Clay Minerals (1968) 7, 311.

T H E O R I G I N O F T H E M O N T M O R I L L O N I T E

O F T H E E U R O P E A N C H A L K W I T H S P E C I A L

R E F E R E N C E TO T H E L O W E R C H A L K O F

E N G L A N D

C. V. J E A N S Sedgwick Museum, Cambridge

(Read at a joint meeting of the Clay Minerals Group and the Groupe Beige des Argiles at Brussels 2 June 1967)

A B S T R A C T : The evidence is reviewed for the various hypotheses put forward to explain the origin of the Chalk montmoriUonite.

Recent X-ray investigation of the mineralogy of the acid-insoluble clay fractions (buffered 2 N acetic acid at pH 3; ~ 2 /~ e.s.d.) of the Lower Chalk of England sheds light on the origin of the montmoriUonite. The relations between the qualitative and semi-quantitative mineralogy of the clay fractions and the facies and stratigraphy in this formation have been studied in detail. Montmorillonite, illite, kaolinite, chlorite, vermiculite, pyrophyUite, mixed-layer minerals, quartz, low-temperature cristobalite and apatite have been identified; their semi-quantita- tive distribution reveals that two main antipathetic assemblages are present, between which all gradations occur. The first is characterized by montmoriUonite, fllite, quartz and by montmorillonite/iUite (M/I) values of 0.7 and above; and the second by iUite, kaolinite, chlorite, vermiculite and by M/I values of below 0"2. The distribution of these assemblages or of any particular mineral does not show obvious relations to the facies or stratigraphy.

There is strong evidence that the second of these assemblages is of detrital origin, introduced into the Lower Chalk seas by currents flowing mainly from areas to the east and south-east of England. There is no evidence to suggest that the montmorillonite and the illite of the first assemblage are of detrital or volcanic origin, and their distribution in the Lower Chalk is best explained by their neoformation in the sediment on the sea floor by precipitation from the porewaters. By extrapolation it is thought that most of the Chalk montmorillonite of clay- grade is of neoformational origin. Some from the Campanian and younger Chalks may be detrital. Locally in N.W. Germany and possibly Poland minor amounts may have been derived from the decomposition of volcanic glass in the Chalk seas.

I N T R O D U C T I O N

The Chalk of the Anglo-Par i s Basin has been a special fascinat ion to the scientifically curious for m a n y decades. Its extreme whiteness, fine grain, fr iabil i ty, pureness and apparent homogenei ty have caught the publ ic ' s imaginat ion. To the scientific investi- gator the ma in quest ion has concerned the na ture and origin of the finest calcite

312 C. V. Jeans

material of the Chalk which is too fine to be identified with the light microscope. Was it a chemical prec ip i ta te - -poss ib ly under organic control---or was it the skeletal

remains of micro-organisms? In 1953 Maurice Black settled this quest ion once and for all by demonst ra t ing that this mater ial represented whole and fragmented calcite skeletons of the coccoli thophoridae, p lanktonic mar ine calcareous algae. Since 1949 another Chalk problem has been growing. In this year Mil lot published the first minera l analysis of the acid-insoluble clay fraction of a sample of European Chalk. Its d o m i n a n t component was montmori l loni te , and since then many other analyses have conf i rmed this dominance. At the present t ime perhaps the most interesting of the Chalk problems concerns the origin of the mineral . Already con- siderable argument has occurred and in the first part of this paper a brief summary is given of the evidence and hypotheses put forward. The second part is an account of the author 's work on the clay mineralogy of the Lower Chalk and contemporary rocks of England and its possible bear ing on the origin of the Chalk montmori l loni te .

M I N E R A L T E R M S

Various terms used in this account are defined below. They include the names used to describe the minerals of the Chalk clays.

Montmorillonite. Smeetite identified as dioctahedral by Heim (1957), Perrin (1957), Schrner (1960) and Weir & Catt (1965).

lUite. Dioctahedral hydromica according to Helm (1957), Perrin (1957), Sch~Sner (1960) and Weir.& Catt (1965).

Kaolinite. Kandite identified as kaolinite by Heim (1957), Perrin (1957) and Schtiner (1960). Chlorite. q-he Chalk chlorite is probably iron-rich. In untreated, glycerolated, and heated

aggregates (440 ~ C) the 14 A diffraction line is considerably less intense than those at 7"1 and 3-54 A. When it is heated to 550~ the intensity of the 14 A line is much enhanced and is more intense than the 7"1 and 3"54 A lines.

Vermiculite. This mineral has been identified by the presence of a 14 A line in untreated and glycerolated aggregates which disappears when heated to 440 ~ C.

Pyrophyllite. This has been identified by lines at 9"0 and 3"02-3"07 A. The former of these is considerably more intense than the latter contrarY to the data given by Brown (1961, Table XIII.3, p. 475). The study of the upper and lower surfaces of the oriented aggregates demon- strates that there are variations in behaviour of this mineral with heat treatment. The diffraction lines of the pyrophyllite from the coarsest material of the clay fraction persist unchanged after heating to 440 ~ C (2 hr) and 550 ~ C (2 hr). Those of the finest material of the clay fraction persist to 440 ~ C but disappear when heated to 550 ~ C.

Mixed-layer clay minerals. A number of weak to very weak diffraction lines have been ascribed to mixedqayer clay minerals. Their spacings range from 50 A downwards. No attempt has been made to identify their type and order of interstratification. Their diffraction lines often occur in only one of the four X-ray diagrams from the sample concerned. Young (1965) has tentatively identified a mixedqayered chlorite-vermiculite mineral in the Lower Chalk of the Leatherhead borehole.

Quartz. Quartz may include high- and low-temperature forms. Small amounts of low- temperature tridymite may also be included as its most intense line at 4-3 A may be masked by the 100 lines of low- and high-temperature quartz.

Low-temperature tridymite. Weir & Catt (1965) have identified this mineral in clays from the Upper Chalk of Sussex.

