Origin of solution seams and flaser structure in upper cretaceous chalks of southern England

Sedimentary Geology, 19 (1977) 107--137 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

ORIGIN OF SOLUTION SEAMS AND FLASER STRUCTURE CRETACEOUS CHALKS OF SOUTHERN ENGLAND

107

IN UPPER

ROBERT E. GARRISON and W.J. KENNEDY

Earth Sciences Board, University of California, Santa Cruz, Calif. (U.S.A.) Department of Geology and Mineralogy, University of Oxford, Oxford (Great Britain) (Received October 17, 1975; revised and accepted August 19, 1976)

ABSTRACT Garrison, R.E. and Kennedy, W.J., 1977. Origin of solution seams and flaser structure in Upper Cretaceous chalks of southern England. Sediment. Geol., 19: 107--137. Flaser chalks consist of small ellipsoidal bodies or lenses of relatively pure chalk surrounded by clay-rich solution seams. The latter may be simple, individual clay partings or composite aggregations of clay partings. These flaser structures formed during late burial diagenesis in response to mechanical compaction and pressure dissolution of calcium carbonate. Dissolution preferentially affected the coccolith-rich, fine-grained matrix of the flaser chalks, leaving coarser skeletal particles largely unscathed; in addition, solution was most intense in those parts of the chalks which were originally most argillaceous. Variations in flaser chalks resulted when the above mentioned processes acted on and modified the products of early diagenetic lithification such as nodular chalks, intraformational conglomerates and incipient hardgrounds. True stylolites produced by pressure solution are present only in these early lithified chalks. The most probable range of burial depths at which flaser structure formed in chalks was approximately 300--2000 m.

INTRODUCTION Small-scale lenticular structures and associated irregular marl seams are widely developed in English Chalks, and are amongst the most conspicuous features of many parts of the sequence (Figs. 4--15). Structures of this type a r e w i d e s p r e a d in f i n e - g r a i n e d c a r b o n a t e r o c k s a n d h a v e b e e n d e s c r i b e d in l i m e s t o n e s o f d i v e r s e r e g i o n s , a g e s a n d d e p o s i t i o n a l e n v i r o n m e n t s (e.g. Mc-

Crossan, 1958; Greenwood, 1960; Barrett, 1964; HOllman, 1964; Hallam, 1967; Garrison and Fischer, 1969; Jurgan, 1969; Jenkyns, 1974; Noble and Howells, 1974). Very similar structures have been described in phosphatic rocks (e.g. Dickert, 1966). Because these structures appear to reflect a widespread type of behavior of fine-grained sediments, their origin is a matter of genera] significance. Fabrics of this type developed in carbonate rocks have been given various names, notably sedimentary boudinage (McCrossan, 1958; Greenwood,

108 1960). German-speaking authors have termed limestone with these structures as Knollenkalk (= nodular limestone) or Flaserkalk (= lens limestone). We have adopted the latter term in this paper, and refer to chalks with this type of diagenetic fabric as flaser, nodular flaser or conglomeratic flaser chalks, depending on their original lithology, for this lithology has strongly influenced the detailed morphology of individual units. Relatively few workers have investigated these structures, but amongst this small group, there seems to be general agreement that they originated by diagenetic alteration of fine-grained sediments. The nature of this alteration process is, however, disputed, as is the question as to whether these are of an early or late diagenetic date. Two main processes have been invoked to explain this fabric: (1) solution-reprecipitation during early diagenesis (Hallam, 1967; Garrison and Fischer, 1969; Jenkyns, 1974) or (2) the differential mechanical responses to stress of interlayered competent and incompetent sediment (Ramberg, 1955; McCrossan, 1958), occurring, by implication, rather late in burial diagenesis. In English Chalks, we find compelling evidence that flaser structure owes its origin to late diagenetic solution and compactional processes acting in concert on lithologies which varied from soft chalk to those which had suffered varying degrees of early lithification. Late diagenetic alteration of soft chalks/nodular chalks/intraformational conglomerates/incipient hardgrounds has thus produced a series of broadly similar fabrics as a result of the same processes acting on dissimilar starting materials.

Area of study Fig. 1 shows the extent of the region studied, and some of the more important localities. Because of the unsatisfactory nature of exposures in many inland quarries, we have concentrated on coastal cliff sections for this work, in particular those on (1) the Norfolk Coast, e.g., Hunstanton (National Grid Reference TF 673415), Weybourne Hope (TG 110438), West Runton (TG 160436), Overstrand (TG 255406), Sidestrand (TG 255404) and Trimingham (TG 298379); (2) Kent: the cliffs between Folkestone and Dover (TR 270384--TR 301394), Dover and St. Margarets (TR 327418--TR 327418), Kingsdown (TR 390485), Joss Bay (TR 398702), Kingsgate (TR 395709), Botany Bay (TR 391712), Foreness Point (TR 384716) and Palm Bay (TR 375714) to Birchington (TR 290700); (3) Sussex: Eastbourne (TV 610980) to Beachy Head (TV 595955), Cuckmere Haven (TV 520975), Seaford Head (TV 495975), Peacehaven (TQ 410007) and Brighton (TV 335034); (4) Isle of Wight (Hampshire): Culver Cliff (SZ 628855), Compton Bay (SZ 360855) to Watcombe Bay (SZ 344855) and Alum Bay (SZ 300850); (5) Dorset: Studland Bay (SZ 040825), Ballard Cliff, Swanage (SZ 040813), Worbarrow Bay (SY 860803), Arish Mell (SY 855803), Mupe Bay (SY 845801), Lulworth Cove (SY 825800), St. Oswald's Bay (SY 810803) and Durdle Door (SY 805804) to White Nothe (SY 772807); and (6) Devon:

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Charton Bay (SY 305899), Haven Cliff (SY 260897), White Cliff, Seaton (SY 235895), Beer Harbour (SY 230891) and Beer Head (SY 227880) to Hooken Cliff (SY 224878). General accounts of these coastal sections can be found in Rowe (1900-1908), Jukes-Browne and Hill (1903, 1904) and Kennedy and Garrison (1975). Specific areas are dealt with by Brydone (1912, 1939), OsbourneWhite (1921, 1924, 1928), Gaster (1929, 1939, 1941), Arkell (1947), Peake and Hancock (1961, 1970), Jefferies (1962, 1963), Smart et al. (1966), and Kennedy (1969, 1970a).

Stratigraphy Fig. 2 summarizes the classic bio- and lithostratigraphic divisions of the Chalk sequence in the area studied, based upon the brief but complete review of British Upper Cretaceous biostratigraphy given by Casey et al. (in press) to whom reference should be made.

Sedimentation and depositional environment Chalks are typically coccolith-rich friable biomecrites containing variable amounts of coarse calcitic skeletal debris such as Oligostegina (calcispheres), planktonic and benthonic Foraminiferida, echinoderm, articulate brachiopod, serpulid polychaete, pteriomorph bivalve (notably Inoceramus) and bryozoan fragments, small amounts of originally siliceous skeletal material (now calcitised), authigenic minerals (glauconite, phosphate), coarse terrigenous material (notably quartz) and clay minerals (chiefly illite, kaolinite and montmorillonite). These constituents are discussed by Sorby (1861), Hume (1893), Jukes-Browne and Hill (1903, 1904) amongst early workers, and latterly by Black (1953), Jeans (1967), Hakansson et al. (1974), Scholle (1974), Hancock and Scholle (1975) and Kennedy and Garrison (1975). Locally, non-calcareous constituents may reach rock-forming proportions, as in the case of the Glauconitic Marl, the phosphatic Taplow Chalk (Wilcox, 1953a, b), and the more argillaceous parts of the Lower Chalk and Plenus Marls. Coarse calcarenitic levels from local units such as in some parts of the Melbourn Rock, Totternhoe Stone and like. Substantial quantities of skeletal aragonite were incorporated in the sediment chiefly in parts of the Lower Chalk and Plenus Marls, where aragonitic fossils are now preserved as composite moulds. At other levels, strong evidence for pre-burial sea-floor dissolution of aragonite (Jefferies, 1962, 1963; Hudson, 1967; Kennedy, 1969; Bathurst, 1975) suggests an original aragonite-free sediment. Regional and faunal studies (Reid, 1962a, b, 1968, 1971, 1973; Kennedy, 1970b; Hakansson et al., 1974; Kennedy and Garrison, 1975) suggest deposition in an outer shelf environment, with water depths in the 50 to 200 or 300 m range, and under normal salinities. Bottom conditions were variable, with evidence of semifluid conditions for much of the time, as well as times

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STAGE

ZONE

LITHOSTRATIGRAPHIC UNIT

Belemnella occidentalis UPPER CHALK

LOWER MAASTRICHTIAN Belemnella lanceolata

Maximum thickness 400m.

