Geological setting of the study area

The simplified geology of the study area is shown on Figure 4.1 (MWR, 1997a). This simplified regional geological map was constructed with the aid of geological maps compiled for the Ministry of Minerals and Petroleum (BRGM, 1992).

The eastern Oman Mountains, which are located around the northern and eastern margins of the Ash Sharqiyah Sands Aquifer, are formed from eight distinct classifications of rocks (Glennie, 1988; MWR, 1997a). These rocks from older to younger comprise:

(i) Pre-Cambrian gneiss and schists of Jabal Jaalan.

(ii) The Huqf (Cambrian) and Haima (Cambro-Ordovician) Group in Ash Sharqiyah Sands. A seismic reflector in Haima, referred to as the Intra Haima, is an important horizon extending laterally below the Sands. These comprise formations which alternate between carbonates and silicaclastics deposited in shallow-marine to terrestrial environment.

(iii)The Hajar Super Group, which is a thick sequence of Late Permian to Late Cretaceous shallow marine carbonates together with Early Jurassic silicaclastics. These rocks are exposed in an isolated outcrop 15km north of Al Kamil at the edge of Jabal al Hajor Ash Sharqi.

(iv) The Hawasina, which are deep marine sedimentary rocks (radiolarian chests and silicified carbonate turbidites) of the same general age as the Hajar Super Group.

These rocks are exposed to the west of the study area on the edge of the northern Ash

Sharqiyah Sands, to the north along the edge of Jabal al Hajor Ash Sharqi, and between Jabal Jaalan and the coast.

(v) The Metamorphic Sole, which is an irregular contact of metamorphic rocks located between the Hawasina and lower portion of the Samail Nappe. The lower part consists of quartzites, schist and marble, and the upper part amphibolites. These rocks do not appear in the study area but do occur some 30km north of the study area (north-west of Mintirib in Wadi Dohir.

(vi) The Samial Nappe, which comprises a thick slice of former oceanic crust and mantle that now overlies the Hawasina and Metamorphic Sole. These rocks do not extend into the study area, but are exposed on the southern mountain front between ad Dahir and Zilaft.

(vii) Shallow-marine and terrestrial Tertiary formations, which may lie unconformably on all other rock units (a-f) above. Fars Group which comprise Miocene-Pliocene marine and terrestrial conglomerates, silts, clays, mudstones, limestones, sands and gravels have been cut by a number of borehole beneath the Ash Sharqiyah Sands These data together with information obtained from the TDEM and Seismic surveys indicate a deepening of the Fars, from west to east, beneath the study area (MWR, 1997a).

(viii) Quaternary alluvium is found throughout the study area in alluvial fans, terraces, wadi channels, and beneath the Ash Sharqiyah Sands and varies in thickness from a few meters to more than 100m. It consists of gravels, sands, and clays, with variable carbonate cementation. It is the main source of potable water in the region.

4.3.1 Alluvial deposits

In the central part of the study area, Tertiary and Quaternary alluvial deposition (see units g and h in the Section above) was heavily influenced by the formation of a basinal depression, probably formed during the late Oligocene to Early Miocene. The alluvial

deposits represent the erosional products of Oman mountains area (up-lifted during the late Oligocene to early Miocene) transported southward by intermittent fluvial action under varying climatic conditions (Maizels and Anderson, 1988). Alluvial deposition in Wadi al Batha Basin is bounded by normal faults in the north, east and south as shown in Figures 4.1, 4.2 & 4.3. Creep has occurred on the faults during the latter part of the Tertiary and possibly also during the Quaternary ages due to the large thickness of alluvium, aeolianite and sands, which has caused subsidence in the basin. The base of the alluvium could be block faulted bedrock (MWR, 1997a). The area of alluvial deposition, however, is not entirely bounded by faulting. The edge of the alluvium is sedimentary east of Al Kamil and Jaalan Bani Bu Hassan, where the deposits gradually thin to zero at the foot of Jabal Jaalan. In the north-west of the study area, the approximate edge of the basin lies between Mintirib and Hawiyah. To the north-west, the alluvium is less than 50m thick and to the south-east the alluvium thickens rapidly to greater than 100m.

4.3.2 Aeolianite and sand deposits

Aeolianite and sand deposits occur on top of the alluvium (see Figures 4.2 and 4.3).

Gardner (1988) and Glennie (1988) recognize three major aeolian sequences:

1. A strongly cemented older aeolianite sequence with low primary porosity, the pores being infilled with low Mg-calcite cement and halite. This sequence is composed predominantly of allochem material containing shell fragments, forams, peloids, and algae, and an insignificant terrigeneous quartzose component. This supports the suggestion of Glennie (1970), that the bulk of the aeolianite had originated by deflation of the near shore areas and continental shelf during periods of low sea level.

2. A coastal or younger aeolianite which is only loosely cemented and porous, retaining an important shelly carbonate component with a strong terriginous component.

3. A sequence of looser linear mega-dunes and meso-dunes, which are large north-south trending linear dunes being 1-2 km wide and ranging from 50 to 100 m high.

The sands consist predominantly of quartz and carbonate. Pye (1984) suggested that the mega-dune were deflated from an exposed coastal plain during lower sea-level during glacial time, an origin similar to that suggested for the older aeolianites.

The existing Ash Sharqiyah Sands groundwater supply project provides water from both the alluvium and aeolianite.