Rory N. Mortimore

Lithostratigraphy for mapping the Chalk of southern England

Recent British Geological Survey (BGS) mapping in Dorset, Wiltshire, Hampshire, Sussex and Kent has shown that the Lower, Middle and Upper Chalk formations of the Chalk Group, can be subdivided into mappable units of member status. The members are recognizable by their lithology and topographic expression, and can be followed readily across open country. Some members have distinctive wireline log signatures. These members will be shown on future BGS maps. In the above areas, the Lower Chalk (Formation), with its traditional boundaries, is retained and divided into two members, a lower, West Melbury Marly Chalk , comprising, in part, the Glauconitic Marl, most of the Chalk Marl up to and including the Tenuis Limestone; and a higher, Zig Zag Chalk , comprising the top of the Chalk Marl, the Grey Chalk and the Plenus Marls. These members can be readily traced from Dorset and Wiltshire into Sussex and Kent. In the Chiltems, the Glauconitic Marl (s.s.) forms a third, basal, mappable member to the Lower Chalk. The Middle Chalk (Formation) consists of two members: the redefined Holywell Nodular Chalk , comprising the Melbourn Rock and overlying Mytiloides shell-detrital chalks; and the New Pit Chalk , a massively bedded chalk with conspicuous marl seams. The traditional concept defining the base of the Upper Chalk (Formation) at the entry of common flint is too variable and unreliable for mapping. Instead, the base of the Upper Chalk is coincident with a revised base to the Lewes Nodular Chalk , defined by the entry of hard nodular chalk in basinal successions, by the base of the Chalk Rock in condensed marginal successions and the base of the Spurious Chalk Rock in south Dorset and the Isle of Wight. The Upper Chalk is subdivided into a basic framework of 8 members. The coarse-grained, rough Lewes Nodular Chalk is succeeded by very finegrained, smooth chalks, marl-free except at the base, with conspicuous bands of large flints, the Seaford Chalk . The overlying Newhaven Chalk is characterized by firm, marly chalk with numerous marl seams and regular, but fewer, bands of flint. The marl seams locally thin or disappear over tectonic highs. In central Dorset, the base of Newhaven Chalk cannot be mapped, leaving an undivided Seaford and Newhaven Chalk , later renamed Blandford Chalk ; the latter name is herein discontinued. In parts of east Kent (Thanet), the Seaford Chalk is overlain by very soft, nearly flint-free chalk, the Margate Chalk . The bases of the Tarrant Chalk and Spetisbury Chalk are defined by the crests of prominent scarp features Ftl and Ft2 in central Dorset. The origin of the features, due to lack of exposure of the feature-forming beds, remains uncertain. These members comprise uniform, firm, white, flinty chalks and collectively, e.g. where not mapped separately, constitute the Culver Chalk of the existing classification. The lower limit of the Portsdown Chalk , with numerous marl seams, is taken at a pronounced negative feature break in central Dorset (base of Ft3 scarp), that approximates to the base of the member as originally defined. The terminal member in Dorset and the Isle of Wight ( Studland Chalk ), comprising soft, white, marl-free chalk with very large flints, is mapped with the Portsdown Chalk as it is not readily separable from the latter unit.

Upper Cretaceous tectonic phases and end Cretaceous inversion in the Chalk of the Anglo-Paris Basin

Three intra-Upper Cretaceous tectonic phases, Stilles Ilsede (Late Turonian-Early Coniacian), Wernigerode (Late Santonian-Early Campanian) and Riedels Peine (latest Lower Campanian) are investigated in the Anglo-Paris Basin. Criteria for recognizing these events include field evidence of slumping, allochthonous chalks; lateral changes in thickness and lithology; seismic evidence for slump horizons; and lacunae closely related to tectonic axes. Further evidence from seismic sections indicates large-scale channel development during these phases but the spatial relationship with tectonic lineaments is more difficult to determine. Each phase rejuvenates the tectonic topography which is subsequently buried by post-tectonic facies changes, for example from Lewes to Seaford Chalk following the Ilsede tectonic phase and Culver to Portsdown Chalk, following the Peine tectonic phase. The tectonic phases are recognized along local tectonic lineaments in contrast to more widespread sea-level fluctuations. New information on the end Cretaceous inversion in the London and Anglo-Paris basins supports the recognition of a London axis of uplift where the chalk at subcrop was more deeply eroded than at outcrop. Maximum inversion in the Weald-Wessex area occurred in East Sussex along the southern margin of the Weald where the thickest chalks had previously been deposited.

