Journal of the Geological Society, London, Vol. 154, 1997, pp. 897–912, 7 figs, 1 table. Printed in Great Britain

Petrology and palaeoenvironmental significance of glaucony in the succession at Whitecliff Bay, Basin, UK

J. M. HUGGETT1 & A. S. GALE2 1Department of Geology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK 2School of Earth Sciences, University of Greenwich, Grenville Building, Central Parade, Chatham Maritime, Chatham, ME4 4AW, UK and Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BB, UK

Abstract: Following the investigations of Odin and others into the distribution of green granules, glaucony has been widely assumed to be a reliable indicator of a fully marine, open shelf environment with a low sedimentation rate. We have investigated the value of glaucony as a palaeoenvironmental indicator through an investigation of the pellets, and their distribution and reworking in the predominantly brackish to shallow marine Tertiary sediments of the , together with a re-evaluation of the sedimentology. Glaucony has apparently formed in situ in all lithofacies from shallow marine to estuarine. Of the three highest glaucony concentrations (all dominated by in situ glaucony) two occur within highstand system tracts, the third is at a sequence boundary. Several important surfaces do not have more than a few percent glaucony, with very variable proportions of mature and in situ pellets. The correlation between glaucony concentration and sequence stratigraphy is most obvious in the and Wittering Formations, where least reworking of pellets has occurred. In the Barton Group there are no major concentrations of glaucony at any of the important stratal surfaces, we believe this more random glaucony distribution is due to limited glaucony formation and reworking of older glaucony. In these sediments ideal conditions for glaucony formation are interpreted to have been: fully marine, 10–30 m water depth, a ‘warm’ temperature plus low sedimentation rate with periodic winnowing to concentrate the pellets. Although most of these conditions for glaucony formation occurred in the Selsey Formation and Barton Group, a factor or factors mitigated against glauconitization. We suggest that this was lowering of the water temperature. The London Clay and Wittering Formations were deposited relatively rapidly (50–60 m Ma"1) and include intervals of estuarine sedimentation, both factors that we believe inhibited glaucony formation. Glaucony maturity reflects the minimum length of time spent in surface sediments, close to the oxic/sub-oxic interface. Point count data and chemical data for glaucony indicate widespread reworking and an overall increase in reworking with time, possibly due to uplift on the monocline. The apparently wide range of conditions in which glaucony will form, and the frequency with which it is reworked, suggest that it is a less useful indicator palaeo-environmental indicator than is commonly supposed.

Keywords: Eocene, glaucony, petrology, palaeoenvironments, sequence stratigraphy.

The glaucony facies and its present day, shallow water coun- nascent stage of glauconitization. The principal substrates for terpart, the verdine facies, occur in areas of slow sedimentation both the glaucony and verdine facies are faecal pellets, shell where there is a suitable substrate, a semi-confined environ- bioclasts, phyllosilicate grains and foraminifera. Glaucony ment, and an abundant supply of iron. The glaucony facies formation commences close to the sediment-water interface, comprises pellets of ferric-iron-rich, glauconitic minerals with with the formation of iron-rich smectitic clay (nascent). The a 10–14 Å basal spacing (Odin & Matter 1981), where 10 Å is intensity and rapidity of glauconitization depends upon the glauconite and 14 Å is 100% expandable smectite. The verdine nature and size of the substrate; the slightly evolved stage may facies comprises physically similar pellets composed of odinite apparently be reached after 104 years, and the evolved stage in (7 Å) and chlorite (14 Å) (the phyllite minerals of Odin 1985). 105 years if the granules are not buried (Giresse et al. 1980; The term glauconite should be reserved for the potassium-rich, Hughes & Whitehead 1987). Whilst Bornhold & Giresse (1985) micaceous end-member of the glauconitic mineral series estimate that glaucony with 3% K takes 3000 years to form. (Grunner 1935; Hendricks & Ross 1941; Burst 1958; Odin & Chemical evolution (uptake of FeIII and K) stops either after a Fullager 1988). long exposure at the sediment-water interface or after burial to Odin & Matter (1981) proposed a genetic classification for several decimeters (Odin & Matter 1981; Odin 1988b). Burial the progressive maturation of glaucony: nascent, slightly prior to the glaucony attaining the fully mature state will evolved, evolved and highly evolved. Potassium content is a preserve the immature state. Because glaucony is slow to form reliable indicator of pellet maturity. Iron content is less useful it is commonly associated with transgressions, where rapid because uptake of iron occurs almost entirely during the deepening starves the shelf of sediment.

