The Geology of Ukhaa Tolgod (Djadokhta Formation, Upper Cretaceous, Nemegt Basin, Mongolia)

摘要

The lithostratigrahy and sedimentology of the fossiliferous Upper Cretaceous strata exposed in the Gobi Desert of Mongolia at Ukhaa Tolgod are described and mapped on aerial photos. Topographic features are also mapped by plane table and alidade.Five lithologic and sedimentologic facies are described: E-1, distinctly cross-stratified sandstone with fine structure, interpreted to represent eolian dune deposits; E-2, vaguely bedded sandstone with cross-stratified concretionary sheets, interpreted to represent eolian dune deposits modified by diagenetic formation of slope-parallel concretionary sheets of pedogenic calcite; S, structureless sandstone lacking concretions or cross-strata, interpreted to represent sandslide deposits generated by mass wasting along the lee slopes; C, conglomerate interpreted to represent basin-margin conglomerates washed into the dune field from adjacent topographic highs; and M, mudstone and siltstone interpreted to represent interdune deposition in ephemeral ponds and lakes. Facies E-2 and S have not been reported previously. Eleven stratigraphic sections at various localities within the Ukhaa Tolgod drainage basin are documented. The exposed composite section consists of about 75 m of pale orange sandstones, greenish-brown conglomerates, and brown siltstones that are products of an arid environment. Four schematic cross sections are documented to illustrate the lateral relationships among the five facies.