|
Stratigraphy and the Channel Tunnel Sequence stratigraphy; the detailed mid-Cretaceous stratigraphy of the tunnelling horizon From GeologyShopOne of the main sites for geological information worldwide . This is one of over 50 link pages plus there are many original articles. Try our site specific SEARCH ENGINE to find the information you want or go to our MAIN INDEX page. Or try our site specific ORIGINAL articles, FREE geological stuff, or use our HOMEWORK AND TUTORIAL GUIDE .(Links to: Channel tunnel facts and brief history; Detailed geology on each side of the Channel Tunnel; Chalk, the basic facts; Chalk, the White Cliffs; Landslips of East Kent)
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
This page is an attempt to provide a summary, partly based on existing published data, of the detailed stratigraphic studies undertaken during the construction of the Channel Tunnel. It has been published with the kind permission of Eurotunnel. This page is still is under construction. The text is in draft form and there are a number of maps and figures in preparation.
Summary During the construction of the Channel Tunnel TML, the contractors, faced a number of problems with regard to the geology and ground conditions encountered during construction. A new approach to quantitative micropalaeontology enabled them to effectively check the horizon of the tunnel relative to the geophysical mapping and horizons that all of the safety probes encountered. It was essential to the understanding of the geological history and sedimentary facies of the area, and it proved very important in the contractual negotiations. This article describes how the techniques evolved over the various site investigations, the results of the studies and the commercial importance of them. The work was essentially completed between 1988 and 1992. It is now published on the web with the permission of Eurotunnel.
Introduction Stratigraphy has always been an essential part of the geological studies for the various Channel Tunnel attempts, the aim being to divide the rock mass into layers of rock, each layer being deposited during the same time period. Despite all the available data accumulated over more than 100 years it became evident during the early stages of the tunnelling operation, that while there was a mass of stratigraphic data available, the stratigraphy of the main tunnelling horizon itself (basal Chalk Marl) was only poorly understood. Difficulty in recognising any subdivisions within the Chalk Marl was compounded underground, since the exposures were often smeared or jigger marked and the uniformity of the Chalk Marl with its light grey colour meant that, when it was viewed under artificial lighting, differentiation of beds became extremely difficult. Even with good access, it proved very difficult to log lithologically on a consistent basis. If TMI. were to subdivide the Chalk Marl of the tunnelling horizon they only had one possible boundary, the Zone 8/9 microfossil boundary of Carter and Hart (1977) on which they could rely. To recognise this datum on a regular basis in the tunnel drives (where it would not always occur), samples had to be collected from vertical sequences underground on a regular basis and resources would have to be assigned for this. At the beginning of operations, no serious attempt was made to recognise this horizon. This was due both to the difficulty of collecting such samples on a regular basis underground and because it would have required additional staff (more than the additional staffing which had already been agreed). Management would not increase staff levels further unless they could be persuaded that this work would be of significant commercial value. No such evidence could be provided, even though geologists and geotechnical engineers logging the underground exposures could not determine the tunnelling horizon from the exposed faces within the lower Chalk Marl with any degree of certainty. To locate the tunnelling horizon against the predicted geological section, which had been produced by geostatistical contouring of the base of the Glauconitic Marl, TML therefore had to rely on vertical downward probes which were drilled at the rear of the Marine Service Tunnel TBM. The aim of these was to locate the base of the Glauconitic Marl. This allowed TML to locate the tunnel precisely against its predicted location, but it did not allow confirmation of which horizon within the Chalk Marl the tunnel was located. While these data were generally adequate during the TBM construction cycle, they were not sufficiently refined for determining the path of the probes drilled ahead of the face of the TBM which did not intersect the Glauconitic Marl, or for the verification of other probe holes. Although trials of downhole surveying of the forward probes, and wireline logging of vertical probes had been made both prior to and during the early stages of the TBM drives, these proved relatively impractical. Not only were they time-consuming, they involved the use of sophisticated equipment likely to lead to expensive and lengthy stand-by-time associated with the uncertain probing times. They were therefore abandoned. To obtain any significant geotechnical data underground, by being able to compare like with like in terms of stratigraphy, TML therefore had to develop an in-house stratigraphy with a 1 to 2 m resolution that could be applied underground, at least in probe holes. To achieve this, TML geotechnical department, with the support of management, decided to embark on an attempt to refine the microbiostratigraphy of the 1964-65 site investigation, correlating it in detail with lithology to allow integration with the wireline logs and therefore other analytical data (especially altimetry). This article briefly discusses the biostratigraphic data obtained and made available to TML during the various site investigations and outlines the various steps that allowed TML to produce a refined stratigraphy which proved essential to the verification of probe holes and finally allowed a detailed depositional model of the tunnelling horizons to be made. This model was then used to construct a refined structural history of the area during the transition from Early Cretaceous to Late Cretaceous deposition. Within this article, stratigraphic names have often been used on an informal basis, as they appeared in much of the design and construction literature. As much of the stratigraphic work had contractual implications, the original terminology had to be referred to and thus it was not possible to modify it to the continually changing academic literature without causing a great deal of confusion to the project management. In addition, most of the literature referred to, for contractual reasons is pre-1985. Only where new literature added significantly to TML's ability to achieve its main aim of providing a refined site-specific stratigraphy (e.g. Gale, 1989) was it adopted. TML's stratigraphic work was finalised in 1991. The emphasis on the type of stratigraphic data has changed over the years with the introduction of more advanced techniques. For the pre-1964-65 attempts, emphasis was put on data from the cliffs on both sides of the Channel, together with sampling of the seafloor as an aid to mapping the continuity of the strata. In 1964-65 the emphasis was on drilling boreholes, stratigraphically biased core logging and micropalaeontology to map the various Chalk beds both on land and beneath the sea. This data was supplemented by unsophisticated and relatively low resolution wireline logs undertaken on selected boreholes and very limited seismic survey data, which were of lesser significance than the borehole data in the overall interpretation. Limited calcimetry and other analytical techniques were also applied to obtain basic engineering data. The site investigations performed in the 1970's and l980's were primarily concerned with obtaining high quality wireline data from a very carefully selected series of boreholes, together with a geotechnically biased core logging programme, a detailed geotechnical analytical programme and high quality seismic data. Only limited biostratigraphic information was obtained from these boreholes and from the 1974 trial tunnels compared to that from the 1964-65 