Low-temperature cristobalite. Diffraction lines have been recorded at between 4"04 and 4-11 A in a few samples from the Lower Chalk. These lines persist after heating to 550~

The origin of the Chalk montmorillonite 313 and are interpreted as being the strongest lines, 101, of low-temperature cristobalite. Cristobalite has been identified by Millot and his co-workers (Duplaix et al., 1960) in untreated and glycerolated clays of French Chalk by the presence of lines between 4.04 and 4"11 A.

Glauconite. The term gtauconite is used in the geological sense for green clays and pellets, which according to Burst (1958) may contain minerals other than glauconite (sensu stricto).

M / I values. The M / I value of a sample is the illite content expressed as a fraction of the montmorillonite content.

Threshold proportion. The threshold proportion of a mineral is the maximum amount which can be present in a clay mixture without being identified by the X-ray methods used.

Intrinsic and non-intrinsic diagenesis. The diagenesis of a sediment can be divided genetically into two parts, the intrinsic and non-intrinsic diagenesis. The intrinsic diagenesis consists of the physical and chemical processes affecting the sediment and is genetically related to its milieu of deposition, its fauna, its porewaters and its own physical and chemical nature. Non-intrinsic diagenesis is controlled by processes and events genetically unrelated to the sediment and the milieu of its deposition These include earthquakes, stress systems causing folding, faulting, jointing and pressure solution, and also the circulation of groundwaters of varying chemistry, and weathering.

Chalk. The Chalk is a marine Upper Cretaceous formation occurring extensively in Northern Europe and ranges in age from Cenomanian to Maestrichtian. Its sediments are here referred to as Chalk, although they mainly consist of chalks, marls and fine-grained limestones.

H I S T O R I C A L S U M M A R Y

The problem of the Chalk montmorillonite dates from 1957. In France Millot, Camez & Bonte (1957) suggested that montmorillonite from chalks of the Paris Basin was neoformed in the Chalk seas. At the same time Heim (1957) concluded that the montmorillonite of chalks from central Germany was of detrital origin.

Later Duplaix et al. (1960) expanded the neoformational hypothesis and suggested that the montmorillonite was formed either at the actual moment of sedimentation or at a slightly later stage during diagenesis. They also put forward the possibility that the cristobalite with which montmorillonite may be associated, might have been neoformed at the same time and under similar conditions. Millot (1964) remarking on Heim's conclusions suggested that the montmorillonite is unlikely to have been derived from continental areas as he thought these were characterized by laterite formation. Although laterites and bauxites underlie Upper Cretaceous rocks in Sweden (Brotzen, 1960), southern France (Lapparent, 1930), Hungary (Vadasz & Fial6p, 1959) and Czechoslovakia (Vachtl, 1950; Soukup, 1954) some of the fresh- water Upper Cretaceous sediments contain montmoriUonite as their dominant clay mineral (Sabatier, 1964). This invalidates Millot's re-interpretation of Helm's results.

These two contrasting hypotheses, may reflect both different approaches and materials. Heim studied the mineralogy of the acid-insoluble residues of 26 samples from the Cenomanian and Turonian Chalk of Central Germany. In the clay fractions montmorillonite was present in only 16 samples in amounts up to 41%; illite and quartz were the dominant components, kaolinite occurred occasionally always in small amounts. Millot and his co-workers dealt with 12 samples from the Turonian and Senonian Chalk of the Paris Basin and Angoumois. Montmorillonite was nearly always the dominant component of the acid-insoluble clay fractions and in some

314 C. V. .leans

instances constituted practically the whole; illite occurred in amounts up to 30%; kaolinite, cristobalite and possibly pyrophyUite were often present as minor com- ponents. A number of diffraction lines between 4"17 and 4"23 A, recorded from some of these samples, were not assigned to any mineral. They may represent the 100 line of low-temperature quartz.

Analyses of English Chalk clays have been published since 1956. Perrin (1957) was the first to demonstrate the general vertical variation in the Chalk; the Lower Chalk (Cenomanian and lowest Turonian) is characterized by variable assemblages containing montmorillonite, illite, kaolinite, vermiculite and quartz; the Middle and Upper Chalks (Turonian and Senonian) are characterized by assemblages dominated by nearly equal amounts of montmofillonite and iUite with lesser amounts of quartz; apatite occurred throughout the sequence. Young (1965) has confirmed these results in Surrey in the Leatherhead borehole and in addition noted the possible presence of a chlorite and/or mixed-layer chlorite-vermiculite and/or a vermiculite in the Lower Chalk. Weir & Catt (1965) made a significant contribution to the origin of the Chalk montmorillonite by carrying out an electron-microscope and X-ray analysis of the acid-insoluble residue from 7 samples of the Upper Chalk of Sussex. They recognized various neoformed minerals (barytes, alkali feldspar, quartz, low-temperature tridymite and apatite) and found the morphology of the montmorillonite to be very constant. They suggested that this might be the effect of the Chalk sea enviroumentmincoming detritus of various compositions having been altered to a constant form.

There are important differences between the published analyses of the acid- insoluble clays of French and English Chalk. The clays from the English Chalk differ by containing (1) montmorillonite up to 72% (Weir & Catt, 1965, sample 4 recalculated to exclude apatite and amorphous material), never approaching 100% as in some of the French material, (2) low-temperature tridymite but not cristobalite as the French Chalk contains, (3) no pyrophyllite, and (4) apatite. French Chalk clays never contain apatite according to published results but this is probably due to the different methods used for extracting the insoluble residues. The French use N/10 hydrochloric acid, which may dissolve the apatite, while the English use buffered acetic acid at pH 3 and above.

Valeton (1960) and Sch/Sner (1960) have described aggregate grains (>6 t~) con- sisting very largely of dioctahedral montmoriUonite from chalk samples from N.W. Germany ranging in age from Cenomanian to Maestrichtian. These aggregate grains have been shown by Valeton (1960) to represent decomposed fragments of volcanic glass. Authigenic silicates (heulandite, melilite, quartz, orthoclase and albite) were recorded in varying amounts in the samples studied by these authors.

L O W E R C H A L K OF E N G L A N D

Since 1961 the author has been studying the relations between the clay mineralogy, lithologies, stratigraphy and macrofossils of the Lower Chalk and contemporary rocks of England in an attempt to determine their origins and the environments

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Surface and subsurface extent of Lower Chalk

Surface and subsurface extent of contemporary rocks

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FIG. 1. Extent of Lower Chalk; localities; extent of detrital and neoformed Clay assemblages; suggested derivation directions for detrital clay.