897

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generally much less. For much of the Eocene, Whitecliff Bay was situated in the most distal part of the estuary complex and the facies developed here show more evidence of open marine conditions than are seen elsewhere in the basin. Nannofossils are present in parts of the marine succession, and provide a detailed correlation between Whitecliff Bay and the Palaeogene NP Zones (Aubry 1983; Aubry et al. 1986). The magnetostratigraphy of the Whitecliff succession has been investigated by Townsend & Hailwood (1985) and Ali et al. (1993). Aubry et al. (1986) proposed a correlation between the southern English Palaeogene stratigraphy and the oceanic record using nannofossils to identify magnetic chrons. Recently Amorosi & Centineo (this volume) have discussed the distribution of glaucony within the Lower to Middle Eocene sequence at Whitecliff Bay. Fig. 1. Location map and outline stratigraphy of the Eocene succession at Whitecliff Bay. The map shows approximate shorelines Sampling and methodology for (1) Wittering Formation, second sequence (2) Earnley Formation (3) Selsey Formation. HF, Harwich Formation; LCF, London Clay For this project, we have only used the exposures on the foreshore Formation; WF, Wittering Formation; EF, Earnley Formation; MF, because they have undergone minimum weathering. Early in the study, Marsh Farm Formation; SF, Selsey Formation; BCF, Barton Clay it became apparent that the succession in the Bracklesham Group Formation; BS, Becton Sand Formation. exposed on the foreshore differed significantly from that described in the cliff by Plint (1983). We have not used any of the available bed Within the tropics verdine takes the place of glaucony in numbering schemes because additional units exist on the foreshore, present day shallow marine and lagoonal environments, but instead have referred to heights above the base of the Harwich occurring in present-day water depths of 15–60 m (e.g. Odin Formation (marked on Fig. 2). Glaucony-bearing sediments were sampled from the Harwich Formation through to the Barton Group. a et al 1988 ; Odin & Sen Gupta 1988; Rao . 1995). Odin and Glaucony pellets concentrated using a Franz magnetic separator co-workers have suggested that glaucony forms in deeper (run with a current of 2 A) were analysed using a Phillips PW1710 water sediments in tropical environments (60–500 m) than in X-ray diffractometer. In order to model what happens when glaucony temperate environments (10–60 m), while below 60 m glauco- is reworked a batch of magnetically separated pellets was left in an nitization may not proceed beyond the nascent stage. Though ultrasonic bath for 4.5 hours, with aliquots of suspension removed data from the Gulf of Guinea suggests that glauconitization of every half hour for XRD analysis. The proportion of 10 Å clay in faecal pellets may have started beneath water as shallow as mixed layer glaucony was estimated from the tabulation provided in 25 m, with evolution continuing during the progressive deep- Moore & Reynolds (1989). ening that resulted from the Holocene transgression (Odin Glaucony pellets and their enclosing sediment have been described using both optical and back-scattered electron microscopy of polished 1988b). Apparently recently formed glaucony has been re- rock thin sections. The scanning electron microscope used was a ported from water depths of 100–500 m at temperate latitudes Hitachi S2500 with an Oxford instruments 860 EDS (energy dispersive (Bornhold & Giresse 1985). The verdine facies is however, spectra) detector. The total proportion of glaucony in a slide was apparently absent from sediments >20 000 years old, and there determined by point counting thin sections (all non-glauconitic grains is no obvious reason why there should be depth control of the were counted together). Glaucony was classified using a combination distribution of green clays; factors which must cast doubt on of appearance in back-scattered electron images and chemistry (deter- the validity of using the present as the key to the past in this mined by EDS X-ray analysis). The various classes were quantified by instance. describing approximately 100 grains per slide, or as many pellets as could be found in the slide, whichever was the lower number. These data are displayed as a bar chart in Fig. 3. Chemical analyses were Geological background carried out using a beam current of 2 ìA, a working distance of 20 mm, a low count rate to minimize beam damage (1500 counts per The cliff and foreshore exposures in Whitecliff Bay (Fig. 1), second) and 150 second counting time. situated on the steep northern limb of the Sandown Pericline, provide the best single section through the Thames, Bracklesham and Barton Groups (Lower and Middle Eocene) Glaucony characterization and classification in the Hampshire Basin. General accounts of the succession are given by White (1921) and Daley & Insole (1984). Classification During the Eocene, the Hampshire Basin was situated near Using BSEM and analytical data from over 500 glaucony the mouth of an easterly and southeasterly draining estuary pellets we have constructed a classification (Table 1) based complex (Fig. 1), an antecedent of the present day Solent principally on observed morphological types, but also taking (Jones 1989). The irregular coastline ran roughly north–south account of chemistry. Unlike previous schemes we have not through the basin, or far to the west of the area at times of attempted to fit all categories of glaucony into a single maximum transgression. In the western part of the basin, glaucony maturation path. For instance vermiform pellets are alluvial and estuarine deposits make up much of the succes- a consequence of glauconitization of decapod faecal pellets, sion, and interdigitate with marine facies which dominate to not an evolutionary stage as implied by the classification the east (Plint 1984; Edwards & Freshney 1987). At times of scheme of Odin & Matter (1981). Not surprisingly the chemi- high sea-level stand, as during deposition of the London Clay, cal composition of vermiform glaucony is the most vari- the entire region was covered by a shelf sea with a probable able, because the pellets retain a vermiform texture until maximum depth of about of 100 m (Murray & Wright 1974), they recrystallize (discussed below). In contrast with older

