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Studies of key and newly discovered sections of the Upper Cretaceous Djadokhta Formation along the southern margin of the Ulan Nur Basin allow a new subdivision based on lithology. The formation and its members were mapped at both Bayn Dzak, an area that includes the Flaming Cliffs, and Tugrugyin Shireh, an area about 50 km to the northwest of Bayn Dzak. Stratigraphic sections at both localities were remeasured. The considerably enlarged formation comprises a lower Bayn Dzak Member, dominated by moderate reddish orange sands with subordinate mudstone units, and an upper Tugrugyin Member, composed of pale orange to light gray sands. Investigations of key sections at Tsonzh and Alag Teer demonstrate the presence of transitional mudstone lenses between these members within the Djadokhta Formation. Two distinct, sandy, sedimentologic facies are recognized in both members. Cross-bedded intervals, occasionally exhibiting wind-ripple cross lamination, document the presence of a Cretaceous dunefield in the Ulan Nur Basin. Structureless intervals are interpreted to represent wet sandy fluvial deposits and debris flows that moved down the dune faces. In the Bayn Dzak Member, lenses of brownish mudstone are interpreted to represent interdune deposition in shallow ponds by fluvial action. Fluvial action is also represented in the Bayn Dzak Member by beds of caliche, which contain conglomerate at the base but fine upward into limestone.
The vertebrate fauna from the Djadokhta Formation is summarized. Although the Bayn Dzak fauna lived somewhat earlier than that from Tugrugyin Shireh based on the superposition of the members, it is not clear how much earlier. The fauna from the Djadokhta Formation has previously been assigned ages from Cenomanian to earliest Maastrichtian. New magnetostratigraphic data document a sequence of normal and reversed magnetozones through the Bayn Dzak Member up into the basal Tugrugyin Member. The presence of reversed magnetozones establishes that the sediments containing the faunas were probably deposited after C34n. The quick stratigraphic succession of normal and reversed magnetozones suggests, but does not clearly establish, that the sediments may have been deposited during the rapid sequence of polarity changes in the late part of the Campanian between about 75 to 71 Ma.
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