Stratigraphy, palaeontology and sedimentology of the Mesozoic formations in the Shiliin Nuruu Syncline (Gobi Altai, Mongolia)

The presented geological map at a scale of 1:75,000 shows a part of the Upper Mesozoic sedimentary belt exposed along the northern slopes of the Mongolian Altai in the Valley of Lakes (Western Mongolia). The Upper Jurassic to Lower Cretaceous formations of the Shiliin Nuruu Syncline, which have both lateral and vertical transitions, reach a total thickness of up to 1000 m. Poorly sorted conglomerates in the bottom of the sequence represent the proximal parts of the alluvial fans and grade up to the cycles of sandstones, mudstones, siltstones, marls, and coal seams of fluvial to lacustrine origin. Relics of terrestrial plants indicate a warm climate. The coalification corresponds to early brown coal. The rich Hauterivian–Aptian palaeontological association containing conchostracans, larvae of giant mayflies, gastropods, bivalves, and fish in the Anday Khudag and the Khulsan Gol formations could be compared with the Jehol Biota in northeast China.