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Fossile remains of genus
Prasopora (Trepostomata bryozoa) in Silurian strata
of the Jabal Bzimah (Libya)
nearby Jabal Zalmah (syn. Jabal Dalma)
Norbert Brügge,
Germany
Dipl. - Geol.
The Howar-Uweinat Uplift forms the eastern border of the Kufra Basin. At the
northern part of this tectonic structure is located the Jabal Zalmah. The
plateau contains sandstones, siltstones and shales of Ordovician to Devonian
age. In the southeastern part of the Jabal Zalmah are preserved transgressive
Carboniferous strata with plant remains. |
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Source: Lyell Collection/Google images
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Geological map
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Glacial traces in
Ordovician
sandstones. Source: Lyell Collection/Google images
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Early Silurian graptolites in the Tanezzuft shales
in SE Libya (Jabal Dalma)
Neodiplograptus
fezzanensis (DESIO, 1940)
indicating a Lower
to Middle Llandovery age
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Cambro-Ordovician Skolithos (Tigillites)
from Jabal Dalma
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Ordovician and Silurian strata
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To a great extent the Ordovician sandstones
preserve the lithological features observed further SE even if morphologically
the yare notably different. The sandstones with quite a high cement content are
more numerous, sometimes passing to quartzite. In the NE area there are also
numerous beds with Tigillites which are, in constrast, much rarer further
to the south.
Ichnostratigraphic correlation of Lower Palaeozoic clastics
in the Kufra Basin (SE Libya)
Seilacher , Lüning , Martin , Klitzsch , Khoja , Craig , 2002
Abstract:
The Lower Palaeozoic deposits in North Africa are dominated by sandstones and
shales which often lack biostratigraphically useful body fossils. "Trilobite"
burrows (Cruziana) partly fill this gap and provide the basis for medium-resolution
stratigraphic interpretations. Several Ordovician-Silurian ichnostratigraphically
significant Cruziana forms have been found and studied in the Kufra Basin (SE
Libya), including C. goldfussi and C. furcifera from the Lower
Ordovician Hawaz Fm. and a new ichnospecies, C. kufraensis, occurring
in transgressive sandstones at the base of the Lower Silurian Tanezzuft Fm. The
upper Tanezzuft Fm. and Akakus Fm. typically contain C. acacensis, a form
that is characteristic of the Lower Silurian of Northern Gondwana.
Petr Štorch (Department
of Palaeontology and Stratigraphy, Institute of Geology, Czech Republic) from
the Kufrah-Field Work 04/05 1999
found 22 samples
of Graptolites from two localities in silty shales of the Jabal Dalma Area
in SE-Libya.
The present material is assigned in one of the most widely distributed and most
common North African graptolites - Neodiplograptus fezzanensis (DESIO).
The species was described from W-Libya by Desio (1940), later recorded by Massa
and Jaeger (1971) from the same area.
Diversity of the graptolite fauna is the lowest possible. All graptolite rhabdosomes
belong to single, though slightly variable biserial species Neodiplograptus
fezzanensis. Such monotonous, often monospecific, always diplograptid-dominated
assemblages are typical of the North African lower and middle Llandovery
strata and account for a rather restrictive environmental conditions (limited
or uniform food resources, lower surface temperatures). The same assemblage,
composed of a number of aligned rhabdosomes of Neodiplograptus fezzanensis
occurs also in shallow shelf sandstones.
Two trilobite exuviae assigned in Calymene aff. blumenbachi (BRONGNIART)
were found in a sample. The exuviae which are preserved in situ, without post-mortal
transport, suggest that some benthic faunal elements survived in presumably oxygen
depleted bottom environments on the Early Silurian North African shelves and
basins. Trilobites are associated with fragmented orthocone nautiloid shell and
graptolite rhabdosomes.
Benthic trilobites and nektic nautiloid shells which associate common though
uniform graptolite fauna may indicate some incursions of better oxygenated water
masses in presumably oxygen depleted offshore environment. Graptolite diversity
was strongly restricted by local environmental patterns in this facies.
The found trace fossils recorded in a sample belong probably to Cochlichnus
and Protovirgularia ichnogenera and account for a shallow water environment
of tidal flat or related facies.
In the area of Jabal Zalmah the Devonian sediments
outcrop with thicknesses generally greater than the southern areas. The Lower
Devonian outcrops are discontinuous while the Middle-Upper Devonian outcrops
form a large, continuous belt, of approximately ENE direction, from Jabal
Qardabah to the southern edge of the Tertiary onlap
Tadrart Formation.
These formation is well exposed in a narrow belt of about 60 km N and NNW of
Jabal Zalmah. In other areas, the Binem directly overlies the Akakus. Lower Devonian
sediments are represented by sandstones, quartzitic, coarse- to medium-grained,
dark reddish to yellowish, with kaolinitic and ferruginous cement, locally with
interbeds of very fine sandstones and micaceous siltstones, soft and greyish,
or of dark, hard ironstone.