site investigation boreholes. Thus, the 1964-65 data forms the bulk of the available biostratigraphic and core log data, while seismic, wireline log and geotechnical data are predominant in the 1970's and 1980's site investigations. The tunnel crosses the Channel entirely within Cretaceous rocks on the northern limb of the Wealden Anticline. Within this succession, it is the strata of the Aptian to Turonian stages that are of relevance and, in particular, the Albian (Gault Clay) and the Cenomanian (Lower Chalk). To subdivide these sedimentary rocks TML. could have used either their lithological character (lithostratigraphy) or their fossil content (biostratigraphy) or a combination of both these techniques. Lithostratigraphic data was obtained by: · logging of the cliff and quarry exposures · detailed borehole core logs · calcimetry analysis of rock samples from surface exposures, boreholes or underground exposures · interpretation of wireline log data · interpretation of seismic survey data. Biostratigraphic data was obtained from: · macrofossils - due to access problems underground the use of macrofossils was restricted largely to surface exposures and selected cored boreholes · microfossils (foraminifera) - while many other microfossil or palynomorph groups could have been applied in this study, due to a combination of the existing large database of foraminiferal data, their suitability for use in boreholes, and the cost and time lag of any alternatives, there was no suitable practical alternative to their use. Major Lithostratigraphic units The upper part of the Folkestone beds which correlate with the Gardes Formation in northern France were present beneath the Gault Clay along the entire length of the tunnels. While the excavations in the UK sector did not occur within the Folkestone Beds, they were significant to the tunnelling operation as a probing hazard due to the high artesian water pressures known to occur within them. The Gault Clay Formation was encountered at several localities during the tunnelling operation, most significantly at Castle Hill where Beds XI to XIII were encountered, the UK Crossover where the Marine Service Tunnel and invert of the Crossover cavern were deliberately located within Zone 6a and the three UK pump stations. Tunnelling within the Gault Clay was generally avoided due to its plasticity and swelling characteristics. In the Channel there are beds (Zone 6a) between the Gault Clay and the Glauconitic Marl that are missing in most of the cliff and inland sections of Kent. They represent a sedimentary cycle or cycles, having a base of sandy glauconitic mudstone passing up into a greenish grey, silty marl rich in mica. The Chalk Marl (Craie Blue) is a dark bluish grey, marly chalk with distinctive sedimentary cycles and a calcium carbonate content generally below 75%. The cycles begin with a thick, dark blue-grey marl with much disseminated pyrite at the base, passing upwards through a medium grey marly chalk to a thin pale hard limestone with sponges. The calcium carbonate content of the Chalk Marl increases from the base, with a consequent decrease in clay content, so that the basal Zone 8 may contain in excess of 40% clay minerals. The basal 2-5 m of the Chalk Marl includes the conspicuous Glauconitic Marl or Tourtia, after the name used by miners in northern France, which forms the base of the Cenomanian stage. It comprises a dark green, marly glauconitic sand or sandstone at the base, which is commonly piped into the underlying Gault Clay. The proportion of marl increases upwards as the proportion of glauconite sand decreases, giving a green, sandy glauconitic marl in the upper part. Despite all of the available data on the Chalk Marl on the English side and unlike the Gault Clay, of which the bed by bed stratigraphy has been known since the work of Price (1874), no accurate lithological log of the Chalk Marl had ever been published that could be used as a reference section before that of Gale (1989). According to Gale, Price (1877) underestimated the thickness of the Chalk Marl by about 15 m; furthermore Kennedy (1969) did not correlate the base of the Abbot's Cliff section with the foundered chalk blocks on the foreshore, and consequently his published log for the Chalk Marl includes a total overlap of approximately 10 metres. Gale (1989) was the first to publish a complete, detailed, lithological log of the Chalk Marl and this was of particular relevance to the work of TML. Microfossils Micropalaeontological analysis 1964-65 The micropalaeontological zonation used in the 1964-65 investigation was a modified version of that developed for the 1958-59 survey (Carter, 1961) which was based on detailed analysis of the microfaunal sequences found in the Dover No. 1 Borehole. It was started by Carter in 1958. as a means of classifying the large number of sea-bed drop samples collected as part of that investigation. The 1958-59 zonation covered all the beds encountered in the Channel between the upper part of the Middle Chalk and the base of the Lower Greensand. A total of 2l stratigraphical zones were recognised by Carter, each identified by its contained fauna. This zonation, which was described in detail by Carter and Hart (1977), is shown in Fig**. In this, they subdivide the Lower Chalk into eight zones, with the basal Zone 7 representing the Glauconitic Marl (Tourtia) and Zones 8,9 and 10 occurring within the Chalk Marl. One of the main reasons for originally developing a micropalaeontological zonation was that relatively few of the beds in the Gault Clay-Chalk sequence defined by Jukes Browne were readily identifiable in cores. In order to recognise these, during the 1964-65 investigation the ratio of planktonic to benthonic foraminifera was worked out for samples taken at approximately one metre intervals down each borehole to be correlated. Individual curves were prepared for each borehole and then correlated. Corresponding maxima and minima were identified in each borehole and connecting lines drawn to produce the final correlation. In most cases the curves showed characteristic maxima and minima which could be correlated from borehole to borehole and in many cases right across the Channel. One of the most distinctive features of the curves is a strong, constant peak known as the 'C' correlation point, which served as a datum when correlations of the Lower Chalk sequence were in doubt. Using the C' correlation line as a datum reduced to the horizontal, eliminated post mid-Cenomanian folding and revealed a series of erosion surfaces at a constant level within the Cenomanian. The areas of maximum erosion have been found to be centres of monoclinal uplifts in the vicinity of Boreholes R005, R100, R210, R265 and T340. Despite the limited application of the work of Carter and Hart (1977) by TML during the construction phase, it proved essential to the design process. It was, however, invaluable to TML in that it allowed TML to describe the distribution of zones and especially Zone 6a spatially using the earlier northern alignment boreholes. Carter and Hart also, significantly, identified that syn-depositional tectonism was associated with the 'C' correlation line. At the time the work was undertaken it was technically very advanced and it certainly achieved its main aim of producing a detailed stratigraphic framework across the Channel. Techniques applied during this study, and especially the planktonic/benthonic ratio plots, have been the subject of much debate and comment by various authors over the years. However, modified planktonic/benthonic ratio plots have been applied successfully in TML's study, where they were applied in combination with other stratigraphic disciplines. Checks made by TML on the earlier work (such as a re-analysis of part of Aycliff No. 1) proved the accuracy of this work. Micropalaeontological analysis 1987 Over the last 20 years, B.R.G.M. have developed a refined biostratigraphical division of the Cenomanian that can be applied to the whole of the Anglo-Paris Basin. The method is based on the chronological evolution of microfaunal associations (foraminifera), and it has been successfully tested in the course of several activities undertaken by B.R.G.M. On the French side of the Channel, following a study of the