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The geological position of the layers in the pharaonic "Ochre
Quarries"
at the Abu Ballas Trail (Western Desert, Egypt)
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December 2010
Norbert Brügge, Germany
Dipl.-Geol.
On the route between Abu Ballas -- a depot of water jugs,
built in the Pharaonic period -- and the Gilf Kebir plateau, excavated outcrops
of red Ochre layers from the Abu Ballas Formation were described. This specially
locality were found and photographed in February 2003 by Giancarlo Negro,
Italia. In November 2005, have precise explored Giancarlo Negro, Vincenzo
de Michele and Benito Piacenza this locality. They come to the conclusion that
the Ochre layers was used by mining in the Pharaonic period. The results were
published in Sahara Journal 2005 and 2007. During an expedition
to the Gilf Kebir in 2009 explored
Richard de Nul,
Belgium, and its companions, the "Ochre Quarries"
again.
The geological context of the Ochre layers
In connection with the Ochre layers
was mentioned the Abu Ballas Formation. This Formation in the Western
Desert of Egypt represents a marine intercalation within fluvial, Nubia-type
sandstones. These deposits document an extensive transgression of Aptian age
flooding the Dakhla Basin from north to south. Due to this first advance of the
Tethyan sea during the Cretaceous, sediments of open and marginal marine provenance
were accumulated in the SE of the basin.
At the western coast line the Abu Ballas Formation is interfinger with the fluvial
Gilf Kebir Formation.
The sedimentary succession of the open marine facies is in sense HENDRIKS
& KALLENBACH (1986) divided into five lithostratigraphic units which reflect
that deposition was largely controlled by one single transgressive-regressive
event. The transgressive sequence in the lower part of the formation comprises
sandstones of a backshore to foreshore environment (unit 1) which are overlain
by offshore claystones (unit 2). The middle portion of the sedimentary succession
consists of silty and sandy strata (unit 3) which are attributed to large-scale
redepositional processes due to intensified bottom currents during the maximum
extension of the sea. These sediments are capped by off-shore originated claystones
(unit 4) grading upwards into siltstones and sandstones of transition-zone to
backshore environments (unit 5), which indicate a gradual regression. This stratigraphic
sequence documenting a retrograding-prograding shoreline due to the transgression
and regression of the Aptian sea, interfingers with silty and sandy deposits
of the marginal marine facies of the Abu Ballas Formation. Comparable to the
lithostratigraphic units 1 and 5 these strata represent a variety of sedimentary
environments ranging from the backshore to the transition zone. They were accumulated
on the western slope of the Kharga Uplift bounding the Dakhla Basin towards the
east, which was partially flooded while the sea had its maximum extension.
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The stratigraphic sequence
of open marine origin which has a thickness of 12.2 m to 27.6 m, is separated
into the following 5 units because of lithologic aspects.
Unit 1
The strata of unit 1 comprise white and grey, fine-grained sandstones which
are strongly bioturbated. These deposits overlying the Six Hills Formation
with a gentle unconformity, where they attain a thickness of 1.5 m. This sequence
generally is small-scale wave-ripple cross-laminated. The foreset laminae
are more less bundled. Individual sets which often have an undalatory lower
bounding surface, and cosets partially are separated by evenly laminated beds
with a thickness of 2-5 cm. These occasionally show desiccation cracks and
are bare of any trace fossils. The uppermost part of the unit is destratified
due to intense bioturbation.
Unit 2
Unit 2 mainly consists of white, grey, green, red and violet, partially mottled,
massive or laminated claystones. The thickness of this unit varies from 1.8
m to more than 7.0 m. These strata are bioturbated and contain shells of brachiopods
(Lingula sp.), pelecypods and gastropods. The middle part of the unit is made
up of white and yellowish grey, laminated, fossiliferous, marly claystones.
Locally the uppermost portion of the sequence is characterized by an increasing
silt and sand content documenting a gradual coarsening upwards of grain sizes.
Lenses and beds of fine-grained sandstone which are horizontally bedded or
small-scale wave- and current-ripple cross-laminated, also occasionally occur
in this part of unit 2. These intercalations attain a thickness of 2 to 10
cm. The partially calcareous sandstone beds contain bioturbations, among which
Rhizocorallium sp. The upper part of this unit is furthermore characterized
by the occurrence of concretionary clay-ironstone layers and of coquinas which
are up to 5 cm thick and occassionally contain intraformational claystone
pebbles.