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classification schemes, we have found considerable chemical not survive reworking and may be totally disaggregated. The overlap between each morphological category. There are gra- green colour of the <2 ìm fraction in green clay-dominated dations, in terms of both appearance and maturity, between all intervals of the Barton Group suggests that this is indeed so. types of glaucony, but in particular between vermiform and The estimates of reworking, which are based on point counting ovoid, and ‘detrital clay-rich pellets with cracks’. pellet fragments, are therefore minimum values, and probably In BSEM images a change in texture is apparent from the underestimates. nascent, weakly evolved and vermiform grains to the evolved For clarity, in Fig. 3 the classification has been simplified to fractured, ovoid and healed fracture pellets (Fig. 4). Fracture- weakly evolved, vermiform, evolved and reworked. filling glaucony is typically maturer than the glaucony pellet it infills (Fig. 4). Ovoid pellets without identifiable filled fractures (Fig. 4) are chemically the most mature. The less evolved Glaucony composition grains have clear particulate constituents, whilst the more mature appear solid, have a uniform appearance and lack XRD data chemical zonation. The change to a ‘solid’ texture, the loss of Most glaucony extracts analysed are made up of either several vermiform structure and apparent homogenization of the discrete compositions of interstratified expandable (smectite)/ composition in healed fracture pellets to chemically uniform non-expandable (glauconite) clay, plus either discrete glauco- ovoid pellets may equate with the recrystallization proposed by nite or smectite, or a continuous range in composition from Clauer et al. (1992) and Stille & Clauer (1994) for glauconitic 50% to 100% smectite. Interstratified or discrete 7 Å or 14 Å material with >4% potassium. clay was not detected in clay pellets from any facies. The In situ glaucony has been identified using the following dominant composition(s) can in most samples be correlated criteria. with the dominant categories of glaucony point counted. End (1) Concentration of glaucony in burrows rather than the member glauconite, without detectable swelling layers corre- host sediment. lates with highly evolved glaucony. Rare kaolinite is either (2) Fragile pellet structure. Immature glaucony is a soft clay detrital or a weathering product (Carson & Kunze 1967). A pellet, unlikely to withstand transport over more than a high proportion of kaolinite, together with goethite and lepi- few metres. Highly fractured evolved glaucony must have docrocite was detected in the clay fraction of weathered fractured in situ (though of course this does not preclude samples (yellow sands). These minerals are a product of pre-fracturing reworking). glaucony weathering (Fig. 4). Reworked glaucony has been identified using the following criteria. (1) Fragments of fractured glaucony. (2) Ragged vermiform pellets; being open-textured vermi- Chemical data form pellets are more easily damaged, and more easily Potassium and iron both increase with pellet maturity. In the identified as reworked, than ‘solid’ pellets. Thames and Bracklesham Groups potassium oxide weight % (3) Sand-size glaucony in either clay or silt dominated sedi- varies from 1% for the most immature pellets to 10% to the ments. most mature (Fig. 6a & b). Iron oxide weight percent varies (4) Rounded grains with the same grain size distribution as from 5% (immature) to 27% (mature) in unweathered pellets. the detrital grains with which they are mixed (Lebauer In the Barton Group the spread of values is much narrower, 1964). with only one or two values outside 4–9% potassium and (5) Cross-bedding: glaucony is associated with slow sedimen- 20–30% iron (Fig. 6). An iron content >27% is indicative of tation, cross-bedding implies a high energy environment weathering (Odin & Matter 1981). Slight weathering does not (Wermund 1961). appear to affect the potassium concentration, but in the most (6) Glaucony, at the immature stage has a similar mineralogy intensely altered pellets (Fe>50%) all potassium has been to the associated clay or faecal pellets. Inclusions of removed (Fig. 6a). The reaction involves removal of ferrous detrital material which are not representative of the iron from the phyllosilicate lattice, and its oxidation to host sediment may therefore indicate reworking (Bell & goethite. The high iron content of weathered pellets suggests Goodall 1967). that iron is conserved, i.e. the goethite precipitates within the confines of the pellet, and that alumina and silica are removed. Amorosi (1995) uses sphericity and immaturity of pellets as Dissolution of the phyllosilicate structure apparently precedes criteria for reworking, whilst Amorosi & Centineo (1997) use, kaolinite formation (Fig. 4). The composition of highly in addition, pellet fracturing. We have not included these weathered pellets is compatible with a mixture of goethite and criteria because we do not believe that these characteristics are a combination of glaucony, expandable clay and kaolinite mainly acquired through reworking. Sphericity is a conse- (substantiated by XRD data). In the early stages of alteration quence of glaucony maturation (Odin & Fullager 1988). the goethite may or may not be visible in BSEM images, Immature pellets may cease to evolve if reworked into a less and the glaucony is morphologically indistinguishable from favourable environment, but we have demonstrated experi- unweathered glaucony. mentally (Fig. 5) that the least mature pellets are the most Silica increases with maturity, indicating a decrease in likely to disaggregate during transportation. Most of the tetrahedral aluminium. Octahedral aluminium decreases with reworked glaucony quantified is fragments of mature fractured increasing iron substitution, to a minimum value of 4% at 27% and vermiform pellets. The only other types recognized were total iron (Fig. 4d). The Mg content is typically 2–4% and does minor sand-size glaucony pellets in clay-rich sediments, and not appear to change significantly with increasing maturity. very rare overgrowths of contrasting composition to their host Sodium and calcium decrease with increasing potassium fix- pellet. Some entire pellets are probably also reworked, but ation in the interlayer sites. All other elements are present in criteria for identifying them remain elusive. Other pellets may amounts <1%.

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Fig. 2. Left.

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Fig. 2. Right.

Fig. 2. The succession in the Thames, Bracklesham and Barton Groups on the foreshore at Whitecliff Bay with position of samples indicated. Stippling indicates glaucony concentration.

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Fig. 3. Stratigraphical variation in glaucony concentration and maturity against changing palaeoenvironmental parameters. Arrows adjacent to the grain size log indicate major stratal surfaces. The sea-level curve and sequence analysis are based on our own observations. To the left of the sea-level curve the letter H with an arrow indicates major hiatuses. The four categories of salininty are (1) freshwater, (2, 3) lower and higher salinity brackish (4) euryhaline (approximately normal marine salinities). Absolute glaucony abundance (narrow bars) and relative abundances of principal glaucony categories (black is reworked; white is immature; light grey is mature; dark grey is vermiform) are shown. The position of the Eocene thermal maximum is shown. GLAUCONY IN EOCENE OF WHITECLIFF BAY 903

Table 1. Classification of glaucony based on BSEM and EDS data

Nearest equivalent Glaucony Back-scatter Odin (1988) morphology coefficient (ç) %Fe %K classification

