Holocene post-volcanic spring deposits in the Jebel Uweinat (SW-Egypt) ?

Norbert Brügge, Germany
Dipl.-Geol.

Update: 21.05.2018


Holocene freshwater carbonate structures in the hyper-arid Gebel Uweinat region of the Sahara Desert (Southwestern Egypt)

by Margarita M. Marinova, A. Nele Meckler, Christopher P. McKay, in Journal of African Earth Sciences 89 (2014) 50–55
http://www.sciencedirect.com/science/article/pii/S1464343X13001763

"During our preliminary exploration (in 2005) of the Gebel Uweinat region, two valleys ("Karkur Talh") were discovered that contain morphologically distinct carbonate structures and at one of the sites form a bench along the valley wall. Both of these valleys are located in likely ancient valley network channels. At Site 1, the carbonate structures form a distinct bench along the sandstone valley wall; the bench is continuous for over 50 m and is about 1 m thick vertically (Site 1, N21°58' / E25°06' , elevation 715 m SL)
The carbonate bench is ‘‘pasted’’ onto the valley wall and is not part of, or aligned with, the stratigraphic sequence. At this location the valley is about 20 m deep. No apparent shorelines were observed.
Site 2 is located about 5 km away in a neighboring valley, and at a slightly higher elevation than the first site (Site 2, N21°56' / E25°04' , elevation 775 m SL). The height of the surrounding, broader valley is about 10 m. The carbonates structures had a similar, distinct morphology, but did not form a bench along the valley walls: they were distributed both along the valley walls and along the valley floor.
Bulk XRD mineralogical analysis of the samples showed carbonates and quartz as the main minerals, with other minor constituents (less than a few percent total weight fraction). The carbonate is in the form of calcite, with less than 5 mol% magnesium. The average total carbonate weight fraction (as determined by acid digestion) is 46% for Site 1, with the three subsamples giving 42.5%, 49.0%, and 46.8% by weight. For Site 2, the average total calcite content was 28% by weight, with the three subsamples giving 29.2%, 28.2%, and 26.0% by weight. The quartz component was likely contributed by the ubiquitous sand sheets and dunes in the region.
Energy dispersive spectrometer (EDS) analyses showed some of the minor mineralogical components to be calcium sulphate, magnetite, ilmenite, (Na, Al) pyroxenes, and orthoclase. The composition at both sites is similar, consistent with their close proximity. The organic carbon content of the samples is below the detection limit of the measurement: less than 0.3 weight percent.
Microscopic examination of the samples from both sites shows a calcite matrix primarily embedded with quartz, with other mineral grains as a minor component. The quartz grains are angular with little rounding, similar to the angular nature of the quartz grains in the surrounding sandstone rocks. The persistent angular grains suggest short transport distances. No fossils or pollen grains were observed.
The calcite structures were dated using radiocarbon dating of the bulk sample. Two subsamples from Site 1 were dated, giving similar dates: 8190 calibrated years BP and 7970 cal. years BP. For Site 2, the carbonate age is 9440 calibrated years BP." (BETA Analytic Inc., Miami)

Fig. 3. Calcite matrix and composition. SEM image of a thin-section of the calcite structures at Site 1.
The matrix is calcite (1), the most ubiquitous incorporated mineral is quartz (2); iron oxides/ilmenite (3, bottom left), pyroxenes (Na, Al pyroxenes in this image; 4), and orthoclase (5) are also present.

"Carbonate-bearing rocks are not present upstream of the study sites. .... No active springs were found upstream, although one spring is known on the other side of the mountain and carbon input from groundwater cannot be fully ruled out. .... The level of the deposits reported here is uniform across the valley wall consistent with formation from lake water. ... The chemical and morphological similarity of these formations to carbonate structures from modern lakes suggests that these lakes contained fresh, standing water. ... Microbial carbonate structures are reported from a diversity of environments."
 

 

The results of the investigations, however, must to be interpreted quite differently:

The age determination of the sinter-like deposits found in Karkur Talh is a big surprise. There are apparently hydrovolcanic deposits formed in the early Holocene period. These sediments are not of microbially origin and were not in a lake deposited, as assumed. These are deposits that presumably originated from local springs with volcanic water have occured, laden with mineral-rich carbonates and dissolved iron. This volcanic water has apparently also carried fragments of metamorphic basement. This means that in the Holocene period, the Oligocene volcanism had a post-repercussion in the region. The mineralogical composition, documented in the article, is highly likely to correspond to metamorphic carbonate from the UM formation (Ultramafic-Mafic and Calc-Silicate Rocks) of the Precambrian basement. Other (sedimentary) carbonates, which could be input, are not present in the subsurface of the entire Western Desert.
The carbonates of the Karkur Talh does only weak or very weak react with formic acid. It contains pyroxene, orthoclase and magnetite as well as unrounded quartz. The proportion of these is high in samples of both sites (46% and 28%, respectively).

