Early Paleozoic Biochronology of the Great Basin, Western United States

摘要

The Ibexian Series, first proposed by Hintze (1982), is the accepted chronostratigraphic unit for the Lower Ordovician of North American usage. The lower boundary stratotype of the Ibexian Series is defined at a point in rock 39.1 meters (128.3 feet) above the base of the Lava Dam Member of the Notch Peak Formation in the Lava Dam Five section of the Steamboat Pass-Lava Dam composite section of Hintze and others (1988). The lower boundary of the Ibexian Series coincides with the lowest observed occurrence of Cordylodus andresi Viira and Sergeeva in Viira and others, 1987, which also defines the base of the Hirsutodontus hirsutus Subzone of the Cordylodus proavus Zone. This horizon is about 0.1 meter (4 inches) below the lowest trilobite sample assigned to the Eurekia apopsis Zone. The top of the Ibexian Series is defined as the base of the Whiterockian Series, which, at its stratotype, is drawn at the base of the Tripodus laevis Conodont Zone and coincidentally with the base of the Paralenorthis-Orthidiella Brachiopod Zone (=Zone L of Ross, 1951). In the type area of the Ibexian, the base of the Tripodus laevis Zone is 6.3 meters below the top of the Wah Wah Limestone.