Groundwater ferricretes

in the Mill Cove Formation (Dunquin Group, Silurian)
Dingle Peninsula, Ireland

Somewhere between half and full size in most browsers

I have never seen anything like this before! What is it? This time I was lucky. There was a field trip leader to tell me. And if by accident I didn't hear what he said, or have forgotten - there is still the Field Trip Guidebook, which says:

"Much of the formation is made up of tabular-bedded mudrocks ... They are generally massive or poorly laminated with ... abundant post-depositional features including complex colour mottling, burrows and dessication cracks, reflecting deposition in a shallow, ephemeral lacustrine or playa-lake setting. The mottled mudrocks were interpreted as groundwater ferricretes by Wright et al. (1992)." (Williams et al. 2000).

It is no problem to spot the colour mottling (reddish iron-stained layers), bioturbation and dessication cracks (running from top right to bottom left) - but what are ferricretes? I have heard of concrete :-D, and of calcrete (see the whitish burrows), but ...

Ferricrete is a term from the pedology or rather paleopedology. "Cretes" are duricrusts, and there are in fact more sorts.

DURICRUSTS
Indurated, or hardened, crusts of various mineral compositions form as a result of weathering processes, or by the redistribution of weathering products:
ALCRETE/ALUCRETE Al₂O₃.3H₂O bauxite, gibbsite
FERRICRETE Fe₃O₃ (laterite), hematite
SILCRETE SiO₂ terrazzo
CALCRETE CaCO₃ calcite, caliche
DOLOCRETE CaMg(CO₃)₂ dolomite
GYPCRETE CaSO₄.2H₂O gypsum
SALCRETE NaCl rock salt, halite

Ferricretes normally form in the soil profile at the water-table when iron-oxides accumulate and cement together filling pore space. Transport and deposition of iron is controlled by fluctuations of the groundwater level. Iron is soluble in - and therefore transported by - reducing acid waters and is precipitated at higher Eh (above 0.4) and/or pH (above 6). (See Eh-pH diagram for iron - e.g. S238, Block3, Part 1). The mottling is reflecting the various grainsizes. The finest particles came from volcanic ashes from volcanoes to the west of the formation, and could explain the high iron content.

Here are some synonyms for ferricrete or ferruginous duricrust: hardpan, ironpan, laterite, ouklip or ngubane.

Ferricretes may form at any depth in the soil profile. Most of the ferricretes in the Mill Cove Formation are exceptional by being less than one cm thick. In fact many of the layers are just about a mm thin.

The term laterite is often substituted for ferricrete but technically it refers to a soil rich in iron oxides and aluminum In the hot humid tropics laterites form most readily between annual high and low positions of the water table. Alternating wet and dry seasons are therefor ideal for laterization.

The latitude position of the Dingle Peninsula in the Silurian was around 30 °S.

In Northern Ireland "fossil" ferruginous laterite is found on top of some of the tertiary lava flows at the Giant's Causeway.

REFERENCES

Wright, V.P., Sloan, R.J., Valero Garces, B. and Garvie, I.A.J. (1992) Groundwater ferricretes from the Silurian of Ireland and Permian of the Spanish Pyrenees. Sedimentary Geology, 77, 37-49
Williams, B, Sloan, R. and Richmond, L. (2000) Shallow marine to fluvial-aeolian interaction in tectonically active basins, Siluro-Devonian (ORS), Dingle Peninsula, SW Ireland. IAS Dublin 2000 Field Trip Guidebook, 71-112

Comments

Dr. Tapan Chakraborty
Geological Studies Unit
Indian Statistical Institute
27 June 2001:

The photograph that is shown in this page has a strong resemblance with the adhesion structures that are common in many of the Proterozoic aeolian deposits that I have worked on. I would particularly refer you to the photographs in the following papers:

1.

Chakraborty & Chaudhuri (1993) Geological Society of London Special Publication No. 72. 127-141. Please refer to fig. 6,7,8. Please notice that the sandstone unit show evidences of abundant diagenetic iron enrichment. But the minutely crinkly laminations in your photograph stongly resemble the adhesion laminae.

2.

Chakraborty & Chakraborty (2001) JSR,71, 106-115. Please compare your photograph with Fig.10 and 16a of this paper.

I reiterate that evidences of diagenetically-formed ferruginous crusts may be encountered in the deposits discussed in the above two papers but the crinkly lamination style common in both of them are definite record of adhesion structures. The crinkly lamination shown in your photograph is perhaps a similar feature that has been affected by post-depositional sedimentary deformation, organic activity/desiccation and may be associated precipitation of Fe-cement.

I shall be very happy to know if your deposit contains any evidences of aeolian activity.

Ole Nielsen
4 July 2001:

At the time I put the deposit down as "marine sediments" in my field notes. The mudrocks are tabular-bedded with typical bed thicknesses of 20-60 cm. They show local wave-ripple cross-lamination. Will be followed up later.

More comments are welcome!

 

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