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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.
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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 (Kufra region) are
preserved transgressive Carboniferous strata with plant remains.
Silurian outcrops. Jabal Eghi
(western Kufra
Basin margin, SE Libya).
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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 |
Cambro-Ordovician Skolithos
(Tigillites)
from Jabal Dalma
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Ordovician and Silurian
strata |
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 (Desio). 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 Neodipl. 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 Longitude 23°52' 30"
and Latitude 25°47' 50" 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: lat 25°34'10" N and long
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
(lat 25°14' N and long 23° 46'30” E), section (lat 25°08'20” N and long 23°
49'15"E) and section (lat 25°12'N and long 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.
 
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 red
sandstones. The lower part of deposits included differently silty shales.
Jabal Bzimah
Lost Oasis Bzimah
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 red 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 under it
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Silty shales of Silurian age
 Thick and thin layers of Paleozoic sandstones
 with remains of
Prasopora bryozoa
 Turbulences in flat water
 Cross-bedding
 
Bryozoan remains in a
shallow marine sandstone
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