This Sheet Description describes the Quaternary and solid geology of the Umm Azzimul 1:100 000 scale geological map. The Umm Azzimul district covers approximately 2700 km2 in the far southeast of the UAE along the border with Saudi Arabia and Oman. This district spans the transition from the extensive sand sea of the Ramlat ar Rabbad in the northwest to the distal alluvial fan sediments and Miocene limestone outcrops along the border with Oman around Al Manadir and Umm Azzimul, and includes the eastern extension of the Liwa megabarchan field.
The oldest rocks in the area are the Miocene Dam Formation limestones and dolomites that form flat or very gently sloping limestone pavements within the interdunes in the far southeast of the district. In the northeast, these are partially overlain by the fluvial sands and gravels of the Hili Formation. These fluvial gravels represent the very distal end of a large alluvial fan system that extends out from the Hajar Mountains. The lithological composition of these gravels reflects their source area in the Hamrat Duru region of Oman, rather than the ophiolite source seen further north. These alluvial fans peter out in a series of continental sabkhas underlain by both fluvial and aeolian sands.
The major part of the Umm Azzimul district consists of aeolian dunes of various morphologies. The dune morphology changes systematically in a south and south-easterly direction across the district, reflecting the migration of the dunes driven by the prevailing wind. In the northwest, the extensive Ramlat ar Rabbad sand sea is comprised of large barchan dunes that get progressively smaller to the south and east. Across the central part of the district, these morph into more discrete megabarchan dunes and dune ridges separated by flat interdune sabkhas. Much of the south-western part of the sheet is occupied by large crescentic megabarchan dunes up to 130 m high that extend west into the Liwa area, whilst in the east, the elongate linear dune ridges, punctuated by numerous star dunes and occasional megabarchans are more common. The star dunes become more frequent close to the Oman border. These dune ridges and star dunes can rise up to 100 m above the surrounding interdunes. Most of the district is sparely populated, with no major urban areas and few roads. The south of the region is also host to the relatively small Qusawira and Mender oilfields.
This sheet explanation provides a summary
of the geology of the Isle of Wight district
(Special Sheet) arising from the British Geological
Survey’s Isle of Wight Integrated
Project. This project, commenced in September
2007 and completed in 2013, sought
to improve the understanding of the nearsurface
geology, and create representational
models of the 3D structure. This will
provide essential framework information
for use by the geological community operating
in this classic area of British geology.
This sheet explanation and accompanying
1:50 000 scale geological map special sheet
are principally aimed at users in academia,
local authorities and statutory bodies, but
also at the large number of ‘geotourists’ that
are such an important part of the island’s
economy. A Special Issue of the Proceedings
of the Geologists’ Association (Geologists’
Association, 2011) introduced by Hopson
(2011) gives further detailed accounts of the
recent work by BGS on the island.
The Isle of Wight (Figure 1), the largest
island in England at 384 km2, is separated
from the mainland by the Solent. This body
of water is essentially the drowned lower
reaches of an extensive Quaternary river
system draining much of southern central
England. The island’s protective presence
offshore of the south coast within the
English Channel strongly influences the
tidal regime within the Solent system and
led directly to the development of Southampton
and Portsmouth as major ports.
Large parts of the island form Areas of Outstanding
Natural Beauty (ANOBs) and a
considerable length of the coastline in the
south-west and north-west of the island is
designated as Heritage Coast (Figure 2).
Topographically the diamond shape of
the island is the direct result of the presence
of a central, east–west orientated
ridge, of complex tectonic origins, founded
on the steeply dipping, moderately hardened,
Chalk Group (Figure 3). The softer
sediments of the Palaeogene forming
lower-lying, ground within the northern,
mainland-facing, part of the island are protected
from extensive tidal erosion by this
Chalk ridge. To the south-west of the central
ridge, sediments of the Early Cretaceous,
forming low cliffs, suffer extensive
erosion from Atlantic storms funnelling up
the Channel. The south-eastern coast, eastward
of St Catherine’s Point, is protected
by a capping of more durable, essentially
horizontal Upper Greensand and Chalk
strata forming the Southern Downs but here
an extensive, deeply seated, landslide (the
largest in north-west Europe) considerably
modifies the coastal geomorphology.
An essential prerequisite for any engineering or hydrogeological investigation of soluble rocks is the identification and description of their characteristic, observable and detectable dissolution features, such as stream sinks, springs, sinkholes and caves. The British Geological Survey (BGS) is creating a National Karst Database (NKD) that records such features across the country. The database currently covers much of the region underlain by Carboniferous Limestone, the Chalk, and particularly the Permo-Triassic gypsum and halite where rapid, active dissolution has caused significant subsidence and building damage. In addition to, and separate from, the identification of specific karst features, the BGS has created a National Karst Geohazard geographical information system (GIS). This has been created to identify areas that may potentially contain karst geohazards. Initially, all the soluble rock units identified from the BGS 1:50 000 scale digital geological map are extracted. Each soluble unit has been given an objective score, interpreted, as based on factors including lithology, topography, geomorphological position and characteristic superficial cover deposits. This national zonation of these soluble rocks can then be used to identify areas where the occurrence potential for karstic features is significant, and where dissolution features might affect the stability of buildings and infrastructure, or where karstic groundwater flow might occur. Both datasets are seen as invaluable scientific tools that have already been widely used to support site investigation, groundwater investigations, planning, construction and the insurance underwriting businesses.
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