HR: 0800h
AN: PP31A-1519    [Abstracts]
TI: Preservation of primary structures and organic matter in Pleistocene spring carbonates, Western Desert, Egypt: Relationship to macroscopic texture and age.
AU: * Pohlman, E
EM: epohlman@rice.edu
AF: Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Smith, J R
EM: jensmith@wustl.edu
AF: Washington University, 1 Brookings Dr., St. Louis, MO 63130 United States
AB: Spring-deposited carbonates (tufas) along the flanks of the Libyan Escarpment in Dakhleh and Kharga Oasis record relatively humid conditions which prevailed in the Egyptian Sahara periodically throughout the Pleistocene. Previous work, particularly Nicoll et al.(1999), has suggested the Western Desert tufas, though certainly displaying evidence of secondary cementation by sparry calcite, aggrading neomorphism, etc. do in many instances preserve primary features, particularly organosedimentary lamination, and a clotted microbial texture. In order to facilitate field-based selection of suitable, unaltered samples for geochemical analysis, we undertook a petrographic examination of tufa samples in order to determine whether certain macroscopic features (e.g., color, porosity, presence of detrital iron oxides, preservation of visible plant casts) could be quantitatively correlated to the degree of diagenesis present in thin sections as indicated by percent calcite spar. We also determined total organic content through peroxide digestion, as younger samples (determined by U-series dating and by geomorphic context) qualitatively appeared to contain both more casts of botanical remains, and better defined microbial textures. Older and more altered tufas also generally had heavier (less organically-influenced) carbon isotopic signatures, further suggesting a relationship between diagenesis, organic content, and age. Petrographic analysis included descriptions of sample texture, spatial relationship of textural elements (e.g., pores, plant casts, detrital material), and frequency of biological inclusions or casts. Point counts were performed to estimate sample mineralogy and porosity. Tufas are predominantly micritic calcite, with little (generally <2%) sparry calcite. Porosity may be as great as 46%. Most samples examined displayed some evidence of primary (generally microbial) textures. The expected relationship between porosity and diagenetic alteration, however, was not found. While some denser (non-porous) tufas are the result of post-depositional infill and alteration of primarily porous tufas, others reflect unaltered, primary fabrics deposited in deep pools or along downstream pool rims. Porous tufas were expected to display substantial evidence of recrystallization, originating at pore margins, but generally showed little to no penetration of diagenetic alteration past the immediate pore boundaries. While general trends in other macroscopic factors were loosely correlated with relative age (e.g., older tufas were paler in color), variation in same-age tufas of different facies is substantial enough to prohibit age classification simply based on these factors. Nicoll, K., Giegengack, R. and Kleindienst, M., 1999. Petrogenesis of Artifact-bearing fossil-spring tufa deposits from Kharga Oasis, Egypt. Geoarchaeology: An International Journal, 14(8): 849-863.
DE: 1039 Alteration and weathering processes (3617)
DE: 1051 Sedimentary geochemistry
DE: 3625 Petrography, microstructures, and textures
DE: 3675 Sedimentary petrology
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005