ANATOMY OF A CAMBRIAN TRILOBITE EXTINCTION EVENT, HONEY CREEK FORMATION, WICHITA MOUNTAINS REGION, OKLAHOMA

At least five major trilobite extinction events are recorded in the Upper Cambrian–Lower Ordovician succession in Laurentian North America. One of these, at the base of Cambrian Sunwaptan Stage, is expressed in the Honey Creek Formation of the Wichita Mountains region. The Honey Creek was deposited in an archipelago of rhyolite islands during a period of rising sea level and onlap of the Carlton Rhyolite. Sharp lateral facies changes, from sandstone rich to carbonate dominated, occur over distances of less than a kilometer, reflecting the influence of local sources of siliciclastics. The extinction occurs through a stratigraphic interval of about four meters, with turnover at the base and the top of the Irvingella major Zone; there is little associated change in sedimentary facies. The base of the zone is marked by the replacement of a diverse fauna of the Elvinia Zone with a low diversity biofacies dominated by Irvingella and Comanchia. Orthid brachiopods also appear in abundance in the extinction interval, and the succession comprises stacked brachiopod and trilobite shell beds. A further drop in diversity occurs at the top of zone, where Irvingella and Comanchia are replaced by Parabolinoides, which occurs in mixed trilobite-brachiopod shell beds. Diversity remains low through an interval of about 10 meters of strata that is marked by the appearance of the trilobite genus Taenicephalus, and a decline in the abundance of orthids. The data from the Honey Creek Formation demonstrate the extinction was a multistep event with both turnover and immigration of trilobites and a “bloom” of rhynchonelliform brachiopods. The abundance of the latter indicates unusual ecological conditions, as rhynchonelliform brachiopods are generally rare components of marine paleocommunities of the Cambrian of Laurentia.