OR/13/006 Mineralogy

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Entwisle, D C, Hobbs, P R N, Northmore, K J, Skipper*, J, Raines, M R, Self, S J, Ellison, R A, and Jones, L D. 2013. Engineering geology of British rocks and soils - Lambeth Group). British Geological Survey. (OR/13/006).

* Geotechnical Consulting Group (GCG)

Introduction[edit]

Knowledge of the Lambeth Group mineralogy and the processes that alter them allows for a more informed appreciation of the sediments at a desk study level, resulting in better designed site investigations. This section gives an account of the mineralogy of the Lambeth Group sediments and its effect on engineering behaviour, based on mineralogical data from a number of sites in the London and Hampshire Basins.

Heterogeneity within the Lambeth Group is not only due to the changes in primary sediment lithology but also, in certain parts, by hard bands such as ferricretes, calcretes and silcretes formed during penecontemporaneous pedogenesis. Pedogenesis alters and transports minerals by washing them through the soil profile (eluviation) and enriching zones where they collect (illuviation). Pedogenic processes also produce the mottled colouring characteristic of some of the Reading Formation clays and the upper part of the Upnor Formation beneath thin Lower Mottled Clay, and may lead to localised changes in clay mineralogy. The clay minerals present will affect behaviour, most notably smectite, which results in higher plasticity clays, generally prone to shrink/swell hazards.

Deoxygenation of the Upnor Formation, which has caused deaths during tunnelling and deep shaft excavation operations in London (Lewis and Harris, 1998) is due to ‘green rust’. There is little information on the mineral as it oxidises very rapidly and may be seen, very briefly when inspecting fresh core (Newman et al., 2013[1]) (see Deoxygenated air in tunnels and deep excavations).

In stiff clay beds, pedogenesis may give rise to the partial infilling and cementing of fissures (possibly originating from post depositional drying and wetting cycles) by iron oxides. Sampling methods may destroy or damage these cemented bonds, resulting in laboratory strength tests underestimating the strength in situ, possibly leading to unnecessarily increased construction costs due to over-design (Hight et al., 2004[2]).

Where both pyrite and calcium carbonate in the form of shell debris or calcrete are present, near surface oxidation of pyrite (e.g. during excavations) may produce sulphuric acid that reacts with calcium carbonate to form gypsum. High sulphate contents derived from subsequent dissolution of gypsum may require the use of sulphate resistant cements.

Data sources[edit]

There is limited published literature on the mineralogy of the Lambeth Group and the stratigraphical control of some of the available information is poor. Most of the work carried out on clay mineralogy in the 1960’s is summarised in Perrin (1971)[3]. Buurman (1980)[4] investigated intra-formational soil horizons. However, recent interest in the Lambeth Group, particularly in London, has resulted in new data, particularly that acquired by Skipper (1999[5], and personal communication), and Knox (personal communication, 1997). Other information has come from work produced for this project (Pearce et al., 1998[6]) and various BGS reports and memoirs and the paper by Huggett and Knox (2006)[7].

As part of the BGS Lambeth Group study, samples were collected from a number of sites for mineralogical, petrographic and geotechnical determinations to supplement existing published data and information held in archived site investigation reports in the BGS National Geoscience Data Centre (NGDC). Most of the samples were collected from the Newbury Bypass construction site in Berkshire, Orsett Quarry in Essex, Upnor Quarry in Kent and cliff sections at Alum Bay, Isle of Wight. Data acquired from these sampling sites was supplemented by similar test data determined on samples from the Jubilee Line Extension Borehole 404T (BGS borehole TQ37NE/2118, [TQ 33638 79604]) selected from the same depths as used by Knox in his study of the clay mineralogy.

Non-clay minerals[edit]

Quartz[edit]

Quartz is usually the dominant non-clay mineral of the Lambeth Formation and is generally present as sand-size particles but may also occur as clay grade material (Gilkes, 1966[8], 1968[9]). Much of the fine to medium sand particles are angular or subangular but coarser grains are usually rounded. Some beds are of almost pure quartz sand. Grains or layers maybe cemented with silica, in some cases forming silcretes such as sarsen stones.

Silica — flint or chert[edit]

Most flint or chert gravels, and occasional cobbles, are found in the Upnor Formation where it is often present at the formation base. These gravels also make up approximately 75% of the pebble beds in the upper part of the Upnor Formation in the central part of the London Basin (Ellison, 1983[10]) and occur sporadically throughout. Elsewhere, flint gravel is very rare, but is found in the upper part of the Upper Mottled Clay sequence beneath the Shepperton Formation (‘Thames Basal Gravel’) and as thin beds in the Woolwich Formation.

Where present, flint gravels are usually rounded, often black, dark grey or green but when affected by contemporaneous pedogenic processes may be brown or white coated and may have a red core; these are porous and weaker than unaltered flints. Some altered flints can be mistaken for chalk. They are usually less than 2 cm across although they may be of cobble size and up to 200 mm across in some places. In the Upnor Formation the gravels show percussion cracks (or ‘chatter’ marks) indicating a high-energy shallow marine or tidal depositional environment. Flint gravels are also present in the form of ‘puddingstones’ and as fragments in some more breccia-like sarsens, with silica being the primary cementing agent and chert usually a minor constituent (see Hard Bands below). Irregular silica may form in voids, such as old root holes, as shown in Figure 3.1.

Feldspar[edit]

Feldspar is usually a minor constituent of the Lambeth Group sediments and may be altered or corroded (Figure 3.2).

Mica[edit]

Mica generally occurs as a minor component. Colourless flakes of muscovite and biotite are often present (Ellison and Lake, 1986[11]) and the clay-grade mica, illite, is a common and important component of the clay mineral assemblage (see Clay mineralogy).

Calcite and calcium carbonate[edit]

Calcite is present as shells in parts of the Upnor and Woolwich formations and may be the dominant material in parts of the Lower Shelly and Upper Shelly Beds where it may form limestone. Due largely to soil forming processes (pedogenesis), calcium carbonate is re-deposited to form concretions that are present as scattered small white calcareous nodules (Bloodworth et al., 1987[12], Edwards and Freshney, 1987[13]). These concretions coalesce and develop into hard bands in the upper parts of the Upnor Formation especially where the overlying Lower Mottled Clay Bed is thin; in the upper part of the Lower Mottled Clay in much of the London area; and in localised pockets, most commonly in sand lenses in the undifferentiated Reading Formation.

Figure 3.1    Irregular, micron-scale silica crystals forming a void lining in the Upper Mottled Clay, Newbury Bypass, 19.70 m above Chalk [SU 4500 6810]. (BGS electron micrograph No. E442S1/02).

Shrinkage cracks may be filled with calcite crystals in parts of the succession, such as at Alum Bay on the Isle of Wight (Buurman, 1980[4]). In these areas the calcite has been removed from the matrix and fossils and re-deposited by percolating calcium carbonate-rich waters. Aragonite, derived from shell debris, is present in the upper part of the Upnor Formation in the Bradwell area (Bloodworth et al., 1987) and in the Chilterns (Bateman and Moffat, 1989[14]).

Figure 3.2    Corroded K-feldspar in the Upper Mottled Clay from Newbury Bypass, 19.7 m, above Chalk [SU 4500 6810]. (BGS electron micrograph No. E442S1/01).

