London - Palaeogene-Paleocence

From Earthwise
Jump to navigation Jump to search

At the beginning of Palaeogene time the London district lay on the edge of a sedimentary basin that included much of the present North Sea, and extended eastwards at least as far as Poland. To the west was the proto-Atlantic Ocean, the development of which was associated with rifting and igneous activity that culminated in early Eocene time about 55 million years ago (Knox, 1994). The Palaeogene deposits were laid down during alternating transgressions and regressions driven by global sea level changes and this broad pattern was overprinted locally by tectonic influences. The general succession (Table 6) is divided into major depositional sequences (Knox, 1996) and related to the magnetic chronology and nannofossil zones.

The strategic importance of understanding the stratigraphical and palaeo-environmental history of the Palaeogene deposits of the London Basin became apparent with the discovery, in the early 1970s, of oil-bearing strata of this age beneath the North Sea. In the local context, this understanding, underpinned by information provided from key boreholes (Table 7) has also played a role in the development of better interpretations of site investigation data for the planning of major infrastructure projects in London. Some of the larger ones have been the development of the London Docklands, the building of underground services, including the London Under­ground, and recently the Channel Tunnel Rail Link.

THANET SAND FORMATION[edit]

The oldest Palaeogene deposit in the London district is the Thanet Sand Formation. Its base is unconformable on the eroded surface of the Chalk Group. The unconformity is not caused by a single event but, based on evidence in the London Basin as a whole, is attributed to erosion and reworking during two or more depositional sequences (Knox, 1996).

The Thanet Sand Formation occurs at depth beneath much of London and the north-east of the region but is absent north-west of a line from Hillingdon [10 85] to Borehamwood [20 95]. The principal outcrops are in the south and east of the region, mainly in outliers around Dartford [52 73], Swanley [51 69], Southfleet [614 711] and Cobham [671 685]. Thanet Sand is also preserved locally in dissolution pipes and hollows in the Chalk peripheral to these outliers, in some cases beneath a cover of Head or Clay-with-flints. The maximum thickness is about 30 m in the vicinity of Stanford-le-Hope [68 82], decreasing to the north-west (Figure 9).

The lowest beds of Thanet Sand lie on a roughly planar dip slope formed by the top of the Chalk. This feature is particularly well developed in the vicinity of the A2 road and to the south of it, around Istead Rise [63 70] and Singlewell [66 71], and around Horton Kirby [57 68]. Pockets of Thanet Sand occur in dissolution ­cavities developed on this surface, but the majority of them do not form features. They are known only from augering, excavations and borehole records; in general they are too small to show on the geological maps. The outliers that constitute the main outcrop of the formation form positive, well-drained features with convex slopes. The basal contact with the chalk is, in many places, at a pronounced concave break of slope. The top of the formation is difficult to place on the basis of features, and is generally located by augering or placed on the evidence of borehole data.

The basal unit, known as the Bullhead Bed, is a conglomerate up to 0.5 m thick. It is variable in lithology, with sporadic rounded flint pebbles up to 50 mm in diameter, and almost unworn nodular flints (‘bullhead flints’) up to 150 mm across set in a dark greenish grey, clayey fine- to coarse-grained sand matrix containing pellets of glauconite up to 1 mm in diameter. The nodular flints are typically coated with dark green crystalline glauconite less than 0.5 mm thick. Small, pale yellow-grey slivers and flakes of unpatinated flint are also present in the matrix. These are resinous in appearance when freshly broken. Small fossils, including fish vertebrae, pelecypod shells, bryozoa and echinoid spines all derived from the underlying chalk are also present.

The bulk of the Thanet Sand is a coarsening-upward sequence, dominantly of fine-grained sand, but clayey and silty in the lower part as illustrated by a typical gamma-ray log signature (Figure 10). The proportion of fine-grained sand ranges from 10 per cent at the base to 60 per cent in the upper beds.

The unweathered sediments are pale to medium grey to brownish grey in colour. They weather at the surface to pale yellowish grey. Contemporary weathering and pedogenic processes locally give rise to a typical podsol profile, with purplish brown weak ferruginous cement developed within 0.8 m of the surface, in the lower part of the profile.