A. Wilmington, Devon. SY/209999 B. Hooken Cliff, Devon. SY/221879 C. Beer, Devon. SY/228888 D. White Nothe, Dorset. SY/772806 E. BaUard Cliff, Dorset. SZ/040811-046812 F. Culver Cliff, Isle of Wight. SZ/630854 G. Eastboume, Sussex. TV/590952 H. Folkestone, Kent. TR/253383-306396 I. Leatherhead, Surrey. TQ/158565 J. Pitstone, Buckinghamshire. SP/935145 K. Totternhor Bedfordshire. TL/985222 L. Cambridgeshire: Cherry Hinton TL/485558, Barrington TL/395507 M Norfolk: Hunstanton TF/673413-679425, Heacham TF/687368 N. Lincolnshire: Dalby TF/405706, Candlesby TF/460682, Welton TF/451691 O. Goodmanham, Yorkshire. SE/898429 P. Yorkshire: Flixton TA/039791, Speeton TA/161751-184746

316 C. V. Jeans

and processes of their sedimentation and diagenesis. These researches are far enough advanced to give an account of the relations between the variations in the clay mineralogy, in lithologies and in various factors of the macrofossils, and to come to certain conclusions about the origin of the montmoriUonite in these rocks. The localities at which these rocks have been studied and the new unpublished strati- graphical scheme used in the construction of Figs 4-9 are shown in Figs 1 and 2 respectively.

W e l l - known marker l)iacTarnmotic horizons not used in this vertical Marker horizons men~- ',los used for "narker. horizong

Doper section toned in this paper in Figs 4-9 Plenus Marls. l ~------'---'~--I(̀ -'--=E---I Base of Melbourn Rock (~ Erosion surface Beds 4 - 6 of Jefferies(1962) at base ~L,-.- ~--=- -

Upper l imit of t h i c k - ~, : - . , : . . '~ . - -s hel led Ha~aster spp

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FIG. 2. Zonal scheme used in Figs 4-9.

G E O L O G I C A L S U M M A R Y

The Lower Chalk and contemporary rocks of England range in age from Lower Cenomanian to lowest Turonian.

Lower Chalk. The Lower Chalk, 50-250 ft thick, outcrops and underlies large areas in east and south England (Fig. 1). Lithologically it consists mainly of very fine-grained homogeneous and nodular limestones, chalks and marls composed of four major groups of components; (1) remains of Coccolithophoridae, (2) spheres (Calcisphaerulidae), foraminifera and other invertebrate skeletal fragments, (3) silicate minerals; mainly fine silt and clay-grade clay minerals and quartz, (4) calcite cement precipitated in the sediments during their intrinsic

The origin of the Chalk montmorillonite 317 diagenesis. Variations in the relative abundances of these constituents are responsible for the main lithological types. The overall distribution of lithologies is shown in Fig. 4.

Two lithological characters need special mention, variations in carbonate content and in average grain size of the original Lower Chalk sediment.

Alternating beds of more and less carbonate-rich chalky limestones and marls is the most dominant lithological facies in the Lower Chalk although in east England it is masked by the diagenetic development of nodules. The variation in carbonate contents reflects differences in the original sediment composition, and during the intrinsic diagenesis the carbonate-rich horizons have been emphasized by preferential calcite porefilling.

The original sediments of the Lower Chalk range in average grain-size f rom conglomerates to fine silts. Their particle size distributions are usually multimodal and are considerably different f rom those of their separate calcite and acid-insoluble fractions (Fig. 3). Very extensive

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FIG. 3. Particle size distribution of total and acid-insoluble (cold N acetic acid buffered at pH 3 with sodium acetate) fractions of Lower Chalk samples from Lincolnshire and Yorkshire. Details of horizons and localities given in Figs 55 and 57 of Jeans (1967).

bioturbat ion is probably partially responsible for the general mult imodal nature al though the formation of various minerals in the sediments after their deposition may also be responsible. Five cycles, defined on their average grain-size, have been recognized in east England. Each starts with a single, or group of, bored or burrowed surfaces, above which coarse-grained chalks occur passing up into the finer-grained ones. Often associated with the base of these cycles is the appearance of a particular group of macrofossils. This group is called the pulse fauna and has also been found at other horizons not associated with the base of the lithological cycles. Southwards from Norfolk the pulse faunas and lithological cycles can be traced to varying extents into the sequences of southern England (Fig. 4).

Contemporary rocks. Contemporary rocks are much thinner, of different lithology and are restricted to south-west England (Fig. 1). They range in thickness f rom 1 to 40 ft and consist of coarser grained sediments such as quartz sands, chalky quartz sands and glauconite- quartz sandy limestones in which sand-grade quartz and other detrital minerals contribute a conspicuous portion.

318 C. V. Jeans If the Lower Chalk and contemporary rocks are considered as a whole two major areas

of relatively thin condensed deposits can be recognized, the Devon-west Dorset region and the Norfolk-Lincolnshire-east Yorkshire region (Fig. 4). They are not sharply defined and pass gradually into regions of thicker sediments. These regions are characterized by levels of condensed deposits; included are well-lithified and relatively well-sorted sand-grade deposits, which are often associated with bored and burrowed surfaces and thin layers of phosphatic material and stromatolites.

Chemical diagenesis

Five chemical processes are known to have affected the sediments of the Lower Chalk and contemporary rocks during their intrinsic diagenesis.

Calcite precipitation in the porespace of the sediment is the most widespread of these processes. It is best developed immediately below certain burrowed surfaces where the original sediment has been converted into a derise rock of low porosity at a very early stage in its intrinsic diagenesis. The nodular chalky limestones and marls of east England are the product of local centres of preferential calcite pore- filling in the sediments.

