Latest Devonian (Famennian, expansa Zone) conodonts and sponge-microbe symbionts in Pinyon Peak Limestone, Star Range, southwestern Utah, lead to reevaluation of global Dasberg Event

Abstract

We document the signature of the late Famennian (Early expansa Zone) multi-phase Dasberg Event, previously termed ALFIE, in Utah and Colorado. The Dasberg was a major event, not a crisis, because it resulted from a eustatic rise that introduced the Etroeungt fauna, consisting of large solitary rugose corals and unusually large brachiopods, from an as yet undiscovered refugium. This “Lazarus” fauna was short-lived; it disappeared within a few million years, as a result of the more intensely studied end-Devonian (D—C) Hangenberg Extinction Event. Our research centers on a meter-thick sponge-microbe symbiont bed at the top of the lower member of the Pinyon Peak Limestone at Elephant Canyon, Star Range, southwestern Utah, USA, which formed during an early phase of the Dasberg Event. This “disaster” bed is well dated by conodont faunas in beds just below and just above. Superficially resembling stromatolites, the meter-thick symbiont bed contains microscopically interlayered sponges and microbialites. Occurring with the symbionts are bryozoans, brachiopods, and megascopic stromatoporoid sponges. This is arguably the first reported occurrence of late Famennian stromatoporoids in North America. Mounds at the top of the biostrome represent the last growth stage of the biostrome, which was arrested by development of a bored hardground. The mounds, which contain the same biota as the symbiont bed, were bored by unknown organisms. By combining our findings at Elephant Canyon with known data on Devonian ∂13C positive excursions, we determined that the biostrome developed at a time of warming during the otherwise cool late Famennian. The presence of a stromatolite-like symbiont bed is suggestive of an extinction event, especially as the environment of deposition represents shallow water, but with normal salinity, allowing growth of a diverse biota. Conodont faunas of the Pinyon Peak Limestone in the Star Range were pivotal in development of a late Famennian conodont biofacies model. Our re-dating of conodont faunas enables an improved correlation of the ∂13C curve between localities in Utah and Colorado.