Acritarch assemblages from the Middle–Upper Ordovician in the Aksu area, northwestern margin of the Tarim Basin, Xinjiang, China

Abstract

Upper Darriwilian to middle Katian (Ordovician) outcrops are well exposed in the Kalpin (Aksu) area, in the northwestern margin of the Tarim Basin, Xinjiang, China. The Dawanxigou and Sishichang sections in the Kalpin area, near the Dawangou Auxiliary Stratotype Section and Point (ASSP), were palynologically investigated. Both sections provide well-preserved acritarch assemblages dominated by acanthomorph, sphaeromorph and netromorph morphotypes, attributed to 36 species and 20 genera. A comparison of the acritarch assemblages with coeval microfloras from other parts of the world confirms a wide cosmopolitism distribution, indicative of a breakdown of acritarch provincialism during the Late Ordovician. Several acritarchs from the Tarim Basin are restricted to the Late Ordovician, and as they are distributed worldwide, they present a potential for global bio-stratigraphical correlations. The acritarch assemblages from the different sections of the northwestern part of the Tarim Basin show slightly variable compositions, probably caused by different palaeo-ecological settings. The diversity and compositional changes of the acritarch assemblages, combined with the sedimentological evidence, suggest a transgressive sequence from the Sargan Formation to the Qilang Formation.