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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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The Pleistocene fauna in limestone on Ambergris Cay is homologous with the adjacent Holocene fauna enabling direct comparison between unaltered and diagenetic samples.
Four facies are recognizable in the limestone: a reef-crest facies (I); a backreef facies (II); a shelf-lagoon facies (III), composed of outer, middle, and inner shelf zones; and a mud-bank facies (IV). The Pleistocene middle shelf zone is oolitic, unlike any nearby Holocene deposits. Facies I and II are biomicrites, III is a biopelmicrite, and IV is dismicrite.
Aragonite persists in corals, mollusks, Halimeda, tunicate spicules, pellets, and ooliths, where not obviously replaced by calcite; magnesian calcite is retained in skeletons of encrusting algae and Foraminifera.
Skeletal materials show four categories of diagenetic alteration: (1) solution; (2) precipitation of carbonate as drusy rims, or coarse sparry mosaics of calcite, or syntaxial overgrowths; (3) replacement of aragonitic gastropods by calcite along a jagged "front," probably with solution and deposition on a minute scale; ghosts of primary structures remain in many places, indicating absence of a major intermediate void stage; replacement of Halimeda and corals such as Montastrea annularis occurs after occlusion of internal pores by sparry calcite; (4) recrystallization (i.e., alteration of crystal form without change in mineralogy), evident in a few pelecypods where local patches of shell have altered to coarse, transverse blades of aragonite in which ghosts of primary structures may o may not persist.
Cementation of the limestone has been achieved through interstitial precipitation of drusy and sparry calcite and through recrystallization.
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