Important in this context is the statement that in the Karkur Talh above ground no metamorphic basement of the UM formation outcroped that could have been eroded. If the theory of post-volcanic springs is correct, they may have been on the uplifted Hassanein Plateau. In the Oligocene, along with the intrusion of granite probably parts of the basement were involved.


Another extraordinary option could be basalt-carbonate as a source. It has recently been discovered that carbonates can form in basalts when it come in contact with CO2-rich water.
In the case of the Jebel Uweinat, where there are basalts, such a CO2-containing hot volcanic water might have been in contact with these basalts in underground. In this context, the "White Spot" basalt on the top of the Hassanein Plateau could be a source. The carbonates formed in the basalt must then have reached the surface in dissolved (hot) state in order to form the found deposits.
If the thesis of aggressive hot volcanic, CO2-rich solutions is correct, which may have formed the carbonates in parts of basalts, rock-forming minerals should be found in the deposits, which is the case. In any case, hot hydro-volcanism is a priority because the carbonates found in the deposits do not react mostly with acid.


Against the announced microbial formation of deposits in these localized and narrow congestion channels with a possible overflow speak the following facts:
 

• Predominantly Fe-containing sandy deposits
• Weak soluble carbonates (calcium sulphate matrix?) with high amount of crystalline components (unrounded quartz, pyroxene, orthoclase, ilmenite and magnetite)
• Very low organic presence in the deposits
• No plant remnants and pollen or algae pillows as well as bacterial structures (stromatolites).

The age determination does not necessarily document the actual age of the found sandy sinter-like deposits. Organically produced carbonaceous deposits should not be present in the Karkur Talh.


The carbonates from the basement can not be organic carbonate. These carbonats in the matrix are at least 1000 million years old and no one yet knows exactly how this carbonates came about. At least not with the participation of calcifying organisms.
Moreover, we must assume that the springs were active only relatively short time, because the deposits were not thick. So no organic carbonate could develop during sedimentation.
Between the two deposits in Karkur Talh a difference of maximal 1500 years was determined. This can only mean that the springs were active at different times, which is unlikely. It is more likely that the organic carbonate were later added at different times.
Today the remains of this sinter-like deposits in the Karkur Talh can be found directly on the edges of the sandstone walls. The deposits, which were originally 1-2 m thick over a length of about 600 m (Site 1), were again eroded by heavy rainfalls.

 

Supplement:
Andras Zboray sent me a sample from Site 1 in 2016. My visual analysis has a surprising result. The sample consists of an accumulation of iron-containing sand. This is documented by the color, weight and hardness of the sample. Parts of sand-free and hard carbonate with undoubtedly non-sedimentary minerals are stored in it. These parts should be correspond to the investigated samples by MARINOVA et al. The examination of the sent sample with formic acid resulted in a weak to very weak reaction.
Part of this sample was sent by A. Zboray to a laboratory in the USA for a new C14 dating (C. McKay). The analysis revealed a significantly younger age than previously determined.
I think that due to the enormous time differences, the dates can not be used to determine the true age of the deposits. It can only mean that the investigated organic carbonates (or carbon) have been intruded as contaminants in the porous deposits during a longer time span. The question now is, what is the true age of the sinter-like deposits? They could be much older than the C14 datings suggest.

 

Author, 2006: Sinter-like remnants at Site 1 Andras Zboray, 2005: Sinter-like remnants at Site 1
Andras Zboray, 2016: Site 1   Site 1

 

 


 

 

 



 


Further post-volcanic spring deposits in the Wadi Wahesh and on the Hassanein Plateau (Jebel Uweinat) ?

Andras Zboray found in 2017 further sinter-like deposits in the Wadi Wahesh (Site 4), and more surprisingly in the sand-filled main basin on the Hassanein Plateau (Site 3). These deposits are identical in appearance and texture with the non-carbonate deposits from the Karkur Talh (Site 1 and Site 2).

In contrast to the sintered sandy spring deposits from Site 1 to Site 4, the spring deposits of the Site 5 surprisingly have a pure carbonate composition:
"I noted already in 2005 travertine-like deposits surrounding a large shelter containing prehistoric rock paintings in the middle section of the Wadi Wahesh. This site is completely different from the previous, and is unique at Jebel Uweinat and consists of a fossil spring with carbonate tufa deposits surrounding a large multi-chambered shelter. The spring clearly emerged from a junction Paleozoic sandstone bed immediately above the upper shelter, and flowed down in several channels as well as over the brow of the shelter." (Andras Zboray)


Lumps cropping out of the surrounding fine silt deposit
 


Lumps in the steeply wadi bed
 

  

Summary of my visually analysis of  Site 1 to Site 4 samples
(send to me by Andras Zboray)

Location
(Coordinates)

Sample

m SL

Sample
type

Characteristics

Decay
in water

Carbonate test in formic acid

Residue

Color Consistency Matrix Minerals*

CO2

Decay

Site 1
(Karkur Talh)
21°57'49.33"N
25°05'56.54"E

 2016 + 715 FeOx Sand
and
Carbonates
light-brown Sintered
chunks,
hard

Sand
and Carbonates
+ dark parts with
magnetite (?)

