Abstract: Marine Dolomitization of Holocene Mudbank Deposits (Cangrejo Shoals), Northern Belize

S. J. Mazzullo, W. D Bischoff, C. S. Teal

Mud-dominated sediments of Cangrejo Shoals mudbank (southern tip of Ambergris Caye) were deposited in a humid-zone, transgressive setting during the last 6500 yrs, entirely in shallow marine environments (bank top and channels). Thickness of sediment within the mudbank complex, which encompasses an area of 11.7 sq km, is 1.56.7 m, and sediments are dominantly (70%) high-Mg calcite. Dolomite occurs pervasively in the subsurface (0.6-4.3 m below sea level) over a large area of the mudbank, and composes 1% to 30% (mean = 5%) of the mud fraction of the sediments. The dolomite is principally interparticle cement whose crystal sizes range from 2-50 microns (mean = 10 microns), and is poorly ordered and nonstoichiometric (Ca_55-62Mg_38-45); isotopic compositions presently are not known. By various lines of evidence the dolomite is believed to be authigenic rather than detrital. The strictly subtidal history of sediment deposition at Cangrejo Shoals (i.e., peritidal facies are absent), and the lack of any mineralogic alteration of enclosing sediments, argue for a subtidal marine dolomitization model. Preliminary analyses of pore water chemistry suggest dolomite precipitation from fluids of near-normal salinity, with nucleation facilitated by high alkalinities induced by bacterial oxidation of organic matter.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994