Page 139 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

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directed toward an overaU understanding of the cultural as weU as natural limits of
settlement and land use. Tlie few kilometers that separate Bahrain from the
mainland are insignificant in terms of the regional geologic framework. Thus, the
islands must be viewed in a general perspective including the Arabian Peninsula.
TTiis perspective gained by first understanding the surficial geology of eastern
Arabia and the sedimentary rocks that underlie the region.

Tertiary Stratigraphy

The landscape of eastern Arabia is underlain by Tertiary sedimentary rock
formations that range in age from Paleocene to Pliocene. These are overlain by
Quaternary sediments of littoral, lacustrine, fluvial, and eolian origin, Tlie
stratigraphic sequence is shown graphicaUy in Figure 21.

Umm er-Radhuma Formation

The oldest near-surface formation present in eastern Arabia and Bahrain is the
Umm er-Radhuma formation which Powers (Powers et al. 1966) describes as a
series of light-colored foraminiferal limestone, dolomitic limestone, and dolomite.
A 3-meter vertical section of the formation is exposed at the Dammam dome near
Dhahran, but a total thickness of 243 m has been described for other areas of
eastern Arabia. In the smaU coasted outcrop, the Umm er-Radhuma is a
yeUowish-gray, very fine to medium grained, crystaUine dolomite, containing zones
of chert nodules and quartz geodes. It dates from Paleocene to early Eocene. A
thick interbed of anhydrite is also present in portions of the formation. Many
Miocene and Pliocene slump features occur near the periphery of structures of
similar age, where solution of anhydrite apparently resulted in local collapse. The
groundwater potential of the Umm er-Radhuma is important to this study. It is a
single, thick hydraulic unit of regional extent and an established gradient that
ranges from water table conditions in its catchment area to flowing artesian
conditions along the Arabian coast and Bahrain. Local anticlinal fold structures
show decreased permeability along their flanks and divert regional water flow
along their axes. Flushing by fresher meteoric water is also restricted by