Dear Aitalokhai,

It is not easy to see on these photographs, but I think, theses deposits could be related to volcanism. The last picture looks like a lapilli tuff, the first picture seems to show convolut bedding. The stratigraphic situaton (underlaying, water-saturated sandstones and overlaying, impermeable claystones) would fit the scenario of convolut bedding as the result of spontantenous dewatering of the sandstones during an (volcanic driven?) earthquake. But the deposits are obviously intensely weathered and coated by Fe oxides, making it hard to visually analyze the deposits. Hope this helps!

All the best,

Elmar

Thanks Elmer. I share the same thoughts with you on soft sediment deformation. The possibility of the sediment being from volcanic origin will be an interesting find since there is no evidence of volvanism in the Late Cretaceous in this part of the Basin in Literature. I will keep an Open mind. Thanks

Looks like the onset of spheroidal weathering to me, if you mean the hammer-scale rounded shapes.

I recommend to make thin sections of the rounded clasts and the cement. Otherwise any conclusion is suspicious.

Need the  general contexte of the outcroup? magmatic? metamorphic or sediment rock?

it may be fault brecciated area?

Yello  and red colors indicat that you hav  the  iron oxyd?

it possible to be a sedimented bloc,   that is very différent lithologically  from the hosted sediment, and after it will be altered différently,

So the general context and the nature of sediment are necessary to interpret that?

Goog Luck?

Dear, I Think this is a deformation produced by the collapse when the organic matter is devolatiliced.

So, as Irina has already suggested,

any determination of the rock only from the photos is untrustworthy without a thin section and/or polished section.

First photo looks like a mixed mass of oxidised zone (developed along a  primary  sulphidisedreplacement zone) and very altered  characteristic pillow-shaped structures, overlain by gentle schistose rocks (thrust sheet)

In second photo, especially in the left  and also below  the hammer seems  a couple of very altered  and deformed characteristic pillow-shaped structures.

As  Irina suggested thin section could be a solution,but the rocks are highly altered.

As a carachteristic of pillow lavas (extrusion of the lava under water)  is having a glassy rim and going  from the rim to its  core the size  of grains increases.

Hit with hammer a piece of yellow mass if it  smells like sulphur.

As previously stated by others is difficult to give a definitive statement only by seeing the photos, nevertheless I think these are not sedimentary deformation structures, but weathering ones developed on volcanic or vulcanosedimetary materials.

Hi Aitalokhai,

you say that "the interval is underlain by poorly sorted cross-bedded sandstones and overlain by black mudrocks." What does the interval itself consist of?

As Irina & Günther have already suggested, make a thin section or a polished rock slab as a start. Studying that should definitively help.

Further questions that may be of use: Are the underlying X-bedded ssts marine or fluvial? Is the overlying mudrock marine? What is the dark infill material in the round (tubular?) structures on your 1st photo? The overlying mudrock or something else? How does the upper & lower surface of the interval in question look like? Sculpted (e.g. loading, as was suggested by Elmar)? Bored? sharp or gradual transition? Judging from only the photo, the rocks on your 3rd photo (the interval? pen cap for scale?) could also be an oolithic grainstone. Did you check if it's a carbonate? Again, a thin section would help!

Last but not least, I have seen palaeosoils that look similar to the yellow interval, so you may want to look closely if you find pedogonic structures or not in order to prove or disprove...

Interesting!

Hope that helps,     Niels

Assuming that this outcrop is somewhere in the Gulf of Benin, Campano-Maastrichian sediments were deposited at an early stage of Atlantic rifting. There may have been some volcanic input in the vicinity, accompanied by crustal tilting. The spheroidal structures look like concretions caused by predominantly iron mineral migration, during and after some soft sediment mobilization.

I agree with the interpretation of a volcano-sedimentary environment. The spheroid structures in the second photo closely resemble altered pillow lavas found in Proterozoic sequences near São Paulo. But this is as opinion from a photo... more studies are necessary.

The last photo clearly shows oolitic texture with iron mineral (possibly siderite) surrounds.

The second photo shows the overlying shale;  a  +/-20 cm, grey unit which is partially concretionary; below this is a darker brown nodular (of pillow-like unit).  Is this volcanic or calcareous?  You did not say. Is the grey unit calcareous?

The first photo looks like nodular siderite with overlying nodular limestone.  At the very top it looks like shale.

So, depending on your observations I asked above:

Photo 1) is nodular oolitic iron stone overlain by nodular limestone and shale,

Photo 2) we need some more information from you

Photo 3) oolitic ironstone probably composed of siderite

Here is a link to a model which looks similar to your sequence (see section on Page 40)

 https://pubs.usgs.gov/bul/b2004/model34f.pdf

Here are some photos of oolitic ironstone:

http://www.open.ac.uk/earth-research/tindle/AGT/AGT_Home_2010/BGS_IronstoneB.html

Here is a reference to Origin of oolitic ironstones:

http://archives.datapages.com/data/sepm/journals/v47-50/data/049/049004/1351.htm?doi=10.1306%2F212F7928-2B24-11D7-8648000102C1865D

If you have any more info on observations please provide us.

I agree with the interpretation of a volcano-sedimentary environment. The spheroid structures in the second photo closely resemble altered pillow lavas found in Proterozoic sequences near São Paulo. But this is as opinion from a photo... more studies are necessary.

The last photo clearly shows oolitic texture with iron mineral (possibly siderite) surrounds.

The second photo shows the overlying shale;  a  +/-20 cm, grey unit which is partially concretionary; below this is a darker brown nodular (of pillow-like unit).  Is this volcanic or calcareous?  You did not say. Is the grey unit calcareous?

