The following preliminary report on Khoton Lake and Biluut geology is excerpted, revised, and updated from Whitelaw, Kortum, et. al.'s 'When Stones Speak: Geologic Influence on the Creation of Petroglyphs at the Biluut Complex in the Altai Mountains of Bayan Olgii Aimag, Mongolia,' Proceedings of the Sixth Annual Hawaii International Conference on Arts & Humanities (HICAH), January 11-14, 2008, Honolulu, HI, 23 pp.
LOCATION AND GEOMORPHIC DESCRIPTION
The Biluut Petroglyph Complex is located in an area centered at 48o 39' 01" N and 88o 18' 20" E. This site is located within the Altai Tavan Bogd National Park in Bayan Olgii aimag in far western Mongolia. The area is bordered by China to the west and Russia to the north. The three high hills of the Biluut Petroglyph Complex are part of a series of hills and moraines that trend eastward away from the eastern shore of Khoton Nuur (Lake). Khoton Nuur is the most northerly of a series of three NW-SE trending glacial lakes that parallel the trend of the high Altai Nuruu (mountain range). The lakeshore elevation is 2083m while the locally highest point of the Altai Nuruu, approximately 8km to the south, is 3507m. The drainage divide on the Altai Nuruu locally defines the border between Mongolia and China.
The petroglyph-bearing hills of the Biluut Petroglyph Complex are designated from west to east as Biluut 1, 2 and 3 (B1, B2, B3). B1 has a long axis extending approximately 2.2km, which trends NW-SE. At its widest it measures 1.2km and it achieves an elevation of 2257m. B2 is 2.2 km long, 0.6 km wide; it trends NNW and achieves an elevation of 2200m. B3 is the largest of the three. It has an arcuate shape, concave to the east, is 2.85 km long and 0.8 km wide. It trends N-S at is northern end, NNW at its southern end, and achieves an elevation of 2320m. B1 parallels the eastern shore of Khoton Nuur. Its lowest petroglyphs are located just 35m from the shoreline. The slopes and summit of B1 consist of exposed rock surfaces strewn with glacial erratic boulders of mostly granitic origin. The lower flanks and lake shore margins are covered by glacial till and reworked and/or wind blown sediments. These sediments give way to a sandy swamp to the north. The southwest flank of B1 has a well developed tarn created behind a glacial moraine dam. The southern margin of B1 gives way to a low SE trending ridge of glacial moraine that terminates at the lake shore.
The narrow valley between B1 and B2 is marked by a poorly developed drainage. Glacial tarns occur within this drainage at both the northern and southern extremities. A much broader valley separating B2 and B3 is marked by a well developed drainage that contains Ut-Khuiten Gol, a year-round icy stream fed by mountains located to the north of the study area. The Khuiten Gol drains into Khoton Lake to the south-southeast of B3. Glacially derived erratic boulders, ranging in size from gravel to boulders >10m in diameter are ubiquitous across B2 and B3.
REGIONAL GEOLOGY AND GEOLOGIC OVERVIEW OF THE Biluut Petroglyph Complex
A detailed geologic map suitable for the scale of this project is not available. Regional geologic studies indicate that this region is dominated by early-Paleozoic sediments of the Central Asian Belt and that it occurs within the Altai-Mongolian tectonic terrain (Chang, et. al., 1996; Dorjgotov, et. al., 2004). These sediments consist of slope and shelf sequences subsequently emplaced by collisional events on to the growing China craton (Dobrestov, et. al., 1994; Lamb & Bardach, 2001). Parts of this sequence were subsequently intruded by Late Paleozoic granites. In Mesozoic times terrestrial sequences of conglomerates, sandstones, siltstones, and shales were deposited in basins across the area. These have been well documented in the Valley of Lakes area some 200km east of Biluut (Sjostrom, et. al., 2001), but not in the Khoton Lake area itself. These Mesozoic sediments are interpreted to be Jurassic non-marine basin fill deposited in a compressional or perhaps strike-slip setting (Sjostrom, et. al., 2001). This region of Central Asia is now undergoing extensional deformation as a consequence of the ongoing India-Asia collision. This has produced a series of transpressional pull-apart basins that are now receiving sediments from higher elevations.
The Biluut study area is bordered by Khoton Nuur to the west which, in turn, is bordered by the high Altai Nuruu. These relationships clearly indicate that Biluut occurs on a down-dropped block within the regional tectonic framework. The local geology is dominated by basal laterally extensive sequences of matrix supported quartz pebble and polymict conglomerates, sub-arkosic to orthoquartzitic sands, silts, and shales. Across the three hills of the primary study area sediments show a general trend of fining upwards towards the east and south. Small shell fossils were found on B3 in 2012, but have yet to be identified, so their age is currently undetermined. However, the interpretation of these sediments as a transitional sequence of fanglomerates to fluvial flood-plain to overlying deltaic sediments is consistent with detailed descriptions of Jurassic basin fill sequences to the east (Sjostrom, et. al., 2001).
