Limestone is common in rocks from all geologic periods of the Phanerozoic era as well as in many Proterozoic assemblages (1). The minerologic and fabric character of these limestones generally reflect the complex biological, physical, and climactic character of the depositional systems under which they were created. Of particular interest are ooids – non-skeletal grains that precipitate predominantly in the warm, shallow waters of the tropics (1). Ooids are defined as spherical, concentric accretions of calcium carbonate, usually less than 2 mm in diameter, developed around a nucleus of some previously existing particle. By understanding modern ooid-producing environments, we can begin to interpret and understand ancient carbonate facies and sediments that include such particles. In a field research trip to Providenciales, Turks and Caicos, samples of oolitic limestone were collected at a series of successive Pleistocene and Holocene beach ridges, and brought back for analysis.
The samples taken from the most recent beach ridges close to the modern shore show a general trend of decreased porosity, increased weathering, and higher coral content, with minor variations in ooid size, when compared to samples from older beach ridges further inland, about 1.2 km away. Observations were also made regarding ooid transport direction, visible as highly reflective sub-aqueous sand bodies in multispectral scanner satellite and high resolution aerial photos. The ooids studied in this project are produced along the southeastern coast of Providenciales and transported westward by longshore currents along the coast. This allows for a progressive sorting of ooid size along Long Bay Beach, with larger, coarser particles observed near the northeastern end of the beach, and smaller, finer ooid particles observed at the southwestern end.
Ooids are spherical or ellipsoid concretions, usually less than 2 mm in diameter, of calcium carbonate and aragonite crystals arranged around a nucleus – typically a small particle such as a quartz grain or shell fragment (2, 3). The crystals can be arranged radially, tangentially, or randomly. Figure 1 illustrates the difference between radial and tangential classifications. The arrangement of the calcium carbonate within the ooid is generally dependent upon the process by which it was formed: physical processes produce ooids with concentric lamellae, and chemical processes produce ooids with radiating crystals (4). Some geologists recognize a third type of ooid structure – the “recrystallized structure.” In this type of ooid, large irregular crystals either converge toward the center or have no special orientation (5).
In addition to variations in structure, ooids can also be classified as lacustrine or marine in origin. A prime example of lacustrine ooid formation is the Great Salt Lake in Utah. Lacustrine ooids are also primarily composed of aragonite, and are often dull, have a radial fabric, and may have a bumpy surface, known as a “cerebroid” surface (1). Marine ooids are generally found in shallow, warm marine environments, often in locations at low latitudes. These ooids usually consist of aragonite rods without crystallographic terminations, oriented tangentially (parallel to the ooid lamination). Aragonite rods in marine ooids have an average length of 1 micron and a maximum length of 3 microns (5). Marine ooids, as opposed to lacustrine ooids, tend to have higher surface polish and tangentially arranged crystals
Marine ooids are formed in shallow marine environments, such as the tropics, in waters that are supersaturated with calcium carbonate. These waters are approximately two meters in depth, although ooids can form in waters of up to 10 to 15 meters in depth (Figure 2) (6). Formation begins when a small sand grain, shell fragment, or other particle becomes coated with calcium carbonate and the particle is kept in suspension by wave or tidal action; this process is sometimes referred to as the “growth phase” (7). The growth phase alternates with a period known as the “temporary resting phase” in which no new material is accreted. Ooids gain successive layers by alternating between growth and resting phases.
During the growth phase, ooids are suspended in the water due to turbulence and collide with other particles. Researchers have determined that the mass lost per impact with another object increases as the cube of the radius, but the mass gained from growth is proportional to the square of the radius. The net result is that eventually, the mass loss will equal or exceed the mass gained, limiting the size of the ooid (8). Numerical models produced by Sumner and Grotzinger (1993) indicate that ooids have a larger radius in higher velocity environments. In other words, the size of ooids is limited primarily by agitation from wave or tidal action. Ultimately, the ooid falls out of suspension, and is washed ashore where it becomes part of the modern beach.
As previously noted, ooids are generally formed in shallow marine environments, when waters become supersaturated with calcium carbonate. Monaghan and Lytle (1956) determined that the most desirable concentration range for ooid formation is between 0.002 moles/liter and 0.0167 moles/liter of calcium carbonate. In addition to concentration, agitation plays a key role in the formation process. Ooids are typically formed “in agitated waters where they are frequently moved as sandwaves, dunes and ripples by tidal and storm currents, and wave action” (9).
Examples of ooid producing environments include the Great Bahama Bank, the Gulf of Suez, Persian Gulf, the Yucatan shelf, Mexico, and Shark Bay, Western Australia. Another prime example, and the basis of this study, is the Caicos Platform, located southeast of the Bahamas. In 1989, geologists Harold Wanless and Jeffrey Dravis led a field trip to Turks and Caicos, and established four basic settings on the Caicos Platform in which concentric ooids occur. The first is the shallow subtidal swash zone, on the interior of the platform; this refers to areas such as the southeastern coast of Providenciales. The second is an area along a large shoal (the Ambergris Shoal) which experiences both wind-wave and tidal agitation. The third setting refers to shallow tidal shoals along the southwestern edge of the platform, and the fourth is the shoreline, including sediment and beach-swash (10).
Field Research in Turks & Caicos
A field research trip similar to the one led by Wanless and Dravis (1989) was conducted in February of 2006, on the island of Providenciales. The goal was to investigate models of mixed carbonate-evaporite deposition as well as the effects of tectonics and climate on modern sedimentation. Furthermore, the field trip aimed to study modern ooid-producing environments as a basis for interpreting and understanding ancient carbonate facies and sediments that include such particles.