Belemnitella mucronota CAMPANIAN Gonioteuthis quadrata Offaster pilula Marsupites testudinarius SANTONIAN

Uppermost Campanian and Maastrichtian present only in Norfolk, in part as glacially transported masses. B a s e of U p p e r C h a l k m a r k e d b y the Chalk Rock handground comp l e x in N o r f o l k , C a m b r i d g e s h i r e , the Chilterns and Berkshire Downs. Elsewhere drawn as base of p l a n u s Zone on f a u n a l g r o u n d s .

Uintacrinus socialis CONIACIAN

Micraster coranguinum Micraster cortestudinarium Holaster planus

TURONIAN

T e r e b r a t u l i n a lata

MIDDLE CHALK Maximum thickness

Inoceramus labiatus 'Horizon A '

Metoicoceras g o u r d o n i Metoicoceras geslinianum

PLENUS MARLS Maximum thickness 8m. Base marked by a regional erosion surface. LOWER CHALK

Calycoceras naviculare

Maximum thickness CENOMANIAN

90m.

Base marked by important nodular beds, the Melbourne R o c k , o v e r m o s t of c o u n t r y .

Acanthoceras jukesbrownei Turrilites acutus Turrilites costatus Mantelliceras di~coni Mantelliceras saxbii

80m.

Arbitrarily divided into Chalk Marl below and Grey Chalk above, C o n d e n s e d b a s a l f a c i e s k n o w n as Cambridge Greensand. Glauconitic Marl or Chalk Basement Bed. Base d i a e h r o n o u s from L o w e r to U p p e r C e n o m a n i a n in S . W . E n g l a n d . w h e r e non-chalk equivalents (Cenomanian Limestone, Wilmington and Haldon Sands} are present.

Hypoturrilites corcitanensis

Fig. 2. Litholand.

and

biostratigraphic

subdivision

of the Chalk

sequences

of southern

Eng-

of firm bottoms and even hardgrounds (Bromley, 1967; Kennedy, 1970b; Reid, 1973; Kennedy and Garrison, 1975). The whole of the Chalk sequence is intensely bioturbated, with trace

112 fossils Thalassinoides, Chondrites, and Planolites abundant, plus Zoophycos at some levels. This intensive bioturbation has destroyed virtually all other minor sedimentary structures, so that the most striking features of the sequence are the small-scale c y c l e s o r sedimentary rhythms present througho u t the succession. These cycles are typically in the tens of centimetre to metre range, and consist either of ABAB type alternations or A -* B gradational sequences, with units within cycles separated by burrowed omission surfaces, or in the case of gradational cycles, the cycles terminated by such surfaces (Bromley, 1975; Kennedy and Garrison, 1975). In their simplest form, these cycles consist of alternations of more and less argillaceous marly chalk, as in the Lower Chalk and Plenus Marls. In the purer, clay-poor parts of the sequence, cycles consist of soft bioturbated chalk, alternating with or grading into chalks variously modified by processes of early lithification, the latter typically terminated by omission surfaces (Kennedy and Juignet, 1974; Bromley, 1975; Kennedy and Garrison, 1975). Both soft, wholly u n c e m e n t e d units and those which have experienced early, sea-floor cementation, have undergone subsequent, late diagenetic solution and compaction processes, and although a continuous gradation exists from soft chalks with scattered cemented areas to units which were completely lithified as a result of sea-floor diagenesis, it is convenient for discussion purposes to recognise the following: (1) Nodular chalks: chalks containing discrete dispersed nodules produced b y the local cementation of sediment below the sediment--water interface. The nodules were never reworked b y current action and do not show signs of exposure on the sea floor such as encrusting epizoans, borings, marginal glauconitisation or phosphatisation. (2) Intraformational conglomerates: these are current-reworked accumulates of nodules derived from the sedimentary reworking of nodular chalks. They differ from nodular chalks in the close packing of intraclasts, the presence of borings, encrustation and mineralisation. (3} Incipient hardgrounds are chalks in which development and growth of nodules proceeded to the e x t e n t of coalescence to produce a semicontinuous or continuous layer of interlocking nodules or, occasionally, a continuous rock band. The upper surfaces of these rock bands show no evidence of exposure on the sea floor. (4) True hardgrounds are lithified layers which have been exposed on a r o c k y sea floor, as is indicated by the presence of borings, epizoans and mineralisation. Early lithification in chalks probably resulted from the precipitation of high-magnesium calcite cement in pore spaces, followed by aggrading neomorphism in which many coccolith fragments were destroyed. Cements are n o w in the form of low-magnesium calcite. The period involved in lithification appears to have spanned tens or hundreds of years, although the hiatus associated with some hardgrounds was of much longer duration. A full discussion of evidence for early lithification and descriptions of field and petrographic characteristics of these chalks is to be found in the

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work of Bromley (1965, 1967, 1968), the summary in Bathurst (1975, Chapter 9), and in Kennedy and Garrison (1975). FIELD RELATIONSHIPS

Flaser chalks typically develop in soft, wholly uncemented chalks, in nodular and conglomeratic chalks or in incipient hardgrounds, with lenses of soft chalk, nodular chalk or nodules themselves being enclosed by marl seams. As demonstrated below, these seams are, in part or in whole, the result of dissolution, and are a soft sediment analog of the stylolites so prominent in well-cemented limestones {Stockdale, 1922, 1926, 1936, 1943; Park and Schot, 1968a, b; Mossop, 1972; Bathurst, 1975, pp. 468--473). These seams lack the fretted form of stylolites, however, being generally linear or undulose clay partings, corresponding to cross-sections of planar or undulating surfaces. These structures closely resemble features described by Barrett (1964) from Oligocene calcarenites in the North Island of New Zealand, and b y Mossop (1972) from Devonian reefal carbonates in Alberta. We have adopted a modified form of their terminology, and would speak of the solution seams, which may be simple and consist of an individual clay parting (e.g., Figs. 3, 6, 9, 10), or composite, consisting of scores or even hundreds of individual partings fused into an apparently laminated sediment sometimes several centimetres in thickness (Figs. 3, 4, 11--15). The areas of chalk between seams we term chalk lenses with the whole structure being termed a flaser, nodular flaser or conglomeratic flaser chalk. This terminology is illustrated in Fig. 3. The precise form of these seams, and hence the morphology of the flaser chalk they produce is dependent on the lithology of the preexisting chalk which the seams modify. In sections of uncemented units such as the Lower Chalk, parts of the Terebratulina lata zone in the Middle Chalk and the softer parts of the Upper Chalk (notably those levels in the Uintacrinus socialis to Belemnella occidentalis zones which are free of nodular developments), the seams appear as wavy or undulating, horizontal, inclined, and sometimes vertical lines, branching and combining with groups of simple seams, so that the whole rock is divided into chalk lenses. Predominantly parallel to beddings (Figs. 4, 5, 8), these chalk lenses typically have thicknesses of a few centimetres (e.g. Figs. 4, 8), and are b o u n d e d b y major composite seams. In detail, however, these may in turn be composed of units with dimensions measurable in centimetres and millime~es only (e.g. Figs. 6, 9, 10). Solution seams show a series of relationships to primary depositional features which indicate they are in part, at least, of secondary origin. On a large scale, inclined and vertical seams cut omission surfaces, erosion surfaces and like, and run across cycles (e.g. in the higher parts of the Lower Chalk at Eastbourne, Sussex). On a smaller scale, seams frequently cut burrows (Fig. 7). Burrows which are cut b y seams at low angles may show displacem e n t across the seam, indicating removal of a layer of rock along the seam,

