THE GEOLOGY OF 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. Karst 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 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 basal Mississippian Sandstone. Originally the St. Joe was considered to be a member of the Boone formation though now it is recognized by most as it is own formation. The St. Joe 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 many karst features: springs, caves, dolines, solutioned fractures, disappearing streams, ect. Structure is important and 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.