The first photo looks like nodular siderite with overlying nodular limestone.  At the very top it looks like shale

Photo 3 oolitic ironstone probably composed of siderite - it is siderite.

I agree with the interpretation of a volcano-sedimentary environment. Maybe an interbedded slumping of calcareous mud or cineryte?

If the rounded structures with concentric layers are not carbonate and are in a volcanic deposit, you might want to consider the possibility that they are accretionary lapilli.

I refer you to other Nigerian geologists' paper on oolitic ironstones in the Campano-Maastrichtian in Leru, southeastern Nigeria :

http://www.ajol.info/index.php/gjgs/article/view/18755

Additionally a link to bibliography which has on Page 8:

Arua, I 1988 in Journal of African Earth Sciences, 1988 describing oolitic ironstones in the Campano-Maastrichtian

 http://nora.nerc.ac.uk/507498/1/agla000090.pdf

Additionally, in this link below at the bottom of Page 975 oolitic ironstone is described in the Nkporo Section at Leru.  

http://nora.nerc.ac.uk/507498/1/agla000090.pdf

And, Figure 12 shows a section of 'your' Mamu Formation, showing no volcanic lithologies.  Where do your outcrop photos fit in this section??

Hi,

I  prefer an interpretion of regolith whatever the former rocks are!  This strongly marks the sequence boundary if it occurs within marine sediments! In other words, it implies a long (over 1 Ma) exposion under air , which caused by great fall of sea-level. 

Thanks all. I have attached photos of  a thin section of this rock.

Robin Tolbert:

The Leru section is in the eastern flank of the basin  which is thicker and is underlain unconformably by sediments of the Abakaliki Basin.

The Western Flank (My study area) is thought to represent a condensed section of the Anambra Basin. Its  Lithostratigraphy is very similar to that of the Southern Bida Basin. The Mamu Formation here is underlain by Pebbly to medium grained arkosic sandstones Lokoja - Basange Formation which is believed to be the time equivalent of the marine Nkporo Shales . The Lokoja - Basange Formation uncomformably overlies Precambrian Basement rocks in the western flank. Unlike the eastern flank, Sediments of the Abakaliki Basin are absent here.

the sandstones below this interval are interpreted as fluvial sandstones. while the grey shales above the interval are not calcereous but carbonaceous. This rock did not effervesce when I did the acid test.  

You need to give more information.  What is the scale?  What minerals are present and/or chemical compositional data.

My first impression is that the first photo shows weathered diagenetic concretions, the second photo possible onion-skin weathering developing from joints, and the third photo shows near spherical structures of a totally different origin. They look a bit like oncolites (algal growths), but that should be in a limestone. They also resemble zoned airfall lapilli. I have no confidence in my answers.

Hi  Aitalokhai 

I Agree with Elmar's answer. Although the outcrop and the pictures are not very clear it looks like convolute bedding produced by fluidization of an unconsolidated sediment during an earthquake. As Elmar mentioned the lithological sequence is favourable to develop this type of structures. These, together with sand dikes, are common features in the fluviatile-shallow marine sequences of Aptian-Albian age of the Lusitanian Basin in Portugal, reflecting the tectonic seismicity related to the opening of the North Atlantic.

Do you have any paleontological information about taxas in the  mudrocks?

Dewathering could develop that kind of convolute bedding;   and you must be sure about the correct facies,   iningbritic or normal sedimentite?

The mineralogy as Mr.  Madeira ask is also needed.

The first photo is unreadable. The second photo looks like a conglomerate. The third photo is very similar to amygdaloidal basalt.

Heavily weathered volcanics, probably basaltic, and in the third picture I agree that this is lapilli tuff.  I agree with Elmar and Leonid.

Ciao

John

the given images depict highly weathered, ferrugenised and deformed sedimentary units obliterating the original texture. please checkout for syn-sedimentary volcanic event in the succession...which seems probable reason for some volcanic like features.

The thin section can explain well

In the light of new data from better quality thin sections, XRD and XRFS data, I wish to say that these are oolitic ironstones with particle size and Fe content increasing upwards. I am yet to classsify these Iron stones. Preliminary review of the data so far obtained suggest close similarity with what has been observed in the Leru section (eastern section of the Anambra basin) and similar aged Agbaja Ironstone (Bida basin)

Thanks everyone for your help

:>) yes, indeed!!

Thanks Robin...

Edegbai,

Permit me to respond to this 2 year old question. Judging from the hammer as scale,the ooliths in the iron stone are even larger than pisolite. The thin section is of the groundmass not the cobbles and boulders (near the hammer and jug).

But of the large pebble/cobble or boulder sized clasts, I think of mud cobble conglomerate that underwent spheroidal weathering to give the circular/ ring structures. Monocraterion yoyo burrows (crab traces) of tidal regime also have similar ring structures and can be as large.

All the best.

Obianuju P. Umeji

Totally lost here. Just learning

Thanks Prof. Umeji

the large structures shown in the field pictures are ferrugenous concretional sandstone. That interval some how marks an ancient subaerial surface and/or transgressive surface that was later subaerially exposed

You should add the location please. Because understanding the Geology of the Area would help significantly.

Edegbai,

Oolitic ironstones, whether in Bida Basin, the eastern section or western part of Anambra Basin were well developed during the Campanian-Maastrichtian. These show the depositional history of the Basin, that the Nkporo Sea shallowed to neritic depths; and before the deposition of the Coal Measures Group, oolitic ironstones of economic value developed. They have been much weathered to the colours you have. The concentric structures are simply those of ooliths or pisoliths depending on size; but your work has confirmed what is on the ground.

Congrats and Good research.

Obianuju P. Umeji