Sabkha Matti, Middle East

Sabkha Matti is one of the largest tectonically-induced sabkha depressions in the region. It transects the western extremity of the Abu Dhabi Emirate and the north east of Saudi Arabia. It is the largest sabkha in the UAE, and is mostly a continental sabkha (Kirkham et al., 2012). At its northern end is a marine-influenced coastal strip, around 8km wide, that is similar in many ways to the Abu Dhabi coastal sabkhat farther east, except that anhydrites are poorly developed. The surface of the southern margin of the coastal sabkha is covered by a deflationary marine shelly lag, which passes into the northern part of the continental Matti sabkha. This region is covered by a veneer of coarse sand and gravel representing the deflation lag from vanished aeolian dunes and former fluvial channels. Salt accumulation (mostly CaSO4 ± NaCl) in this and all other sabkhas is controlled by a combination of capillary evaporiatation and eolian deflation (see sabkha hydrology).

Coast-parallel supratidal salt flats in the marine margin of the sabkha grade southwards into inland salt flats of the continental eolian sabkha, with no distinct break in surface topography. Capillary salts (gypsum and halite) are growing in this inland sabkha zone, wherever shallow water-tables occur across vast expanses hosted by deflationary eolian sand sheets. The inland sabkha (BWh), with an area of 2,950 km2, extends southwards for about 150 km and, in this distance, the land surface rarely rises more than 40–60 m above sea level. There is little information available on rainfall for the inland areas of Sabkha Matti, but measurements taken on the coast show that average annual rainfall is less than 40 mm and further inland it is less. In the 1930s, Bedouins told western surveyors that the salt flats were named after someone named "Matti", who had disappeared while trying to cross them.

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Sabkha Matti is a regional groundwater sump, located between the Rub Al Khali and the Qatar Arch in the southern central Arabian Gulf (in part after Alsharhan and Kendall, 2003; Landsat image courtesy of NASA).

Like the Rann of Kutch, Sabkha Matti occupies a former fluvial valley, which in the Miocene was a freshwater river system. Today this region of saline mudflats is a groundwater sump to the Qatar Arch. It also defines the margin to the modern accretionary eolian sabkha sheet along the edge of the Rub al Khali, and so is adjacent to a set of dry mudflats and sand sheets. This, in turn, passes east and downwind into an active sand sea (Rub Al Khali portion of the Arabian Erg) that is ultimately piling up sand at the foot of the Oman Mountains.

Evaporation of occasional ephemeral brine sheets reforms the salt crust, which for most of the year covers large areas of Sabkha Matti. Somewhat thicker salt crusts (1 ± 5 cm), with large megapolygonal pressure ridges, characterise the Sabkha Matti surface situated within 5–10 km of the modern coast. Further inland, thinner salt crusts mark the Sabkha Matti surface (≈1 cm thick) and smaller, lower-relief, pressure ridges, along with more numerous blisters and petees, interact with windblown sediments to form characteristic haloturbated, slightly reddened “leopard-skin” structures (Evans &Kirkham, 2005).

“Leopard skin” constitutes a hummock-and-hollow relief up to a metre in vertical scale. The hummocks tend to be darker in colour than the hollows, hence the derivation of the “leopard skin” descriptive term. Within half a metre of the surface, the hummocks, in particular, are generally underlain by concentrations of fibrous halite, gypsum and palygorskite (Figure). Locally, selenite gypsum can protrude through the surface in the “leopard-skin” terrain.

Several modern gypsum volcanoes have also been observed in Sabkha Matti (Kirkham et al., 2012). They reach up to a metre in height and create gypsum cones in the sabkha up to 50 m in diameter around a central vent. The volcanoes seem to occur near the centres of large depressions on Sabkha Matti and are perhaps an artesian indicator. The gypsum is likely of both Miocene and Quaternary origin. At least some of the volcanoes are known to be modern, as they spread across recent vehicle tracks.

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Polygonal salt crust at Matti surface away from highway.
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Polygonal crust at Sabkha Matti surface, looking north taken from the coastal highway where expansion of crust interacts with highway embankment to create rectangular crests in crust.
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A shallow trench in "leopard-skin" terrain revealing a subsurface mixture of palygorskite and gypsum, concentrated within soft aeolian quartz sand, Sabkha Matti. Handle of knife is 11cm long (after Kirkham et al., 2012)
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