Geology topics
Publications and source records attributed to E.A. Shinn.
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A succession of elevated ridge deposits on the south Florida margin was mapped using high-resolution seismic and side-scan sonar imaging in water depths ranging from 50 to 124 m. The ridges are interpreted to be subtidal shoal complexes and paleoshorelines (eolian dune or beach) formed during the last sea-level transgression. Oolitic and skeletal grainstones and mixed skeletal-peloidal-ooid packstones were recovered using a research submersible. All of the grains are of shallow-water or intertidal origin, and both marine and nonmarine cements were identified.
During or immediately following the transit of Hurricane Andrew (August 23-24, 1992) across the northern part of the Great Bahama Bank, thin laminated beds of carbonate mud were deposited in high-energy subtidal channels (4 m depth) through the ooid shoals of south Cat Cay and Joulters Cays. During our reconnaissance seven weeks later, we observed lime-mud beds exposed in the troughs of submarine oolite dunes and ripples. The mud layers were underlain and locally covered by ooid sand. The mud beds were lenticular and up to 5 cm thick. Their bases cast the underlying rippled surface. The layers were composed of soft silt- and sand-sized pellets and peloids and in some areas contained freshly preserved Thalassia blades and other organic debris along planes of lamination. The beds had a gelatinous consistency and locally had been penetrated by burrowers and plants. Layers of lime mud had also settled on bioturbated, plant-stabilized flats and in lagoonal settings but were quickly reworked and made unrecognizable by the burrowing of organisms. Thicker, more cohesive (and therefore older) mud beds and angular mud fragments associated with ooids from Joulters Cays have similar characteristics but lack fresh plant fragments. We infer that these older beds were similarly deposited and thus record the passage of previous hurricanes or tropical storms. Storm layers are preserved within channel sediments because migrating ooids prevent attack by the burrowing activity off organisms.
The porous limestone bedrock, thin sediment cover, and tectonic stability of the Florida Platform during the past 15 ka BP provide an exceptionally suitable setting for reconstruction of paleoshorelines and onshore projection of future shorelines in a rising-sea scenario. Paleoshorelines for 8, 6, 4 and 2 ka BP show that 1) a series of limestone islands formed, then drowned, along the outer platform; 2) a distinct trough, called Hawk Channel, separated the outer islands from shore; 3) the lower Keys flooded earlier and more rapidly than the rest of the Keys; and 4) Florida Bay and tidal passes through the middle Keys into the bay developed within the past 4 ka BP. Projection of future shorelines onto land shows that most land forming the Florida Keys would flood in a rise of 1 to 2 m and that a rise of little more than 5 m would submerge all land. Offshore reefs would die, while nearshore reefs would shift landward as the mainland shoreline migrated northward. -from Authors
Three craters, created by the explosion of nuclear fusion devices, were mapped, sampled, core drilled and excavated with airlifts at Enewetak Atoll in the Marshall Islands by using scuba and a research submersible. The craters studied were Mike, Oak, and Koa. Tests took place near sea level at the transition between lithified reef flat and unlithified lagoonal sediments, where water depth ranged from 1 to 4 m. Craters produced by the blasts ranged from 30 to 60 m in depth. The purpose of our study was to determine crater diameter and depth immediately after detonation. Observations of submerged roadways and testing structures and upturned crater rims similar to those characteristic of meteor impacts indicate that the initial, or transient, craters were smaller than their present size. At some later time, while the area was too radioactive for direct examination, the sides of the craters slumped owing to dewatering of under lying pulverized rock. Core drilling of crater margins with a diver-operated hydraulic coring device provided additional data. On the seaward margin of the atoll, opposite Mike, a large portion of the atoll rim approximately the size of a city block had slumped into the deep ocean, leaving a clean vertical rock section more than 400m high. An abundance of aggressive grey reef sharks displaying classic territorial behavior prevented use of scuba at the Mike slump site. The two-person submersible R.V. Delta provided protection and allowed observations down to 300 m. During the 6-week period of study, we made more than 300 scuba and 275 submersible dives. Mapping was with side scan sonar and continuous video sweeps supplemented by tape-recorded verbal descriptions made from within the submersible. A mini-ranger navigation system linked to the submersible allowed plotting of bottom features, depth and sediment type with spatial accuracy to within 2 m.
