GEOCHEMISTRY, MINERALOGY, AND MORPHOLOGY OF FOSSIL WOOD FROM THE LATE PALEOCENE-EARLY EOCENE CHICKALOON FORMATION, ALASKA

We analyzed permineralized fossil wood from a stratigraphic section of the Chickaloon Formation in the Wishbone Hill area of the Matanuska coalfield. Five representative samples from various positions in stratigraphic section were selected to undergo bulk chemical, mineralogic, and morphologic investigation.  Utilizing the XRF bulk chemistry technique, it was discovered that a major gradational change in the chemistry of the permineralized wood samples occurred with respect to stratigraphic section.  The chemical composition of samples high in section tended to contain high levels of calcium, which declined in abundance lower in stratigraphic section, whereas iron followed the opposite trend. This change correlates well with the observed mineralogic differences of the samples.  X-ray diffractograms revealed that in fossil wood high in the stratigraphic section the mineralogy of the samples consisted mostly of calcite with some ankerite.  With increasing depth in the section the prevalence of the ankerite phase increased as the calcite phase decreased.  Low in stratigraphic section, siderite and ankerite were the dominant minerals. Morphologically, the samples also exhibited a change with stratigraphic position. Samples collected higher in the section exhibiting better wood preservation, (including anatomical features) and evidence of greater brittle deformation. Samples low in section exhibiting poorer preservation, anatomical distortion and a high-degree of plastic deformation. The observed trends could be an artifact of a variety of processes that influence porewater chemistry. These include changing proportions of marine-and freshwater input to the basin, a difference in the timing of the permineralization process, or even differences in the original chemistry of the wood that was ultimately permineralized.