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TXV24

Hi, 

 

Bit of a geological question here, I recently took this photo of some of the Upper Hamstead Member strata exposed on a headland at Bouldnor Cliff whilst out collecting. I really like this spot as the colour variation in the beds is really interesting. I've heard that the colour mottling in mudstones such as these can be indicative of the paleo-environmental conditions they were deposited in. Generally speaking these muds were deposited in ponds, lakes, and sluggish waterways on a low lying coastal plain. However, would it be correct to presume the redder areas indicate more arid conditions i.e. a period when the Hampshire Basin coastal plain was very dry and the other green and grey beds periods in which the environment on the plain was wetter? 

 

Thank you, 

 

Theo 

 

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Kane

A very good question. Judging by the picture, it does appear that there were some alternating cycles. Perhaps, in posing a follow-up question that might reveal whether the red or green bands are dry or wet periods, have you noticed a difference in faunal assemblage between the two?

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WhodamanHD

I would guess high levels of iron oxide typically would mean slower moving bodies of water like lakes or streams, giving the iron time to settle into the banks or the bottom.

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ynot

I always heard that red beds indicate terrestrial deposit and gray/black beds indicate marine.

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TXV24

@Phevo @WhodamanHD That would definitely explain it as there are occasional iron stone bands which must have formed through similar environmental conditions. 

 

@Kane I've never been able to recover any extensive vertebrates from the 'red bed' other than small seams of fish bones, whereas the grey/green clays above have produced large mammals (namely anthracotheres). 

 

 

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jpc

red also indicates more oxidation.  Whether that means a drier environment, I leave that to the geologists in the crowd.  

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EMP

I was actually just thinking about this! I saw some nice looking bands in an exposure of the Potomac Group deposits (early Cretaceous) with the same kind of red-gray-red coloring. 

 

The red coloring comes from iron (iii) oxide (usually Fe2O3 or hematite). The iron is often naturally present in the environment, and what happens is that it becomes deposited in the layers much like other sediment types. Over the years, this iron can become exposed to oxygen (usually through air contact) at which point it reacts and forms the oxide. Red coloring is often indicative of terrestrial deposits, as others have said. 

 

The gray layers is likely due to a higher standing of the water, or at least a more anoxic environment at the time of deposition. This could be that sea levels had risen to temporarily cover a low-lying, near sea location or that it became covered by slow moving bodies of water, like a pond or sluggish stream. 

 

It's important to also note, however, that freshly exposed, iron rich formations will also undergo oxidation and turn a reddish color (as is common with the local Mahantango and Rose Hill Formations). Usually it's possible to differentiate rocks that were oxidized around deposition from those oxidized more recently by the fact that if you split the later type open the red coloring won't continue all the through, whereas it would with the former. 

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TXV24

@EMP Thank you for such a detailed response, it's interesting you mention the exposure to air as a lot of horizons (some which aren't iron oxide rich) contain mammal bones with rodent gnawing suggesting that many of the muds in the Hamstead Member this were at least periodically exposed, so the right conditions for it to be oxidised were definitely present. 

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