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.
Diagenetic origin of quartz silt in mudstones and implications for silica cycling
Mudstone—the most abundant sedimentary rock type, composed primarily of clay- or silt-sized particles—contains most of the quartz found in sedimentary rocks. These quartz grains, which are chemically and mechanically resistant and therefore preserve their characteristics well, have long been considered to be derived from the continental crust. Here we analyse quartz silt from black shales in the eastern USA, dating back to the Late Devonian period (about 370 million years ago), using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe. Our results indicate that up to 100% of the quartz silt in our samples does not originate from the continental crust. Instead, it appears to have precipitated early in diagenesis in algal cysts and other pore spaces, with silica derived from the dissolution of opaline skeletons of planktonic organisms, such as radiolaria and diatoms. Transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 Myr ago and molecular evidence for a much earlier appearance between 266 or even 500 Myr ago. Moreover, if many other mudstone successions show similarly high proportions of in situ precipitated—rather than detrital—quartz silt, the sedimentary record in mudstones may have been misinterpreted in the past, with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica.
Sedimentology, petrology, and gas potential of the Brallier Formation: upper Devonian turbidite facies of the Central and Southern Appalachians
The Upper Devonian Brallier Formation of the central and southern Appalachian basin is a regressive sequence of siltstone turbidites interbedded with mudstones, claystones, and shales. It reaches 1000 meters in thickness and overlies basinal mudrocks and underlies deltaic sandstones and mudrocks. Facies and paleocurrent analyses indicate differences between the depositional system of the Brallier Formation and those of modern submarine fans and ancient Alpine flysch-type sequences. The Brallier system is of finer grain size and lower flow intensity. In addition, the stratigraphic transition from turbidites to deltaic sediments is gradual and differs in its facies succession from the deposits of the proximal parts of modern submarine fans. Such features as massive and pebbly sandstones, conglomerates, debris flows, and massive slump structures are absent from this transition. Paleocurrents are uniformly to the west at right angles to basin isopach, which is atypical of ancient turbidite systems. This suggests that turbidity currents had multiple point sources. The petrography and paleocurrents of the Brallier Formation indicate an eastern source of sedimentary and low-grade metasedimentary rocks with modern relief and rainfall. The depositional system of the Brallier Formation is interpreted as a series of small ephemeral turbidite lobes of low flow intensity which coalescedmore » in time to produce a laterally extensive wedge. The lobes were fed by deltas rather than submarine canyons or upper fan channel systems. This study shows that the present-day turbidite facies model, based mainly on modern submarine fans and ancient Alpine flysch-type sequences, does not adequately describe prodeltaic turbidite systems such as the Brallier Formation. Thickly bedded siltstone bundles are common features of the Brallier Formation and are probably its best gas reservoir facies, especially when fracture porosity is well developed.« less