Mudstone or Grainstone: SEM Observations of Processes and Patterns of Pelletal Hardening, Bahamas

Kelly L. Bergman and R. Pamela Reid
University of Miami, Miami, FL

Aragonite pelletal grains, ubiquitous in the shallow-water environment of Great Bahama Bank, range from soft to hard despite a probable common fecal origin. In the geologic record, pellets may be preserved as distinct grains or may be compacted to form muddy beds invariably affecting their reservoir potential. At present, there are no predictive criteria for how or where pellets are preserved. To address this problem, sediment samples collected from Great Bahama Bank were preserved for organic material in 5% formalin then described in thin section using a polarizing light microscope and examined using a high-magnification SEM.

Results show pellets are softest in the lee of Andros Island and increase in firmness with distance away from Andros Island. This trend corresponds to observed variations in sedimentary facies: as mud content decreases and peloidal content increases, pellet hardness increases. SEM analyses show that soft to firm pellets contain cauliflower-shaped, anhedral crystals which appear as an outer layer on the pellet surface and are interpreted as minimicrite formed from the recrystallization of aragonite needles. Very firm to hard pellets contain blocky micrite, an abundance of mucilage containing small precipitates, and numerous rod-shaped crystals.

The hardening of pellets is interpreted as a diagenetic recrystallization process. Aragonite needles recrystallize to minimicrite in early stages of hardening and to blocky micrite in later stages, which may involve organic processes. The results of this study suggest it is possible to predict spatial patterns of the diagenetic process of pelletal hardening and the preservation potential of pelletal grains.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005