Page 143 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

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sometimes results in a resistant layer that forms prominent escarpments. The top
of the Alat is marked by an unconformable contact with the overlying Hadrukh
formation. The Alat is an important aquifer along the Arabian coast and in Bahrain,
where it is referred to as the "Zone A aquifer." The underlying Alat marl is
distinctive by both its orange color and a sharp contact with adjacent units.
Lithologically, the unit consists of limonite-stained, slightly dolomitic marl and
forms an impervious base to the Alat member.
Like the Alat member, Powers subdivides the Khobar member into an
upper Khobar limestone and a lower Khobar marl, In Bahrain, Willis (1967)
recognized only the limestone, which he referred to as the "brown crystalline
limestone." At the Dammam dome, the Khobar limestone is about 8 m thick and is
underlain by 1.5 m of marl. Willis shows a 33.5 m sequence for the Khobar member
in Bahrain. The Khobar limestone is a brown to buff, massive, porous, finely
crystalline, dolomitic limestone containing a lower band of chert nodules. These
silicified beds form resistant rimrocks in the outcrop area. The Khobar Member is
also defined on the basis of fossils. The top of the member is a Nummulites-
bearing limestone below the sharp contact with the Alat marl. The base is marked
by an Alveolina-bearing limestone member. The Alveolina bearing bed is less well
defined in Bahrain. The lower limit of the Khobar member is marked by the "shark's
tooth shale," an informal term used by Willis (1967) for the Midra shale member,
the lowermost unit of the Dammam formation. This shale, like the Alat marl,
forms an impervious base to a porous and permeable limestone. The shark's tooth
shale makes an easily recognized division between the Dammam formation and the
chalky limestones of the underlying Rus formation.
Artesian conditions exist for the Alat and Khobar limestones throughout
the Arabian coastal zone. The Khobar is the major aquifer for both al-Hasa and
Bahrain. Tlie Alat limestone is a less prolific but significant aquifer for both al-
Qatif and Bahrain, although flows are erratic.
A major unconformity separates the Dammam formation from the
overlying Miocene and Pliocene rocks. TTiis is especially apparent in the thickening
of the Dammam formation downdip and away from the major structures at