| Popis: |
The Lower Triassic Virgin Limestone Member of the Moenkopi Formation provides a holistic picture of the lag phase of the biotic recovery in the aftermath of the end-Permian mass extinction. This unit records deposition in siliciclastic and carbonate nearshore to mid-shelf palaeoenvironments in the western United States during latest Early Triassic (Spathian) time. Nearshore siliciclastic-dominated units are characterized by a relatively diverse trace fossil assemblage, including vertical traces such as Rhizocorallium and Arenicolites , but record low ichnofabric indices. Middle shelf carbonate-dominated units, however, contain a low diversity trace fossil assemblage dominated by simple horizontal traces and tend to record high ichnofabric indices. Subtidal microbial structures occur as wrinkle structures in siliciclastic strata and stromatolitic and thrombolitic buildups in carbonate strata. The strong variations in trace fossil assemblages, ichnofabric indices, and microbial development across the onshore-offshore transect demonstrate that an examination of a variety of facies in different palaeoenvironments is required to reveal the true picture of the biotic recovery. In addition, the prolonged absence of deep bioturbation during the Early Triassic and the distribution of microbial fabrics in subtidal carbonates and siliciclastics indicates a return to Early Phanerozoic-style substrates and suggests that latest Early Triassic shelf environments continued to experience environmental stress, perhaps episodically, for 4–7 million years after the end-Permian mass extinction. |
