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PSOoids and Grapestone – A Significant Source of Carbonate Mud from Caicos Platform*

 

Noelle Van Ee1 and Harold R. Wanless2

 

Search and Discovery Article #50163 (2009)

Posted February 20, 2009

 

*Adapted from poster presentation at AAPG Annual Convention, San Antonio, TX, April 20-23, 2008.

 

1 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL

2 Geological Sciences, University of Miami, Coral Gables, FL

 

Abstract

Samples of aragonitic oolitic and grapestone sand from agitated shoal, platform and beach environments of Caicos Platform were assessed for grain durability in tumblers. After one week of tumbling with equal weights of 1mm spherical glass spheres, two to seven percent of the oolitic sand had abraded to mud size, aggregate grains releasing most of the mud. Additional durability assessments were made by tumbling only the carbonate sands to determine if they are capable of significantly abrading themselves in the absence of siliciclastic material. In samples that are mostly grapestone aggregates, 2-3 percent of the sand sample was reduced to mud in one week. Samples containing over 85 percent well-rounded, glossy oolitic grains produced 0.3-0.4 percent of mud from their sand fractions in a week.

Scanning Electron Microscope analysis showed that grapestone breaks down by abrasion of the aragonitic marine cement between the constituent grains and by abrasion around pre-existing micro-bore structures. In ooids, observed breakdown is by extension of pre-existing micro-bore structures and grain surface irregularities. The mud produced consisted of broken aragonite needles, most less than three microns in length. The size of the mud component produced is extremely fine and may reflect the common milkiness associated with the waters of agitated shoals.

This study suggests that the in situ growth of ooids and grapestone grainstone sediment bodies is associated with the production of at least an equivalent amount of carbonate mud (Figure 4). This significant source of carbonate mud has been overlooked in both modern and ancient marine settings.

 

 

 

uAbstract

uFigures

uBackground

uMethods

uResults

uConclusions

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uBackground

uMethods

uResults

uConclusions

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uBackground

uMethods

uResults

uConclusions

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uBackground

uMethods

uResults

uConclusions

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uBackground

uMethods

uResults

uConclusions

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uBackground

uMethods

uResults

uConclusions

uAcknowledgements

uReferences

 

Selected Figures

Figure 1. Milky colored water resulting from suspended very fine carbonate mud. Photo courtesy Kelly Gibson.

Figure 2. Aerial photograph of Ambergis Shoal. Photo courtesy Harold R. Wanless.

Figure 3. Aerial photo of Turks and Caicos Platform. Green X’s mark sample locations. Photo courtesy Harold R. Wanless.

Figure 4. Ooids moving along the bottom of Ambergris Shoal. Photo courtesy Harold R. Wanless.

Figure 5. Even without a silica abrasive, carbonate sand grains produced mud-size particles. This self-abrasion process produced <0.5% of mud from sand fractions in the oolitic sample (WSO) but 3-3.5% in the grapestone (GS). GS1 and WSO1 are identical in composition to GS and WSO, respectively.

Figure 6. Before abrasion. A) Surface textures of grapestones, as seen using Environmental Scanning Electron Microscopy (ESEM), show aggregate grains held together by a loose mesh of aragonite needles. This cement can incorporate other material, such as the diatom test seen in the lower right hand corner. B) Oolitic samples have both smooth surface textures and highly porous surfaces with microborings and intricate micro-topography. These features cause weaknesses in what would otherwise be a durable, spherical grain.

Figure 7. After abrasion. Breakdown trends in the samples are apparent under ESEM. Exposed cement between the grains in the aggregates is preferentially abraded so that, after abrasion, samples in the 500-1000 micron range appear to be much knobbier; with constituent grains protruding excessively out of a reduced cement matrix (see inset).

Figure 8. ESEM micrograph of resulting mud, which is extremely fine and consists of broken aragonite needles, most less than three microns in length.

Figure 9. Underwater photos of agitated shoals and resulting milkiness of water from suspended very fine carbonate mud. Photos courtesy Harold R. Wanless. Similar effect as shown in Figure 1.

Background

In the geological record, there is a greater quantity of carbonate mud (Figure 1) than carbonate sand, yet there has been a paradoxical focus on sand-sized particles in the literature (Matthews, 1966). Currently, there are four proposed models for the origin of carbonate mud. They are summarized by Matthews (1966) as: (1) the production of aragonite needles by physical precipitation from waters of abnormally high salinity and carbonate saturation; (2) post mortem disintegration of calcified green algae; (3) the production of mud-sized skeletal debris by predominately physical processes of particle-size reduction in agitated environments; and (4) by predominately biological reduction (bio-corrosion) in quiet-water environments.

The purpose of this study was to demonstrate that ooids and grapestones from Turks and Caicos Islands (Figure 3) are capable of abrading themselves under natural conditions, suggesting that the formation of grapestone and ooid grainstone bodies are associated with a significant amount of mud production. Fabricius (1977) noted close chemical and isotopic similarities between ooids and grapestones and aragonitic mud. He interpreted this to mean that both mud and ooids are primarily inorganic precipitates (Fabricius 1977). However, based on the findings of this study, the similarity may be because the lime mud was generated from the physical breakdown of ooids and grapestones. Mud from the Bahamian archipelago is known to consist of aragonite needles, a few microns in length, however it is debated if the origin of the mud is inorganic or algal (Tucker and Wright, 1990).

