Page 147 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

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presented by Doornkamp, Brunsden, and Jones (1980:316-27), who show a broad
array of raised beach features ranging in age from 6900 to 3100 radiocarbon years
ago. Uplift of the ridge during the past 6000 years has recently been disputed by
McClure and Vita-Finzi (1982) who suggest that region has been relatively stable
over this period.
Because artesian water is the critical resource for Bahrain, geologic
structures which distribute or allow these subsurface waters to reach the surface
are pertinent for this cultural study. The commonly recognized structures that
affect water distribution in Bahrain are fold trends and fractures in the surrounding
rocks. Fractures may also allow interconnection of aquifers with the surface.
Fractures occur in two major forms, joints and faults. Both types of fractures
result from stresses to which the rock has been exposed. Faults, however, show
differential movement along the fracture plane. In Bahrain, joints are related to
tectonic stresses and are systematically distributed about folds. Joints found
perpendicular to fold axes are parallel to the compressional stress and are termed
extension joints. These result from a slight elongation of rocks parallel to the fold
axis during compression. Release joints form at right angles to the compressional
stress and parallel to fold axes when the compressional stresses are released.
Other types of joints, termed shear fractures, form at high angles to the axial
plane of the fold. In laboratory experiments, the angle between shear fractures is
bisected by the compressional stress. This angle is always less than 90° and is
generally about 60° (Billings 1972).
Systematically arranged joint networks were measured for Bahrain during
field studies. Strike and dip were measured for fractures encountered in the field.
These data were plotted on the lower hemisphere of an equal area net. A point
plot of poles normal to each joint plane was determined and converted into a
density diagram showing the relative percentage of poles grouped at particular
orientations (Billings 1972). The distribution of poles is shown on Figure 23. The
most frequently encountered joint set (12-18%) was found to trend N60°E. Other

prominent joint sets were oriented at N40°E, N18°E, N22°W, and N40°W these
made up 8-12% of the assemblage. The first of these, the N40°E set, is often