Lacustrine and palustrine facies in the Bembridge Limestone (late Eocene, Hampshire Basin) of the Isle of Wight, southern England

PALAEO ELSEVIER

Palaeogeography, Palaeoclimatology,Palaeoecology128 (1997) 111-132

Lacustrine and palustrine facies in the Bembridge Limestone (late Eocene, Hampshire Basin) of the Isle of Wight, southern England Ildefonso Armenteros a, Brian Daley b, E m m a Garcia a a Departamento de Geologia, Facultad de Ciencias, Universidad de Salamanca, 37071, Salamanca, Spain b Department of Geology, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 3QL, UK Received 12 December 1995; revision 3 July 1996; accepted 14 August 1996

Abstract

The carbonate lake system represented by the late Eocene Bembridge Limestone Formation (Isle of Wight, southern England) comprises two main facies: lacustrine and palustrine. These facies are arranged vertically in transgressive/regressive cycles, corresponding to a model of carbonate lakes of low gradient and fluctuating margins with extensive palustrine fringes. These lakes developed in a coastal plain of limited relief, where two paleogeographical domains predominate. A lacustrine domain is represented in northern and eastern sections in the Isle of Wight by cycles representing upward passage from central lacustrine sub-facies (marl/marly limestone) through marginal lacustrine sub-facies (biomicrite) to sub-facies indicative of exposure (brecciated-nodular limestone). A palustrine domain, predominant in west Wight, is represented by a predominantly pedogenic palustrine facies with clottedpeloidal to ooidal textures and some intercalated laminated (laminar calcrete) horizons. Within the pedogenic facies, textures have evolved from those of unaltered biomicrites through clotted-peloidal to dense ooidal to open ooidal textures. A third domain, developed in close geographical proximity to more saline waters, is represented by gypsiferous lake-margin facies found in north Wight in which microlenticular gypsum developed post-depositionally as a result of evaporation following subaerial sediment exposure. In the lacustrine domain, there appears to be an association between cyclicity and changes in base (sea) level reflecting a likelihood that the lakes were part of a paralic, coastal plain complex with distal marine connections. Alternatively, the cyclicity might have developed as a result of the alternation of wetter and drier climatic periods.

Keywords: lacustrine; palustrine; carbonate; Late Eocene; Hampshire Basin; England

1. Introduction

Over the last twenty years or so, a number of examples of ancient lacustrine sequences have been described which show more or less pronounced similarities to the Bembridge Limestone, the subject of the present paper. These developed in shallow, low-energy carbonate lakes with gentle gradients from littoral to more central zones and

where lake levels were subject to fluctuation (Freytet, 1973; Freytet and Plaziat, 1982; and Platt and Wright, 1991 ). In these low gradient carbonate lake systems, marginal shallow facies are dominated by a characteristic suite of exposure features which led Freytet (1973) to define a lacustrine model in which two facies associations could be recognised: lacustrine and palustrine. Palustrine facies record the modification of the original lacus-

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L Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111 132

trine sediments by pedogenic processes. Such carbonate facies associations have been recognised from Cretaceous and Tertiary strata, mainly in France, Spain and the USA, and are associated with several tectonic settings (Table 1). Most examples are found in continental depositional settings. Few, such as the Bembridge Limestone (Daley and Edwards, 1990) are associated with paralic and transitional environments (see also Freytet and Plaziat, 1982 and, for a present-day example, Platt and Wright, 1992). Prior to the stratigraphical revision of Daley and Edwards (1990) and the recent detailed study of the outlier at Headon Hill (Hooker et al., 1995), no comprehensive account of the Bembridge Limestone had been published since White (1921). In recent years, the recognition that this formation has in places been pedogenically modified has given rise to a renewal of interest (Marshall et al., 1987, 1988; Daley, 1989; Garcia G6mez, 1990; Armenteros et al., 1992). The aim of this paper is to describe and interpret the various facies of the formation, their cyclic arrangement and significance as palaeogeographic and palaeoclimatic indicators.

2. Stratigraphic context In the Hampshire Basin of southern England, over 600 m of predominantly clastic Palaeogene strata rest unconformably on the Chalk (Upper Cretaceous). They represent an interval of sedimentation in excess of 20 Ma, from around NP9 NP10 (earliest Eocene) to NP23 (earliest Oligocene) (Costa and Downie, 1976; Martini, 1972; Aubry, 1986) and comprise the products of repeated transgressions and regressions. Marine conditions predominated until late Eocene, around NP18 times, after which non-marine conditions became dominant. The distinctive character of the succession representing the latter phase was recognised early by Forbes (1853) who named it the "Fluvio-marine Formation". Now known as the Solent Group (Insole and Daley, 1985), it comprises three formations: in ascending order, the Headon Hill Formation, the Bembridge Limestone Formation and the Bouldnor Formation (Fig. 1).

The Bembridge Limestone is the most laterally extensive of a number of freshwater limestones within the Solent Group. The others occur within the Headon Hill Formation, but are thin and/or only of local importance and do not warrant formational status. Although the Bembridge Limestone is the most extensive Palaeogene freshwater limestone north of the Paris Basin, where such deposits are thicker, more widespread and well described, it has until recently not received the attention that it deserves.

3. The Bembridge Limestone The Bembridge Limestone occurs only in the Isle of Wight. At its maximum development, the formation reaches just over 9 m in thickness, but elsewhere thins considerably (see isopachyte map in Insole and Daley, 1985, fig. 23). It consists of several beds of cream-coloured, predominantly freshwater limestone (characterized by the pulmonate gastropod Galba) with subordinate marls, although the formation as a whole is "split" into "upper" and "lower" limestones by a central sequence of green and black muds, at least in part of brackish water origin, as indicated by the presence of the bivalve Corbicula, and the gastropods Potamides and Melanoides. There are many small coastal exposures of Bembridge Limestone, although the quality of the sections is frequently poor, especially along the northern coast of the Isle of Wight, as a result of recent land slippage. The major localities and their correlation are shown in Fig. 2 (after Daley and Edwards, 1990, fig. 12). Of these, three were selected for the present study to emphasise contrasting sedimentological developments. These are Whitecliff Bay (the stratotype), Gurnard and Prospect Quarry (see Fig. 3A, B and C, respectively). That the Bembridge Limestone is of predominantly freshwater origin was recognised some years ago from the common occurrence of freshwater pulmonate gastropods, particularly Galba and to a lesser extent Planorbina, with the close proximity of land indicated by the presence of a land gastropod fauna (Pain and Preece, 1968). Recent work by Hooker et al. (1995) on the mammalian fossils

Alluvial systems Local alluvial terrigenous influx Prograding alluvial fan/fluvial systems

Alluvial fan Paralic to transitional environments Alluvial fans

Foreland Small graben Foreland next to Iberian Range Foreland next to Central System Foreland Subsiding basin Foreland basin (N Pyrenees) Foreland

Low-lying marshland

Intracratonic basins with active fronts Foreland Basin

Foreland basin (S. Pyrenees) Subsiding zone

Fluvial plain (paralic environment)

Foreland

Between distal fluvial and brackish marsh-lagoon complex

Alluvial fans

Distal alluvial plain in an endoreic basin Alluvial/fluvial systems

Prograding alluvial fans and alluvial plains Alluvial/fluvial systems

Prograding alluvial fan

Alluvial fan and fluvial systems

Foreland

Graben type

22 23

Freshwater carbonate marsh with dispersed lakes.

10, 20, 21

18, 19

17

15, 16

14

13

12

11

10

9

7, 8

5, 6

4

3

2

1

Author

Shallow lacustrine and palustrine environments with evaporites in the basin center. Shallow freshwater lakes (ocasionally saline lakes)

Marginal and central carbonate and evaporitic lacustrine environments Shallow lakes, swamps and pedogenesis areas; minor evaporites and coals Swamp (palustrine), marginal nearshore, evaporitic flat-saline lake Shallow palustrine and pedological domaines

Lacustrine and paludal environments (paleolake Flagstaff) Littoral and central lacustrine environment Alluvial plain, lacustrine and palustrine environments. Lacustrine (littoral), palustrine (supralittoral) and alluvial plain environments Fluvial plain, lacustrine nearshore (paludal)offshore and evaporitic lakes Lacustrine and palustrine environments

Open lacustrine and marginal lacustr./ palustrine environments; minor evaporites Palustrine environment and permanent lake Shallow perennial lake with peripheral paludal areas Palustrine and lacustrine environments

Shallow freshwater carbonate lake

Lacustrine environments

Authors: (1) Valero Garcrs et al., 1994; (2) Platt, 1989; (3) Normati and Salomon, 1989; (4) Brown and Wilkinson, 1981; (5) Freytet, 1973; (6) Freytet and Plaziat, 1982; (7) Stanley and Collinson, 1979; (8) Wells, 1983; (9) Lablanche, 1982; (10) Corrochano and Armenteros, 1989; (11) Arribas Mocoroa, 1986a, Arribas Mocoroa, 1986b; (12) Nickel, 1982; (13) Daley and Edwards, 1990; (14) Szulc et al., 1991; (15) Platt, 1992; (16) Bersier, 1958; (17) Cabrera et al., 1985; (18) Freytet, 1965; (19) Menillet, 1980; (20) Armenteros, 1986; (21) Alonso et al., 1992; (22) Molenaar and De Feyter, 1985; (23) Platt and Wright, 1992.