Escalles Borehole by Marie in 1960 (unpublished), in which a rough comparison was made with Carters initial results, Andreieff (1964, unpublished)) prepared an initial biozonation of Cap Blanc Nez, which was later refined and completed by Amedro et al (1978). Comparison of the English and French studies proved to be difficult however, because the authors did not use the same stratigraphic marker species (or called them by different names) and assigned different names to the same assemblages. At Dover, Hart et al identified a narrow zone of Rotalipora reicheli at the top of the Chalk Marl, also identified in the Boulonnais by Andreieff (1964, unpublished) and Amédro et al (1978) over a thickness of 2 to 3m. This biozone was of major interest, since it was known to occur across the Channel and its narrowness aided accurate correlation. In the 1986-87 site investigations attempts were made to recognise this biozone. However, since it was only possible to recognise it in 7 of the 14 boreholes analysed, this entire study was of very limited value when compared to the earlier studies of the 1964-65 boreholes which were largely reverted to for the purpose of Development Study 17. The Folkestone Warren Gale (1989) describes the Folkestone Warren as an area of landslipped undercliff, which extends along the coast eastwards from Folkestone to Abbot's Cliff, a distance of nearly 2 km. Its maximum width is approximately 500 m, and it is bounded on the landward side by high cliffs of Lower and Middle Chalk, which rise to 150 m above OD. Slipping is initiated in the underlying Gault Clay. The Gault Clay (Albian) is at present exposed only intermittently on the foreshore in the Warren, near the base of the landslipped part of the succession. The extent of outcrop depends on sand and shingle movement. In the past, before the construction of a concrete toe weighting in East Wear flay, successive movements of the landslip provided vast foreshore outcrops of the highly fossiliferous Gault. The high cliffs at the back of the Warren display spectacular, but rather inaccessible sections through the Lower and Middle Chalk. The Plenus Marl is conspicuous on account of its slightly recessive weathering, and pale greenish-yellow colouration. The very white, nodular, Melbourn Rock immediately above weathers out prominently. In the autumn of 1987, heavy seas scoured weed and shingle from the foreshore of East Wear Bay producing exceptional exposures of large landslipped blocks of Gault Clay (Albian), Glauconitic Marl and Chalk Marl (Cenomanian). These sections allowed the detailed stratigraphy of the Chalk Marl at this location to be studied for the first time, and a correlation established between Abbot's Cliff and the foreshore sections in East Wear Bay (Gale, 1989). The work of Gale proved very important to TML as he firstly recognised the problems involved with the previously published lithostratigraphic work while also providing the first continuous and detailed lithological log of the Glauconitic Marl and Chalk Marl (Fig.**). Unfortunately this work was not available until after tunnelling had commenced. Database Summary The work of Carter and Hart between 1958 and 1977 allowed a detailed zonation to be proposed, comprising 21 zones, for the stratigraphic interval from the Lower Greensand to the Middle Chalk. In addition to this zonation, they developed the use of planktonic/benthonic ratio plots which allowed the definition of a number of distinct datums, including the 'C' correlation line. Lithological studies combined with accurate palaeontological collecting of both macro and microfossils, allowed Robaszynski and Amédro (1986) to develop a multi stratigraphical chart for the Cretaceous of the Boulonnais (Fig. **). They divided the Aptian to Turonian into fifteen formations, which could be further subdivided into stratigraphical units given letters between A and O. The five formations which make up the Cenomanian comprised a total of eight stratigraphical units, plus the lowest horizon of the largely Turonian Grand Blanc Nez Formation. Following on from the work of Robaszynski and Amedro (1986). and for the purposes of Development Study 17, the B.R.G.M. (1987) grouped the stratigraphical units into the recognisable facies of Glauconitic Marl, Chalk Marl, Grey Chalk and White Chalk, that have been used consistently throughout the many site investigation campaigns for the Channel Tunnel. However, it was recognised by them that in some cases the reclassification of the formations did not appear to have any advantages for easy correlation of the vast amount of Channel Tunnel borehole data collected during this phase of site investigation, and may even have made the correlation more difficult. The detailed stratigraphic correlation produced for the French sector and the work of Carter and Hart (1977), although important to the design process, proved largely irrelevant to the detailed stratigraphic correlation of the underground exposures of the British sector once tunnelling commenced (Fig. 26.1). This was for several reasons: · The work of Robaszynski and Amédro (1986) was largely irrelevant as where it was defined on the French side of the Channel Zone 8 (the main tunnelling horizon on the UK side) was considered to be either absent or represented by a different lithology. · The confusion of Chalk Marl lithostratigraphic data from the cliffs between Dover and Folkestone, largely due to the presence of landslips, was not fully recognised until the work of Gale (1989); · Carter and Hart (1977) defined only one consistently recognisable zonal boundary (8/9 boundary) within the main UK tunnelling horizon, the lower part of the Chalk Marl (Fig. **). Because of these problems, TML geotechnical department undertook a very detailed but selective analytical programme which allowed TML to relate the geotechnical parameters through the calcimetry profiles to the stratigraphy along the entire length of the UK sector tunnels. There were three main elements to this study which combined the techniques of the B.R.G.M. in utilising both the palaeontological and lithological information to define rock units; the quantitative approach of Carter and Hart to micropalaeontology; and macropalaeontology. In the Gault Clay Formation, the Zones of Carter and Hart (1977) in association with the Bed numbers of Price (1874) have been used to define the main stratigraphic units that have been applied by TML (Fig. **). thus avoiding the introduction of even more stratigraphic terms to an already confused nomenclature. The first steps towards the revised stratigraphy were made due to TML's requirement for the verification of underground probe holes. Verification of underground probing During excavation of the tunnels and associated caverns and pump stations, TML geotechnical department were responsible for the verification of ground conditions ahead of excavation. This was achieved by both examining cored boreholes and the cuttings collected during the destructive drilling of boreholes. For several of these programmes detailed stratigraphic knowledge was essential. In these, since calcimetry was regarded as a crude method for the correlation of boreholes and sections, and because of the impracticability of using wire-line geophysical equipment underground in probeholes drilled at all angles, it was decided to use microfossils (foraminifera) in order to obtain a detailed correlation of the tunnelling horizons. These were applied to the following drilling programmes: · forward probing ahead of the Marine Service Tunnel · vertical probes downwards in the Marine Service Tunnel and the Land Service Tunnel · probes inclined upwards at the UK Crossover cavern · vertical probes drilled downwards in the pump station shafts · vertical boreholes drilled upwards from the Marine Service Tunnel during a segment survey · vertical boreholes drilled both upwards and downwards from the service tunnels for contractual purposes. Of all these drilling programmes, it was the analysis of the continuously cored Crossover probe holes which first proved the potential for utilising microfossils as a stratigraphic tool to verify the underground boreholes. The Crossover boreholes were analysed for microfossils with the sole purpose of verifying the stratigraphic