The composition and appearance of lime mud in the Bahamian archipelago is consistent with that of the mud produced in this experiment. This suggests that ooids and grapestones need to be added to skeletal debris as grains that produce mud predominantly by physical processes of particle-size reduction in agitated environments. Indeed, ooid shoals, like other carbonate environments that are subjected to wave and/or current agitation, should be expected to produce lime mud that in most cases will be transported to quiet water for deposition (Matthews, 1966).

Methods

Durability Analysis

  1. Sand samples were collected from agitated, bare bottom, shallow-marine environments around the Caicos Platform.
  2. Samples were rinsed, sieved into standard size fractions >125 microns, dried and weighed.
  3. Samples were subjected to abrasion in small, rubber-lined tumbling barrels for one week according to the method of Wanless and Maier (2007). One trial included an equal weight 1000 micron spherical abrasive and one did not.
  4. After abrasion, samples were sieved, dried and weighed.
  5. Raw data of the before and after weights of each size fraction were used to calculate the percent weight change according to the formula:

[(weight of size fraction before tumbling) – (weight of size fraction after tumbling)] x 100%
(total weight of sample after tumbling)

Environmental Scanning Electron Microscopy (ESEM)

  1. Pre- and post-abrasion samples were mounted on carbon-backed stubs and palladium coated.
  2. Samples were analyzed under high vacuum mode using a Philips XL30 ESEMFEG at the University of Miami’s Center for Advanced Microscopy.
  3. Analysis focused on surface grain textures to locate any inherent weaknesses in pre-abrasion samples and evidence of breakdown in post-abrasion samples.

ESEM Results

Before Abrasion
Surface textures of grapestones, as seen under ESEM (Figure 7), show aggregate grains held together by a loose mesh of aragonite needles. Oolitic samples have both smooth surface textures and highly porous surfaces with microborings and intricate micro-topography. These features cause weaknesses in what would otherwise be a durable, spherical grain.

After Abrasion
Breakdown trends in the samples are apparent under ESEM (Figure 8). Exposed cement between the grains in the aggregates is preferentially abraded so that, after abrasion, samples in the 500-1000 micron range appear to be much knobbier; with constituent grains protruding excessively out of a reduced cement matrix. Where the ooids themselves are abraded, the breakdown appears to be an extension or expansion of pre-existing holes or weaknesses from boring or dissolution. Boring algae, fungi and bacteria are known to produce pits and grooves on the surface of ooids (Margolis and Rex, 1971; Newel et al., 1960; Harris et al., 1979). These pits are often infilled, but grain irregularities and weaknesses result (Gaffey, 1983).

Mud
The mud produced by this process is extremely fine and consists of broken aragonite needles, most less than three microns in length (Figure 8). Because the mud is so fine, it will remain in suspension for a long time. This mud may reflect the milkiness commonly associated with agitated shoals (Figures 1 and 9).

Conclusions

  1. Abrasion of oolitic and grapestone sand produces a significant amount of mud in carbonate systems.
  2. ESEM work suggests that mud is formed primarily by the preferential erosion of aggregate cement.
  3. This source of carbonate mud has been overlooked in both modern and ancient marine settings.

Acknowledgements

I would like to thank my undergraduate committee members, Dr. Harold Wanless, Dr. Donald McNeill and Dr. Daniel DiResta. ESEM analysis was facilitated by Dr. Terri Hood, Dr. Patricia Blackwelder and Husain Alsayegh. I am also grateful for the support and encouragement of the undergraduate Marine Science and Geology students, faculty and staff.

References

Fabricius, F.H., 1977, Origin of Marine Ooids and Grapestones, in H. Fuchtbauer, A.P. Lisitzyn, E. Seibold and J.D. Milliman, eds. Contributions to Sedimentology, Stuttgart: E. Schweizerbart’sche Verlasbuchhandlung.

Harris, P.M., R.B. Halley, and K.J. Lukas, 1979, Endolithic Microborings and Their Preservation in Holocene-Pleistocene (Bahama-Florida) Ooids: Geology, v. 7, p. 216-220.

Margolis, S. and R.W. Rex, 1971, Endolithic Algae and Micrite Envelope Formation in Bahamian Oolites as Revealed by Scanning Electron Microscopy: Geological Society of America Bulletin, v. 82, p. 843-852.

Matthews, R.K., 1966, Genesis of recent lime mud in Southern British Honduras: Journal of Sedimentary Petrology, v. 36, p. 428-454.

Newell, N.D., E.G. Purdy, and J. Imbrie, 1960, Bahamian Oolitic Sand: The Journal of Geology, v. 68, p. 481-497.

Tucker, M.E. and V.P. Wright, 1990, Carbonate Sedimentology: Oxford: Blackwell Scientific Publications, 482 p.

Wanless, H.R. and K.L. Maier, 2007, An Evaluation of Beach Renourishment Sands Adjacent to Reefal Settings, Southeast Florida: Southeastern Geology, v. 45, p. 25-42.

 

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