Glauconization and /o r glauconite precipitation is not an uncommon process. Glauconite sands are often present at the base of the Lower Chalk (Fig. 4) and occasional grains are not infrequent at other horizons. A bright green clay pigment, insoluble in buffered acetic acid at pH 3, is conspicuous in many of the acid-insoluble clay fractions, as a lining to certain burrows in the chalk sediment, as blebs and streaks where pink and red coloured chalks have suffered calcite porefilling and dis- colouration below well-defined burrowed surfaces, and as a stain to fossils and chalk surfaces exposed on the original Lower Chalk seafloor.

Alkali feldspar precipitation may be a widespread phenomenon. Maurice Black has noted considerable abundances of euhedral crystals of plagioclase feldspar in the Lower Chalk of Cambridgeshire. He has optically identified these as albite- oligoclase, near to albite.

Limited phosphatization and silicification of the sediment has occurred locally. There is no evidence to suggest that the bottom waters of the Lower Chalk sea

and the pore waters of its sediment were anything other than mildly alkaline in nature. The calcite fraction of the sediment shows no indications of having been corroded during its intrinsic diagenesis. The only calcite dissolution to have occurred has been caused by pressure solution during late non-intrinsic diagenesis.

C L A Y M I N E R A L O G Y

Samples were systematically collected from all the Lower Chalk and contemporary rock sequences studied so as to represent any obvious or cryptic variation in the facies. The acid-insoluble residues of 160 samples were extracted by dissolution in buffered acetic acid at pH 3 and above, and from these the clay-grade fractions were separated by sedimentation and mineralogically analysed by X-ray diffraction. The horizons and localities of these samples are shown in Fig. 5.

The origin of the Chalk montmorillonite 319 Method

The samples were finely ground with a mechanical roller-crusher. A portion of each, sufficient to give 5-6 g of clay-grade acid-insoluble material, was dissolved in cold buffered 2 N acetic acid at pH 3 and above. After removal of any organic matter by treatment with 20 vol hydrogen peroxide, the insoluble residues were dispersed in 0"1% buffered sodium hexametaphosphate solution at pH 8 by mechanical shaking for at least 2 hr. A portion of the clay-grade fraction ( < 2 ~ e.s.d.) was extracted by sedimentation, centrifuged onto two glass slides using the tubes of Perrin (1955), and the resulting oriented aggregates were analysed by standard X-ray diffraction methods using two Brindley-Robinson type cameras. X-ray diagrams for each sample were taken from the untreated, glycerolated (2 hr in glycerol vapour at 110 ~ C), 440 ~ C heated (2 hr) and 550 ~ C heated (2 hr) aggre- gates. Only the upper surfaces of the aggregates were X-rayed. Semi-quantitative analysis was carried out using the external binary mixture method as discussed by Brindley (1961). There was no rounding off of the estimated percentages. The standards used were those of the School of Agriculture, University of Cambridge.

In a series of trials the lower surfaces of the aggregates were X-rayed, and the semi-quantitative results were found to be considerably different from those of the upper surfaces, supporting the experiments of Gibbs (1965). It was found that montmorillonite decreased, while iUite, kaolinite, chlorite and quartz increased in amount towards the lower surface. At the moment it is not possible to generalize on the overall trends.

Results

The following minerals have been identified:

Major components Range of weight Montmorillonite 0 - 80 IUite 20 - 84 Kaolinite 0 - 29 Quartz 0 - 51 Apatite occurred in many samples but its percentages were not estimated.

Minor components Vermiculite; chlorite; mixed-layer clay minerals; pyrophyllite and low-temperature cristobalite (probably minor components).

The semi-quantitative distribution of iUite is shown in Fig. 6; of kaolinite, chlorite and vermiculite in Fig. 7; of montmoriUonite and pyrophyllite in Fig. 8; and of quartz and cristobalite in Fig. 9. A close inspection of these figures and comparison with Fig. 4 shows that there is no straightforward relationship between the clay mineral distribution and the lithological facies or the stratigraphy as defined by the marker horizons. It should be particularly noted that there is no relationship between the clay mineralogy and either the lithological cycles or pulse faunas of east England.

There is an obvious similarity between the distribution patterns of illite and

320 C. V. Jeans

kaolinite on one hand and between montmorillonite and quartz on the other. These two patterns are antipathetic to each other.

The illite distribution pattern can be divided into two parts. This can be seen in Fig. 5--here the illite content of each sample has been plotted as its fraction of the montmorillonite contents of the same sample (M/I). The first part consists of the chalk in which the M / I value is below 0"2, the second consists of the chalk in which the M / I value is 0"7 or above. There is an intermediate zone of chalk in which values range from 0"2 to below 0"7. Values in the second part rarely rise above 2"0, the majority lie between 1"0 and 1"5.

The distribution of samples with 10% or more of kaolinite is similar to that of samples with 50% and more illite. Even better correlation occurs between these kaolinite-rich samples and distribution of those with M / I values of below 0"7.

The majority of samples containing vermiculite and /or chlorite contain 10% or more kaolinite. The distribution of these two minerals relative to the two main masses of chalk containing 10% or more kaolinite, Culver Cliff and the Kent- Norfolk area, is somewhat different and is worthy of special comment. The kaolinite- rich chalk of the trecensis subzone of Culver Cliff contains both vermiculite separately and mixed with chlorite. Chalks at equivalent horizons in Dorset and Eastbourne contain chlorite but lack vermiculite and the abundance of kaolinite. In the Kent- Norfolk region the kaolinite-rich chalk samples from Folkestone usually contain a mixture of vermiculite and chlorite but may contain these minerals separately; in the Leatherhead borehole, according to Young (1965) the situation is similar; but in Cambridgeshire and Norfolk they contain only vermiculite. Vermiculite has not been found in samples having less than 10 % kaolinite.

Pyrophyllite and low-temperature cristobalite have been identified in two of three samples from the varians zone of Buckinghamshire. They are associated with clay mineral assemblages lacking in quartz and dominated by approximately equal amounts of montmorillonite and illite.

Mixed-layer clay minerals have occasionally been recorded. Their very weak diffraction lines and their irregular appearance in the X-ray diagrams from the same sample suggests they occur in only very minor amounts.

Interpretation o] clay variation

(1) The absence of any obvious relation between the facies and distribution of any of the clay minerals suggests that their presence or abundance was not con- trolled by either the lithologies and their intrinsic diagenesis or by factors which affected the distribution of the macrofossils.