Iron-bearing minerals[edit]

There are a wide range of iron-bearing mineral present in most of the Lambeth Group including pyrite, limonite, haematite, magnetite/ilmenite, goethite, jarosite, lepidocrocite and leucocene. These minerals provide most of the colour variation, most notably within the mottled beds of the Reading Formation and some upper parts of the Upnor Formation. Pyrite, FeS2, was probably the dominant iron bearing mineral present at deposition, the oxidation of which, particularly in the Reading Formation and parts of the Upnor Formation, would have occurred during or shortly after deposition by sub-tropical weathering, soils pedogenesis and biological activity. During dry periods, the oxidation process would be enhanced by lowering of the ground water table to allow the introduction of air. In clay-rich deposits dry conditions could cause shrinkage and induce cracks, thus increasing the depth of oxidation. For example, fissures in the Reading Formation may be coated in a different coloured mineral to the material surrounding it. Air, and material from above, can also be introduced too much greater depths by burrowing animals such as crustaceans, and the rotting and removal of roots. Some of the most vivid examples of colour contrasts have been formed in this way (Figure 3.3). Iron minerals are also redeposited in ferricrete (see Hard bands) such as the Winterbourne Ironstone of north Kent and in shrinkage cracks and other voids such as root holes (Figure 3.4).

Figure 3.3    Examples of mottling in the Reading Formation from the Newbury bypass construction site [SU 4500 6810] (right) and from Alum Bay, Isle of Wight [SZ 3054 8520] (left).
Figure 3.4    Anhedral to occasionally micro-botryoidal goethite lining voids in the Lower Reading Formation, 17.6 m above the Chalk, Alum Bay, Isle of Wight [SZ 3052 8522]. (BGS electron micrograph No. D821S1/06).

The oxidised iron minerals may be reduced by the activity of bacteria consuming organic material such as roots or by water logging, as illustrated in Figure 3.5.

Figure 3.5    Variation in colour of the Reading Formation, showing blocks with a red interior and grey reduction exterior. Star Brickworks, Knowl Hill [SU 8160 7970].

In the Reading Formation at Alum Bay, Buurman (1980)[4] found that the mottling is due to local sharp boundaries between iron-rich and iron-poor spots. He found that the iron was transported and accumulated in favourable conditions (pyrite where organic material was present and other iron minerals in oxidised zones).

Contemporary oxidation occurs where reduced deposits are exposed. This can be seen in the sands of the Upnor Formation, which are generally green due to the presence of glauconite but change to the typical ‘pepper and salt’ appearance on exposure. The grey deposits of the Woolwich Formation oxidise to grey brown or brown, and if calcium carbonate is present may form gypsum.

Pyrite is the main iron mineral in grey and black deposits such as those found in the Woolwich Formation and some parts of the Upnor Formation. It is often associated with deposits containing fossil remains such as shells, lignite, wood or roots.

A yellow mineral, jarosite, colours a sandy clay at, or near the base of, the lignitic beds of the lower Woolwich Formation at Newhaven. Jarosite, KFe2(SO4)2(OH)6, is an alteration mineral that is usually associated with oxidation of pyrite.

Red or dark red colouring or mottling is usually due to hematite, Fe2O3, (see Figure 3.3 and Figure 3.5) which is the oxidised and dehydrated form of iron. It is likely to be present where a bed was subjected to longer periods of surface or near surface exposure resulting in drying and dehydration. It is commonly seen in the upper parts of the Lower and Upper Mottled Clay, in burrow deposits and root holes. The strong red colour requires only small quantities of hematite and in some cases this may be below the detection limit of x-ray diffraction apparatus. Earthy hematite rosettes also form linings in voids in the Reading Formation (Figure 3.6).

Figure 3.6    Earthy hematite rosettes forming thin linings in voids in the Lower Mottled Clay, 18.6 m above Chalk, Alum Bay, Isle of Wight [SZ 3055 8523]. (BGS electron micrograph No. D822S1/04).

Most of the common yellow-brown and brown ferric oxides, often known as limonite, belong to the goethite species, FeO(OH). It is probably the most common form of oxidised iron in the Reading Formation and is also found in the Upnor Formation. It may be present as a minor or trace mineral and can be found with siderite in the Bradwell Borehole [TM 0530 8507], or with hematite or lepidocrocite at Newbury. Goethite is usually formed as the result of weathering of iron or iron-containing minerals such as pyrite, siderite and glauconite, under oxidising conditions. It may be found in bioturbated material in the clays of the Reading Formation and Upnor Formation, in drying cracks and in root holes. In sands it may be ubiquitous, as in the upper part of the Upnor Formation and the lower and middle part of the Upper Mottled Clay in the Bradwell Borehole BH 202 (BGS borehole TM00NW/27) [TM 0530 8507].

Lepidocrocite, FeO(OH) has the same chemical formula as goethite but a different (g) structure. It usually dark brown with an ‘earthy’ texture and is rather uncommon. In the Lambeth Group, lepidocrocite has been found as a minor or trace clay-size mineral in a 2 to 3 m section in the middle part of the Lower Mottled Clay at Newbury and in a sample of Lower Shelly Clay from the Lower Upnor Quarry. It is rarely found in gleyed calcareous soils and infrequently in groundwater gley soils, but is common in surface-water gley soils (Karim and Newman, 1986[15]).

Glauconite[edit]

Glauconite is a green potassium iron silicate mineral that imparts the greenish colour typical of the Upnor Formation. It also occurs occasionally in some of the coarser parts of the Reading and Woolwich formations. Glauconite forms as an alteration product of detrital biotite mica or other parent materials, by marine diagenesis in shallow water under reducing conditions; it is characteristic of some sands, sandstones (such as greensand), and impure limestones such as the Zig Zag Chalk and West Melbury Marly Chalk formations of the Grey Chalk Subgroup of Southern England. It forms up to 30% of parts of the Upnor Formation where glauconite grains are of similar size or slightly larger than the quartz grains, between 0.1 to 0.3 mm, and are rounded or subrounded (Figure 3.7). On weathering and oxidation glauconite breaks down to form brown-red limonite or yellow-brown goethite. When fully weathered these sediments are light greyish brown to orange brown, speckled with dark green grains of glauconite, as seen at outcrop.

Figure 3.7    Example of the clay-rich glauconitic sands from the Upnor Formation. The glauconite grains, dark grey, are rounded and some have oxidised (pale grey) (BGS borehole TQ37NW/2118, [TQ 33638 79604], 38.01 m). (BGS electron micrograph No. D783P1/02).

Gypsum[edit]

Gypsum is not a primary mineral in the Lambeth Group but is formed during modern weathering where pyrite and calcium carbonate are found together, and is most likely to occur in the Upnor and Woolwich formations. Pyrite is typically found in association with sediments deposited in anoxic conditions and often with organic matter that restrict its oxidation (see Iron-bearing minerals). However, during modern weathering pyrite oxidises to produce sulphuric acid that reacts with calcium carbonate, often derived from shell fragment, producing gypsum. It is likely that all the gypsum in the Lambeth Group is formed in this way: Pyrite + oxygen + water ® Iron II sulphate + sulphuric acid

FeS2 + 7O2 +2H2O ® 2FeSO4 + 2H2SO4 (1) Sulphuric acid + Calcium carbonate® Gypsum + water + carbon dioxide H2SO4 + CaCO3 + 2H2O ® CaSO4.2H2O + H2O + CO2 (2)

Gypsum may have formed as a part of the sub-tropical weathering process shortly after deposition but, because it is soluble, would have been removed from the near surface by water movement. It is rarely described in the Lambeth Group and there are only a few references to gypsum in the BGS National Geotechnical Properties Database, all associated with the Lower Shelly Clay. In cliffs at Newhaven, Skipper (1999)[5] describes gypsum in a thick lignitic clay bed about 0.80 m above yellow (jarositic) very sandy clay (Figure 3.8). In a section exposed in the Upnor Brick and Stone Quarry at Lower Upnor in Kent [TQ 7590 7110], a lignitic bed at the base of the Woolwich Formation Lower Shelly Clay contains occasional shell moulds and abundant gypsum as selenite crystals. It is likely that these parts of the bed were originally shelly and have now been replaced by the gypsum.