Detrital clay-rich pellets, with inclusions Low to moderate, variable within 5.2–8.5 2.2–5.6 Nascent of mica, quartz, apatite, feldspar pellets Minor inclusions, no fractures Moderate, often internally variable 14.3–21.5 4.1–5.6 Slightly evolved Detrital clay-rich pellets, with cracks Low to moderate 10.8–18.3 0.86–8.3 Slightly evolved Vermicular pellets, usually without Low to moderate, not variable within 25.4–31.0 1.7–7.1 Slightly evolved inclusions pellets Dense, ovoid pellets with extensive Moderate to high, some variability 12.7–28.0 2.0–6.8 Evolved fracturing, inclusions very rare within pellets Dense, ovoid pellets, inclusions very rare High, rarely variable within pellets 20.8–25.9 4.7–9.0 Evolved Ovoid pellets with filled fractures, no High, but higher in fractures 27.0–28.0 5.4–7.7 Highly evolved inclusions (higher in fractures) Replacement rims on quartz High 21.1–21.9 6.1–8.35 None

Clay expandability is omitted because this classification is based on observation of individual pellets rather than the bulk, modal composition.

Stratigraphy and sedimentology of the Eocene sediments Sand of King (1981) which possibly represents a lowstand. The at Whitecliff sandy, sparsely glauconitic horizons identified as the bases of B1 (38.5 m, mostly weakly evolved and vermiform glaucony) Thames Group and B2 (48 m, mostly evolved and vermiform glaucony) re- This unit includes the Harwich Formation (Ellison et al. 1994) spectively represent transgressive surfaces at the base of a third and the London Clay Formation for which Whitecliff Bay Ypresian sequence; elsewhere in the basin Division B is under- serves as stratotype (King 1981). King recognized five divisions lain by a flint pebble conglomerate (King 1981) interpreted within the London Clay of both the Hampshire and London here as a transgressive beach deposit. Weathered glaucony Basins, labelled A–E, which are widely used in correlation in from the Bognor Sand has been reworked at the base of B1. both the London and Hampshire Basins, and identified nine The fourth sequence in the London Clay commences at the transgressive levels within the Thames Group. Plint (1988) base of Division C, which is a transgressive lag of flint pebbles briefly discussed the sedimentology of the highest part of the in a glauconitic sandy matrix. Divisions C2 and D1 consist London Clay, and Ali et al. (1993) described the magneto- of structureless or cross-bedded yellow sands deposited in stratigraphy of the London Clay. Murray & Wright (1974) estuarine channels during the early stages of a transgression described the foraminifera, and from these concluded that the (the Portsmouth and Whitecliff Members). Lags of clay clasts maximum depth of deposition (in Division B of King 1981) (reworked clay drapes) are common and Ophiomorpha is 20–100 m. abundant. Glaucony is present as rare weathered pellets. The The Harwich Formation (Ypresian) is 3 m thick and com- basal bed of Division D2 is a flint pebble conglomerate prises cross-bedded fine-grained glauconitic sands (mostly (134 m), representing a transgressive beach resting on a wave cracked or ovoid evolved glaucony) containing marine ravinement surface. D2 is a grey muddy siltstone with rare molluscs, interbedded with silts and clays. The basal bed weathered glaucony at the base reworked from the underlying includes abundant concretions (cm+) reworked from the weathered sands, and in situ, mostly mature glaucony filled underlying Reading Formation and sparse black flint pebbles. burrows at 143–144 m. Division E is a medium sand, glauco- Glaucony occurs in concentrations of 10–15% and locally nitic at the top (<5%, mostly mature and vermiform). The top more in burrow fills, where vermiform glaucony (a replace- of the London Clay (151 m) is an erosion surface, immediately ment of faecal pellets, Tapper & Fanning 1968; Pryor 1975) is below it weathered glaucony is present. particularly abundant. We interpret the Harwich Formation at Whitecliff as representing a condensed transgressive systems tract deposited in shallow marine conditions. Bracklesham Group The London Clay Formation (Ypresian) is 135 m thick at Fisher (1862) provided a detailed account of the stratigraphy Whitecliff Bay, and is made up of sands, silts and clays of this group, and Eaton (1976) described the dinoflagellate deposited in marine, lagoonal and estuarine environments. stratigraphy against a lithological log. The sedimentology of Most glaucony occurs in transgressive systems tracts, where it the Bracklesham Group was the subject of a detailed study by is concentrated above transgressive surfaces. At Whitecliff Plint (1983). He identified five transgressive levels, interpreted Bay, Division A1 is absent (King 1981). The lower part of the as eustatic sea-level rises. Subsequently (Plint 1988) elaborated formation at Whitecliff comprises mostly clay, the upper part the sedimentological model, and extended the study down into is predominantly sand (Fig. 2), but within this overall trend the London Clay and up into the Barton Group. In the model there are four upward coarsening units (A–D of King 1981). (Plint 1988, fig. 4), lagoonal muds and sands accumulated Division A2 rests non-sequentially upon the Harwich Member, behind a transgressive barrier bar, and represent the first stage and is the lower part of the second sequence in the Thames of transgression. Subsequently, the wave-dominated barrier Group. Division A2 is mudrock with intermittent metre spaced migrated landwards, generating a marine ravinement surface, sandy partings containing <3% mostly immature glaucony. on which a lag of pebbles accumulated. Above, a succession of Division A3 is a yellow–orange sand (17–21 m), the Bognor open marine facies was deposited, until sea-level fall created a

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Fig. 4. (a)–(f).

