Publication Date:
2002-01-01
Description:
The southern Arabian (Persian) Gulf is at present the site of extensive carbonate sedimentation, as was the case during Pleistocene interglacial marine highstands. During glacial lowstands the basin was subaerially exposed, and aeolian sedimentation predominated. Most of the southern Arabian Gulf floor is underlain by Quaternary carbonates, and scattered outcrops may be found onshore. These belong to three formations: the aeolian Ghayathi Formation, the continental Aradah Formation and the marine Fuwayrit Formation. The Fuwayrit Formation consists of three members, separated by subaerial exposure surfaces. These are, from the base upwards, the shallow marine Futaisi and Dabb'iya Members, and the aeolian Al Wusayl Member. Offshore, at least six Quaternary sequences are present within the uppermost 50 m of sediment. No reliable direct age dates have been acquired from Pleistocene shallow marine or coastal deposits in the southern Arabian Gulf. It has therefore been necessary to infer the ages of these sediments by a comparison of their stratigraphy and elevation with deposits known from other parts of the world. We regard this approach as valid because the southern Gulf coastline lacks evidence for significant widespread neotectonic uplift, and halotectonic effects are localized. This comparison indicates that the Fuwayrit Formation was deposited during the last interglacial (oxygen isotope substage 5e), as (1) these sediments represent the youngest pre-Holocene marine deposits, and (2) they are found at an elevation correlative with many substage 5e deposits from other parts of the globe. Sedimentary evidence reveals two highstands during this period, peaking at around 1.5 m and 6 m above present sea level, respectively. Offshore sediments indicate that sea level did not fall as far as 24 m below present level in the intervening regression. Following the second highstand, sea level fell to more than 23 m below present level, before briefly rising once again (late isotope stage 5). This later highstand probably peaked between 17 and 7 m below present level. The sequence underlying the Fuwayrit Formation was probably deposited during the penultimate interglacial (late oxygen isotope stage 7). It is also likely that the Ghayathi Formation aeolianites were largely sourced from this sequence. Facies analysis of offshore core sediments indicates that sea level reached at least 15 m below present level during this period. Widespread evidence exists for a Holocene sea level higher than at present in the southern Arabian Gulf, indicating that it peaked at 1-2 m above present level, c. 5.5 ka bp. Pleistocene deposits preserved in the southern Arabian Gulf provide a record of changing palaeowinds and palaeoclimates. Currently, the region experiences a hyper-arid to arid climate, with facies patterns dominated by the northwesterly shamal wind. The Ghayathi Formation was originally deposited under an arid climatic regime, which allowed the sediments to remain unconsolidated. The dunefield was later remodelled under conditions of increasing wind speed, with a change in wind direction from NNW to WNW. These changes are thought to reflect the onset of glaciation. Palaeocurrent directions from the Al Wusayl Member, combined with sedimentary evidence from the Futaisi and Dabb'iya Members, indicate that during the peak of the last interglacial the prevailing wind (the palaeo-shamal') blew from the NE. Compelling evidence for a pluvial episode during this period is provided by abundant and widespread dissolution (palaeokarstic) pits found in the top surface of the Futaisi Member, believed to represent the former positions of abundant trees or large plants.
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