Figure 3.8    Pseudomorphs after gypsum in the Lower Shelly Beds at Newhaven [TQ 7590 7110].

Although gypsum is most likely to form in parts of the Woolwich and Upnor formations it has been seen where reworked calcareous nodules mix with lignite, for instance at the mid-Lambeth Group Hiatus at Alum Bay (Skipper 1999[5]).

Clay mineralogy[edit]

General clay mineral trends and distribution[edit]

The clay mineralogy of the Lambeth Group usually comprises smectite, illite, kaolinite and chlorite; mixed layer clays and very rare halloysite have also been identified. A summary of the clay mineral assemblages typically found in the Lambeth Group formations is given in Table 3.1 and outlined below for the London and Hampshire Basins.

Table 3.1    Summary of typical Lambeth Group clay mineralogy.
Lambeth Group London Basin, North and East Hampshire Basin South and west Hampshire Basin
Upper Woolwich Formation (Upper Shelly Clay) Mixed clay assemblage with increasing smectite and decreasing kaolinite upwards Not present
Upper Mottled Clay and lower Woolwich Formation (Laminated Beds and Lower Shelly Clay) Mixed clay assemblage, smectite, illite and kaolinite Illite major
Kaolinite major
Smectite minor increasing at top
Lower Mottled Clay Smectite dominant or major,
Illite major or minor,
Kaolinite minor to absent but sometimes locally dominant particularly at top
Illite major,
Kaolinite major,
Smectite minor but locally dominant at top,
Rare exotic and mixed layer clays
Upnor Formation Smectite dominant Illite minor
Kaolinite minor to absent
Illite major, Kaolinite major, Smectite minor sometimes major

London basin[edit]

A summary diagram of the relative abundance of clay minerals for samples from Jubilee Line Extension borehole 404T (BGS borehole TQ37NW/2118, [TQ 33638 79604]) is shown in Figure 3.9 and other sites in Figure 3.10 provide a general guide to the clay mineralogy of the London Basin.

The clay mineralogy of the Upnor Formation and Lower Mottled Clay typically contains dominant or major smectite with minor illite, although illite content is usually higher in the north and west in the upper part of the Lower Mottled Clay. Kaolinite is usually a trace or minor component and is often absent in the east Essex area.

Above the major mid-Lambeth Group Hiatus, in the Woolwich Formation and Upper Mottled Clay (see The Lambeth Group sequence), the clay assemblage becomes more mixed. Kaolinite and illite content tend to increase and smectite decreases upwards through Lambeth sequence 3 (lower Woolwich and Upper Reading sequences). In central London illite and smectite tend to have similar contents, whilst chlorite becomes more important.

There is limited data for the Upper Shelly Clay of the Upper Woolwich Formation sequence 4. In central London smectite content increases upwards as illite and chlorite reduce. At the top of the Upper Shelly Clay sequence smectite is the major to dominant clay mineral.

Figure 3.9    Relative percentages of clay mineral for samples from Jubilee Line Extension borehole 404T (BGS borehole TQ37NW/2118 [TQ 33638 79604)].
Figure 3.10    Clay mineralogy of borehole core in the London Basin.

The clay minerals of the Hampshire Basin show a general trend of decreasing smectite content to the south west, with the north of the basin being similar to the western part of the London Basin. The Upnor Formation is smectite-dominated with illite and minor kaolinite (Edwards and Freshney, 1987b[16]). The rest of the succession contains major illite with smectite and kaolinite in approximately equal quantities, minor amount of chlorite may also be present. To the south, at Studland and Alum Bay, the Basement Beds contain a mixed assemblage of approximately equal quantities of smectite, illite and kaolinite, or major illite with minor smectite and kaolinite. Above these beds a majority of the succession usually contains major kaolinite with minor illite and smectite. However, in the upper part of the Lower Mottled Clay smectite is the dominant clay mineral with minor or trace kaolinite and illite (Buurman, 1980[4]; Pearce et al., 1998[6]). Chlorite is absent or in trace quantities.

In the east of the basin at Felpham, the clay assemblage is more typical of that in the London Basin having high smectite content in the lower part and smectite with illite and kaolinite in the upper part (Skipper, personal communication 1999). This is not surprising as at the time of deposition the north and east of the Hampshire Basin and the London Basin were part of a continuous marginal depositional area.

The simple models of clay mineral distribution described above are general trends. Due to the complex nature of the deposition, pedogenic alteration and the tectonic and volcanic activity during the late Palaeocene detailed studies have shown rapid local and possible regional changes. A good example is Alum Bay, described by Gilkes (1966[8], 1968[9]), Buurman (1980)[4] and Pearce (1998)[6]. Investigating the clay mineralogy of the early Tertiary of the Hampshire Basin, Gilkes (1966[8], 1968[9]), and analysed nine samples from the cliff section and found them to contain mainly illite and kaolinite with minor or trace smectite. Buurman (1980)[4], studying pedogenesis, analysed forty-five samples including at least one sample from each identifiable bed, apart from the basal unit which was obscured at the time. Most of the results showed a clay mineral assemblage of illite and kaolinite. However, a marked contrast in clay mineralogy occurs within a few metres of the top of the lower Reading Formation. Here, a two to three metre thick mottled clay bed contains increasing quantities of smectite that becomes the dominant clay mineral at the top of the bed. At the top of the Lower Mottled Clay kaolinite is the dominant clay mineral. More recent analysis of samples from this section for this project (Pearce et al., 1998[6]) showed a similar smectite peak. A smectite peak also occurs near the top of the Lower Mottled Clay in Central London (Knox, personal communication, 1997).

Rapid changes in the clay mineral assemblage occur elsewhere. At the Lower Upnor Quarry [TQ 757 712] and Orsett Pit [TQ 673 808] the typically smectite-rich Upnor Formation contains a kaolinite-rich zone associated with a palaeosol horizon. In the lower Reading Formation at the Newbury Bypass section the clay minerals change from smectite-rich to kaolinite-rich or illite-rich and smectite-poor within a few metres (Skipper, 1999[5]; Pearce et al., 1998[6]).

In the Hampshire Basin, the Bunker’s Hill Borehole [SU 3038 1498] contained a very unusual clay mineral assemblage about 3 m above the Upnor Formation, consisting of halloysite with random mixed-layer chlorite-vermiculite and chlorite-smectite.

Detailed clay mineralogy[edit]

London Basin[edit]

The major London Basin sites are summarised in Figure 3.10.

Essex
Shotley Borehole, Shotley, Suffolk (BGS borehole TM23SW/19, [TM 24390 34600]) (Huggett and Knox, 2006[7]).

The Shotley Borehole (Figure 3.10) contains about 3.3 m of Upnor Formation and the five analyses is dominated by smectite with minor illite and kaolin, which is not present in the lower part of the formation. Smectite content peaks near the top of the Upnor Formation where is comprises nearly all the clay minerals. The Reading Formation is 11.7 m thick and the 26 analyses show that illite is the major clay minerals with minor smectite and lesser quantities of kaolin.

Bradwell Borehole BH202, Bradwell, Essex (BGS borehole TM00NW/27, [TM 0053 0851]) (Bloodworth et al., 1987[12]).

