CHAPTER 2 SURFACE SEDIMENTS
Published:January 01, 2010
The Surface Sediments equates to the Quaternary Superficial Deposits of Hughes Clarke (1988, his figure 3). Never described or published as a single complete unit they comprise any sediments younger than the Fars Group. The simplified geological map of Oman (Figure 1.1) provides an overview of the surface geology.
Lithology and surface expression: The main building blocks of what constitutes the land area of Oman are the desert and coastal plains separated by mountain ranges. The Al Hajar Mountain range dominates the northern coastline with the peaks of Jabal Al Akhdar in the western Al Hajar Mountains exceeding 3,000 m (Jabal Shams). The desert plateau of Central Oman is bounded to the south by the Qara Mountains (reaching up to 1,675 m and extending into the Hadhramaut of adjacent Yemen). The country is fully situated in the band of tropical deserts that spans the earth between latitude 10°–30° north and south of the equator. Not surprisingly around 80% of Oman consists of desert, most of which comprises extensive gravel plains, with subtle depressions through which wadis (dry, shallow watercourses) wind their way. In the desert climate of Oman, these wadis only flood rarely, at most a few times a year.
Alluvial fans flank the Al Hajar Mountains in the North of Oman and the Qara Mountains in the southwest. Short fans cover the Batinah Coast in the north and the Salalah plain in the south. Much bigger fans, up to 200 km long, interfinger with dune sands of the Rub’ Al-Khali in the west, covering much of Oman’s interior desert plateau (the Najd) at an altitude of some 200 m or less. The ophiolite-rich older gravels in North Oman are partly altered to dolomite, which has been named ‘barzamanite’ (Maizels, 1988). Wadis drain from the southern and northern mountains into the interior, with a number of them ultimately leading to the large interior sabkha of the Umm As Samim; at approximately 58 m altitude the lowest area of Interior Oman and the adjacent Rub’ Al-Khali Desert. Subsurface flow from underlying Cenozoic limestones aquifers contribute a significant part of the current water budget in the Umm As Samim, with only occasional surface water inflow from the northern mountains (Heathcote and King, 1998).
Much of the western borderlands of Interior Oman are part of the Rub’ Al-Khali with extensive sand dunes (Glennie, 2005). The Al Sharqiyah Sands (formerly known as the Wahiba Sands), in the east of Central Oman, is the other true sand desert in Oman. The remainder of Oman’s interior is a desert plateau that developed on underlying flat lying Cenozoic carbonates at an altitude of some 100 to 200 m. Away from the mountain ranges the flat desert plains have received very little sediment over time, with strong winds removing the fines. These plains have become well known as a meteorite treasure-trove, where some of the rarest Martian meteorites have been found. Locally the underlying limestones are extensively karstified, forming sinkholes where groundwater levels in the limestone aquifers are close to the surface. Noteworthy is the Khoshilat Magandeli or Majlis Al Jinn Cave, the second largest cave in the world, in the Selma Plateau, north of Sur in the Eastern Hajar Mountains (some 300 by 200 m wide and 120 m high, or some 4 million m3 in size).
Surface Sediments are fully covered in the BRGM series of Geological Maps and associated Explanatory Notes (Le Métour et al., 1993).
Sequence stratigraphy: Part of Megasequence AP11 (Sharland et al., 2001).
Age: Quaternary, ca. 1.8–0 Ma (NB. base Pleistocene may be extended to 2.6 Ma, see discussion in Ogg et al., 2008).