Sedimentologic and geophysical study of the stratigraphy and development of modern carbonate islands, Cotton Key, Florida.

摘要

Cotton Key, located just north of Upper Matecumbe Key, Florida is a triangular shaped island in the Southeastern region of Florida Bay. This Recent carbonate island, covered in mangroves, is dominated by skeletal wackestones, 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 flat, hardened Pleistocene bedrock. However, two separate 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 techniques on these types of deposits. The use of high-resolution seismic imaging at ultra-shallow depths may have the potential to complement ground-penetrating radar, in areas where materials exhibiting high electrical conductivity, such as clays, prevent the effective use of ground penetrating radar, and give us a better understanding of the three-dimensional geometry of coastal sediments. Seven sediment cores were first used to evaluate the sediment stratigraphy and paleo environment chronology. Two seismic lines of densely sampled, 15-centimeter interval, geophones and a near-source, inelastic deformation, seismic impulse source were laid. Preliminary geophysical results indicate that varying acoustic-based geophysical devices were effective at shallow depths in modern carbonate environments.