Smaine:

Looks like Flat-Pebble Conglomerate. You need to check the mineral composition of the pebbles and the age of the horizon yielding these specimens.Please see link here:

http://link.springer.com/chapter/10.1007%2F978-3-642-75829-4_28#page-1

Best

Syed

Thank you Sayed Abbas Jafar

This sample dont contain carbonats (negatif reaction with Hcl) it hardnes >=7.

Best

Smaine:

In order to make a realistic assessment of this Sandstone rock (since carbonate Flat-Pebble Conglomerate is ruled out), please furnish details like Age, thickness of unit and locality including mineral composition of sandy intraclasts and the underlying Sandstone (?calcareous or not).

Best

Syed 

Dear Syed Abbas Jafer

It is a sandstone, with a grainy texture, probably of Cambrian age, or Precambrian?. .

There are many Cambrian outcrops in north of the sampling area, the two figures show the sampling zone (central Hoggar). Other details I, will communicate them after the vacation.

Best

Smaine

Dear Smaine ,

If the  the rock is sandstone and if these features are confined on the bedding planes (as it appears from the Photo), then check the lateral extent of the feature.  Probably it can be intraformational conglomerate/breccia. You need to check the composition of these angular fragments and need to trace them.

 M. G. Kale

Sedimentary structures

Molds after gypsum/anhydrite

Dear Dr. Chellat,

you provided us with an overview of the bedding plane and a section cutting the bedding plane more or less perpendicular. The process leading to this mineral association, in general terms, was only operative on the bedding plane and did not infiltrate or interact with the bedrock, described as sandstone. The term intraformational related to the formation of the arenaceous bedrock cannot be applied in this case. It is a post-depositional bedding-plane parallel mineralization. The clue is a more detailed description of the mineralogy of these structures, which I cannot give an answer to. If you provide more detailed information on this issue we will get a step closer to the solution of this enigmatic structure. A mineral which comes close to the mineralization as to the outward appearance is barite or a pseudomorphosis of a sulfate. Think of desert roses or so. The mineralization did not evolve as a void-filling type but its growth was mechanically impacted.

Best regards

H.G.Dill

Dear Smaine,

This is a sort of "flat pebble conglomerate" common in the Lower Devonian from western Morocco into Algeria. These are thin beds that include large, hollow attachment structures of the crinoid Siphonocrinites. These attachment structures ("roots') appear either as large bulbous structures or are often broken up by wave activity into flat disk-like fragments and accompanying columnals from the stems. Wave imbrication stacks the fragments into an imbricated bioclastic conglomerate. These fragments are harder than such sulphates as barite. They are calcareous, but their composition is so dense that the fragments must be powdered (scratched) so that they can react actively with hydrochloric acid and produce bubbles. These are not Precambrian--Lower Paleozoic.

Best,

Ed L. 

Smaine and Ed Landing:

If the assumption of Smaine is correct that these features are coming from Precambrian-Cambrian level and the flat pebbles are actually of carbonate composition, then chances of these being thin tempestites flat-pebble conglomerate are strong. Age and mineralogy holds key to interpret the genesis of these structures.

Best

Syed

Dear gentlemen, you complete details of this sample:

After laboratory confirmation, the dark grain hardness is 7 indicating a quartz, white grain and casks have a hardness approximately equal to 6.5 I guess it is a feldspar, I tried the effect on Hcl entire rock with concentrations of 5% to 10% no reaction. Tiny grains of black mica biotite are present, the Central Hoggar pickup area is known by massive magmatic, metamorphic and meta sediments. I guess if you allow me dear gentlemen named a Graywack but the surface structure appears to have caused a reshuffle during the deposition.

Thank you very much for your contributions.

I have seen empty molds after gypsum or anhydrite that look like these features in Neogene carbonate sediments of the Dead Sea Rift. In the material shown, the molds could be filled with fine siliciclastics, but more probably the evaporite crystals were diagenetically silicified. Other sulphates or an origin similar to desert-rose type crystals are also possible. Y-shaped sections are readily derived from crystal twinning especially in sulphates, but I do not see how FPC can give Y-shaped sections. Nor do FPC fragments orient themselves normal to bedding.--CB