Mazzullo, S. J.1
(1) Wichita State University, Wichita, KS
ABSTRACT: Carbonate Platform Stratigraphy and Evolution, Pliocene to Holocene, Northern Offshore Belize
Exposed Pleistocene limestones and samples from a 285-meter borehole on Ambergris Caye are the basis of reconstruction of platform evolution since the Middle Pliocene. The oldest rocks encountered (at 234-285 m) are basal Middle Pliocene. They, and overlying strata to 186 m, are slightly cherty, chalky lime mudstone to biowackestone, with re-transported shelf fossils, overlain to 136 m by biopackstone. They are interpreted as shallowing-upward, fore-platform slope deposits. Compositionally they are low-Mg calcite with 10-30% dolomite. Dolomite isotope values (ä18O 2.6-3.8‰ PDB; ä13C 1.0-3‰ PDB) suggest precipitation from circulating paleo-seawater. Shift in whole-rock C-O isotope values at the top of the section may mark the Pliocene-Pleistocene boundary. Overlying strata to 6 m comprise at least 11 cycles of biopackstone to grainstone with two recognizable unconformities. The rocks are inferred to be outer shelf to platform-edge reef deposits. They are entirely calcite below 15 m depth, and overall upward-depletion in C-O isotope values (from -4 to -7.5‰ ä13C and -4 to -6‰ ä18O ) suggests overprint of latest Pleistocene to Holocene meteoric alteration on the rocks. Exposed and near-surface Pleistocene rocks, comprising similar lithologies and environments, are radiometrically dated at 125,000-134,000 BP and represent oxygen isotope stage 5e. Mineralogy down to 15 m is low-Mg calcite and aragonite. The stratigraphic section records possible ramp architecture in the Pliocene, and a shift to reef-rimmed platforms thereafter. Inferred facies development, and overall whole-rock C-O profiles in the section, are similar to those in cores from the Bahamas, suggesting similarity in sequence-stratigraphic development and diagenesis.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.