RAPID TURNOVER OF TRILOBITE FAUNAS RELATED TO STRATIGRAPHIC CONDENSATION IN A LOWER ORDOVICIAN "BIOMERE"

Shallow subtidal carbonates of the House Limestone, southern House Range, Utah, preserve an unparalleled record of the Early Ordovician recovery of trilobite faunas following an extinction interval at the end of the Sunwaptan Stage. Apparent turnover and replacement of diverse trilobite species assemblages in the youngest Red Canyon Member of the formation occurs on a meter scale and, taken at face value, is far more rapid than the diversification of trilobites in the Late Cambrian “biomeres”. However, the succession includes numerous complex, amalgamated hardgrounds that appear to represent condensed intervals. The hardgrounds consist of irregular to scalloped surfaces with cm- to dm-scale relief that are overlain by lags of intraclastic rudstone; successive hardgrounds often cut down and truncate both underlying surfaces and rudstones. Commonly, the rudstones are relict sediment confined to topographic lows in the hardground surfaces. Although similar to surfaces that have been interpreted as karst, the hardgrounds occur in successions that lack such peritidal features as planar microbial laminites, fenestral fabrics or desiccation cracks, and are most likely of subtidal origin. Major hardgrounds bracket the stratigraphic ranges of discrete trilobite assemblages; few species cross hardgrounds and individual assemblages have ranges of no more than four meters. Stratigraphic condensation has accentuated the rate of apparent taxonomic turnover, so that a punctuated faunal succession reflects a discontinuous rock record. Diversification recorded in the House Limestone is terminated by a major extinction event that is associated with an influx of terrigenous clastics at an apparent sequence boundary. The data indicate that “biomere” type diversification and extinction patterns extend into the Ordovician but caution against a literal interpretation of apparent turnover rates in local stratigraphic sections.