Mysterious
concretions in Devonian layers on the Northern Plateau
of Jebel Uweinat (Sudanese part)

Norbert Brügge, Germany
Dipl.-Geol.

Update:
01.02.2020


During the trekking-tour in March 2019 to the Northern Plateau of the Jebel Uweinat, Andras Zboray and his group discovered unusual spherical siliceous concretions in exposed layers of Devonian sandstone. Location: 21°57'43.2"N / 25°00'55.9"E. The following photos were sent me by the participants Max Damberger, Gabor Merkl and Clemens Scheidhammer.

So far, the origin of these structures cannot be scientifically explained. Nor can the question be answered whether the formation is of post-sedimentary or hydro-volcanic. The known scientific explanations for a sedimentary formation of concretions are not really clarified and may can not be considered in this case.
It would therefore be of great interest first to determine the detailed mineral components of the concretions and their coating before the obviously complicated mechanical and chemical processes are involved in the formation of this structures can be understood.

Therefore, only an optical inventory can be presented at the moment. This includes the very qualified observations by Andras Zboray: The concretions appear to form around a central nodule.
A test with HCl was negative".


The location can be described as a cavant in a wall of Devonian sandstones, on the bottom of which a pack of concretions can be seen, encased by an apparently sandy, but brighter cement. This concentrically laminated coatings can include several concretions.
It can be clearly seen that there is a broken edge (pic 7) on the floor between the Devonian sandstone and the brighter material. This is an indication of the young emergence of this layer-breaking cavant, including its filling. The possible continuation of the structure in depth cannot be determined.

Of great importance for the genesis of the deposit is its location on one of several tectonic faults on the plateau. That would speak for a post-volcanic spring, as expected (Map-1)


An astonishing accumulation of weathered concretions (pic 15), far from the Northern Plateau, on a plain  between Karkur Murr and Karkur Talh, and debris from the upper branch of Karkur Talh (pic 9, 11, 12/13, 14) lead to the assumption that occur more such structures in the Jebel Uweinat.

Particularly revealing debris come from the upper branch of Karkur Talh (Map-2):

  • Cutted concretion reveal inside bright inclusions of still unknown material  & Cutted concretions to be infiltrated with iron solutions. Location 21°57'34.76"N / 25°02'9.74"E (pic 9)

  • Cutted concretions reveal a dark, still unknown material inside. Location 21°57'12.4"N / 25°01'45.2"E (pic 11)

  • Cross sections of large concretions with radial traces that formed between a small nucleus and periphery. Location: 21°57'03.4"N / 25°01'3.5"E (pic 12, 13)

  • A remain of sandy solution-flow. Location: 21°57'13.42"N / 25°01'37.8"E (pic 14)


Map-1


Map-2

Pic-15

 

         

 

1

2

3

4

5

6

7
Location on the Northern Plateau (21°57'43.2"N / 25°00'55.9"E)  
8 9   10

11

12

13 14
Edge with broken bedrock Bullets at slope near the plateau (upper Karkur Talh)  Debris at slope near the plateau (upper Karkur Talh Debris with radials near the plateau (upper Karkur Talh)  Sandy solution-flow near the plateau (upper Karkur Talh)

 



12a. Cross sections of large concretions with radial tracks
 that formed between a small nucleus and periphery

 
9a.
This cutted concretions  to be infiltrated with iron solutions
9b.This cutted concretion reveal inside bright inclusions of still unknown material


14a. A remain of  sandy solution-flow



11a. 
This cutted concretions reveal inside a dark of still unknown material


           

10a. The enlargement of the bullet surface shows puzzling details regarding structure and composition

 
 
A look at the inner structure of the find with the radial tracks reveals astonishing things:
  • The arrangement of the grains within the radial and raised tracks is ordered and sometimes looks like a pigtail (pic 12b, 13c).
  • It cannot be confirmed that the overall grains is a typical accumulation of exclusively rounded grains of sand.
  • At some places, veins are visible that have been filled with solutions.(pic 12c)
  • There are areas in which the bright colored groundmass contains dark colored inclusions. (pic 12d, 12e, 13b).
  • The dark inclusions resemble those in the post-volcanic deposit of Site 1 in Karkur Talh, in which pyroxene and orthoclase were detected.
  • An elongated nodule has a concentric filling (pic 13a)

12b

 

12c

 

12d

 

12e

 

13a

 

13b

 

13c