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CHALK 2. SOFT CHALK

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5. FLASER CHALK

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~.- LATE BURIAL DIAGENESIS Fig. 3. (1) S c h e m a t i c representation of flaser structure developed in a soft chalk, to illustrate t e r m i n o l o g y used. (2--7) Influence of primary lithology on the f o r m of flaser chalks. Late burial diagenesis of soft chalks (2) produces a flaser chalk (3) with elongate lenticular lenses. If the chalk has u n d e r g o n e local early c e m e n t a t i o n , resulting in a nodular chalk (4), late diagenesis results in solution seams being c o n c e n t r a t e d into areas of u n c e m e n t e d chalk. Dissolution and associated c o m p a c t i o n produces a nodular flaser chalk with a secondary clay-rich matrix. The chalk lenses may be discrete nodules or lenses of u n c e m e n t e d chalk, or m a y be c o m p o s i t e . S e d i m e n t a r y r e w o r k i n g of early lithified nodules p r o d u c e d intraformational c o n g l o m e r a t e s (6). Late burial diagenesis leads to a fabric superficially similar to (5), although close inspection reveals that intraclasts and n o t nodules are involved; solution seams are c o n c e n t r a t e d into areas of u n c e m e n t e d chalk, again producing a secondary clay matrix. Intraclasts in c o n t a c t have undergone pressure solution and their j u n c t i o n s are stylolitic (7).

Fig. 4. An horizon of c o m p o s i t e solution seams and associated unit of flaser chalk in the Offaster pilula z o n e U p p e r Chalk at Seaford Head, Sussex. Arrows indicate a high angle seam which truncates the horizontal seams and appears to offset them. F a i n t b u r r o w m o t t l i n g below the main level of flaser chalk indicates that seams developed along a slightly argillaceous level above an omission surface in an otherwise pure chalk sequence. Scale given by coin, which is 2.6 cm in diameter.

Fig. 5. R h y t h m i c a l l y b e d d e d Terebratulina lata zone, Middle Chalk, Ballard Cliff, Swanage, Dorset. Flaser structure is well deveIoped in this weakly nodular chalk, and solution seams are c o n c e n t r a t e d in the originally more argillaceous parts of cycles, diagenesis thus accentuating primary r h y t h m i c i t y . Scale is given by h a m m e r head, which is 15 cm long.

Fig. 6. Cut section normal to bedding in an argillaceous unit from the upper part of Upper Cenomanian Calycoceres naviculare zone Lower Chalk, Culver Cliff, isle of Wight, Hampshire. The chalk has been intensively burrowed, and faint burrow mottled texture is cut by abundant oblique solution seams which branch and re-combine giving the appearance of cross-lamination. Scale bar equals 2 cm.

Fig. 7. Cut section normal to bedding in a bioturbated marly unit from the Lower Chalk at Truleigh Manor Farm, Edburton, Sussex. A slightly argillaceous chalk has been piped by pure chalk in Chondrites burrows, and development of horizontal solution seams has produced a thick marl seam of partially secondary origin by dissolution of burrowed sediment. Seams tend to avoid burrow infillings, but oblique or vertical burrows are cut, as indicated by arrows. Scale bar equals 2 cm.

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Fig. 8. Well-developed flaser structure and composite solution seams developed in soft Terebratulina lata zone Middle Chalk, Compton Bay, Isle of Wight, Hampshire. Scale given by hammer head, which is 15 cm long.

as is also observed with stylolites. In many cases, however, seams 'wrap around' burrows, and surfaces of seams sometimes display well-preserved burrow systems in relief (e.g. Kennedy, 1967, figs. 7E--F). Seams frequently show similar relationships to echinoid tests, fish teeth or the pre-fossilised hardened moulds of ammonites, and in these cases, the clay-coated surface of the burrow or fossil may be covered in slickensides. The surfaces of oblique seams may also be so marked. In sequences with well~eveloped differences in clay content between c o m p o n e n t members of rhythms, seams are typically more a b u n d a n t in what were originally more argillaceous members (Fig. 5), the seams thus accentuating the primary r h y t h m i c i t y . In thick marl seams with high clay contents (Figs. 9 and 10), the development of solution seams is such that the original bioturbated fabric is highly modified into a secondary, diagenetic lamination, with burrow infillings of more calcareous chalk relatively unaffected. Fig. 7 shows an example from the Lower Chalk at an early stage, with bioturbation still recognisable; m a n y thick marl beds in the Terebratulina lata zone can be shown to be primary features in t h a t t h e y show signs of burrow mottling and are piped into the chalk below, b u t cut surfaces of these beds (Figs. 9 and 10) show faint laminations which are clearly of secondary origin. We have seen no examples of solution seams which are themselves cut by

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120 Fig. 10. Detail of marl bed shown in Fig. 9, from the Terebratulina lata zone, Middle Chalk, Langdon Bay, northeast of Dover, Kent. Chalk-filled C h o n d r i t e s burrows at A and B are strongly compacted but largely unaffected by solution seams. Larger burrow at C, is, however cut by diffuse seams, and indicates that faint lamination in adjacent matrix is indeed diagenetic. Upper margin of chalk lens at D is sharply defined, and high clay content of adjacent matrix (black) indicates maximum dissolution at this point, in contrast to lesser dissolution and hence diffuse lens junction in pressure shadow at ends of lens, as at E. At F are a series of solution seams resembling a set of small conjugate fractures. Scale bar is 2 cm.

burrows, nor truncated by erosion surfaces, nor disturbed by slumping of the type described b y Kennedy and Juignet (1973). In nodular chalks and incipient hardgrounds (Figs. 11--15) the form of seams and the overall appearance of the resultant flaser chalk becomes increasingly irregular. Although the scale of chalk lenses is similar, the length : height ratio decreases and some lenses have length : height ratios of as little as 0.5 (Fig. 10). The solution seams typically develop in the softer, uncemented matrix between nodular and cemented areas of sediment. In weakly nodular chalks, the seams are simply undulose (Fig. 11); as nodulaxity increases to the state represented by incipient hardgrounds, the seams become increasingly irregular, 'wrapping around' nodules to form flaser chalks with gross fabric recalling 'chickenwire anhydrite'. The progression

Fig. 11. Flaser structure developed in nodular Terebratulina lata zone. Middle Chalk, Worbarrow Bay, Dorset. Scale given by hammer head, which is 15 cm long.

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Fig. 13. Detail of composite solution seam at the Plenus Marls/Middle Chalk junction, Lower Froyle, Hampshire. Note variability in deformation and degree of disintegration of chalk lenses from top to b o t t o m of field. Surface cut normal to bedding. Scale bar equals 2 cm.

Fig. 14. Terebratulina lata zone Middle Chalk, Compton Bay, Isle of Wight, Hampshire. The Spurious Chalk Rock hardground is just below the hammer head; above, flaser structure is well developed in a nodular chalk. Scale given by hammer head, which is 15 cm long.

