CONODONTS AND BIOSTRATIGRAPHY OF THE GUNTER SANDSTONE MEMBER OF THE GASCONADE FORMATION IN SOUTHERN MISSOURI

The Gunter Sandstone forms the basal unit of the Gasconade Dolomite in southern Missouri. It comprises beds of quartzose, medium- to thick-bedded, commonly cross-bedded, and sometimes dolomite-cemented sandstone, and thin- to medium-bedded, sometimes quartz-silty to quartz-sandy dolomite. The Gunter ranges from 5 to 15 meters thick in the region and is a regionally significant aquifer. Because the underlying Eminence Dolomite and overlying middle and upper members of the Gasconade originally were thought to lack sandstone beds, the Gunter served as an easily recognizable marker bed for mapping. Our recent geologic mapping has demonstrated that the Eminence Dolomite does indeed locally contain some quartz sandstones in its upper part, some of which are physically indistinguishable from typical Gunter sandstones. Conodont studies from several measured sections from Camden to Carter Counties, including the type section of the Gunter, show that the upper Eminence ranges in age from the Eoconodontus or Proconodontus Zone to at least the Clavohamulus elongatus Subzone of the Cordylodus proavus Zone. These uppermost Cambrian strata are unconformably overlain by the Gunter sandstones sensu stricto. These basal Gunter sandstones yield only unidentifiable, well-rounded fragments of conodonts. Dolomitic beds immediately higher contain scant, non-diagnostic faunas; the lowermost of these which can be assigned a confident age range from the uppermost Cambrian Cordylodus intermedius Zone to the lowermost Ordovician Cordylodus angulatus Zone. A chertified zone (silcrete?) capping the Eminence at some sections would appear to indicate an extended period of non-deposition following the time of the C. elongatus Subzone at which these Eminence beds are dated. We tentatively conclude that the overlying basal Gunter sandstone represents a transgressive event that is widely-recognized over much of Laurentia – the Stonehenge Transgression event of authors.