The Bradwell Borehole BH202 (Figure 3.10) records about 2.8 m of Upnor Formation comprising a lower 0.7 m of sand and 2.1 m of clay, and 3.2 m of clay from the Upper Mottled Clay (Knox, personal communication, 1997). The lowest part (Upnor Formation) is glauconitic and the clay mineralogy is dominated by smectite with a little illite. Smectite is also the major clay mineral in the lower part of the overlying Reading Formation but illite becomes a more important constituent and kaolinite occurs as a trace clay mineral. In the upper part of the succession, smectite content reduces and illite is the dominant clay mineral with minor quantities of smectite and kaolinite.

Wormingford Mere Borehole, BGS Borehole TL93SW/1 [TL 9267 3262] and Bures Borehole, BGS borehole TQ05SE/2 [TL 9120 3399] (Ellison et al., 1986[11]).

The Wormingford Borehole and Bures Borehole (Figure 3.10) record a lower bed (4.4 and 4.5 m thick respectively) comprising green and red mottled glauconitic clayey fine to medium sand of the Upnor Formation. Above is about 5 m predominantly of stiff to very stiff, red to purple, with vertical veining and mottles of orange and pale greyish blue clay with silt beds and sandy silt beds of the Reading Formation. The clay assemblage of the Upnor Formation is dominated by smectite, which becomes significantly less abundant in the overlying Reading Formation where illite is the dominant clay mineral. Kaolinite, rarely present in the Upnor Formation, becomes more common further up the Reading Formation succession, in conjunction with decreasing smectite.

BGS Dowsetts Farm Borehole, BGS Borehole TL32SE/38 [TL 3806 2079] (Moorlock and Highley, 1991[17]).

During an appraisal of fuller’s earth resources in England and Wales, Lambeth Group samples from the BGS Dowsetts Farm Borehole at Colliers End, Essex were initially tested for whole rock specific surface area using the 2-ethoxyethane method (Carter, 1965). As a part of this assessment those samples with specific surface area values of greater than 240 m2/g, inferring a smectite content of >30%, were examined by X-ray diffraction analysis. Only clays were analysed. The highest specific surface areas were present in the lower part of the Lambeth Group, that is in the clay facies of the Upnor Formation and the lower part of the Reading Formation, indicating dominant or major smectite. X-ray diffraction analyses confirmed major smectite and low or trace quantities of illite and kaolinite.

Chiltern Hills (Bateman and Moffat, 1987[18]).

Bateman and Moffat (1987)[18] carried out a study of the petrography of the Lambeth Group of the Chiltern Hills to the north and north west of London, including a number of outliers. They describe the clay mineralogy of thirteen samples from ten sites. Most of the exposures sampled were small, shallow (<3 m) and degraded, although some were from brick or sand pits. The samples were classified as ‘Bottom Beds’, ‘Sand’ or ‘Clay’; the former are from the Upnor Formation and the other two are from the Reading Formation but it is not possible to identify which part of the where in the Reading Formation they come from. Of the four samples identified as from the ‘Bottom Beds’, most contained major illite with minor ‘expansibles’. Kaolinite was a minor or trace mineral. The ‘expansibles’ were vermiculite, mixed layer illite-smectite or vermiculite/smectite. Only one sample contained the more typical smectite>illite>kaolinite clay assemblage. The ‘clays’ from the Reading Formation contained major smectite and sometimes with mixed layer smectite-vermiculite or other mixed layer clay. Illite was present as a minor or trace mineral and kaolinite as a trace component.

The six ‘sand’ samples were more varied with major illite, kaolinite or smectite and little or no vermiculite. A sample from Hedgemoor [SU 977 944] contained no smectite.

Central London
Channel Tunnel Rail Link (CTRL) Borehole A2, BGS borehole TQ38SW/2201, [TQ 3296 8051] and Jubilee Line Borehole 404T, BGS borehole TQ37NW/2118 [TQ 3363 7960], (Knox, personal communication, 1997)

The clay mineralogy of the <4 mm fraction of the two Lambeth Group reference sections in London, the Channel Tunnel Rail Link (CTRL) Borehole A2 and Jubilee Line Borehole 404T, show similar trends. Borehole 404T is more complete (Figure 3.9 and Appendix 1) and records an approximately 17 m thick sequence comprising all the main units of the Lambeth Group, ranging from the lower and upper Upnor formations through to the Upper Shelly Clay of the Woolwich Formation. The clay mineral assemblage of the lower and upper Upnor Formation and the Lower Mottled Clay (Lambeth sequences 1 and 2) are dominated by smectite, usually with minor quantities of illite and smaller amount of kaolinite. Chlorite is sometimes present. Above, in the Upper Mottled Clay and Lower Shelly Clay and Laminated Beds (Lambeth sequence 3), there is a reduction in smectite content and an increase in illite and kaolinite. Smectite content again increases in the Upper Shelly Beds (Lambeth sequence 4).

Staines borehole, Staines, Surrey, (BGS borehole TQ07SW/156, [TQ 0360 7240] (Huggett and Knox, 2006[7])

The Staines borehole was considered to contain less than 1 m of Upnor Formation and 21 m of Reading Formation Figure 3.10. The single sample tested form the Upnor Formation contain major illite with minor quantities of smectite and kaolin. The 45 analyses on 21 m of Reading Formation shows that the lower and middle part is dominated by illite with minor smectite, kaolin and chlorite. However, 3 m horizon within the upper part contains only smectite only Above, smectite and illite are major clay minerals with minor kaolin.

Newbury Bypass (Pearce et al., 1998[6]; Huggett and Knox, 2006[7])

A lithological and mineralogical correlation of a composite section of a cutting through the Lambeth Group at the Newbury Bypass site is presented in Figure 3.10. The lower part of the section was sampled in detail whereas there were few samples taken in the upper part.

The Lambeth Group in the Newbury Bypass section is approximately 25.5 m thick and comprises about 2.5 m of Upnor Formation sands and clays overlain by about 11 m of Lower Mottled Clay and 12 m of Upper Mottled Clay of the Reading Formation. The major clay mineral is usually smectite with minor or trace illite and kaolinite. However, there are parts of the sequence that contain major illite or kaolinite and these occur more commonly within the Upper Mottled Clay. The exceptions are below.

Starting about 3.5 m above the top of the chalk, is the base of a 0.8 m coarsening up sequence. The silty clay contains major smectite, whereas the flaser-bedded sand, silt and clay at the top of the sequence contains major kaolinite with minor illite and trace smectite. About 4.9 m above the top of the chalk is 2 m thick pale grey sand channel infill. About 0.5 m above the base the infill the sample contained major illite with minor kaolinite and trace smectite, but at the top of the sand body smectite was the major clay mineral. Smectite content tends to decrease towards the top of the Lower Mottled Clay. Only three samples were collected above the mid-Lambeth Group Hiatus, in the Upper Mottled Clay, only three samples were collected. Just above the hiatus a sample of mottled grey and light yellowish brown clay contained major kaolinite with minor smectite and illite. A sample of brownish orange sandy clay nearly six metres above the previous sample contained major smectite with minor kaolinite and illite and 0.8 m above, brown sandy clay contained major illite and kaolinite with minor smectite.

Hampshire Basin[edit]

The major Hampshire Basin sites are summarised in Figure 3.11.

Coastal sites
Studland Bay, Dorset [SZ 044 824] (Gilkes, 1966[8], 1968[9]) The lower part of the Lambeth Group at Studland Bay was studied by (Gilkes, 1966[8], 1968[9]). This part of the sequence equates to the basal fluvial glauconitic sands (possibly reworked Upnor Formation) and iron cemented sands, pedogenically altered sands and silts and fluvial channel sands, Lower Mottled Clay (Skipper, 1999[5]). The lower fluvial sands contain approximately equal quantities of smectite, illite and kaolinite. Smectite is the main clay mineral at the base of the pedogenically altered sands and silts with minor illite and kaolinite. However, in the upper part of these silts the clay mineralogy is similar to the basal beds. In the lower part of the fluvial channel sands the only reported clay mineral is mixed layer illite-smectite. The rest of these sands comprise illite and kaolinite with trace or no detectable smectite.

