Biostratigraphic correlation of the Ordovician black shales in Tarim Basin and its peripheral regions

Abstract

Ordovician black shales are widely distributed in the Tarim Basin and its peripheral regions, and some of them may serve as potential hydrocarbon source rocks. The present study of the Ordovician graptolite fauna from these shales, together with the yielded conodonts and chitinozoans etc., permits a refined correlation of the rocks. Based mainly on a new collecting of the graptolites and a study of the faunas in Kalpin and Kuruktag regions, and the successful identification of the new graptolite material from a few drill cores within the basin, we are able to update our knowledge of the Ordovician in the regions, and draw some conclusions: (1) The most widespread distribution of the black shales in the Tarim Basin and its peripheral regions, which correspond to the Nemagraptus gracilis Zone, may be related to a global sea-level rise during this time interval. (2) Black shales of Ordovician occur most frequently and extensively in the Kuruktag (also spelled as Quruq Tagh) region in eastern Tianshan Mountains, spanning Tremadocian to mid-Katian (D. spiniferus Zone) temporally and extending southeastwards into Manjiaer Depression. The black shales of the D. spiniferus Zone may even extend into the central Tarim Basin. (3) Three different bio- and litho-facies belts (Bachu: carbonate platform and reef belt; Kalpin-Aksu: marginal platform and upper slope belt; Wushi: slope belt) are recognized in the northwestern Tarim Basin. (4) The internationally well-correlated Saergan black shale, which has been considered to possess high potential for hydrocarbon source rock, may possess a restricted distribution in the Kalpin and Aksu areas.