Sedimentology of cherts in the Early Proterozoic Wishart Formation, Quebec–Newfoundland, Canada

The siliciclastic Wishart Formation of the Early Proterozic Labrador trough is high-energy shelf deposit. Wishart sandstones contain both interstitial chert with textures of void-filling cement and thin chert intercalations contaminated with siliciclastic mud. Although volumetrically minor, these cherts occur in several thin, areally extensive stratigraphic mud. Although volumetrically minor, these cherts occur in several thin, areally extensive stratigraphic intervals. The Wishart contains intraclasts of both the chertcemented sandstone and the impure chert layers (as well as several other types of chert sand and gravel). This suggests the cherts formed penecontemporaneously, which is consistent with the absence of any signs of replacement in all but one of the chert types and the clear-cut distinctions between chert types, even where they are side by side in a single thin section. The origin which appears to be most compatible with available evidence is that the cherts represent silica precipitated from thermal waters that rose through the sediments of the Wishart shelf and discharged precipitated from thermal waters that rose through the sediments of the Wishart shelf and discharged into suprajacent seawater. A biogenic origin is unlikely in view of the lack of appropriate organisms during the Early Proterozic and the rapidity with which the cements formed. A volcanogenic origin is likely because volcaniclastic tezxtures are plentiful in associated formations but absent from the Wishart. Precipitation induced by evaporative concentration in unlikely in view of the widespread evidence of tidal currents and the lack of evidence of desiccation in the Wishart. Finally, the cherts are not restricted to the lowest-energy faces, and therefore they presumably did not accumulate as a background sediment. Deposition of silica above the sediment/water interface was probably made possible by ambient concentrations of silica that were significantly higher than those of Phancrozoic seawater. Cherts with similar textures occur in other Early Proterozoic sediments, most notably arenitic or granular iron-formations

Characterization of hard‐to‐differentiate dune stratification types in the Permian Coconino Sandstone (Arizona, USA)

Dune stratification types, which include grainfall, grainflow and ripple lamination, provide a record of the fine‐scale processes that deposited sediment on palaeo‐dune foresets. While these facies are relatively easy to distinguish in some cross‐bedded sandstones, for others – like the Permian Coconino Sandstone of northern and central Arizona – discrete stratification styles are hard to recognize at the bedding scale. Furthermore, few attempts have been made to classify fine‐scale processes in this sandstone, despite its renown as a classic aeolian dune deposit and Grand Canyon formation. To interpret depositional processes in the Coconino Sandstone, cross‐bed facies were characterized using a suite of sedimentary textures and structures. Bedding parameters were described at multiple scales via a combination of field and laboratory methods, including annotated outcrop photomosaics, strike and dip measurements, sandstone disaggregation and laser‐diffraction particle analysis, high‐resolution scans of thin sections, and scanning electron microscopy. Cross‐beds were observed to be laterally extensive along‐strike, with most dip angles ranging from the mid‐teens to mid‐twenties. While some cross‐bed sets are statistically coarser near their bases, others exhibit no significant vertical sorting trends. Both massive and laminated textures are visible in high‐resolution scans of thin sections, but laminae contacts are commonly indistinct, making normal and reverse grading difficult to define. Diagenetic features, such as stylolite seams and large pores, are also present in some samples and might indicate alteration of original textures like detrital clay laminae and carbonate minerals. Observed textures and sedimentary structures suggest that the cross‐beds may consist of grainflow and grainfall deposits, but these remain difficult to differentiate at outcrop and thin‐section scales. This characterization of fine‐scale processes will play a critical part in the development of depositional models for the Coconino Sandstone and elucidate interpretations for similar cross‐bedded formations