ANALYSIS OF FOSSIL CONCENTRATIONS WITHIN THE PINYON PEAK LIMESTONE, GREAT BASIN, U.S.A.: IMPLICATIONS FOR THE ECOLOGICAL EFFECTS OF THE LATE DEVONIAN MASS EXTINCTION

The Late Devonian mass extinction was one of the most ecologically devastating Phanerozoic extinctions. Although the effects of this extinction are well known from studies examining the decline of reef ecosystems, the normal marine benthic soft-substrate realm was also heavily affected. Examination of Devonian and Mississippian aged limestones from the western U.S. demonstrates that there was a major faunal shift across this extinction from a brachiopod dominated soft substrate fauna in the Devonian to a crinoid dominated soft-substrate fauna in the Mississippian. However, new data from the Famennian aged Pinyon Peak limestone indicates that a crinoid dominated soft-substrate system was established by the Famennian.

The Pinyon Peak limestone crops out at localities across western and central Utah, and is interpreted as having been deposited in a shallow shelf environment (Bocko, 1997). This unit is typically between 90 and 120 meters thick, and is characterized by fine-grained, thin (10’s of cm) laterally continuous beds that form ledges and small ridges. Four localities of complete or nearly complete sections of the Pinyon Peak were logged. Although a few thin packstones are present, the majority of the lithology is composed of lime mudstones and wackestones. Fossils include brachiopods, gastropods, and rugose corals. However, the largest portion of material is fine-grained (< 1 mm) crinoid debris, which forms distinct beds in portions of the outcrop, and is responsible for a large portion of the fine-grained material that forms the majority of this unit. Although these fine-grained encrinites are quite different from the typical coarse-grained crinoidal sands of other encrinites, they nonetheless indicate significant input of echinoderm material.

The encrinites within this Famennian limestone are in marked contrast to Frasnian aged materials from similar depositional environments that lack large amounts of crinoid debris, but they do resemble Mississippian aged encrinites. Thus the findings of this research have important implications for understanding the ecological changes associated with the Late Devonian mass extinction, since the major change in faunal dominants within the soft-substrate benthic realm signals the development of a new benthic ecosystem.