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

Norbert Brügge, Germany
Dipl.-Geol.

Update: 15.01.2018


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. The lack of significant rounding of the grains suggests short transport distances.
The morphology and composition at Site 2 are similar.

"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 through which volcanic water laden with dissolved iron has risen. 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 a metamorphic carbonate silicate from the UM formation (Ultramafic-Mafic and Calc-Silicate Rocks) of the Precambrian basement. This metamorphic carbonate silicate does not react with formic acid. It contains pyroxene, orthoclase and magnetite as well as unrounded quartz. The proportion of these dark carbonate silicates were 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 edge of the uplifted Hassanein Plateau. In the Oligocene, along with the intrusion of granite probably parts of the basement were involved. Springs could have been in a disturbed zone at the edge of the plateau (right).

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

• Predominantly sandy deposits
• Fragments of metamorphic Calc-Silicates of the basement with high proportion of unrounded quartz, pyroxene, orthoclase and magnetite
• Very low organic presence in the deposits (only contaminants)
• No plant remnants and pollen or algae pillows as well as bacterial structures (stromatolites).
• Structure is not comparable to stromatolites in Canadian Pavilion Lake. The seemingly textured surface is the result of erosion (different hardness of the deposits internally).


The age determination does not necessarily document the actual age of the found sandy sinter-like deposits. The organic substance required for the C14 analysis is less than 0.3% and therefore should only be considered as a contaminant. Organically produced carbonates should therefore not play a role in Karkur Talh.
The pieces from the basement can not contain any organic substance. These (metamorphic) 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 calcareous organisms.
Moreover, we must assume that the springs were active only relatively short time, because the deposits were not thick. So no organic development could occur during sedimentation.
Between the two deposits in Karkur Talh a difference of approximately 1400 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 contaminants 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 2-3 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. Dark and sand-free hard parts of undoubtedly non-sedimented material 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 momentary weak reaction.
Recently, 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 much younger age than previously published.
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 have been intruded as contaminants in the porous deposits during a longer time span by rainfalls. The question now is, what is the true age of the sinter-like deposits? They could be much older than the C14 datings suggest.

 

New:
In the Wadi Wahesh and on the Hassanein Plateau, there are more, not yet investigated areas with such sinter-like deposits that have now been researched and sampled in November 2017 by Andras Zboray (Site 3 to Site 5).

 

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).


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

Characteristics

Test in formic acid

Residue

Color Consistency Matrix Minerals*

CO2

Carbonate matrix

Decay

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

 2016 + 715 light-brown sintered
chunks,
hardened

 sandy
in FeOx.
+ dark calc-silicate inclusions
--------------------------->

Calcium sulphate, Magnetite,
Ilmenite Pyroxenes, Orthoclase

shortly
weak
No Yes
(very slow)

Quartz grains (rounded), FeOx. coated, same larger free quartz grains (rounded)
+ dark calc-silicate inclusions

 2018-1/A
 2018-1/B persistently
weak
No

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

 2018-2 + 778 dark-brown  sintered
chunks,
crumbly

 sandy
in FeOx.

No

shortly
weak
No Yes
(quick)

FeOx.-coated small rounded quartz grains, some larger free quartz grains (rounded)
no calc-silikate inclusions

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

 2018-3 + 1338  beige to
light-brown
 hart
but crumbly

 sandy

No

persistently
moderate
Yes Yes

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

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

 2018-4 + 925  beige to
light-brown
very hard

bright
calc-silicate ?

Yes
like site 1

shortly
very weak
Yes ?
(not resolvable)
No

No
(matrix contains some rounded quartz grains)

*Marinova et al. 2014

 

 

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)


On January 10, 2018, I examined a sample of this deposit with my modest possibilities.Visually, the deposit resembles a previous lime 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.
My prognosis
: 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 (sludge) of marble from the Proterozoic basement (Jebel Kamil) or the Infra-Cambrian (Jabal Arkenu), that have been solved by aggressive volcanic water. In context, the paintings are much younger than the deposit.