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Fig. 4. (g)–(i)

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erosion surface at Whitecliff, with most of the 35% glaucony in burrows, and consequently likely to be in situ. Magnetochron 22n may be missing on a hiatus at this level (Aubry et al. 1986; Plint 1988). The transgressive higher part of the formation includes laminated intertidal sands, silts, clays and a conspicu- ous rooted lignite (the ‘Whitecliff Bay Bed’; 198 m), which represents a freshwater marsh (Collinson pers. comm.). The highest concentration of immature glaucony was found in the Wittering Formation. Fig. 5. Length of time in ultrasonic bath plotted against % 10 Å The Earnley Formation (Lutetian) consists of 30 m of bio- clay, mixed layer 10–14 Å clay, and % 10 Å in the mixed layer clay turbated glauconitic sands and silty sands containing a diverse for disaggregated glaucony pellets. The clay % do not total 100% fauna of marine bivalves, gastropods, and large benthic because other clays not plotted are also present. foraminiferans which are concentrated in storm-deposited lenticles. The glauconitic base of the Earnley Formation lowstand erosion surface. Although Plint did not use sequence (206.7 m) is burrowed into intertidal muds and represents a stratigraphic terminology, his erosion surfaces correspond to maximum flooding surface, and the formation is interpreted as type 2 sequence boundaries, overlain directly by transgressive a condensed highstand deposit, formed offshore to a north– surfaces at the base of the lagoonal facies. The marine ravine- south coastline in 10–30 m of water (Plint 1983). The glaucony ment surfaces correspond to maximum flooding surfaces, concentration increases to 10–30%, most of which appears to overlain by highstand deposits. In this paper, we broadly be in situ and evolved. Numerous breaks in sedimentation are follow Plint’s model, but are aware that new work on estuaries represented by burrowed omission surfaces. The abundance of and incized valley systems (e.g. Zaitlin et al. 1994; Dalrymple the large foraminiferan Nummulites, which housed symbiotic et al. 1992, 1994) may justify a reinterpretation of the zooxanthellae, is evidence of warm conditions within the Bracklesham Group. Aubry et al. (1986) briefly described the photic zone (Adams et al. 1990). Although the macrofauna is nannofossil and magnetostratigraphy of the succession. In characteristically marine, evidence from the low foraminiferan contrast to the Thames Group, early transgressive deposits diversity (Murray & Wright 1974) and oxygen isotope data were intertidal muds, poorly suited to glaucony formation from corals (Swart & Rosen pers. comm.) suggest slight because of reduced salinity. At times of maximum flood- hyposalinity. The top of the Earnley Formation is eroded in ing barrier bars were broken down, and above ravinement the Hampshire Basin (Edwards & Freshney 1987) and repre- surfaces highstand sands were deposited in shallow (10–30 m), sents a sequence boundary overlain by a transgressive surface warm, open sea conditions. Supply of sand was very at the base of the Marsh Farm Formation (Lutetian) which limited, and storm reworking extensive, these conditions were comprises 15 m of transgressive estuarine laminated muds, silts evidently ideal for glaucony formation as they resulted in pellet and one thin fine marine sand. This transgressive surface has concentrations of 10–40%. negligible glaucony. The top of the Marsh Farm includes up to The Wittering Formation (Ypresian) is 56 m thick on the five rooted soils perhaps representing saltmarsh. Glaucony is foreshore at Whitecliff and includes a wide range of marine, extremely scarce throughout the formation and is mostly estuarine and lagoonal facies which are lithologically diverse reworked or weakly evolved. and include two incomplete sequences. The basal transgressive The Selsey Formation (Lutetian) includes fully marine sands, part of the lower sequence comprises estuarine laminated silts and clays and is laterally variable in development at sandy clays and a unidirectionally cross-bedded sand, above Whitecliff. The formation contains numerous breaks in sedi- which lies a marine ravinement surface with burrows filled mentation, and as interpreted here, contains three partial mostly by evolved glaucony (170 m), at the base of the second sequences. Despite these breaks, glaucony is much less abun- sequence. Weathered glaucony reworked from the Whitecliff dant than in the Wittering or Earnley Formations (Fig. 3). The Member is present at the base of the formation. The higher base of the lowest sequence (252 m) is a sand with mostly sequence comprises glauconitic and silty sands containing evolved glaucony filled burrows and scattered dark rounded marine molluscs, passing up into unfossiliferous highly flint pebbles and marine molluscs which rests on a marine glauconitic sands (mostly weakly evolved and evolved ravinement surface. A muddy glauconitic sand (258.5–260 m, glaucony). The sequence is terminated by an erosional surface mostly evolved glaucony) containing sparse black flints and with burrows filled mostly by evolved glaucony (187.5 m) abundant bivalves (Amussium) is interpreted as a transgressive which is interpreted as a sequence boundary immediately surface at the base of a second Selsey sequence. A thin overlain by a transgressive surface. This is the most glauconitic cross-bedded sandstone (the Tellina Sand, 280.5–281.9 m) was