W. Wyoming-SE Idaho, USA, Lower Cretaceous S. France, Languedoc, Upper Cretaceous~pper Eocene Uiuta Basin, USA, upper Paleocene-lower Eocene S.W. Paris Basin, France, upper Eocene E. Duero Basin, Spain, upper Eocene Oligocene Tajo (Madrid) Basin (Spain), Palcogene Southern Pyrenees Spain, upper Eocene Hampshire Basin, S. England, upper Eocene Narbonne Basin, SE France, upper Oligocene Swiss Molasse Basin, west Switzerland, upper Oligocene S Ebro Basin, NE Spain, Oligocene-Miocene S. Paris Basin, France, upper Oligocene lower Miocene Duero and Madrid Basins, Spain, middle-upper Miocene Pietrarrubia Basin, N. Italy, Messinian, upper Miocene Florida Everglades, USA, Holocene

Alluvial plain

Extensional rift

Foreland basin

Appalachian Basin, Pennsylvanian cyclothems, Pennsylvania, USA Cameras Basin Iberian Range, Spain, Lower Cretaceous Cameras Basin (as former example)

Related depositional systems Distal fluvial (anastomosed to single-channel) Distal alluvial environments

Basin type

Example

Table 1 Examples and main characteristics of shallow, low-energy, low-gradient carbonate lacustrine systems with fluctuating levels

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SOLENT

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WHITECLIFFBAY

PRIORYBAY

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F HBOURNE

WOODVALE

;HT

Fig. 1. Geological map of the Isle of Wight, showing the outcrop of the Bembridge Limestone Formation and its position in the Isle of Wight succession. Main sections studied: Whitecliff Bay; Gurnard and Prospect Quarry. NP (nannoplankton) zones show approximate age.

NP 9/10

NP 18

:IMATION JLDNOR 1 ABRIDGE ESTONEF. kDONHiLL

L~ Ka

t~

!

I. Armenteros et aL /Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111-132

115

WHITECLIFF B A Y

I

I Limited palustrine development (LP)

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GURNARD

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I Clastic Sediments, mostly muds

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Limestone, "",':'i'! well lithified

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Fig. 2. Correlation of the Bembridge Limestone Formation. (After Daley and Edwards, 1990).

suggests woodland or forest in the adjacent hinterland whilst noting the absence of any other indication of trees except for the land snails. The concentration of hydrobiid gastropods at some horizons (Daley and Edwards, 1990) is now considered to imply a degree of brackishness at intervals, whilst the occasional occurrence of foraminifera (Murray and Wright, 1974) suggests at least the proximity of the sea, if not relatively short-lived raised salinities from time to time. Mineralogical studies by Daley (1989) support such possibilities. Lithologically, the carbonate sediments of the Bembridge Limestone vary from soft, clay-bearing carbonates (marls, marly limestones) to well lithifled limestones. They mainly comprise low magnesian calcite, with late ferroan calcite cementation in the eastern and northeastern localities. Clay

content is generally < 2% and:comprises dioctahedral smectite, illite and smaller amount of kaolinite. Illite, frequently degraded, predominates in the lacustrine facies, whereas smectite, illite and smaller amounts of kaolinite characterise the brackish muddy horizons. In the pedogenic facies, it has proved difficult to extract sufficient clay minerals for clear identification, although smectite is present. Chert nodules occur locally and comprise quartz and chalcedonic silica (lutecite and chalcedonite) (Daley, 1989). Telodiagenetic goethite is locally present in the Gurnard section (Fig. 3B). 4. Carbonate facies of the Bembridge Limestone

The carbonate sediments present within the formation can be assigned to three facies: lacustrine

L Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111 132

116

7

, 8w ~

5

2

t~-C

3

2

@ #

ol rl

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c, 6,

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a

Gr C 5r I HHF

c

KEY

w

FOSStLS

LITHOLOGY

A

Corbicula

Ooidal - pelok~,l limestone Clotted peloidal

J

Potamldes

Biomicdte

<~

Lymnaea( Gatba)

Intraclastlc limestone

~,

Hydrobiids @

Brecclated Laminar ( crust ) lirneslo ne

w

a

planorbids

~:)

Land gastropods

Marly limestone

Broken shells

Marl Black horizons Argillites Sand

Foraminilers

8 <

6r

Ostracods Char~y~e~

e Cherl nodules

~

•0

~erruginous concentrations snd tiesegang ringa

~

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Casts of foSSils

a

POSTDEPOSITIONAL FEATURES

6

SEDIMENTARY STRUCTURES AND CYCLES,

J

rhizoliths

° I

i

vug porosity =

?,.~

"i

~r/

0

fipp~es

w

C:

paraltel lamination geopelaJ infiliing burrows

@ "~

erosive surface

vermltorm fabnc and assc~iated taminar cr LLel

Sha$1odng - upward cycle

pseudornoq:)hs afte¢ gypsum

p: pst ust rine; ML: marginal lacust dne; CL: central lacustrine; B.E : brackish environment B: black: 8r: brown' G: grey: Gr greenl 0 ochrous: W: white; D: dark (D-G: dark grey): 8tBr: alternation of B and Br.

I. Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111-132

facies, gypsiferous lake-margin facies and palustrine facies. 4.1. Lacustrine f a c i e s

This facies is represented by three subfacies which developed as a result of primary sedimentation in a lacustrine environment. They contain no or few indications of subaerial exposure, but are weakly modified in places by processes which took place not far below the surface. This facies predominates in the eastern and northern outcrops (Whitecliff Bay, St. Helens, Gurnard), (Figs. 1 and 3A,B). 4.1.1. M a r l / m a r l y limestones

These graduate from slightly green or greenish white marls to grey or dark grey, gastropod-rich non-consolidated marly limestones. They form tabular units 10-50 cm thick, are massive or diffusely laminated and have gradational boundaries. They usually succeed intraclastic limestones and pass up into biomicrites. They are well represented in the lower part of the Whitecliff Bay succession (Fig. 3A) and at the base of the Gurnard sequence (Fig. 3B). These lithologies comprise more or less fossil-bearing (gastropods and charophytes), wackestone-textured micrites, with a significant presence of clay, dispersed grains of quartz 0.05-0.15 mm across and sometimes, subrounded biomicritic intraclasts 0.06-1 mm in diameter. 4.1.2. Biomicrites

These comprise massive, compact and wellcemented limestones, cream or grey to dark grey in colour, forming tabular beds 0.5-1.0 m thick. Diffuse horizontal stratification, tens of centimeters thick, highlighted by an alternation of dark and light horizons may occur. The biomicrites usually contain vug and, sometimes, root-formed porosity. G o o d examples of this lithotype occur in

117

the Whitecliff Bay section, where it occurs above marly limestones. The biomicrites have a mudstone to wackestone texture, with fossil fragments dispersed in a homogeneous micritic matrix (Fig. 4a). Within this, there are diffuse patches of a spongy-clotted micritic texture as well as dark mottles related to the presence of organic matter. The most common fossils are gastropods (Galba, hydrobiids and some planorbids), followed by calcified stems and gyrogonites of charophytes, ostracoda (sometimes with paired valves) and foraminifers. The biomicrite has a vesicular to tubular porosity (pore diameter: 0 . 5 - 6 m m ) as well as zig-zag pores 40-150 ~tm across, displaying vesicular widening. The larger cavities are wholly or partially filled with round, wall-derived fragments (0.04-1 mm in diameter) set in a fine to medium sparitic cement. The geopetal nature of such fills is apparent where their upper parts are filled by a drusy calcite mosaic or calcitic silt cemented by fine sparite, giving a clotted texture. Mouldic fossil porosity may also be present.

4.1.3. Intraclastie limestones

These are best developed in the Whitecliff Bay profile, where five intraclastic horizons may be recognized (Figs. 3A, 4b), each underlain by an erosion surface sometimes showing karstic features. These limestones contain grey subangular clasts up to 5 cm in diameter of varied composition that are mixed and not always related to the underlying substrate: fossiliferous micrites and intramicrites, nodularised micrites, micrites with calcite pseudomorphs after gypsum, laminar and brecciated limestones. The clasts are dispersed within a grey green marly matrix. Thin sections show that intraclasts down to silt size occur in a matrix comprising silty particles of micritic nature and fine sparite in varying proportions.

