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ABSTRACT: New Interpretation of Vadose Diagenesis in Light of Microbial Fabrics

K. R. Stank, E. A. Burton, M. P. Weiss, H. G. Machel

Vertical profiles across interfaces between soil horizons and limestone of the Antigua Formation (Antigua, British West Indies) were investigated by scanning electron microscope, thin section and chemical techniques. Our observations indicate that algae and fungi play more significant roles in the development and redistribution of porosity in caliche horizons than has been recognized in previous models.

Evidence of dissolution reactions increases toward the soil contact where the sediments have highly porous pelletal textures. Biological mediation of these reactions is evident in degradation of allochems and etching of spar surfaces along uncalcified hyphae. Vermicular textures produced by calcified fungal hyphae are most abundant at the sediment-limestone boundary and decrease downward, grading into biomicrites with traces of meniscus cements. Masses of broken calcified hyphae with calcite rhombs nucleated on their outer surfaces commonly replace the original micrite matrix and form a chalky grainstone fabric, thus causing significant changes in sediment permeability and porosity. Tangential needles increase in abundance concomitantly with organic material, and their morphologies ar a likely product of selective organic poisoning of crystal faces. Other crystal forms associated with organics include pore-lining calcite microspar and densely intergrown dendritic calcite.

Structures (e.g., calcified filaments) evident in these profiles also have been described from certain marine facies. This suggests that organic mediation of carbonate precipitation and dissolution reactions may be significant in those environments as well. Chemical and textural criteria are being tabulated for use in differentiating subaerial from submarine algal and fungal structures in the rock record.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990