NEW GRAPTOLITE AND CONODONT FAUNAL DATA FROM THE MIDDLE ORDOVICIAN DAWANGOU SECTION, TARIM BASIN, WESTERN XINJIANG, CHINA, AND ITS IMPLICATIONS FOR BIOGEOGRAPHY

The Middle Ordovician Sargan Formation that is well exposed at the Dawangou Section in the Tarim Basin (Xinjiang Autonomous Region) of western China consists of graptolite-rich black shale with thin limestone interbeds that contain conodonts. Previous studies by Zhou et al., 1992, Wang and Zhou, 1998, and Bergström et al., 1999, provided graptolite and conodont biozonations for the Dawangou section, which was subsequently chosen as the auxiliary Global Stratotype Section and Point (GSSP) for the base of the Nemagraptus gracilis Zone and the base of the Upper Ordovician Series. We have recently conducted a detailed re-examination of the graptolite collections from the Sargan Formation, updating the graptolite identifications and refining the existing graptolite biozonation. In addition to providing important new graptolite information, our study has also discovered abundant conodonts on some bedding plane surfaces, which allows for a more precise conodont zonation of the lower Sargan Fm. The lowermost part of the Sargan Fm. contains a rich graptolite fauna that can be referred to the Pterograptus elegans Zone. At 4.23 m above the base of the formation Pseudamplexograptus distichus and Cryptograptus tricornis first appear, and are closely followed by Hustedograptus vikarbyensis and Didymograptus murchisoni. The latter taxon appears late in the Tarim region relative to its FAD in Great Britian. A distinct faunal change occurs at 8.32 m with the debut of Dicellograptus vagus, and other dicellograptids. Nemagraptus gracilis first appears at 10.11 meters above the base of the formation. The graptolite faunal succession is most similar to the Middle Ordovician succession of Baltoscandia. The conodonts present on the black shale surfaces are typical of deep water, North Atlantic assemblages, and include the taxa Periodon aculeatus, Pygodus anitae, P. serra, P. anserinus, Protopanderodus varicostatus, and Walliserodus sp. This is first report of P. anitae from the lower Sargan Fm. and its presence provides a precise conodont age of upper E. suecicus Zone for this interval. Like the graptolite fauna, the conodonts exhibit strong affinities to Baltoscandian faunas. The faunal similarities indicate that Baltica and Tarim likely occupied similar paleolatitudinal positions during the Middle and early Late Ordovician.