Fig. 3. Bembridge Limestone profiles. A. WhitecliffBay. Lacustrine facies dominant; cycle3 includes the major brackish element of the formation, whilst cycles4-7 are incompletelydeveloped. B. Gurnard. Cycles 1-3 are incompletelydeveloped,whilst cycle4 is in part brackish in origin. C. Prospect Quarry. Pedogenicfacies dominant; superimpositionof successivepedogenicchanges renders the recognitionof cyclesmore difficult,but four polygeniccyclesare tentativelyidentified(HHF= Headon Hill Formation;BF= Bouldnor Formation).

118

~ Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111 132

Fig. 4. a. Biomicrite with abundant gastropods fragments and some ostracod remains. Note clotted-peloidal zone (arrow). PPL (plane polarized light). Whitecliff Bay. b. Intraclastic limestone on eroded surface in dark biomicrite which has been exposed (now brecciatednodular facies). The boundary (underlined) is draped by a thin laminar crust. Whitecliff Bay, 5.75 m above base. Scale is 10 cm long. c. Biopelsparite mainly made of dark pellets, horizontally orientated ostracod valves and some foraminiferal tests (F= mililiolids). PPL. Gurnard. d. Patches of biomicrite with remains of gastropods and ostracods in a matrix containing calcite pseudomorphs after lenticular gypsum. PPL. Gurnard. e. White lenticular to rhomboidal forms are silica pseudomorphs after primitive crowded interstitial gypsum. The black lenticular forms are deeply stained by goethite. The enclosing material is a biomicrite with a diffuse goethitic impregnation. PPL. Gurnard. f. Clotted-peloidal and ooidal limestone. White "pseudointraclasts" (true pedorelicts) comprising clotted micrite/biomicrite are surrounded by dense and open ooidal texture (darker material). Polished section. Prospect Quarry.

I. Armenteroset al./Palaeogeography, Palaeoclimatology, Palaeoecology128 (1997) 111-132 4.1.3.1. Interpretation of lacustrine facies. The absence of any features indicative of exposure supports a view that the marls and marly limestones were deposited at a time when the lakes were at their deepest. Marls horizons represent times of greatest (albeit fine) clastic terrigenous influx, while the marly limestones may reflect increasing organic activity and carbonate saturation of the lacustrine mass whereby allochtonous sediments are gradually replaced by biogenic and photosynthesis-induced precipitated carbonates. The succeeding biomicrites with their numerous, almost exclusively freshwater biota characteristic of lacustrine environments (cf. Paul, 1989), the almost general absence of lamination and the predominance of depositional mudstone/ wackestone textures indicate low-energy freshwater sedimentation in holomictic lakes with oxygenated bottom waters and mostly biogenic carbonate production. Much of the calcite could have been derived from encrustation around charophytes, the breakage of skeletal material (molluscs, ostracods,...) and/or precipitated biochemically (see Brown and Wilkinson, 1981). The presence of charophyte stems, which are fragile structures, suggests low-energy shallow water (<10 m: Menillet, 1980; Fl~gel, 1982; Cohen and Thouin, 1987; Platt, 1989). The mottles of clotted texture probably reflect a faecal origin by the agglomeration of lime mud (Freytet and Plaziat, 1982). The scarcity of terrigenous material may indicate that the lakes were surrounded by wide flats acting as filters to clastic transport (Freytet, 1973) and/or that there was little mechanical erosion in the hinterland. Shallow water is also indicated by the rootlet porosity (probably related to submerged plants), and diagenetic vadose features. It would thus represent a sublittoral environment in carbonate lakes of low gradient and energy. The occasional presence of foraminifers is somewhat anomalous and at first sight incompatible with an essentially freshwater biota. However, there is evidence from elsewhere in the formation that more saline waters were close by, and it may be that, being small in size, foraminifera might have been introduced by birds, wind (Paul, 1989) or from nearby marine lagoons during severe storms.

119

The clasts of the intraclastic limestones were derived from earlier lithified carbonates subaerially exposed at the lake margins during regressives phases, but which were subsequently subject to erosion during resubmergence as lake levels rose once more. On initial flooding, energy at the lake margins may have been moderate to high, but exclusively local clast provenance (including both pedogenically altered and unaltered material from the previous cycle), poor clast sorting and subangularity all indicate transport over short distances in the littoral realm.

4.2. Gypsiferous lake-margin facies This facies represents circumstances where lacustrine carbonate sediments were subsequently modified following subaerial exposure and the concentration and evaporation of saline pore waters. The development of gypsum as a result of such processes is now recorded by the presence of calcite and silica pseudomorphs. Two subfacies are considered below, although they have a number of features in common.

4.2.1. Pelsparites-biomicrites, with pseudomorphs after gypsum This subfacies is present only within the "upper" limestones at Gurnard which overlie brackish marls and muds and are succeeded by the brackish muds of the overlying Bouldnor Formation (Fig. 3B). The pelsparites have a packstone texture, comprising pellets (similar in size and shape to the pellets of Hydrobia, Flfigel, 1982) and ostracod shells (Fig. 4c), together with silt-size, quartz grains (about 3%). Foraminifers (miliolids and discorbids) and smaller amounts of gastropod and charophyte remains occur. The pelsparites pass laterally into biomicrites with gastropods (hydrobiids) and foraminifers as above, which contain anastomosed centimetric patches formed of calcite pseudomorphs after lenticular gypsum (Fig. 4d). 4.2.2. Silicified biomicrites, with pseudomorphs after gypsum This subfacies is only developed at the top of the "lower" limestone at Gurnard section (Fig. 3B)

120

I. Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111 132

where microlenticular gypsum and chert nodules have developed within lacustrine biomicrites. The "host" rock is a fossiliferous wackestone with gastropods (mainly Galba) and, to a lesser extent, foraminifers (discorbids, miliolids), ostracods, charophytes and wood fragments. It contains vertical "root-moulds", calcite pseudomorphs after microlenticular gypsum (20 100 gm in length) and is pseudobrecciated. The chert nodules are brown, roughly lenticular in shape, some 15 x 5 cm in size and occur within a ferruginised, ochrous limestone. These nodules are in fact made of quartz (30-40%), goethite (30-50%) and calcite (10-20%). The major silica minerals are megaquartz and chalcedonic silica, comprising lutecite and chalcedonite (Daley, 1989). In thin section, the nodules display a microsparitic (3-4 ~tm crystal size) mosaic, stained an ochre colour by diffuse goethite, and siliceous replacements of gastropods, ostracods and microlenticular gypsum (Fig. 4e). The nodules are pervaded by a network of millimetric "strings", sometimes densely intermingled, that encompass numerous ghosts of lenticular gypsum, aligned along the string traces, and partially pseudomorphed by silica.

gypsum are probably related to burrowing (Truc, 1980; Freytet and Plaziat, 1982). The presence of chert within this facies is quite compatible with the above, since it is known that length slow chalcedony can develop under alkaline groundwater conditions (Folk and Pittman, 1971 ). Whilst Daley (1989) proposed a post-depositional pedogenic origin for both the gypsum and the length slow chalcedony, the horizontal disposition of chert lenses at the top of "lower" limestone at Gurnard (Fig. 3B) suggests a silcrete origin from a water table (Thiry and Milnes, 1991) related to a paleoexposure surface inmediately above. The silicification may have developed by the evaporite pumping of siliceous groundwater moving slowly under high water table conditions within the carbonate sediments flats towards the lake (Summerfield, 1983; Armenteros et al., 1995). The absence of dolomite can be explained by the SO~- content of the pore fluids; sulphate contents of even less than 5% of seawater normal can prevent dolomite precipitation (Baker and Kastner, 1981). Where chert is present in this facies, it may reflect the mixing of meteoric and sea water which elsewhere has been associated with the development of chert and the replacement of calcite and gypsum (Knauth, 1979).

4.2.2.1. Interpretation of gypsiferous lake margin facies. The gypsiferous lake margin facies repre-

4.3. Palustrinefacies

sents circumstances where primary lacustrine carbonates have been subsequently modified by the growth of microlenticular gypsum following their exposure and the evaporation of brackish pore waters. The faunal association (hydrobiids and foraminifers) and the textural features suggest that deposition of pelsparite-biomicrite occurred in shallow lakes with slightly brackish waters, and the pellets have a certain similarity in size and shape to the pellets of Hydrobia (Fl~igel, 1982), something clearly compatible with the presence of abundant hydrobiids fragments. The contemporaneous exposure of the shallower areas of the brackish lakes and the evaporitive loss from the subjacent water table (Bowler and Teller, 1986; Cody and Cody, 1988) led to the intrasediment growths of lenticular gypsum seen now as calcite pseudomorphs in the gypsiferous biomicrite. The patches of crowded calcite pseudomorphs after

This facies is represented by limestones displaying a range of features recording different pedogenic responses to the subaerial exposure of the lacustrine facies. Three subfacies may be broadly recognized: brecciated-nodular limestones, clottedpeloidal-ooidal limestones and laminar limestones, with each containing a variety of textural features. Textural variation is particularly marked in the clotted-peloidal-ooidal limestones and provides a major key to understanding the pedogenic evolution. This facies is especially well developed in Prospect Quarry (Fig. 3C) and locally in the Whitecliff Bay and Gurnard profiles and characterises the palustrine environment associated with the fluctuating lake margins. A more detailed petrographic analysis and interpretation of this facies will be considered elsewhere (Armenteros and Daley, in prep.).

