Facies and Diagenesis of Upper Tertiary-Pleistocene Platform Carbonates in a 300m Borehole, Southern Ambergris Caye, Belize

Cynthia A. Burtnett

Well cuttings from an ~300 m-deep borehole drilled by the Belize government on Ambergris Caye were examined for inferred depositional environments, mineralogy, and stable oxygen-carbon isotopic compositions. The recovered section composes carbonates of the Barton Creek and (?)Sand Hill Formations, presumably of upper-most Tertiary to Pleistocene age. Location of the wellbore to the west of the exposed Pleistocene and superposed Holocene platform margin reef suggests continuous platform deposition. Comparison to modern facies indicates that the entire section is, in fact, mostly outer-shelf reef and immediate back-reef facies. Four unconformities are recognized in the upper 150m of section on the basis of microbreccias and calcretes, and they truncate stacked, shallowing-u ward cycles within the section. Depleted ^sgr¹⁸O and ^dgr¹³C compositions of whole-rock samples from the upper 150 m, which are highly porous and LMC-dominated, range from -2.5 to -5 o/oo and -3 to -7 o/oo (PDB), respectively. Unconformities are therefore believed to be subaerial exposure surfaces.

Dolomite (replacive and cement) occurs below 150 m within HMC-dominated limestones that also contain aragonite. Dolomite generally increases with depth from 8-28%, its composition varies from Ca_51-57, and its mean isotopic composition is ^dgr¹⁸O = 3.2 o/oo and ^dgr¹³C = 2.5 o/oo. There are no depth-related trends in dolomite stoichiometry or isotopic composition. Although the timing and site of dolomitization are not known, available data suggest formation fron circulating marine fluids of near-normal composition.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California