horizon at which the probe holes terminated (for safety purposes to ensure that the probes reached their destination horizon, the Crossover crown). This study resulted in a TML modified Carter and Hart (1977) zonation, which was first applied in 1989 and more formally defined in an internal document in 1990 (Fig. **). The initial success of this verification work allowed TML to apply the technique to the verification of all other probe holes and most significantly to the verification of the probe holes ahead of the Marine Service Tunnel. This analytical work proven very important to the safe progress of the tunnels. It also saved substantial monies which otherwise would have had to be spent on other more expensive methods of verification, all of which would have also involved substantially greater down time for the Marine Service Tunnel TBM. The continued programme of verification work allowed TML. to gradually refine the methodology in order to maximise the results obtained and minimise the reporting time (e.g. the Marine Service Tunnel forward probe samples were all reported on within 24 hours of completion of probing). The refined zonation allowed Colin Harris to process and analyse a total of typically 10 to 12 samples per day. In contrast to the TML methodology adopted, if the analytical method chosen had involved complete microfaunal analysis of samples as is normal in industrial practice, the results would not have been obtained in time to allow the ground ahead of the probing bulkhead to have been verified. This would have resulted in either the TBM stopping until the results were available. These studies eventually allowed TML to define the following methodology for use in the contractual study of boreholes. There were three basic premises on which the methodology was based in order to attempt to produce a refined stratigraphy along the entire length of the UK tunnel drives: · that an integrated approach was essential both practically to provide the desired results and contractually to avoid unnecessary interdisciplinary debate · as neither the application of planktonic or benthonic foraminifera to the stratigraphy of the tunnelling horizon had provided the required stratigraphic resolution a radically new approach had to be tried: selective quantitative micropalaeontology with an emphasis on the planktonic component of the assemblage · to achieve this detailed sampling of every sedimentary cycle was required. Borehole sampling To ensure accurate correlation based on regional foraminiferal assemblage changes, at least one sample from each Chalk Marl sedimentary cycle was taken, at a sample interval no greater than I m and preferably less than 0.5 m. This was always done in association with detailed lithological core logging (Figs **). All borehole contamination was removed from each sample. Sampling equipment was cleaned after each sample was taken to avoid contamination. The sample depth was marked on the borehole log. The remaining core was then labelled with the depth and wrapped in clingfilm and a labelled marker was placed in the sample position. Sample processing Sample preparation comprised crushing the sample to no greater than 5 mm in diameter. 65 ml of crushed rock from each sample was then dried in an oven at 1200C for a minimum of 3 hours, or until dry. The dry sample was then soaked in white spirit for 15 minutes, when the excess liquid was poured off. The saturated sample was then totally immersed in water until completely disaggregated, and washed through a 75 micron sieve using household detergent to remove any excess white spirit. The wet residue was then removed from the sieve and dried in an oven. At each processing stage the sample equipment was cleaned to avoid sample Contamination. Sample examination This was undertaken using a Nikon binocular microscope with 10 eye pieces giving a maximum magnification of x30. Residues were placed in a nest of 500, 250, 180 and 125 micron sieves and sieved ensuring maximum sample splitting. The separate sieve residues were then placed on a tray for examination. Taxonomic counts Carter (1961) and Carter and Hart (1977) only defined two species, Pseudotextulariella cretosa and Flourensina intermedia of major zonal significance in the Glauconitic Marl and basal Chalk Marl (Figs **). However, they did illustrate the value of detailed taxonomic counts in the understanding of Cenomanian stratigraphy. Based on this work, TML developed a series of key taxonomic counts (Figs **). These counts incorporate the most reliable benthonic zonal marker taxa and the most significant planktonic genera, together with the main faunal components and are summarised below. · Arenaceous/Calcareous Benthonic/Planktonic taxa. The entire 500 micron sieve fraction was counted and added to 100 specimens counted from the 250 micron sieve fraction (plot Micropal 1). · Arenaceous/Calcareous Benthonic/Planktonic taxa. A minimum of 100 specimens from the 180 micron sieve fraction were counted (plot Micropal 2). · Arenobulimina spp/Flourensina intermedia/Pseudotextulariella cretosa. All of the specimens in the 500 micron sieve fraction were counted and added to 40 specimens (if present) from the 250 micron sieve fraction (plot Micropal 3). · Hedbergella spp/Praeglobotruncana spp/Rotalipora spp. All of the specimens in the 500 micron sieve fraction (usually zero) were added to 40 specimens (if present) picked randomly in the 250 micron sieve fraction (plot Micropal 4). Each sample count was then converted into a percentage and presented graphically as illustrated by Carter and Hart (1977). In the Gault Clay detailed taxonomic counts were not required, as the application of the zonal scheme of Carter and Hart (1977) provided the required stratigraphic resolution. The interpretation of the quantitative results in the Chalk Marl was based on the variations in the microfaunal assemblages illustrated on the various graphs (Figs **). The following criteria were used to define datums of correlative value from these graphs. · 'Floods' of a planktonic genus. The genera Praeglobotruncana and Rotalipora occur commonly at several horizons and may comprise almost all of the planktonic assemblage recorded in the 500 and 250 micron sieve fractions. At other horizons they may be totally absent. · Abundance of planktonic foraminifera. Planktonic foraminifera show significant variations in abundance in both the Glauconitic Marl and in the Chalk Marl. · Extinction of a benthonic species. The last consistent occurrence (ignoring sporadic occurrences of the species higher in the succession) of Flourensina intermedia defines the top of Zone 8. · Abundance of benthonic foraminifera. Variations in the overall benthonic assemblage and abundance of species such as Pseudotextulariella cretosa can be of value locally. The TML subdivision of the tunnelled horizons Since the initial micropalaeontological zonation developed in order to verify the stratigraphic level of the probe holes within the tunnels, a substantial amount of work has been carried out on the macrofossil content of selected cores, together with lithological logging of all the cores and some detailed work on the lithostratigraphic marker horizons defined by Gale (1989) from the foreshore at Abbot's Cliff. This work has been incorporated into the TML zonation and hence the zonation is no longer purely a microfossil zonation, it is a combined microfossil, macrofossil and lithological zonation. The evidence from each discipline is discussed separately below. The conclusions integrate these various sources of data and compares them with previous zonations. The resolution of the zonation is also discussed. Macrofossil datums In order to relate the macrofossil work of Gale (1989) on the foreshore at Abbot's Cliff to the tunnelling horizons, C. J. Wood, analysed two type sections in detail, boreholes BCA 1 and PH 1 (both cored) and assisted in sampling the foreshore localities of Gale. In addition to the 'M' marker horizons of Gale one other datum proved to be of significant value: · Lowest occurrence of thick shelled inoceramid fragments: the end downhole occurrence of conspicuous shell fragments, particularly inoceramids. This datum was identified in boreholes BCA1 and PH2 (Fig. 26.8). At both localities this datum proved to