Fossile remains of genus Prasopora (Trepostomata bryozoa) in Silurian strata of the Jabal Bzimah (Libya)
--- nearby Jabal Dalma (syn. Jabal Zalmah) ---

Norbert Brügge, Germany
Dipl. - Geol.

Jabal Dalma (Zalmah)
The Howar-Uweinat Uplift forms the eastern border of the Kufra Basin. At the northern part of this tectonic structure is located the Jabal Dalma (Zalmah). The plateau contains sandstones, siltstones and shales of Ordovician to Devonian age. In the southeastern part of the Jabal Dalma are preserved transgressive Carboniferous strata with plant remains.

Geological map

  Lüning et al, 1999: Silurian sections


Cambro-Ordovician strata


"Detailed outcrop studies at the flanks of Kufra Basin reveal the nature of glacially-related sedimentation and post-depositional deformation styles produced in association with the Late Ordovician glaciation, during which ice sheets expanded northward over North Africa to deposit the Mamuniyat Formation. At the SE basin flank (Jabal Asba), the Mamuniyat Formation is sand-dominated, and incises interfingering braidplain and shallow marine deposits of the Hawaz Formation. The glacially-related sediments include intercalations of mud-chip bearing tabular sandstones and intraformational conglomerates, which are interpreted as turbidite and debrite facies respectively. These record aggradation of an extensive sediment wedge in front of a stable former ice margin. An increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites. A widespread surface, bearing streamlined NW-SE striking ridges and grooves, punctuates this succession. The structures on the surface are interpreted to have formed during a regional north-westward ice advance. Above, siltstones bearing Arthrophycus burrows, and Orthocone-bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath. By contrast, the northern basin margin (Jabal Dalma) is appreciably different in recording shallower water/paralic sedimentation styles and major glaciotectonic deformation features, although facies analysis also reveals northward deepening. Here, a siltstone wedging from 8 to 50 m toward the north was deposited (lower delta plain), succeeded by climbing ripple cross-laminated sandstones up to 60 m in thickness (distal through proximal delta mouth bar deposits) with occasional diamictite interbeds. These rocks are deformed by thrusts and > 50 m amplitude fault-propagation folds, the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation. Integrating observations from both basin margins, a model of fluvial-dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed."
Le Heron et al, 2009 and Le Heron et al, 2010

Glaciotectonic deformation

Late Ordovician (Hirnantian) glaciation

Glacial traces. Source: Lyell Collection

Facies details in the Mamuniyat Formation in the northern part of Jabal

Skolithos (Tigillites)


Typical surface tracks (Planolites) at the top of the Mamuniyat Formation

Ripple marks


"Field studies in the Jabel Zalmah region, at the northern flank of Al Kufrah Basin provide new insights into the stratigraphy and evolving depositional environments of Cambro-Ordovician through Devonian sediments. At the base of the succession, the Hawaz Formation is about 300 m thick and is characterised by frequently Skolithos-burrowed, fine to medium grained sandstones with occasional silty intervals and rare conglomerates deposited in a generally high-energy tidal shelf setting. Bioturbation is pervasive, and includes spectacular Cruziana traces produced by Early to Mid Ordovician trilobites at the bottom and the top of the Hawaz Formation respectively. These strata were incised during the late Ordovician (Hirnantian) glaciation. Ice sheets flowing over Jabel Zalmah cut a palaeo-topography of up to 100 m relief. This relief is overlain by a coarsening upward succession of clastic sedimentary rocks, recording re-advance of an ice sheet, which overrode the uppermost sandstones. Ice sheet retreat was accompanied by marine transgression and erosion, which left a thin transgressive lag-conglomerate, capped sharply by siltstones of the Tannezuft Formation. Progradation resumed after post-glacial maximum flooding, depositing tide, storm, and river-influenced parasequences of a major delta complex (200 m thick Acacus Formation). Fluvial incision, and the development of a disconformity, preceded the deposition of the Tadrart Formation (50 m thick), a succession of trough-cross bedded braided river deposits. Transgression, and the re-establishment of shallow or marginal marine systems, is represented by the extensively bioturbated Binem Formation in which Zoophycos burrows offer a tentative Middle Devonian stratigraphic fix.."
Le Heron et al, 2011

Outcrop character and sedimentary facies of the Hawaz Formation in the westernmost part of Jabel Zalmah.
A: view of ~ 200 m high outcrop representing the complete thickness of the Hawaz Formation.
B: view of a large, composite sigmoidal bedform, built from trough cross-bedded and planar laminated sandstones, at the base of the Hawaz Formation.
C: planar cross strata in medium-grained sandstone.
D: Skolithos burrows (vertical fabric) crosscutting planar cross-bedded sandstones.
E: large-scale Thalassanoides trails.
F: Cruziana.
G: recumbent appearance of folded trough cross-strata (notebook for scale).
H: large-scale overturned and folded cross strata. The scale of the deformation allows a current shear mechanism to be ruled out, and a liquefaction explanation is hence preferred.

