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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
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.
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).
[(weight of size fraction before tumbling) – (weight of size fraction after tumbling)] x 100%
Environmental Scanning Electron Microscopy (ESEM)
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.
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.