The samples used in this study were collected along Long Bay Beach, Providenciales, and samples of oolitic limestone were collected at successive Pleistocene beach ridges (Figures 3 and 4). Loose oolitic sand was collected by hand, and oolitic limestone samples were taken at road cuts using a rock hammer. The samples were then impregnated with epoxy, sliced and polished into thin sections, and analyzed using an optical microscope.
Samples from the most recent beach ridges close to the modern shore show a general trend of decreased porosity, increased weathering, and higher coral content, with minor variations in ooid size, compared to samples from the older beach ridges, further inland. The samples span a distance of about 1.2 km from the modern shore to the oldest beach ridge, and represent up to six beach ridges. Additionally, samples along Long Bay Beach were compared with samples at Extreme Point to characterize ooid transport in this region.
Porosity and Cementation
The younger beach ridges (Figure 5, samples TC6 R1 and TC6 R2), have notably greater porosity and less cementation than the older beach ridges. The youngest samples have approximately 40% porosity while the older ridges exhibit as little as 5% porosity. Consequently, the older samples contain much more calcium carbonate cement than the younger samples. This progression is evident in samples TC6 R1-6.
It is also interesting to note that TC6 R1 (Figure 5) revealed signs of banding – zones of more cement and zones of less cement (more pore space).
Weathering and Ooid Structure
As expected, the older ridges show more signs of weathering, whereas the younger ridges feature more intact ooids. The younger ridges have clear outlines and unfragmented particles, as opposed to broken and dissolving cement and particles, such as that found in sample TC6 R6.
Coral Fragment Content
In addition to increased cement and increased weathering, the older beach ridges contain more coral fragments. This is evident in samples TC6 R5 and TC6 R6, as well as a sample collected on a previous research trip from a much older ridge, T&Cp1 (Figure 6). Only about 5% of sample TC6 R6 is composed of debris from coral reefs, but this amount is significantly greater than samples TC6 R1-4, which contain no coral fragments.
Although there are variations in ooid size among the successive ridges, there is no distinct trend. Average sizes range from 0.3 – 1.5 mm in diameter. However, there is a notable size difference among samples found along Long Bay Beach and at Extreme Point. Ooids in the Extreme Point sample (Figure 7, sample TC6 EP+20) are visibly much smaller than those found further to the northeast along Long Bay Beach. Ooids at Extreme Point are approximately 0.25 mm in diameter, compared to approximately 0.5 mm for those at Long Baby Beach.
Finally, observations were made regarding ooid transport direction, visible as highly reflective sub-aqueous sand bodies in multispectral scanner satellite and high resolution aerial photos. Observations revealed that the ooids are transported westward along the coast.
The results were consistent with our predictions. Porosity was expected to decrease with age, as the ridges become more compacted. Cement was also expected to increase with age because of the time it takes to form this chemically precipitated material. The young ridges are relatively uncemented and look similar to loose oolitic sand. The extent of weathering and ooid structure is also in line with our predictions. As with any rock exposed to the vadose environment, the degree of weathering increases over time due to physical and chemical processes. As a result of weathering, the structure of the ooids within the rock becomes increasingly less defined due to compaction and dissolution.
Coral fragments were expected in the old beach ridge samples. The material most likely came from the barrier reef to the north, incorporated into older beach ridges as the island rose relative to sea level. Younger beach ridges and ooids on the modern shore do not contain these fragments because they have been isolated from the barrier reef by the island, as the land mass increased. Therefore more recent beach ridges appear to be more pristine.
The size of the ooids in the beach ridge samples is also worthy of note. There is no distinct trend between successive beach ridges, but there are clear variations between samples. The average size of the particles, 0.3-1.5 mm in diameter, is fairly consistent with ooids found in the Bahamas and Caicos Platform (10). The most striking difference is not in the beach ridge samples but rather the samples taken along Long Bay Beach and at Extreme Point. The Extreme Point ooids were visibly much smaller (~0.25 mm) than those found further to the northeast along Long Bay Beach (~0.5 mm), and suggest that the size variations are due to sorting, as a result of transport.
The transport of ooids along the southeastern coast of Providenciales is due to longshore currents that carry ooids to the west. This allows for a progressive sorting of ooid size along Long Bay Beach, with larger, coarser particles observed near the northeastern end of the beach, and smaller, finer ooid particles observed at the southwestern end. The findings in the samples are consistent with predictions made from aerial photos – the westward migration of particles suggests that the particles become increasingly smaller the further they are transported.
Overall, findings were consistent with Wanless and Dravis (10), and samples were similar to those collected in an earlier field research trip, led by Johnson (2001).
Areas of Future Research
The current research provides a solid foundation for future studies of ooid production and transport on the Caicos Platform. In particular, the correlation between characteristics of ooid-producing environments and the size, type, and composition of ooids formed is important to explore. Variations in nucleus composition, water depth, water current, or even the concentration of carbonate in the water could be investigated at other ooid-producing environments on the Caicos Platform described by Wanless and Dravis (10). Additional research may also be conducted about the zones of cementation in the beach ridge samples, such as ridge 1, to determine what factors caused the cement to form in bands rather than continuously throughout the sample. Future research in these areas would serve to broaden our understanding of the Caicos Platform, as well as modern ooid producing environments.
Limestones commonly occur in rocks in every geologic period of the Phanerozoic and many from the Proterozoic (1). The minerologic and fabric character of these limestones reflect the complexities of the depositional systems under which they were created, serving as indicators of biological, physical, and climactic variations. By understanding modern ooid-producing environments, we can begin to interpret and understand the ancient carbonate facies and sediments that included such particles.
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