be at the base of the TML Middle Unit in association with M2. Lithostratigraphic datums The lithostratigraphic marker (M) horizons, M1 to M8 inclusive identified by Gale (1989) on the rotated foreshore block at Abbot's Cliff were sampled (Fig. **). An attempt was made by Wood to recognise these marker horizons in boreholes BCA1 and PH 2. However, this proved difficult due mainly to the lateral variation of the succession between the wavecut exposure and the boreholes. Fortunately, all of the significant datums (M2 to M4) proved to be associated with distinctive microfossil assemblages and could therefore be easily recognised by micropalaeontological analysis. Table ** is a comparison of the characteristics of these horizons as defined by Gale, together with the microfossil and calcimetric characteristics which allowed them to be recognised in the tunnel drives. The datums M3 and M4 were easily recognised in all of the boreholes which penetrated them. The recognition of M2 proved problematical because, in the type section on the rotated block, this marker occurs directly above the Glauconitic Marl/Glauconitic Chalk Marl, but directly below a very cyclic sequence of dark marls and limestones (TML Chalk Marl Middle Unit). Consequently, it was originally thought that M2 was equivalent to the limestone on top of the Glauconitic Marl/Glauconitic Chalk Marl at other localities. However, this would have placed it up to 10 m below the very cyclic sequence (Middle Unit) in boreholes such as BCA l, thus implying a major missing section (comprising poorly rhythmic silty marls of the TML Lower Unit) on the foreshore. This contradiction was resolved with the drilling of LU1, which proved an almost identical lithological succession and similar location of datums to that of the rotated slab at Abbot's Cliff. This borehole proved that the difference between the Abbot's Cliff and the succession recorded in BCA1/Craelius 1 was due either to an unconformity or to a facies change but not to a fault as had previously been considered possible. Once this had been established it was possible to correlate the TML datums of the Lower Unit to horizons within the Glauconitic Marl on the foreshore, thus demonstrating a major facies change. This confirmed that M2 was situated at the base of the cyclic Middle Unit and not at the top of the Glauconitic Marl, as had earlier been suspected. Microfossil zones (Carter and Hart, 1977) In their publication 'Aspects of mid-Cretaceous stratigraphical micropalaeontology', Carter and Hart define Zones 3 to 14 of the Middle Albian to Lower Turonian age. Of these zones, only Zones 5 to 10 are relevant to the present study and of these by far the greatest amount of tunnel drive occurred in Zones 7, 8 and 9. Albian Zones (Fig. **) · Zone 5 (inclusive 5a): this is the oldest Zone to be identified in the present study. It was recorded in many of the vertical probe holes; excavation occurring within it in the pump stations, at the landward end of the land drives and at Castle Hill. Because it was the basal zone that required recognition in the present study further subdivision of it was not required. · Zone 6: the base of this Zone is characterised at East Wear Bay by a glauconitic horizon termed Bed XII by Price (1874). An expanded sequence of calcareous mudstone (Bed XIII) is present above Bed XII, these two beds are assigned to Zone 6. This sequence has been variously subdivided by many authors including a tripartite subdivision into 6L (6 lower), 6M (6 middle), and 6U (6 upper) by Hart (TML internal reports by Hart, 1988). This classification was subsequently applied by TML. This allowed the recognition of the replacement of Bed XIII by condensed glauconitic facies, typically 1 m thick, under the majority of the tunnel drive (Fig. **). · Zone 6a: this sequence was first recognised as a distinctive, discrete depositional unit by Carter and Hart (1977) who described it as having 'a peculiar geographical distribution'. They assigned it to Zone 6a and regarded it as transitional between the Albian and Cenomanian. However, they did note that where it occurred in this northwestern part of the English Channel, it was sharply truncated by the overlying Glauconitic Marl. As this Zone is so distinctive and so localised no subdivision of it was required for the present study. Cenomanian Zones Zones 7, 8, 9 and 10: these zones, established by Carter and Hart, comprise the greater part of the Cenomanian succession below the mid-Cenomanian non-sequence (Fig. **). · Zone 7/8 boundary: Carter & Hart (1977) noted themselves that Zone 7 coincided with the Glauconitic Marl, and that the abundance of large Lituolacea that are characteristic of it are facies controlled. They also noted, that in the absence of the Glauconitic Marl lithology Zones 7 and 8 are not easily separated. Due to these limitations Zone 7/8 boundary: Carter and Hart (1977) noted themselves that Zone 7 coincided, TML decided not to attempt to recognise this boundary as a datum. · Zone 8/9 boundary: the top of Zone 8, the 8/9 zonal boundary, is based on the extinction of Flourensina intermedia and the incoming of Pseudotextulariella cretosa. This was identified in the initial TML work as a discrete datum, with potential for long range correlation. All subsequent work undertaken by TML has proved this, with the quantitative approach to the two zonally significant species allowing the boundary to be placed precisely in each borehole (Figs. **).	TML regarded the quantitative approach as essential to the accurate recognition of this boundary because Pseudotextulariella cretosa, or forms that have close affinity to it, are present in Zone 8 and have been recorded in Zone 7. In addition, F1ourensina intermedia was recorded as a rarity above the 8/9 boundary. The boundary in this report has been placed at the last consistent occurrence of Flourensina intermedia, which normally coincides with the first consistent occurrence of Pseudotextulariella cretosa. In many boreholes the 8/9 zonal boundary is situated directly above a distinctive shallowing cycle recognised on the basis of a decrease in planktonic abundance and a concomitant increase in the arenaceous component of the foraminiferal assemblage. This shallowing cycle assisted TML in placing the 8/9 zonal boundary; it is especially well developed in boreholes MU4 (Fig. **) and MU6 below the 8/9 zonal boundary. However, because the 8/9 boundary itself is so consistent in its occurrence, this shallowing cycle was not defined as a formal datum. · Zone 9/10 boundary: this boundary was defined by Carter and Hart at the last occurrence of Marssonella ozawai, Lingulogavelinella jarzevae and on their range chart, but not in the text, Quinqueloculina antiqua. However, they themselves recorded that Lingulogavelinella jarzevae occurred in Zone 10. The early TML investigations indicated that this zonal boundary was diachronous, varying in position between two distinct datums which are now termed M3 and M4 (Gale, 1989). The use of this zonal boundary was therefore dropped by TML in favour of the recognition of the datums M3 and M4. Additional TML datums 'K' and 'L' (Table **) have also been defined above and below this boundary providing very good stratigraphic resolution for this sequence. The difficulty of picking the Zone 9/10 boundary consistently is shown by the thicknesses of Zone 9 recorded in the 1964-65 site investigation boreholes (Fig. **). These varied between 2 and 16 m, with a 'characteristic' thickness in many boreholes of between 4 and 5 m. No such variation in thickness of Zone 9 was recorded in the present study. · Zone 10/11 boundary: no tunnels intersected this boundary in the UK sector. In addition, none of the TML probe holes penetrated this horizon and therefore this boundary was not studied further by TML. To summarise, for the Upper Albian TML used Zones 5, 5a, 6 (6L, 6M and 6U in selected vertical probes) and 6a as defined by Carter and Hart (1977) without modification. In the Chalk Marl itself only the 8/9 boundary could be applied. The TML datums and the TML stratigraphic units (1992) Cenomanian The stratigraphic work undertaken by TML has identified a number of micropalaeontological datums which, in association with the other datums defined, has allowed a very detailed model for the stratigraphy of the Lower Cenomanian of the tunnelled horizons to be established. All of these datums are defined in Table ** (Fig. **). The definition includes a type section for each datum. Once a correlation along the length of the UK tunnels had been established based on the TML datums (Figs **). the TML stratigraphic Units could be defined (Fig. ** and Table **). Two criteria were used in their definition: · the presence of erosional contacts or non-sequences recognised during the detailed core logging undertaken by TML(Fig. **); · palaeoclimatic or sea level changes which are reflected in significant changes in the gross lithology (Fig.