The sediments are intensely bioturbated so that primary sedimentary structures such as lamination are generally lacking. Bioturbation structures are identified as wisps of relatively dark grey clay and silty clay in hand specimen and in exposures. Dark grey to black mangan­ese-rich silt has been observed in the linings of sinuous burrows up to 8 mm in diameter. Scattered oblique and near-vertical burrows also occur in the top 1 to 1.5 m. These are filled with glauconitic sand derived from the overlying Upnor Formation.

Faint bedding is seen in places, in weathered exposures, and some fine lamination is recorded near the top of the formation in the Crystal Palace Borehole (at 152.2 m to 145.8 m depth). Glauconite grains and flakes of white mica are sparsely distributed throughout. Beds weakly indurated by iron oxide have been described in north Kent, and irregular to oblate masses of siliceous sandstone (colloquially known as ‘doggers’) have been recorded in the vicinity of Thurrock and Grays. Irregular nodules of pyrite less than 5 mm across occur rarely, presumably replacing small wood fragments, and Prestwich (1852) described gypsum, presumably from the dissolution of pyrite, at Blackheath.

Grain size analyses of the sediments (Figure 11) reveal that the succession as a whole is remarkably uniform and well sorted; better sorted beds occur towards the top. The sand grains consist almost exclusively of quartz, and are mainly angular and subangular, with only a small ­proportion of subrounded grains and flint chips. Montmorillonite is generally the dominant clay mineral, thought to be, in part, a weathering product of penecontemporaneous volcanic ash falls (Knox, 1994).

Thin sections indicate the presence of corroded feldspar, minor randomly orientated white mica, chlorite and ilmenite. Authigenic pyrite and glauconite clasts are rare. Some over-sized voids may be caused by dissolution of framework grains. Apatite occurs as fine sand grade detrital grains in samples from central London, but is absent in Crystal Palace and Stanford le Hope boreholes, presumably due to dissolution by acidic groundwater (Hallsworth, 1993).

The heavy mineral assemblage of the Thanet Sand has been studied in samples from the BGS boreholes at Stanford-le-Hope and Crystal Palace (Morton, 1982), and Borehole 404T and Borehole CTRL A2 (Hallsworth, 1993; see Table 7). The lower half of the formation contains a rich and diverse suite of heavy ­minerals, 42 per cent of which are unstable, dominated by an epidote-garnet-hornblende assemblage. This is thought to be the primary heavy mineral suite. A more stable suite dominated by zircon, rutile and tourmaline and a moderately stable garnet dominates the upper part of the formation. These variations are thought to be due to weathering dissolution of the more unstable minerals by meteoric acidic ground­water during post­depositional subaerial exposure.

Representative sections[edit]

There are few natural exposures. The basal few metres of the Thanet Sand can be seen in the top part of ­exposures in former Chalk quarries at Grays [609 792] and Swanscombe Western Quarry [606 728]. The top part of the formation is exposed in sand pits near Orsett [673 806]. Almost complete sequences through the entire formation are held by the BGS from Borehole CTRL A2 in south-east London and Jubilee Line Extension Borehole 404T (see Table 7 for details).

Biostratigraphy[edit]

Calcareous nannoplankton are the most important fossils for regional correlation (Aubry, 1986; Knox et al., 1994). Foraminifera (Curry, 1965) are not useful for stratigraphical purposes, but indicate that deposition occurred in a cool sea at a depth of less then 50 m. Molluscs are not well preserved; they occur sporadically throughout the formation, and include cold water genera (see Ward, 1978). Jolley (1992) has used palyno­floral assemblages for regional correlation, relating them to the sequence- stratigraphy scheme of Haq et al. (1987). He recognised that the bulk of the formation in this district was laid in the seventh (T7) of nine onlapping sequences that constitute the formation and its correlatives in the London Basin as a whole. It is possible that the younger T6 and T5 sequences may be present, particularly in the east but the evidence is inconclusive.

Environment of deposition[edit]

The deposits are interpreted as inner shelf to coastal in origin. In general, they are well sorted, indicating considerable winnowing and reworking prior to deposition. Grain shape suggests a rather juvenile origin with only minor recycling from older sedimentary formations. The heavy mineral assemblage indicates an input from a single source area rich in an epidote-garnet-amphibole assemblage, and most likely to be the Moine and Dalradian metasediments in Scottish highlands (Hallsworth, 1993).

