Papers by Twichell, D.C.

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Locker, S.D. , Brooks, G.R. , Hine, A.C. , Davis Jr., R.A. , Twichell, D.C. , and Hafen, M. . 1999. Compilation of geophysical and sedimentological data sets for West-Central Florida Coastal Studies Project. USGS Open File Report OF 99-0539
Hine, A.C. , Brooks, G.R. , Davis Jr., R.A. , Duncan, D.S. , Locker, S.D. , Twichell, D.C. , and Gelfenbaum, G. . 2003. The west-central Florida inner shelf and coastal system: a geologic conceptual overview and introduction to the special issue. Marine Geology, v. 200, p. 1-17
This paper provides an overview for this special publication on the geologic framework of the inner shelf and coastal zone of west-central Florida. This is a significant geologic setting in that it lies at the center of an ancient carbonate platform facing an enormous ramp that has exerted large-scale control on coastal geomorphology, the availability of sediments, and the level of wave energy. In order to understand the Holocene geologic history of this depositional system, a regional study defined by natural boundaries (north end of a barrier island to the apex of a headland) was undertaken by a group of government and university coastal geologists using a wide variety of laboratory and field techniques. It is the purpose of this introductory paper to define the character of this coastal/inner shelf system, provide a historical geologic perspective and background of environmental information, define the overall database, present the collective objectives of this regional study, and very briefly present the main aspects of each contribution. Specific conclusions are presented at the end of each paper composing this volume.
Donahue, B. , Hine, A.C. , Locker, S.D. , Twichell, D.C. , and Tebbens, S. . 2003. Late Holocene estuarine inner shelf interactions; is there evidence of an estuarine retreat path for Tampa Bay, Florida?. Marine Geology, v. 200, p. 219-241
The purpose of this study was to determine if and how a large, modern estuarine system, situated in the middle of an ancient carbonate platform, has affected its adjacent inner shelf both in the past during the last, post-glacial sea-level rise and during the present. An additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier-island shorelines. Through side-scan sonar mosaicking, bathymetric studies, and ground-truthing using surface grab samples as well as diver observations, two large submarine sand plains were mapped ? one being the modern ebb-tidal delta and the other interpreted to be a relict ebb-tidal delta formed earlier in the Holocene. The most seaward portion of the inner shelf studied consists of a field of lobate, bathymetrically elevated, fine-sand accumulations, which were interpreted to be sediment-starved 3D dunes surrounded by small 2D dunes composed of coarse molluscan shell gravel. Additionally, exposed limestone hardbottoms supporting living benthic communities were found as well. This modern shelf sedimentary environment is situated on a large, buried shelf valley, which extends eastward beneath the modern Tampa Bay estuary. These observations plus the absence of an incised shelf valley having surficial bathymetric expression, and the absence of sand bodies normally associated with back-tracking estuarine systems indicate that there was no cross-shelf estuarine retreat path formed during the last rise in sea level. Instead, the modern Tampa Bay formed within a mid-platform, low-relief depression, which was flooded by rising marine waters late in the Holocene. With continued sea-level rise in the late Holocene, this early embayment was translated eastward or landward to its present position, whereby a larger ebb-tidal delta prograded out onto the inner shelf. Extensive linear sand ridges, common to the inner shelves to the north and south, did not form in this shelf province because it was a low-energy, open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies. The distribution of bedforms on the inner shelf and the absence of seaward-oriented 2D dunes on the modern ebb-tidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf.
Edwards, J.H. , Harrison, S.E. , Locker, S.D. , Hine, A.C. , and Twichell, D.C. . 2003. Stratigraphic framework of sediment-starved sand ridges on a mixed siliciclastic/carbonate inner shelf; west-central Florida . Marine Geology, v. 200, p. 195-217
Seismic reflection profiles and vibracores have revealed that an inner shelf, sand-ridge field has developed over the past few thousand years situated on an elevated, broad bedrock terrace. This terrace extends seaward of a major headland associated with the modern barrier-island coastline of west-central Florida. The overall geologic setting is a low-energy, sediment-starved, mixed siliciclastic/carbonate inner continental shelf supporting a thin sedimentary veneer. This veneer is arranged in a series of subparallel, shore-oblique, and to a minor extent, shore-parallel sand ridges. Seven major facies are present beneath the ridges, including a basal Neogene limestone gravel facies and a blue-green clay facies indicative of dominantly authigenic sedimentation. A major sequence boundary separates these older units from Holocene age, organic-rich mud facies (marsh), which grades upward into a muddy sand facies (lagoon or shallow open shelf/seagrass meadows). Cores reveal that the muddy shelf facies is either in sharp contact or grades upward into a shelly sand facies (ravinement or sudden termination of seagrass meadows). The shelly sand facies grades upward to a mixed siliciclastic/carbonate facies, which forms the sand ridges themselves. This mixed siliciclastic/carbonate facies differs from the sediment on the beach and shoreface, suggesting insignificant sediment exchange between the offshore ridges and the modern coastline. Additionally, the lack of early Holocene, pre-ridge facies in the troughs between the ridges suggests that the ridges themselves do not migrate laterally extensively. Radiocarbon dating has indicated that these sand ridges can form relatively quickly (~1.3 ka) on relatively low-energy inner shelves once open-marine conditions are available, and that frequent, high-energy, storm-dominated conditions are not necessarily required. We suggest that the two inner shelf depositional models presented (open-shelf vs. migrating barrier-island) may have co-existed spatially and/or temporally to explain the distribution of facies and vertical facies contacts.
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