A UNIQUE MIDDLE ORDOVICIAN K-BENTONITE BED SUCCESSION AT ROSTANGA, S.SWEDEN

An approximately 8.5 m thick sequence of upper Viruan (upper Middle Or dovician) shales, mudstones, and limestones in an outcrop at Kyrkbacke n near Rostanga in W-central Skane contains 19 K-bentonite beds, sever al of which are as much as 40-67 cm thick. Thirteen of these beds are in the upper part of the Sularp Fm., four in the Skagen Fm., and two q uestionable beds in the Mossen Fm. Evidence from macrofossils, chitino zoans, and conodonts are used for biostratigraphic age assessment of t he K-bentonite succession. Regional comparison of the sequence with th ose at Kinnekulle (Kullatorp), Koangen, and Tommarp suggests that its total stratigraphic thickness is smaller than those at the two former sites but the thicknesses of several of the Kyrkbacken ash beds are gr eater than those in similar stratigraphic positions in the other succe ssions. The K-bentonites at Kyrkbacken have a similar clay mineralogy and major and trace element composition as other Ordovician K-bentonit es, and these data indicate that the parental magma was of felsic, pro bably rhyolitic composition. Based on amphibole geobarometry, the magm a chamber is interpreted to have been at a depth of 14-20 km. The rela tively large number of unusually thick ash beds of Middle Ordovician a ge makes the easily accessible Kyrkbacken outcrop unique not only in B altoscandia but, as far as we are aware, also on the entire northern h emisphere, and only one comparable exposure is known in the southern h emisphere, namely in the Precordillera of northern Argentina.