DJADOKHTA FORMATION CORRELATIVE STRATA IN CHINESE INNER-MONGOLIA - ANOVERVIEW OF THE STRATIGRAPHY, SEDIMENTARY GEOLOGY, AND PALEONTOLOGY AND COMPARISONS WITH THE TYPE LOCALITY IN THE PRE-ALTAI GOBI

Because the Bayan Mandahu redbeds of Inner Mongolia share similar sedi mentary facies and fossil assemblages with the Djadokhta Formation of pre-Altai Gobi, the two units are interpreted as stratigraphic correla tives, both of Campanian age. Sedimentary facies indicate that the Bay an Mandahu redbeds were deposited in semiarid, alluvial to eolian envi ronments. An assemblage of fossil vertebrates found in the Bayan Mandu hu consists of ceratopsian, ankylosaurian, and theropod dinosaurs; tur tles; crocodiles; and small lizards and mammals. Six different kinds o f fossil vertebrate eggs are present. The most common fossil vertebrat es occur in association with eolian deposits and are interpreted as th e remains of autochthonous ''faunal'' components, many of which died i n situ during sandstorm events. In contrast, rare and fragmentary spec imens of large dinosaurs occur in coarse-grained alluvial deposits and are interpreted as the remains of allochthonous faunal components. Th e low diversity of this fossil assemblage and overall small to medium size of its constituents indicate a relatively stressed paleoenvironme nt, an interpretation which is compatible with our sedimentological co nclusions. A diverse trace fossil assemblage is present and includes r hizoliths and endogenic traces. Endogenic traces are well preserved an d typically associated with eolian deposits, suggesting that the depos its were at least seasonally damp and cohesive. The opinion that the L ate Cretaceous Gobi Basin was a large inland lake, still advocated by some authors, cannot be maintained within the context of our sedimento logic and paleontologic data. In contrast with the perennial lacustrin e sedimentation that was characteristic of the underlying Lower to low er Upper Cretaccous units in the Gobi Basin, the Bayan Mandahu redbeds and correlative Djadokhta Formation mark a pronounced shift toward eo lian and intermittent lacustrine sedimentation in an increasingly arid climate.