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Figure 4.
Geometric similarity might, but need not, mean similarity of process. (A) Cross-bedded Jurassic aeolian sandstone near Boulder, Utah, USA. Width of view ~15 m. (B) Cross-beds in fluvial Pleistocene basaltic sands near Mono Lake, California, USA. Width of view 50 cm. Bedding in both A and B formed in turbulent, high-energy environments where the Reynolds number was likely >104, and thus grain inertia dominated. (C) Intersecting modal layering in Cretaceous granodiorite near Mack Lake, California, USA. Width of view 60 cm. How these features form is unknown, but the extremely high viscosity of silicate liquids means that Reynolds numbers were likely 10–6 or less. Therefore, viscous forces were dominant, rendering impossible the sorts of grain interactions that produce crossbedding (Glazner, 2014). (D) Intersecting bands of diagenetic iron oxide in sandstone from the Triassic Chinle Formation, Utah, USA. Oxide layers do not correspond to depositional layering. Width of view 5 cm. Although these examples are geometrically similar, the erosive turbulence that truncated bedding in sedimentary rocks (A, B) cannot happen in highly viscous granitic magmas (C) and is irrelevant to the chemical processes that produce diagenetic banding in sandstones (D). The chemical processes that produced banding in the sandstone (D), however, may be relevant to banding in granodiorite (C).
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