Typical features of the Mamuniyat Formation at the contact with the Tanezzuft Formation.
A: micro-fractured sandstones, interpreted to record deformation beneath cold-based ice. B: chaotic folding within coarse-grained sandstone, interpreted as soft-sediment, glacially related sediment deformation.
C and D: nature of the contact between the Mamuniyat Formation and the overlying Tanezzuft Formation. This can be summarised as typically sharp, yet undulose at the outcrop-scale.
E: Hummocky cross-stratified sandstones and interbedded siltstones at the base of the Tanezzuft Formation. These facies are also common further up section into the Acacus Formation.

General outcrop views and sedimentary facies within the Acacus and Tadrart formations.
A: typical outcrop pattern of the Acacus Formation.
B, C: beautifully exposed parasequences within the lower part of the Acacus Formation.
D: sharp-based sandstone cutting down into underlying parasequence.
E: middle part of the Acacus Formation. In the background, each of the clearly defined benches of sandstone mark the top of a 2-8 m thick coarsening upward cycle.
F: discrete interval of Skolithos bioturbation, typical of the upper Acacus Formation.
G: Arthophycus linearis (worm burrows).
H: cross-stratification in lower levels of the Tadrart Formation.


Silurian strata

Sections G, F, H, C, D and B (Lüning et al, 1999)

Section E (Lüning et al, 1999)

Early Silurian graptolites in the Tanezzuft shales in SE Libya (Jabal Dalma)
Neodiplograptus fezzanensis (DESIO, 1940)
indicating a Lower to Middle Llandovery age

Sample of Diplograptus

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.

Devonian strata

In the area of Jabal Dalma 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 Dalma. 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 Dalma. 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).

Le Heron et al, 2012: High resolution facies analysis and sequence stratigraphy of the Siluro-Devonian succession of Al Kufrah basin

Source: - Journal of African Earth Sciences

General view of the Silurian and lower Devonian strata in Jabal Zalmah (N25°43'37" - E23°33'59"). The uppermost part of the Tannezuft formation and the Akakus formation is mostly exposed along extensive cliffs and consists of heterogeneous shale to fine-grained sandstones in which 8 m-thick massive sandy bars have been observed and used as regional stratigraphic markers. Benches are interpreted as being equivalent shoreface storm dominated deposits (bars A, B, D, F and G) or tidal flat to estuarine deposits (bars C, E and H). Bars are sharp-based with underlaying heterogeneous inter-bars and represent key stratigraphic forced regression surfaces. Lower Devonian fluvial sandstones of the Tadrart formation (red strata) unconformably overlie Silurian Akakus formation





Regional correlation transect of the Siluro-Devonian succession between Jabal Zalmah and Jabal Asba.  Geographical coordinates of section 2 (N25°43′37″ - E23°33′59″), section 4 (N23°11′02″ - E24°29′21″) and section 5 (N22°55′17″ - 24°20′58″).

The uppermost Tanezzuft formation and the Akakus formation successions contain storm-related deposits.
(a) Amalgamated hummock cross-stratified fine-grained sandstones forming massive sharp-based equivalent shoreface deposits (Jabal Zalmah).
(b) Amalgamation of symetrical vortex straight-crested oscillatory ripples in fine to medium grained sandstones (Jabal Asba).
(c) Interferent polygonal co-genetic ripples or “bricks and tiles” pattern-like ripples (Jabal Zalmah).
(d) Stacked assymetrical climbing ripples sets forming centimetric thick tabular sandy beds. Facies may include “ball and pillow” structures (Jabal Asba).
(e) Details of an aggrading pattern of in-phase symmetrical climbing ripples within well-sorted fine sandstones (Jabal Asba).
(f) Teichnicus sp. burrowing a thin wave-rippled sandy bed (Jabal Asba).
(g) Tabular millimetric-thick silty sandy benches interbedded with muddy siltstones (distal upper offshore).
(h) Details of undulated fining-upward storm graded sandy and silty layers burrowed by Skolithos sp. (distal upper offshore).

