DETAILED GEOLOGIC MAP FOR FORUM QUADRANGLE, ARKANSAS INCLUDING A KARST INVENTORY

A mapping thesis of Forum quadrangle has been undertaken to understand the karst terrain that has formed in response to various lithologic and structural controls. The Forum quadrangle is located in Northwest Arkansas in Madison County centered on the town of Forum. Physiographically it is located in the Ozark Plateaus Province. The Ozark Dome uplifted in the Pennsylvanian in response to the Ouachita orogeny. Karsting has formed in a predictable manner. Porosity differences between limestone, dolostone, sandstone, and shale units are easily observed in the surface character of all units. A grid like pattern of jointing is evident in most of the units allowing for groundwater concentration along discrete flow paths, thus focusing karst formation. The Mississippian St. Joe Limestone (Msj) contains the bulk of these karst forms. It is a relatively pure crinoidal limestone with thickness ranging up to about 33 meters. It is overlain by the Mississippian Boone Formation (Mb), a cherty limestone of about 91-106 meters thick, and is underlain by the Ordovician St. Peter Sandstone (Osp) to the east and the Ordovician Everton Formation (Oe) to the west. Originally the Msj was considered to be a member of the Mb though now it is recognized by most as it is own formation. It has a high density of karst forms and is a prominent bluff former allowing it to be easily recognized and mapped individually. Haley (1973) produced a photo-geologic map of the quadrangle under the Arkansas Geologic Commission. A review of this map is in order to determine all lithologies in detail, and to better define structural controls within the region. The karst inventory was applied to the new geologic map. The karst inventory includes the locations of all karst features such as; springs, caves, dolines, disappearing streams, ect. Since structure is so important here the orientations of all jointing and faulting were located in order to better understand groundwater relationships between karst features. These observations have allowed development of conceptual models based on porosities, permeabilities and structural controls. The interpreted digenetic evolution allows speculation on the conditions for speleogenesis and the type of cave development.