In the upper part the beds are 30-50 cm thick while, towards the base, the thickness
increases rapidly to 2-3 m. The sandstones are normally cross-bedded and with
parallel laminations and foresets. The Lower Devonian Sediments of this area
seem to represent mostly the Tadrart. The greatest thickness of 100 m and partial
thicknesses of 22 and 25 m were measured.
Binem Formation. In the northeastern side the Binem Formation outcrops
over large extensions, forms low reliefs and is quite dismembered by faults;
the total thickness must be measured on various dissected and scattered hills.
In this area the thickness of the Binem Formation increases considerably and
shows some lithological changes with respect to the southern areas.
The main lithological changes refer to the occurrence of arenaceous, multicoloured
clay in the lower part of the formation, with thicknesses of some tens of metres
and the occurrence of slightly silty clay in the highest part of the formation
with a thickness of over 22 m. These silty, greenish, laminated clays, locally
with Brachiopoda, have been found in two sections. The best exposures
of the Binem Formation are at four sections with thickness of 60 to 125 m.
Fossil traces are locally extremely abundant and represented mainly by Zoophycus.
Asterosoma sp. is sometimes common at the bottom of sandstone beds overlying
clay or siltstone. The most important fauna was found at a section in central
part of Jabal Zalmah. At 25°47' 50"N and 23°52' 30"E of the Binem type
locality displays nuculouid bivalves oft he genus Kufralana and Paleoneilo of
lagoonal to deltaic facies and some Gastropoda. (Termier et al. 1980).
On account of their lithological, sedimentological,
palaeontological and environmental characteristics they were referred to the
Dalma Formation (from the high hill of Jabal Zalmah (Jabal Dalma), NNE
of Kufra with coordinates: 25°34'10"N and 23°55'15"E. The Carboniferous sandstones
form the highest reliefs of the Palaeozoic outcrops and it is in one of these,
at Jabal Zalmah, that the type section has been chosen for the Lower Carboniferous
in the Kufra basin.
The sequence exposed at Jabal Zalmah, however, is not representative of the whole
formation and the reference section must be completed with some additional sequence
which outcrops southwards, in scattered hills: section (25°14' N and 23°46'30”E), section (25°08'20”N and 23°49'15"E) and section (25°12' N and 23°45' E).
The outcropping of the Dalma Formation is here reduced by erosion and the unconformable
onlap of the Continental Mesozoic.
At Jabal Zalmah the outcropping section is 388 m thick. The base of the relief,
and of some nearby reliefs, is represented by sandstones, quartzitic, whitish,
slightly argillaceous and poorly cemented and generally thin bedded. These sandstones
unconformably overlie the Devonian greenish shales and siltstones, sometimes
with Brachiopoda. Upwards they are overlain by about 290 m of sandstones,
quartzitic, coarse- to fine-grained, brown-reddish to yellowish and greyish,
generally poorly cemented and thick-bedded, cross-bedded, with some irregular
bed or lenses of fine, whitish to pink quartz conglomerate.
Uncommon plant remains, mainly Lepidodendron, are found here. The
upper 90 m are formed of interbeddings of sandstones, as above, generally very
thick bedded, and shales and siltstones, slaty, very hard, dark reddish-violet
to grey, sometimes with common traces of Licopodophyta (Lepidodendron, Sigillaria
etc.).
In contrast the outcrops of another section are represented by over 50 m of
shale, greenish, silty, soft, fissile, with interbeddings (l 0-30 cm) of sandstones,
fine-grained, hard, with abundant ferruginous cement, oolitic and fossiliferous
(small Pelecypoda and fossil traces). Southward, these sandstones and silty shales
are unconformably overlain by the Continental Mesozoic.
Lost Oasis Bzimah
The Jabal
Bzimah is an isolated sandstone - plateau, located outside in the desert nearby
Jabal Zalmah (Dalma). During a trekking - tour in 2005 we had a camp at the Jabal
Bzimah. This plateau is found approximately 70 km in distance from the Jabal
Zalmah. The upper part of the deposits are partly massive reddish sandstones.
The lower part of deposits included differently silty shales.
The plateau
of Bzimah probably was uplifted in the Upper Devonian period by the Bretonian
tectonic event likewise as Jabal Zalmah, Jabal Uwaynat and Gilf Kebir (Egypt).
The reddish sandstones are stratified thick and thin. Cross-beddings are not
rarely available. The lower part of the mighty profile contains frequently siltstones.
During a short-time excursion I have searched fossils. I have found only some
remains of Silurian bryozoa of Genus Prasopora in thin layers of
sandstones above the siltstones. Bryozoans (Moss Animals) are sessile aquatic
animals forming mosslike colonies of small polyps. Each having a curved or circular
ridge bearing tentacles and reproduce by budding. Bryozoans are found in the
deeper intertidal to subtidal zones, attached to firm substrate, and also in
brackish water.
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Massive Paleozoic
sandstone on the plateau
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Silty shales
of Silurian age
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Silty shales in the foreground
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Silty shales
on base the sandstones
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Flat water Turbulences
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Cross-bedding
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Thick and thin
layers of Paleozoic sandstones
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with remains
of Prasopora bryozoa
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Bryozoan remains
in a shallow marine sandstone
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