptly decreases its number from average 193.4 pieces/g to less than 50 pieces/g at around 48 and 38 ka. The ratio of sponge spicule also decreases up to 2 %. In addition, number of charcoal grain also decreases from average 2675 pieces/g to less than 100 pieces/g at the same periods. The ratio of plant fragments including charcoal grain also decreases to less than 10 % and minimum ratio is 1 % at 34.4 m in depth.
Pediastrum is an inhabitant of shallow water environment, because it produces energy by photosynthesis. It was also detected from RB core at the same depth, and its number reaches maximum of 45 pieces/g at 35.5–34.5 m depth.
Similar abrupt increase of Phytoliths of Bambusoideae at two horizons are detected at 54–52 and 47–44 m in depth of TC, and at 35 and 31–30 m in depth of DPTC cores (Fig. 2a). When Phytoliths of Bambusoideae is dominant, number and ratio of sponge spicule and plant fragment decrease same as in RB core. Thus this event is not a local phenomenon but widespread in the whole lake.