Z Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111-132

4.3.1. Brecciated-nodular limestones Such limestones are typically situated immediately underlying erosive surfaces overlain by intraclastic beds (Fig. 4b) although they may also be succeeded by marls or marly limestone. They generally rest gradationally upon biomicrites or intramicrites. The brecciated lithologies are in fact pseudobreccias, comprising "pseudoclasts" developed in situ and separated by a complex pattern of sedimentfilled fissures. A common feature is the presence of filled planar fissures (cf. planar voids of Brewer, 1964, and Bullock et al., 1985) mm to cm in width and more or less interconnected. In thin section, irregular networks of fissures are recognised ranging from those 50-200 tam across which wedge out laterally (joint planes of Brewer, 1964) to those comprising more complex nets of flat and curved planes (craze planes) about a centimetre wide containing apparently wall-derived microbreccias. This subfacies contains scattered nodules (pedological features with no particular internal fabric, Freytet and Plaziat, 1982). Of these, three somewhat different types may occur: allothic (sedimentary relicts and pedorelicts), disorthic (well differentiated from the surrounding matrix) and/or orthic (texturally transitional to the surrounding matrix) (after Wieder and Yaalon, 1974), rounded and circular to subcircular in section (0.3 4.0 mm in diameter), which may show circumnodular cracks. Vugs, channels and mouldic porosity are also present. 4.3.2. Clotted-peloidal-ooidal limestones These lithologies, best represented in the western outcrops of the formation (for example in Prospect Quarry, Fig. 3C), form compact, tabular layers, 30 50 cm in thickness, dark grey to light brown in colour, and are characterised by a range of secondary (postdepositional) textures. Scattered remnants of the slightly altered original sediment are present as 0.1-25mm "pseudointraclasts" which have been displaced by the physical processes leading to soil development. These mainly comprise soft white limestone and are commonly biomicritic (Fig. 4f). With regard to the grade of pedality (sensu Bullock et al., 1985), the textures may be related to two end members: clotted-

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peloidal texture (where differentiation of peds-the nodulisation of Freytet, 1973, and Freytet and Plaziat, 1982-- is incipient) and peloidal-ooidal texture (with well differentiated elements). The former consists of mainly small (<20 gm) micrite peloids (cf. Friedman and Sanders, 1978) which have developed from a homogeneous matrix to form a clotted, grumelar texture, similar to the crumb structure of Bullock et al. (1985), although more complex (Fig. 5a). Whilst the clotted-peloidal lithologies have an essentially wackestone texture, the peloidal-ooidal texture is that of a packstone-grainstone formed by peloids and ooids. The peloids range in size from 10-100 gin. The ooids (pedological ooids of Freytet and Plaziat, 1982; ooids and pisolites of Hay and Wiggins, 1980; pisoids of Calvet and Julia, 1983, etc.) are spherical to ovoid particles from 0.05 mm to 1 cm in diameter comprising a nucleus with a thin cortex development (either a coating or peripherial differentiation, sensu Freytet and Plaziat, 1979). Sorting is often poor and packing varies considerably. Fabrics with dense packing, where small particles (peloids) fill the spaces between ooids, contrast with "open fabrics" with considerably greater interparticle porosity (Fig. 5b). Fossils, mainly freshwater pulmonate (planorbids, lymnaeids) and other snails, occur commonly. They are represented by casts, but also occur within the pseudointraclasts. A variety of porosities (vug, mouldic, interparticle, intraparticle) occur associated with the clotted-peloidal-ooidal textures. In the Prospect Quarry section, there is evidence for the former existence of much larger cavities. Here complex, downward-narrowing and intraclast-filled vertical and oblique cavities 1-9 cm wide (cf. pseudomicrokarst cavities: Plaziat and Freytet, 1978; Freytet and Plaziat, 1982) clearly cut the other textures (central part of the section, Fig. 3C). Many of the infills show reverse grading. Former, nearhorizontal cavities (channels) occur more or less filled with pseudointraclasts and other particles differentiated "in situ". Both these, and to some extent the oblique and vertical cavities, display geopetal interparticle infilling with a dirty calcitic silt below and a sparry, drusy cement above. Not infrequently, the mechanically deposited silt occurs

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~ Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111 132

Fig. 5. a. Transition from dense texture (clotted micrite) to a more open texture showing a development of peloids and incipient ooids. The porosity comprises channels of variable width and concertina-like outline (root casts), (arrow). PPL. Prospect Quarry. b. Ooidal subfacies. The large ooid is constitued by a clotted-peloidal nucleus and a clotted micritic coating. The two smaller ooids in the central part (arrow) show a laminate micritic coating (C). The poor sorting and polymodal distribution of sizes are well represented. Vug and interparticle porosities are present. PPL. Prospect Quarry. c. Hand specimen showing five unconformable sets with finely wavy lamination in which the vermiform structure is present. The upper boundary is overlain by white clotted-peloidal carbonate (C) and the lower boundary is underlain by ooidal carbonate (O). Base of Prospect Quarry profile, d. Typical vermiform fabric with circular, ovoid and more irregular sections presenting a diffuse layered micritic wall. The base of some vesicles contains clotted micrite (= vadose calcite silt, arrow). PPL. Base of Prospect Quarry profile.

"perched" at a variety of levels in the larger ooidfilled cavities.

4.3.3. Laminar limestones These limestones form thin discontinuous bands (up to 10 cm thick) containing subhorizontal laminae, complicated by irregular undulation and truncation (Fig. 5c). Sharp but irregular surfaces separate them from either the brecciated-nodular, clotted-peloidal or peloidal-ooidal limestones. The laminar limestones tend to have a localised development and their occurrence is unpredictable. The

best examples occur in Prospect Quarry and other western localities (Cliff End and Tapnell Farm Quarry), but thin developments also occur in the Whitecliff Bay succession. Alternations of finely wavy millimetric to submillimetric laminae occur in sets (1-3 cm high by 2-5 cm long) which differ in colour and texture, and are discordant relative to each other. Some of the laminar limestones are disrupted by brecciation. Microscopic examination reveals two transitional structures: one spongy (called vermiform fabric in this paper) and the other dense. The

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vermiform structure, characterized by circular, ovoid and elongate cross sections (0.2 mm in diameter) (Fig. 5d), is developed in millimetric laminae, whilst the dense structure (which in many cases appears to have developed transitionally from it) comprises a thinner and more regular alternation of light and dark laminae up to 0.5 mm thick. These two fabrics have a complex relationship, but in general the vermiform fabric is clearly discordant with the better laminated dense fabric. At the tops and bases of the laminar limestones, there is a clear indication that the above textures are replacing the adjacent brecciated-nodutar and clotted-peloidal-ooidal lithologies (Armenteros and Daley, in prep.).

4.3.3.1. Interpretation of palustrine facies. The palustrine facies comprises variable, texturallydefined pedogenic responses to the exposure of the lacustrine facies, mainly biomicrites rich in typical limnic fossils and terrestrial gastropods. These textures together define a characteristic facies asocciated with palustrine environments surrounding shallow carbonate lakes with fluctuating margins (Freytet, 1973). The presence of the fissuring and its association with nodule development (brecciated-nodular texture) is well documented in marginal areas of shallow and fluctuating lakes (Freytet, 1973; Brown and Wilkinson, 1981; Freytet and Plaziat, 1982). This facies records the earliest stage in a process of modification of the lacustrine facies as a result of pedogenesis. A more advanced degree of pedogenic change corresponds to the clotted-peloidal/peloidal-ooidal textures which display a range of progressively more evolved textures, where secondary porosity and glaebules (peloids and ooids) are ultimately developed to the extent that the parent materials may become highly modified and unrecognizable. The glaebular development is directly influenced by processes associated with repeated drying and wetting in palustrine zones (Freytet, 1973). Leaching and further internal mechanical distribution of material clearly also played a part in the development of the palustrine facies of the Bembridge Limestone, giving rise to the most developed textures (peloidal-ooidal). The