Fig. 4. (a) Very slightly evolved glaucony pellet from a mud-dominated estuarine channel fill in the Wittering Formation. Fe=10.5%, K=1.7%. Scale bar=25 ìm. (b) Slightly evolved, fractured glaucony from the sand below E3 in the Wittering Formation. The top right sector of this pellet may be missing, if so it is reworked. Fe=18–25%, K=4.8–7.3%. Scale bar=50 ìm. (c) Slightly evolved glaucony with abundant inclusions, from mud-dominated estuarine channel fill (172.5 m), Wittering Formation. The clay fabric, clearly apparent in (a) is by this stage diminished, though still visible. Fe=21.5%, K=5.6%. Scale bar=50 ìm. (d) Vermiform glaucony from a mud-dominated estuarine channel fill in the Wittering Formation. This pellet is deformed by compaction. Fe=31% (i.e. weathered), K=7.1%. Scale bar=30 ìm. (e) Evolved spheroidal pellet with cracks, Earnley Formation. Scale bar=100 ìm. (f) Evolved spheroidal pellet, Selsey Formation. Scale bar=100 ìm. (g) Pellet with fractures containing glaucony which is more mature than the pellet (arrow). The white patches are NaCl crystals from recent seawater. From the base of B2, London Clay. Scale bar=100 ìm. (h) Quartz grain with marginal alteration to glaucony (arrow), Wittering Formation. Scale bar=50 ìm. (i) Reworked vermiform glaucony, Harwich Formation. Scale bar=60 ìm. (j) Weathered glaucony (Fe=50%, K=0.3%) from 31.2 m, grey silty sand, Division A3, London Clay. This pellet is reworked from the underlying, oxidised yellow sand. The pyrite in the fractures formed during re-burial. Scale bar=50 ìm. (k) Glaucony oxidised to kaolinite and goethite in yellow sand at the top of the Barton Sand. Scale bar=50 ìm.

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Fig. 6. EDS X-ray data, plotted as oxide wt % for glaucony pellets. <4% K oxide and <19.7% Fe oxide is the range for nascent and slightly evolved glaucony, >8% K oxide and>22% Fe oxide is evolved and highly evolved glaucony, 4–8% K oxide and 19.7–22% Fe oxide is the range of overlap. Fe oxide >27% indicates weathered pellets. (a) % K oxide plotted against % Fe oxide, London Clay Formation (b) % K oxide plotted against % Fe oxide, Bracklesham Group (c) % K oxide plotted against % Fe oxide, Barton Group (d) % Al oxide plotted against %Fe oxide, data from all formations.

interpreted by Plint (1988) as a hummocky cross- stratified common (<3%). At 316.5 m, nummulites appear in a thin product of lowstand winnowing. The highest 6 m of the Selsey glauconitic sand, which is laterally equivalent to the transgres- Formation, fine silty glauconitic sands (mostly reworked and sive base of the Barton Clay at the type section at Barton on evolved, with the percentage reworking increasing towards the Hampshire coast (Hooker 1988). This horizon therefore the top) resting on an erosional surface, are interpreted as the represents the thin transgressive systems tract of a second transgressive part of a third Selsey sequence. Barton sequence, and is overlain by green clays with marine molluscs of the succeeding highstand. These clays are truncated by an erosional surface, interpreted as a sequence Barton Group boundary (331.5 m) on which rests a lenticular flint conglom- Curry (1937) provided a brief account of the stratigraphy of erate, interpreted as a basal transgressive lag of the third the Barton Clay, Freshney et al. (1990) described the contact Barton sequence. with the underlying Selsey Formation and Bujak et al. (1980) The lower part of the Becton Sand Formation (339 m) the dinoflagelate stratigraphy. The Barton Clay and basal comprises weakly glauconitic fine sands, interpreted as a Becton Sand contain four partial sequences according to our highstand deposit, overlain (at 353.3 m) by an erosively based study, but we are not sure how these relate to the transgressive- thin glauconitic medium sand containing many small clasts of regressive cycles described by Hooker (1988) from the strato- reworked Reading Formation red clay. This represents the type in the Hampshire Basin. There are no major transgressive lag of a fourth Barton sequence. concentrations of glaucony in the Barton Group, and both immature and vermiform glaucony are rare above 310 m. The concentration of evolved and reworked glaucony is higher Glaucony reworking overall in the Barton Group than in any other part of the Overall, the degree of reworking increased with time (Fig. 3), succession. perhaps reflecting uplift related to development of the Isle of The transgressive base of the Barton Clay Formation rests Wight Monocline. The uniformly green colour of the Barton on a burrowed omission surface at the top of the Selsey Clay is inferred to be due to dispersed glaucony platelets. In Formation and is a glauconitic muddy silt (<5% glaucony) the Bracklesham Group reworked fragments are most abun- containing occasional flint pebbles. The overlying clays and dant (the point count data indicates a minimum of 10–25%) silty clays (292–316.5 m) contain a marine fauna have a in laminated estuarine mudrocks overlying sandy estuarine distinctive green-blue tinge, though glaucony pellets are un- channel fills. High glaucony concentrations are principally

