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

Norbert Brügge, Germany
Dipl.-Geol.

Upload: May 2015
Update:
03.09.2018


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:
 

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. 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).

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

• Predominantly Fe-containing sandy deposits
• Weak soluble carbonates 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 of the sinter-like deposits found in Karkur Talh is a big surprise. That does not necessarily document the actual age of this sandy sinter-like deposits. Organic produced carbonate deposits should not be present in the Karkur Talh. The conditions for a permanntly still water in the Jebel Uweinat are missing. The Jebel is a sandstone plateau. Impermeable layers are missing that would allow a longer accumulation of groundwater. Rainwater seeps and/or flow away into the deeper foreland.

Therefore, we must assume that the deposits were associated with hydro-volcanic springs. Moreover, we must assume that such springs were active only relatively short time. So no organic carbonate could develop during the sedimentation in a basin.
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 as contaminant.
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.

 

There are two options of origin of non-organic carbonates, formed in the early Holocene period by post-volcanic springs:

  • Volcanic water has apparently carried fragments of metamorphic basement. The mineralogical composition, documented in the article, can correspond to metamorphic carbonate from the UM formation (Ultramafic-Mafic and Calc-Silicate Rocks) of the Precambrian basement or "Infra-Cambrian" marble. Other (sedimentary) carbonates, which could be input, are not present in the subsurface of the entire Western Desert.
    Important in this context is the statement that in the Karkur Talh above ground no metamorphic basement of the UM formation or "Infra-Cambrian" outcroped that could have been eroded. If this theory 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.
    Such carbonates can not be organic carbonate because they are at least 1000 million years old.

  • The most interesting and probably option is the origin of the carbonates from the Earth' mantle (hypabyssal), ascended along faults. The genesis can be explained by the ascent of hot hydro-volcanic water, loaded with a mix of FeOx and fragments of porphyritic carbonates from a older dyke, containing rock-forming minerals. The degree of mixing with sand was dependent on the transport distance on the surface. After loss of water, the mixture hardened. Compare the photo of thin section in Marinova et al. (2014). The Jebel Uweinat is structurally located on uplifted Archean Basement within the East Sahara Ghost Kraton. In this context, my latest discoveries:
    1)
    Lamprophyre dykes of Karkur Talh and Wadi Wahesh
    2)
    Kimberlite breccias and -pipes in the Gilf Kebir region
    3)
    Large diamonds in the Qaret-el-Hanash structure nearby the Silica-strewnfield in the Great Sand Sea


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 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:
Andras Zboray:
"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."


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 5 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

Quartz
Magnetite,Ilmenite
 Pyroxenes
Orthoclase

Only
sandy
parts
Persistently
weak
to
very weak
No
or
very weak

Quartz grains (rounded), some larger free quartz grains (rounded)
FeOx-pigments
Some extracted minerals

 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 Carbonate  beige to
light-brown
Very hard

Carbonate

Yes

No Persistenly
very weak
No
or
very weak

Some extracted minerals
Some rounded quartz grains

Site 5
(Wadi Wahesh)
21°53'24.96"N
25° 2'53.10"E

2018-5

+ 890 Carbonate light Hard

Carbonate

No

No intensive Yes

No residue

*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 pigments. The carbonates contain high amount of diverse minerals. The carbonates may also contain insoluble dark parts like Schlieren (Site 1). The sample from Site 3 deviates. The carbonates are more soluble and contain neither minerals nor dark parts.
The character of the carbonates is still puzzling. Due to the limited solubility and the high proportion of minerals but must be concluded on their volcanic origin. These carbonates can be characterized as crystalline (compare the photo of  thin section in Marinova et al.).

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 perhaps detritus of marble from the Infra-Cambrian (see 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, which are difficult to dissolve in acid, contain many mineral components, and are embedded in ironoxid-rich sand. These carbonates can be characterized as crystalline.

It is certain that the calcareous deposits not have been organic 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.

The most interesting and probably option is the origin of the carbonates from the Earth' mantle (hypabyssal), ascended along faults. The genesis can be explained by the ascent of hot hydro-volcanic water, loaded with a mix of FeOx and fragments of porphyritic carbonates from a older dyke, containing rock-forming minerals. The degree of mixing with sand was dependent on the transport distance on the surface. After loss of water, the mixture hardened (Site 1 to Site 4).
Site 5 documents a spring from which pure calcium carbonate (without rock-forming minerals and FeOx) has ascended and thus is a mysterious special case.

The Jebel Uweinat is structurally located on an uplifted Archean Basement of the East Sahara Ghost Craton. The mechanism for the formation of calcareous deposits is therefore hydro-volcanic (hydrothermal is the wrong term here). It is classified as post-volcanic because volcanic activities are not known since the Oligocene.

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 of 7000 years (latest data).