Hump Cross-Bedding and the Recognition of Ancient Parabolic Dunes with Examples from the Miliolite, Western India

Cross-Bedding is a common sedimentary structure in the global stratigraphic record with planar and trough cross-Bedding occurring more frequently than hummocky Cross-Bedding. Whereas the latter is typical to marine environments the former is observed both in aeolian and Sub-Aqueous environments (fluvial and marine). A fourth variant of Cross-Bedding is described here that is characterized by Convex-Up structures in the depositional dip direction and Hump-Like structure across the depositional dip direction. In plan view the structure has beds that show radial fanning out dip azimuths with dip amounts varying between 25° and 30°. This structure is termed here as 'hump crossbedding' owing to its hump like geometry across the depositional dip direction. These bedding geometries may be reconciled with sediment migration along the front of a parabolic dune. Since parabolic dunes do not form in subaqueous environments, hump Cross-Bedding becomes an essential aid in distinguishing aeolian from shoreface deposits in coastal environments. The recognition of this structure becomes invaluable towards reconstructing depositional environments and deducing their palaeoenvironmental significance from Quaternary deposits such as those found in the Bahamas, Bermuda and Saurashtra.