SEDIMENTOLOGIC AND GEOPHYSICAL STUDY OF THE STRATIGRAPHY AND DEVELOPMENT OF MODERN CARBONATE ISLANDS, COTTON KEY, FLORIDA

Cotton Key, just north of Upper Matecumbe Key, Florida, is a triangular shaped island in the Southeastern region of Florida Bay. The Recent carbonate island, covered in mangroves, is dominated by skeletal wackstones, gray to grayish brown in color up to 2.5 meters thick overlying Pleistocene limestones. To the north of Cotton Key is the boundary of the Everglades National Park containing numerous similar carbonate islands. The classic interpretation of the 2-dimensional geometry of the sediments beneath the islands and mud banks is described as a single shallowing upward sequence related to Holocene sea level rise flooding over a flat, hardened Pleistocene bedrock surface. However, under certain circumstances, two separate shallowing upward sequences may be preserved beneath Florida Bay Islands, an observation with serious implications for standard shallowing upward models of carbonate tidal flat deposits.

The purpose of this study is to reexamine the deposits beneath Cotton Key in Florida Bay using a combination of closely spaced sediment cores and shallow geophysical imaging techniques. This study will be the first to test the feasibility of using ultra-shallow seismic on these types of deposits. Nine sediment core samples transecting the island were used to evaluate the sediment stratigraphy and paleo environment chronology. Two seismic lines of densely sampled, 5-centimeter interval geophones and a near-source, nonelastic deformation seismic impulse source were laid. The practical implementation of high-resolution seismic imaging at ultra-shallow depths may have the potential to complement ground-penetrating radar, chiefly in areas where materials exhibiting high electrical conductivity, such as clays, prevent the effective use of GPR, and give us a better understanding of the three-dimensional geometry of coastal sediments. Preliminary geophysical results indicate that acoustic-based geophysical devices with high dominant-frequency content ranging from 450 Hz to 200 kHz were effective at depths. Potential applications of these results exist in hydrogeology and environmental, Quaternary, and neotectonic geology studies.