123

Fig. 15. Flaser structure in nodular I n o c e r a m u s labiatus zone, Middle Chalk, Little Beach, near Beer Head, South Devon. Scale given by coin, which is 2.6 cm in diameter.

shown in Figs. 11--15 does far more justice to these fabrics than words. In detail, the seams in nodular chalks show complex relationships. As in soft chalks, seams cut across primary depositional features such as bedding planes and omission surfaces, and they cut and truncate burrows {Fig. 12). In many nodular chalks, the seams form a secondary matrix to nodules, and in detail, seams are distorted around nodules, resembling a laminated clay matrix compacted around cemented nodules, which themselves resisted compaction. Cross-cutting relations noted above indicate, however, that these seams are in part secondary, whilst sections frequently show individual 'laminae' cutting burrows and nodules (Figs. 9 and 10). In some cases, solution seams in matrix pass into stylolites where they cut through nodules. Fractured surfaces frequently show that nodules bear clay coatings and are covered with slickensides. Where discrete nodules are in contact, interpenetrant stylolites may develop, whilst in nodular chalks and incipient hardgrounds, where the nodule matrix had appreciable quantities of clay at the time of deposition (e.g., levels in the Terebratulina lata and Holasterplanus zones), seams are typically better developed than at other, clay-poor levels. Seams may also follow individual Thalassinoides burrows filled with clayrich sediment when these reside in a clay-poor matrix (Fig. 15). As with solution seams in soft chalks, we have never seen seams in nodular chalks or incipient hardgrounds which are cut b y burrows, truncated by erosion surfaces, or affected by slumping.

124

125 PETROGRAPHY

Composition and texture in soft chalks, nodular chalks and chalk hardgrounds The typical soft chalk in thin section is a sparse foraminiferal biomicrite with a bioturbated fabric in which there is random orientation of planar and linear grains (e.g., echinoderm and bivalve fragments, sponge spicules) due to disturbance b y burrowing animals (Fig. 16A). Within a micritic matrix composed largely of coccoliths and coccolith fragments (Black, 1953; Hancock and Kennedy, 1967) are various skeletal grains, most prominently whole tests of planktonic Foraminiferida and calcispheres (Oligostegina). Skeletal fragments of benthonic organisms include echinoderms, Foraminiferida, inoceramids and other bivalves, brachiopods, calcitised sponge spicules, as well as small quantities of ostracode carapaces and bryozoan fragments. Also present in generally small amounts are fragments of phosphatic fossils (teeth, bones) as well as phosphatic and glauconitic pellets. The compositions and textures described above are typical not only for the soft chalks, b u t also occur in nodular chalks and in the chalks of hardgrounds (Kennedy and Garrison, 1975). Chalk hardgrounds, however, may be substantially modified b y secondary glauconitisation and phosphatisation.

Flaser chalks Compositionally, flaser chalks seem to be virtually identical to the typical soft chalk described above, at least at the level of thin-section observations. The main difference is that some of the solution seams have higher concentrations of coarse skeletal grains compared to other kinds of chalk, and compared to adjacent chalk lenses. This difference appears to be the result of preferential dissolution of coccolith micritic matrix. Texturally, however, the flaser chalks show significant differences, nearly all of which can be attributed to compaction. The most prominent difference is the comparatively high degree of preferred orientation of planar and linear grains, most of which lie parallel or nearly so to bedding (Fig. 16B). Fig. 16. Photomicrographs of chalks. Scale bars equal 500 pm. A. Soft chalk from the Holaster planus zone, Upper Chalk, Charnage Quarry, Wiltshire. Skeletal particles include planktonic Foraminiferida, calcispheres and fragments of echinoderms and bivalves. Thin section is cut normal to bedding; note random orientation of elongated particles, a fabric produced by bioturbation. B. Argillaceous chalk in a solution seam of a flaser chalk from the Terebratulina lata zone, Middle Chalk, Langdon Hole, Dover, Kent. Thin section cut normal to bedding. Note preferred orientation of elongated particles (mostly echinoderm fragments) parallel or subparallel to bedding. C. Flaser chalk from the Terebratulina lata zone, Middle Chalk, locality as in B. The deformed inoceramid shell in the center lies between a chalk lens and the adjacent solution seam, and attests to mechanical flowage between these two parts of the chalk.

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In many cases these grains seem to define patterns of sediment flowage. This is particularly evident within the marly matrix of nodular flaser chalks developed from nodular chalks and .incipient hardgrounds where chalks cemented at an early stage were preserved (cf. Kennedy and Garrison, 1975). Within these nodules, the random bioturbated fabric was locked in place by early cementation, whereas the surrounding matrix remained unlithified during burial and may even have flowed plastically around the hardened nodule during compaction. In other cases, however, b o t h flasers and the adjacent marl seam have the same preferred alignment of elongate grains. Both thus must have been relatively unconsolidated during compaction so that elongate grains could be rotated from the random positions imposed during burrowing to orientations more nearly parallel to bedding. This t y p e of reorientation of skeletal grains is restricted to those parts of flaser chalks which lack signs of early lithification, and further implies that the fine-grained matrix in these chalks remained in a plastic state into the late stages of burial diagenesis. Crushed and broken grains, as well as coarse skeletal grains which impinge against each other, are also prominent characteristics of many flaser chalks, b u t are generally lacking in most other chalks (however, see Wolfe, 1968, and SchoUe, 1974, for some exceptions). These characteristics likewise can be attributed to the effects of compaction. Some of the coarser skeletal grains, such as echinoderm fragments, are in contact with each other along irregular contacts that appear to be slightly brecciated on a fine scale -- as if there had been mechanical fragmentation as the grains were compressed together. There is no evidence of stylolitic solution seams in grain-to-grain contacts of this kind. Other skeletal grains are involved in a spectrum of deformational effects. Tests of planktonic Foraminiferida seem to have been especially fragile during compaction, and the upward facing parts of these tests are c o m m o n l y fractured or collapsed while the parts below remain intact. This type of breakage occurred in some instances when a robust grain like an echinoderm fragment was compacted onto the t o p of a more fragile shell. Tests of benthonic Foraminiferida and calcispheres, in contrast, seem to have been relatively more resistant to compactional breakage or crushing. Most resistant of all were the coarsest skeletal grains, b u t even these were not immune. A few bivalve fragments have prominent compactional fractures or other indications of severe deformation. A striking example is the inoceramid shell fragment shown in Fig. 16C. The shell lies along the boundary b e t w e e n a thin solution seam and a chalk lens, and it appears to have been deformed by differential m o v e m e n t between the seam and the lens. Both the folding and the fragmentation of the shell result from small offsets along boundaries between individual prisms in the outer shell layer. Compactional fractures lace some echinoderm fragments and divide them into small segments with slightly different crystallographic orientations, each with a somewhat different extinction position under crossed nicols. Elongate

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phosphatic skeletal particles seem to have been especially susceptible to fracturing during compaction, and it is rare to find one that is unbroken. Some of the most advanced stages of compactional crushing are found in calcarenitic layers where coarse skeletal fragments very c o m m o n l y impinged upon and crushed foraminiferal tests. Even here, however, completely crushed and flattened foraminiferal shells are very rare, probably because most of them had become filled by micrite prior to compaction. That micritic matrix material served to absorb much of this compressional stress preferentially through compaction and flowage is well shown by the textures in the sparse biomicrites of the flaser chalks; compared to the calcarenites, these have far fewer crushed and broken grains. We observed no clear-cut evidence that pressure solution has affected any of the coarser skeletal grains in flaser chalks. There are some grains with what appear to be corroded or etched margins, but the interpretation of these is equivocal. Nothing as obvious as stylolites cutting across grains was observed. We are thus led to conclude that any pressure solution which affected the marl seams in these rocks must have been restricted largely to the fine-grained matrix.

