Arenig trilobite associations and faunal changes in Southern Shaanxi, China

Abstract

Stratigraphic ranges of 22 Ordovician trilobite taxa are recorded across the Chaochiapa and Siliangssu formations (Arenig) at two localities in southern Shaanxi, representing shelf facies close to the northwestern margin of the Yangtze Platform, with additional material recorded from two spot-sampling localities in the same region. These are assigned to at least three benthic trilobite associations: a deep inner shelf Neseuretus Association, occurring in clastic facies, and outer shelf Trinucleid and Nileid-Asaphid associations, occurring in clastic-carbonate and carbonate facies, respectively. The occurrence of a possible fourth association, a Taihungshania Association apparently occurring in shallow inner shelf clastic facies, is also suggested. Stratigraphic occurrence of these associations at the collection localities is interpreted as reflecting the Arenig transgression. Although many of the recorded trilobite species are widespread across the Yangtze Platform during the Arenig, most of the benthic associations recognized herein are restricted to southern Shaanxi. The sediments in this region are interpreted as having been deposited under slightly shallower palaeobathymetric conditions than those in western Hubei during the Arenig, although an older record of a cyclopygid trilobite from the Siliangssu Formation may challenge this idea.