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.
Evidence for high-energy events and shallow-water deposition in the Chattanooga Shale, Devonian, central Tennessee, USA
Abstract The Upper Devonian Chattanooga Shale of central Tennessee, a classical black shale, was deposited in an epicontinental setting, west of the Appalachian foredeep. Its finely laminated and highly carbonaceous nature is commonly interpreted to indicate deposition in comparatively deep and stagnant water. Interbeds of bioturbated greenish-gray shale, indicating oxygenated bottom waters, are commonly ascribed to pycnocline fluctuations. However, laminated fine sand and silt and hummocky cross-stratification (HCS) at the base of some of these beds indicates interaction of storm waves with the seabed, and suggests that greenish-gray shale beds are post-storm mud drapes. Other interesting features are inclined-undulose erosion surfaces that are conformably overlain by shale beds, sets of inclined shale beds that suggest low-angle cross-bedding, and clearly and uniformly developed alignment of clay particles (magnetic fabric studies). These observations show that the seabed was at times subject to prolonged erosion by bottom currents (erosion surfaces), agitation and reworking by storm waves (HCS and greenish-gray shale beds), and sediment transport by long-lived bottom currents (particle alignment). The epicontinental sea setting and the presence of HCS and other storm-produced features suggest a relatively shallow water depth (possibly only a few tens of meters). Together with abundant evidence of variably strong bottom currents and bioturbation of black and gray shale beds this suggests that abundant planktonic organic matter production rather than stagnant bottom waters are the primary cause for black shale formation.
Prevalence of Headache Within a College Student Population: A Preliminary Analysis
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