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associated with transgressive surfaces. The proportion of Barton Clay. The decreasing rates of deposition probably reworked glaucony is also high on transgressive surfaces, have a tectonic component, related to uplift of the Isle of indicating shoreward reworking. Throughout the entire succes- Wight–Purbeck structure in the mid-Eocene (see Plint sion, the similar composition of reworked and in situ pellets 1982). This reduced accommodation space through the time implies that the reworking is restricted to local (Hampshire the Selsey Formation, the Barton Clay and Barton Sand Basin) Eocene glaucony. Reworking of glaucony from fully Formations were being deposited. marine to marginal and brackish environments is indicated Using published data for glaucony maturation times in shelf throughout the Bracklesham and Barton Groups by the environments (Bornhold & Giresse 1985; Odin & Matter 1981; significant proportion (10% to around 70%) of reworked Odin 1988b), it may be possible to estimate the length of time glaucony fragments in lagoonal and estuarine channel fill represented by erosion surfaces and for the cm-thick silty sands sediments overlying shelf sands. However to assume that in the mudrocks of Divisions A and B in the London Clay. The glaucony in non-marine facies is reworked because the sedi- immature glaucony in the latter suggests a duration of 3000– ments are non-marine (cf. Amorosi 1995) is to assume that 10 000 years. For surfaces on sediments with predominantly glaucony cannot form in non-marine facies. mature glaucony in burrows, i.e. in situ, as at 2–3 m (TS), 48 m The disaggregation experiment (Fig. 5) was undertaken to (TS), 170 m (MFS), 187.5 m (SB/TS), 206.7 m (MFS), 252 m investigate what happens when glaucony pellets are reworked. (SB) and 316.5 m (TS), a duration of at least 105 to 106 years The composition of the clay which is disaggregated first is is likely. Other glauconitic horizons dominated by less mature dominated by 10 Å clay (illite and glauconite) and mixed layer surfaces with in situ glaucony, and perhaps representing less 10–14 Å clay (illite-smectite or glauconitic smectite). Over the time, occur at 38.5 m (TS), 81 m (TS) and 173 m (minor 4.5 hours of the experiment the proportion of 10 Å clay erosive surface within a HST). dispersed decreases to zero, and the mixed layer clay becomes dominant (90% of the clay dispersed). The composition of the mixed layer clay shows a slight but steady increase, from 50% Sedimentary facies to 70% in the proportion of 10 Å interlayers. This is inter- preted as indicating initial dispersal of immature pellets of It is evident from Fig. 3 that maximum concentrations of glauconitic smectite and glauconitic illite (with 50% glaucony apparently in situ glaucony occur in open marine sediments. interlayers). The decrease in the expandability of the mixed Minimum concentrations occur in estuarine facies (Wittering layer clay with time in the ultrasonic bath is interpreted and Marsh Farm Formations). The presence of nascent to as dispersal of progressively maturer pellets with increased slightly evolved glaucony in both very shallow marine and reworking. The last pellets to be dispersed were of uniform estuarine sediments suggests in situ formation and either burial composition (smectitic glauconite). Had the experiment been prior to maturity, or slower than normal rate of formation due continued we predict that the dispersed clay would eventually to the unfavourable nature of the environment. Glaucony been end-member glauconite without any smectite interlayers. formation and maturation is apparently favoured by coarser The narrow compositional range of Barton Group EDS X-ray grain size as well as low sediment deposition rate (Giresse et al. analyses around mature glaucony compositions (Fig. 6c) sug- 1980; Odin & Matter 1981). Concentration in coarser sediment gests that with few exceptions glauconitization was not occur- is corroborated by our assessment of the distribution of mature ring, but rather reworking was only preserving the more glaucony in the Eocene sediments at Whitecliff Bay. Whether durable, mature glaucony from the underlying beds. The green this is a result of winnowing, or preferential formation in colour of the matrix throughout the Barton Group mudrock coarser-grained sediments is unclear. intervals implies that disaggregated glaucony pellets have been extensively reworked without oxidation. Depth of glaucony formation At Whitecliff Bay, a summation of faunal and sedimentologi- Palaeoenvironmental controls on glaucony formation cal evidence leads us to suggest that glaucony formation Sediment accumulation rates occurred in water depths less than 60 m, but was concentrated in 10–30 m. There is sufficient evidence for in situ formation Depositional rates for the Whitecliff Bay section are shown as that extensive reworking from >60 m may be discounted. a graphic correlation plot (Fig. 7) against the Palaeogene time Present-day glauconitization appears to be occurring scale, taken from Berggren et al. (1995). Correlation lines with principally at water depths of 60–500 m in warm shelf seas, Whitecliff are based on nannofossil zones taken from Aubry with 7 Å clay pellets forming in shallower water (Odin 1988b; et al. (1986), and magnetozones (Townsend & Hailwood 1985; Porrenga 1967). It is perhaps surprising therefore that we have Aubry et al. 1986; Ali et al. 1993). Because of the dominance of detected no 7 Å or 14 Å green clay pellets. Burst (1958) and silt and fine sand in the succession, differential compaction is Wermund (1961) both recorded ‘abundant’ green clay pellets unlikely to affect the shape of the curve, and it is interpreted as from estuarine and lagoonal sediments of Eocene age, with approximating to primary depositional rate. Quoted rates are rare occurrences in deltaic and fluviatile sediments. Both inevitably mean values for periods of sedimentation and of authors report a 7 Å and 14 Å clay component to the pellets, non-deposition. Deposition was very slow in the Harwich and Burst goes so far as to correlate these clays with near-shore Formation, and Division A1 of the London Clay is missing and brackish environments. (King 1981). However, the London Clay and Wittering Formation were relatively rapidly deposited (rates of 50–60 m Ma"1) up to the hiatus on which chron 22n is missing. The Earnley Formation registers depositional rates of about 30 m Salinity Ma"1. Above the Wittering Formation depositional rates fell Salinities for the Whitecliff Bay Eocene succession have steadily to a minimum value of 6–7 m Ma"1 in the lower been placed in four categories, based largely upon evidence

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Fig. 7. Graphic correlation plot of Eocene succession in Whitecliff Bay (metres, horizontal axis) against the timescale of Berggren et al. (1995; millions of years, vertical axis). This gives an impression of changing depositional rates associated particularly with mid-Eocene inversion of the Purbeck–Isle of Wight structure.

from molluscs, foraminiferans (Murray & Wright 1974) and includes some of the most glauconitic beds in the succession palynomorphs (Eaton 1976), with additional information on with apparently high concentrations of in situ glaucony, selachian (shark and ray) tooth assemblages (D. Ward pers. yet faunal and isotope data imply slight hyposalinity. In comm.). Salinity is apparently a major factor in glaucony estuarine environments (Wittering and Marsh Farm For- formation (Odin & Matter 1981; Odin & Fullager 1988), mations) the presence of rare entire, immature (and therefore although not all the major concentrations reported here are in fragile relative to mature glaucony) pellets (Fig. 4) suggests sediments of fully marine salinity. The Earnley Formation in situ formation.