LAMBETH GROUP[edit]

The formal term Lambeth Group has been adopted in recent years (Ellison et al., 1994) to replace the Woolwich and Reading Beds of earlier authors (see for example Whitaker, 1889; Hester, 1965). The group is divided into three formations and several informal lithological units (Table 8). The relationship between these informal units is most complex in the central part of the district, coincident with central and south-east London (Figure 12).

The Lambeth Group crops out in narrow tracts around the margins of the main mass of the Palaeogene, as outliers in the south-east of the district and as small inliers close to the margin, notably in the Lea valley [38 85]. It also occurs beneath superficial deposits principally between Camberwell [33 77] and Docklands [38 79]. No distinctive landforms are associated with the Lambeth Group, but it crops out on concavo-convex slopes or locally on the hill tops in some outliers. Minor springs occur at the top of clay-dominated units, particularly the Lower Shelly Clay.

The Lambeth Group is more than 20 m thick in the south-west, increasing to 30 m at Southwell [119 799] (Figure 13). In the south-east part of the main outcrop, and in the outliers east of Chiselhurst [46 70], the top part of the succession was removed by erosion before the deposition of the overlying Harwich Formation (see p.38). The amount of erosion is very variable (Dewey et al., 1924). Locally, for example at Plumstead Common [45 78], only the Upnor Formation is preserved, and in some of the Palaeogene outliers, for example at Kevingtown [484 675] and near Swanley [513 686], all the Lambeth Group has been removed.

The lithological variation of the Lambeth Group at outcrop is documented in considerable detail, particularly in the earlier editions of the Geological Survey memoirs (Whitaker, 1872, 1889), but also in accounts of field excursions made by the Geologists’ Association and in the publications of local societies such as the Croydon Natural History Society, and Essex Naturalist. They provide a wealth of information on sections in pits and railway cuttings that are no longer available for study. Since 1990, new information has become available in the form of complete cores of the Lambeth Group obtained during the course of site investigations in Central London. These, together with the interpretation of numerous other borehole records and the examination of contemporary open sections, have provided a regional appreciation of the variability of the group and its constituent formations, and is the basis for the informal division described below. The lithological variation in the group is illustrated by cross-sections on Figure 14, and in graphical logs at particular localities on Figure 15.

The best and most accessible exposure of the Lambeth Group in the London district is Charlton Sand Pit at Maryon Park [419 786] (Figure 15), now preserved as an SSSI (Daley and Balson, 1999). Other good sections, which are not permanent at the time of writing because they occur in or close to active quarries, are in the Orsett Tarmac Pit near Walton’s Hall, Orsett [673 803], Orsett Cock Pit [656 811] (Plate 1) (Ellison, 1979) and Swanscombe Eastern Quarry [605 730] (Figure 15).

Upnor Formation[edit]

The formation, present everywhere at the base of the Lambeth Group, is impractical to map in detail. It is difficult to identify the top of the formation by augering and in many places it is built on or is obscured by more recent sandy wash. The thickness is well documented in boreholes (Figure 16) and exposed sections, but in some borehole logs it is not possible to determine as the Upnor Formation cannot be separated from the Thanet Sand.

The Upnor Formation rests unconformably on Thanet Sand, overlapping onto Chalk north-west of a line from Northolt [13 84] to Borehamwood [20 95]. There may be a weak seep­age at the contact due to the higher silt and clay content in the Thanet Sand. In exposures the base is generally well defined, and burrows extend up to 2 m below the contact. A basal bed of rounded flint pebbles is usually present. In the east, relatively intense bioturbation has resulted in a gradational junction. In contrast with the Thanet Sand, the Upnor Formation contains slightly coarser grade sand (Plate 2) and the lower beds may be gritty and contain fragments of sub­angular flint (less than 1 mm across).

The formation consists of fine- to medium-grained sand with a variable proportion of glauconite, beds and stringers of well-rounded flint pebbles, and minor amounts of clay. At outcrop the sediments are pale grey-brown to orange-brown, speckled with dark green grains of fine to medium sand grade glauconite. At depth, the sediments are mainly dark grey and dark greenish grey. In the west, the entire formation is noticeably green caused by a relatively high glauconite content. In north, west and central areas of the district, the highest part and locally the entire thickness of the Upnor Formation is oxidised to a range of brown, orange, red and purple-brown colours, as a result of emergence and pedogenesis during deposition of the overlying Reading Formation. This period of pedogenesis also gave rise to the localised development of carbonate concretions, either in the form of hard irregular masses or powdery patches up to 0.5 m in diameter, and the development of a clay matrix derived by translocation from the over­lying deposits.

