London - Concealed strata
Cambrian and Tremadoc clastic deposits, proved in boreholes in Buckinghamshire and Hertfordshire (Molyneux, 1991), probably extend beneath the north-west of the district, where they are overstepped by Upper Devonian strata (Figure 2). Rocks of Ordovician age, such as the Caradoc strata proved in Bobbing Borehole, 19 km to the east, and Arenig strata proved by Strat-A1 Borehole, 18 km to the south-west, are probably absent in the district (Smith, 1987).
Nonmarine Silurian strata of Prˇídolí age were proved in the Streatham Common Borehole (Tables 1; 2), and in the north-west turbidites of Silurian age may be present, similar to those proved in deep boreholes in East Anglia and north-east Kent (Molyneux, 1991; Sumbler, 1996). However, most of the Palaeozoic strata known in the district are nonmarine sandstone of Devonian age; the thickest proved succession is 509 m in Willesden No. 1 Borehole. Lower Devonian (Emsian) sedimentary rocks were proved in Beckton Gasworks Borehole, and other boreholes prove Middle to Upper Devonian rocks of Givetian or Frasnian age (Table 2).
Carboniferous Limestone is proved in the Warlingham Borehole, south of the Addington Thrust (Figure 2) identified as a Variscan structure (Keary and Rabae, 1996).
Strata of Triassic age are probably not present in the district, but thin sequences are recorded in the Weald Basin to the south, including conglomeratic limestone and red and grey mottled mudstone proved in the Warlingham Borehole that was tentatively regarded as Triassic– Jurassic in age (Worssam and Ivimey-Cook, 1971).
Movement on the major growth faults that bounded the Weald Basin governed the distribution and thickness of strata of Jurassic age. They occur only in the south of the district (Table 3; Figure 3) and consist of mudstone with some interbedded limestone and sandstone. The deposits thicken appreciably south of grid line 70; 12 m were proved in the Streatham Common Borehole compared with 1051.6 m in Warlingham Borehole, 15 km to the south (Table 3). This change in thickness is possibly due to syndepositional growth faulting. A similar pattern is illustrated by seismic reflection data to the south of the region (Whittaker, 1985). In general terms, successive Jurassic formations overlap farther onto the London Platform. Some of the younger formations may have originally extended right across it and were removed by contemporary erosion during periods of low sea level and uplift in late Jurassic or early Cretaceous times. The main variation in this pattern is due to syndepositional movement on faults bounding a small graben beneath the eastern part of the Thames estuary (Figure 3) where Oxford Clay is preserved but younger Jurassic strata have been removed by erosion (Owen, 1971).
Earth movements during the latest part of the Jurassic period and the early part of the Cretaceous resulted in the elevation of the London Platform high above sea level, and further subsidence of the Weald Basin to the south. Jurassic strata were eroded from the platform, exposing the Palaeozoic basement rocks, and accumulated in the basin to form the Wealden Group (Table 4). Mud and sand were deposited in an environment of swamp with lakes and lagoons of varying salinity and crossed by meandering river channels. The Wealden Group occurs along the southern boundary of the region, and is proved in boreholes at Addington, Hartley Bottom and Russell Hill. The succession, largely of interbedded mudstone and sandstone, is 256.6 m thick in the Warlingham Borehole (Worssam et al., 1971), considerably thinner than in the central Weald Basin, although several faults noted in the borehole core may indicate that part of the succession has been faulted out.
