New granitic ring-dyke intrusion was found in the Jebel Uweinat (Sudan)


Norbert Brügge, Germany
Dipl. - Geol.
 

Update: 23.04.2013


In the Jebel Uweinat - on the territory of the Sudan - can be found a new remarkable crater.
Contrary to previous assumptions, this is not an explosive crater structure and not younger than the others in the Gilf Kebir and Jebel Uweinat region known crater-like structures that have emerged in the Oligocene. (white spot, Clayton craters, Gilf Kebir crater field, etc.)

The circular crater has a depth of up to 100 m and a diameter of approximately 600 m. The southern rim of the crater is down eroded. The geographical position of the crater's centre is 21�53'13"N / 25�10'14"E.

Andras Zboray, Hungary told me: "Mark Borda has been into that crater last November (2007), he showed me photos of the rock at the centre bottom which is unmistakably granite. I had the same thoughts as you, but in light of this new information we all have to thing of something else. I plan to visit it on one of the forthcoming trips."

Now Andras Zboray has realized his plan in April 2012. The results of the exploration you can see below.
 

    

         

 
Andras Zboray has realized his plan to explore the crater in April 2012 along with a group of like-minded. It is now document that the crater is a granitic intrusion. The special is a towering circular ring-dyke at the outer edge of the intrusion.

Now Andras Zboray told me that the ring-dyke of the crater consist of two distinct igneous rocks: A hard rock lies on top of the dyke, and a soft tuff-like material is below in the dyke. The tuff-like deposits also characterize the eastern foothills of the crater within several 100 m.

On Andras Zboray's website the crater and the volcanic embossed foreland with multiple photos are well documented. Gabor Merkl, participant of the field trip, has sent me eight further photos.
 

Typical "Woolsack weathering" of granite on bottom of the crater
 

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Northeastern foothills of ring-dyke

 

Outer flank of the ring-dyke

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Outer flank of the ring-dyke (2)

 

Crater inside with eroded southern dyke-rim (3)

 

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Crater inside

 

Fallen down rhyolite (?)

 

 Conception

I assume that the ring-dyke is slightly younger than the granite intrusion in the center.
The ring-dyke consist of two different varieties of eruptiva. On the rim rests a rhyolite (granite equivalent) and below in the dyke a trachyte (syenite equivalent). The rhyolite intrusion is younger and went up through the trachyte (or ash) layer. Tuff or ash is it probably not, even if it looks like stratification. Would it eruptive ash, then should they be widely distributed in all directions, also on the bottom of the crater itself. Inside the crater is missing ash or tuff.
In addition, the southwestern part of the ring-dyke is embedded in older sediments above the basement unit.
The trachyte eruptiva flowed several hundred meters in the northeastern direction. But a detailed investigation is necessary to clarify whether it actually trachyte, or possibly tuff is.

Photos: Andras Zboray and Gabor Merkl