GETTING MIXED SIGNALS: EFFECTS OF SEDIMENTARY REWORKING ON SAMPLED DIVERSITY IN THE ORDOVICIAN OF THE BASIN AND RANGE (UTAH AND NEVADA)

Reconstructing temporal and environmental diversity trends is one of the principal goals of paleoecology. Because of concerns about the interpretation of synoptic datasets, paleoecologists have increasingly focused on local (alpha) diversity patterns. However, these are also subject to numerous complications. Sedimentary reworking by storms and biological activity can influence sampled diversity patterns by homogenizing faunal patchiness patterns and by mixing the remains of individuals which did not live contemporaneously. It can also alter the nature of the substrate and facilitate colonization by previously excluded taxa. Though long recognized as an issue, reworking effects on sampled diversity have rarely been quantified. We present data from co-occurring beds representing storm sedimentation and "background" sedimentation in the Ordovician Pogonip Group of Utah and Nevada. Samples come from the shallow water mixed carbonate-clastic Fillmore and Wah Wah formations and from the more offshore, clastic-dominated Kanosh and Ninemile formations. Each sample represents a single bed <10 cm in thickness. Comparison of rarefaction curves demonstrates that in each formation, sampled richness is significantly higher in grainstone event beds than in co-occurring shales and wackestones. In shallow carbonate-clastic settings, the difference is primarily ascribable to increased temporal and spatial mixing; the faunal composition of grainstone samples is similar to that of the pooled shale and wackestone sample sets. In more offshore, clastic-dominated settings, shale and grainstone faunas are relatively dissimilar; the higher sampled diversity of grainstone event beds results largely from inclusion of pedunculate brachiopods, crinoids, bryozoans, and other taxa excluded from soft substrates. These results confirm that even within a single depositional environment, type and rate of sedimentation can exert a strong influence on sampled diversity. This should be taken into account when examining alpha diversity gradients across sedimentologically disparate units.