The sands may be completely bioturbated with no primary bedding, and perpendicular and subhorizontal burrows filled with sand of contrasting colour to the bioturbated matrix. Rare fragments of carbonaceous material occur also. A few impersistent seams of grey clay and angular clasts derived from such seams have been recorded within dominantly bioturbated beds. Other parts of the succession are well bedded and with horizontal planar lamination, ripple lamination, hummocky and planar cross-bedding, and clay drapes. Stringers of well-rounded flint pebbles occur on a few bedding surfaces and there are beds of pebbly sand, mostly less than 0.3 m thick. Thin seams of grey clay, angular clay clasts and rounded balls of clay are also present. Ophiomorpha and Macaronichnus burrows are typical in these beds (Plate 3). Clay-dominated units, up to 0.3 m thick, contain relatively small amounts of sand, arranged in flaser ­lamination and with lenticular cross-­lamination. These strata are well exposed in quarries at Orsett Cock [657 811] and Orsett Tarmac Quarry [673 805].

The flint pebbles that occur throughout the formation are generally less than 30 mm in diameter but may ­exceptionally reach 200 mm, with a black to dark purple cortex and pale grey interior. They are typically well rounded, elongate, spheroidal to flattened spheroidal and in many cases with a slight concavity, presumed to be a result of pressure solution. Many have crescent-shaped percussion marks.

Pebble-dominated units occur principally at the base and top of the formation. In borehole cores, these pebble beds are almost invariably disturbed or only partly recovered, but information on bedding and other details have been recorded from exposure. The pebble bed at the base of the formation is impersistent and up to 1 m thick. A local development of pebble beds occurs in the basal part of the formation at Orsett in south Essex where the pebbles are arranged in large bedforms, in total about 9 m thick, and dipping at 21° to the east (Plate 1). Individual beds fine upwards and many are clast supported with imbrication. These pebble beds are well exposed at Orsett Cock Quarry [657 810] (Ellison, 1979). At the top of the formation in central and south-east London, there is a persistent pebble bed up to 3 m thick (see Figures 12; 15). Pedogenesis and the precipitation of calcrete have altered the matrix in places.

The oyster Ostrea bellovacina is the most common mollusc as in most places aragonite-shelled molluscs are not preserved. Sharks’ teeth also occur sporadically.

Woolwich Formation[edit]

The Woolwich Formation rests on the Lower Mottled Clay of the Reading Formation (Figure 12).

Lower Shelly Clay[edit]

This unit, characterised at outcrop by finely comminuted shell debris in a clay soil, is the most easily distinguished of the informal units of the Lambeth Group. It crops out principally in south-east London (Figure 17). In general, the formation thickens from central London towards the south-east, reaching a maximum of 6 m.

The Lower Shelly Clay rests disconformably on the Lower Mottled Clay of the Reading Formation. The base is sharp with burrows up to 10 mm in diameter extending to a depth of 1 m into the underlying strata. The top of the unit is generally a sharp or transitional with the Laminated beds or the Upper Mottled Clay.

The dominant lithology of this unit is dark grey to black clay that contains abundant shells. In east London, there is an increase of medium grade sand in the matrix. Some beds, up to 1 m thick, consist almost entirely of shells forming a weakly cemented coquina in places. One of these beds (0.5 m thick) was proved in a borehole near Stratford. The basal few centimetres of the unit is relatively sandy and commonly contains oyster shells. A bed dominated by oysters, encrusted with bryozoa and cemented in places, occurs locally about 1 to 2 m above the base of the Lower Shelly Clay (Dewey and Bromehead, 1921; Tracey, 1986). These shell-dominated beds indicate that sediment input was low, thus allowing the development of shell banks, and they may represent a maximum flooding surface. A few beds of brownish grey clay occur sporadically throughout, particularly in the higher parts of the unit. These are slightly cemented with siderite. Finely comminuted carbonaceous debris occurs in places; some of it is ­pyritised.

Lignite (usually less than 0.3 m thick) is commonly seen at the base of the Lower Shelly Clay in the south-eastern part of the outcrop (Figure 15). It consists of soft, brownish black, organic mud with small lignitic wood clasts. At Shorne [678 697], the lignite is up to 2 m thick and displays a cleat (closely spaced joints) similar to a sub-bituminous coal. It is interbedded with pale grey, leached, medium-grained sand and pale grey clay with lignitic wood fragments and small listric fractures, similar to seatearth.

