I have spent a great deal of time on this blog celebrating stratigraphic correlations and stressing their importance in understanding what happened during the Quaternary in West Wales and in SW England in particular. These are just a few of my posts:
https://brian-mountainman.blogspot.com/2015/12/quaternary-stratigraphy-in-pembrokeshire.html
https://brian-mountainman.blogspot.com/2014/01/the-joy-of-stratigraphy.html
https://brian-mountainman.blogspot.com/2017/02/the-west-angle-enigma.html
https://brian-mountainman.blogspot.com/2016/09/rhosyfelin-in-its-regional.html
https://brian-mountainman.blogspot.com/2016/09/devensian-stratigraphy-of-south-west.html
https://brian-mountainman.blogspot.com/2018/10/abermawr-new-quaternary-stratigraphy.html
Without stratigraphic matching and comparisons, we would not be able to work out common histories of events and environmental conditions -- but there are limitations and even dangers that we should be aware of. There are different sorts of stratigraphies -- and we have already, on this blog, bemoaned the fact that our understanding of the Quaternary in Wales has probably been set back by decades by this weird thing called "aminostratigraphy" -- involving the correletion of layers of sediment in different locations according to the "amino acid ages" given to contained in shell fragments. When you do that sort of thing with an immature dating technique, you are asking for trouble --and in Wales we have had trouble in bucket-loads........
When we talk of "lithostratigraphy" we are on safer ground -- after all, this is the basis of most solid rock correlation and dating, although fossil content, geomagnetic signatures and degree of metamorphism are among the other factors used by geologists in sorting out (for example) the Permian from the Pre-Cambrian. Here is the Wikipedia definition:
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of the gross physical character of a rock. It is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.
In Pembrokeshire the lithology of a rock if often a simple indicator of where it lies in the sequence. Even the least experienced geologist can differentiate between the red sandy rocks of the Old Red Sandstone, the fossiliferous grey limestones of the Carboniferous Limestone, and the black shiny anthracites of the Coal Measures. Most can see the differences between igneous and sedimentary rocks, although metamorphics can be more difficult. But when it comes to shales and mudstones, most of us would have difficulty in differentiating, in hand specimens, between Ordovician samples from Ceibwr and Millstone Grit samples from near Haverfordwest.
When it comes to the Quaternary, things are also very difficult, since different events or climatic / environmental conditions will give rise to deposits with similar lithologies or appearances but with widely differing ages (an old stratified slope deposit might well look very similar to a fresh one). So it might be difficult to sort out what is what unless there are very clear stratigraphic sequences on display which can be used for lateral correlations. The Devensian Irish Sea till, for example, always occurs in the same stratigraphic situation (where we can see it) but it differs markedly in its physical characteristics, from one place to another, depending on what the "parent material" might have been, and depending on the glaciological conditions that obtained at the time of deposition. In cases like this my own instinct is to minimise the number of lithostratigraphic labels I use, since it would not help anybody if I was to refer to the Abermawr till, the Druidston till, the Newport till and the Gwbert till (and many others) by different names. When I was writing my doctorate thesis in 1965, I tried always to refer to the Irish Sea till, but I did qualify that by referring to two sub-categories -- the coastal facies (made mostly of marine muds containing sea shells) and the land facies (made largely of locally derived rock fragments in a sandy and silty matrix).
So to the bigger problems, and to the reason for my use of the word "curse."
Unfortunately there are some geomorphologists who have made a specialism out of making lithostratigraphic correlations and publishing an endless stream of "lithostratigraphic regional and national correlations" in which the correlations keep on changing. Confusion reigns, in a certain section of the specialist literature. Prof DQ Bowen has probably published more than anybody else in this field, and some of the terms he has applied to some of the deposits have, as far as I know, never been used by anybody else. However, in some cases, because he is the "specialist", others have deferred to him and his work, without first checking carefully on reliability. So we have the use of these terms: formations, members and beds. That's all very fine, and conforms to geological good practice, but it gets a bit mystifying as far as Joe Public is concerned. In SW Wales we find things labelled as belonging to the St Asaph Formation and the Elenid Formation (correlated with Oxygen Isotope stage 2) and then lower in the sequence the Penfro Formation (correlated with OIS 16).
