The actual origin of the travertine-like deposits of
the Tassili, Tadrart and Acacus Mts. in Algeria and Libya

Norbert Brügge, Germany

Upload: 16.03.2020

1. Accacus Mts. (Libya)

Publications about ....

Geomorphological Map of the Tadrart Acacus Massif ...... (Libyan Central Sahara)

Zerboni, A. Perego & M.  Cremaschi -- Link to this article:

"Along the wadi banks on the inner parts of the Tadrart Acacus Massif, at an elevation ranging from 900 to 1000 m a.s.l., calcareous tufa were identified (Cremaschi et al., 2010; Zerboni & Cremaschi, 2012). Outcrops of this kind of deposits were found within rock shelters, along vertical fractures inside relict solutional tunnels, and on vertical cliffs; they are related to the horizontal boundary between sandstone and thin, impermeable beds of shale or fine-grained sandstone beneath (Cremaschi et al., 2010). In the central part of the Tadrart Acacus, tufa deposits occur as thin laminated crusts on the floors of rockshelters and belong to the pool and barrage types; in the upper reaches of the wadis, up to several tens of meter long alignments of massive flowstone line vertical walls (Figure 8). An alignment of calcareous tufa on a vertical cliff along the upper reach of wadi Imha. In the latter case stalactites may be 2 m long. The microscopic properties of tufa are discussed by Carrara, Cremaschi, and Quinif (1998) and Cremaschi et al. (2010), with several U/Th and radiocarbon dates placing their deposition to the Early Holocene (Cremaschi et al., 2010). Due to the topographic and geologic position of tufa and their sedimentological characteristics, we define them as spring tufa and their occurrence indicates the position of ancient springs that were active during humid phases."

Figure 8. Upper part of Qued Imha
Comment: Probably eroded remains on the roof of a volcanic spring with younger (?) stalactites

The calcareous tufa in the Tadrart Acacus Mt. (SW Fezzan, Libya) -- An early Holocene palaeoclimate archive in the central Sahara
 M. Cremaschi,  A. Zerboni, Ch. Spötl, F. Felletti --
ELSEVIER / Palaeogeography, Palaeoclimatology, Palaeoecology 287 (2010) 81–94

"The tufa deposits were discovered during a geoarchaeological survey in the Tadrart Acacus mountain conducted from 1994 to 2005, when almost all the wadi of the massif were examined. Nineteen tufa outcrops were identified, and six of them were now sampled for laboratory analyses.
The calcareous tufa deposits crop out at the base of rock shelters and niches, originated by undersapping processes and related to the horizontal boundary between sandstone and thin, impermeable beds of shale or fine-grained sandstone beneath. Tufa also formed along vertical fractures inside relict silicakarst tunnels. In both cases tufa deposits occur as thin (3–20 cm thick) laminated crusts at the floors of the rock shelters and belong to the pool and barrage types. They also line the walls and roofs of the cavities and tunnels as massive flowstone. Pool-, barrage-, and flowstone-type tufa is uniformly white to light brown and well lithified. The laminar structure, observed macroscopically, also clearly appears at the microscopic level: cryptalgal and bacterial structures are embedded in a dense, whitish yellow, micrite groundmass, which appear as very thin, planar or wavy "stromatolithic" laminae containing calcified algal filaments, fan-shaped branching algae and bacterial clumps. Compact tufa is laterally discontinuous and grades into highly porous moss tufa consisting of tubules and phytoclasts. In a few cases tufa also impregnates clastic deposits, such as aeolian sand and detrital fragments of sandstone occurring mostly at the base of the sequences.

Six samples were U/Th dated. A single date is available for the sample 05/56, a massive, sand-rich flowstone, located above a small spring pool. A sequence of five dates (four U/Th and one AMS 14C) was obtained on the sample from site 02/99, which is the thickest (130 mm) deposit identified; samples were taken from the top and the base of each hand specimen and in correspondence to the main lithological discontinuities. Two dates each were performed at the base and at the top of the samples from the sites 94/96 and 94/49, obtained from flowstones, and on sample TH128 (pool and dam type). U concentrations of the Tadrart Acacus tufa deposits are high (4–12 ppm) and U-series analyses gave consistent results, with the exception of sample TH96-2 (very high 230Th/238U ratio).
An AMS 14C age was performed on the lower part of sample 02/99 and gave 8360±50 14C yr BP (9500–9250 cal. yr BP (2σ) using the INTCAL04 calibration curve of Reimer et al., 2004.
This result is in good agreement with the U/Th data, and supports their reliability. The available dates indicate that tufas were deposited between c. 9605 and 7634 yr BP. Most of the sedimentation occurred between 9500 and 8100 yr BP; a single date hints towards a slightly younger age in one location (7634 yr BP)."

