Geologic map of Forum quadrangle with a karst inventory, Madison County, Arkansas

A photo-geologic map of the Forum 7.5-minute quadrangle was produced for the revised state geologic map (Haley, 1973). A review of this map was warranted to determine all lithologic details, and better define structural controls within the region. This study describes the areal geology of the Forum 7.5-minute quadrangle, Madison County, Arkansas; it was undertaken to remap the quadrangle, and to better define the karst terrain that has formed in response to various lithologic and structural controls in this area. Within this portion of the Ozark Plateau Physiographic Province, karst has formed in a explainable, but sometimes unpredictable manner. Porosity differences between limestone, sandstone, chert, dolostone, and shale are discernable in the surface character of all exposed units. A grid-like pattern of jointing is evident in most of the units, allowing groundwater to concentrate along discrete flow paths, thus focusing formation of karst. The Mississippian St. Joe Limestone contains the bulk of these karst forms. This limestone is relatively pure with a maximum thickness of 90 feet. The Boone Formation overlies the St. Joe as a thicker limestone with bedded chert. Total thickness of the Boone ranges from 160--260 feet. This formation contains karst features, but they are dominantly internal structures connecting groundwater flow paths from land surface to the St. Joe. Perching of groundwater focuses springs and associated caves to be concentrated at the contact of with the St. Joe and the underlying sandstone.;Originally, the St. Joe was a member of the Boone Formation, but is now recognized as a formation. It has a high density of karst forms and is a prominent bluff former, allowing it to be easily recognized and mapped, although the upper contact with the Boone is not always observable. A karst inventory was added to the geologic map, and includes the locations of all karst features, such as springs, caves, sinks, and disappearing streams. Insofar as structure is important in contributing to hydrogeology within the region, the orientations of all observed jointing and faults are summarized to understand the relation between groundwater and karst features. These observations allowed development of conceptual models and a groundwater flow map of the region. The interpreted diagenetic evolution provides the basis for speculation on dominant controls affecting speleogenesis and the type of cave development.