Page 141 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

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structure. Thus, water quality is variable, ranging from good quality near surface
outcrops to poor quality high in dissolved hydrogen sulfide in the less well flushed
areas.

Rus Formation
The Rus formation is exposed in the central portion of the Dammam dome and in
Bahrain (fig. 22). It has been assigned an early Eocene age by Powers (Powers et al.
1966). In general, the Rus is composed of a lower unit of 21 m of yellowish-gray,
microcrystalline dolomite and dolomitic limestone containing quartz geodes and
large light colored chert nodules, a middle unit of massive yellowish-gray
microcrystalline limestone with calcite geodes, and an uppermost zone of 25 m of
yellowish-gray, microcrystalline dolomitic limestone and white chalky limestone.
Boulder sized brown to black chert nodules are sometimes found near the top of the
formation and may form a lag gravel when the limestone has weathered away
(Tleel 1973). Anhydrite is also present within the formation. Here, too, Powers
suggests that solution of anhydrite may account for the complexly settled rocks
seen at the surface around the Dammam dome. Because the Rus formation is
largely an evaporite sequence, it has little porosity, although limestones at the top
and the bottom of the formation may be waterbearing.

Dammam Formation

The Dammam formation is the most often exposed sequence in eastern Arabia; its
outcrops occur along the flanks of both the Dammam and Bahrain domes. Powers
(Powers et al. 1966) assigns a Middle Eocene age to the formation. Although it lies
conformably on the Rus formation, its surface is marked by a regional
unconformity. The Dammam is subdivided into five members. The uppermost or
Alat member is composed of an upper limestone approximately 9 m thick which
overlies about 6 m of marl. In Bahrain, these same units are referred to as the
Alat limestone, and the Orange marl (Willis 1967). The Alat member is generally
white in color and is composed of a very porous, finely crystalline, chalky, and
partially dolomitized limestone. Local silicification at the surface of the member