Dewey and Bromehead (1921, p.20) recorded a ‘freshwater bed’ of limited distribution, within the middle of the ‘shell beds’. This bed is almost certainly the ‘Paludina Limestone’, a marker horizon that has been used to show that the Upper Shelly Clay probably rests directly on the Lower Shelly Clay in south-east London in the vicinity of Petts Wood and St Mary Cray [45 68] (Figure 14; Whitaker, 1872, p.116), and prob­ably also at Shorne [678 697] where there is a particularly thick development of shell beds (see Figure 15).

Characteristically few species occur in this unit; they are mainly oysters (Ostrea tenera), corbiculid bivalves such as Corbicula cordata and the cerithiid gastropods Brotia melanoides and Tympanotonos funatus.

Laminated beds[edit]

This unit generally rests conformably on the Lower Shelly Clay and has a similar distribution (Figure 18). It is 5 m thick south of Stratford. The base may be sharp, a rapid gradation up from the Lower Shelly Clay or, locally, interfingering with Lower Shelly Clay. In some of the eastern part of its occurrence it passes up into and locally on interfingers with the Upper Mottled Clay (Figure 14; Crystal Palace Borehole, Figure 15). A second unit of Laminated beds occurs higher in the succession around Lewisham where it was formerly known as ‘Striped loams’ (Dewey et al., 1924) (Figure 15). The stratigraphical relationships of the laminated beds are uncertain but they probably have an erosive base, cutting down through the Upper Shelly Clay.

The Laminated beds consist of thinly interbedded fine- to medium-grained sand, silt and clay with scattered intact bivalve shells. Beds are generally less than 50 mm thick and typically finely laminated. Sedimentary features include lenticular bedding, ripple lamination, burrows and some bioturbated, structureless beds. Localised bodies of sand are generally up to 1.4 m thick, but locally up to 5 m (for example Borehole 404T, Figure 15). They occur particularly in south-east London around Lambeth and Bermonsey and may have originated as channel fills. More extensive bodies of sands are present between Docklands and Stratford. Typically the sand is pale olive to pale brown, medium grained, well sorted and cross-laminated, with some clay drapes and scattered bivalves. Thin beds of colour mottled clay and silt (Upper Mottled Clay) occur within the Laminated beds between Docklands and Stratford (see Figure 14).

At Abbey Wood [484 787], shelly medium-grained sands underlying the Harwich Formation are significant for their well-preserved fauna, which includes mammals. The strata are included in the Laminated beds, although their age and precise stratigraphical relationship are uncertain (Hooker 1991).

Clay beds with leaf impressions have been recorded from the ‘Striped loams’ at Loam Pit Hill, Lewisham [375 761] (Whitaker, 1866).

Upper Shelly Clay[edit]

The main occurrence of the unit is in south London (Figure 19). South-east and north-east of this, the Upper Shelly Clay is proved only sporadically in boreholes and is inferred to be preserved in shallow depressions below an erosion surface at the base of the Harwich Formation. The unit is up to about 3 m thick.

The base of the unit is sharp and it rests disconformably on the Upper Mottled Clay (for example Borehole A4A, Figure 15), or where it rests on Laminated beds the contact may be a rapid gradation. It is likely that beds equivalent to the Upper Shelly Clay are present farther south-east than is shown on Figure 19 but, in the absence of the intervening Upper Mottled Clay unit, they cannot be distinguished from the Lower Shelly Clay in sections or borehole cores. One exception to this is in the Crystal Palace Borehole where the base of the Upper Shelly Clay is taken at a thin lignite bed (see Figure 15). In some borehole records, the Upper Shelly Clay was formerly interpreted as the basal beds of the London Clay Formation.

This unit consists mainly of grey shelly clay with thinly interbedded grey-brown silt and very fine-grained sand with scattered glauconite grains, passing south-eastwards to mainly sand (Figure 19). Bioturbated beds, sand-filled burrows and clay rip-up clasts (less than 5 mm in diameter) are characteristic, and locally there is a weakly cemented shell bed (up to 0.43 m thick) containing Ostrea. Between Bermondsey and Lewisham, a more or less continuous bed of grey limestone with an earthy texture is known as the Paludina Limestone (see Figures 14; 15). The bed is generally 0.1 to 0.3 m thick, up to a maximum of 1.89 m, and contains unbroken and comminuted gastropods Hydrobia, Planorbis and Viviparus, which indicate deposition in a freshwater lake. A thin bed of lignite occurs locally at the base of the unit at Crystal Palace (Figure 15).