I have referred to the Penfro Formation before, and have expressed strong concerns about its reliability as a label:
If we look at the definitive entry for this "ancient till" in the BGS lexicon, under the Albion Glacigenic Group as the "parent unit", we find that the type localities are West Angle Bay and Llandre Quarry in Pembrokeshire. That is an extraordinary error on the part of the geologists, since there is NO ancient till exposed at either site, and the sands and gravels at West Angle are demonstrably Devensian -- and therefore have nothing to do with the Albion Glacigenic Group.
The critical central part of the West Angle exposure. Here we see the erosional contact between the older deposits (partly interglacial) to the south and the younger (partly glacial) deposits to the north. Dixon, Bowen and others might not have seen this contact clearly, and so they assumed (erroneously) that some glacial deposits are older than the grey silt and clay series.
There are admittedly some glaciofluvial gravels at Llandre that might be old, but neither DQB nor anybody else has ever published a full description of them, and there is nothing at the site to tie the gravels into a regional sequence.
Type localities need to be stable, accessible and clearly tied into a regional stratigraphy. Llandre is useless as a type locality for anything; if West Angle is used as a type locality for anything, it should be for the Devensian or the Ipswichian, and most definitely not for the Penfro Formation or the Anglian glaciation.
Then we come to another problem of lithostratigraphy. Once somebody has studied a region in detail and published what might be deemed to be "definitive" studies of the sedimentary sequence, with "formation" or "member" labels attached, and type localities identified, there is a tendency for those labels to be over-used or applied in a non-critical fashion. I became very much aware of this during my short visit to the Isles of Scilly a couple of years ago, as I looked at innumerable coastal sections on all of the main islands. In a QN article I described and interpreted the deposits as best I could, given the limitations of time and resources:
https://www.researchgate.net/publication/328413421_Evidence_for_extensive_ice_cover_on_the_Isles_of_Scilly/figures?lo=1
Right from the start, I had problems in fitting the sequence and the characteristics of the deposits into the stratigraphic sequence devised by James Scourse in 1991 and subscribed to (with minor modifications) by others since then. There were two problems, one relating to the choice of type localities, and the other to the choice of stratigraphic labels. I had problems with the "Watermill Sand and Gravel" on the basis that Watermill Cove is not a very good type locality and that the raised beach sediments are neither sandy nor gravelly. I had problems with the "Scilly Till" type locality at Bread and Cheese Cove since the till there is associated with glacigenic structures which are not all that widespread and since other till exposures elsewhere are perhaps more typical of the northern fringes of the islands. The "Porthloo Breccia" I can live with, in that it is always in the same stratigraphic position and since Porthloo is a good type locality. I had problems with the "Tregarthen Gravel" and the "Hell Bay Gravel" labels since they did not seem to have consistent characteristics and since some exposures were not at all gravelly. The labels seemed to me to be surplus to requirements. I was not too sure about the "Bread and Cheese Breccia" either, since in places it had exactly the same characteristics as the Porthloo Breccia. And neither was I sure about the "Old Man Sandloess" since it too had rather variable characteristics from one exposure to another.
Clay-rich till exposed at Chad Girt, in a rocky gully on White Island, Isles of Scilly. Should this be called "Scilly Till", or something else.....?
In my QN paper I used these Scourse terms sparingly, just where I thought they might help in understanding the stratigraphic sequence -- but I have been heavily criticised for occasionally placing the deposits I observed into the "wrong" category in the "accepted lithostratigraphic sequence" !! Hmmm. As far as I am concerned, I have not accepted anybody's sequence, and will not accept any sequence unless it seems to me to be either accurate or useful. In ice wastage environments depositional circumstances -- at exactly the same moment in time -- from place to place can vary enormously, and that needs to be recognized. It is a singularly fruitless exercise to become worked up about the application -- or non-application -- of predetermined labels when the only questions worth asking are these: What happened? And when?
Hence my scepticism about lithostratigraphic labelling. There are bad labels, and bad type localities. If we become obsessed by labels and type localities, we can easily be dragged into a scenario in which we "force" the sediments we find into predetermined boxes -- and in the process lose scientific objectivity. If labels are worth using, they will survive. If some of them are causing more trouble than they are worth, they should be dumped.
No comments:
Post a Comment