It is an understatement to say that just getting to the Bogda Mountain localities where Paleozoic–Mesozoic rocks are exposed is easy. It is not. Travel requires 4×4 vehicles with very high clearances, able to sustain the gravel bedload and barforms of the ephemeral braided (anabranching) river beds that serve as the only “main roads” in the region. This movie gives you a brief idea about the ride quality, which goes on for kilometers, and the channel depth when the rivers are at banks full stage, which occurs during short times of high rainfall and flash flooding. In years past, it was not uncommon to be stranded on the floodplain waiting for the waters to recede, or to be caught in a torrent of water rushing down gradient and hoping that barforms would not be topped and vehicles washed downstream. On days when rain was predicted, it was better to wait before thinking about travel.
North Tarlong
The regional depth of weathering and high structural attitude of these rocks necessitates traveling to exposures cropping out in the valleys. This is the case in North Tarlong where access to this area was hampered/restricted by a locked gate, which required some on-site engineering. Once the bar was moved, a descent of ~150 meters along a (single) very narrow and crumbly road became a parallel universe reminiscent of some of the best roller coaster rides (without the free fall excitement). At the base of the valley was a single shepherd whose home consisted of a few yurts, mud-brick animal quarters with corrugated tin roofs, and a vegetable garden irrigated by a small stream that watered the riparian vegetation marking its path.
The high dip angle of the beds exposes the bedding surface of a coarse conglomerate river system in which logs are preserved by (petrifaction) permineralization with silica. This type of plant-fossil preservation has been found in <2% of the latest Permian to Middle Triassic rocks in the Bogda Mountains, and coming across a large assemblage is uncommon and rare. The preserved logs vary in length and orientation (white arrows), and are scattered along the outcrop. The dip angle, along with the instability of the weathered sandy conglomerate, precluded any possibility of mapping them to get a better understanding of the assemblage. Due to a prohibition of even having a drone in the province, aerial imaging and mapping of the assemblage remains a challenge.
Central Tarlong
Once our drivers maneuvered to a familiar parking site from previous field work, a bit of orienteering was required to figure out our footpath route. Dehydration and heat stroke were always possibilities due to the absence of any shade or cover, and return trips to the vehicles were necessary to restock field packs or to get out of the sun. Bogda winds can be high, and these will erase any evidence of vehicle activity within one or more days, making even more important to have gained a sense of visual cues and markers during travel.
The structural attitude of rocks in this part of the Bogda Mountains is very steep. Well, not very steep but extremely steep. Resistant sandstone beds are oriented on a near vertical basis with intervening weathered mudrock increasing their prominence. All mudrock is deeply weathered requiring even greater excavation depths to “find” relatively “solid” rock to sample for plant fossils. Conversely, though, the deep weathering profile coupled with the resistant preservational mode of vertebrate fossils makes these easier to spot as surficial objects to be collected.
Macrofossil plants, on the other hand, are infrequently preserved in these rocks and are only found in <4% of the rock record, and limited to sediments that were deposited in either open lake or lake delta deposits. The high structural attitude and deep weathering profile of Central Tarlong lake deposits renders recovery of plant fossil very (very) difficult. The relief in stress during unearthing of these rocks has resulted in their fragmentation, with pieces not much larger than a few centimeters recoverable. Each blackened object on the bedding surface of these lake deposits, positioned in a near vertical orientation, is a plant remain. Their fragmentary nature can easily be seen in the sample to the left of the arrow.
South Tarlong
The structural geology of the Bogda Mountains is complex and often obscured by the depth of weathering and monotonous coloration in several areas. Structural relationships are easier to identify when prominent, resistant sandstone channel bodies “pop out” of the surrounding weathered mudstone as is the case in South Tarlong. Here, it is easy to identify a synclinal axis (yellow line) that is truncated by faulting (black line with arrows). Although such structural relationships can be identified, correlation of rocks on either side of them is made more difficult because of a dearth of volcaniclastics with which to place geochronometric constraints on the successions. Along with marker beds, where available, high resolution, stratigraphic cyclicity has been employed to understand these latest Permian and earliest Triassic rocks.
In South Tarlong, a thin coal and organic-rich mudstone is a marker horizon that can be traced across this area in the latest Permian. All these rocks are highly weathered as a consequence of the intense surface heat in the summer and winter temperatures approaching freezing. Snow caps the Bogda Mountain peaks, even in June, but rarely accumulates along the flanks or in the Turpan depression in the coldest month of January. As a consequence of these extremes coupled under desert conditions, the weathering profile exceeds 1 m across most of the piedmont. This condition requires excavation at least to that depth just to access highly fragmented rocks for their description and, where present, plant fossils. Recovered plant fossils are typical late Permian wetland elements.