Most of the Upper Shelly Clay contains disarticulated bivalves of more marine-tolerant species and a generally greater diversity of fauna than in the Lower Shelly Clay.

Reading Formation[edit]

The Reading Formation rests on the Upnor Formation in the centre of the district and passes laterally into the Woolwich Formation in the central and south-eastern outcrops of the Lambeth Group. In the central part of the London district the formation is divided into two leaves, the Upper and Lower Mottled Clay, separated by the Woolwich Formation (Figures 12; 14). The Reading Formation as a whole is present in most of the region, but is thin or absent in the north-east. The Lower Mottled Clay persists in the entire area of the Reading Formation. The Upper Mottled Clay occurs only between Walthamstow and Merton (Figure 20) because of ­nondeposition or erosion prior to the deposition of the ­succeeding Harwich Formation.

The formation reaches a maximum of about 20 m in the south-west of the district, thinning progressively ­eastwards, where it passes laterally into and interfingers with the Woolwich Formation (Figure 12).

The boundary of the Lower Mottled Clay with the underlying Upnor Formation is usually diffuse and difficult to place precisely because of clay translocation and colour mottling caused by pedogenetic processes that affected the top of the Upnor Formation. The base of the Upper Mottled Clay has, in many places, a similarly vague contact with the Laminated beds, except around Stratford where the two units interfinger.

The bulk of the formation consists of unbedded, colour mottled, silty clay and clay. This characteristic lithology was formerly called the ‘Reading Beds’ or ‘plastic clay’. Colours include pale brown and pale grey-blue, dark brown, pale green, red-brown and crimson, depending on the oxidation state of the sediments. The clays contain numerous fissures, many of them listric, which give rise to a blocky texture. Thin, black, carbonaceous clays are recorded locally in the west of the district in the middle of the sequence (see Figure 14). Beds of colour-mottled silt and sand constitute up to 50 per cent of the unit, particularly in the east. The colour is dominantly of brown hues, red hues being less prevalent than in the clays. These beds are thinly laminated in places with small burrows and root traces, and minor brecciation caused by soft sediment deformation. Beds of well-sorted sand, mainly in the west of the region, are recorded in borehole logs but are not known in detail. Evidence from adjacent areas to the west suggests these occupy discrete channels.

The Lower Mottled Clay typically shows purple-red hues. The top part of the unit contains irregular-shaped, hard, splintery and soft, powdery carbonate nodules up to 0.5 m in diameter. East of Stratford, the principal lithology is turquoise to dark green and brown mottled, structureless slightly clayey sand with minor amounts of irregularly iron cemented and partially cemented ­calcareous clayey sands; the beds as a whole become increasingly sandy in an easterly direction. (Figures 12; 20).

The Upper Mottled Clay is identified principally in cores in central and east London; it consists largely of mottled clay, silty clay and silt with colours similar to those of the Lower Mottled Clay, but the purple hues are absent.

Mineralogy of the Lambeth Group[edit]

The non-clay minerals of the Upnor Formation are domi­­nated by quartz with variable amounts of glauconite, some alkali feldspar, chert, minor mica, traces of collophane and calcite. The quartz grains are typically subangular and well sorted (Plate 3). At outcrop glauconite grains in some places are oxidised to yellow-brown goethite and ilmenite. The clay mineral assemblage is generally dominated by smectite with subordinate illite and mixed layer illite-smectite; kaolinite is less than 10 per cent (Figure 21).

The Upnor Formation is characterised by an abundance of stable heavy mineral grains dominated by zircon, along with rutile and tourmaline. Morton (1982) inferred that they are derived from the south, probably Armorica and/or the Ardennes–Rhenish massifs. Hallsworth (1993) found compositionally diverse pyrope garnets in the Upnor Formation, chemically different from those in the Thanet Formation. They are derived either from a source area with heterogeneous rocks or by recycling of detritial minerals from several source areas.

