New Stratigraphic Subdivision, Depositional Environment, and Age Estimate for the Upper Cretaceous Djadokhta Formation, Southern Ulan Nur Basin, Mongolia

Abstract

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). 2 NO. 3498 AMERICAN MUSEUM NOVITATES 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 de-position 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 magne-tostratigraphic data document a sequence of normal and reversed magnetozones through the Bayn Dzak Member up into the basal Tugrugyin Member. The presence of reversed magne-tozones 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.

A New Specimen of the Ornithischian DinosaurHaya griva, Cross-Gobi Geologic Correlation, and the Age of The Zos Canyon Beds

“…Dingus et al (2008) carefully documented the geology of the Ukhaa Tolgod beds and suggested a Campanian age for these sediments. This is in general agreement with previous studies of this and other Djadokhta localities (Lillegraven and McKenna, 1986;Averianov, 1997;Gao and Norell, 2000;Dashzeveg et al, 2005). These correlative Djadokhta localities include the red sandstones at Bayn Dzak (Berkey and Morris, 1927) and the white sands of Tugrikin-Shireh (Fastovsky et al, 1997), both in the Middle Gobi near the town of Dalandzadgad.…”

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

“…Yet, similar to the classic Djadokhta exposures at Bayn Dzak, the sequence of strata at Ukhaa Tolgod reflects a mixture of facies E-1, E-2, F, and S in the lower part of the section, with prominent beds of Facies C near the top (Gradziń ski et al, 1969(Gradziń ski et al, , 1977Gradziń ski, 1970;Dashzeveg et al, 2005). Dashzeveg et al (2005) refer the exposures at Bayn Dzak to the Bayn Dzak Member of the Djadokhta Formation.…”

Dinosaur morphological diversity and the end-Cretaceous extinction

“…In the three-bin approach, if taxa were known from the first and third time bin, then they were also assumed to be present in the intervening time bin. The Djadokhta Formation is here considered to be late Campanian in age and the overlying Nemegt Maastrichtian, following the latest work on Asian Late Cretaceous stratigraphy 49,50 . There is some uncertainty regarding whether the Nemegt is entirely Maastrichtian or if its base may be late Campanian in age.…”