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peloid/ooid/intraclast filled cavities must reflect periods of considerable solution. The reverse graded fills indicate the downwashing of granular elements (perhaps in part due to the availability of effective porosity in the underlying sediments cf. Braithwaite, 1983), whilst the succession of "perched" silts of the geopetal fills, particularly in the more horizontal channels presumably reflect more quiescent phases when fines could settle out. Similar textures have been described in many earlier accounts of calcretes (James, 1972; Read, 1974; Braithwaite, 1975, 1983: figs. 5C,D; Harrison ant' Steinen, 1978: figs. 7A-D; Ballais and Vogt, 1979; Arakel, 1982; Calvet and Julia, 1983; Vogt, 1984: pl. 1, fig. 1; Armenteros, 1989; Wright et al., 1993: Figs. 4-6; among many others) as well as in carbonate palustrine associations (Freytet, 1975; Freytet and Plaziat, 1979; Menillet, 1980; Freytet and Plaziat, 1982; Nickel, 1982; Arribas Mocoroa, 1986a,b; Platt, 1989; Alonso et al., 1992; Platt, 1992). In these environments, Freytet and Plaziat (1979, 1982) refer to the formation of similar ooidal fabrics, that are attained by the development of a lacustrine carbonate mud subjected to severe degrees of exposure: original micritic mud--*nodulization--, curved fissuring--,coating of the nodule surfaces~isolation of the nodules, a sequence of development which the present authors consider also ocurred in the Bembridge Limestone (Fig. 6). The alveolar and vermiform features of the laminar texture are typical and well documented features, both in recent and fossil laminate crusts and the presence of the root vestiges indicates their formation by plants in a vegetated soil, as proposed by Wright et al. (1988). Similar laminated-vermiform structures have been also described in palustrine environment (Menillet, 1980; Wright et al., 1988; Platt, 1989; Alonso et al., 1992). Overall, the laminar limestones represent a localised replacement of the other previously developed pedogenic lithologies, a feature documented in recent calcretes (Klappa, 1979). Where dense lamination replaces vermiform fabric, destruction of fine detail in the latter has been replaced and the lamination only remains. This is due to epigenetic neomorphism during a later diagenetic phase.

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Micrite

:.::

,r.,.~,, Clotted micrite ~

Glaebular differentiation Ooid

c3 Peloid

Sparite Curved void

Compound ooid Channel and vug porosity

5) COMPOUND OOIDAL MICROSTRUCTURE

•°

,~. ,~c

Reflects a new pedogenic phase

4) OOIDAL MICROSTRUCTURE Well developed secondary interglaebular porosity _

0 0

3) CLOTTED - PELOIDAL MICROSTRUCTURE

E

Clear glaebular differentiation

0 i-

0 a.

2) CLOTTED MICROSTRUCTURE

e"

Not well differentiated glaebules

0

e"

1) ORIGINAL BIOMICRITE ( Sometimes with sedimentary crumb structure )

I

5 mm.

I

Fig. 6. Successive stages of the progressive modification of primary lacustrine facies (fossiliferous wackestones, biomicrites) toward~ increasingly evolved pedogenic facies. The passage from stage 4 5 represents a pedogenic discontinuity. See Fig. 3 for key to symbols

Z Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111-132

5. Stratigraphical cyclicity and palaeogeography In Whitecliff Bay, where the lacustrine f a c i e s is best developed, the vertical succession of subfacies is clearly cyclic. Here, seven lacustrine cyclothems may be recognized, ranging in thickness from 0.6 to 3 m. Where completely developed, a typical cycle comprises in ascending order: (a) intraclastic facies overlying an erosional surface; (b) marl/marly limestone; (c) biomicrite; (d) brecciated nodular limestone; (e) laminated limestone horizons (Fig. 3A). Elements d and/or e may be absent where removed by erosion associated with the development of the next cycle, whilst towards the middle of the succession, there is a temporary reversion to clastic brackish sedimentation. Lower down the Palaeogene succession in the Hampshire Basin, the cyclicity of the predominantly clastic Bracklesham Group has been attributed to eustatic changes (Plint, 1983; cf. Vail et al., 1977). Whilst there is no clear evidence that a similar cause gave rise to the lacustrine cycles described here, this remains a possibility. After all, the evidence, both palaeontological and mineralogical, that periodic rises in salinity occurred, suggests that the Bembridge Limestone did not accummulate in isolated bodies of freshwater, but rather the inner predominantly freshwater parts of a sluggish lacustrine/lagoonal/estuarine complex (cf. the Everglades freshwater marshes and lakes, linked to the marginal marine waters of Florida Bay, Platt and Wright, 1992). In such circumstances, a transgression following a sea-level rise would account for the "middle" muds of the Bembridge Limestone with their clearly brackish faunal association (Daley and Edwards, 1990). Smaller rises and falls in sea level might simply influence water levels in the more lacustrine parts of such a paralic complex but could, by generating trangressions and regressions and the consequent variation in water depth, provide an explanation for the cyclicity. Sea level rises would initially have produced the intraclastic limestone with subsequent deepening giving rise to the marls and marly limestone. Later shallowing, as base level fell again, would have led first to the development of biomicrites with subsequent exposure leading to brecciation and, if sufficiently pro-

125

longed, glaebular differentiation (formation of peloids and ooids) (Fig. 7). Such a transgressive/regressive pattern is analogous to that which gave rise to somewhere similar cycles in the Palaeogene of Languedoc (see Tucker and Wright, 1990, fig. 4.77). An alternative explanation for the cyclicity is to invoke the alternation of pluvial and somewhat dryer climatic phases, highstand being associated with the former and lowstand and exposure with the latter. The cyclicity may also represent the alternation of open and closed lacustrine systems, since the marly sediments of the lower part of the cycles could have been deposited in open conditions, with the limestones of the upper part of the cycles deposited in closed lakes, as happens in some other cases (Van Houten, 1964; Olsen, 1986). More recently, Wright and Platt (1995) have suggested that, whilst no direct evidence of seasonal fluctuation is apparent, palustrine cyclothems may reflect variation in sediment aggradation and exposure-modification in a seasonal wetland context. However, whatever the cause of cyclicity, an evaluation of the various lithotypes supports a scenario of inundation and erosion, deepening, shallowing and subaerial exposure. As already explained, the marls and marly limestone represent periods when the lakes were at their deepest, with the latter perhaps reflecting increasing organic activity and the deposition of biogenic and photosynthesis-generated precipitated carbonates. The overlying biomicrites with their substantial, almost exclusively freshwater biota, indicate that the lakes had become carbonate-saturated (hence the high biogenic carbonate production) and reached their greatest degree of isolation. Scarcity of terrigenous clastics perhaps indicates that the lakes were surrounded by wide flats acting as filters (Freytet, 1973) and/or that there was little mechanical erosion in the hinterland area. The latter would very possibly have been of low topography, perhaps comprising earlier exposed Palaeogene strata, whilst any exposures of the Cretaceous Chalk would have provided negligible clastic material that might be transported by sluggish streams likely in the large, low-lying Palaeogene basin of southern England. The brecciated and nodular limestones overlying

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--Palustrine

Lacustrine A s s o c i a t i o n - -

,,

Association

Seasonal highest highstand

INCREASING EXPOSURE

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--

-

.

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Highstand r

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- - -

Lowstand .Seasonal lowest Iowstand

Ooidal-peloidal texture Laminar texture

Clotted-peloidal texture Breceiated nodular texture

Biomicrite Intraclastic limestone

Marl / marly limestone

Fig. 7. Hypothetical section to show the spatial relationship of lacustrine and pedogenic facies associations. See Fig. 3 for key to symbols.

the biomicrites reflect maximum lowstand and exposure in a marginal, palustrine setting. The gypsiferous lake-margin facies, found only in the the Gurnard area, comprises microlenticular gypsum-bearing limestones containing brackishwater faunal elements (foraminifers and hydrobiid gastropods of probable stenohaline tendency) as well as the remains of freshwater pulmonates Galba and Chara gyrogonites. Such an anomalous association is most readly explained by the former lateral juxtaposition of environments with differing salinities. Both the "middle" muds of the Bembridge Limestone (Fig. 3B) and the overlying Bembridge Marls Member (Bouldnor Formation), with their brackish faunas, indicate the proximity of the sea and it seems likely that a context for this facies is one where deposition occurred in the upper reaches of a sluggish lagoonal/estuarine system. Fluctuating salinity or post-mortem transportation would account for the mixture of the biotic elements. During periods of low water and consequent sediment exposure, surface evaporation could have led to the lateral migration of saline interstitial waters from which the gypsum was precipitated. In the palustrine Jacies, mainly represented by clotted-peloidal to peloidal-ooidal textures, it is difficult to distinguish facies sequences in the field, but micromorphological analysis leads to the recognition of the following evolutionary series

starting with lacustrine biomicrite and continuing through brecciated-nodular (sc-netimes absent)--* clotted-peloidal--,peloidal-ooidal textures (Fig. 6). The laminar fimestone is found associated with the two last members of this series. It tends to have a localized development and its occurrence is unpredictable. In general, a progressive pedogenic development from lacustrine facies is seen to give rise to successively more evolved pedogenic facies, better differentiated glaebules and increasing interaggregate and interglaebular porosity.