Coastal chalk cliff instability in NW France: role of lithology, fracture pattern and rainfall

Abstract Coastal retreat has been studied along 120km of French Channel chalk coast from Upper Normandy to Picardy. During the investigation period, 1998–2001, 55 significant collapses were recorded. Of these 5.5% were very large-scale, 34.5% large-scale, 34.5% medium-scale and 25.5% small-scale collapses. Observations indicate that the larger the collapse size the greater the coastal cliff retreat. Four types of cliff failure were observed: (1) vertical failures in homogeneous chalk units; (2) sliding failures where two superimposed chalk units were present; (3) wedge and plane failures mainly recognized in the UK in formations with stratabound fractures; (4) complex failures in cliffs with more than one style of fracturing. Rainfall in relation to the timing of cliff collapse indicates two periods that trigger a collapse. The first occurs about one month after heavy rainfall within poorly fractured chalk and the second occurs when a dry period is interrupted by sharp rainfall in cliffs with major karst features (pipes etc). Medium to small-scale cliff collapses were, in some cases, caused by marine erosion at the base of the cliff creating a notch. A key factor controlling the type of collapse is the lithostratigraphic unit, while the extent of the collapse scar may be controlled by fracture type.

Coastal cliff geohazards in weak rock: the UK Chalk cliffs of Sussex

Abstract Geohazards related to chalk coastal cliffs from Eastbourne to Brighton, Sussex are described. An eight-fold hazard classification is introduced that recognizes the influence of chalk lithology, overlying sediments and weathering processes on location, magnitude and frequency of cliff collapses. Parts of the coast are characterized by cliffs of predominantly a single chalk formation (e.g. Seven Sisters) and other sections are more complex containing several Chalk formations (Beachy Head). Rock properties (intact dry density or porosity) and mass structure vary with each formation and control cliff failure mechanisms and scales of failures. The Holywell Nodular Chalk, New Pit Chalk and Newhaven Chalk formations are characterized by steeply inclined conjugate sets of joints which lead to predominantly plane and wedge failures. However, the dihedral angle of the shears, the fracture roughness and fill is different in each of these formations leading to different rock mass shear strengths. In contrast the Seaford and Culver Chalk formations are characterized by low-density chalks with predominantly clean, vertical joint sets, more closely spaced than in the other formations. Cliff failure types range from simple joint controlled conventional plane and wedge failures to complex cliff collapses and major rock falls (partial flow-slides) involving material failure as well as interaction with discontinuities. Other hazards, related to sediments capping the Chalk cliffs, include mud-slides and sandstone collapses at Newhaven, and progressive failure of Quaternary Head and other valley-fill deposits. Weathering, including the concentration of groundwater flow down dissolution pipes and primary discontinuities, is a major factor on rate and location of cliff collapses. A particular feature of the Chalk cliffs is the influence of folding on cliff stability, especially at Beachy Head, Seaford Head and Newhaven. A new classification for cliff collapses and a new scale of magnitude for collapses are introduced and used to identify, semi-quantify and map the different hazards. Climate (and climate change) and marine erosion affect the rate of development of cliff collapse and cliff-line retreat. This was particularly evident during the wet winters of 1999–2000–2001 when the first major collapses along protected sections of coastline occurred (Peacehaven Cliffs protected by an undercliff wall; Black Rock Marina the Chalk cliffs and the Quaternary Head). It is the geology, however, that controls the location and scale of erosion and cliff failure.

Chalk physical properties and cliff instability

Abstract Physical properties such as porosity and intact dry density (IDD) are compared with strength testing in relation to the Chalk formations in the cliffs of the English Channel. Natural moisture contents are close to saturation moisture contents for chalks with intact dry densities above 1.70 Mg/m3. Below this IDD, the natural moisture contents show a much greater range and greater divergence from the saturation line. There is also an indication that certain types of chalk retain water at saturation level while others gain and lose water more readily. Strength tests (Point Load Index, Brazilian Crushing Strength and Uniaxial Compressive Strengths) show up to four times reductions in strength between dry (higher strength) and saturated (lower strength) samples. Absence of a strong correlation between density and strength is interpreted as resulting from either mineralogical differences in the samples and/or textural differences between different chalks. The variation in physical properties and strength in the different chalks forming the cliffs indicates the strong stratigraphical and sedimentological controls on mechanical performance of the material and mass in cliff failures.