Unit 3
The deposits of unit 3 comprise siltstones and sandstones which have a thickness
of 1.0 to 2.3 m . Yellowish grey and red, horizontally bedded siltstones with
trace fossils varying in thickness from 1.0 to 2.2 m. These strata often are
clayey and partially contain claystone layers. The siltstones occasionally
are calcareous again. They alternate with bioturbated, fine-grained sandstone
beds, which show small-scale wave-ripple cross-lamination and which are up
to 10 cm thick. The uppermost 0.1 m of unit 3 within this section consists
of an intraformational conglomerate overlying a minor erosional unconformity.
This unit is almost completely made up of red and violet, bioturbated, fine-grained
sandstones. These sediments which attain a maximum thickness of 2.1 m
are small-scale current- and wave-ripple cross-laminated. In the latter section
they overlie a grey, massive siltstone bed with a thickness of 0.2 m.
Unit 4
The strata of unit 4 again are made up of grey, green, yellowish brown, red
and violet, partially mottled, massive or laminated claystones. They vary
in thickness from 3.4 m to 6.9 m. These sediments contain various brachiopods,
pelecypods, gastropods and trace fossils. In the uppermost 1.0 to 2.0 m of
the unit horizontally bedded siltstone layers with a thickness of 1-5 cm and
intra-formational conglomerates with a thickness of 5-15 cm occur. The intraformational
claystone pebbles have a maximum diameter of 2.5 cm. A fining-upward sequence
with a thickness of 0.4 m appears in this part of unit 4. It consists of a
basal intraformational conglomerate (10 cm) erosively overlying claystones,
which upwards grade into grey, bioturbated siltstones (15 cm). The latter
strata are horizontally bedded in the lower part and small-scale wave-ripple
cross-laminated in the upper part.
They upwards grade into white, laminated, silty claystones (5 cm) which in
turn are erosively capped by horizontally bedded siltstones (10 cm). Nearly
this entire unit which comprises variegated, massive claystones with a total
thickness of 5.6 m, is destratified by roots penetrating the sequence from
the overlying Sabaya Formation. Only the lowermost 0.5 m of the unit which
consists of laminated, silty claystones, is not uprooted.
Unit 5
The deposits of unit 5 generally are made up of siltstones and sandstones
attaining a thickness of 5.0 to 15.6 m. Although the strata of this sedimentary
succession in all sections studied have more or less the same lithofacial,
stratofacial and ichnofacial characteristics, conspicuous regional variations
occur.
The lower part of the unit 5 comprises grey, bioturbated siltstones with an
upward increasing sand content. Coinciding with the gradual coarsening of
grain sizes, even lamination progressively is replaced by small-scale current-
and wave-ripple cross-lamination. Within these siltstones numerous grey and
red, massive or laminated, thin claystone layers are intercalated. The upper
portion of unit 5 in this section is made up of grey, reddish brown and violet,
fine-to medium-grained sandstones which are partially strongly bioturbated.
These deposits are mainly evenly laminated or small-scale current- and wave-ripple
cross-laminated. The uppermost part of this sedimentary succession consists
of grouped sets of large-scale cross-bedded psammites alternating with small-scale
current- and wave-ripple cross-laminated strata, which fill up a local through-shaped
depression. The individual large-scale cross-stratified sets within this sequence
have an erosional, planar or scoop-shaped lower bounding surface. The angular
relation of the lithologically homogeneous, tabular and concave-upward foreset
laminae to the lower bounding surface is discordant.
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Reference profile through the Abu Ballas
Formation
in sense BARTHEL & BOETTCHER, 1978
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The geographic position of the "Ochre
Quarries": 24°12'48''N / 27°11'28''E
Sources:
W. BARTHEL & R. BÖTTCHER -- Abu Ballas Formation (Tithonian/Berriasian;
Southwestern Desert, Egypt) a significant lithostratigraphic unit of the former
"Nubian Series"
Mitt. Bayr. Staatsslg. Paläontologie hist. Geol., 18, 1978, München,
Germany
Ronald BÖTTCHER -- Die Abu Ballas Formation (Lingula
Shale) (Apt ?) der Nubischen Gruppe Südwest-Ägyptens.
Eine Beschreibung der Formation unter besonderer Berücksichtigung der Paläontologie
Berliner geowiss. Abh. (A), 39, 1982, Berlin, Germany
Ronald BÖTTCHER -- Environmental model of the shallow
marine Abu Ballas Formation (Aptian, Nubia Group) in South-Western Egypt
N. Jb. Geol. Paläont. Abh., 196, 3, 1985, Stuttgart, Germany
F. HENDRIKS & H. KALLENBACH -- The offshore to
backshore environments of the Abu Ballas Formation of the SE Dakhla Basin
(Western Desert, Egypt)
Geologische Rundschau 75/2, 1986, Stuttgart, Germany
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