Surface and subsurface sedimentary structures produced by salt crusts

Abstract

The growth and subsequent dissolution of salts on or within sediment may alter sedimentary structures and textures to such an extent that it is difficult 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 comprehensive 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 Oman. These salt flats are affected by tidal‐marine, alluvial and aeolian depositional processes and include both clastic‐ and carbonate‐dominated surficial sediments. The efflorescent and precipitated salt crusts in these areas 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 assume the surface morphology of underlying features, such as ripples or biogenic mats. A variety of small‐scale textures were observed: pustular growths, hair‐like spikes and irregular wrinkles. Evolution of these crusts over time results in a variety of distinctive sedimentary fabrics produced by salt‐growth sediment deformation, salt‐solution sediment collapse, sediment aggradation and compound mixtures of these processes. Salt‐crust processes produce features that may be confused with aeolian adhesion structures. An example from the Lower Triassic Ormskirk Sandstone Formation of the Irish Sea 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 sheetfloods modified by soft‐sediment deformation and bioturbation. Close inspection of laminations seen in core reveals many of the same sedimentary fabrics seen in SE Arabia associated with salt crusts. This facies is the product of salt growth on aeolian sediment and is not of fluvial origin.

A classification scheme for fluvial–aeolian system interaction in desert-margin settings

“…In cases where flood waters are saline and where ponded water evaporates or infiltrates only slowly, salts such as calcium carbonate, gypsum, halite or potash may be precipitated (Valyashko, 1972). For example, the salt flats of Umm as Samim, close to the eastern border of the Rub' Al-Khali Sand Sea, Oman, occur in a low-lying area between the alluvial fans to the north, the aeolian dunes of the Rub' Al Khali to the west and south (Figure 2, Goodall et al 2000). If the outer edge of the aeolian dune field gradually expands over time via dune migration, aeolian deposits may become juxtaposed over flood deposits.…”

Intermontane eolian sand sheet development, Upper Tulum Valley, central-western Argentina

“…The formation of surface evaporites may follow two main processes, efflorescence and precipitation. Efflorescence of salt crystals occurs where salt accumulates by direct crystallization onto sediment grains as a result of the evaporation of saline ground water adhering to those grains, and precipitation forms by the evaporation to dryness of ephemeral ponds of rainwater (Goodall et al 2000). The adhesion of sand and silty mud to salt crystals occurs by the hygroscopic action of evaporites (Kocurek & Fielder 1982), and the bicolored pattern seen on the surface of this structure is given by the high proportion of adhering dark windblown dust on white salt crystals.…”

“…In the area close to the south extremity of the sand sheet (transitional area from sand sheet to playa environment), where the surface is periodically damp, a variety of adhesion features are found, including adhesion ripples, adhesion warts, and evaporitic-adhesion structures (Kocurek & Fielder 1982;Olsen et al 1989;Goodall et al 2000). Adhesion ripples and warts are small structures, 0.3 -2 mm high, less than 5 mm in wavelength, characterized by very fine-to fine-grained sand adhered to the damp surface.…”

Spatial-Temporal Distribution of Soil Salt Crusts under Saline Drip Irrigation in an Artificial Desert Highway Shelterbelt

“…A soil salt crust (SSC) is a special layer of topsoil with high concentrations of soluble salts, which mainly include Na + , Mg 2+ , Ca 2+ , Cl´, and/or SO 4 2´[ [1][2][3]. It is significantly different from physical and biological soil crusts.…”

“…It is significantly different from physical and biological soil crusts. SSCs develop in some regions with shallow groundwater levels [1,2,[4][5][6], saline-water irrigation [3,7], and saline surface and groundwater [8].…”