Whitecliff Bay, Isle of Wight [SZ 4639 8580 to 4639 8585] (Gilkes, 1966[8], 1968[9]; Huggett and Knox, 2006[7]).

Eight samples tested by Gilkes (1966[8], 1968[9]) from Whitecliffe Bay from the Reading Formation were all dominantly illite and kaolinite with minor amounts of smectite and occasional trace chlorite. Smectite content increases in the uppermost beds. The fourteen samples from the 34.5 m exposure reported in Hugget and Knox (2006)[7] (Figure 3.11) was generally dominated by illite with moderate amounts of smectite and kaolin with rare chlorite. Within the upper part of the succession smectite was absent with increased illite content.

Alum Bay, Isle of Wight (Gilkes, 1966[8], 1968[9]; Huggett and Knox, 2006[7]; Buurman, 1980[4]; Pearce et al., 1998[6]; Skipper, personal communication, 2003).

Alum Bay provides probably the best cliff exposure of Reading Formation deposits and because of this has been studied by a number of workers. The cliff exposes a c.41 m sequence of Upnor Formation sands (up to 2 m thickness at the cliff base) overlain by c.19 m of Reading Formation Lower Mottled Clay and c.20 m of Upper Mottled Clay. Gilkes (1966[8], 1968[9]) tested nine samples and found that the clay mineral assemblage consisted mainly of illite and kaolinite with minor amounts of smectite and occasional traces of chlorite. Buurman (1980)[4] took forty-five samples at Alum Bay to investigate the palaeosols of the Reading ‘Beds’ but did not sample the pedogenically altered sands at the base. X-ray analyses, carried out on less than 1 mm fractions, showed results similar to those of Gilkes, that is most of the sequence is dominated by illite and kaolinite with smectite a minor or trace mineral and often absent from the lower 10 m; chlorite is also occasionally present as a trace mineral. However, almost half way up the sequence, in a zone containing a ferricrete, kaolinite is the dominant clay mineral and illite and smectite are absent or present as only minor constituents. This coincides with the top of the Lower Reading Formation and the lowest part of the Upper Reading Formation (Skipper, 1998). The smectite content appears to be cyclic (Figure 3.11). In the lower 10 m there is a low amplitude increase and decrease in smectite, that is, from absent to trace to absent. Above is an approximately 5 m cycle where smectite becomes a minor to a trace constituent. In the next 3 to 4 m smectite increases to become the dominant clay mineral corresponding to a marked decrease in illite content. In the ferricrete zone, smectite and illite contents reduce to either trace or absent constituents in contrast to a marked dominance of kaolinite. For 17 m in the Upper Mottled Clay above the kaolinite-dominated ferricrete zone, smectite again forms a minor component with illite. In the upper 3 m of this sequence smectite content increases until it becomes the major clay mineral.

Pearce et al. (1998)[6] report on twenty-two samples tested from the Alum Bay coastal section Skipper (personal communication, 2003) on twelve samples and Huggett and Knox (2006)[7] eight analyses. These three sets of analyses found similar trends to Buurman (1980)[4], although Pearce found greater proportions of kaolinite and Skipper found more smectite throughout.

Felpham, Sussex [SZ 942 989] (Skipper, Personal communication, 2003)

At Felpham the clay mineralogy of the Upnor Formation is dominated by smectite with illite and kaolinite as trace or minor clay minerals in the upper part of the formation. Within the Lower Mottled Clay smectite is the dominant or major clay mineral with minor illite and kaolinite. However, near the top of this unit smectite again becomes dominant. The lower Woolwich Formation, represented by the ‘Felpham lignite bed’ (Bone, 1986[19]) contains a lower clay with sphaerosiderite and upper lignitic clay has major smectite, and minor illite and kaolinite. The Upper Mottled clay has major illite with minor smectite and kaolinite. Chlorite is occasionally present in the upper half as a minor or trace mineral.

Newhaven [TQ 444 000] (Pearce et al., 1998[6]).

Two samples from the cliff at Newhaven were analysed as part of the present study, one from the sandy clay above the ferricrete zone and the other from a gypsiferous, dark brown silty clay with rootlets (Lower Shelly Beds). The lower sample contained major illite with minor smectite and trace kaolinite whereas the upper sample contained major smectite with minor kaolinite and illite.

Inland sites
Bunker’s Hill Borehole, BGS borehole SU31SW27 [SU 3038 1498] (Edwards and Freshney, 1987b[16])

The sequence recorded for the Bunker’s Hill Borehole (Figure 3.11) consists of a basal c.2 m thickness of Upnor Formation sands overlain by c.22 m of Reading Formation deposits. The basal Upnor sequence is smectite-dominated with minor illite. Two metres above the Upnor Formation boundary the lower Reading Formation sequence comprises a 3 to 4 m thick layer of atypical clay minerals dominated by halloysite with minor or trace quantities of mixed layer chlorite-vermiculite and chlorite-smectite. Halloysite is commonly found in the tropical residual red clays of East Africa. This unusual clay mineral assemblage may represent the product of subtropical weathering of volcanic ash. Above this assemblage illite is the main clay mineral with minor smectite and kaolinite, and trace chlorite. However, over a 2 m zone at the top of the borehole kaolinite reduces to a trace or minor component with trace chlorite accompanied by a corresponding increase in smectite content within the still illite-dominant assemblage.

Shamblehurst Lane Borehole, BGS borehole SU41SE336, [SU 4927 1456] (Edwards and Freshney, 1987b[16]) The clay mineralogy of the Reading Formation, Lower and Upper Mottled Clay from the Shamblehurst Borehole (Figure 3.11) is a consistent assemblage comprising major illite with minor smectite and kaolinite and occasional minor amounts of chlorite and/or vermiculite.

Knowl Manor brick pit [SY 973 975] The precise stratigraphic positioning of Reading Formation samples from the sand and brick pit sites at Knowl and Michelmersh is unclear. However, the clay mineralogy of two samples tested from the Knowl Manor pit contained major kaolinite with minor illite and minor or trace smectite.

Michelmersh Brick Pit [SU 345 259] The only sample acquired from the Michelmersh pit contained major smectitic with minor illite and kaolinite.

Figure 3.11    Clay mineralogy of boreholes and natural sections in the Hampshire Basin.

Origin of the clay minerals[edit]

The major clay minerals of the Lambeth Group are probably mainly detrital in origin. Illite and chlorite were mostly derived from the erosion of rocks and subsequent redeposition, although chlorite may also be derived from the alteration of fine-grained volcanic material (Knox, 1996a[20]). The other two major clay minerals, smectite and kaolinite, are also detrital but were formed by the contemporaneous weathering and alteration of other minerals.

Gilkes (1969) considered the origin of the two clay mineral assemblages he encountered: the illite-kaolinite suite with little or no smectite or chlorite, and the smectite-illite suite with lesser quantities of kaolin and minor amounts of chlorite. The former could be considered as typically non-marine, and the latter as marine. However, the presence of smectite in non-marine samples indicates that this is not always the case. He concluded that the distribution of the clay minerals showed a clear geographical influence, with illite-kaolinite sediments in the west and illite- smectite in the east. He further suggested that this reflected two different sediment sources. the kaolinite-rich material being derived from granitic rocks of the Cornish Massif (with variation in kaolinite content thought to be due to the degree of tropical weathering and erosion), and the smectite-rich sediments being derived from the Chalk (from the insoluble fraction produced after erosion and dissolution in a wet sub-tropical climate). Smectite formed from altered ash was considered to be of minor importance.