Rising sea level in Aptian times flooded the Weald Basin and the southern margins of the London Platform, so that the Lower Greensand Group overlaps the Wealden Group to rest on Jurassic strata (sheets 270, 271). The Lower Greensand occurs in the south of the district, thickening south-west into the Weald Basin. It also occurs in a graben (Owen, 1971) beneath the Thames estuary in the east of the district. At Warlingham the Lower Greensand Group is 94 m thick and includes strata correlated with all formations recognised in the thicker successions in the Weald Basin. At the base the Atherfield Clay Formation is about 7.5 m thick; it passes up into the Hythe Formation (25.5 m thick) of Aptian age, which consists of calcareous greenish grey sand, sandstone and thin sandy mudstone with chert at the top. The succeeding Sandgate Formation rests disconformably on the Hythe Formation marking a marine transgression in the nutfieldiensis Zone. The Sandgate Formation (1.3 m thick) comprises fine-grained glauconitic sand that is characteristically more argillaceous elsewhere in the Weald. The succeeding Folkestone Formation (59.7 m thick) comprises coarse-grained sand of jacobi Zone age. This is interpreted as sand-wave deposits like those exposed in the Weald between Reigate and Sevenoaks. Although only small amounts of coarse-grained sand were recovered from the Warlingham Borehole, the gamma-ray logs suggest a relatively uniform sand lithology throughout.
North of Warlingham, (for example in the Richmond, Addington, Russell Hill and Carshalton boreholes), the Lower Greensand is thinner, and the constituent formations cannot be identified with certainty. The sequences are dominated by sand that is variably glauconitic and calcareous with bioturbation in places. These lithologies resemble aspects of the Atherfield, Hythe and Sandgate formations rather than the Folkestone Formation. For example in the Richmond Borehole, 3.2 m of hard, grey, calcareous, glauconitic, shelly sandstone with granules and pebbles of chert may correlate with the Hythe Formation or possibly the Bargate Formation that occurs at the base of the Sandgate Formation in northern parts of the Weald.
The Gault Formation was deposited following a marked deepening of the sea in mid-Albian times. It rests disconformably on the Lower Greensand, overstepping northwards onto Jurassic and then Palaeozoic rocks and extending across the entire London Platform, the earliest Cretaceous formation to do so. The Gault Formation is 50 to 70 m thick in most of the district, but ranges from 40 to 94 m, although there is no obvious regional trend (Figure 4). The formation consists mainly of grey mudstone, with variable amounts of silt, in which primary bedding structures are not clearly preserved because of bioturbation. Small, greyish buff concretions or nodules rich in calcium phosphate may be developed around fossils or burrows and seams of bluish black, commonly phosphatic, pebbles and reworked fossils mark non-sequences; these represent debris winnowed from the sediment during periods of erosive current action.
A distinction is made in some boreholes between dark grey mudstone of the Lower Gault, which is of mid-Albian age, and paler, more calcareous mudstone of Upper Gault, of late Albian age (Figure 4). The Lower Gault (less than 10 m thick) is known in detail only in boreholes in the Thames estuary (Owen, 1971) and is absent in the north where it was removed by erosion in late Albian times. In the Thames estuary boreholes, the basal 2.5 m or so consist of glauconitic grey mudstone, gritty in places, with a pebbly and indurated basal bed, and black phosphatic nodules throughout. They are succeeded by about 6 m of medium to dark grey and fawn grey mudstone, which is burrowed and shelly in places and contains seams of phosphatic nodules. A short period of tectonic activity during the earliest part of the late Albian gave rise to slight uplift of the London Platform and movement on the faults that trend east–west across the district (Figure 3). This led to the removal of the Lower Gault from the northern part of the district (Figure 4) The succeeding Upper Gault is between 48 and 52 m thick. It consists of mid to pale grey shelly mudstone, glauconitic in the top 2.5 m, and with a bed (2.44 m thick) containing ‘small black phosphatic nodules’ in the middle of the succession (Owen, 1971).
The Upper Greensand Formation consists of glauconitic, calcareous fine-grained sandstone and siltstone that was deposited during the late Albian. It is present in the central part of the district, but elsewhere it passes laterally into the Upper Gault or is absent having been removed by erosion prior to deposition of the Chalk Group (Figure 4). The maximum thickness is 16.7 m in the Warlingham Borehole. In some borehole records the Glauconitic Marl, which marks the base of the succeeding Chalk Group, may have been mistakenly classified as Upper Greensand.