Early Diagenetic Silica Deposition in Algal Cysts and Spores: A Source of Sand in Black Shales?
ABSTRACT Devonian black shales deposited on the North American craton contain abundant Tasmanites cysts. Although these are typically flattened because of compaction, a small proportion are filled with diagenetic silica. The latter are spherical to ellipsoidal (0.1-0.5 mm), and filled with chalcedony, microquartz and megaquartz, and single quartz crystals. Chalcedonic cyst fillings are preserved best in cheat and phosphate nodules, whereas megaquartz and single quartz crystals are most common in shale matrix. Together with colloform textures, this suggests that the various silica types originated from recrystallization of early diagenetic silica deposits. Thin sandstone beds in the Chattanooga Shale (e.g., Bransford Sandstone) contain abundant quartz sand that is much coarser than the detrital quartz component of the underlying black shales. Because of this, their quartz component is thought to have been transported over considerable distances from the basin margin. However, because certain shale horizons contain as much as 10% silicified cysts that upon reworking could have yielded quartz grains of fine to coarse sand size, the quartz component of these sandstone beds may actually have formed in situ. Indeed, petrographic examination of the sandstone beds shows them to contain quartz grains with morphological and textural features of "cyst" quartz (e.g., rounding, sphericity, chalcedony, pyrite inclusions, lobate grain margins). Thus, silica deposition in algal cysts may provide a significant component of intrabasinal quartz sand in shale sequences. Distinction of this type of quartz from extrabasinal detrital quartz is important to the reconstruction of the depositional history of shale sequences.
Petrography of Shales : A Survey of Techniques by
With clays typically being the most abundant constituent of shales, their study and identification has long figured prominently in the petrographic study of shales. Useful texts are those by Correns (1938), Müller, 1967, Grim (1968), Millot (1970), Weaver (1989), Serra (1990), Jasmund and Lagaly (1991), and Blatt (1992). They cover aspects such as classical clay mineralogy, applied clay mineralogy, clay geology, clay deposition, clay in engineering geology, geochemistry, clay diagenesis and logging responses. When dealing with clay minerals one has to keep in mind that they are typically metastable (Lippmann, 1979), a circumstance that makes precise determination often difficult (e.g. mixed-layer clays, glauconite group). Besides the composition-centered, general classification schemes mentioned in the introduction to this book (e.g. Ingram, 1953; Dunbar and Rodgers, 1957; Folk, 1965; Picard, 1971; Lewan, 1979; Blatt, Middleton, and Murray; 1980; Lundegard and Samuels, 1980; Potter et al, 1980), several authors have presented classification schemes that were designed to meet specific needs of a region, basin, or stratigraphic unit. Conze (1984) defined six types of fine-grained sediments in the Upper Carboniferous of the Ruhr basin, based on grain size, sedimentary structures, and organic carbon content. Nuhfer et al. (1979) presented a classification Devonian shales of the eastern US that relies heavily on features observed in X-radiographs. Mineralogical-geochemical classifications of some genetic kaolinite types, resulting from bedrock weathering and recycling of clays from older sediments, were presented by Lasch and Thiergärtner (1981). Classifications in which textural characteristics are emphasized over compositional parameters were presented by Schieber (1989, 1994b) for shale successions with comparatively small compositional variations.
HANDBOOK OF OCEANOGRAPHIC TABLES
Abstract : The tables are divided into four sections: General Mensuration Information Related to the Oceans, Data on Oceans not Related to Geography, Data on Oceans Related to Geography, Tables for Computation and Conversions.