Variability of Middle Triassic vermicular limestone ichnofabrics: the Anisian Hisban Formation in Jordan

Abstract

The strongly bioturbated Middle Triassic (Anisian) limestones, referred to as vermicular limestone, are the dominant lithology of the Hisban Formation in Jordan. This limestone exhibits the monotonous Oravaichnium– Planolites ichnofabrics, which are known and common in the northern Peri-Tethys and the Germanic Basin. The most abundant and widespread trace fossil here is Oravaichnium carinatum, which is produced by bivalves. Specimens of O. carinatum from the Hisban Formation are typically strongly elongated in the vertical axis and have a smooth surface, whereas specimens from the Germanic Basin mainly have a pear-shaped cross-section. Nevertheless, both morphologies and transitional morphotypes between these ichnospecies are present in both areas. The difficulty in distinguishing between Oravaichnium and Planolites causes the intense bioturbation by bivalves to be underestimated. The wide occurrence of the Oravaichnium ichnofabric in the northern and southern Peri-Tethys suggests that small burrowing bivalves played a significant role in the colonization and infaunalization during the long-term recovery of the benthos after the Permian–Triassic crisis. They experienced intense development during the Middle Triassic, when they were responsible for extensive bioturbation.