POST-EXTINCTION BIOFACIES OF THE FIRST CARBONATE RAMP OF THE EARLY JURASSIC (SINEMURIAN) IN NE PANTHALASSA (NEW YORK CANYON, NEVADA, USA)

摘要

The end-Triassic mass extinction was global, severe, and accompanied by worldwide disturbance to carbonate ramp and platform sedimentation. We examine the earliest known Jurassic carbonate ramp produced in the back-arc basin along NE Panthalassa following the extinction event to determine the biotic constituents and timing of local ecological recovery. Field observations, fossil surveys, and microfacies analysis focused on the Ferguson Hill Member (Hettangian and Sinemurian) of the Sunrise Formation in the New York Canyon area of west central Nevada, USA. In the Hettangian, post-extinction biosiliceous sedimentation extended to the inner ramp, where an ooid and grapestone shoal marked the outermost extent of a narrow belt of carbonate sedimentation. An early recovery phase in the late Hettangian is characterized by widespread, laterally homogeneous, demosponge-dominated level-bottom sedimentation across the mid-to inner-ramp, in addition to limited trophic tiering (sessile epifaunal suspension feeding and mobile infaunal deposit feeding), substantial ramp aggradation, and poor settling conditions for competitive benthic colonizers (e.g., corals, crinoids, infaunal bivalves). Within 1.6-2 Myr after the extinction (in the early Sinemurian), a late recovery phase is recognized by the appearance of epifaunal grazers (gastropods, echinoids) and suspension feeders (crinoids, solitary scleractinian corals), phototrophic microbialites (oncoids, and possibly photosymbionts within corals), and infaunal deposit or suspension feeders (bivalves). Although the late recovery faunas included more trophic levels than pre-extinction carbonate ramp habitats, development and progradation of the first Jurassic carbonate ramp still relied heavily on sponge, microbialite, and abiotic mineralization.