**) and fossil assemblage (Figs ***). The degree of resolution of the zonation is an important consideration when drawing conclusions regarding the subtle variations in depositional thickness of Units within the Lower Cenomanian and their structural implication. In this study the accuracy in locating any one datum is dependent upon the type of data to define that datum. Where a datum is based on micropalaeontological criteria, it is the sample interval that determines the degree of accuracy. This is normally less than 0.5 m and hence the accuracy is normally less than ±0.5 m. In the most recent boreholes analysed, the sample interval averages 0.3 m and the accuracy is therefore ±0.3 m. Where a datum is based on the calcimetry data, the accuracy is again dependent on the sample interval, which in this instance averages 15 cm. Hence, the accuracy is ±0.15 m. As stated above, the lithological logs have also been used to place boundaries. Here the accuracy of the datum is controlled by the accuracy of the logging and surveying. A figure of ±0.05 m is assumed. If the datum is based on more than one set of evidence or where there are multiple datums (F and G and H/M2 and J/M3 and K/M4 and L) the accuracy is thought to be greater than if it was based on a single datum. The maximum inaccuracy of the Lower/Middle and Middle/Upper Unit boundaries is ±0.15 m as these are based on both calcimetry and lithological data. They are supported by microfossil datums. The Basal/Lower Unit boundary is based on microfossil evidence and the accuracy of it is therefore ±0.3 m. The base of the Basal Unit is placed using lithological evidence and the accuracy is therefore ±0.05 m. Table*** Albian As the Units within the Albian are very distinct, the work required to provide results of similar resolution to those obtained for the Cenomanian was not as great. The top of Zone 6a and the base of Zone 6 are normally placed on lithological evidence, hence an accuracy of ±0.05 m. The base of Zone 6a is based on a combination of microfossil and lithological evidence and hence the maximum in accuracy of this boundary is ±0.15 m. Summary · The relationship between the TML Units and previous significant work is illustrated on Figs **. From this figure it is evident that none of the previous authors recognised the major divisions of the Chalk Marl identified by TML. · Figure ** shows the location of the TML Datums within these Units. This figure, when compared to Fig.** indicates the much greater resolution achieved by TML for the tunnelling horizon than by any of the previous authors. · This zonation has allowed TML to resolve the stratigraphy of the foreshore section in relation to the successions encountered during tunnelling (Fig.**). · It has allowed TML to resolve the relationship (Fig.**) between the macrofossil and lithostratigraphic work of Gale (1989), and the modified Carter and Hart (1977) microfossil zonation used by TML. · By recognising several isochronous events, TML has been able to divide the tunnelled horizons into discrete layers of rock of the same age, thereby allowing depositional models to be proposed and facies variation ascertained. · The accuracy of most of the Unit boundaries is ±0.15 m. The refined stratigraphic zonation has allowed TML to subdivide the sedimentary sequences encountered within the tunnel drives into a number of isochronous layers (Units) of rock, all of which were deposited within a marine environment. These TML Units (Basal, Lower, Middle and Upper) varied considerably in thickness across the study area (Figs **). The Middle Unit is characterised by extreme cyclicity which is represented both on 'Micropal 1 and 2' and on the calcimetry plots. This Unit varies significantly in thickness, in its palaeoenvironment and in the numbers of limestone/mudstone interbreeds. Much of the variation in thickness occurs between M2 and M3. The Lower Unit is typically represented by a homogeneous sequence of very uniform depositional environment. It is characterised by an almost symmetrical planktonic/benthonic 'Micropal 2 plot, with the planktonic abundance generally Increasing from D to F and gradually decreasing from H to M2. Lateral variation in thickness may he marked, as with the increased thickness of this unit at Shakespeare Cliff (e.g. boreholes BCA 1 and Craelius 1). Marked facies variations occur within it, the Unit becoming noticeably glauconitic (Abbot's Cliff foreshore section and borehole LU2) and consequently it is not always readily separable on a lithological basis from the Basal Unit. This Unit is bounded at its base by the normally distinctive erosional contact ~which forms the base of the Glauconitic Marl. It incorporates two distinctive transgressive/regressive cycles including IML datums A, B, and C and is bounded at its top by TML datum D. The Basal Unit normally comprises the Glauconitic Marl and Glauconitic Chalk Marl, the top normally coinciding with the cessation of glauconitic deposition in the study area. It varies greatly in thickness, the percentage of glauconite content, in density and in hardness. The basal erosional contact becomes less distinct away from structural highs. Depositional controls There are three factors that have controlled deposition of the highest Gault Clay and the lowest Chalk Marl which should be considered when interpreting the variations in the lithology of the TML stratigraphic Units. These are: · sea level control (water depth) · climate control · structural control Sea level control It has long been accepted that major sea level changes occurred in the latest Albian and earliest Cenomanian (Hancock and Kauffman, 1979) and that these played an important role in controlling the deposition of sediment. The sea level changes are thought to have had worldwide significance, especially affecting marginal marine and shallow shelf deposits, but also being recognisable in deeper marine deposits. In the latest Albian, major periods of transgression occurred, that were reflected by changes in the microfossil assemblages, especially by changes in the relative abundance of the planktonic component (Carter and Hart, 1977). The Albian transgressions reached an acme within Zone 6a. At the time of the later site investigations it was generally accepted that the Chalk Marl was deposited under uniform transgressive conditions, with the first major sea level rise at the mid-Cenomanian non-sequence. However, it is now known that there were significant fluctuations in sea level change within Early Cenomanian times. It is this factor that controlled some of the regional datums which allowed the recognition of the TML Units. Climate control It has recently been suggested by many authors (e.g. Hart, 1987) that the sedimentary cyclicity of the Chalk of southern England was controlled by changes in productivity reflecting climatic changes. These climatic changes are thought to be periodic in the Milankovitch Band, being induced by variations in the Earth's orbital cycles. This theory provides an explanation for the fact that the Chalk Marl cycles can be correlated across the present study area and even, according to Gale (1989) throughout the northern Anglo-Paris Basin. However, while orbital forcing may be a significant control of intra-Unit datums, TML considered that the main Unit boundaries were the result of major changes in water depth or to regional tectonism, the local consequence of which may have been a change in the palaeoenvironment. Structural control Robaszynski and Amedro (1986) suggested that Cretaceous sedimentation in NW France was controlled locally by a mosaic of tectonic blocks bounded