WEYLA OF THE SUNRISE FORMATION

Weyla (Nielsen, 1963) New York Canyon, Nevada

A lovely example of the large bivalve, Weyla (Nielsen, 1963), from the earliest known Jurassic Ferguson Hill Member (Hettangian and Sinemurian) of the Sunrise Formation in the New York Canyon area of west-central Nevada, USA.

The end-Triassic mass extinction was global, severe, and accompanied by worldwide disturbance to carbonate ramp and platform sedimentation. We see the effects played out in the Ferguson Hill Member of the Sunrise Formation. These outcrops are the result of 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.

The Ferguson Hill Member (Hettangian and Sinemurian) of the Sunrise Formation in the New York Canyon area of west-central Nevada, USA has a lovely counterpart in the Rockies of British Columbia, Canada, explored over three field seasons in the early 2000s before being closed off as a provincial park.

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.