Page 155 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

-131-

reconstructed. Tlius, the Alat limestone thickens downdip along the flanks of the
dome. In reality, much of the Alat limestone in the updip areas has been eroded
away. Powers et al. (1966) would place an Oligocene erosional phase to mark this
contact between the Alat and the Hadrukh. From this interpretation, a still greater
erosional phase may have occurred between the Hadrukh and Dam formations. This
implies an early to mid Miocene date for exposure of the dome to subaerial erosion.
A blanket of Quaternary unconsolidated sediments cover the Neogene
complex. From the relationship of the faulted rocks, the possible date of faulting
lies between the Pliocene and early Quaternary. This dating relates to Kassler^
(1973) interpretation of faulting around the Bay of Salwa or Gulf of Bahrain and his
report of closely spaced post Pliocene faults near Abu Dhabi. In both cases the
faulting was along the Zagros fold trends. On this basis, the faulting shown on
Figures 26 and 27 relates to early Pleistocene orogenic movements. These faults
closely approximate the N40°W joint set suggested earlier as a release jointing
feature. It is therefore reasonable to suggest that normal faulting followed a
relaxation of compressional stresses, thereby allowing individual fault blocks to
settle. Alternatively, solution of evaporites in the underlying formations could be
used as a mechanism to explain settlement along preexisting joints.
Continuity of these faults cannot be displayed, because they are not
visible in outcrop along the slopes of the central dome. A survey of the rimrocks
surrounding the central portion of the dome did not reveal intersecting fault
planes. With the exception of one fracture-controlled wadi incised into the
northwest flank of the dome (fig. 26), there is no recognizable trace of fault
control in surface erosion patterns. This wadi, which intersects the coastal plain
west of the village of Ali, is structurally controlled along some part of its
north-northwest, south-southeast trending valley. Bapco geophysical studies point
to disrupted rock units at depth in this location, but no recognizable faulting can be
seen in outcrop. On the other hand, closely spaced vertical joint sets are found
underlying the wadi. Also of significance is an extinct artesian spring found at the