5.1. Palaeogeographic domains Two rather different facies distributions predominate (Fig. 8). In the east, lacustrine sedimentation is represented by shallowing cycles, where the lower parts (marls, biomicrites) correspond to central lacustrine sedimentation and the upper (brecciated-nodular limestones and laminated limestone horizons) indicate marginal lacustrine areas subject to exposure. Such sequences represent shallow low-energy lakes with low-gradient margins. Only the tops of the cycles provide evidence of incipient development of palustrine signatures. In the west, primary sediments are poorly developed, with remants of dark biomicrites representing sedimentation in very shallow and ephemeral lakes encircled by marshy areas and intermittently water-covered. The major character of the facies

I. Arrnenteros et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111-132

127

Brackish

Environment

1

-

-2

ENE

;:;~,;.,,'-.;_,, '~:, ,~ *, ~'e, ¢ ,k

#/

Y:_

_ ~ -

uds

rle t

2Kin

I

1 Fig. 8. A possible palaeogeographical reconstruction of the Bembridge Limestone environment. 1 = lacustrine facies; 2 = gypsiferous lake margin facies; 3 = palustrine facies. See Fig. 3 for key to symbols.

here arises from the repeated exposure and subsequent pedogenic modification of the carbonate substrate. Superimposed pedogenic features hinder the recognition of primary facies since deposition alternates with pedogenic intervals leading to modification of these sediments. Consequently, even if not completely blurred, diastems are not easily recognized at the top of the elemental palaeosols associated with polygenetic sequences of soils (Freytet and Plaziat, 1982, fig. 35BII; Meyer, 1987). The intensity and longevity of subaerial exposure controls the development of the sedimentary-pedogenic "cyclothems" (sensu Freytet and Plaziat, 1982, fig. 42; Freytet, 1984, fig. 6). There is a higher rate of sedimentation and more prolonged submergence in the east than in the west (Daley and Edwards, 1990), whilst the opposite is true as far as the extent of contemporaneous exposure is concerned. 5.2. Climate

At one time, it had been widely accepted that, on the basis of palaeobotanical evidence, the south-

ern British area had been consistently humid tropical to subtropical during the whole of the Palaeogene (Reid and Chandler, 1926, 1933; Pallot, 1961). Daley (1971) had, however, suggested that since the Bembridge Limestone and adjacent strata correlated with the Eocene Montmartre Gypsum of the Paris Basin, it might be expected that periodically dry conditions would have occurred in the British area. The existence of gypsum pseudomorphs in the Bembridge Limestone at Gurnard supports this view, since they are thought to have been produced interstitially through the evaporation of groundwater during periods of subaerial exposure (cf. Bowler and Teller, 1986). Although some features of the palustrine facies of the Bembridge Limestone can form in calcretes developed in a cool temperate climate (Strong et al., 1992), the overall palaeobotanical context of the Solent Group indicates a warm climate, whilst features such as desiccation cracks and gypsum pseudomorphs suggest dry phases. Hence, a comparison with calcretes formed in semiarid warm to hot temperatures seems

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appropiate (e.g., James, 1972; Harrison and Steinen, 1978; Arakel, 1982; Calvet and Julia, 1983). However, since Hooker et al. (1995) have established that, for at least some of Bembridge Limestone times, the hinterland was one of open woodland or forest, dry phases must have alternated with periods sufficiently wet to allow for the latter to develop. Hooker et al. (1995) themselves recognised that some mammalian faunas indicated drier, less equable circumstances but presumably these were much less so than those associated with the formation of the calcretes. It follows then that whilst annual seasonal variation may have occurred, the association of the calcretes with evidence for a wooded hinterland (including the land snails as well as the mammals) necessitates the postulation of alternating wetter and drier phases of considerably greater duration, probably hundreds to thousands of years. Moreover, the association provides an illustration of how the products of one set of conditions can be superimposed on those developed many years previously and in quite different circumstances. This more recent work not only supports Daley's suggestion (Daley, 1971) of alternating dry and wet periods but raises the possibility of a different explanation for the cycles considered earlier. With dry conditions facilitating calcrete formation and wetter times leading to maximum lacustrine development, the upper and lower parts of the cycles might represent drier and wetter climatic phases respectively.

6. Summary and conclusions Evidence derived from field relationships and petrographic study shows that the carbonate rocks of the mainly but not exclusively freshwater Bembridge Limestone may be assigned to three facies. One is predominantly "primary" in origin and reflects sedimentation in shallow carbonate lakes (lacustrine facies). The other two reflect modification of the primary freshwater carbonates, firstly, in association with syndiagenetic changes related to the migration and evaporation of interstitial sediment water (gypsiferous lake-margin facies) and secondly, where alteration took place

in palustrine environments in association with soil development (palustrineJ'acies) (Fig. 7). The palustrine facies reached its maximum extent in the west of the Isle of Wight, whilst the lacustrine and gypsiferous lake-margin facies are best developed in the east and north respectively (Fig. 8). The lacustrine facies has been shown to be characteristically cyclic in origin, where inundation (associated with erosion and intraformational conglomerates) was followed initially by deepening, and then by shallowing and finally by subaerial exposure. A number of cycles have been recognized, one of which includes clastic sediments of indisputably brackish water origin. The cause of the cyclicity remains unclear. One possibility is that the cycles were influenced by rises and falls in sea level (cf. Vail et al., 1977), with that containing the muds of the "middle" Bembridge Limestone, with their brackish fauna, representing the most significant rise. A second is that the cycles reflect an alternation of drier and wetter climatic phases. It is difficult to determine the duration of the individual cycles witin the Bembridge Limestone. However, some attempt may be possible by comparison within estimates for similar cycles from the Lockatong Formation (Van Houten cycles of Olsen, 1986). Compared with those from the Bembridge Limestone (averaging 2-3 m thick) these cycles are somewhat thicker (3 7 m) for which Olsen suggested a duration of some 20,000 years (ie., the length of the Milankovitch precession cycle). In Whitecliff Bay, only the tops of the lacustrine cyclothems show evidence of the early stages of pedogenic modification. In contrast, the western localities are dominated by palustrine facies. Here, the primary facies, with its well developed terrestrial gastropod fauna, probably accumulated in very shallow ephemeral/seasonal lakes. However, with the almost complete obliteration of this facies by repeated exposure and pedogenic modification, only thin remnants remain. Within this palustrine, complex pedogenic fabrics developed, reflecting a textural evolution from clotted-peloidal through dense to open peloidal-ooidal fabrics. Vertical sequences reveal the superimposition of a number of pedogenic phases. A feature of the evolution is the progressive loss of carbonate material both as

I. Armenteros et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111 132

a result o f dissolution and the interstratal downwashing o f solids particles leading to intraclastfilled cavities with reverse graded and geopetal fills. Alternating drying and wetting appear to provide an explanation for the range o f textures present. Together with the small n u m b e r o f c a r b o n a t e units present in the underlying H e a d o n Hill Formation, the Bembridge Limestone contrasts m a r k e d l y with the clastic sediments which dominate the Solent G r o u p and indeed the whole o f the local Palaeogene succession, whilst unlike these other carbonate units, it is unique in its lateral continuity. It m u s t therefore reflect a time when, clastic input was at a minimum, p r o b a b l y due to a c o m b i n a t i o n o f tectonic stability and extensive low-lying hinterland facilitating minimal clastic transportation. A l t h o u g h at one time t h o u g h t to be o f fresh water origin t h r o u g h o u t , apart f r o m the "middle m u d s " , it is n o w clear that n o t only was there saline water nearby, but that periodically s o m e w h a t brackish conditions persisted in some areas during deposition o f the Bembridge Limestone. Hence the theory that the Bembridge Limestone was laid d o w n in isolated lakes can no longer be maintained. M u c h m o r e likely is that it accumulated in the p r e d o m i n a n t l y (but n o t exclusively) freshwater u p p e r reaches o f a sluggish lacustrine/lagoonal/estuarine system, in a coastal plain area o f low relief. A m o d e r n analogue m a y possibly be the Florida Everglades, where lowlying vegetated freshwater marshland, s w a m p and freshwater or brackish lagoons are situated at the l a n d w a r d margins o f a large, very low gradient drainage system (Platt and Wright, 1992).

Acknowledgements Analytical w o r k was funded by Project C I C Y T PB-920069. I. A r m e n t e r o s and E. G a r c i a t h a n k the E r a s m u s Project and the University o f Salamanca for partially financing the field w o r k in the Isle o f W i g h t (September, 1989 and O c t o b e r / N o v e m b e r , 1991), as well the D e p a r t m e n t o f Geology, University o f P o r t s m o u t h for its excellent hospitality and access to the D e p a r t m e n t a l specimen collection. Brian Daley wishes to t h a n k

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the University o f P o r t s m o u t h for its continuing support for research on the geology o f the Isle o f Wight. We are gratefully to Drs. P. B u u r m a n , N. Platt and V.P. Wright for valuable criticism and suggestions which have facilitated the improvement o f earlier drafts o f this paper, whilst useful c o m m e n t s by Drs. P. Freytet, F. Surlyk and an a n o n y m o u s reviewer have greatly helped to improve the final version.

