Early Diagenetic Silica Deposition in Algal Cysts and Spores: a Source of Sand in Black Shales

Devonian black shales that were deposited on the North American craton contain abundant Tasmanites cysts. Although these are typically flattened because of compaction, a small proportion of cysts is filled with diagenetic silica. The latter are spherical to ellipsoidal (0.1-0.5mm), and filled with chalcedony, microand megaquartz, and with single quartz crystals. Chalcedonic cyst fillings are preserved best in chert 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 that are found in the Chattanooga Shale (e.g., Bransford Sandstone) contain abundant quartz sand that is much coarser than the detrital quartz component of 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. INTRODUCTION Mudrocks contain the largest share of quartz in the sedimentary column (Blatt, 1970). Quartz grains in mudrocks are thought to be an extrabasinal detrital component, carried into the basin by flowing water and winds. Potential sources are 1) fine-grained low-grade metamorphic rocks, 2) fracturing of coarser grains during weathering and soil formation; 3) abrasion during transport; 4) formation of quartz during clay diagenesis; 5) recrystallization of biogenic opal (Blatt, 1987), as well as combinations thereof. Of these, the first is considered the main source of quartz in mudrocks, whereas recrystallization of biogenic opal is not thought to contribute significantly (Blatt, 1987). Within mudrocks and shales, large detrital grains are thought to provide particularly valuable information concerning provenance and dispersal patterns (Potter et al., 1980). For example, coarse quartz sand in the middle of a shale basin would imply that the sand was derived from the shoreline and moved offshore by currents for possibly hundreds of kilometers. I demonstrate here that in shales, diagenetic silica deposition in algal cysts and similar microfossils can produce quartz grains in the fine to coarse sand grain size range. With quartz grains of that size present, winnowing of a shale by waves could produce sandstone beds without the necessity of strong currents to move material over large distances. GEOLOGIC SETTING This study is based on petrographic examination of black shale samples from the Late Devonian Chattanooga Shale (central Tennessee and southern Kentucky) and New Albany Shale (southern Indiana). These shales were deposited in the Appalachian Basin, and their exposed thickness is approximately 10m in central Tennessee and 35m in southern Indiana. The shales commonly are interpreted as having accumulated in stagnant and comparatively deep water e.g. Potter et al., 1982). Recognition of undulose submarine erosion surfaces and hummocky cross-stratified (HCS) silt and sand beds (Schieber, 1994), however, as well as the presence of widespread bone beds (Conkin and Conkin, 1980), suggests relatively shallow water. These black shales were deposited at the distal end of a westward thinning clastic wedge. Throughout it, they contain sporelike microfossils that commonly are referred to as Tasmanites (e.g. Winslow, 1962). In the thinnest and most distal portions of this clastic wedge, in central Tennessee and south-central Kentucky, the average accumulation rate of these shales was on the order of 103mm/year. This figure is based on thickness of stratigraphic sections, conodont data (Ettensohn et al., 1990), and age calibrations of Devonian conodont zones (Harland et al., 1990, Fordham, 1992). The very slowly deposited black shales of above distal areas contain the Tasmanites cysts with internal deposits of diagenetic silica that are described in this paper. Tasmanites probably represents the cyst stage (phycoma) of fossil algae with affinity to modern planktonic green algae (Prasinophyta; Tappan, 1980). The latter can take on two distinct appearances during their life cycle, changing from motile quadriflagellate cells to cysts in which new motile cells develop (Tappan, 1980). Whereas the motile cell has no skeletal mineralized structures and consequently has little chance to become fossilized, the cyst walls consist of a complex "lipoid" substance that resists chemical breakdown (Tappan, 1980). What is found in the geologic record are probably cysts that settled to the bottom after their contained motile cells were released (Tappan, 1980). Tasmanites and similar forms (Family Tasmanitaceae) are found in deposits from the Precambrian to the Holocene. Spherical prior to compaction, they are characterized by thick walls that are perforated by canals that open as pores to their interior (Tappan, 1980). In Late Devonian rocks of eastern North America, Winslow (1962) recognized several different species of Tasmanites, ranging in size from 0.05-0.81mm.