concretions in Devonian layers on the Northern
of Jebel Uweinat (Sudanese part)
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
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
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
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
Cutted concretions reveal a dark,
still unknown material inside. Location 21°57'12.4"N / 25°01'45.2"E (pic
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)
12a. Cross sections of large concretions
with radial tracks
that formed between a small nucleus and periphery
This cutted concretions to be infiltrated
with iron solutions
cutted concretion reveal inside bright inclusions of still unknown material
14a. A remain of sandy solution-flow
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
- 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
- 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
Site 1 in Karkur Talh, in which pyroxene and orthoclase were
- An elongated nodule has a concentric filling (pic 13a)