LATE CRETACEOUS DINOSAUR-BEARING PALEOENVIRONMENTS OF BUGIN-TSAV, SOUTH-CENTRAL GOBI DESERT, MONGOLIA

For 70 years, Russian paleontologists scoured the Gobi Desert looking for spectacular fossil localities. Among the most spectacular is Bugin-tsav, a five square mile locality exposing about 50 m of Campanian-Maastrichtian-aged sandstones and mudstones. The accepted Russian interpretation of this locality was that it is lacustrine, based upon the presence of bivalves and turtles. Our study there suggests an alternative interpretation for this locality: a meandering fluvial environment. Present at Bugin-tsav is a cross-stratified sandstone facies showing concave-upward channeliform geometry. Unionid bivalve fossils are preserved exclusively in lenticular, coarse sandstone deposits with concave-upward bases, mudstone intraclasts, and unionid shells. We interpret both of these to be the remnants of channel fills; the coarse-grained unionid-bearing units possibly representing channelized crevasse splays. The locality exposes spectacular, three-dimensional arcuate structures bearing inclined heterolithic strata in cross-section, interpreted here as exhumed point-bar deposits. The mudstone deposits preserve high-chroma, red paleosols with root horizons. The paleosols bear caliches, suggesting that the original soil periodically underwent wetting and drying. Some of the mudstones preserve laminations, suggesting suspension settling. These are interpreted to represent ephemeral floodplain lakes. Dinosaurs are not aquatic creatures, although their remains have been found on the margins of water bodies. At Bugin-tsav, however, no clear sedimentary marginal indicators have been found, and evidence suggests that dinosaur carcasses were deposited along the toes of point bars. Nonetheless, dinosaurs lived as well as died on Bugin-tsav point bars, for eggs containing oviraptosauird embryos are associated with the crescentic deposits. It is a tremendous privilege to be able to honor Ed Belt here. The work that we report could not have happened without Ed's having taken one of us (Fastovsky) out into the wilds of the Fort Union and Hell Creek Formations for several weeks, giving a clinic on how it's done right.