by faults with vertical displacements on two main orientations, N110 and N30 degrees. However, in the Albian and Cenomanian there is minimal documented evidence for significant tectonic activity although some minor pulses have been proposed at the end of the Albian and at the mid Cenomanian non-sequence. This has convinced many authors that this period was one of relative structural quiescence with changes in climate and sea level being the major controlling influences on deposition. Evidence presented here proves that this was not the case, and that in the UK sector as in NW France both the local thickness of the TML Units and most of the facies changes within them were indeed controlled by the movements, albeit very minor, of local structural blocks. Variations in the TML stratigraphic units The significant variations of each of the TML stratigraphic Units (Figs **) within the UK sector are described in the following section. Upper Gault Clay, Zones 5/5a These zones form the bulk of the Upper Gault beneath the majority of the UK tunnel drives (Fig. **). Zone 5a is generally a thin zone on top of Zone 5 and is commonly absent (Carter and Hart, 1977). At the top of Zone 5/5a there is a distinct, regionally present, uneven burrowed contact above which occurs the basal bed (Bed XII) of Zone 6. The sharpness of the contact together with the common absence of Zone 5a suggests that a minor but regionally extensive erosive event occurred at this boundary. Upper Gault Clay, Zone 6 The base of Zone 6 is characterised by a regionally occurring but often very thin glauconitic horizon termed by Price (1874) Bed XII of the Gault. The presence of Bed XII at the base of Zone 6 under the entire tunnel length suggests that deposition of Zone 6 began on a very fiat, stable shelf. Deposition of Zone 6 continued as very thin, mainly glauconitic facies between chainage km 16 and km 37. However, there is a marked change at km 16, westward of which there was rapid basement subsidence with a thick sequence of calcareous mudstone being deposited, after the initial glauconitic deposition. Within the Zone 6 glauconitic facies of the platform there is evidence for at least two periods of non-deposition. The presence of these horizons together with common phosphate throughout the facies is consistent with condensed deposition. This was proven in VBL 21 (Fig. **) where all of Carter and Hart's Zone 6 subdivisions (6L, 6M and 6U) could be recognised in a single 0.5 m thick section. The top of Zone 6 where it is overlain by Zone 6a is marked by a generally weakly developed, burrowed contact with phosphates above. Some authors have considered that the lack of a developed Zone 6 across much of the channel is due to erosion associated with either the basal Zone 6a unconformity or the basal Glauconitic Marl unconformity. The evidence of a very condensed Zone 6 with discrete, very thin beds and intra Zone 6 unconformities suggests that this is not so. The weakly developed basal Zone 6a unconformity supports this view. Upper Gault Clay, Zone 6a At the base of Zone 6a a glauconitic horizon with sporadic phosphates is commonly present passing upwards into micaceous, calcareous mudstone. Above this level Zone 6a shows considerable variation in thickness, reaching a maximum between km 25 and 29 at the UK Crossover. In this area the presence of a condensed Zone 6 below Zone 6a, indicating that 6a sediments were deposited not in an erosive channel but were laid down in a subsiding, fault controlled channel or graben-hike structure (Fig. **). On the margins of the graben the thinner deposits of Zone 6a are generally glauconite-rich, commonly merging with both the base of the Glauconitic Marl and the sediments of condensed Zone 6 and consequently requiring very detailed analysis to prove their presence (Fig. **). Zone 6a deposition is restricted to the graben and its adjacent margins (Fig. **). At all other localities it was either never deposited or it is represented by a very condensed section which is extremely difficult to recognise. At the end of 6a deposition, the depositional low within the graben was pushed upwards as a response to lateral tectonic forces, becoming inverted. Subsequent deposition in consequence indicated a depositional high. This minor compressional phase probably resulted in the distinct burrowed, contact which is present regionally at the base of the overlying Glauconitic Marl. Chalk Marl, Basal Unit The deposition of glauconite was predominant in the Basal Unit resulting in the Glauconitic Marl and Glauconitic Chalk Marl being deposited. The glauconite percentages vary from over 90% in some of the highly Glauconitic Marl to 5 to 10% in the Glauconitic Chalk Marl. The Basal Unit is normally characterised by two distinct cycles. A basal transgression (TML Datum A) is followed by a marked regression (TML Datum C). The decrease in depth normally coincides with the peak glauconite abundance; it is followed by a further water depth increase and then another decrease (TML Datum D). It is this latter datum which normally marks the cessation of glauconite deposition in the study area. Spatially the pattern of distribution (thickness) of the Basal Unit is substantially different from that of the underlying Units, especially in the area of the UK Crossover, which represents a distinct depositional high. Here, the Basal Unit is characterised by a thin, well cemented Glauconitic Marl, in contrast to the thicker, expanded, clay rich Glauconitic Marl on the margins of the high. Further from the high, between km 21 and km 17, there is again a thinning which, in this instance is associated with a reduction in the percentage of glauconite in the Glauconitic Marl (boreholes BCA 1 and 2). This indicates that at this location the Basal Unit is developed in a distal facies. From km 28.4 to km 37 there is generally a gradual thickening with only minor changes in facies, indicating a uniform structural regime. Chalk Marl, Lower Unit The base of the Lower Unit is marked by a transgressive pulse which terminated glauconitic deposition in most areas, Only on the landward depositional high from km 15 to km 17 do glauconitic facies extend into the Lower Unit as for example the rotated foreshore block and borehole LU 1. In addition, the Lower Unit here is relatively condensed. Between km 17 and km 21 a distinct thickening of the Lower Unit occurs. In this discrete depocentre the sediments are characteristically homogeneous (borehole BCA 2). The 1974-75 tunnel was excavated entirely within these thick homogeneous sediments of the Lower Unit. From km 21 to km 37 the Lower Unit is of relatively uniform thickness. however, it thins over the UK Crossover high, with minor thickening on either side of the high. From km 30 to km 37 the uniformity in thickness of the lower Unit suggests very uniform basement subsidence, with this section again acting as a single structural block. The Lower Unit is characterised by homogeneous calcareous mudstones with poorly developed, generally nodular, spongiferous limestones. The homogeneity of the sediments is due to the stability of the palaeoenvironment with only minor fluctuations in water depth. The uniformity of the planktonic assemblage, with almost no Tethyan species, indicates consistent palaeocurrents throughout this Unit. There would appear to be only one significant shallowing pulse in the middle of the Lower Unit (TML Datum C). Chalk Marl, Middle Unit The Middle Unit shows marked variation in thickness, being relatively thin over the UK Crossover high and thickening in the depocentres at km 22 to km 24 and at km 37. Landward, this Unit is relatively uniform in thickness, again only thickening towards the depocentre at km 22 to km 24. From km 28 to km 37 the Middle Unit gradually thickens, indicating that the latter section acted as a single tectonic block during deposition. The Middle Unit is characterised by extreme cyclicity, with lithologies varying from weak, calcareous clay-rich mudstones to strong spongiferous limestones. This is well illustrated by the variation in calcimetry recorded in the segment survey boreholes (Fig. **). The cycles are thought to have been initiated by a major shallowing following the deposition of the Lower Unit, followed by rapid fluctuations in water depth