--> Carbonate Dissolution within the Meteoric and Mixing Zone Diagenetic Environments: Porosity Development within Late Pleistocene Reef and Reef-Associated Lithologies, Southeastern Barbados, by K. L. Canter and J. D. Humphrey; #90986 (1994).

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Abstract: Carbonate Dissolution within the Meteoric and Mixing Zone Diagenetic Environments: Porosity Development within Late Pleistocene Reef and Reef-Associated Lithologies, Southeastern Barbados

Karen Lyn Canter, John D. Humphrey

Extensive porosity development has occurred in response to dissolution within both meteoric and mixing zone diagenetic environments as demonstrated by borehole studies of late Pleistocene reef and reef-associated limestones from southeastern Barbados. The conclusions of this study are based on petrologic and petrographic observations from two continuously cored boreholes drilled immediately landward of sea-cliff exposures at Foul Bay and Shark's Hole, Barbados. The Foul Bay locality was selected because it is the locus of meteoric phreatic discharge for most of southeastern Barbados and accommodates a high-flow freshwater lens. Shark's Hole receives a subordinate amount of freshwater discharge; however, meteoric diagenetic modification is also common. Widespread dissolution and porosi y enhancement resulted in fair to poor core recovery at the meteoric-marine mixing zone. Core recovery diminished significantly when the borehole penetrated the present-day mixing zone at the Foul Bay locality. A similar, although less dramatic shift in core recovery was also recorded when the mixing zone was encountered at Shark's Hole. Thus, variations in porosity development as well as diagenetic fabrics can be directly related to the intensity of both meteoric and mixing zone diagenesis.

Both cores contain reef-associated sediments which were deposited during the 194 ka sea-level highstand. Subsequent to deposition, uplift and subaerial exposure were followed by meteoric diagenesis during the 122 ka glacio-eustatic highstand. This period of meteoric diagenetic modification resulted in the formation of vuggy biomoldic to cavernous secondary porosity. The Holocene sea-level rise followed, emplacing a freshwater lens and a mixing zone at its present position. High-flow meteoric diagenesis tends to result in widespread dissolution and calcitization, with lesser amounts of calcite cementation. The mixing zone diagenetic environment is also characterized by extensive dissolution. At Foul Bay, widespread dissolution of Acropora cervicornis occurred throughout the reef and fo e-reef facies, whereas more robust coral forms (e.g., A. palmata, Montastrea annularis) were pervasively calcitized. Dissolution of A. cervicornis also occurred at the Shark's Hole; however, the dissolution fabrics are not as pervasive and many skeletons were merely replaced by chalky LMC. Cementation within the mixing zone is relatively uncommon and limited to partial dolomitization of red algae and other HMC allochems and matrix.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994