Abstract

Dolomite formed within Holocene (<5600 yrs old), shallow-subtidal carbonates in the Cangrejo Shoals mudbank in Belize precipitated as cements from interstitial fluids of normal-marine compo-sition. Active circulation of seawater through the sediments supplied the necessary Mg for dolomitization, which was promoted by elevated pore-water alkalinity resulting from oxidation of organic matter. Rapid dolomitization here promotes the commonly dismissed notion that significant quantities of dolomite can form syndepositionally, in shallow-subtidal deposits, from reactions with seawater of normal-marine salinity and Mg-Ca concentration.

By analogy, syndepositional dolomitization likely also affected some ancient, micritic, shallow-subtidal deposits, such as Pennsylvanian and Permian phylloid algal reefs in the Permian Basin. Pervasive syndepositional dolomitization of such deposits results in an inherent diagenetic potential to create reservoirs with complex, secondary pore systems. Typical pore types expected to occur in such reservoirs would include biomoldic, vuggy, intercrystalline, and in some cases, primary intraparticle porosity.