Quartz
Magnetite,Ilmenite
 Pyroxenes
Orthoclase

Only
sandy
parts
Persistently
weak
to
very weak
Yes
(very weak)

Quartz grains (rounded), some larger free quartz grains (rounded)
FeOx-pigments
Diverse minerals (also calcium sulphate crystals !?)
insoluble magnetite (?) rich parts

 2018-1/A
 2018-1/B

Site 2
(Karkur Talh)
21°56'13.64"N
25°04'9.06"E

 2018-2 + 778 FeOx Sand dark-brown Sintered
chunks,
crumbly

 Sand

No

Yes
(quick)
Shortly
weak
---

FeOx.-coated rounded quartz grains, some larger free quartz grains (rounded)

Site 3
 (Hassanein Plateau)
21°54'43.09"N
25°04'1.43"E

 2018-3 + 1338 Carbonates  beige to
light-brown
Hard

Carbonate

No

No Persistently
moderate
Yes

Rounded quartz grains, some larger quartz grains (rounded)
Low FeOx.-pigments

Site 4
(Wadi Wahesh)
21°53'47.08"N
25°02'55.06"E

 2018-4 + 925 Carbonates  beige to
light-brown
Very hard

Carbonate

Yes

No Persistenly
very weak
Yes
(very weak)

Minerals
(some rounded quartz grains)

*Marinova et al. 2014


First Resume
: The sinter-like deposits of Site 1 to 4 are of the same (hydrovolcanic) origin but different in the consistency. These are sand-grains deposits, which decay in water, which were sintered together with weak to not acid-soluble carbonates. The whole transitionless mixture is more or less interspersed with iron oxide. The carbonates contain high amount of diverse minerals. The carbonates may also contain dark (magnetite (?) dominated) insoluble parts like Schlieren (Site 1).
The sample from Site 3 deviates. The contained carbonates are more soluble and contain neither minerals nor dark parts. The character of the carbonates is still puzzling. But it have to, because demonstrably contained FeOx in the residue, have arisen in the context of the sand-sinter. Their limited solubility may be due with the high amount of minerals. The irregular distribution of the various parts within the deposit speaks against a stillwater milieu during sedimentation.

I examined also a sample of  Site 5 deposit with my modest possibilities. Visually, the deposit resembles a previous carbonate mud that was permeated by gas bubbles. The sample has a light to slightly reddish color, is hard and firm baked. It is completely dissolvable in formic acid. There are no residues of sand grains, minerals or other inclusions.

Here is to see an outcrop of deposits of a former hot hydrovolcanic spring. In the absence of sedimentary limestones in the subsoil of the surrounding, it has to be assumed that it is detritus (mud) of marble from the Infra-Cambrian (Jabal Arkenu), that have been solved by aggressive volcanic water. In context, the paintings in the shelter are much younger than the deposit.
 

Conclusion

The five deposits with the calcareous deposits discovered so far in the Jebel Uweinat are located morphologically in former erosion valleys below the Hassanein plateau (4) or on the plateau itself (1). The deposits are different. Site5 is clearly a pure calcareous spring deposit. The material of the other deposits is heterogeneous. The carbonats, some of which are difficult to dissolve in acids, contain iron-rich sand and various minerals and components of crystalline or metamorphic rocks.
The calcerous deposits not have been organically evolved because the conditions for still water in the Jebel Uweinat are missing. The Jebel is a sandstone plateau with no
impermeable layers that would allow a longer accumulation of groundwater. Rainwater seeps and/or flow away into the deeper foreland.
The origin of the calcareous matrix of sedimentary limestones is not possible because such deposits are absent in the Jebel Uweinat and the surrounding area. Therefore, these must come from the uplifted basement of the East Sahara Ghost Craton, which lie at a shallow depth below the Paleozoic sandstones.
Another source to consider are carbonates, which can be formed by conversion to basaltic intrusions (eg White Spot).
The mechanism for the formation of calcareous deposits is hydro-volcanic (hydrothermal is the wrong term here). It is classified as post-volcanic because volcanic activities are not known since the Oligocene.
By rising aggressive volcanic water, transformations on basalts or dissolved calc- silicates of the basement could have come to the surface.The pure calcarous spring deposit (Site5) is a special case. As source, calcareous deposits of the Infra-Cambrian are postulated.
If the scenario described is correct, it is not possible to determine the age of the deposits using the C14 method. The data obtained are based on the measurement of organic components of contamination. Their age varies on a wide scale
(latest data).