Volcanic ash deposits are now well documented within the North Sea and in the Thanet Formation and Ormesby Clay Formation below the Lambeth Group, and in the Thames Group above. Knox and Harland (1979)[21], Knox and Morton (1983[22], 1988 [23]) and Jolley and Morton (1992) identified two major phases of explosive volcanism within this area. Phase 1 (mid Palaeocene) occurred during the deposition of the Thanet and Ormesby Clay formations. The smectite-rich clay facies of the Ormesby Clay Formation comprises clays of very high to extremely high plasticity with liquid limits up to 172% and plasticity indices up to 116% (Cox et al., 1985[24]). Phase 2 occurred during the latest Palaeocene to earliest Eocene. Phase 2.1, the least active phase, probably correlates to the lower half of the Lambeth Group but may also include the Lmb-3 sequence (upper Reading and lower Woolwich formations) (Knox, 1996a[20]). After a period of limited pyroclastic activity volcanism resumed in phase 2.2 and is recorded in volcanic ash bands of the Harwich Formation and lower part of the London Clay Formation. Ash layers have not been observed in the Lambeth Group. However, smectite is usually the dominant or main clay mineral in the lower part of the Lambeth Group below the mid-Lambeth Group Hiatus within the London Basin and parts of the Hampshire Basin. The smectite is probably derived from the reworking of volcanic ashes that were deposited, eroded and altered, and subsequently redeposited. Other evidence for a pyroclastic origin for the smectite comes from the presence of the halloysite clay assemblage in the Bunker’s Hill Borehole, which is thought to have formed by in situ subaerial weathering of underlying volcanic material, probably ash (Edwards and Freshney, 1987b[16]). However, no evidence for a direct volcanic origin has been found during mineralogical or scanning electron microscope investigations.

Two smectite peaks are present in the sample test data from borehole JLE404T, one in the Upnor Formation and the other in the Reading Formation, Lower Mottled Clay. The latter may correlate with those from the CTRL A2 borehole and Alum Bay. This peak may be due to a short-term increase in pyroclastic activity. However, this correlation must be considered as circumstantial because regional and local erosion events and deposition rates were probably different. The likely source of the pyroclastic material is the Greenland-Faeroes Province (Knox, 1996b[25]).

Kaolinite in the Lambeth Group was formed as a result of rock weathering during moderate to high climatic temperatures (10–20°C) with abundant or seasonal precipitation as indicated by palaeobotany data (Wolfe, 1980). Leaching of the host rock in this climate produces residual soils rich in clay minerals of the kaolin group along with iron and aluminium sesquioxides. Quartz is usually unaffected and remains, in general, as sand grains. In favourable conditions these residual soils may be 40 to 50 m or more thick. Kaolin may have formed in situ or transported from the weathered rock of the hinterland. Contemporaneous deep-sea sediments from the central North Sea show trends of higher kaolinite content in deposits of similar age to the Lambeth Group above the mid-Lambeth Group Hiatus. Equivalent deposits to the Upnor Formation contain no or little kaolinite (Knox, 1996a[20]). Evidence from scanning electron microscopy shows that the majority of the kaolinite is very fine grained and intimately mixed with the other clay minerals, lending support to a purely detrital origin. The higher quantities of kaolinite in the south and west of the Hampshire Basin are probably derived from sub-tropical weathering and erosion products of granites in southwest England (Cornubia) and northwest France (America).

However, there are some anomalous, discrete layers of kaolinite-rich materials that occur in otherwise smectite-rich deposits, for example in the Upnor Formation at Upnor, Kent. A sample from the Upnor Formation in the Upnor pit (UQ3, pale grey clayey sand) contained predominantly kaolinite with trace smectite and no illite. This sample was from just beneath a soil horizon and it is, therefore, likely that kaolinite formed in situ by pedogenesis during a period of near surface sub-tropical weathering in a relatively freely draining soil. There is also some evidence for the in situ formation of kaolinite for example in the upper part of the Lower Mottled Clay in the Alum Bay succession and at a site near the M40. Samples from these sites show rare delicate ‘booklets’ of kaolinite (Figure 3.12 and Figure 3.13). These delicate forms are unlikely to survive erosion and deposition and are considered to have formed in situ. Note that in both cases the kaolinite ‘books’ are silt-size and well developed and are likely to have developed by alteration of detrital micas (Psyrillos et al., 1999[26]) during pedogenesis.

Figure 3.12    A rare, well developed silt-size kaolinite ‘book’ within the clay matrix of major kaolinite and minor smectite and illite of the Reading Formation from a site on the M40 [SU 947 895]. (BGS photomicrograph No. E444S1/03).
Figure 3.13    Rare, silt-sized corroded silt-size kaolinite ‘book’ from the Lower Mottled Clay, Alum Bay, Isle of Wight, 17.6 m above the Chalk. (BGS photomicrograph No. D821S1/03).

The contrast in smectite distribution between most of the Lambeth Group and the south and west of the Hampshire Basin is probably due to the distance from the pyroclastic source, the pathway of deposition and the rate of deposition. In the London Basin the source of the Upnor Formation sediments was mainly the Mesozoic rocks of the Midlands or the equivalent rocks of Mainland Europe with some reworking of the Lambeth Group (Hallsworth, 1994[27]). The lower Woolwich and upper Reading Formation deposits have some different mineral characteristics, including a restricted garnet suite, in comparison with the lower Lambeth Group that indicates other sources of sediments, such as the Armorican or Cornubian massifs plus material from the Midlands. The rocks in the south and west of the Hampshire Basin are probably derived from the Amorican and Cornubian Massifs, whereas those to the north and east are similar to those in the London Basin.

Dramatic changes in the clay mineralogy may occur during the mid-Lambeth Group Hiatus or after erosional events, particularly if the new material source is from a different area or a different type such as continental or marine, or weathered differently. Also, biogenic activity mixes sediments from different origins and may also result in different clay mineral assemblages.

Pedogenic alteration[edit]

The Lambeth Group, in particular the Upnor Formation and the Reading Formation, have undergone alteration. As described above, mottling indicates penecontemporaneous tropical weathering. Buurman (1980)[4] identified such soils as pseudogleys. The iron of these soils is mobilised in the ferrous state during periods of high water table and after moving a short distance, it precipitates and re-oxidises when the water table falls (Duchaufour, 1982). This forms rusty patches or concretions and grey or yellowish bleached patches, which give a mottled appearance. The transition between the two colours can be sharp or gradual. The iron compounds and the resulting colour depend on the degree of hydration. Periodic wetting and drying may lead to the alteration or the decomposition of clay minerals; however, the in situ alteration of the clay minerals appears to be limited, partly because the low permeability of the clay facies restricts the movement of ions. Alteration of minerals in the clays is generally restricted to near contemporaneous bioturbated parts, such as in or near burrows or old root holes, and shrinkage cracks and may go down a metre or more below surface. Examples of root holes from the Upnor Formation include open holes (Figure 3.14) with clay particles aligned or infilled with well-sorted clean sand (Figure 3.15). The former is an open root hole with clay particles aligned around the root channel in the Upnor Formation at the Newbury Bypass site. The latter is a section through a root channel filled by well-sorted clean sand grains in the Upnor Formation at Orsett Quarry. Pseudogley soils are usually restricted to landscapes of low relief.

