Ooids and Grapestone - A Significant Source of Carbonate Mud from Caicos Platform
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. This significant source of carbonate mud has been overlooked in both modern and ancient marine settings.
AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas