Page 171 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

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amplified the more constant overflow from the oasis. Similarly, increased aridity
increased evaporation from these lakes, even though spring-fed, causing drops in
lake level and possible deposition of evaporites.
Within this same chain of lakes and rivers draining the oasis, two more
areas of datable stratigraphy are exposed. These are near al-Ayun at the northwest
portion of the oasis, and further downstream in the vicinity of Abqaiq. Near
al-Ayun, a buried stream channel 35 m wide is exposed along drainage canal Dlaf.
This channel was incised into marsh silts that overlie gray-green silty sand. The
channel deposits are marked by black organic silts dated at 715 + 135 B.P. (A.D.
1255), which are overlain by gray clayey silt and sandy silt containing a Melanoides
tuberculata fauna, indicating quiet lacustrine conditions. The deposits are capped
by eolian fine- to medium-grained sands related to dune encroachment. This
stratigraphic section indicates artesian flow and increased runoff from the oasis
and the surrounding catchment areas during the medieval period. This runoff
doubtless fed the river and lake system downstream, while subsequent lacustrine
deposits suggest the creation of lakes or marshes in outlet channels possibly related
to the encroachment of dunes from the north. The overlying eolian sands mark the
final filling of the outlet channel.
South of this exposure are two others that relate to buried northern
outlets. The first shows a similar buried channel 1.5 meters below the present
ground surface. The bed of this channel is marked by deposits of fine to medium
sand and fine gravel. Like the section above, this channel was later filled with
eolian silt followed by deposition of marsh sediments. Although no dating was
possible because of contamination by modern root growth, a datable sequence
nearby showed the same sequence. Here the edges of the stream channel were
marked by black organic silt overlying marsh deposits. The organic silt, dated to
670 + 135 B.P. (A.D. 1300), is overlain by eolian silts and finally tan silts with a
gastropod fauna. Both dated sections point to high runoff in medieval times carried
by channels that were later choked by eolian deposition.