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Thursday, 8 November 2018

On the significance of cementation

The locations of cemented till exposures in Foxhole Cove, Gower.  Are they Later Devensian in age, or much older?  (Photo:  Prof Danny McCarroll /  QRA)

Occasionally on this blog I have mulled over the significance of cemented or concreted deposits -- which might also be referred to as duricrusts, hardpans, duripans or "indurated layers".  In Pleistocene deposits they involve the solution and mobilisation of minerals in groundwater and then their later precipitation because of oxidation, supersaturation or some change in chemical or physical circumstances -- including evaporation, temperature change or infiltration.  In the UK there are two main types of cement: (a) calcite, association with the mobilisation of calcium carbonate in limestone, chalk and related rocks; and (b) iron oxide and manganese oxide cement, sometimes associated with waterlogging and soil formation processes.  In the former case, the cement is normally coloured grey or white, and in the latter the colours range from orange to foxy red through to black.

Here are four of my earlier posts:

On the cemented water-lain deposits near Ceibwr and Witches Cauldron:

On the cemented till and brecciated slope deposits at Black Mixen, Lydstep:

On the cemented raised beach and sandrock at Broad Haven South:

On the cemented till at Watch-house Bay and Foxhole Cove, Gower:

My interest in this issue was triggered by the fact that the editors and contributors to the QRA Gower Field Guide in 2015 attach no significance to the fact that some of the tills described in coastal sections around the Gower coast are cemented, and others are not.  We all know that hard-pan formation can occur in quite recent deposits in what appears to be more or less random circumstances;  I have described small-scale iron and manganese oxide concretions and staining in many sediment sequences, including the Pleistocene sediment sequences at Rhosyfelin, Abermawr,  Fopston, West Angle, Llangolman and elsewhere.   Sometimes these hardened and stained layers appear to be related to old water-table positions.  But the solid cementation of varied deposits several metres thick is another matter entirely, and almost always when such cemented deposits are described in the literature there is an assumption that the cemented deposits are OLD whereas the overlying uncemented deposits are YOUNG.  That is, of course, perfectly sensible........

In many locations across SW Britain the raised beach assumed to date from the Last Interglacial is solidly cemented:

Cemented raised beach at Carn Morval, Isles of Scilly

Cemented raised beach (Gower)

Cemented raised beach (Torquay)

Cemented raised beach, Broad Haven South (Pembs)

Associated with the assumed Ipswichian raised beaches (which are admittedly not always calcite-cemented!)  there are cemented head or slope breccia deposits, sandrock and some deposits that seem to be of fluvial origin. The specialist in this area is Ian West of Southampton University:

West, I.M. 1970. Carbonate Cementation of Some Raised Beaches and Blown Sands of Great Britain. Unpublished M.Sc. Thesis, Liverpool University, 257pp.

The sandrock deposits assumed to be linked in age and environmental contexts with the raised beaches have been heavily studied. One of the more interesting recent studies is this one:

Facies analysis and diagenesis of late Pleistocene shoreline sands, Saunton, North Devon
Sophie Young
The Plymouth Student Scientist, 2012, 5, (2), 486-543

Stratified sandrock (now thought to be Ipswichian beach sands, for the most part) above the famous pink erratic at Saunton sands, Devon.

A photo from 1962 of the sandrock sequence at Bloody Basin, Saunton.  A classic unconformity!

There are abundant exposures of cemented limestone breccia (lower head) in Pembrokeshire, for example on Caldey Island and at Broad Haven South and Lydstep.  At the latter location, as described on this blog, the concreted limestone breccia (which has come from rockfalls and scree accumulations) is several metres thick, and covers an extensive area at Black Mixen, even roofing over a cave.  This material cannot be stratigraphically linked with either the raised beach platform or the cemented raised beach.  But it does overlie a cemented till:

Limestone bedrock, cemented till and overlying cemented limestone breccia at Black Mixen, Lydstep

One of the first things we learn in geology is that TIME is probably the most important factor in determining which sediments are consolidated and transformed into solid rock, and which ones are not. So it has to be true that the cementation processes referred to in my first paragraph need time, and that the most solidly cemented deposits are most likely (unless we can find other explanations) to be the oldest.

We know that the Lydstep cemented till is pre-Devensian, because it is capped by thick cemented brecciated deposits, and because very close by we can see a Devensian till in a Carboniferous Limestone context which is completely fresh and uncemented.  In this case there is no other explanation that withstands scrutiny.   But what about the cemented tills on Gower?  And what about the cemented deposits at Ceibwr and Witches Cauldron?  The jury is still out........ and we need cosmogenic dating.

I have done a lot of digging around to see what there is in the literature about RATES OF CEMENTATION -- but there seems to be remarkably little research on this topic, from anywhere in the world.   Maybe I have missed crucial articles -- in which case I will appreciate comments from others who are in the know!

It also occurs to me that cave science may have some valuable pointers to what happens out in the fresh air.  There are many places in caves in the limestone districts of the UK where roof falls and collapses have dumped brecciated limestone fragments in large caves and where cementation has then occurred as time has passed.  Are there any good examples which have dates attached?

Weren't the lads who were rescued from that cave in Thailand sitting on a pile of rubble from an old roof collapse?

All advice gratefully  received........

