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

Norbert Brügge, Germany
Dipl.-Geol.

Update: 23.03.2020

During the trekking-tour in March 2019 to the Northern Plateau of the Jebel Uweinat, Andras Zboray and his group discovered unusual spherical concretions in exposed layers of Paleozoic 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.

There is a striking resemblance to special "cannonballs" in sandstones from Utah (eg Molen Reef, Emery county). The formation of these structures is declared to be epigenetic. Despite some theories, the mechanism for the emergence of such concretions is not really clarified.
It would therefore be of great interest 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 dilute warm HCl was negative."

The location can be described as a cavant in the Paleozoic sandstone, on the inner wall and bottom of which a pack of concretions can be seen, encased by an apparently pure (?) sandy, laminated fine cement. These laminated coatings in the pool can include several of concretions.

It can be clearly seen that there is a broken edge (pic 7) between the Paleozoic sandstone and the brighter (?) material in the pool. This can be an indication of the younger formation of this layer-breaking deposit, including its filling. The possible continuation of the structure in depth cannot be determined.

Of importance for the genesis of the deposit is its place on one of several tectonic faults on the plateau. That could speak for post-volcanic influencing (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 places with concretions in the Jebel Uweinat.

Due to the appearance, it could be assume a variable age of the cocretions in the entire Uweinat. Some may have been formed in the Tertiary (tectonic movements in the area). The pool on the plateau could be younger (maybe late Quaternary, and post-volcanic influenced ?)

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 a mysterious 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 groundmass contains darker colored parts. (pic 12d, 12e, 13b).
  • An elongated nodule has a concentric filling (pic 13a)

12b

 

12c

 

12d

 

12e

 

 13a

 

13b

 

 13c