Abstract

Lower Triassic strata in the western United States and northern Italy have yielded the first reported occurrence of microbially mediated wrinkle structures in shallow subtidal siliciclastic paleoenvironments since the Cambrian. We hypothesize that wrinkle structures formed under reduced infaunalization conditions during the aftermath of the end-Permian mass extinction. These wrinkle structures occur in siliciclastic sedimentary rocks in association with hummocky cross-stratification and, in some cases, trace fossils such as Asteriacites, Rhizocorallium, Planolites, and Gyrochorte that indicate a subtidal paleoenvironment. Wrinkle structures were found in the Campil Member in the Werfen Formation of northern Italy and in the Virgin Limestone Member of the Moenkopi Formation and the upper member of the Thaynes Formation exposed in the western United States. The presence of wrinkle structures is likely related to a unique taphonomic window that allowed for the preservation of these delicate features on storm-dominated siliciclastic shelves. Wrinkle structures commonly formed in a variety of subtidal paleoenvironments during the Proterozoic–Cambrian, but thereafter became restricted to intertidal-supratidal and deep-sea environments. This restriction has been attributed to the increase in infaunalization of metazoans following the Cambrian radiation. The proliferation of wrinkle structures in subtidal settings during the Early Triassic suggests that infaunal bioturbation was reduced after the end-Permian mass extinction and that this reduction lasted for millions of years.

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