The very limited occurrence of mixed-layer clay minerals does not give support to the possibility that any of the clay minerals have been formed from any other of the clay minerals of the assemblages.

(2) Detrital origin o] kaolinite, illite, chlorite and vermiculite. Judging by the conditions controlling the formation of clay minerals in vitro at atmospheric pressure and low temperature (Millot, 1964, gives summary) the alkaline milieu of the Lower

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The origin of the Chalk montmorillonite 321

Chalk sea and sediment would have been unsuitable for the neoformation of kaolinite, but might have been suitable for the neoformation of montmorillonite, vermiculite, chlorite, illite and quartz. Therefore it is most likely that kaolinite is of detrital origin.

The similarity in the distribution patterns of kaolinite, illite with M / I values up to 0"7, chlorite and vermiculite suggests they are of a common detrital origin.

(3) Detrital onigin ]or montmoriUonite? The antipathetic relation between the montmorillonite pattern on the one hand and those of kaolinite, illite (M/ I -(0-7) , chlorite and vermiculite on the other is that to be expected if this segregation has been controlled by differential settling under increasingly saline conditions. White- house, Jeffrey & Debrecht (1960) have investigated the behaviour of the common clay minerals in saline solutions up to full oceanic salinities (18). They found that iliite and kaolinite reached their maximum sedimentation rates of 15"8 and 11-8 m/day respectively at a chlorinity of approximately 2. Chlorite had rates intermediate between illite and kaolinite. Vermiculite settled slightly faster than illite at chlorinities in excess of 10. Montmorillonite had very different characters, its settling rate increased gradually over the whole chlorinity range investigated (0-18). The maximum rate of 1"3 m/day was reached at full oceanic chlorinity (18). These results suggest that if a suspension containing illite, kaolinite, chlorite, vermiculite and montmorillonite was transported from freshwater through increas- ingly brackish water and finally into fully marine water the following differential mineral separation would occur. The fast-settling minerals--illite, kaolinite, chlorite, vermiculite--would be deposited in relatively large amounts in the brackish and marine environments. While montmorillonite would be deposited in much 'smaller quantities and its proportion in the suspension would increase seawards and with distance from the point at which salinity-controlled differential settling becomes effective.

If the montmorillonite was detrital and derived from the same source as the fast-settling clays, differential settling would have caused an antipathetic relationship between the montmorillonite and the other minerals. Montmorillonite would only have to occur as a minor component in the freshwater clay assemblage to allow it to be concentrated as the dominant component of the clay suspension and sediments far distant from the point at which the detritus was discharged into the Lower Chalk sea. In the marine environment the clay minerals would have been sorted according to their settling properties.

The fast-settling minerals--illite, kaolinite, chlorite and vermiculite--would have been deposited preferentially in the higher energy environments. But there is no evidence indicating that they are more abundant in the coarser grade sediments of the Lower Chalk and contemporary rocks and this indicates that the detrital hypothesis for the origin of montmorillonite is untenable.

(4) Montmorillonite of volcanic origin? Montmorillonite is a common breakdown product of volcanic ash in the marine environment. Perhaps the best examples are the extensive montmorillonite bentonite beds common in the Upper Cretaceous of the North American Great Plain, which were derived from penecontemporaneous

322 C. V. Jeans

explosive andesitic vulcanism in the Rocky Mountain region and the area immediately to the west.

No evidence has been found in the Lower Chalk and contemporary rocks of England suggesting that the montmorillonite is of volcanic origin. No glass shards or other volcanic detritus have been discovered. None of the extensive thin marl bands--one has been traced along the outcrop for over 90 miles--show any evidence of having originated from single ashfalls; their clay mineralogy does not differ from that of the chalk immediately above or below.

There is still the possibility that all the montmorillonite was derived from the background fall-out of volcanic dust originating from the total world activity at the time. The European area would have made very little contribution as there is evidence of only very meagre volcanicity [Germany--Valeton (1959, 1960), Dorn & Br~tutigam (1959); Poland--Sujkowski (1931), Valeton (1959); Lesser Caucasus-- Nalivkin (1960)]. The main contribution must have come from the abundant andesitic vulcanism associated with the Rocky Mountains and the eastern seaboard of Russia. The decomposition of the fine volcanic dust would have had to have taken place after its inclusion in the chalk sediment, otherwise sorting would have occurred and a relation between the sediment grade and the clay mineralogy would be present. In the author's opinion this background fallout of volcanic ash would not have been sufficient to provide all the montmorillonite.

(5) Montmorillonite o/neoformation. The only possibility left is that the mont- morillonite is of neoformation, precipitated in the sediment of the Lower Chalk seafloor after any reworking had taken place. The reactions giving rise to this mineral were not controlled by the variations in conditions reflected in the varying faunas and lithologies and their intrinsic diagenesis.

(6) Origin of non-detrital illite. It has already been concluded that the illite in samples with M/I values of less than 0"7 is of detrital origin. If the remainder of the illite was of similar origin it would be very difficult to explain the distribution of M/I values. If it came from the same source as the kaolinite its proportion in the clay should decrease rapidly with increasing distance from the regions of high kaolinite contents; but if it was derived from other detrital sources one would expect there to be regions of low M/I values not related to the distribution of high kaolinite. Neither of these patterns are evident and the possibility of a volcanic or neoformational origin has to be considered.

Illite is only an occasional and minor component of the decomposed volcanic ashes of the North American Upper Cretaceous. The absence of volcanic detritus and the abundance of this mineral in the Lower Chalk suggests it is of neoformational origin and formed in the sediment in a manner similar to montmoriUonlte.

Fifty-one of the seventy-six samples with M/I values of 0"7 and above have values between 1-0 and 1.5 suggesting the conditions controlling the neoformation of illite and montmorillonite were relatively constant. Only occasionally is the value of 2 reached and exceeded, and no doubt this reflects variations in composition of the sediment's porewaters from which these minerals were probably precipitated.

