OR/15/042 Geological setting

From Earthwise
Jump to navigation Jump to search
MAURICE, L, FARRANT, A R, BUTCHER, A AND ATKINSON, T C. 2015. Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015. British Geological Survey Internal Report, OR/15/042.

The geology of the Pang-Lamborn catchments is relatively simple. Most of the area is underlain by gently dipping Upper Cretaceous Chalk which crops out across much of southern and eastern England, forming the quintessential English rolling ‘downland’ landscape (Figure 3). In the Pang-Lambourn catchments the Chalk Group dips gently to the south-southeast at 1-2°. To the north, it forms a distinctive escarpment reaching a maximum elevation of around 280 m, overlooking the clay lowlands of the Thames valley. To the south the Chalk is unconformably overlain by Palaeogene sands and clays.

The Chalk can be divided into two subgroups: the Grey Chalk and the White Chalk. The former consist of two constituent formations (Table 1), whilst the White Chalk is divided into five formations. The latter are generally defined by the flint content and the presence or absence of thin marl seams. Most of the Pang and Lambourn catchments lie on the dip slope of the Chalk, mostly on the Seaford Chalk Formation, although locally a few metres of the Newhaven chalk is preserved beneath the Paleogene unconformity.

Figure 3 Topography of the Pang-Lambourn catchment. Topography is NEXTMap Britain elevation data from Intermap Technologies. Elevation in metres.
Table 1 Summary of lithostratigraphy of the Chalk of the Pang Lambourn catchment
Formation Typical composition Thickness (m)
White Chalk Subgroup
Newhaven Chalk Chalk with marl seams, some flint 5–10 m
Seaford Chalk Chalk with few marl seams, much flint 50 to 80
Lewes Nodular Chalk Nodular, gritty chalk, marls, much flint 35 to 80
New Pit Chalk Chalk with marl seams, sparse flint 35 to 50
Holywell Nodular Chalk Nodular, gritty chalk, with marl seams 25 to 35
Grey Chalk Subgroup
Zig Zag Chalk Grey chalk, some marly, some limestones 35 to 50
West Melbury Marly Chalk Grey marly chalks, some hard limestones 15 to 30

In the south of the catchments, outliers of the overlying Palaeogene strata are preserved, forming low hills and ridges. These outliers are composed of mottled clay and sands of the Reading Formation (part of the Lambeth Group), overlain by the London Clay Formation, locally capped by river terrace gravels. These generate surface streams which sink into the underlying Chalk, and resurge at large springs in the valley.

Figure 4 North-south geological cross section across the Chalk outcrop in the Yattendon- Blue Pool area.