Surface and subsurface sedimentary structures produced by salt crusts

The growth and subsequent dissolution of salts on or within sediment may al ter sedimentary structures and textures to such an extent that it is diffic ult to identify the depositional origin of that sediment and, as a result, the sediment may be misinterpreted. To help to overcome such problems with investigating ancient successions, results are presented from a comprehensi ve study of the morphology and fabrics of three large areas of modern salt flats in SE Arabia: the Sabkhat Matti inland region and the At Taf coastal region, both in the Emirate of Abu Dhabi, and the Umm as Samim region in Om an. These salt flats are affected by tidal-marine, alluvial and aeolian dep ositional processes and include both clastic- and carbonate-dominated surfi cial sediments. The efflorescent and precipitated salt crusts in these area s can be grouped into two main types: thick crusts, with high relief (> 10 cm) and a polygonal or blocky morphology; or thin crusts, with low relief ( < 10 cm) and a polygonal or blister-like appearance. The thin crusts may as sume the surface morphology of underlying features, such as ripples or biog enic mats. A variety of small-scale textures were observed: pustular growth s, hair-like spikes and irregular wrinkles. Evolution of these crusts over time results in a variety of distinctive sedimentary fabrics produced by sa lt-growth sediment deformation, salt-solution sediment collapse, sediment a ggradation and compound mixtures of these processes. Salt-crust processes p roduce features that may be confused with aeolian adhesion structures. An e xample from the Lower Triassic Ormskirk Sandstone Formation of the Irish Se a Basin demonstrates how this knowledge of modern environments improves the interpretation of the rock record. A distinctive wavy-laminated facies in this formation had previously been interpreted as the product of fluvial sh eetfloods modified by soft-sediment deformation and bioturbation. Close ins pection of laminations seen in core reveals many of the same sedimentary fa brics seen in SE Arabia associated with salt crusts. This facies is the pro duct of salt growth on aeolian sediment and is not of fluvial origin.