Summary o f petrographic evidence on the nature and origin of flaser chalks Although field observations, as previously discussed, provide striking evidence for considerable post-depositional solution in the flaser chalks, few indications of this are observed in thin-sections. Most obvious at this level of observation is the evidence for mechanical deformation during compaction. Based on this evidence we may surmise t h a t the matrix of most of these chalks remained in an unconsolidated and plastic condition into the later stages of burial diagenesis. Consequently, coarse skeletal grains were able to rotate within this matrix to stable positions relative to the compressional stress field and to impinge against each other. Both matrix and grains were apparently able to flow plastically for at least short distances. The most highly deformed grains (Fig. 16C) are those that lie at the boundaries between chalks of differing compositions or physical consistencies; the intensity of this deformation seems to have resulted from stresses imposed by differential flowage during compaction. Petrographic evidence suggests the marl seams were especially plastic and m a y have been susceptible to flowage. Because we observed little or no evidence for solution of coarse grains in the flaser chalks, we conclude that the intrastratal solution evident in outcrops must have occurred primarily within the fine-grained matrix of these chalks. This is supported by the concentrations of relatively insoluble coarser skeletal debris in some marl seams, compared to their abundances in adjacent chalk lenses. COMPARISON WITH SELECTED NODULAR LIMESTONES

The solution seams and chalk lenses which constitute flaser chalk are similar to structures described in other limestones by different authors. Explana-

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tions for these structures have generally fallen into one of two categories. They have either been attributed mainly to processes involving mechanical flowage or mainly to processes involving solution--reprecipitation of calcium carbonate. McCrossan (1958) described features very similar in size and appearance to flaser chalks in thinly interbedded argillaceous limestone and calcareous shale of Devonian age in Alberta. He called these features sedimentary 'boudinage' structures, and attributed their origin to differential flowage between relatively more competent calcareous layers and more plastic, clay-rich layers. He postulated that the 'boudins' were remnants of calcareous layers which were pulled apart as adjoining clay-rich layers moved laterally during unconfined compaction near the depositional interface. It seems, however, that McCrossan did not distinguish between two wholly different processes. His chief examples of flowage (1958, figs. 1, 2, 6, and 7) are in fact probably the result of differential compaction around ellipsoidal early diagenetic concretions in the more calcareous levels of an interbedded limestone-shale sequence. The 'boudinage' shown in McCrossan's fig. 3 is a solution-modified nodular limestone analogous to our nodular flaser chalks. It is important to emphasize that many other reports of so-called sedimentary boudinage in unmetamorphosed rocks describe structures very different from those discussed by McCrossan (1958) and also different from flaser chalks. Most of these reports describe structures more akin in form and size to metamorphic boudinage, with the individual boudins being of the order of tens of centimetres in maximum dimension rather than a few centimetres. Structures of this sort in limestones have been explained as the result of differential flowage during tectonic movement (e.g. De Sitter, 1964, pp. 284-285) or during compaction (Greenwood, 1960). Theoretical and experimental explanations of boudinage structure are given by Ramberg (1955) and StSmgard (1973). They have emphasized that it results from the differential response of interlayered competent and incompetent beds to tectonic or compactional stress. Voigt (1962) has described complicated structures formed during early diagenetic deformation of semi-lithified, fine-grained carbonate sediments of Late Cretaceous age in northern Germany. Among these structures are ellipsoidal to irregular, boudin-like bodies which he called phacoids. Although most of the phacoids are relatively large bodies, many of which contain internal laminae which are highly folded, some in aggregate closely resemble flaser structure in chalk (e.g., see plate 24, figs. 26 and 27 of Voigt, 1962). Voigt's phacoids are, however, clearly of deformational origin, although some of his illustrations do show the beginnings of solution seam development (e.g., Voigt, 1962, plate 22, fig. 2, pp. 1--24). Flaser structure is also very common in red, nodular, cephalopod-bearing limestone of Paleozoic and Mesozoic age. Explanations for the nodularity in these rocks have mostly included some form of carbonate dissolution. For example, Nevill (1962) has described a red nodular limestone of this sort, the

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Lower Carboniferous Cork Red Marble in Ireland; he interpreted the nodules as limestone pebbles in a conglomerate, and thought the rock was modified by development of stylolitic solution seams at the edges of the pebbles during tectonic deformation. These rocks thus appear analogous to nodular and conglomeratic flaser chalks. Tucker (1974 and references therein) described red pelagic limestones of Devonian age in southern France which have a distinctive nodular structure called 'griotte texture'. They closely resemble the Alpine Ammonitico Rosso described below, and Tucker attributes their origin to a combination of early lithification and dissolution, compaction, and pressure solution during burial. The most widely discussed of these red limestones are those of the Jurassic Ammonitico Rosso facies in the circum-Mediterranean Alpine chains (see extensive discussion in Jenkyns, 1974). Except for their deep red colour, many of these limestones very closely resemble nodular and conglomeratic flaser chalks (see illustrations in Fabricius, 1962; Garrison and Fisher, 1969; Hudson and Jenkyns, 1969; Jurgan, 1969; Bernoulli, 1972; Bernoulli and Jenkyns, 1974; Jenkyns, 1974). Hollman (1962, 1964) attributed the nodularity of these rocks to early lithification of carbonate mud followed by some dissolution, both on the sea floor, and following burial. This view was generally subscribed to by Garrison and Fischer (1969) who considered many of the nodules as solution remnants. Other workers (e.g. Lucas, 1955a, b; Hallam, 1967; Jenkyns, 1974) have emphasized 'unmixing' (Sujkowski, 1958) as an important process which results in the diagenetic separation of calcium carbonate from argillaceous sediment into the limestone nodules of the Ammonitico Rosso facies. Jenkyns (1974) in particular has argued for solution of fine-grained aragonite in such a sediment, followed by rhythmic reprecipitation as magnesium calcite into nodules during early diagenesis. Such magnesium calcite nodules have recently been discovered in cores of Pleistocene sediment from the eastern Mediterranean (Milliman and Miiller, 1973; Miiller and Fabricius, 1974). Nearly all workers are in agreement that compaction has modified the nodular structure of the Ammonitico Rosso limestones, mainly by inducing flowage of the clay-rich matrix around the hard limestone nodules. In describing Jurassic Ammonitico Rosso recovered in cores of the Deep Sea Drilling Project in the North Atlantic, Bernoulli (1972, p. 812) has emphasized the role of plastic deformation in enhancing and modifying earlier formed nodules. Ammonitico Rosso is clearly to be compared with nodular and conglomeratic flaser chalks, where the lithologies from which the flaser chalk developed included early lithified nodules, either in situ or reworked. There is, however, no widespread Ammonitico Rosso analog of flasers developed in wholly soft chalks, as described here. Noble and Howells (1974) have also described nodular limestones w h i c h are nearly identical to the nodular flaser chalks mentioned previously. These are of Silurian age in New Brunswick. In the authors' view, the nodules

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developed during early diagenetic cementation just below the sea floor. Subsequent compaction modified these structures through flowage of the surrounding argillaceous lime mud which remained unlithified burial. Barrett (1964) has described residual seams formed by intrastratal solution in Oligocene calcarenites of New Zealand. These seams are rather closely spaced and undulating layers lie subparallel to bedding. Some branch and bifurcate (e.g. Barret, 1964, figs. 3, 4) and most closely resemble flaser structures developed in soft chalks. Flaser structures in coccolith-rich chalks have been recovered in many cores of the Deep Sea Drilling Project. Among these, we would cite DanianMaastrichtian flaser chalks in Core 23, Site 216, Leg 22 (sub-bottom depth of 329.5--339 m; Von der Borch et al., 1974) and Early Miocene chalk in Core 27, Site 223, Leg 23 (sub-bottom depth of 487--496 m; Whitmarsh et al., 1974). Amongst other limestones which have been modified by the late dissolution of carbonate, we would also cite examples described by Oldershaw and Scoffin (1967), examples discussed by Bathurst (1975} and those illustrated by Benavides C~ceres (1956), Mossop (1972) and Fiirsich (1973). Plessmann (1964, 1966) emphasized the role of solution-reprecipitation of carbonates in producing lenticular structures as well as foliation in tectonically stressed carbonate rocks. ORIGIN OF SOLUTION SEAMS AND F L A S E R CHALK

