THE BARUUNGOYOT-NEMEGT TRANSITION (UPPER CRETACEOUS) AT THE NEMEGT TYPE AREA, NEMEGT BASIN, SOUTH CENTRAL MONGOLIA

The Baruungoyot and Nemegt formations interfinger through a minimum of 23 meters of stratigraphic section at Nemegt. This interfingering stratigraphic interval comprises the lower one-half of the Nemegt Formation's stratigraphic exposure in the area. This interfingering formational contact, as well as an upsection increase in conglomeratic sediments in the Nemegt Formation are evidence for progradation of the Nemegt clastics. We infer that during Nemegt time, uplift in the paleo-Altan Nemegt source area increased both sediment supply and the depositional gradient, which, in tum, decreased accommodation. Decreased accommodation resulted in a preservational bias for channel-fill and sheet flood deposits. During progradation, a multi-kilometer-wide Nemegt meanderbelt episodically shifted its position to the southeast, and occasionally back-stepped to the northwest, thus leaving a succession of interfingering Baruungoyot and Nemegt tongues in the stratigraphic record. The meanderbelt hosted medium-size channels (~6 m deep and 75 m wide) that flowed to the southwest. Southeast of the meanderbelt, seasonally wet-dry distal-basin environments of the Baruungoyot Formation continued to exist during the time represented by the interfingering interval. The interfingering interval is the most vertebrate-fossil rich part of the stratigraphic section in this area (Nemegt meanderbelt tongues in particular). Rich fossil OCCUITences in this interfingering interval indicate that many of the Nemegt and Baruungoyot vertebrates from this interval co-existed.