Page 152 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

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Figure 25 maps the subsurface topography on the top of the Alat marl
using these water well data. The orange color of this stratigraphic unit made it a
common marker bed to field geologists who studied the sections. Thus, the contacts
of the Alat marl with both the Alat limestone and Khobar limestone were sharp and
easily recognized. The topographic surface shown on Figure 25 is drawn at a five
meter contour interval. Nonetheless, it shows the expected increased depth to the
Alat marl away from the center of the dome, as well as certain other
characteristic trends. A prominent northwest-southeast trending ridge, for
example, appears along the northeast coast, while a similarly oriented depression is
found along the northwest coast. Closer examination suggests an orientation
parallel to the trend of Zagros folds. Finally, the surface outcrop pattern of the
Alat limestone was drawn based on this array of borings. This outcrop pattern is
shown on Figure 26, the surficial geologic map of Bahrain, which reveals a family
of closely spaced, normal faults trending approximately N45°W. Distinct offsets of
the Alat limestone are conspicuous as surface features. The zone of most intense
faulting occurs along the north coast and west of Manama. The spacing between
faults increases to the southwest suggesting a lesser degree of faulting in this
direction. These closely spaced faults compare with similar fractures noted for Abu
Dhabi by Kassler (1973). The north coast fault zone is also expressed by the
,tblocky" outline to the intertidal zone shown on Figure 26, which shows the same
north west-southeast trend. While these shallow water features are often assumed
to be coral reefs, their spatial patterns point to underlying faulted Eocene and
Neogene rocks.
A cross section of the northeast coast in Figure 27 shows the spacing and
displacement of normal faults. The unconformity which separates the marls of the
Neogene complex from the underlying limestones is recognizable. The relationship
between the sandy limestone described for the surface of the Alat limestone and
the Neogene complex is open to interpretation. Powers et al. (1966) describe this
sandy zone as the Miocene Hadrukh formation in Saudi Arabia and note that it is
separated from the underlying Alat by an erosion surface. This was impossible to
determine from the data available, although a great degree of erosion could be

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