References Alonso, M.A., Calvo, J.P. and Garcia del Cura, M.A., 1992. Palustrine sedimentation and associated features-granification and pseudo-microkarst in the Middle Miocene (Intermediate Unit) of the Madrid Basin, Spain. Sediment. Geol., 76:43 61. Arakel, A.V., 1982. Genesis of calcrete in Quaternary soil profiles, t-Iutt and Leeman Lagoons, Western Australia. J. Sediment. Petrol., 52: 109-125. Armenteros, I., 1986. Estratigrafia y Sedimentologia del Ne6geno del Sector Suroriental de la Depresi6n del Duero (Aranda de Duero-Pefiafiel). Dip. Salamanca, 471 pp. Armenteros, I., 1989. Alteracidn del sustrato y encostramientos carbonfiticos ligados a la discontinuidad Cretficico-Terciaria en el borde este del Sistema Ibdrico Central (Espafia). Stud. Geol. Salmanticensia Vol. Esp., 5: 13-54. Armenteros, I., Bustillo, M.A. and Blanco, J.A., 1995. Pedogenic and groundwater processes in a closed Miocene basin (northern Spain). Sediment. Geol., 99: 17-36. Armenteros, I., Recio, C. and Daley, B., 1992. Sedimentology and stable isotope study of the lacustrine Bembridge Limestone (upper Eocene), Isle of Wight, southern England. In: 3rd Congr. Geol. Esp., 8th Simp. Latinoam. Geol., 1, pp. 27-38. Arribas Mocoroa, M.E., 1986a. Petrologia y anfilisis secuencial de los carbonatos lacustres del Pale6geno del sector N de la Cuenca Terciaria del Tajo (provincia de Guadalajara). Sedimentaci6n Continental en Espafia. Cuad. Geol. Ib6r., 10: 295-334. Arribas Mocoroa, M.E, 1986b. Pedogenetic facies in a lacustrine sedimentation model: paleogene of the Tertiary Tajo Basin. In: R. Rodriguez-Clemente and Y. Tardy (Editors), Geochemistry and Mineral Formation in the Earth Surface. CSIC-CNRS, Madrid, pp. 339-350. Aubry, M.-P., 1986. Palaeogene calcareous nannoplankton biostratigraphy of north western Europe. Palaeogeogr. Palaeoclimatol. Palaeoecol., 55: 267-334. Baker, P.A. and Kastner, M., 1981. Constraints on the formation of sedimentary dolomites. Science, 213:214 216. Ballais, J.L. and Vogt, T., 1979. Crofites calcaires quaternaires du pi6mont nord des Aur6s (Alg~rie). Rech. G~ogr. Strasbourg, 12:23 34.

130

I. Armenteros et al./Palueogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111 132

Bersier, A., 1958. Sdquences d6tritiques et divagations fluviales. In: 5th Congr. Int. Sediment. Eclogae Geol. Helv., 51(3): 854-893. Bowler, J.M. and Teller, J.T., 1986. Aust. J. Earth Sci., 33: 43-63. Braithwaite, C.J.R., 1975. Petrology of palaeosols and other terrestrial sediments on Aldabra, Western Indian Ocean. Philos. Trans. R. Soc. London, 273: 1-32. Braithwaite, C.J.R., 1983. Calcrete and other soils in Quaternary limestones: structures, processes and applications. J. Geol. Soc. London, 140: 351-363. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. Wiley, New York, 470 pp. Brown, R.E. and Wilkinson, B,H., 1981. The Draney Limestone: Early Cretaceous lacustrine carbonate deposition in western Wyoming and southeastern Idaho. Contrib. Geol. Univ. Wyo., 20(1): 23-31. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G. and Tursina, T., 1985. Handbook for soil thin section description. Waine Res. Publ., Albrighton, 152 pp. Cabrera, L., Colombo, F. and Robles, S., 1985. Sedimentation and tectonics interrelationships in the Paleogene marginal alluvial systems of the Ebro Basin. Transition from alluvial to shallow lacustrine environments. In: 6th Eur. Reg. Meet. Excursion Guidebook, pp. 395 494. Calvet, F. and Julia, R., 1983. Pisoids in caliche profiles of Tarragona (N E Spain). In: T. Peryt (Editor), Coated Grains. Springer, Berlin, pp. 456-473. Cody, R.D. and Cody, A.M., 1988. Gypsum nucleation and crystal morphology in analog saline terrestrial environments. J. Sediment. Petrol., 58(2): 247 255. Cohen, A.S. and Thouin, C., 1987. Nearshore carbonate deposits in Lake Tanganyika. Geology, 15:414 418. Corrochano, A. and Armenteros, I., 1989. Los sistemas lacustres de la cuenca Terciaria del Duero. Acta Geol. Hisp., 24(3/4): 259 279. Costa, L. and Downie, C., 1976. The distribution of the dinoflagellate Wetzeliella in the Palaeogene of North-Western Europe. Palaeontology, 19:591 614. Daley, B., 1971. Some problems concerning the Early Tertiary climate of southern Britain. Palaeogeogr. Palaeoclimatol. Palaeoecol., 11: 177-190. Daley, B., 1989. Silica pseudomorphs from the Bembridge Limestone (Upper Eocene) of the Isle of Wight, Southern England and their palaeoclimatic significance. Palaeogeogr. Palaeoclimatol. Palaeoecol., 69:233 240. Daley, B. and Edwards, N., 1990. The Bembridge Limestone Formation (Late Eocene), Isle of Wight, Southern England: A Stratigraphical Revision. Tertiary Res., 12(2): 51 64. Flagel, E., 1982. Microfacies Analysis of Limestones. Springer, Berlin, 633 pp. Folk, R.L. and Pittman, J.S., 1971. Length-slow chalcedony: a new testament for vanished evaporites. J. Sediment. Petrol., 41 (4): 1045-1058. Forbes, E., 1853. On the fluvio-marine tertiaries of the Isle of Wight. Q. J. Geol. Soc. Lond., 9:259 270. Freytet, P., 1965. S~dimentation microcycloth6mique avec

crofites zonaires fi Algues dans le Calcaire de Beauce de Chauffour-Etrechy (S. et O.). B.S.G.F., (7) 7: 309-313. Freytet, P., 1973. Petrography and paleo-environment of continental carbonate deposits with particular reference to the Upper Cretaceous and Lower Eocene of Languedoc (Southern France). Sediment. Geol., 10:25 60. Freytet, P., 1975. Quelques observations p6trographiques sur les calcaires continentaux rencontrds 'fi l'excursion de mai 1974 de I'A. G. B. P. : faci6s lacustres, modifications p6dologiques, Microcodium. Bull. Inf. Geol. Bassin Paris, 12(2): 15 23. Freytet, P., 1984. Les s6diments lacustres carbonat6es et leurs transformations par 6mersions and pddogenbse. Importance de leur identification pour les reconstructions pal6ogeographiques. Bull. Cent. Rech. Explor.-Prod. Elf-Aquitaine, 8, 1:223 -247. Freytet, P. and Plaziat, J.C., 1979. Les ooides continentaux: diversit6 des formes, des modes de formation, Rech. Geogr. Strasbourg, 12: 69-80. Freytet, P. and Plaziat, J.C., 1982. Continental carbonate sedimentation and pedogenesis. Late Cretaceous and Early Tertiary of Southern France. Contrib. Sedimentol., 12, 213 pp. Friedman, G.M. and Sanders, J.E., 1978. Principles of Sedimentology. Wiley, New York, 792 pp. Garcia G6mez, E., 1990. Sedimentologia de la Formacion Calizas de Bembridge. Eoceno superior continental de la Isla de Wight (Reino Unido). Grado de Salamanca, 82pp. (unpubl.) Harrisom R.S. and Steinen, R.P., 1978. Subaerial crusts, caliche profiles, and breccia horizons: comparison of some Holocene and Mississippian exposure surfaces, Barbados and Kentucky. Geol. Soc. Am. Bull., 89: 385-396. Hooker, J.J., Collinson, M.E., Van Bergen, P.F., Singer, R.L., De Leeuw, J.W. and Jones, T.P., 1995. Reconstruction of land and freshwater palaeoenvironments near the EoceneOligocene boundary, southern England. J. Geol. Soc. Lond., 152: 449-468. Hay, R.L. and Wiggins, B., 1980. Pellets, ooids, sepiolite and silica in three calcretes of southwestern United States. Sedimentology, 27: 559-576. Insole, A. and Daley, B., 1985. A revision of the lithostratigraphical nomenclature of the Late Eocene and Early Oligocene strata of the Hampshire Basin, Southern England. Tertiary Res., 7:67 100. James, P.N., 1972. Holocene and Pleistocene calcareous crust (caliche) profiles: criteria for subaerial exposure. J. Sediment. Petrol., 42(4): 817 836. Klappa, C.F., 1979. Calcified filaments in Quaternary calcretes: organo-mineral interactions in the subaerial vadose environment. J. Sediment. Petrol., 49(3): 955 968. Knauth, L.P., 1979. A model for the origin of chert in limestone. Geology, 7: 274-277. Lablanche, G., 1982. Les calcaires lacustres pal6og6nes de la Champagne berrichonne. Doc. BRGM, 49, 127 pp. Marshall, J., Paul, C.R.C. and Wright, V.P., 1987. Biotas, formation and diagenesis of Tertiary palustrine palaeosols.