Figure 3.14    Open root channel with clay particles oriented around channel wall. Upnor Formation, Newbury Bypass, 1.5 m above Chalk. (BGS photomicrograph No. E431S1/04).

The following sequences of processes were identified by Buurman (1980)[4]:

  • Sedimentation, sometimes with reworking of the top of the underlying sediments;
  • Emergence, drying, consolidation, burrowing cracking and structural formation, with the segregation of iron along root holes. In better drained soils the clays are mobilised, forming oriented bodies, resulting in horizons of dense clay accumulations;
  • Segregation of iron when the soil is saturated;
  • Submergence producing accumulations of pyrite in former root holes, voids and sites containing organic material. This may have occurred during periods of rising groundwater and a new sedimentation phase;
  • Oxidation of pyrite.
Figure 3.15    Root channel infilled with sand with clay particles oriented around channel wall. Upnor Formation, Newbury Bypass, 1.5 m above Chalk. (BGS photomicrograph No. E431S1/04).

This sequence is repeated many times. The oxidation of pyrite may occur during any subsequent phase of emergence. Different minerals are produced by different rates of oxidation. Hydroxides are produced by slow oxidation and jarosite during rapid oxidation. The dominance of pseudogley features indicates terrestrial conditions. This sequence probably originated in an environment of intermittent sedimentation and soil formation. Most of the beds of the Reading Formation at Alum Bay show alteration due to pedogenesis, indicating that for most of the time the soil formation kept pace with sedimentation and that sedimentation was slow. The soils also show a pattern of increasing degrees of soil development that occurred during periods of falling base level/water table. These soils were probably deposited as overbank fines in a floodplain environment. They were seasonally waterlogged but as the water table or base level fell, leading to increasing emergence, a degree of soil development occurred (Skipper, 1999[5]).

Hard bands[edit]

The Lambeth Group is a Tertiary deposit and has not been buried to any great depth after deposition nor has it been greatly altered by tectonic activity; it is considered that most of the hard bands were formed during periods of subaerial exposure and changes in the height of the phreatic surface (water table or perched water table). Different hard beds normally tend to be limited by both stratigraphy and area.

Three types of hard bands are encountered, iron oxide-cemented (ferricrete), calcium carbonate-cemented (calcrete), and silica-cemented (silcrete). However, not all the hard bands were formed by significant post depositional mineral dissolution, movement and precipitation. Shelly bands, notably in the Lower and Upper Shelly Clay, may form local limestone bands; for example. The fresh water ‘Paludina Limestone’ of the Upper Shelly Clay is one of the more persistent biogenic limestones. Cemented shell layers can be seen in the cliff section at Newhaven. These limestones may have undergone some recrystallisation producing a stronger, more coherent rock.

The hard bands formed by pedogenesis are generally found in the upper part of the Upnor Formation, particularly where it was at or near the surface during the mid-Lambeth Group Hiatus, and the upper metre or so of Lower Mottled Clay. Iron, carbonate and silica cements occur in many other parts of the Lambeth Group and occasionally form coherent hard beds. Although most of the hard bands are of a specific type a combination of cemented material can sometimes occur. For example Figure 3.16 shows alteration of clays of the Lower Mottled Clay to form silica along with the formation of calcite concretions and pervasive iron oxide staining.

Figure 3.16    Heavily altered clay from the Lower Mottled Clay. The original clay has been patchily altered to leave a siliceous matrix (dark grey) and is pervasively stained by iron oxide (light grey). Calcite rhombs (mid grey) form concentrations. Sample JLE 404T, 34.20 m.b.g.l. (BGS photomicrograph No. D777P1/01).

Calcium carbonate and calcrete[edit]

Calcium carbonate nodules are present in parts of the Upnor and Reading formations. They coalesce into more coherent cemented beds (calcrete) up to 1.6 m thick in the Upnor Formation, where the overlying Lower Mottled Clay is thin, and in the Lower Mottled Clay in central and east London. Calcareous nodules are also seen near Arundel in the Hampshire Basin. In east London calcite veins are present in the clays of the Lower Mottled Clay and described in ground investigation reports as very weak to weak mottled green-grey, purple grey and brown mudstone. The calcretes and calcareous nodules vary between very weak to moderately strong, brown, light grey brown, bluish-grey, grey or white, sometimes crystalline limestone. Calcretes may have been more widespread and a possible precursor to silcretes such as the Hertfordshire puddingstones, which were then altered to silcretes as conditions changed (Skipper, 1999[5]).

Core from the Jubilee Line Extension (BH JLE 404T) contained two zones of calcrete, one near the top of the Upnor Formation and the other in the Lower Mottled Clay, which is about 1.30 m thick. The calcrete in the Upnor Formation is about 0.30 m thick as shown in Figure 3.17.

Figure 3.17    Core from the Upnor Formation with the description showing a calcrete, described as highly weathered limestone. Borehole JLE 404T (TQ 3363 7960), 35.30 to 36.00 m.b.g.l.

Accumulations of calcium carbonate usually exhibit a dense, continuous micritic groundmass. Different areas of the nodules contain different densities or size of calcite crystal creating a mottled fabric and the growth is dispersive (Wright, 1990[28]). Figure 3.18 shows pyrite and calcite concretions from the upper part of the Upnor Formation that may have replaced the original clay matrix. The presence of calcite, pyrite and fractured quartz crystals indicates that the calcrete must have formed in anoxic conditions of high alkalinity. A mechanism for grain breakage associated with calcareous concretions indicates rapid crystallisation from calcium carbonate saturated water (Skipper, 1999[5]). The structure of an Upnor Formation calcrete nodule from Limehouse, London (TQ 362 809), shows equant calcite crystals (Figure 3.19), which is consistent with a freshwater environment. Occasional quartz crystals in a calcite matrix indicates an expanded detrital grain framework where the quartz has been pushed apart by the formation of the calcite (Figure 3.20), indicating that the formation of the calcite was near surface.

Figure 3.18    Core from the Upnor Formation with the description showing a calcrete, described as highly weathered limestone. Borehole JLE 404T (TQ 3363 7960), 35.30 to 36.00 m.b.g.l.
Figure 3.19    Equant calcite crystals in the Upnor Formation calcrete, which is consistent with pedogenesis in a freshwater environment, Limehouse, London (TQ 362 809).
Figure 3.20    A few quartz crystals in a calcite matrix in an Upnor Formation calcrete. This represents an expanded detrital grain framework where the quartz has been pushed (blown) apart by the formation of the calcite, suggesting that the formation of the calcite was near surface. Limehouse, London (TQ 362 809).

Silica cementing and silcrete[edit]

The hardest and strongest of the hard bands are the silcretes. The sarsen stones and puddingstones (conglomerates) of southern England are probably mostly derived from the Lambeth Group, although some may be from Eocene deposits such as the Harwich, Bagshot and Barton formations (Summerfield, 1983[29]; Summerfield and Goudie, 1980[30]). Most are found to the south of a line from Lowestoft to the Severn Estuary. The distribution of most of the silcretes relate to the original outcrop of the Lambeth Group. Many are post erosion relicts found as isolated block or in groups on the Chalk or in Clay-with-Flints on the chalk away from the current outcrop of the Lambeth Group (P201387). The conglomerates are commonest to the north and west of London; for instance in Hertfordshire (Hertfordshire Puddingstone), and further west towards High Wycombe (Bradenham Puddingstone). In Hampshire and Sussex the conglomerates often contain angular flints (flint breccia). All the puddingstones are derived from the gravel beds of the Upnor Formation. Whereas other sandstone silcretes are also likely to be from the Upnor Formation sand beds but some may originate from the sand facies of Lower Mottled Clay. Other examples may not be in situ, as they may have been utilised in walls, buildings or rockeries as this stone may be one of the few strong rocks available in the area. The sarsens in the Ipswich area have similar heavy detrital minerals to the Reading Formation (Boswell, 1927[31]).