or climate throughout the Middle Unit. The water depth generally increased through the Middle Unit. Pulses of Tethyan planktonics which are restricted to individual sedimentary cycles occur throughout the Unit conforming rapid changes in the palaeoenvironment. Although cyclicity is characteristic of the Middle Unit, there is also evidence for lateral variation in the degree of the cyclicity. Towards the landward end, the degree of cyclicity in the upper part of the Middle Unit diminishes, while in the depocentre from km 22 to km 24 the individual cycles are very clearly defined throughout the Middle Unit. In the middle of the Channel around km 37 cyclicity is present, but the correlation of these cycles (Fig. **) with those between km 22 and km 24 indicates them to be second order cycles with a greater number occurring within the same given time interval Chalk Marl, Upper Unit No complete sections of the Upper Unit were analysed by TML and the upper boundary of this Unit has not therefore been defined. However, all of the analysed sections within the Unit do illustrate similar uniformity in their calcimetry profiles (Fig. **). Evidence from wireline logs and from Abbot's Cliff suggest that the Upper Unit continues to the two prominent limestones which occur below the Cast Bed of Price (1874). This is clearly illustrated by Gale (1989). The base of the Upper Unit is marked by a distinctive change in the planktonic foraminiferal assemblage, with Tethyan genera occurring consistently and commonly above (Fig. **). The change in the planktonic assemblage coincides with the loss of the distinctive cyclicity of the Middle Unit. Cycles do occur, but they are characterised by relatively minor calcimetry variations and an associated lack of either well defined limestones or dark marls. It is this relatively homogeneous, weakly cyclic Chalk Marl Unit that is exposed in the Abbot's Cliff section. Late Albian non-sequences Two major non-sequences of significance to TML's study occur in or at the end of the Upper Albian. at the boundary between Beds XI and XII (Zones 5 and 6 of the Gault Clay) and at the base of the Glauconitic Marl (Zone 6a and the Basal Unit boundary). These horizons could be the result of either major sea level (water depth) changes, climatic changes, tectonic activity, or a combination of all of these factors. Major fluctuations in palaeoenvironment are indicated by variations in the planktonic/benthonic foraminifera ratios at many horizons within the Upper Albian and Lower Cenomanian (Figs **). Even within Zone 6a Hart (1988) recognised two major cycles, the latter of these having a similar planktonic percentage to the basal transgressive (deepening) pulse of the Glauconitic Marl (TML datum A). Where changes in water depth have been recorded within the Late Albian and Early Cenomanian sediments encountered in the tunnel drives they are not always associated with non depositional or minor erosional surfaces. This would suggest that changes in the palaeoclimate, particularly water depth, are not the primary cause of these non-sequences. In contrast, these two non-sequences can he demonstrated to be directly associated with major changes in the rates of basement subsidence, both in time and space. This would imply that these non-sequences are primarily of tectonic origin. The layering of the Upper Gault Clay and Chalk Marl into a number of isochronous Units or sequences (Figs **) by TML has allowed the recognition of significant thickness variations within each of the Units. These thickness variations indicate that basement subsidence occurred at different rates within many of the Units and that the rates of basement subsidence at the same locality may have been substantially different between successive Units. The six TML stratigraphic Units of Upper Albian to Lower Cenomanian age can be recognised along the entire length of the UK tunnel alignment. Of these, the four Lower Cenomanian Units have distinct geotechnical characteristics as defined by their calcimetry profiles. (Fig. **). Evidence of variations in thickness of the Units is often supported by the facies changes within the Units, the best example of this being the change in the glauconitic facies of the Basal Unit across the UK Crossover (km 25-km 30). Both the marked variations in thickness of the Units and the facies changes within the Units suggest some structural control of deposition. Cyclicity of the Units is controlled by both water depth and climatic changes, although they are not the only causes, since both primary and secondary cycles can be recognised. Because of the nature of both the water depth and climatic control, many of these changes can be recognised regionally. The Lower/Middle and Middle/Upper Unit boundaries are the result of significant water depth changes, hence their distinctiveness. Other datums such as TML Datum K, are the result of climatic changes. The two major unconformities of the Upper Albian are now considered by TML to be partly associated with mainly the result of tectonic activity rather than purely changes in water depth or climate, changes that were occurring during the deposition of each individual sedimentary cycle. The use of selective quantitative micropalaeontology which could be integrated with detailed lithostratigraphy and macrofossil data proved to be highly successful at the Channel Tunnel, providing a framework on which probe holes could be verified. It also provided new stratigraphic and structural information which proved of value in resolving the contractual dispute over adverse tunnelling conditions. Where deposition has occurred in shallow shelf seas of Cretaceous to Quaternary in age it has often proved difficult for stratigraphers to apply planktonic zonation schemes, owing both to the sporadic occurrence of planktonic specimens and the depth stratification of species, which results in many of the zonally significant soecies not occurring in shallow shelf sediments. This has often meant that, as with Carter & Hart (1977), that stratigraphers resort to attempting to define a zonation based on benthonic species. Everyone is aware of the massive problems associated with this method and the resulting zonation schemes are often difficult to accurately apply and may prove highly diachronous. Quite amazing and often totally useless correlations have often been made and presented, particularly before 1990. It is now recognised that a more integrated approach is required. Unfortunately, the original work of Carter and Hart with regard to their planktonic benthonic ratio charts was not particularly well received by the geological community at the time. The work at the Channel Tunnel has proven that if this work had been taken more seriously at the time and more research had been undertaken into its merits then the commercial application of microfossils would have advanced at a far quicker rate than it actually has. The technique applied at the Channel Tunnel not only provided highly accurate isochronous datums, it provided more datums to correlate than had been defined before. Datums could be correlated over tens of kms and while not all of the datums could be correlated across facies changes many of them could be. This would suggest that the technique is applicable over much greater distances and across much greater facies changes than at the Channel Tunnel. There is certainly no reason why this technique cannot be applied to all of the Chalk sequences of Europe or indeed to other shallow shelf sea deposits worldwide. Both for research purposes and commercially, the major benefit would be the larger (approximately) double the number of samples that could be analysed daily with obvious cost and turnover benefits. Because of the detailed sampling required it could only be considered viable in the oil industry where expanded sequences of sediments are present, such as the Tertiary shallow marine basins of SE Asia, or where specific horizons are targeted with more detailed sampling. Such basins do exhibit marked changes in their planktonic abundances with many major flooding events. Such floods which represent major palaeoenvironmental changes, could therefore be used to define isochronous horizons for correlation purposes.
|
|||||||||||||||||||||||||||||||||||||||||||||||