Red triangles: Examinated deposits (6)

Comment: Profile-B depicting a groundwater table in an ideal (but unrealistic) saturated sandstone aquifer in the Holocene

Comment: Probably an eroded volcanic spring

Contradiction and new proposal

Consolidated bedrocks (also sandstone) of Paleozoic age cannot absorb groudwater. Porosity and permeability are very low. No aquifer in the true sense can form. Rainwater can only seep away along cracks in the sandstone and accumulate in cavities etc. The infiltration rate into the "aquifer" of the Acacus Mts. was consequently low despite intensive rainfalls and could not lead to a saturated true aquifer with a high uniform water table, as it was declared. Most of the rainwater ran off along the valleys towards the foreland. A true “groundwater level” would also have been significantly lower, since it is determined by the level of the foreland. The formation of permanent small lakes at barriers was possible, but pent-up water would have been exposed to constant exchange with rainwater, so carbonates could not form autochthonous.

It is crucial, however, that there is no convincing geological evidence for the thesis of carbonate precipitation through solution processes in imaginary or at least hardly available carbonate layers. There are therefore no prerequisites for the meteogenic formation of travertine in this mountain region. The only realistic possibility is the formation of these deposits by feeding them from volcanic springs containing carbonates, and which may be older than the Holocene, probably the Pleistocene. Such young hydro-volcanic activities have nothing to do directly with the volcanoes in the region (eg Adrar volcano southern of the Tassili Mts).

The usability of the determined age of the deposits with the U/Th-dating method is questionable because the reliability of the method is restricted in an "open system". The deposits could have later been contaminated by leachate in the rainy phase of the Holocene. An indication of this is the likely new formation of flowstones (stalactites) in shelters. The suspected organic residues appear to be phantoms.

The data found can only relate to the period of erosion, renewed deposition (eg stalactites) and/or contamination. Since there is no U / Th older data to before 9,500 yr BP, we should assume that the age of the original carbonates cannot be determined using the U/Th method. (Pleistocene ?)

Pure carbonates can be seen in photos of the shelters. The microphotographs presented in the publication are from other locations, of which there are no photos. In a table one speaks of flowstones, presumably samples of secondary stalagmites and stalactites. These can also contain contaminats and foreign material. We can suspect that they were used for the analyzes.

Deposits of a sand and gravel mix in drainage valleys were consolidated by carbonate muds from eroded volcanic springs.
Such deposits can be found in the Qued Djerat (Tassili Mts., Algeria) or the Qued Tin Uded in the Algerian Tardrart Mts. The age of these deposits is likely to be the same or younger than that in the Acacus Mts. (Holocene).


2. Tassili Mts. (Algeria)

Andras Zboray was back in Algeria in November 2019 and examined some mysterious carbonate deposits in the Qued Djerat (Tassili Mountains) again. He had documented his first finds in November 2014 photographically. These deposits can be traced a good 20 kilometers along the Oued side, although they are not continuous. They are found regularly and seem to mark a river terrace , as they rise with the valley floor. Carbonate mud have cemented a mix of sand and gravel. The predict eroded volcanic carbonate springs were not sought because the planned route only followed the valley.

Locations 1-8 im Qued Djaret

Loc 1

Loc 2

Loc 3

Loc 4

Loc 5

Loc 6.1

Loc 6.1

Loc 6.2

Loc 6.2

Loc 7

Loc 8


3. Tadrart Mts. (Algeria)

Andras Zboray has also found in 2019 another similar series of outcrops in a Qued Tin Uded in the Algerian Tadrart Mountains. Location: 23°54'14"N / 10° 55'20"E. This seems to indicate that these strange deposits are not limited to the Tassili Plateau. It is very plausible that carbonate mud, mobilized from still to be found older volcanic springs, has solidified the sand and debris-leading terrace deposits in drainage valleys. These kind of deposits also can be compared with some deposits in the Jebel Uweinat  (Sudan).


4. Conclusion
coming soon