(7) Origin of silica minerals. Although the distribution pattern of quartz is

The origin of the Chalk montmorillonite 323 antipathetic to those of the detrital minerals there is no similarity with the distri- bution of montmorillonite. This can be seen by comparing the extent of 5% and more quartz with that of either 25% and more montmorillonite or of M/ I values of 0"7 and above. There is considerable overlap in the Yorkshire-Norfolk and Eastbourne-Dorset areas but there is no similarity in the Kent-Cambridgeshire region.

There is only scant indirect evidence for the origin of the quartz. Samples from Lincolnshire and Yorkshire, the particle size distribution of which have been analysed, contain considerable amounts of silicified sand-grade calcite material and also numerous subangular silt-grade quartz grains; the latter without doubt of detrital origin. This suggests that the quartz of the clay fraction may have originated by the silicification of the finest calcite material and also from detrital sources. Until more information is available no satisfactory conclusions can be drawn about the origin of the clay-grade quartz.

Low-temperature cristobalite, identified in two samples, may be of neoformational origin.

(8) Source o[ detrital clays. The distribution of samples containing 10% and more kaolinite indicates that two currents or current systems introduced detrital clays into the Lower Chalk sea. One being responsible for the high kaolinite con- tents in the trecensis subzone of Culver Cliff and the other for those of the sequences of Folkestone, Leatherhead. Buckinghamshire, Cambridgeshire and Norfolk.

It is possible to determine the general flow direction of these currents by con- sidering the relationship between the three-dimensional shape of the chalk with the high detritus composition and the stratigraphical divisions. The conclusions thus obtained can be independently checked if one assumes that the detrital clays, once in the Lower Chalk sea, would reach their threshold proportions in an order governed by their original abundance in the clay suspension, their differential settling rates and by the distance from their source. The independent neoformation of illite and montmorillonite in the sediment has been of considerable use as it effectively increased by dilution the threshold proportions of all the detrital clays, thus allowing flow lines to be more exactly defined. The fullest compliment of detrital clays would occur at a point closest to the source---current system or entry-point of detritus into Lower Chalk Sea--and with increasing distance away various of these minerals would 'disappear' as they reach their threshold proportions.

The three-dimensional form of the detrital-rich chalk evident in the trecensis subzone of Culver Cliff is not known. It certainly does not extend for any distance to either the west or east as it is not present in the Lower Chalk of Eastbourne or of the Dorset coast. Some idea of the differential settling trends and the threshold proportions can be obtained by considering all the chalk of the Dorset coast and Eastbourne equivalent to and including the chalk of Culver Cliff rich in detrital clay (Fig. 5). Kaolinite descends below 10% at the same point as the M/ I values reach or exceed 0"2. Vermiculite, occurring with chlorite in the detrital-rich chalk of Culver Cliff, reaches its threshold proportions at approximately the same time as the kaolinite contents descend below 10%. Chlorite reaches its threshold pro-

324 C. 111. Jeans

portions later and occurs outside the 10% kaolinite contour in the sequences of Eastbourne and the Dorset coast. This distribution pattern of detrital clays is con- siderably different from that associated with the second current discussed in the next paragraphs.

The approximate three-dimensional form of the occurrence of samples with 10% and more kaolinite from the Lower Chalk sequences of Folkestone, Leatherhead, Buckinghamshire and Bedfordshire, Cambridgeshire and Norfolk can be deduced from Fig. 5. Initially this body of chalk rich in detrital clay was laid down probably in two separate areas. At first it was only deposited in the Cambridge-Norfolk area while elsewhere chalk with neoformed illite and montmorillonite was being laid down. At a slightly higher horizon detrital clay sedimentation was initiated in the Folkestone-Leatherhead region and this later fused to the first area, which had by then been considerably reduced in extent only occupying the Norfolk area. The latest appearance of detrital clay sedimentation was in Buckinghamshire where it first appears just above the base of the subglobosus zone. The withdrawal of the zone of detritus-rich clays occurred first in Bedfordshire and Norfolk, then Cam- bridgeshire and finally Leatherhead. At the end of Lower Chalk times this zone still occupied the Folkestone area. This distribution is easiest explained by con- sidering that initially the detrital clays were borne to the Cambridgeshire-Norfolk area by westerly flowing currents. Slightly later they were introduced into Kent and Surrey by westerly and north-westerly flowing currents which later spread detrital clays over the whole Kent-Norfolk area. The effect of these detritus-rich currents started to wane first in Buckinghamshire and Norfolk, then Cambridge- shire, Leatherhead, and at the beginning of the Turonian they only affected the Kent area. Whether the withdrawal of detrital clays from the Lower Chalk sea was controlled by the decrease in the amount of detrital material being supplied from the penecontemporaneous continental source areas, or by an actual withdrawal of the currents bearing detrital clays from the English area and their re-routing to other regions, is not known. The flow directions suggested for these currents is supported by evidence from the threshold proportions of chlorite and vermiculite. The fullest cc.mpliment of detrital minerals is restricted to the Lower Chalk of Folkestone and possibly Leatherhead. Here both chlorite and vermiculite may occur while in the Cambridgeshire and Norfolk sequences only vermiculite occurs. This is due to the chlorite reaching its threshold proportion before vermiculite and confirms that the current system flowed in a general manner from Kent and Surrey towards Cam- bridgeshire and Norfolk.

The detrital clays at the base of the Lower Chalk in the Cambridgeshire-Norfolk area are similar to those from higher horizons in the same area. They are thought to have been introduced by westerly flowing currents, the original detritus of which contained chlorite, but by the time this area was reached it had already fallen below its threshold proportion. There is no evidence suggesting that this detrital clay came from the north-west borne by a south-easterly flowing current system.

The mineral distribution in the Kent-Norfolk chalk rich in detrital clay is con- siderably different from that of the Dorset-Eastbourne region. The M ! I values reach

The origin of the Chalk montmorillonite 325

0"2 and above before the kaolinite contents drop below 10%. Chlorite reaches its threshold proportion before vermiculite and with few exceptions both these minerals have passed their threshold proportions before kaolinite drops below 10%. These differences are thought most likely to represent original differences in the relative quantities of the minerals in the two main source current systems. Although they could possibly be caused by variations in the differential settling properties of the minerals of these two currents. The ratios of kaolinite to illite and of vermiculite to chlorite were markedly greater in the currents associated with the Norfolk- Folkestone detritus than with the Dorset-Eastbourne detritus.