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Temperature affect glaucony has apparently only affected the top metre of Although there are no published oxygen isotope analyses of preserved sediment in divisions A3 and D1 of the London material from the succession at Whitecliff Bay, various authors Clay. The majority of the weathered horizons (yellow sands have published temperature curves for the Eocene based on and brown clays) contain mostly fresh glaucony. This implies ä18O values from mollusc shells collected in NW Europe that glaucony is more resistant to oxidation than either pyrite (Buchardt 1978) and benthic foraminifera from deep sea core or siderite, which are absent from all weathered sediments and (Oberhansli & Hsu 1986; Katz & Miller 1991; Pak & Miller are presumably a prime source of the goethite. 1992). Although absolute temperatures derived from the iso- topes are not always reliable, the trends shown by isotope Summary values are quite consistent. These demonstrate a rise in tem- peratures through the Ypresian, culminating in a late Ypresian (1) The range of conditions in which glaucony will form is to early Lutetian peak (NP11–13), followed by a fall through wider than previously thought. the late Lutetian (later part of NP14). Temperatures fell slowly (2) The principal difficulty in determining the conditions in or remained approximately level through the latest Lutetian, which glaucony has formed is that of identifying glaucony then fell again in the Bartonian. The ä18O curve provided by which has been reworked from one environment to another, Pak & Miller (1992) can be superimposed onto the Whitecliff and glaucony which has been reworked from older to younger Bay succession using nannofossil zones (NP) from Aubry et al. sediments. The former may be particularly common on trans- (1986) to provide correlation. The result suggest that the gressive surfaces. We have defined criteria for estimating the Eocene thermal maximum falls within the upper part of the degree of reworking, but few of these are unequivocal. London Clay (Divisions C–E) and in the Wittering and (3) The boundary conditions for glaucony formation are Earnley Formations. Absolute temperatures for this interval broader than those for intense glauconitization, which may be in NW Europe derived from ä18O data for foraminifera quite narrow. (4) In the Eocene of Whitecliff Bay, ideal conditions for (Buchardt 1978) fall in the interval of 25–30)C, which is probably too high, and may indicate diagenetic re-setting of glaucony formation are: the ä18O ratio. Foraminiferal evidence from the upper part of + approximately full marine salinity the Selsey Formation in the nearby and closely similar succes- + 10–30 m water depth sion at Bracklesham Bay led Murray & Wright (1974) to + sea-surface temperature >20)C + periodic winnowing to concentrate the pellets. suggest warm summer temperatures of about 18)C. The decrease in both the absolute abundance of glaucony (5) Although most of these conditions for glaucony and the proportion of presumed in situ pellets which com- formation occurred in the Selsey Formation and Barton mences in the Selsey Formation and continues through the Group, much of the glaucony in these sediments is believed Barton Group may be a consequence of climatic cooling. This to be reworked. A factor or factors mitigated against is contrary to the conclusion of Odin & Matter (1981) that glaucony formation which we suggest that this was lower water glaucony formation is temperature independent. In other temperature. respects the Selsey Formation and Barton Group are most (6) In the London Clay most glaucony is in transgressive suitable candidates for glaucony formation, being marine systems tracts, immediately above individual transgressive and having lower sedimentation rates than almost any other surfaces. Increased current activity associated with transgres- marine interval in the succession. Unfortunately there is no sion produced a winnowed substrate of periodically disturbed reliable modern analogue for the control of temperature on (storm reworked) sand grade detritus, which provided tempor- glaucony distribution: the importance of odinite in modern ary, but ideal, habitats for glaucony formation. In the green sediments makes all comparison with the recent dubious. Bracklesham Group, lower overall sea-levels meant that the At the present day, glauconite forms in the tropics at early transgressive deposits of each sequence were intertidal muds – poorly suited to glaucony formation because of c15)C (Odin 1988a), down shelf from the verdine facies reduced salinity. Maximum flooding caused breakdown of which requires bottom water temperatures of 20–25)C (Odin 1988a). barrier bars, and above ravinement surfaces, highstand sands were deposited in shallow (10–30 m), warm, open sea con- ditions of almost full marine salinity. Supply of sand was very limited, and storm reworking extensive; these conditions were Weathering of glaucony ideal for glaucony formation and lead to the high concen- Glaucony weathering has been believed to occur in water trations of the Wittering and Earnley Formations. Compar- shallower than 30 m, in a sub-tropical climate (Odin & able sea-levels and environments in the Selsey Formation did Fullager 1988) and as deep as 120 m on the Gulf of Guinea not lead to such high concentrations of glaucony. In the shelf (Odin 1988b). The presence of nascent and otherwise Barton Group there are no major concentrations of glaucony apparently in situ glaucony in Bracklesham Group sediments pellets. We therefore agree with the recent work of Amorosi & deposited in <30 m water depth implies that this is not Centineo (1997) that sea-level has an impact on glaucony necessarily the case. Weathered glaucony was detected in formation, but we consider other factors (see point 4 above) horizons which may have been deposited above wave base, or are equally important. even partially emergent. The presence of weathered glaucony (7) Glaucony is widely used as a palaeoenvironmental in the London Clay Division E marine mudrock, indicates that indicator, however, the apparently wide range of conditions in the weathering of Division C and D estuarine sands predates which glaucony will form, and the frequency with which it the transgression which followed deposition of D2, and is not is reworked, suggest that it is a less useful indicator than is entirely, or even at all, recent. The presence of pyrite in commonly supposed. weathered glaucony fractures indicates subsequent sulphate (8) At the present day, the glaucony facies appears to have reduction during re-burial. Weathering sufficiently intense to been displaced to deeper water by the verdine facies. This

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