Our studies of solution seams and flaser structures in the English Chalks lead to the following conclusions about their origin: (1) Both the chalk lenses and solution seams which together constitute the flaser structure are clearly of post
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Neugebauer, 1974). Thus, the most likely burial depths at which flaser structure forms in chalks are between approximately 300 and 2000 m. This estimate is in general accord with the burial depths at which flaser chalks are found in cores of the Deep Sea Drilling Project. (c) Intrastratal solution was most marked within layers which had a higher primary c o n t e n t of clay minerals than adjacent layers. The sites of future solution seams were thus, in some cases, predestined by primary deposition of clay-rich chalk layers. (d) Localisation of most intensive intrastratal solution in clay-rich layers accords well with the predictions of Weyl (1959). He suggested that the rate of pressure solution should be increased by clay films between grains. This is mainly because the rate of diffusion in these clay films would be much more rapid than the diffusion rate in a solution film between clean mineral grains. (e) The intrastratal solution occurred exclusively in the coccolith-rich micritic matrix of the chalk and rarely if ever affected coarser skeletal grains. (f) Some simple solution seams within chalk lenses may have been originally compactional fractures along which solution became concentrated. The fractures may have acted as conduits for pore fluid, thus locally increasing the diffusion rate. (g) Some of the calcium carbonate which was dissolved may have been reprecipitated locally, perhaps at the ends of the individual chalk lenses where they are often transitional into the solution seams. This is analogous to the precipitation of secondary minerals in the low pressure 'shadows' at the ends of some metamorphic boudins (cf. StSmg~rd, 1973). The chalk lenses also provided many more carbonate nuclei on which the dissolved calcium and carbonate ions could be precipitated (Matter, 1974, p. 441). (h) Judging b y the sharpness of the contact between the chalk lenses and the adjacent solution seams, most solution was concentrated at the tops and b o t t o m s of the flasers, chiefly the tops. (i) The flattened, ellipsoidal shape of many chalk lenses is a result in part of compaction, as is the high degree of grain alignment and grain breakage in flaser chalks. This implies that the fine-grained matrix in both the solution seams and the chalk lenses developed in non-nodular facies remained in an unconsolidated, plastic state into the later stages of burial diagenesis. (j) Chalks with flaser structure have far more grain alignment and breakage due to compaction than do any other kinds of chalks, including soft chalks. This implies that the intensive compaction was in some way related to genesis of the solution seams and chalk lenses. Compactional flattening and dissolution of calcium carbonate probably were in part contemporaneous processes. Removal of CaCO3 b y solution would have created void space in solution seams thus promoting additional compaction (see discussions of the processes and effects of compaction of fine-grained argillaceous and carbonate sediments in Brown, 1969; Rieke and Chilingarian, 1974; Chilingarian and Wolf, 1975). (k) Contemporaneous compaction and dissolution acted in concert to pro-

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duce chalk lenses which in some cases became markedly elongated parallel to bedding and riddled b y simple solution seams. Continued solution expanded the simple seams into composite ones, and led to progressive disintegration of the chalk lenses so that some lenses were completely destroyed, others reduced to remnant shreds or to diffuse, ghost-like patches. (1) The compactional effects in these flaser chalks bear some resemblances to chalks deformed b y tectonic stresses. Mimran (1975) has shown that modification of the fabric in deformed Cretaceous chalks from Dorset occurs in t w o stages. Mechanical compaction and rearrangement of grains dominates in the first stage, whereas pressure solution, leading to redistribution and removal of CaCO3, prevails in the second stage. The total volume loss resulting from the t w o stages can be as much as 70% in highly deformed chalks. (m) In unlithified or poorly lithified sediments like most of the chalk, interdigitating complex stylolites do not develop in response to pressure solution. Instead, the solution is more diffuse and spread through a thicker layer of sediment (e.g. millimetres instead of microns) to produce solution seams. (4) In a number of cases, pre-burial, early diagenetic cementation of chalk at or near the sediment--water interface produced hardened chalks which responded in a different manner to burial diagenesis. Instead of the kind of flaser structures in soft chalks which we considered above, we noted the following variations: (a) Some irregular to roughly spherical nodules were apparently fully lithified prior to burial (cf. Noble and Howells, 1974; Kennedy and Garrison, 1975} b u t lay within unlithified chalk. These nodules remained relatively immune to pressure solution during burial diagenesis. Such solution acted preferentially on the enclosing unlithified chalk, converting it to solution seams. Relatively unflattened nodules thus reside within solution seams (cf. Figs. 3, 11--15), although some nodules are b o u n d e d by true stylolites and bear slickensides. (b) Some nodules cemented during early diagenesis were uncovered and reworked b y currents on the sea floor. The fine unlithified chalk between them was thus winnowed away to produce an intraformational conglomerate c o m p o s e d of resedimented, hard chalk nodules. During later burial compaction, pressure solution along areas of contact between nodules produced stylolitic contacts. (c) For similar reasons, the hard chalks in some mature hardground layers are also traversed b y stylolites with more or less random orientations. In the English Chalks, therefore, the evidence is clear that one prerequisite for the development of true stylolites is complete lithification. (5) The fact that solution seams and chalk lenses are mostly inconspicuous or absent in the relatively pure parts of the Upper Chalk suggests that these chalks were either subjected to insufficient compactional stresses or that their comparative purity precluded development of flaser structure. We suggest that b o t h factors were important, b u t that the compositional one was

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the more significant. As noted above, the presence of clay minerals seemed to enhance intrastratal solution in response to compactional pressures. And differentiation into clay-poor and relatively plastic clay-rich layers, on the one hand, and chalk lenses, on the other hand, provides contrasts in mechanical competency during compaction. ACKNOWLEDGEMENTS

Collaboration on this project was rendered possible by the award of a Fellowship from the J.S. Guggenheim Memorial Foundation to Garrison, and a Fellowship from the Lindemann Trust to Kennedy, whilst research was in part supported by an award from the Petroleum Research Fund (PRF 5692AC2) administered by the American Chemical Society. All are gratefully acknowledged.

REFERENCES Arkell, W.J., 1947. The geology of the country around Weymouth, Swanage, Corfe, and Lulworth. Mere. Geol. Surv. U.K., 386 pp. Barrett, P.J., 1964. Residual seams and solution in Oligocene shell calcarenites, Te Kuiti Group. J. Sediment. Petrol., 34: 524--531. Bathurst, R.G.C., 1975. Carbonate Sediments and Their Diagenesis. Developments in Sedimentology, 12. Elsevier, Amsterdam, 2nd ed., 658 pp. Benavides C~ceres, V.E., 1956. Cretaceous System in northern Peru. Bull. Am. Mus. Nat. Hist., 108: 353--494. Bernoulli, D., 1972. North Atlantic and Mediterranean Mesozoic facies: a comparison. Initial Reports of the Deep Sea Drilling Project, 11. U.S. Government Printing Office, Washington, D.C., pp. 801--822. Bernoulli, D. and Jenkyns, H.C., 1974. Alpine, Mediterranean and Central Atlantic Mesozoic facies in relation to the early evolution of the Tethys. In: R.H. Dott Jr. and R.H. Shaver (Editors), Modern and Ancient Geosynclina] Sedimentation. Soc. Econ. Paleontol. Mineral. Spec. Publ., 19: 129--160. Black, M., 1953. The constitution of the Chalk. Proc. Geol. Soc. London, 1499: 81--86. Bromley, R.G., 1965. Studies in the Lithology and Conditions of Sedimentation of the Chalk Rock and Comparable Horizons. Thesis, London University. Bromley, R.G., 1967. Some observations on burrows of Thallassinidean crustacea in chalk hardgrounds. Q. J. Geol. Soc. London, 123: 157--182. Bromley, R.G., 1968, Burrows and borings in hardgrounds. Medd. Dan. Geol. Foren., 18: 248--250. Bromley, R.G., 1975. Trace fossils at omission surfaces. In: R.W. Frey (Editor), The Study of Trace Fossils. Springer, New York, N.Y., pp. 399--428. Brown, P.R., 1969. Compaction of fine-grained terrigenous and carbonate sediments -- a review. Bull. Can. Pet. Geol., 17: 486--495. Brydone, R.M., 1912. The Stratigraphy of the Chalk of Hants. Dulau and Co., London, 116 pp. Brydone, R.M., 1939. The Chalk Zone of Offaster pilula. Dulau and Co., London, 8 pp. Casey, R., Dilley, F.C., Hancock, J.M., Kennedy, W.J., Neale, J.W., Rawson, P.F., Wood, C.J. and Worssam, B.D., 1974. A correlation of the Cretaceous rocks of the British Isles. Q. J. Geol. Soc. London, in press.