Summary ol interpretations These interpretations suggest that the kaolinite, chlorite, vermiculite and con-

siderable amounts of illite was detritus introduced by two main currents or current systems into the Lower Chalk sea in which considerable amounts of iUite and montmorillonite were being neoformed in the sediment during its intrinsic diagenesis (Fig. 1). In the immediate vicinity of these currents the amount of detrital clays deposited was sufficient to dominate the clay fraction of the sediment. With in- creasing distance from the currents this dominance rapidly decreased and one passed into regions characterized by neoformed montmorillonite-illite clay assemblages. One of the currents, the flow direction of which is unknown, was responsible for the chalks of Culver Cliff which are rich in detrital clays. The other one, flowing in westerly and north-westerly directions, was responsible for the detrital clay-rich chalks of the Norfolk-Folkestone region. The qualitative mineralogy of the detritus of both currents was similar consisting of kaolinite, illite, chlorite and vermiculite. The proportions of these various minerals was markedly different and is now reflected in their distribution. The kaolinite-illite and vermiculite--chlorite ratios were higher for the detritus of the current system that affected the Folkestone- Norfolk area.

Considerable amounts of montmorillonite and illite were neoformed in the chalk sediment of the regions not dominated by detrital clay sedimentation and were occasionally accompanied by the formation of low-temperature cristobalite and pyrophyllite. These neoformations were not controlled by factors affecting the variations in lithologies and faunas. They must have occurred as precipitates from the alkaline porewaters of the sediments at a stage during the intrinsic diagenesis post-dating any reworking by bottom currents. Variations in the composition of the porewaters may have been responsible for the proportions of montmorillonite and illite precipitated. At the moment little can be said about the origin of the quartz.

CLAY M I N E R A L A S S E M B L A G E S OF T H E

E U R O P E A N C H A L K

In the European Chalk four main clay mineral assemblages have been recognized between which all gradations occur. The minerals of assemblages 1 and 2 are F

326 C . V . Jeans

thought to have been neoformed in the Chalk sediment during its intrinsic diagenesis, The montmoriUonite and illite of assemblage 3 are considered to be of similar origin but no well-founded suggestions can be put forward to explain the quartz. Assemblage 4 consists nearly exclusively of detrital clay minerals with only trace amounts of quartz and neoformed montmorillonite and iUite.

Special mention should be made about the methods used in deciding the amount of detrital minerals present in an assemblage. Kaolinite, chlorite and vermiculite have always been considered as detrital. The amount of detrital illite is determined by considering the M/ I values. If they are below 0"2 the illite is dominantly detrital, if 0"7 and above it is dominantly of neoformational origin. Values between 0"2 and 0"7 probably represent illite assemblages with considerable amounts of both detrital and neoformed material.

The samples containing montmoriUonite aggregates studied by Valeton (1960) and SchSner (1960) have not been included in these assemblages. They deserve special consideration as part or all of their clay-grade montmorillonite may have originated from the decomposition of volcanic detritus in the Chalk sea; this has not yet bccn demonstrated. There arc two possible ways by which the clay-grade montmorillonite could have been derived from volcanic detritus; (i) the direct breakdown of volcanic detritus to clay-grade montmorillonitc; (2) the break-up of montmorillonite aggregates into their component clay-grade particles. At the moment there is little evidence for or against these two possibilities. Sch6ncr has demon- strated that the second of these is unlikely--the aggregates were shaken for 20 hr in a dilute solution of ammonia without any dissaggregation occurring. It is very unlikely that more effective conditions for disaggregation would have been present in the Chalk Seas.

Assemblage 1. Montmorillonite is the dominant mineral; illite may occur in very minor or trace amounts; cristobalite may be present as an accessory. Samples 3, 5 and 6 of Duplaix et al. (1960) belong here. There are no known representatives from the English Chalk.

Assemblage 2. Montmorillonite and illite are the dominant constituents; M/ I values lie between 0"7 and 1"5; pyrophyllite, low-temperature cristobalite and quartz (? neoformed) may occur as accessories. Most of the samples of Middle and Upper Chalk analysed by Pert'in (1957, 1964) and Young (1965) belong to this assemblage. It is also well represented in the varians zone of the Lower Chalk of the Folkestone- Cambridgeshire area.

The analyses of Weir & Catt (1965), 1 and 2 of Duplaix et al. (1960) and of Millot, Camez & Bonte (1957) are intermediate between assemblages 1 and 2. The

c l a y mineral assemblages determined by Weir & Catt (1965) contained accessory neoformed low-temperature tridymite and quartz.

Assemblage 3. The dominant constituents are montmoriUonite, illite and quartz. M / I values range from 0"7 to 1"5. It is characteristic of the Lower Chalk of the Lincolnshire-Yorkshire area and of parts of the sequence in the Eastbourne-Dorset area. A single sample from the Upper Chalk of Goodmanham (Pen-in, 1964) may belong here. Samples OTSK and STL of Heim (1957) are included in this assemblage.

The origin of the Chalk montmorillonite 327

Assemblage 4. This assemblage is characterized by the dominance of detrital minerals. In the English Chalk it is only found in the Lower Chalk: here illite and kaolinite are the main constituents, chlorite and vermiculite occur as accessories; montmorillonite and quartz may also be present in trace amounts. Samples ETSK, LTL, ETL, LTMI-z, ETMI_8, ECK 1-3 and ECP 1-4 of Helm (1957) are considered to belong to this assemblage, but in these examples illite and quartz are the dominant detrital minerals while kaolinite only occurs occasionally in very minor amounts,

Samples LTSK, OTL, HTL, SCK, OCP and OCM of Heim (1957) are inter- mediate between assemblages 3 and 4.

