Vadose Diagenesis of Quaternary Eolianite Sequences, San Salvador Island, Bahamas
J. A. Beier
Vadose alteration of eolian grainstone results in the formation of a characteristic vertical sequence of diagenetic fabrics. Eolianite sequences of San Salvador Island, Bahamas, are divided into zones of minor, moderate, and intense alteration. The zone of minor alteration, in which eolian sediments may be partly to totally lithified by micrite or spar cement, grades upward into a zone of moderate alteration, where many grains have a micrite coating, and random needle fibers of calcite and calcified tubules are common. Caliche pisolites and rhizomorphs are found at the top of the moderate alteration zone, along with stringers of laminated calcrete. The zone of intense alteration at the top of the vertical sequence is characterized by deposition of laminated calcrete, tota micritization of the original grainstone, the development of terra rosa sediments, precipitation of ferroan spar, and brecciation.
Analysis of whole-rock samples from two Pleistocene(?) eolianite sequences yield vertical profiles of ^dgr13C, ^dgr18O, percentage of total organic carbon, ^dgr13Corg, and trace-element compositions (wt. % of Fe2O3 and MgO, and ppm Sr) through the zones of progressive alteration. Trends toward enrichment in Fe2O3, 12C, and 16O, and an increase in organic carbon occur gradually through the moderate alteration zone and are extreme in the intense alteration zone.
Microsampling of diagenetic features for isotopic analysis (including micrite and spar cements in the zone of minor alteration, random needle-fiber calcite, pisolites, rhizomorphs, calcrete, and ferroan spar and micrite) provides useful information to determine the processes responsible for developing specific diagenetic fabrics and the timing of these events.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.