SEQUENCE STRATIGRAPHY AND CAMBRIAN TRILOBITE EXTINCTIONS

In the Great Basin and Oklahoma, the overall context of trilobite extinction at the base of the Late Cambrian Sunwaptan Stage (“Ptychaspid Biomere”) is one of rising sea level. Basic principles of sequence stratigraphy suggest a predictable relationship between relative sea level change and apparent tempo of faunal change. Major facies shifts associated with platform drowning or back-stepping will include condensed intervals, so that extinctions will inevitably appear to be abrupt events. An example occurs in eastern Nevada, where the base of the Sunwaptan Stage (Irvingella major Zone) is marked by a sharp shift from shallow subtidal carbonates to deeper subtidal facies. The I. major Zone is a strongly condensed interval (< 10 cm thick) of grainstones, packstones and rudstones at the top of the shallow subtidal succession, so that the turnover of trilobites associated with the upper and lower boundaries of this zone appears remarkably swift. In contrast, areas in which sedimentation keeps pace with the rate of increase in accommodation space will show little or no change in facies at the extinction event. The Wichita Mountains of Oklahoma provide an example. Here, the Honey Creek Limestone records deposition in an archipelago of igneous islands that was gradually submerged during the late Steptoean and early Sunwaptan. As sea level rose, shallow water environments persisted and sedimentation rates remained high through the extinction interval. The succession comprises sandy bioclastic carbonates with no discernable facies changes. However, the I. major zone is expanded into a two meter interval of stacked trilobite and orthid brachiopod rudstones. Although still geologically rapid, the perspective gained on the extinction in Oklahoma differs from the nearly instantaneous event apparently recorded in Nevada. One of the more vexing issues in the analysis of Cambrian trilobite extinctions has been the absence of a consistent signature of environmental change. The data presented here indicate that contrasts between regions in the extent of facies change are simply consequences of the local sequence-stratigraphic architecture. Further progress in understanding the nature of these extinctions will require integration of regional stratigraphic analyses with faunal studies.