Hi all,

I'm currently digging a small possibility in South Wales. It is completely full of mud and rocks but there appears to be a layer of ironstone within the fill. So I'm wondering about how this occurs and if anyone on here can offer any insights. Would it have been deposited from iron mineral-rich waters at some point when the sediments were being laid down? Or, would it have been washed in as small rocks from somewhere else?
Just curious about how such layers may originate but also if it is possible to infer anything about what the cave system may have been like before becoming blocked and filled with sediments.

TIA

Hi Mudman,

Are you to the south of the coal fields ? Hematization is widespread in fractures in limestone to the south. Mined in Garth and Llanharry. Thought to be influenced by the overlying hematite rich Triassic Keuper Marl - now mainly denudated apart from a strip adjacent to the Bristol Channel.

Robin

RobinGriffiths said:

Hi Mudman,

Are you to the south of the coal fields ? Hematization is widespread in fractures in limestone to the south. Mined in Garth and Llanharry. Thought to be influenced by the overlying hematite rich Triassic Keuper Marl - now mainly denudated apart from a strip adjacent to the Bristol Channel.

Robin

Click to expand...

Hi Robin,

Thanks for getting back.
No, this dig is on the NE edge or thereabouts.

Two possibilities spring to mind.

Firstly, the ironstone layer may have been deposited by iron rich 'ochrous' waters (like the red stained streaks you see in drainage culverts)

Alternatively, the layer may be what is sometimes referred to in surface sediments as a hardpan.  These form in sediments at the boundary between oxygenated sediments, ie. those where the pore spaces between particles contain free oxygen, and anoxic sediments, ie. reduced /no oxygen.  Haematite is precipitated at this boundary and can often look like it is part of the sedimentary sequence particularly when the layers are near horizontal or where there is large variation in particle size between layers.    These haematite layers are usually up to a couple of cm's thick but can be quite thicker particularly if the boundary fluctuates due to recharge or dewatering.  The ironstone in these layers usually looks 'sandy' as the iron crystals tend to be relatively small but they can form homogenous layers dependent on conditions before and after precipitation.  Colour is a good indicator of oxygen levels in the layers of mud, oxygenated muds usually being brown to red and the reduced muds being much greyer, and usually lying beneath the 'hardpan' (although conditions can be reversed, particularly if the lower layers are coarse grained), although if one or other of the conditions spreads throughout the whole mud sequence this can change.

Given that iron is a relatively common mineral the waters do not necessarily have to be sourced from iron rich areas such as the Triassic as mentioned above but could easily be derived from the Namurian gritstones and shales or even from the formation of peat on the surface.

May thanks Robin and Paz for taking the time to reply, you have both given me something to think about.

Paz, what you said was intersting as I had assumed that deposition would occur due to ponding of water, not within the sediments themselves. If I understand correctly, what you say is that the ironstone nodules will be created within the sediments, not on top of them and then be covered by later sedimetation.

Unfotunately, I don't think we shall be progressing this dig. The dig is located in a quarry and there is a large amount of fracturing of the rock both in the walls and above the mud fill. The roof appears to be supported by the mud fill and removal of this would in all likelyhood result in many tons of rock falling down onto the diggers. It's a shame really as I'm pretty certain that there is quite a bit of potential with this dig.

More than likely though, we'll leave it a couple of months and then go back for another look.