Calcareous nannofossils from Eastbourne (southeastern England) and the paleoceanography of the Cenomanian–Turonian Boundary interval

Abstract The Cenomanian–Turonian (C–T) boundary interval is marked by one of the most prominent perturbations of the Mesozoic carbon cycle, Oceanic Anoxic Event 2 (OAE2). Increased fertilization of surface waters caused by greater fluvial input of nutrients may have caused the widespread deposition of organic-rich black shales during the OAE2 (productivity model). Alternatively, sluggish oceanic circulation may have enhanced stratification of the water column favoring the preservation of organic matter due to anoxic bottom-water conditions (preservation model). In order to gather evidence for the driving mechanism behind the deposition of the OAE2 black shales, calcareous nannofossils from the midlatitudinal Holywell section (Eastbourne, southeastern England) were studied. Ten bioevents, including last occurrences of six species and first occurrences of four, were recognized throughout the 11-m-thick interval. Preservation of calcareous nannofossils was moderate to good in all studied samples. The C–T interval here contains an abundant (mean 2.4 × 109 specimens/g sediment) and highly diverse (mean 58 spp./sample) calcareous nannoflora, with Watznaueria, Zeugrhabdotus, Biscutum, and Prediscosphaera the most common taxa. The most remarkable change in assemblage composition through the OAE2 is the decrease of Biscutum spp. Low abundances of Biscutum, combined with elevated numbers of Watznaueria spp. and/or Polycyclolithaceae, indicate reduced surface-water fertility during the OAE2 in midlatitudinal European shelf areas. A reduction of primary productivity seems to be quite common in midlatitudinal sections, whereas calcareous nannofossils and geochemistry indicate an increase in primary productivity in low-latitudinal sections. It is therefore likely that the origin of the OAE2 in mid latitudes was caused by sluggish ocean circulation, which intensified stratification. Reduced rates of mixing prevented the oxygenation of bottom waters in these regions, causing black shale deposition.

Chalk: its stratigraphy, structure and engineering geology in east London and the Thames Gateway

Abstract The geology of the Chalk beneath east London and the Thames Gateway is reviewed and key features affecting engineering geology are summarized. In particular, the variable stratigraphy preserved beneath the sub-Palaeogene erosion surface, the evidence for syndepositional tectonics in the Chalk, and the recognition of tectonic fractures and strata-bound fracture systems are emphasized. The contrasting physical properties of chalk and flint are discussed and the depth of weathering in the subcrop and outcrop are compared and contrasted. The information gained from separate ground investigations is combined to suggest that there are regions in east London where better quality chalk and less permeable ground are present between regions of poorer quality chalk with higher permeability, closely related to zones of faulting.

Chalk engineering geology – Channel Tunnel Rail Link and North Downs Tunnel

Agreat part of the Channel Tunnel Rail Link (CTRL) is constructed through the Chalk and the ground investigation for the CTRL has yielded a mass of new information on allaspects of the Chalk. A precise bed by bed lithostratigraphy obtained from cored boreholes has aided engineering description, classification, decision making on design of machines and construction methods/specifications. Correlation of Chalk marker beds between boreholes drilled for the tunnels beneath London illustrated the influence of sub-Tertiary erosion and of faulting onpreservation of different stratigraphic levels in the Chalk. These different stratigraphic levels affect materials through which the tunnels will be constructed andconsequently tunnel vertical alignment or choice of construction method. The marker bed stratigraphy in the Thames Tunnel boreholes has enabled the same stratigraphic levels to be identified in local quarries and detailed analyses to be carried out for the design of Tunnel Boring Machines. Using individual marker beds, a detailed ground profile was constructed for the North Downs Tunnel which allowed fault zones to be predicted accurately and the different rock mass character of the Chalk formations to be delimited for numerical modelling, design zones and construction monitoring. The project also provided the opportunity to evaluate the CIRIA Chalk Grading scheme.

The engineering description of chalk: its strength, hardness and density

The description of chalk for engineering purposes is reviewed and recommendations are made for a refined method for the field determination of chalk strength, hardness and density. The proposed method incorporates field descriptions, the current CIRIA intact dry density divisions and the strength terms of BS5930:1999. The recommendations arise from work on chalk tunnels where it was found that the previous methods of field assessment gave unreliable results. Specific fieldwork was carried out to develop the proposed system on a wide range of Middle and Upper white chalks in southern England.

Multiscale fracture analysis along the French chalk coastline for investigating erosion by cliff collapse

Abstract Coastal cliffs of Upper Normandy and Picardy are eroded by cliff collapses of various sizes. This paper presents a multi-scale analysis of the pre-existing fractures embedded within the Cretaceous chalk. About 20 representative sites equally spaced along the 120km long coastal section were analysed and compared to a continuous structural analysis of the coast derived from aerial photographs taken in 1986. Ancient collapses interpreted on the aerial photos were compared to the pre-existing fracture content. Regional faults, pre-1986 collapse location and fracture density are spatially correlated. However, recent collapses observed on the field between 1998 and 2001 did not systematically correlate to the pre-existing fracture occurrence and therefore, there is no clear link between recent collapse and the regional faults.