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Wichita State University, Wichita, Kansas

Abstract: Syndepositional Dolomitization of Shallow Marine Deposits, Cangrejo Shoals, Northern Belize

Dolomite cement comprises an average of ~12% of Holocene, subtidal carbonate sediments in an areally extensive, nearshore mudbank in northern Belize. It is present in sediments along the margins of tidal channels and within the interiors of broad banks between channels, throughout the maximum 7.6 m-thick section of subtidal deposits. Salinity of surface seawater here is 38 o/oo, and that of pore fluids in dolomitic sediments is 38-40 o/oo. Mean dolomite d18O composition is 2.1 o/oo [PDB], and is enriched by 0.7 o/oo relative to that of seawater and pore fluids. There is no change in dolomite d18O composition or of pore fluids with depth or location on the mudbank. These data suggest dolomitization in normal-salinity pore fluids. Three subpopulations of dolomite are recognized based on d13C compositions: (a) those with a mean of -1.0 o/oo [PDB], which overlaps that of pore fluids and which is depleted relative to that of nondolomitic sediments and surface seawater; (b) those with mean d13C of ~3.0 o/oo [PDB], which is close to that of non-dolomitic sediments and unaltered pore fluids; and (c) those with anomalously enriched compositions (up to +11.6 o/oo [PDB]). There is no correlation of dolomite d13C composition with depth, location, sediment texture, or amount of organic matter in the sediment. These data suggest that dolomitization is facilitated by both microbial sulfate reduction and methanogenesis; and that dolomitization has attended reversible organodiagenetic reactions in the sediment over time.

AAPG Search and Discovery Article #90921©1999 AAPG Mid-Continent Section Meeting, Wichita, Kansas