SURFICIAL AND BEDROCK GEOLOGY OF THE EASTERN PORTION OF THE U.S.G.S. 1:100,000 SCALE MARIANNA QUADRANGLE, NORTHWESTERN FLORIDA

The near surface geology of the eastern half of the U.S.G.S. 1:100,000 scale Marianna Quadrangle, which was mapped by the Florida Geological Survey as part of a two-year STATEMAP project, is composed of Eocene to Holocene carbonate and siliciclastic sediments. Within this area, geologic processes include a combination of fluvio-deltaic and marine deposition, erosion, dolomitization, and karstification. Several structural, sedimentological, and geomorphic variables are unique to the area and have affected the near surface expression and interpretation of the geology of the region. Two important structural features are recognized: the Chattahoochee Arch – a northeast/southwest-trending high that exposes Eocene and Oligocene carbonates, with younger strata thinning around the arch; and the Apalachicola Embayment/Gulf Trough - an elongated basin that widens southwestward, towards the Gulf of Mexico, and narrows to the northeast into Georgia. Gulf water moved through this strait from the Middle Eocene through Oligocene, flowing across the eastern panhandle of Florida, through southern Georgia and to the Atlantic Ocean. This structure crosses through the southeastern part of the study area, and noticeably influenced sedimentation patterns in the region.

The Eocene to Miocene carbonate units exposed in the area have regional stratigraphic significance, and historically have been identified, correlated, and interpreted in many different ways. Some previous investigators relied heavily on fossils to establish formations and correlate facies within this region, a practice which has led to some lithostratigraphic confusion. Detailed mapping of the carbonate units allows for a better understanding of the relationships between cave development, karstification, groundwater quality, and regional geology in the area.

In the northeastern portion of the study area, where the siliciclastics thin and the carbonates are near the surface, numerous karst features, springs, and caves are present. These karst features often allow for direct recharge of surface water to the Floridan Aquifer System. This infiltration of surface water influences the ground-water quality of springs in the area as recognized by increased nitrate concentrations in the spring water.