The first Early Ordovician graptolites and marine incursions in eastern Alborz, Iran

Abstract

AbstractGraptolites have been collected from sections through Lower Ordovician strata in northern Iran. At the Saluk Mountains, in the Kopet–Dagh region, mudrocks yielded fragmentary tubaria ofRhabdinoporasp. cf.R. flabelliformis, indicating the presence of lower Tremadocian strata there; stratigraphically, they lie between two limestone beds with the euconodontCordylodus lindstromi. At Simeh–Kuh in the eastern Alborz Mountains (Semnan Province), upper Tremadocian – lower Floian strata include laminated dark mudstones that contain restricted graptolite faunas, mainly of small declined didymograptids; these are thought to represent incursions of plankton during periods of marine highstands. The lower major flooding surface in Simeh–Kuh coincides with an invasion of the graptolite biofacies and an incursion ofHunnegraptus? sp.; the second major flooding surface is associated with an incursion ofBaltograptus geometricus. They were most probably synchronous with those in the lower part of theHunnegraptus copiosusBiozone and at the base of theCymatograptus protobalticusBiozone in the of the Tøyen Shale Formation succession of Västergötland, Scandinavia, suggesting that observed characters of sedimentation were eustatically controlled.