P201387    Sarsens in Clay-with-Flint. The stones are surrounded by red clay derived from the Lambeth Group with clay and flint around. Lee Gate, near Wendover, Buckinghamshire [SP 895 055]. (BGS photograph P201387, 1911).

Silica-cemented rocks are well known in parts of the Lambeth Group as localised zones of silicification, such as in the ferruginous sands in the London Basin and sandstones of the Hampshire Basin. More significant silcretes have been found in the Lambeth Group as partially and fully silicified breccia or sandstone in the Rotherhithe tunnel (Barrow, 1919[32]), at Bushey Station (Hopkinson and Kidner, 1907[33]), in a well section at Neasden (Whitaker et al., 1872[34]), near the base of the Lambeth Group (Upnor Formation) east of Siblets Wood (Sherlock et al., 1922[35]), the Great Western and Great Central Joint Railway cuttings at Gerrard’s Cross (Sherlock and Pocock, 1924[36]) and in gravel pits near Bernards Heath, St Albans (Kerr, 1955[37]). Puddingstones were found in chalk swallow-holes with sands of the Upnor Formation (Sherlock and Pocock, 1924[36]).

A list of localities where silcrete has been attributed to the Lambeth Group is shown in (after Potter 1998[38]).

The formation of the puddingstones and possibly some of the sarsens probably occurred after formation of calcrete that was subsequently removed and replaced by silica (Skipper, 1999[5]). The red, pink and brown colours of some of the flints probably occurred under alkaline conditions present during carbonate deposition. The mixture of colours and textures of flint gravels indicate that varying degrees of calcrete formation affected the flints prior to silicification.

The silcretes are generally found on the edges of the basin areas that have better drainage and extended periods of subaerial exposure. However, silicification occurs elsewhere very locally at mm or cm scale, often associated with biogenic disturbance such as root holes and other voids, usually in the Lower and Upper Mottled Clay of the Reading Formation. This is illustrated in Figure 3.22 which shows a back-scatter electron micrograph of clay that has been altered to siliceous matrix from borehole JLE404T (34.20 m). The void, probably a root hole, is lined by colloform silica that has subsequently been lined by iron oxide. The same material contains heavily altered clay (Figure 3.22) that has been patchily altered to leave a siliceous matrix that has been previously stained by iron oxide (light grey). There are also concentrations of calcite rhombs (mid grey).

Table 3.2    Silcretes attributed to the Lambeth Group (mostly Upnor Formation)
(After Potter, 1998[38]).
Approximate location Reference Notes
West of London Prestwich, 1854[39] Conglomerate
West of London Sumbler et al., 1996[40]
London Woodward, 1891[41]
East London Barrow, 1919[32]
North and east London Barrow and Wills, 1913[42] In situ
North London Bromehead et al., 1925[43]
Cuffley, Herts Pocock and Fortescue Wells, 1914[44] Conglomerate In situ
Hertford, Herts. Sherlock and Pocock, 1924[36] Conglomerate
Radlett, Herts Whitaker, 1864[45]
Radlett, Herts Whitaker, 1875 Conglomerate
Radlett, Herts Hopkinson, 1884 Conglomerate In situ
Radlett, Herts Woodward, 1909
Radlett, Herts Barrow et al., 1914[46] Conglomerate In situ
Rickmansworth, Hertfordshire Hopkinson, 1909
Ruislip and Radlett, Herts Whitaker, 1899[47]
St Albans, Herts Hopkinson, 1892[48]
St Albans, Herts Catt and Moffat, 1980[49] Conglomerate In situ
Watford, Herts Kidner, 1907[50] Conglomerate In situ
Herts. Whitaker, 1899[47] Conglomerate In situ
Newbury, Berks. Adams, 1873[51] In situ
High Wycombe, Bucks Sherlock et al. 1922[35]
Luton, Beds Sherlock, 1922[35] Conglomerate In situ
Pinner, Middlesex Gallois, 1993 Conglomerate In situ
Greys, Essex Holmes, 1904
Ipswich, Suffolk Boswell, 1927[31]
Sudbury, Suffolk Boswell, 1929[52]
Sudbury, Suffolk Pattison et al., 1993[53]
Woodbridge, Suffolk Boswell, 1928[54]
Suffolk Boswell, 1915[55]
Basingstoke, Hants. White, 1909[56]
Figure 3.22    ]Detail of a clay from the Lower Mottled Clay showing alteration to a siliceous matrix. The void is lined by colloform silica and subsequently by iron oxide. Sample JLE 404T [TQ 3444 7941], 34.20 m. (BGS photomicrograph D777P1/03).

Iron cementing[edit]

The movement of iron is very common in the Lower and Upper Mottled Clay and results in most of the colour variations in these beds. The major iron cemented beds are generally found in east London and north Kent. The best example is the ‘Winterbourne Ironstone’ which is attributed to the Lower Mottled Clay (Ellison et al., 1994[57]) or the Upnor Formation (Gamble, 1985[58]). In the Lower Upnor Pit [TQ 785 711] a series of iron cemented beds are found below the Lower Shelly Clay and are detailed in Table 3.3 (Kennedy and Sellwood, 1970[59]).

The Winterbourne Sand Pit [TR 065 570], south of Winterbourne, and Iron Hill Sand Pit [TR 064 582], north of Winterbourne, northwest of Canterbury, Kent also contain well developed iron cemented beds. As at the Lower Upnor Pit, the main ironstone is from the Upnor or Lower Mottled Clay and is generally about 0.50–0.60 m thick and described as very dark brown, orange or red, ferruginous, coarse to medium-grained sands, sometimes silty with irregular hard-pans and lenticular masses of well-cemented ferruginous sandstone or carstone. Here too, the Lower Woolwich beds contain limonite-cemented nodules. Thin bands of iron-rich angular to sub angular gravel are found also near the mid-Lambeth Group Hiatus at Alum Bay [SZ 305 824], and were observed along erosional surfaces within the Lambeth Group during the construction of the Newbury Bypass.

Table 3.3    Description of iron-cementing from the Lower Upnor Pit (TQ 757 711).
Stratigraphy Thickness, m Description
Lower Shelly Clay 0.60 Laminated black and brown sandy clay with gypsum, underlying a ‘line’ of ironstone nodules.
Upnor Formation 0.27 Iron-cemented sandstone.
Upnor Formation 0.55 White sand with low-angle cross-bedding. Occasional baryte rosettes. Large Ophiomorpha penetrates this bed, arising from the base of the sandy clay.
Upnor Formation 0.18 Sandy ironstone.
Upnor Formation 1.90 Hard, massive, purple sandstone with scattered small flint pebbles. Burrowed, and passing down into purple and yellow cross- and ripple-drift bedded sands.
Upnor Formation Below ‘Typical Upnor Formation’.

Recent weathering[edit]

The oxidation of minerals containing reduced iron such as glauconite and pyrite is seen where these minerals are exposed or near surface where oxygen is available due to reduced saturation and ingress of air or where oxygenated water flows through the deposit. In the Upnor Formation there is a change from pale to dark green to yellow-brown and an often speckled ‘pepper and salt’ appearance due to the weathering of glauconite, often to goethite and or limonite. Oxidation of pyrite, found in the Upnor and Woolwich formations, produces sulphuric acid, which reacts with calcium carbonate commonly occurring as in shells, to form gypsum (see Gypsum).

References[edit]

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