Local Geology of the Biluut Petroglyph Complex
All three hills of the study area are dominated by clastic sediments. The west face of B1 is dominated by matrix-supported quartz pebble and polymict conglomerates that occur interbedded with sands and minor shales. These give way to well bedded and relatively continuous sands and shales to the south and east. Channel forms and cross-bedding is common in these sequences. B2 is characterized by a similar sequence. The fining upwards sequences, channel forms, and cross-bedding are consistent with interpretation as an aggradational basin fill sequence, perhaps deltaic in nature. B3 also preserves a sequence that fines to the east. However, it lacks significant volumes of conglomerate and is, instead, dominated by sands and shales on its western flanks and shales to the east. B1 preserves a simple structure with bedding showing uniform dips to the northwest at an average angle of 52.5o NW. Bedding on B2 also uniformly tilts to the northwest at an average dip of 49o NW. A sub-vertical foliation becomes apparent, particularly in the shales, on the east side of B2. This easterly increase in deformation becomes even more apparent in B3, which exhibits considerable structural deformation and heavily distorted beds across the entire hill. The increase in structural complexity towards the east is at least in part due to the increasing volume of fine grained sediments and a resultant reduction in lithologic competency.
Rock Color and Varnish Development
Fresh faces of these sediments preserve a variety of colors ranging from red-browns (Munsell 2.5Y 3/1) for the conglomerates and some sands, to greens (Munsell Gley 1 6/10Y) and grey-greens (Munsell 2.5 Y 2/1) for the sands and shales. These rocks weather to shades of red, brown, and grey-black to aqua blue-green and green. These surfaces are, in turn, modified by a variety of rock varnishes that vary significantly in terms of their development, from being wholly absent to being highly developed, smoothly polished and shiny. The contrast in color and hue between the underlying rock, especially when pulverized by pecking, and the weathered and/or varnished surfaces appears to have been a key attraction for the rock artists.
Glacial Influence on the Biluut Petroglyph Complex
Evidence for glacial activity is ubiquitous throughout the Biluut study site. Clearly visible in the higher elevations, modern glaciers feed Khoton Nuur, which was itself carved out by glacial action. Locally, tarns, glacial moraines, and striated pavements dominate the landscape. Huge numbers of glacial erratics, transported and abandoned boulders of mostly granitic origin, dot the landscape and occur at all elevations from lake level to the ridgelines and summits of all three hills. These sizeable boulders clearly indicate that recent local glacial maximums produced sufficient ice to entirely cover the area and abrade a series of terraces and panels onto the Biluut Hills. Striation directions indicate general ice flow from NW to SE. This is consistent with trends of the valleys in the area and with the location of the High Altai, a likely source of much of the glacial ice to the northwest. The presence of large numbers of Bronze Age and some pre-Bronze Age petroglyphs make it highly probable that the last local glacial maximum pre-dates the pre-Bronze (Neolithic, Mesolithic, and Paleolithic) era.
Geologic Controls on the Development of Planar Surfaces for Rock Art
Planar surfaces are developed on bedrock exposures across the Biluut hills for a variety of reasons. These include surfaces developed by the abrasive action of glacial ice, exposed bedding planes, and joint (fracture) planes. These planar surfaces are significant, as they were favored by the local rock artists. Glacial surfaces exhibit general NW to SE trends and vary from sub-vertical to sub-horizontal in orientation. Different lithologies show little apparent variation in resistance to glacial polishing. In many situations, alternating beds of conglomerates, sands, and/or shales were planed down to a single flat surface. Planar surfaces have also been created by a variety of structural geologic processes. The dip of beds to the northwest and the consequent exposure of bedding planes on west-facing slopes created flat surfaces utilized by the ancient image makers. The structural tilt, NW-SE motion of glacial ice, and surface topography have combined to create significant numbers of planar rock surfaces on the western slopes of B2, but very few such surfaces on the eastern escarpment. Structural modification of B2 sedimentary sequences has provided a third mechanism of planar surface development. Here tilting of beds has been accompanied by joint surface (fracture) plane development. These planes are a response to regional structural stress and generally grow perpendicular to strike. These, too, were utilized by the petroglyph artists.
Geologic Field Methods
Geologic studies were carried out in July 2007 via traditional field methods. Outcrop location, lithology, and structural information were mapped onto a base map created from a 1:200,000 scale Russian military map which was digitally enlarged to 1:25,000 scale. Bedding dip, glacial pavement surfaces, joint plane, and panel orientation data were all measured with a Brunton Transit Compass. Colors of both rock and petroglyph surfaces were classified based on comparison to a standard Munsell soil color chart. Grainsize was determined by visual comparison to a standard sedimentary grain size chart. Initially, depth and profile of the pecked surfaces were classified by naked-eye observation and hand measurements.