MAGNETOSTRATIGRAPHIC CORRELATION OF LATE CRETACEOUS DINOSAUR-BEARING LOCALITIES IN THE NEMEGT AND ULAN NUUR BASINS, GOBI DESERT, MONGOLIA

Despite the paleontological significance of the well-known and long-studied dinosaur faunas of the Gobi Desert, Mongolia, a poorly constrained stratigraphic framework and high endemism have ensured that they have been only broadly assigned in age to the Upper Cretaceous, although several common taxa have been recognized in both North America and Asia. Over the last three years extensive paleomagnetic sampling has been carried out in the Early and Late Cretaceous rocks of the Nemegt and Ulan Nuur basins in central and western part of the Gobi Desert in an attempt to refine the chronostratigraphy of the region.

A magnetostratigraphic study of the Barun Goyot and Nemegt Formations of the Nemegt Basin has been completed. The paleomagnetic samples uniformly maintain a measurable magnetic signal up to 625 degrees C in a thermal demagnetization regime, the reversal data are antipodal, clearly differentiated from the present day field direction, and are completely in accordance with that expected for a mid-latitude, northern hemisphere sampling locality. The sections can be quite reliably correlated to the Geomagnetic Reversal Time Scale (GRTS) and clearly lie in the Campanian/Maastrichtian interval that extends from the uppermost part of subchron C33n, through chron 32 into the lower half of chron 31.

A series of shorter sections from the Djadoktha Formation (including the famous Bayn Dzak or “Flaming Cliffs” section) have been recently analyzed from the Ulan Nuur and Khar dzagin els Basins. The reversal pattern found is more difficult to correlate to the GPTS as the sections are much shorter and often quite widely separated, but aided by the regional stratigraphic relationships, a distinct reversal sequence is emerging that allows us to correlate the sections in a preliminary way to the late Campanian through Maastrichtian interval that ranges from C32 to C31. In addition an Early Cretaceous (Aptian?) lacustrine sequence with a rich biota of plants, insects and feathers has been measured, and found to be entirely normal in polarity. Palynofloral analysis indicates that this sequence is pre-Albian in age, and it has been tentatively assigned to C34n (Cretaceous Long Normal). These results indicate that magnetostratigraphy has enormous potential in the Gobi region to correlate Cretaceous rocks between the far-flung intracratonic basins.