『Abstract
At least six glaciations are purported to have affected North
Africa and the Middle East region over the last one billion years,
including two in the Cryogenian (Neoproterozoic), Hirnantian (Late
Ordovician), Silurian, Carboniferous and Early Permian events.
The sedimentary record associated with these glaciations, together
with the intensity to which each has been investigated, is highly
variable. As hydrocarbon exploration proceeds aggressively across
the North Africa and Middle East regions, we review the relationship
between glaciation and hydrocarbon accumulations.
With the exception of Oman, and locally Egypt, which were tectonically
active both during the Neoproterozoic and Early Palaeozoic all
glaciations took place along an essentially stable passive continental
margin. During the Neoproterozoic, two glaciations are recognised,
referred to as older and younger Cryogenian glaciations respectively.
Both of these Cryogenian events are preserved in Oman; only the
younger Cryogenian has been reported in North Africa in Mauritania
and Mali at the flanks of the Taoudenni Basin. The process of
initial deglaciation in younger Cryogenian glaciations resulted
in incision, at least locally producing large-bedrock palaeovalleys
in Oman, and the deposition of glacial diamictites, gravels, sandstones
and mudstones. as deglaciation progressed “cap carbonates” were
deposited, passing vertically into shale with evidence for deposition
in an anoxic environment. Hence, younger Cryogenian deglaciation
may be associated with hydrocarbon source rock deposits.
Hirnantian (Late Ordovician) glaciation was short lived (<0.5
Myr) and affected intracratonic basins of Mauritania, Morocco,
Algeria, Libya, Egypt and Saudi Arabia. The organisation of the
glacial sedimentary record is considered to be controlled at the
basin-scale by the location of fast-flowing ice streams active
during glacial maxima, and by the processes of meltwater release
during glacial recession. In these latter phases, subglacial tunnel
valley networks were cut at or near the ice margin. These tunnel
valleys were filled in two main phases. The initial phase was
characterised by debris flow release, whereas during later phases
of ice retreat a range of glaciofluvial, shallow glaciomarine
to shelf deposits were laid down, depending on the water depth
at the ice front. Production of linear accumulations of sediment,
parallel to the ice front, also occurred between tunnel valleys
at the grounding line. In Arabia, the geometry of these features
may have been influenced by local tectonic uplift. As glaciogenic
reservoirs, Hirnantian deposits are already of great economic
significance across central North Africa. Therefore, an appreciation
of the processes of ice sheet growth and decay provides significant
insights into the controls on large-scale heterogeneities within
these sediments, and in analogue deposits produced by glaciations
of different ages.
Deglacial, Early Silurian black shale represents the most important
Palaeozoic source rock across the region. Existing models do not
adequately explain the temporal and spatial development of anoxia,
and hence of back shale/deglacial source rocks. The origins of
a palaeotopography previously invoked as the primary driver for
this anoxia is allied to a complex configuration of palaeo-ice
stream pathways, “underfilled” tunnel valley incisions, glaciotectonic
deformation structures and re-activation of older crustal structures
during rebound. A putative link with the development of Silurian
glaciation in northern Chad is suggested. Silurian glaciation
appears to have been restricted to the southern Al Kufrah Basin
in the eastern part of North Africa, and was associated with the
deposition of boulder beds. Equivalent deposits are lacking in
shallow marine deposits in neighbouring outcrop belts.
Evidence for Carboniferous-Permian glaciation is tentative in
the eastern Sahara (SW Egypt) but well established on the Arabian
Peninsula in Oman and more recently in Saudi Arabia. Pennsylvanian-Sakmarian
times saw repeated glaciation-deglaciation cycles affecting the
region, over a timeframe of about 20 Myr. Repeated phases of deglaciation
produced a complex stratigraphy consisting., in part, of structureless
sandstone intervals up to 50 m thick. Some of these sandstone
intervals are major hydrocarbon intervals in the Omani salt basins.
Whilst studies of the Hirnantion glaciation can provide lessons
on the causes of large-scale variability within Carboniferous-Permian
glaciogenic reservoirs, additional factors also influenced their
geometry. These include the effects of topography produced during
Hercynian orogenesis and the mobilisation and dissolution of the
Precambrian Ara Salt. Deglacial or interglacial lacustrine shale,
with abundant palynomorphs, is also important. Whilst both Cryogenian
intervals and the Hirnantian-Rhuddanian deglaciation resulted
in the deposition of glaciomarine deposits, Carboniferous-Permian
deglaciation likely occurred within a lacustrine setting. Hence,
compared to shales of other glacial epochs, the source rock potential
of Carboniferous-Permian deglacial deposits is minimal.
Keywords: glaciation; glacial depositional systems; hydrocarbon
accumulation; North Africa; Middle East』
Contents
1. Introduction
2. Overview of glacial depositional systems
2.1. Glacioterrestrial versus glaciomarine environments and
deposits
2.2. The main controls on sediment geometry
2.3. Role of “thermal regime” and ice flow rate
2.4. The sedimentological products of ice sheet decay
2.5. Tunnel valleys
2.6. The glacial depositional sequence
3. Hydrocarbon source-rock development in glacial epochs and under
conditions of de-glaciation
3.1. Transgressive, organically enriched shales
3.2. Role of coastal palaeogeography/prevailing wind in black
shale generation
3.3. Competing processes of black shale deposition and isostatic
rebound
3.4. Maximum flooding surface black shales
3.5. Summary
4. Cryogenian glaciations
4.1. Cryogenian glacial deposits: strength of the evidence
4.2. Stratigraphy and sedimentary record of Cryogenian glaciations
4.2.1. Mauritania, Algeria and Mali
4.2.2. Arabian Peninsula
4.3. Relationship between glaciation and hydrocarbon accumulations
4.3.1. Mauritania, Algeria and Mali
4.3.2. Oman
5. Hirnantian (Late Ordovician) glaciation
5.1. Hirnantian glacial deposits: strength of the evidence
5.2. Stratigraphy and sedimentary record of Hirnantian glaciation
5.2.1. Mega-morphology of a glacial shelf: ice sheet grounding
lines in North Africa
5.2.2. Recognition of ice sheet grounding lines in Arabia
5.2.3. Tectonic controls on palaeo-ice sheet behaviour: Arabian
examples
5.2.4. Ice sheet reconstructions
5.3. Relationship between glaciation and hydrocarbon accumulations
5.3.1. Hirnantian deposits as glaciogenic reservoirs
5.3.2. Deglacial source rocks
6. Silurian glaciation
6.1. Strength of the evidence
6.2. Relationship between glaciation and hydrocarbon accumulations
7. Carboniferous-Permian glaciations
7.1. Strength of the evidence
7.2. Stratigraphy and sedimentary record of Carboniferous-Permian
glaciations
7.2.1. Egypt (Gilf El Kebir)
7.2.2. Oman and Yemen
7.2.3. Saudi Arabia
7.3. Relationship between glaciation and hydrocarbon accumulations
7.3.1. Egypt
7.3.2. Oman, Yemen and Saudi Arabia
7.4. Summary
8. Conclusions
Acknowledgements
References