I. Armenteros et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 111-132 Abstr. 8th Meet. Carbonate Sedimentol., Univ. Liverpool, July 1987, pp. 14-18. Marshall, J., Paul, C.R.C. and Wright, V.P., 1988. Diagenesis in Tertiary palustrine carbonate palaeosols. Abstr. SEPM Midyear Meet., Columbus, Ohio, August 1988. Martini, E., 1972. Die Gattung Eosphaeroma (Isopoda) im europaischen Alttertiar. Senkenberg. Lethaea, 53: 65-79. Menillet, F., 1980/81. Le lithofaci6s des Calcaires de Beauce (Stampien sup6rieur et Aquitanien) du bassin de Paris (France). Bull. B.R.G.M., 2, 4, 1: 15-55. Meyer, R., 1987. Pal6oalterites et pal6osols. Bureau de recherches g6ologiques et mini6res (Manuels M6thodes, 13). BRGM, Orleans, 163 pp. Molenaar, N. and De Feyter, A.J., 1985. Carbonates associated with alluvial fans: an example from the Messinian Colombacci Formation of the Pietrarubbia Basin, northern Marche, Italy. Sediment. Geol., 42:1 23. Murray, J.W. and Wright, C.A., 1974. Palaeogene foraminifera and palaeoecology, Hampshire and Paris Basins and the English Channel. Spec. Pap. Palaeontol., 14:1 29. Nickel, E., 1982. Alluvial-fan-carbonate facies with evaporites, Eocene Guarga Formation, Southern Pyrenees, Spain. Sedimentology, 29: 761-796. Normati, M. and Salomon, J., 1989. Reconstruction of a Berriasian lacustrine paleoenvironment in the Cameros Basin (Spain). Palaeogeogr. Palaeoclimatol. Palaeoecol., 70: 215 223. Olsen, P.E., 1986. A 40-million-year lake record of Early Mesozoic orbital climatic forcing. Science, 234: 842-848. Pain, T. and Preece, R.C., 1968. The land Mollusca of the Bembridge Limestone. Proc. Isle Wight Nat. Hist. Archaeol. Soc., 6: 101-111. Paul, C.R.C., 1989. The molluscan faunal succession in the Hatherwood Limestone Member (Upper Eocene), Isle of Wight, England. Tertiary Res., 10:147 162. Pallot, J.M., 1961. Plant microfossils from the Oligocene of the Isle of Wight. Thesis. Univ. London (unpublished). Platt, N.H., 1989. Lacustrine carbonates and pedogenesis: sedimentology and origin of palustrine deposits from the Early Cretaceous Rupelo Formation, W. Cameros Basin, N. Spain. Sedimentology, 36: 665-684. Platt, N.H., 1992. Sedimentology and stable isotopes of fresh water carbonates from the Lower Freshwater Molasse (Oligocene), western Switzerland. Sediment. Geol., 78: 81-99. Platt, N.H. and Wright, V.P., 1991. Lacustrine carbonates: facies models, facies distributions and hydrocarbon aspects. In: P. Anaddn et al. (Editors), Lacustrine Facies Analysis. IAS Spec. Publ., 13: 57-74. Platt, N.H. and Wright, V.P., 1992. Palustrine carbonates and the Florida Everglades: toward and exposure index for the fresh-water environment. J. Sediment. Petrol., 62(6): 1058 1071. Plaziat, J.-C. and Freytet, P., 1978. Le pseudomicrokarst p~dologique: un aspect particulier des pal~o-p6dog~neses developp6es sur les d6pots calcaires lacustres dans le tertiare de Languedoc. C. R. Acad. Sci, 286, Ser. D, pp. 1661 1664. Plint, A.G., 1983. Facies, environments and sedimentary cycles

131

in the Middle Eocene, Bracklesham Formation of the Hampshire Basin: evidence for global sea level changes?. Sedimentology, 30: 625-653. Read, J.F., 1974. Calcrete deposits and Quaternary sediments, Edel province, Shark Bay, Western Australia. Mem. AAPG, 22: 250-287. Reid, E.M. and Chandler, M.E.J., 1926. The Bembridge FloraCatalogue of Cainozoic Plants in the Department of Geology, 1. Br. Mus. Nat. Hist., Lond., 206 pp. Reid, E.M. and Chandler, M.E.J., 1933. The Flora of the London Clay. Br. Mus. Nat. Hist., Lond., 561 pp. Stanley, K.O. and Collinson, J.W., 1979. Depositional history of Paleocene-Lower Eocene Flagstaff Limestone and coeval rocks, central Utah. AAPG Bull., 63:311-323. Strong, G.E., Gile, J.R.A. and Wright, V.P., 1992. Holocene calcrete from North Yorkshire, England: implications for interpretating paleoclimates using calcretes. Sedimentology, 39:333 347. Summerfield, M.A., 1983. Silcrete. In: A.S. Goudie and K. Pye (Editors), Geochemical Sediments and Geomorphology. Academic Press, London, pp. 59 91. Szulc, J., Roger, P.H., Mouline, M.P. and Lenguin, M., 1991. Evolution of lacustrine systems in the Tertiary Narbonne Basin, northern Pyrenean foreland, southeast France. In: P. Anaddn et al. (Editors), Lacustrine Facies Analysis. IAS Spec. Publ., 13:279 290. Thiry, M. and Milnes, A.R., 1991. Pedogenic and groundwater silcretes at Stuart Creek opal field, South Australia. J. Sediment. Petrol., 61:111-127. Truc, G., 1980. Evaporites in a subsident continental basin (Ludian and Stampian of Mormoiron-Pernes in southeastern France) sequential aspects of deposition primary facies and their diagenetic evolution. In: Evaporite Deposits. Technip, Paris, pp. 61-71. Tucker, M.E. and Wright, V.P., 1990. Carbonate Sedimentology. Blackwell, Oxford, 424 pp. Vail, P.R., Mitchum, R.M. and Thompson, Ill, S., 1977. Seismic stratigraphy and global changes of sea level, part 4: Global cycles of relative changes of sea level. In: C.E. Payton (Editor), Seismic Stratigraphy--Application to Hydrocarbon Exploration. Mem. AAPG, 26: 83-97. Valero Garc6s, B.L., Gierlowski-Kordesch, E. and Bragonier, W.A., 1994. Lacustrine facies model for nonmarine limestone within cyclothems in the Pennsylvanian (Upper Freeport Formation, Appalachian Basin) and its implications. In: A.J. Lomando et al. (Editors), Lacustrine Reservoirs and Depositional Systems. SEPM Core Workshop 19, pp. 321-381. Van Houten, F.B., 1964. Cyclic lacustrine sedimentation, Upper Triassic Lockatong Formation, central New Jersey and adjacent Pennsylvania. Geol. Surv. Kans. Bull., 169: 497-531. Vogt, T., 1984. Probl6mes de gen6se des crofltes calcaires quaternaires. Bull. Cent. Rech. Explor. Prod. Elf-Aquitane, 8(1): 209 221. Wells, N.A., 1983. Carbonate deposition, physical limnology and environmentally controlled chert formation in Paleo-

132

I. Armenteros et al./Palueogeography, Palaeoclimatology. Palaeoecology 128 (1997) 111-132

cene-Eocene lake Flagstaff, Central Utah. Sediment. Geol., 35: 263-297. White, H.J.O., 1921. A short account of the geology of the Isle of Wight. Mere. Geol. Surv. G.B. Wieder, M. and Yaalon, D.H., 1974. Effect of matrix composition on carbonate nodule crystallization. Geoderma, 11: 95 121. Wright, V.P. and Platt, N.H., 1995. Seasonal wetland carbonate sequences and dynamic catenas: a re-appraisal of palustrine limestones. Sediment. Geol., 99: 65-71.

Wright, V.P., Platt, N.H. and Wimbledon, W.A., 1988. Biogenic laminar calcretes: evidence of calcified root-mat horizons in paleosols. Sedimentology, 35:603 620. Wright, V.P., Turner, M.S., Andrews, J.E. and Spiro, B., 1993. Morphology and significance of super-mature calcretes from the Upper Old Red Sandstone of Scotland. J. Geol. Soc. London, 150: 871-883.