The Gobi is the great desert that extends along southern Mongolia and northern China. Its northern extent is marked by a series of mountain ranges called the Gobi Altai (Altai means ‘golden’). The mountains appear desert-like, with usually nothing higher than small shrubs growing on them, if anything at all, but isolated patches of forest show was was, and what is still possible. The mountains appear as long northwest-southeast chains, separated by much drier basins. The landscape reminds me a lot of my Central Otago home in New Zealand, but with the Gobi Altai being on a much larger scale.
The mountains are made from old, hard rock. The basins are full of sands and gravels that have eroded off the mountains. Dry though it may be, when it does rain in the Altai the rain can be incredibly intense. Streams that are mostly dry become raging torrents of mud and rocks. Camps get washed away, people die. These are classic conditions for the formation of alluvial fans – an integral part of Mongolian geology. Most of the basins are lined with giant fans. They start at a prominent valley – a notch in the mountain chain, and expand out for kilometers. From the outer base of the fan to the head can be hundreds of meters. From a distance they can look so smooth, but up close the fan surface is covered by steep-sided gullies. These make driving across them a sheer Hell and basically a stupid idea. Most tracks in the Altai are around the bottom edge of the fan, avoiding the fine sediment that blankets much of the more central parts of the basins, and makes bogging a serious problem. If travelling round the base of the fan is not an option, the next best bet is to try and get above it.
Deep gullies running down the fans can have nice exposures of the strata that make up fans. These are typically alternations of gravels and finer beds. The gravels were mostly emplaced by mass-flow processes following intense rainfall events. This was far-beyond silt in suspension or cobbles bouncing along the bottom of streams. Instead, water and rocks mixed to form a kind of wet-concrete like slurry which rushed out of the mountains and spread out over the fan, the rocks being dropped as the water finally filtered away. Finer-grained beds of mud are when floods funnelled down gullies and then overflowed, spreading only the smallest particles. In the dry periods dust-storms redeposited this material as a mantle of silt.
The beds in the fan-gullies parallel the surface of the fan – angling up to where the fan starts, as you would expect. However, along the southern edge of the Khantaishir Range – a sort of satellite of the Altai, the basin is rimmed by what appear to be a dramatic series of multi-coloured ‘teeth’. As geologists, these drew myself and colleagues like a moth to a flame. It turns out that the ‘teeth’ are the result of beds of gravel that have been rotated, so they almost stand on their ends, and then eroded. The basic structure of the gravels looks just like that in the fans, and they probably formed in much the same way – as fans on the edge of the Altai. A closer look at them showed they told a story – of the birth of the Altai themselves.
First question, when faced with beds that are nearly vertical – is working out which way is ‘up’. In this case it was pretty simple to look at the pattern of cross-cutting. Then it became apparent that there was a distinct change in lithology from bottom to top. The oldest gravels were dominated by quartz pebbles – and had good, fluvial cross-beds. However, the youngest beds were dominated by greenschist and cross-beds were absent. The earliest strata seem to record the erosion of a quartz-gravel rich landscape (I’ve seen these in a few parts of the Altai) by genuine river flow. This landscape may date back to the flat ‘peneplain’ that covered the area in the Late Cretaceous-early Cenozoic. The flat-tops of many of the Altai mountains preserve this peneplain. This erosion, probably recording very early uplift of the Altai. Then as the mountains really started to grow, deeper schist became exposed and eroded, and mass-flow took over. Then the mountain uplift seems to have overwhelmed even this area – and it was tilted. Following this, the fans we see today established themselves.
It is a nice little window into Mongolian geology. The Gobi Altai are thought to be related to the collision of India with Eurasia around the Eocene, although they may date back to this time, or perhaps be a late Cenozoic phenomenon (Cunningham et al. 1996; Owen et al. 1999). Unfortunately there seemed to be no fossils to give any clues to the age of the ‘teeth’. Maybe time for a closer look?
Cunningham, W. D., Windley, B. F., Dorjnamjaa, D., Badarngarov, J. & Saandar, M. 1996: Late Cenozoic transpression in southwestern Mongolia and the Gobi Altai-Tien Shan connection. Earth and Planetary Science Letters 140: 67-81.
Owen, L. A., Cunningham, D., Windley, B. F., Richards, B. W. M., Rhodes, E., Dorjnamjaa, D. & Badamgarav, J. 1999: Timing of formation of forebergs in the northeastern Gobi Altai, Mongolia: implications for estimating mountain uplift rates and earthquake recurrence intervals. Journal of the Geological Society: 457-464.