DEPOSITIONAL-ENVIRONMENTS AND FACIES TRANSITIONS OF DINOSAUR-BEARING UPPER CRETACEOUS REDBEDS AT BAYAN-MANDAHU (INNER-MONGOLIA, PEOPLES-REPUBLIC-OF-CHINA)

Campanian redbeds near Bayan Mandahu consist of six lithosomes, which in turn comprise twelve lithofacies. The distribution of these lithosomes along a 12 km escarpment resolves into three coeval paleogeographic zones representing a proximal-to-distal depositional gradient. Zone 1, the most proximal, consists of alluvial, lacustrine, and eolian sediments. It is interpreted as an eolian influenced, distal alluvial-fan or braid-plain environment. Farther basinward, zone 2 is dominated by structureless eolian sandstones and caliche profiles with minor occurrences of alluvial. lacustrine, and eolian dune deposits. It is interpreted as a zone of vertically aggrading eolian sand, stabilized by vegetation and a high water table. Zone 3, the most distal, is dominated by eolian dune and structureless eolian deposits with lacustrine and interdune fluvial deposits. It is interpreted as the southern margin of an eolian dune field that was dominated by transverse and barchan dunes. U-shaped, mature caliche profiles can be traced for up to 1.5 km and are abundant throughout zones 2 and 3. They indicate lengthy depositional hiatuses and a swaley paleotopography. Djadokhta Formation paleoenvironments and fossil assemblages in south-central Mongolia compare closely with zones 2 and 3 at Bayan Mandahu, indicating a consistency in climate and depositional processes throughout the Gobi Basin during Campanian times. The depositional environments of modern arid and semiarid basin margins in Central Asia (e.g., Junggar, Tarim) are excellent analogues for the Campanian environments and paleogeography of the Gobi Basin margin.