134 Chilingarian, G.V. and Wolf, K.H., 1975. Compaction of Coarse-Grained Sediments, I. Developments in Sedimentology, 18A. Elsevier, Amsterdam, 552 pp. De Sitter, L.U., 1964. Structural Geology. McGraw-Hill, New York, N.Y., 551 pp. Dickert, P.F., 1966. Tertiary phosphatic facies of the Coast Ranges. In: Bull. Calif. Div. Min. Geol., 190: 289--304. Fabricius, F.H., 1962. Faziesentwicklung an der Trias/Jura-Wende in der Mittlere Nifrdlichen Kalkalpen. Z. Dtsch. Geol. Ges., 113: 311--319. Fiirsich, F., 1973. Thalassinoides and the origin of nodular limestone in the Corallian Beds. Neues Jahrb. Geol. Pal~iontol. Monatsh., 1973: 136--156. Garrison, R.E. and Fischer, A.G., 1969. Deep water limestones and radiolarites of the Alpine Jurassic. In: G.M. Friedman (Editor), Depositional Environments in Carbonate Rocks: A Symposium. Soc. Econ. Paleontol. Mineral. Spec. Publ., 14: 20--56. Gaster, C.T.A., 1929. Chalk zones in the neighborhood of Shoreham, Brighton and Newhaven, Sussex. Proc. Geol. Assoc., 39: 328--340. Gaster, C.T.A., 1939. The stratigraphy of the Chalk of Sussex II. Eastern area -- Seaford and Cuckmere Valley to Easthourne with zonal map. Proc. Geol. Assoc., 50: 510-526. Gaster, C.T.A., 1941. The Chalk zones of Offaster pilula and Actinocamax quadratus. Proc. Geol. Assoc., 52: 210--215. Greenwood, R., 1960. Sedimentary boudinage in Cretaceous limestones of Zimapan, Mexico. Int. Geol. Congr., Rep. 21, Session Norden, part XVIII, pp. 389--398. Hakansson, E., Bromley, R. and Perch-Nielsen, K., 1974. Maastrichtian chalk of northwest Europe -- a pelagic shelf sediment. In: K.J. Hsi] and H.C. Jenkyns (Editors), Pelagic Sediments: On Land and Under the Sea. Spec. Publ. Int. Assoc. Sedimentol., 1: 211--233. Hallam, A., 1967. Sedimentology and palaeogeographic significance of certain red limestones and associated beds in the Lias of the Alpine region. Scott. J. Geol., 3: 195-220. Hancock, J.M. and Scholle, P.A., 1975. Chalk of the North Sea. In: A. Woodlan (Editor), Petroleum and the Continental Shelf of Northwest Europe: the Geology and the Environment. Applied Sciences Press, London, pp. 409--422. Hollman, R., 1962. Uber Subsolution und die Knollenkalke des Calcare Ammonitico Rosso Superiore in Monte Baldo (Malta; Norditalien). Neues Jahrb. Geol. Pal~/onth. Monatsh., 1962: 163--179. Hollman, R., 1964. Subsolutions-Fragmente (zur Biostratinomie der Ammonoidea im Malm des Monte Baldo/Norditalien). Neues Jahrb. Geol. Pal~ontol. Abh., 119: 22--82. Hudson, J.D., 1967. Speculation on the depth relations of calcium carbonate in recent and ancient seas. Mar. Geol., 5: 473--480. Hudson, J.D. and Jenkyns, H.C., 1969. Conglomerates in the Adnet Limestones of Adnet (Austria) and the origin of the 'Scheck'. Neues Jahrb. Geol. Pal~ontol. Monatsh., 1969: 552--558. Hume, W.F., 1893. Chemical and Micromineralogical researches on the Upper Cretaceous Zones of the South of England. Whitehead, Morris and Co., London, 103 pp. Jeans, C.V., 1967. The origin of the montmorillonite of the European Chalk with special reference to the Lower Chalk of England. Clay Miner., 7: 311--329. Jefferies, R.P.S., 1962. The palaeoecology of the Actinocamax plenus subzone (lowest Turonian) in the Anglo-Paris Basin. Palaeontology, 4: 609--647. Jefferies, R.P.S., 1963. The stratigraphy of the Actinocamax plenus subzone (Turonian) in the Anglo-Paris Basin. Proc. Geol. Assoc., 74 : 1--33. Jenkyns, H.C., 1974. Diagenetic origin of red nodular limestones (Ammonitico Rosso, Knollenkalke) in the Mediterranean Jurassic: a diagenetic model. In: K.J. Hsfi and H.C. Jenkyns (Editors), Pelagic Sediments: On Land and Under the Sea. Spec. Publ. Int. Assoc. Sedimentol., 1: 249--271. Jukes-Browne, A.J. and Hill, W., 1903. The Cretaceous Rocks of Britain, 2. The Lower and Middle Chalk. Mem. Geol. Surv. U.K., 556 pp.

135 Jukes-Browne, A.J. and Hill, W., 1904. The Cretaceous Rocks of Britain, 3. The Upper Chalk. Mem. Geol. Surv. U.K., 556 pp. Jurgan, H., 1969. Sedimentologie des Lias der Berchtesgadener Kalkalpen. Geol. Rundsch., 58: 464--501. Kennedy, W.J., 1967. Burrows and surface traces from the Lower Chalk of southern England. Bull. Br. Mus. Nat. Hist° (Geol.), 15: 127--167. Kennedy, W.J., 1969. A correlation of the Lower Chalk of south-east England. Proc. Geol. Assoc., 80: 459--551. Kennedy, W.J., 1970a. A correlation of the uppermost Albian and Cenomanian of southwest England. Proc. Geol. Assoc., 81: 613--677. Kennedy, W.J., 1970b. Trace fossils in the Chalk environment. Geol. J., 3 : 2 6 3 - - 2 8 2 (Special Issue). Kennedy, W.J. and Garrison, R.E., 1975. Morphology and genesis of nodular chalks and hardgrounds in the Upper Cretaceous of southern England. Sedimentology, 22: 311-386. Kennedy, W.J. and Juignet, P., 1973. Carbonate banks and slump beds in the Upper Cretaceous (Upper Turonian to Santonian) of Haute Normandie, France. Sedimentology, 21: 1--42. Lucas, G., 1955a. Caract~res p~trographiques de calcaires noduleux, ~ facies a m m o n i t i c o r o s s o , de la r~gion mediterran4enne. C. R. Acad. Sci., Paris, 240: 1901--1911. Lucas, G., 1955b° Caract~res g~ochimiques et m6chaniques du milieu g~n~rateur des calcaires noduleux, ~ facies a m m o n i t i c o r o s s o . C. R. Acad. Sci., Paris, 240: 2000--2002. Matter, A., 1974. Burial diagenesis of pelitic and carbonate deep-sea sediments from the Arabian Sea. In: R.B. Whitmarsh, O.E. Weser, D.A. Ross et al., Initial Reports of the Deep Sea Drilling Project, 23. U.S. Government Printing Office, Washington, D.C., pp. 421--469. McCrossan, R.G., 1958. Sedimentary 'Boudinage' structures in the Upper Devonian Ireton Formation of Alberta. J. Sediment. Petrol., 28: 316--320. Milliman, J.D.'and Miiller, J., 1973. Precipitation and lithification of magnesian calcite in the deep-sea sediments of the eastern Mediterranean Sea. Sedimentology, 20: 29--46. Mimran, Y., 1975. Fabric deformation induced in Cretaceous chalks by tectonic stresses. Tectonophysics, 26: 309--316. Mossop, G.D., 1972. Origin of the peripheral rim, Redwater Reef, Alberta. Bull. Can. Pet. Geol., 20: 238--280. Milller, J. and Fabricius, F., 1974. Magnesian-calcite nodules in the Ionian deep sea: an actualistic model for the formation of some nodular limestones. In: K.J. Hsii and H.C. Jenkyns (Editors), Pelagic Sediments: On Land and Under the Sea. Spec. Publ. Int. Assoc. Sediment., 1: 235--247. Neugebauer, J., 1974. Some aspects of cementation in chalk. In: K.J. Hsii and H.C. Jenkyns (Editors), Pelagic Sediments: On Land and Under the Sea. Spec. Publ. Int. Assoc. Sedimentol., 1 : 149--176. Nevill, W.E., 1962. Stratigraphy and origin of the Cork Red Marble. Geol. Mag., 99: 481-491. Noble, J.P.A. and Howells, K.D.M., 1974. Early marine lithification of the nodular limestones in the Silurian of New Brunswick. Sedimentology, 21 : 597--609. Oldershaw, A.G. and Scoffin, T.P., 1967. The source of ferroan and non-ferroan calcite cements in the Halkin and Wenlock Limestones. Geol. J., 5: 309--320. Osbourne-White, H.J., 1921. A short account of the geology of the Isle of Wight. Mem. Geol. Surv. U.K. Osbourne-White, H.J., 1924. The geology of the country near Brighton and Worthing. Mem. Geol. Surv. U.K., 219 pp. Osbourne-White, H.J., 1928. The geology of the country near Ramsgate and Dover. Mere. Geol. Surv. U.K., 98 pp. Park, W.-C. and Schot, E.H., 1968a. Stylolites: their nature and origin. J. Sediment. Petrol., 38: 175--191.

