Geology topics
Publications and source records attributed to Gerhard W. Leo.
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Undisturbed cores of saprolite developed on crystalline rocks of the Piedmont Province in Fairfax County, Virginia have been obtained using a combination of Shelby tubes, Denison sampler, and modified diamond core-drilling. The principal purpose of the core study is to correlate variations in chemistry, mineralogy and texture with engineering properties throughout the weathering profile. Coring sites were chosen to obtain a maximum depth of weathering on diverse lithologies. The rocks investigated include pelitic schist, metagraywacke, granite, diabase and serpentinite. Four to twelve samples per core were selected, depending on thickness of 1) the weathering profile (from about 1 m in serpentinite to more than 30 m in pelitic schist) and on 2) megascopic changes in saprolite character for analysis of petrography, texture, clay mineralogy andd major element chemistry. Shear strength and compressibility were determined on corresponding segments of core. Standard penetration tests were performed adjacent to coring sites to evaluate engineering properties in situ. Geochemical changes of saprolite developed from each rock type follow predictable trends from fresh rock to soil profile, with relative Increases in Si, Ti, Al, Fe3+ and H20; variable K; and relative loss of Fe 2+, Mg, Ca, and Na. These variations are more pronounced in the weathering profiles over mafic and ultramafic rocks than metagraywacke. Clay minerals in granite, schist and metagraywacke saprolite are kaolinite, dioctahedral vermiculite, interlayered micavermiculite, and minor illite. Gibbsite is locally developed in near-surface samples of schist. Standard penetration test data for the upper 7 m of saprolite over schist and metagraywacke suggest alternations between stronger and weaker horizons than probably reflect variations in lithology including the presence of quartz lenses. Results for granite saprolite are most consistent but indicate lower strength. Shear strength increases fairly regularly downward in the weathering profile. The engineering behavior of diabase saprolite is controlled by a dense, plastic, near-surface clay layer (montmorillonite and kaolinite)overlying rock which is weathered to a granular state (grus), while engineering properties of serpentinite are determined by a very thin weathering profile.
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Petrographic thin sections have been analyzed for their major-element composition using a fusion-dilution technique and measuring the intensity of X-rays by means of the electron microprobe. The balsam-mounted thin sections were removed from the glass slides by soaking them in methylene chloride. The freed sections were mixed with twice their weight of Li 2 B 4 O 7 , and fused at 1100° C. A fragment of the resulting glasslike bead was mounted for probe analysis. Both wavelength and energy-dispersive detector systems were used for quantitative determinations of elements sodium through iron. Because the samples and standards are diluted and fused, powdered rock standards may be used as reference materials. The calibration curves obtained by plotting X-ray intensity versus concentration of the analyte are linear over the concentration ranges used in this work and have an overall range of error of 2 to 8 percent. Although some of the analytical values show excessive scatter for petrographically similar rocks, in general the analyses are acceptable given the sample size and analytical uncertainties. Bulk chemical analyses were made of 18 thin sections, including altered diabase, amphibolite, and calc-silicate hornfels produced by progressive contact metamorphism and associated with metasomatic magnetite deposits in the Samli area, western Turkey. Normative plots indicate that (1) diabase and amphibolite are compositionally related and are similar to average compositions of basalts and orthoamphibolites, and (2) calc-silicate hornfels appears to have been derived in part from amphibolite and in part from crystalline limestone that underlies much of the region.
The Kuluncak-Sofular area, located about midway between Sivas and Malatya in east-central Turkey is underlain by a variety of sedimentary, volcanic, and intrusive rocks. The sedimentary rocks have been deposited on a pre-Campanian serpentinite basement and include Cretaceous conglomerate, graywacke, tuff, and limestone; Eocene arkosic sandstone, conglomerate, and limestone; and Miocene limestone and dolomite. K-Ar ages determined for volcanic and intrusive rocks from the same area are 75.5 m.y. for alkalic diabase that intrudes the Upper Cretaceous sedimentary rocks; 74.3 and 71.1 m.y. for trachyte that partly overlies and partly intrudes the same Upper Cretaceous sequence; 65.2 m.y. for alkalic syenite that intrudes Upper Cretaceous limestone; 18.7 to 16.8 m.y. for andesite and basalt that overlie middle to late Eocene sedimentary rocks; and 14.1 m.y. for a dacite plug that cuts Miocene limestone. © 1974 Geological Society of America.
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