Examination of 16 artificial reef structures in depths ranging from 30-120m indicated that the highest numbers of fish are found around reefs in water shallower than 46m. Fewer fish, especially those with tropical coral reef affinities, <46m was probably caused by a thermocline. Algae and reef community encrusters, abundant on shallower structures, were absent below 46m. Structures that penetrated above the thermocline, such as upright oil rigs and a hopper barge, were also effective reefs. The open structure and high profile of the rigs enhance their use as artificial reefs by providing a range of well-aerated habitats. Greatest diversity and numbers of fish were observed at the Miami sewer outfall. -from Authors
The Florida reefs have evolved over the last 7000 years during rising sea level. An ancient reef existed from sand shoals on which Miami and Key West are now sited before the last glaciation (100 000 to 125 000 years ago). During the glacial period sea level fell to >300 feet below its present level. Since sea level rose again invading the south Florida shelf again, corals have returned. The bulk of the Florida reef tract is composed of carbonate sand, some coming from corals but most from calcified algae. Future scenarios are considered for the Florida reefs assuming sea level to remain stable or to continue to rise. -J.G.Harvey
Suggests a greater production of coralline sand (rather than slower Halimeda growth) results from a marked acceleration of bioerosion of abundant dead reefs in the area. -from Authors
Artificial compaction of in-situ cores of sediments resulted in: 1) reduction of sediment thickness by 50 percent and more; 2) reduction of initial porosities of 65 to 75 percent to 35 to 45 percent; 3) creation of megascopic textures almost identical to many ancient lime mud- and wackestone; 4) creation of organic, wispy "stylolite-like" layers; 5) chemical compaction, evidenced by thin sections showing quartz grains piercing mollusc shells without causing fractures and SEM evidence of solutional interfitting of 1 to 4-mu m-size aragonitic carbonate grains; 6) obliteration of pellets and birdseye or fenestral voids in those sediments where early cementation was lacking; obliteration of identifiable marine grasses and vertical "root" tube voids; 8) mashing of sediment-filled circular burrows to produce ellipsoidal structures. Significant mechanical compaction resulted from pressures simulating less than 1,000 ft of burial. Increasing loads to more than 10,000 ft did not significantly increase compaction. Chemical compaction was detected only in cores compacted to pressures greater than 10,000 ft of burial. These experiments suggest that chemical compaction would begin at much shallower depths given geologic time. Experiments that caused chemical compaction lend support to the hypothesis that cement required to produce a low-porosity/low-permeability fine-grained limestone is derived internally. Dissolution, ion diffusion, and reprecipitation are the most likely processes for creating significant thicknesses of dense limestones. Continuation of chemical compaction after significant porosity reduction necessitates expulsion of connate fluids, possibly including hydrocarbons.--Modified journal abstract.
An offshore drilling site in an area of extensive live-coral bottom off NW Palawan Island, Philippines, was examined 15 months after well completion. Porites lutea growth rates showed that little suppression of head coral growth could be attributed to drilling. Diver observation, however, together with analysis of sampling transect photomosaics, revealed 70-90% reduction in foliose, branching, and plate-like corals in an iron-stained area that extended out from the wellheads in a 115 X 85-m ellipse.-from Authors
Six of 12 core holes drilled at Looe Key Reef (24°37'18"N. 81°24'24"W) by a diver-operated coring device penetrated a spur and groove system. Drilling indicated that: (II the spurs and grooves formed over at least 5 m of carbonate reef sand: (2) the underlying Pleistocene surface is essentially flat and therefore could not control or initiate spacing of spurs or grooves; (3) only the thin seaward ends of spurs are rooted on underlying bedrock: and (4) the interior of the Millepora-encrusted spurs is composed primarily of Acropora palmata. a species no longer abundant on this reef. From the drilling of Looe Key Reef and from other observations along the reef tract, we propose that most shallow spurs and grooves in active coral reef areas of the Caribbean are constructional in origin and not initiated or controlled by bedrock topography. Spurs and grooves in non-coral reef areas adjacent to shorelines, however. are clearly of erosional origin and have a close spacing distinctly different from spurs and grooves known to be of constructional origin. These observations indicate that spurs and grooves in deeper (> 15 m) fore-reef areas off Florida, which have the same geometry as the shoreline features. are erosional in origin and therefore formed on a shoreline during a lower stand of sea level.
Carbon and oxygen isotope analysis through a 30-year (1944 to 1974) growth of Montastrea annularis from Hen and Chickens Reef (Florida Keys) shows a strong yearly variation in the abundances of both carbon-13 and oxygen-18 and a broad inverse relationship between the two isotopes. Normal annual dense bands are formed during the summer and are characterized by heavy carbon and light oxygen. "Stress bands" are formed during particularly severe winters and are characterized by heavy carbon and heavy oxygen. The isotopic effect of Zooxanthellae metabolism dominates the temperature effect on the oxygen-18/oxygen-16 ratio. The isotopic results on the deep-sea solitary coral Bathypsammia tintinnabulum , where Zooxanthellae are nonexistent, indicates that the abundance of the heavy isotopes carbon-13 and oxygen-18 is inversely related to the growth rate, with both carbon and oxygen approaching equilibrium values with increasing skeletal age.
Core drilling and examination of underwater excavation on 6 reef sites in south Florida and Dry Tortugas revealed that underlying topography is the major factor controlling reef morphology. Carbon-14 dating on coral recovered from cores enables calculation of accumulation rates. Accumulation rates were found to range from 0.38 m/1000 years in thin Holocene reefs to as much as 4.85 m/1000 years in thicker buildups. Cementation and alteration of corals were found to be more pronounced in areas of low buildup rates than in areas of rapid accumulation rates. Acropora palmata, generally considered the major reef builder in Florida, was found to be absent in most reefs drilled. At Dry Tortugas, the more than 13-meter thick Holocene reef did not contain A. palmata. The principal reef builders in this outer reef are the same as those which built the Pleistocene Key Largo formation, long considered to be fossilized patch reef complex.