Bars C, E and H of the Akakus formation in Jabal Zalmah and the uppermost part of the Akakus succession in Jabal Asba constitute sharp-based tide-related strata.
(a) Highly bioturbated horizontal and tabular decimetric thick sandy sheets (Jabal Zalmah).
(b) Bioturbated cross-bedded fine-grained sandstones with planar foresets of megaripples (Jabal Zalmah).
(c) Lee side face tangential foresets of migrating megaripples scoured at the top by erosive surface (Jabal Zalmah).
(d) Close-up of lingoid crested unidirectional current ripples train (Jabal Zalmah).

(e) Concave-up scour surface truncating a stoss-side megaripple, underlain by wave-generated ripples (Jabal Asba).
(f) Aggrading in-phase symmetrical climbing wave-generated ripples into a fine to medium – grained sandy bench (Jabal Asba).
(g) Lenticular to wavy bedding in a muddy to fine-grained sandstone assemblage (Jabal Asba).
(h) Small asymetrical gutter cast located at the base of a centimetric thick coarse – to medium – grained cross bedded sandy bed (Jabal Asba).

Photographs illustrating facies of the Tadrart and Binem formations.
(a) Massive reddish fluvial cross bedded sandstones of the Tadrart formation (Jabal Asba).
(b) Details of planar cross-stratified fine-grained sandstones (fluvial bar preserved in the Tadrart formation) (Jabal Zalmah).
(c) Heretogeneous hummocky cross-stratified succession of the Akakus formation truncated by reddish cross bedded fluvial Tadrart formation sandstones (Jabal Zalmah).
(d) Illustration of the top Silurian–base Devonian unconformity at the Akakus–Tadrart formations transition (Jabal Zalmah).

(e) Cross-bedded sandstones of the middle Devonian Binem formation significantly affected by numerous Skolithos sp. burrows (Jabal Asba).
(f) Close-up of a “Tigillites” facies in the Binem formation (vertical burrowing of long Skolithos ichnofabric) (Jabal Asba).
(g) Zoophycos sp. significantly preserved into the uppermost Binem formation (Jabal Asba).
(h) Spirophyton isp. preserved into the uppermost part of the Binem formation (Jabal Asba).


Carboniferous strata

On account of their lithological, sedimentological, palaeontological and environmental characteristics they were referred to the Dalma Formation (from the high hill of Jabal Dalma (Zalmah), 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 Dalma, that the type section has been chosen for the Lower Carboniferous in the Kufra basin.
The sequence exposed at Jabal Dalma, 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 Dalma 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.


 Jabal Bzimah (Bzemah)

Lost Oasis Bzimah

The Jabal Bzimah is an isolated sandstone - plateau, located outside in the desert nearby Jabal Dalma (Zalmah). 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 Dalma. 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. 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.

Massive Devonian (?) sandstone on the plateau


Silty shales of Silurian Tanezzuft Formation

Silty shales in the foreground


Silty shales on base the sandstones

Flat water Turbulences


Thick and thin layers of Silurian sandstones of Akakus Formation

with remains of Prasopora bryozoa

Bryozoan remains in a shallow marine sandstone of Jabal Bzimah

These circular-shaped fossils are Prasopora bryozoa, was found in situ in the Sugar River Formation, part of the Ordovician Trenton Group (Mohawk river valley, state New York). These are some of the largest examples of this marine animal found anywhere in the world.

The bryozoa Prasopora aff. gotlandica from Jabal Bzimah

Order Trepostomata Ulrich, 1882

Genus Prasopora Nicholson & Etheridge, 1877
Free discoid to hemispherical masses. Zooecia lined by overlapping cystiphragms connected by diaphragms.
Ordovician - Silurian

Silurian occurrences:
P. codonophylloides Yang & Xia, 1976, Early Silurian China (Yunnan)
P. gotlandica Hennig, 1908, Silurian Sweden (Gotland)
P. juvenalis Modzalevskaya, 1979, L. Silurian Russia
P. parmula Foerste, 1887, Silurian USA
P. propria Modzalevskaya, 1979, L. Silurian Russia