Eustatic sea-level patterns from the Lower Silurian (Llandovery Series) of southern Norway and Estonia

Abstract

This study demonstrates the utility of checking for eustatic sea-level events correlated between passive epicontinental seas and those on the margin of orogenic activity. Bathymetric profiles interpreted on the basis of stratigraphic data from the Lower Silurian (Llandovery Series) in five districts of the Oslo region of southern Norway are compared with similar data derived from five bore-hole sites in the Baltic region of Estonia. These regions preserve extensive marine strata deposited on the Silurian continent Baltica. The data are tested for intra-regional and inter-regional conformity. Despite the fact that the Oslo region was affected by the Caledonide Orogeny, the same four highstands in sea level were recorded there as on Estonia9s comparatively stable platform. Expressed in terms of the established geochronologic standard and its faunal zones, coeval highstands in sea level occurred across Baltica during latest Rhuddanian time, in mid-Aeronian time (as marked by the basal Monograptus sedgwickii or Stricklandia lens progressa zones), in early Telychian time ( M. turriculatus or S. laevis zones), and in late Telychian time near the Llandovery-Wenlock boundary. This timing corresponds to four highstands in sea level previously documented in North America and to at least three high-stands found in South China. Claims for Early Silurian eustasy now rest on data collected from three independent paleocontinents. If the Llandovery Epoch lasted 10 m.y., then sea-level cyclicity was on the order of 2.5 m.y. A combination of tectono-eustastic and glacioeustatic causes probably was responsible for these cycles.