The clay minerals of the Lower Mottled Clay (Figure 21) are dominated by well crystallised smectite that may be derived, at least in part, from the alteration of volcanic ash. The Upper Mottled Clay and clays in the Woolwich Formation are characterised by a mixed clay assemblage, with less smectite and more illite, kaolinite and chlorite. Evidence from scanning electron microscopy shows that much of the kaolinite is very fine grained and intimately mixed with other clay minerals, indicating a detrital origin. Some of the kaolinite is arranged in delicate ‘booklets’ that would not survive agitation during erosion and deposition. Therefore, it is assumed to be authigenic in origin, produced by the weathering of smectite and illite as a consequence of pedogenesis.

The characteristic colour variability of the Woolwich and Reading formations is due to the different states of oxidation and hydration of the iron minerals that con­stitute only a few per cent by volume of the sediment. In unoxidised grey and black deposits, pyrite is usually the main mineral. Jarosite, a yellow alteration mineral associated with pyrite, occurs in lignitic beds. In the Reading Formation, the most common form of oxidised iron mineral belongs to the goethite species and imparts yellow-brown and brown colours, and is commonly referred to as limonite. Red colours are due to extremely small quantities of hematite, which developed when the sediment was subjected to long periods of subaerial exposure that resulted in drying and dehydration. Earthy hematite nodules less than 5 mm in diameter are also present in places.

The heavy mineral composition of the Woolwich and Reading formations is similar (Hallsworth, 1993), and is dominated by a zircon-rutile-tourmaline suite. There is a low but variable proportion of the less stable minerals epidote, apatite and spessartite garnet. The variablity of the epidote and apatite was probably caused by dissolution during local emergence and pedogenesis (Morton, 1982). The provenance of the heavy minerals is equivocal. Morton (1982) regarded it as Scottish, similar to the Thanet Sand, suggesting that the heavy minerals were carried by currents from the north and reworked in an estuarine environment. However, the spessartite garnet suggests derivation from a different source, dominated by amphibolite facies metamorphic rocks. Armorica is one such area, although the metamorphic grade is lower than is normally associated with spessartite garnet. The Cornubian granites are another potential source, although they should yield a heavy mineral suite containing a higher proportion tourmaline than is found in the Woolwich and Reading formations. Spessartite garnet occurs also in Coal Measures (Westphalian) sandstones, but there they are associated with other garnets of different composition that do not occur in the Woolwich and Reading formations.

Palaeontology of the Lambeth Group[edit]

BGS and the Natural History Museum hold large collections of macrofossils from the Lambeth Group, brought together mainly before the end of the 19th century. Rundle (1970), Hooker (1974) and Tracey (1986), among others, have documented more recent collections; Collinson and Hooker (1987) have reviewed the flora. Molluscs in particular are valuable as indicators of the environment of deposition, but there is no established biozonation of the Lambeth Group as a whole. Calcareous nannofossils in the lower part of the Upnor Formation in central London (Ellison et al., 1996) have been interpreted as nannoplankton (NP) zone NP9, enabling correlation with sequences elsewhere in north-west Europe (Aubry, 1986; see Table 6). Another potential tool for biostratigraphical correlation is the use of marine and terrestrial palynomorphs (acritarchs, dinoflagellate cysts, pollen and spores). A mammal fauna discovered in shelly sands at Abbey Wood probably equivalent to the Upper Shelly Clay, has been used for correlation with other European sites (Hooker, 1996).

Environment of deposition of the Lambeth Group[edit]

The regional distribution of deposits of the Lambeth Group was recognised as cyclic in nature by Stamp (1921), an idea developed further by Hester (1965) and Ellison (1983). Four depositional sequences separated by unconformities are now recognised (Knox, 1996; Table 6). The sediments as a whole were laid down in a coastal or possibly estuarine setting (Figure 22) in which small fluctuations in sea level led to marked changes in ­environment.

Following a period of regression and weathering of the top of the Thanet Sand, the lowest beds of the Upnor Formation were laid down in transgressive littoral to sublittoral marine, tidal conditions. The abundance of glauconite, most noticeable in the west of the district, indicates periods of sediment starvation, presumably in areas remote from active currents. The Upnor Formation as a whole is interpreted as highstand deposits in which marine flooding events are marked by pulses of glauconite deposition, winnowing and low sediment input (Ellison et al., 1996).

