4 research outputs found
Characterization of hardâtoâdifferentiate 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 fineâscale processes that deposited sediment on palaeoâdune foresets. While these facies are relatively easy to distinguish in some crossâbedded sandstones, for others â like the Permian Coconino Sandstone of northern and central Arizona â discrete stratification styles are hard to recognize at the bedding scale. Furthermore, few attempts have been made to classify fineâscale 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, crossâbed 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 laserâdiffraction particle analysis, highâresolution scans of thin sections, and scanning electron microscopy. Crossâbeds were observed to be laterally extensive alongâstrike, with most dip angles ranging from the midâteens to midâtwenties. While some crossâbed sets are statistically coarser near their bases, others exhibit no significant vertical sorting trends. Both massive and laminated textures are visible in highâresolution 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 crossâbeds may consist of grainflow and grainfall deposits, but these remain difficult to differentiate at outcrop and thinâsection scales. This characterization of fineâscale processes will play a critical part in the development of depositional models for the Coconino Sandstone and elucidate interpretations for similar crossâbedded formations