Evolution of depositional settings in the Torrey area during the Smithian (Early Triassic, Utah, USA) and their significance for the biotic recovery

Abstract

This work focuses on well-exposed Lower Triassic sedimentary rocks in the area of Torrey (south-central Utah, USA). The studied Smithian deposits record a large-scale third-order sea-level cycle, which permits a detailed reconstruction of the evolution of depositional settings. During the middle Smithian, peritidal microbial limestones associated with a rather low-diversity benthic fauna were deposited seaward of the tidal flat siliciclastic red beds. Associated with siliceous sponges, microbial limestones formed small m-scale patch reefs. During the late middle to late Smithian interval, the sedimentary system is characterized by tidal flat dolostones of an interior platform, ooid-bioclastic deposits of a tide-dominated shoal complex, and mid-shelf bioclastic limestones. Microbial deposits, corresponding to sparse stromatolites formed in the interior platform, are contemporaneous with a well-diversified marine fauna living in a seaward shoal complex and mid-shelf area. The nature and distribution of these Smithian microbial deposits are not related to any particular deleterious environmental condition, highlighting that observed patterns of biotic recovery after the end-Permian mass extinction were directly influenced by depositional settings. Facies evolution and stratal stacking patterns allow us to identify large, medium and small-scale, as well as elementary depositional sequences. Large-and medium-scale sequences are consistent with sea-level changes, whereas small-scale and elementary sequences are better explained by autocyclic processes.

Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna

“…S5 to S11) are by far the most unexpected taxon. Although isolated spicules and specimens of Early Triassic siliceous and hypercalcified sponges are frequently found in the western U.S. basin ( 23 – 28 ), all complete sponge specimens recovered from the Paris Biota belong to the leptomitid protomonaxonids. These sponges differ markedly from all other sponge taxa reported so far from the Early Triassic, including the western U.S. basin ( 24 – 26 ).…”

Palaeobiogeographical distribution of Smithian (Early Triassic) ammonoid faunas within the western USA basin and its controlling parameters

“…; Olivier et al . , ) characterized by the presence of the iconic ammonoid genus Anasibirites and other typical late Smithian ammonoids (Lucas et al . ; Brayard et al .…”

Controlling factors for differential subsidence in the Sonoma Foreland Basin (Early Triassic, western USA)

“…The southern part of the SFB, characterized by low subsidence rates, exhibits coarse clastic sedimentation in the Moenkopi Group with the presence of conglomerates and sandstones (Figs 3a, 4, 5b, c, e, 12; e.g. Gabrielse, Snyder & Stewart, 1983; Olivier et al 2016) of density 2.5–2.8 kg cm −3 (Manger, 1963; McCulloh, 1967; Sclater & Christie, 1980; Tenzer et al 2011). The top of the Moenkopi Group consists of thick microbial limestone beds (Figs 3a, 4, 5e; e.g.…”

“…The top of the Moenkopi Group consists of thick microbial limestone beds (Figs 3a, 4, 5e; e.g. Olivier et al 2014, 2016; Vennin et al 2015). These limestones bear a density of c. 2.6–2.8 kg cm −3 (Manger, 1963; McCulloh, 1967; Sclater & Christie, 1980; Tenzer et al 2011).…”