Deposition of the upper part of the Upnor Formation followed a lowering of sea level that may have led to the removal of some of the earlier deposits. Contemporary emergence is indicated by the presence of local silcretes (Kerr, 1955) and clasts of silica-cemented conglomerate in pebble beds at the top of the formation. These clasts are typical of the ‘Hertfordshire Puddingstone’ that, in association with silica cemented sandstones (sarsens), were widespread to the north and west of the district (see Potter, 1998 for a review). This period of emergence marked the beginning of deposition of the Reading Formation on an alluvial floodplain that was liable to intermittent floods and water table fluctuation. Periodic emergence led to oxidation, pedogenesis and the development of the characteristic red coloration, but the instability of this environment precluded the development of extensive colonisation by vegetation. Estuarine and fresh water palynomorphs in the sandier parts of the Lower Mottled Clay in the east are evidence of intermittent encroachment by the sea onto the alluvial floodplain. A temporary rise in sea level led to the establishment of lagoonal and estuarine conditions and the deposition of the Lower Shelly Clay and Laminated beds in the central and eastern parts of the district. Sand bodies in these units contain a brackish water palynomorph assemblage consistent with deposition in estuarine tidal channels. This sequence culminated with a return to continental conditions and deposition of the Upper Mottled Clay. A second rise in sea level and renewed flooding resulted in the deposition of brackish lagoonal and estuarine sediments (Upper Shelly Clay and ‘striped loams’). This period of deposition was ­terminated by uplift and erosion that removed much of the Lambeth Group sediments in the north and east of the district.

HARWICH FORMATION[edit]

This term was introduced by Ellison et al. (1994) to include all sediments between the Lambeth Group and the London Clay Formation. They were formerly differentiated by Prestwich (1854) as the ‘Basement-bed of the London Clay’, and subsequently divided by Whitaker (1866) into the Basement-bed sensu stricto, the Oldhaven Beds and the Blackheath Beds. In the London district, strata formerly mapped as Blackheath Beds are now included in the Harwich Formation.

The Harwich Formation occurs throughout the region (Figure 23), reaching a maximum thickness of 10 to 12 m around Orpington and Chiselhurst. South and east of London, where the deposits were formerly mapped as Blackheath Beds, there are numerous descriptions of former exposures recording very variable thicknesses over quite short distances; this probably indicates an irregular base. Elsewhere in the district, most of the information on the Harwich Formation is from borehole records, which indicate that the Harwich Formation is less than 4 m thick, strata formerly referred to the Basement Bed of the London Clay. In the north-east, an incomplete thickness of 6.88 m was proved in the Stock Borehole [7054 0045], and in the south-east the Stanford-le-Hope Borehole [6965 8241] proved 4 m.

The base is sharply defined, and forms an erosive contact on the Lambeth Group. Locally, in outliers at Kevingtown [485 675] and Swanley [515 686], the Harwich Formation oversteps onto the Thanet Sand Formation.

Glauconitic fine-grained sand and pebble beds of rounded black flints are the principal lithologies with, in places, common disarticulated and broken shells of marine to brackish fauna (see Dewey et al., 1924). The proportion of pebbles varies considerably. Calcareous, ferruginous and siliceous cements occur locally in beds and masses up to several metres thick (for details see Dewey and Bromehead, 1921; Dewey et al., 1924), particularly at outcrop in the south-east of the district. In the north-east the Harwich Formation is dominated by relatively fine-grained sand and is generally less pebbly. The succession is known in detail only in Stock Borehole [7054 0045] where grey-green silty fine-grained sand with scattered broken shell fragments and stringers of black flint pebbles are recorded. At outcrop in south Essex, the Harwich Formation consists of sand and pebbly sand that is green-grey in colour, weathering to pale yellow-brown, highly glauconitic and fine to medium-grained. Calcareous mollusc fossils and scattered sharks’ teeth are typical although the shells are decalcified in places.

In areas where pebble beds dominate the sequence they consist of a series of superimposed channels (Tracey, 1986; Figure 24), with large foresets composed almost entirely of flint pebbles, imbricated in places, and rare pebbles of siliceous sandstone (similar to sarsen stone). The pebble beds include clasts up to 150 mm in diameter but generally less than 20 mm.

Harwich Formation pebble beds are best exposed in former quarries, now Sites of Special Scientific Interest (SSSI), at Charlton [419 786] and Elmstead Rock Pit, Chiselhurst [423 706], and in a small pit at the Inn on the Lake [675 699] near Gravesend.

Geology of London - contents[edit]