136 Park, W.-C. and Schot, E.H., 1968b. Stylolitization in carbonate rocks. In: G. Mfiller and G.M. Friedman (Editors), Recent Developments in Carbonate Sedimentology in Central Europe. Springer, New York, N.Y., pp. 66--74. Peake, N.B. and Hancock, J.M., 1961. The Upper Cretaceous of Norfolk. Trans. Norfolk Norwich Nat. Soc., 19: 293--339. Peake, N.B. and Hancock, J.M., 1970. The Upper Cretaceous of Norfolk. In: G.P. Larwood and B.M. Funnel (Editors), The Geology of Norfolk. Headly Bros., London, pp. 293--339. Plessmann, W., 1964. Gesteinsl6sung, ein Hauptfaktor beim Schieferungsprozess. Geol. Mitt., 4: 69--82. Plessmann, W., 1966. L6sung, Verformung, Transport und Gefiige (Beitr~ige zur Gesteinsverformung im nordSstlichen Rheinischen Schiefergebirge). Z. Dtsch. Geol. Ges., 115: 650--663. Ramberg, H., 1955. Natural and experimental boudinage and pinch and swell structures. J. Geol., 63: 512--526. Reid, R.E.H., 1962a. Sponges and the Chalk Rock. Geol. Mag., 99: 273--278. Reid, R.E.H., 1962b. Relationship of faunas and substratum in the palaeoecology of the Chalk and the Chalk Rock. Nature, 194: 276--277. Reid, R.E.H., 1968. Hexactinellid faunas in the Chalk of England and Ireland. Geol. Mag., 105: 15--22. Reid, R.E.H., 1971. The Cretaceous rocks of north-eastern Ireland. Ir. Nat. J., 17 : 105-129. Reid, R.E.H., 1973. The chalk sea. Ir. Nat. J., 17: 357--375. Rieke, H.H., III and Chilingarian, G.V., 1974. Compaction of Argillaceous Sediments. Developments in Sedimentology, 16. Elsevier, Amsterdam, 424 pp. Rowe, A.W., 1900--1908. The zones of the White Chalk of the English Coast. Proc. Geol. Assoc., 1 6 : 2 8 9 - - 3 6 8 (I: Kent and Sussex); 1 7 : 1 - - 1 7 6 (II: Dorset); 1 8 : 1 - - 5 1 (III: Devon); 1 9 : 1 9 3 - - 2 9 6 (IV: Yorkshire); 2 0 : 2 0 9 - - 3 5 2 (V: The Isle of Wight). Schlanger, S.O. and Douglas, R.G., 1974. The pelagic ooze--chalk--limestone transition and its implications for marine stratigraphy. In: K.J. Hsii and H.C. Jenkyns (Editors), Pelagic Sediments: On Land and Under the Sea. Spec. Publ. Int. Assoc. Sedimentol., 1: 117--148. Scholle, P.A., 1974. Diagenesis of Upper Cretaceous Chalks from England, Northern Ireland and the North Sea. In: K.J. Hsii and H.C. Jenkyns (Editors), Pelagic Sediments: On Land and Under the Sea. Spec. Publ. Int. Assoc. Sedimentol., 1: 177--210. Smart, J.G.O., Bisson, G. and Drewry, G.E., 1966. Geology of the country around Canterbury and Dover. Mere. Geol. Surv. U.K., 337 pp. Sorby, H.C., 1861. On the organic origin of the so-called 'crystalloids' of the Chalk. Ann. Mag. Nat. Hist., 8: 193--200. Stockdale, P.B., 1922. Stylolites: their nature and origin. Indiana Univ. Studies, 9: 1--97. Stockdale, P.B., 1926. The stratigraphic significance of solution in rocks. J. Geol., 34: 399--414. Stockdale, P.B., 1936. Rare stylolites. Am. J. Sci., 32: 129--133. Stockdale, P.B., 1943. Stylolites: primary or secondary? J. Sediment. Petrol., 13: 3--12. StSmgard, K.-E., 1973. Stress distribution during formation of boudinage and pressure shadows. Tectonophysics, 16: 215--248. Sujowski, Z.L., 1958. Diagenesis. Bull. Am. Assoc. Pet. Geol., 42: 2692--2717. Tucker, M.E., 1974. Sedimentology of Palaeozoic pelagic limestones: the Devonian Griotte (southern France) and Cephalopodenkalk (Germany). In: K.J. Hsii and H.C. Jenkyns (Editors), Pelagic Sediments: On Land and Under the Sea. Spec. Publ. Int. Assoc. Sedimentol., 1: 71--92. Voigt, E., 1962. Friihdiagenetische Deformation der turoner Pliinerkaik bei Halle/Westf. als Folge einer Grossgleitung unter besonderer Beriicksichtigung des Phacoides-Problems. Mitt. Geol. Staatsinst. Hamburg, 31: 146--275.

137 Von der Borch, C. et al., 1974. Site 216. In: C. v o n d e r Borch, J.G. Sclater et al., Initial Reports of the Deep Sea Drilling Project, 22. U.S. Govt. Printing Office, Washington, D.C., pp. 213--265. Weyl, P.K., 1959. Pressure solution and the force of recrystallization -- a phenomenological theory. J. Geophys. Res., 64: 2001--2025. Whitmarsh, R.B. et al., 1974. Site 223. In: R.B. Whitmarsh, O.E. Weser, D.A. Ross et al., Initial Reports of the Deep Sea Drilling Project, 23. U.S. Government Printing Office, Washington, D.C., pp. 291--381. Wilcox, N.R., 1953a. Some coprolites from phosphatic chalks in south-eastern England. Ann. Mag. Nat. Hist., 6: 369--375. Wilcox, N.R., 1953b. The origin of leds of phosphatic chalk with special reference to those at Taplow, England. 19th Int. Geol. Congr., Algiers, C.R., pp. 119--133. Wolfe, M., 1968. Lithification of a carbonate mud: Senonian Chalk in Northern Ireland. Sediment. Geol., 2 : 263--290.

Note added in proof: Structures resembling the chalk lenses herein described were produced recently by laboratory compaction of burrowed modern carbonate sediment at a pressure of 556 kg/cm 2 . The experiment and resulting structures are reported and illustrated in the following article:

Shinn, E.A., Halley, R.B., Hudson, J.A. and Lidz, B.H., 1977. Limestone compaction- an enigma. Geology, 5: 21--24.