ORIGIN AND SILICA DIAGENESIS OF TEREDOLITES-BORED WOOD IN TRANSGRESSIVE DEPOSITS, EOCENE TALLAHATTA FORMATION, WESTERN ALABAMA

Silicified angiosperm wood entombed in a porcelanite concretion has been recovered from the basal part of a siliceous claystone sequence in the Eocene Tallahatta Formation, western Alabama. This wood is characterized by Teredolites longissimus, borings produced by wood-digesting teredinid bivalves (shipworms). Like many bored fossil wood occurrences in the Gulf coastal plain, this bored substrate appears to be associated with a phase of sea-level rise and possibly could have originated as an in situ stumpground at or near a transgressive surface. Silicification of the wood and elements of the borings is related to remobilization of silica from originally diatom-rich ambient sediments. Petrographic and x-ray diffraction analyses indicate that silica phases and replacement textures (e.g., amorphous silica, opal-CT lepispheres, and chalcedony) are variable and govern the histological detail preserved in the wood. The disposition of silica phases in the wood, and concretion growth, likely reflect temporal or diffusion-controlled spatial gradients in dissolved silica concentrations of mineralizing fluids. Teredolites tunnels effectively increased the surface area of wood in contact with reactive sediments and/or silica-rich interstitial fluids and, as a consequent, enhanced wood preservation.