Physical, chemical, and biologic mediation of rock fabrics along a subaerial surface : Antigua Formation (Oligocene), Antigua, British West Indies

Includes bibliographical references (pages [234]-253).Vertical profiles across interfaces between soil horizons and limestone of the Antigua Formation (Antigua, B.W.I.) were investigated by scanning electron microscopy, petrographic, isotopic, and geochemical techniques. Observations indicate that soil microbes play significant roles in the development and redistribution of porosity along the subaerial surface. Previous models of caliche formation have only considered abiotic processes of calcite accumulation. This investigation has shown that biota are primarily responsible for redistribution of carbonate in caliche horizons on Antigua, B.W.I.. Measured soil profiles on Antigua are typical Rendzina profiles, varying in thickness from less than 0.5 meter to over 1.5 meters where soils are forming on marls. Caliche is presently forming at scattered locations where soil and its associated microbes come in contact with underlying rock. The depth of caliche is independent of net water balance based on infiltration and evapotranspiration models. Total organic carbon generally decreases rapidly below the “A” horizon; however, in some cases, root mats develop at the sediment/rock interface. Evidence of dissolution increases near the subaerial surface, where the rock exhibits highly porous pelletal texture. Rb and Sr values are indicative of meteoric diagenesis, and isotopically light carbon signatures indicate organic mediation of dissolution/precipitation reactions in the upper 1.5 meters of sediment. Biological mediation of rock/water reactions is evident by textural evidence, such as degradation of allochems and etching of spar surfaces along filaments associated with root hairs. Vermicular textures produced by calcified filaments are most abundant at the sediment/limestone boundary, where alteration of rock fabric by calcified filaments is most prevalent. Masses of broken filaments 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 porosity. Tangential needle cement increases in abundance concomitantly with organic material, in the C soil horizon. Vadose zone cements deep in the profile include pore-lining calcite microspar and botryoidal chalcedony. Dissolution of chalcedony occurs where meteoric waters become saturated in calcite, and the pH is conducive to silica dissolution, in the soil horizons. Calcite cement morphologies may result from the combination of evaporative concentration of soil solutions (which increases Mg poisoning of crystal faces), and the production of HC03' in oxidizing soil horizons, or from slow precipitation from saturated solutions at depth, or from reactions catalyzed by biotic activity. This investigation indicates that biota may be responsible for the redistribution of calcite in caliche horizons on Antigua, B.W.I.M.S. (Master of Science