Cave collapse breccia -- apparently uncemented and therefore young?



It happens all the time -- one does a post and then discovers -- too late -- all sorts of interesting material in the literature.  In this case, two articles about the conditions in which calcite cement becomes abundant in the environment, leading to the cementation of previously unconsolidated sediments.  Both of the articles cited below make the point that sea-water conditions are important on west-facing coasts, leading to variable concentrations of minerals and in turn influencing the frequency of calcium carbonate trapped in marine organisms including sea shells. There is some relationship with the sea floor geology.  But to put it crudely, the higher the concentration of shelly material in beach sand and offshore sediments, the greater the likelihood of the cementation of sediments in the neighbourhood.  That all makes good sense.  But it does not mean that the importance of the "time factor" is reduced.  After all, on a coastline such as that of Pembrokeshire or Cornwall, conditions now will be broadly similar to the conditions that obtained during previous interglacials -- and the only significant variable (apart, maybe, from temperature and salinity oscillations) will be the position of the shoreline as glacial / interglacial shifts have occurred.   It is still the case, after perusing these articles, that the most solidly cemented sediments an any given locality will be the oldest.

Howie, F.M.P. and Ealey, P.J. 2010. An appraisal of Quaternary calcium carbonate deposits in Cornwall. Geoscience in South-West England, 12, 233-239.
Proceedings of the Ussher Society


Cornwall’s calcium carbonate-rich deposits consist predominantly of coastal Holocene beaches, beachrock, dunes, eolianites, tufa and speleothems, offshore coastal maërl beds and late Pleistocene littoral deposits. The majority of these calcareous deposits in Cornwall, with the exception of some tufas and speleothems, were clearly not derived from within Cornwall itself but are largely the product of a regional offshore north-east Atlantic CaCO3 budget controlled by cyclical marine transgressions and regressions coupled with local climate factors operating during the Quaternary.


The Late Pleistocene to Holocene sands and gravels (Evans, 1990), covering the Devonian/Carboniferous bedrock in the 30 km coastal shelf around the Cornish coast are well-mixed with carbonate skeletal material with “a high preservation potential under present conditions, despite the evidence of bioerosion, disturbance by storm waves and transport of material by currents” (Stride et al., 1999) and are undoubtedly subject to entrainment in the strong tidal pulses prevalent around the south-west UK peninsula. Farrow and Fyfe (1988) considered that much of the north-west European shelf “represents a modern-day equivalent of the ‘calcareous shale’ facies common in the geological record” with the mud fraction of Holocene sediments on the shelf containing in the region of 10–20% CaCO3 rising to over 50% CaCO3 on supratidal mud-flats. The availability of calcium and carbon would therefore tend to sustain the production of shelly biota, the remnants of which are undoubtedly subsequently deposited and recycled on a large number of beaches around the Cornish coast as high- calcium carbonate skeletal shell sands. These sediments lie adjacent to a number of beaches, many of which are composed extensively of calcareous shell sand. These are, in turn, closely associated with carbonate dune fields, beachrock, tufa and, in coastal caves, speleothems. The available evidence indicates that these sediments and deposits have developed where there appears to be no association with limestone bedrock or calcareous head and therefore may be causally interrelated.

The possible effect of a north-east Atlantic influenced carbonate cycle in operation during the Pleistocene (Marine Isotope Stage 7 and/or 5e) is suggested by the occurrence of Quaternary calcum carbonate deposits around exposed headlands along the north and west Cornwall coast and north Devon coast (West, 1973; Gilbert, 1996) where mainland limestones do not occur. In contrast, the Quaternary calcareous raised beaches, cemented sands and tufas along the south Devon and south-west coasts of Wales are associated with limestone bedrocks and carbonate-rich head deposits which may have influenced their diagenesis (West 1970, 1973).

Fossil calcareous maërl accumulations are known from sediments of Miocene to Recent age on continental shelves and are used as stratigraphic markers and indicators of palaeoenvironmental conditions (Foster, 2001). The calcareous endoskeleton of the living algae forms annual growth bands similar to tree rings which make maërl a potential palaeoclimate proxy utilising, for example, Mg/Ca ratios in the individual growth bands. Maërl deposits from north France, Norway, Scotland and Ireland have produced useful on, for example Holocene climatic changes (Freiwald et al; 1991). The palaeoclimatological potential of maërl from Cornwall has not been studied in any detail.


Carbonate cementation of some Pleistocene temperate marine sediments

First published: May 1973

Sedimentology, Volume 20, Issue2
Pages 229-249


Carbonate cementation of some carbonate and quartz sands in three raised beaches of temperate origins was investigated. The carbonate of the cements was found to have been derived from the dissolution of skeletal debris. The sandstones, so produced, now possess only low‐magnesium calcite, but the original sediments, like adjacent modern beach and blown sands, probably contained low‐magnesium calcite, aragonite and some high‐magnesium calcite, all of skeletal origin. In meteoric water the dissolution has occurred of all carbonate within minute, tubulelike, volumes of sand. Concurrent deposition in adjacent volumes of sand of low‐magnesium calcite formed cements that are irregularly nodular or uneven on a small scale. Aragonite within the minute nodules has been replaced paramorphically by low‐magnesium calcite. Additional local carbonate cements were formed at later dates, around and within solution pipes.

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