C O N C L U S I O N S ON T H E O R I G I N OF T H E M O N T M O R I L L O N I T E

OF T H E E U R O P E A N C H A L K

The conclusion has been reached by a priori reasoning that the montmoriUonite of the Lower Chalk and contemporary rocks of England has been neoformed in the chalk sediment during its intrinsic diagenesis. There seems to be no reason why a similar origin should not be extrapolated to most of the Chalk montmoriUonite as nearly all of the clay mineral assemblages of the higher horizons are represented in the Lower Chalk. Only the first assemblage, dominated nearly exclusively by montmorillonite, has not been identified. It must not be forgotten that the Campanian and later Upper Cretaceous freshwater deposits of southern France contain appreci- able amounts of montmorillonite. This is likely to have found its way as detritus into the Chalk seas where it would be concentrated by differential settling, and would augment the already abundant montmoriUonite being neoformed in the chalk sediment. It is probably no coincidence that the Campanian and Maestrichtian samples from Angoumois contain a practically pure montmorillonite clay (Duplaix et al., 1960).

In Germany the decomposition of volcanic detritus on the Chalk sea floor may have locally given rise to clay-grade montmorillonite. In the Polish Chalk the occasional presence of volcanic detritus suggests it may also contain some mont- morillonite of similar origin.

These conclusions on the origin of the Chalk montmorillonite are partly in agreement with the neoformational hypothesis of Millot and his co-workers (Millot, Camez & Bonte, 1957; Duplaix et al., 1960; Millot, 1964) but it is not possible to agree with most of their supporting evidence. The main points of departure are listed below.

(1) The Chalk is primarily a bioclastic deposit, not biochemical as Millot claims, and the neoformation-of montmorillonite is in no way causally related to the actual skeleton-forming processes of the coccolithophoridae and other organisms. If the calcite skeletal matter played a role in these neoformational processes in the Chalk sediment it seems probable that any other fine-grained calcite material would have been equally suitable.

(2) The presence of flints or phosphates is not closely connected with the forma- tion of montmorillonite.

3 2 8 C . V. J e a n s

(3) There is no close re la t ionship between the abundance of coarse quar tz grains and of det r i ta l c lay minera ls . This can be seen in the c lay minera l analyses of the Cenoman ian sands and sandy l imestones of Devon; they conta in a b u n d a n t coarse quar tz sand bu t are poo r in det r i ta l c lay minera ls c o m p a r e d to the par ts of the L o w e r Cha lk rich in det r i ta l clays.

(4) A l though later i tes and bauxi tes were widespread on the E u r o p e a n land-masses penecon temporaneous with the Cha lk there was more than sufficient montmor i l lon i t e in the over lying f reshwater depos i t s to supply an abundance of this minera l to the Cha lk sea.

A C K N O W L E D G M E N T S

I am particularly indebted to Dr R. M. S. Perrin of the School of Agriculture, University of Cambridge, for his help and for allowing my use of his X-ray and laboratory facilities; and to my supervisor Maurice Black of the Sedgwick Museum, University of Cambridge, without whose help this research would never have been carried out. The final preparation of this paper was carried out during the tenure of a N.E.R.C. Research Fellowship.

R E F E R E N C E S

BRINDLE",' (3.W. (1961) The X-ray Identification and Crystal Structure of Clay Minerals (G. Brown, editor), Chap. XIV, p. 489. Mineralogical Society, London.

BROTZErq F. (1960) The Mesozoics of Scania, S. Sweden, Guide to excusions A21 and C16, Int. geol. Congr., XXI Norden.

BROWN G. (1961) The X-ray Identification and Crystal Structure of Clay Minerals (G. Brown, editor), Chap. XIII. p. 469. Mineralogical Society, London.

BURST J.F. (1958) Am. Miner. 43, 481. DORN P. & BR~UTIQAM F. (1959) Abh. braunschw, wiss. Ges. 11, 1. Dtrel.~Ix et al. (1960) Bull. Serv. Carte g~oL Als. Lorr. 13, 157. GraBS R.J. (1965) Am. Miner. 50, 741. HEIM D. (1957) Heidelb. Beitr. Miner. Petrogr. 5, 302. JEANS C.V. (1967) The Cenomanian Rocks o/England, Ph.D. thesis, University of Cambridge. JEFFERIES R.P.S. (1962)Palaeontology 4, 609. LAPPARErzr J. DE (1930) Les bauxites de la France m~ridionaIe. M6m. Carte G6ol. d6t. Ft., Paris. MILLOT G. (1949) Th~se Sci. Nancy et G$ol. Appl. Prospec. Min. 2, 172. MmLOT G. (1964) Geologie des Argiles, pp. 234, 390. Mason et Cie, Paris. MILLOT G., CAMEZ T. & But te A. (1957) Bull. Carte gdol. Als. Lorr. 10, 25. NALIVKIN D.N. (1960) The Geology o/ U.S.S.R., p. 120. Pergamon Press, Oxford. PEmUN R.M.S. (1955) Clay Min. Bull. 2, 307. PERR]N R.M.S. (1957) Clay Min. Bull. 3, 193. PEI~L~ R.M.S. (1964) Analysis o/ Calcareous Materials, p. 207. Society for the Chemical In-

dustry, Monograph 18, London. SAB^TmR G. (1964) Bull. Soc. /r. Min~r. Cristallogr. 87, 101. SCH6NER H. (1960) Beitr. Miner. Petrogr. 7, 76. SOUKUP J. (1954) Geotechnica (Czechoslovakia Ostred. Ostav Geol.) 18. SUJKOWSKI Z. (1931) Spraw. pol. Inst. geol. 6, 485. VACnTL J. (1950) Geotechnica (Czechoslovakia, Ostred. ~stav Geol.) 10.

The origin of the Chalk montmorillonite VADASZ E. • FOLOI' F. (1959) El Sistema Cretacico, 1, 221. Int. geol. Congr. XX Mexico. VALETON I. (1959) Neues db. Geol. Paliiont. Mh. 5, 193. VALETON I. (1960) Mitt. geol. Stlnst. Hamb. 29, 26. WEre A.H. & CATT J.A. (1965) Clay Miner. 6, 97. WmTEHOUSE U.G., JEFFREY L.M. & DEBREcrrr I.D. (1960) Clays Clay Miner. 7, i. YOUNG B.R. (1965) Bull. geol. Surv. Gt Br. 23, 110.

329