Page 188 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

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saturated groundwater during seasonal drying. This case does not, however,
account for in-phase climbing ripples which argue against seasonal drying out of
the lake. Seasonal evaporation, on the other hand, could account for the formation
of more concentrated solutions allowing seasonal precipitation of these evaporites.
The southern basin became progressively saline through time as runoff and
erosion continued to enrich the solutions. TTCie time of actual evaporite deposit is
i!
uncertain, although the evidence for the expansion of Arabian lakes during the late
Pleistocene, as well as the 22,800 B.P. date from lacustrine deposits to the north
suggest a temporally similar history. Other evidence points to deposition in an arid
or semi arid environment—somewhat different from our current view of a period of
greater moisture on the Arabian Peninsula. Thus, the dessication which gave rise
to these evaporites may have taken place in the arid interval between 17,000 and
9000 B.P. that McClure identified in the Rub al-Khali. This agrees in part with a
date of 14,090 + 270 B.P. that Doornkamp, Brunsden, and Jones (1980:188) obtained
from fossil gastropod shells in the lacustrine sediments.
Later lacustrine deposits occupy much of this basin. On the northern edge
of the playa and exposed along wadi banks is a thin surface layer of sandy silt
overlying gray medium to coarse, well-rounded eolian sand. A typical stratigraphic
relationship is described as section TLW2 in Figure 33. Although continuous
exposures do not exist, the surface of the lacustrine silts are lower in elevation
than the thick anhydrite beds a few hundred meters to the south. At TLW4, for
example, the evaporite sequence is contained between elevations 10 m and 12 m.
Lacustrine deposits at TLW2, on the other hand, are found below elevation 11 m.
The lowermost sediment in TLW2 is an eolian sand, upon which were found
large, dark-gray flint waste flakes of undetermined cultural affinity. At the
interface between the eolian sand and the overlying silts is a well-defined oxidized
layer indicating subaerial weathering under more moist conditions. In other areas,
this oxidized zone is directly overlain by a dark gray, organic silt with a gastropod
fauna containing Melanoides tuberculata. At TLW2, a thin bed of buff, fine sand
with the same fauna separates the organic silt and oxidized zones. A radiocarbon
date obtained from the organic silt was found to be 8450 + 175 B.P., well within the