Umm as Samim, Oman

Umm as Samim sabkha (BWh) is fed sediment from dunes to the west, alluvium from the Oman Mountains to the east, and groundwater from the limestone aquifers of the Tertiary Umm Er Radhuma Formation to the east and south. At around 3,000 km2 in area (55 m elevation), Umm as Samim is one of the largest sabkhas on the Arabian Peninsula. Known locally as the 'Mother of Poisons' or the 'Mother of Worries, Umm as Samim is a groundwater sump along the boundary of the eastern edge of the Arabian erg, with alluvial fans to the east, with many widyan draining from Al Hajar (Oman Mountains). Wadi Aswad, Wadi Musallim, Wadi Majhul, Wadi Umayri and Wadi Haliban all periodically feed the Umm as Samim sump.

Units hosting Holocene capillary salts at Umm as Samim are typically less than a few meters thick. Today, the water source is dominated by alluvial seeps, but before 6,000 years ago, it was a much less saline hydrology. Then the sump was a perennial lake. The initially closed sabkha depression or playa formed via deflation in the gypsiferous ‘Fars Group’ sometime before 30 ka. During the Late Pleistocene, from 30 to 20 ka, this closed depression became a saline lake, and fluvial flow brought in detrital silt and clay. A drier climate prevailed from 20 to 15 ka, and the lake changed to a sabkha. Wetter conditions returned towards the end of the Pleistocene, from 15 to 12 ka, and a perennial saline lake was re-established, whereby halite was dissolved, but gypsum remained. In the interval from 12 to 9 ka, sabkha conditions were again established with deposition of displacive gypsum and minor halite in a sandy matrix. During the Early Holocene wet phase, from 10 to 5.5 ka, a saline lake formed again, only to terminate in the arid capillary gypsum/halite conditions from 5.5 ka to the present-day.

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Umm as Samim is a regional groundwater sump at the  south western foot of the Oman Mountains

Today the Umm as Samim sump experiences occasional floods, mostly sourced from the nearby Oman Mountains. As a sheet of floodwaters dries, a typical ephemeral saline pan progression in capillary-fed salt crusts develops. A thickening salt layer passes from thin salt sheets to polygonal-overprinted salt crusts (A) to thick pressure-ridged megapolygonal features (B). Much or all of this surface crust will redissolve in the next significant flood event. Throughout the desiccation process, the regional saline water table lies only tens of centimetres beneath the surface (C). During the flood stage, small karst swallow holes develop in response to cavities created by the dissolution of underlying salt. These rill-rimmed features form similarly to the Ojo (eyes) of the Andean salars of South America.

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Umm as Samim, Oman. A) Network of salt polygons with an accumulation of salt crystals within desiccation cracks around the polygon margins. Grey–white salt crust within the polygons. Black notebook for scale. B) Irregular and disrupted salt polygons with heaved and thrust salt ramparts. (pressure ridges). Black notebook for scale C) Groundwater table visible in as brine pool in manmade excavation, with the surrounding almost flat surface of the Samim sabkha. D) Surface collapse feature on a crenulated Sammim with runoff rill disappearing into the sinkhole (After Fookes and Lee, 2019).
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