A TROPICAL RECORD OF TERMINAL FAMENNIAN EVENTS: COLORADO’S DYER FORMATION

The prolifically fossiliferous Dyer Formation of Colorado is of interest because it contains the signature element of the end-Devonian extinction, the Hangenberg Event, and because it may provide insights into the effects of this event on local marine communities. In the White River Uplift of north-central Colorado, large Dyer bedding planes permit pre-, syn-, and post-extinction characterization of infauna, epifauna, and pelagic communities, in tandem with changes in depositional environments, seawater chemistry, and relative sea level.

The base of the Dyer represents a regional flooding surface, and the lower member of the unit is dominated by mottled micritic and nodular lime mudstones, wackestones, and packstones. Intraformational conglomerates, contorted bedding, and quartz granule-rich grainstones also occur. Strata bear abundant corals, brachiopods, orthocone nautiloids, crinoids, gastropods and bryozoans, as well as teeth of holocephalians, acanthodians, elasmobranchs and conodonts. Rare asteroids, sponges, stromatoporoids, stromatolites, and arthrodire plates occur. Trace fossil diversity is low, but strata are well bioturbated. The lower member of the Dyer was likely deposited in a broad, low-relief, tropical open marine setting during transgression or eustatic highstand.

The upper member of the Dyer is dominated by laminated dolomitic mudstones and is nearly devoid of body fossils. Rare brachiopods and trace fossils occur at the base of the upper member. Minor intraformational conglomerates, polygonal cracks, oscillation ripples, and nodular chert horizons occur. Distinctive solution collapse and paleokarst are known from the upper member and are associated with initiation of the Hangenberg Event. These features are internally consistent with deposition under more restricted, evaporitic, and/or shallow conditions.

The Dyer is conformably overlain by a thin, often lensoidal sandy dolostone or dolomitic sandstone, capped by brecciated dolomite and cherty dolomite. Lime grainstones and wackestones occur but are notably barren of fossils.

The Dyer succession’s features suggest extinguishing of a flourishing shallow tropical habitat. This demise immediately precedes the Hangenberg Event and suggests the Dyer system is a sensitive precursor to changing seawater conditions.