Trilobite bio- and ecostratigraphy of the Tremadocian Djupvik and Kopingsklint formations (A. serratus trilobite Zone) on southern Oland, Sweden

摘要

Trilobites of the late Tremadocian Apatokephalus serratus trilobite Zone, the Ceratopyge fauna, are abundant and widely distributed across Baltoscandia. During the Tremadocian they occur in the initial stable carbonate deposits on the platform (the Bjorkasholmen Formation in the west; Djupvik and Kopingsklint formations in the east). Two sections at Ottenby and Degerhamn, southern Oland, Sweden, were investigated for trilobite abundance distribution. At Degerhamn and Ottenby the fauna is restricted to 70 and 78 cm respectively. In the Degerhamn quarry the fauna appears in the Djupvik Formation. At both localities the abundance distributions are similar, with an initial dominance of Ceratopyge acicularis and Shumardia pusilla, followed by a marked shift to a dominance of nileid species (Symphysurus angustatus, Varvia longicauda, Niteus limbatus). Comparisons with the Oslo Region showed a remarkably similar distribution pattern, and three biofacies are recognized. In the Oslo Region the Ceratopyge-Shumardia biofacies, the Bienvillia biofacies, and the nileid biofacies are developed. Only two of these are present on Oland, where the short lived drowning represented by the Bienvillia biofacies in the Oslo Region is not recorded. Within the nileid biofacies, Nileus and Varvia are more frequent on Oland than in the Oslo Region. The three biofacies may be applied to other areas of the Baltoscandian platform where this facies is present and potentially be used to discriminate depth gradients.