High-magnesium calcite-dominated muddy sediments on subaerially exposed rays in northern Belize are being pervasively altered to and lithified by low-magnesium calcite in the meteoric environment. Such diagenesis, occurring in sediments less than 1000-1500 years old, is indicated by the rapid loss of Mg from muds and foraminifera, progressively lighter mean isotopic compositions in LMC-cemented muds relative to those in unaltered marine muds, and concurrent reduction in Sr and increase in Mn concentrations. The fabrics of these LMC deposits are micrites and microspars to pseudospars with pore-filling LMC cements of similar crystal size. These fabrics are similar to calcitized aragonite-dominated precursor micrites except that aragonite relics in microspar or pseudospar are relatively rare in these samples.

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