Dolomite is very abundant in Holocene, subtidal carbonate mudbank deposits in humid northern Belize, where its occurs over an area of l0 km². It is present in sediments along the margins of tidal channels and within the interiors of broad banks between channels, throughout the maximum 6.7 m-thick section of subtidal deposits. Average amount of dolomite is 5.4% (range 1-30%), and dominant mode of occurrence is as cement of average 5 µm crystal size.

Salinity of surface seawater is 38, and that of pore fluids in dolomitic sediments is 38-40. Mean d¹⁸O of dolomite (2.1PDB) is enriched by 0.7 relative to that of surface seawater and associated pore fluids. There is no change in d¹⁸O composition of dolomite or of associated pore fluids with depth or location on the mudbank. These data suggest dolomitization in pore fluids of normal-marine salinity. In contrast, three sub-populations of dolomite are recognized based on d¹³C compositions: (a) those with a mean of -1.0PDB which overlaps that of pore fluids and which is depleted relative to that of non-dolomitic sediments and surface seawater, (b) those with mean d¹³C close to that of non-dolomitic sediments and unaltered pore fluids (~3.0PDB); and (c) those with anomalously high d¹³C compositions (up to +ll.6PDB). There is no correlation of d¹³C with depth, location, sediment texture, or amount of organic matter in the sediment. These data indicate that dolomitization can occur in little-modified pore fluids and also in fluids whose chemistry is modified by either sulfate reduction or early methanogenesis.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah