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The mineralogy of palaeogene sediments in Northeast Kent (Great Britain)
Sedimentary Geology-Elsevier Publishing Company, Amsterdam-Printed in The Netherlands
THE MINERALOGY OF PALAEOGENE SEDIMENTS IN NORTHEAST KENT (GREAT BRITAIN) A. H. WEIR ANDJ. A. CATT Pedology Department, Rothamsted Experimental Station, Harpenden, Herts. (Great Britain) (Received May 3, 1968) (Resubmitted August 28, 1968) SUMMARY The Palaeogene deposits of northeast Kent are approximately 700 ft. thick and consist of unconsolidated clays and loamy sands, which are mainly marine in origin. The mineralogical composition of fine sand (50-250 p), silt (2-5 p, 5-20 p and 20-50 #) and clay ( < 2 p) fractions from samples representing the main subdivisions of the succession is described, and possible sources of the detritus and mode of origin of some non-detrital minerals are discussed. The detritus was derived ultimately from three main sources: (a) the Chalk; (b) metamorphic rocks (containing staurolite and kyanite) and granites, possibly of Armorican massifs; (c) metamorphic rocks containing garnet, epidote and amphiboles. Clay fractions in most of the deposits are composed of montmorillonite with subsidiary mica, but kaolinite occurs in beds containing abundant detritus from possible Armorican sources. The main non-detrital minerals are glauconite, jarosite, pyrites, low-temperature tridymite and clinoptilolite, and detrital clay micas in the sandy sediments have authigenic growths of a layer silicate mineral. The main clay-rich subdivision of the succession (the London Clay) is weathered to great depths; mineralogical effects of this weathering include oxidation of pyrites, alteration of detrital biotite and formation of selenite and jarosite. INTRODUCTION The Palaeogene deposits of the London Tertiary Basin cover large areas both north and south of the estuary and valley of the River Thames as far west as Hungerford, but this paper concerns only those exposed on or near the north Kent coast from the River Medway eastwards to Thanet. These are divisible into six main lithological units, which are (in ascending order) the Thanet Beds, Woolwich Beds, Oldhaven Beds, London Clay, Claygate Beds and Lower Bagshot Beds. The Thanet Beds are separated from the underlying Senonian Chalk by the Bullhead Bed, a thin deposit of glauconitic sandy clay containing abundant flint nodules. This comparatively local succession within the Anglo-Paris-Belgian Basin, which Sediment. Geol., 3 (1969) 17-33
18
A. H , W E I R A N D ,1. A. C ~ I ' !
is approximately 700 ft. thick, comprises the Thanetian, Sparnacian and Ypresian Stages (CURRY, 1966). The mineralogical survey reported in this paper is part of a study of the origin, distribution and weathering of loess and associated Quaternary deposils in Kent, many of which contain material derived from local Tertiary sediments in addition to far-travelled wind-blown silt. The composition of sand fractions fi'om Palaeogene beds in the London and Hampshire parts of the Anglo-Paris-Belgian Basin was described by DAVIES(1915, 1916), BOSWELL(1915, 1923), BAKER(1920), GROVES (1928), WALD~R(1964) and others, but these accounts are scarcely adequate for provenance and other studies of the Quaternary deposits. Knowledge of the composition of silt fractions (2-50/~ equivalent spherical diameter) is especially important in work involving loess, and investigation of clay fractions (<2/~) is necessaryin weathering studies. Mineralogical analysis of the Palaeogene sediments was therefore extended to include all size fractions. This full analysis also provides more information of the conditions in which the Palaeogene sediments themselves were deposited and of the ultimate source of the detritus than could be obtained from the study of sand fractions alone. L O C A T I O N OF SAMPLES
Fig. 1 shows the approximate location of samples taken from six exposures of Palaeogene beds in northeast Kent. The Bullhead Bed (sample BB2) and basal Thanet Beds (TB 7) at Upnor were sampled at Tower Hill, a locality described by PITCHER et al. (1958, pp.12-13); the highest Thanet Beds (TB 8) and Woolwich Beds samples (WS 2, WS 3) at Upnor were taken from Beacon Hill Quarry and represent beds 17, 11 and 7 of the succession described by STINTON (1965). The highest Palaeogene deposits (upper part of the London Clay, Claygate Beds and Lower Bagshot Beds) occur in eastern Kent only on the north coast of the Isle of Sheppey. The least incomplete succession in these beds is that seen at East End (Grid Reference: TR 970733), where the highest 20-30 ft. of the London Clay (represented by LC 5) are much sandier than lower horizons exposed at Sheiford and Herne Bay, and pass upwards by alternation into the fine sands and loams of the Claygate Beds. Fossils are rare and the base of the Claygate Beds is probably best drawn below a prominent bed, 4-5 ft. thick, of brown and buff fine sand (sample CB 1) containing a few clay partings. This is overlain by 20-24 ft. of fine sands and loams with numerous grey and yellow sandy clay partings; CB 2 combines several sand and clay partings from near the middle of this bed. The base of the Bagshot Beds probably occurs immediately below 5 ft. of yellow and orange sands, which are divided into two main courses by a thin yellow clay parting; BS 1 is from the upper sand course. The overlying parts of the Bagshot Beds are very disturbed. A sample (LC 4) was taken approximately 180 ft. below the top of the London Clay near Warden Point, Sheppey (TR 015728). Sediment, Geol., 3 ( 1 9 6 9 ) 17--3 ~
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
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-I Fig. 1. Geographic and stratigraphic location of analysed samples from the Palaeogene deposits of northeast Kent.
The succession at Mockbeggar (BB 1, TB 6) was briefly described by DINES et al. (1954, p.80), and Shelford Quarry, where the basal London Clay (LC 1, LC 2) and Oldhaven Beds (OB 2) were sampled, was described by SMART et al. (1966, pp.193, 197, 198). The cliffs east of Herne Bay provide the best exposure of the Palaeogene deposits in Kent. The lowest visible part of the Thanet Beds is here a blue-grey, shelly, clayey sand (TB 4), which is exposed at low tide on the foreshore east of Bishopstone Point. This is overlain by approximately 40 ft. of Sediment. Geol., 3 (1969) 17-33
20
A.H.
W E I R A N D J. A. ( A I ' t
grey, yellow and blown sands and loams (TB 5) containing a thin impersisten', sandstone, which lies 30 ft. below the top of the Thanet Beds and forms a prominent ledge near the cliff base from Bishopstone Point eastwards. The overlying Wooiwich Beds (represented by WS 1) are lithologically similar to the upper parts of the Thanet Beds, but are separated from them by a pebbly clay containing a rich fish fauna (GuRR, 1962). The base of the Oldhaven Beds is marked by a more pro ~ minent pebble bed; sample OB 1 was taken approximately 12 ft. above this. WHITE (1928, pp.49-51) summarised earlier work on the Thanet Beds exposed at Pegwell Bay near Ramsgate. Here, as at Mockbeggar, the Bullhead Bed consists of 3-6 inches of glauconitic sandy clay containing many unworn flint nodules overlying 0.5-I .5 inches of partly-shattered tabular flint, which in turn rests directly on the Chalk. The lowest horizon above the Bullhead Bed is represented by sample TB 1 and consists of 8-12 inches of glauconitic sandy loam with pink streaks near the top and a few flint nodules. This is succeeded by 1.5 ft. of mottled pinkish-brown sandy loam with glauconitic streaks near the base; the intermingling of these two beds is attributed to animal burrowing. Sample TB 2 was taken 3 ft. above the base of the overlying bed, which comprises 11-12 ft. of well-bedded, buffsilts and sandy clays, and TB 3 is fiom the upper, bluish-grey part of the 55 ft. bed mentioned by WHITE (1928, p.50). WmTAKER (1872, p.56) recognised five main divisions (beds a-e) of the Thanet Beds in Kent. Confusion has been caused by some authors failing to distinguish between the Bullhead Bed and the lowest of Whitaker's divisions (bed a or the "base-bed"). The base-bed contains the Bullhead Bed plus a variable thickness (generally < 5 ft.) of clayey greensand in which flints occur only sporadically, but DINES et al. (1954, p.76) equated the whole of the base-bed with the Bullhead Bed. GARDINER'S(1888) mineralogical study of the base-bed was probably confined to sand fractions from the clayey greensand and did not include the Bullhead Bed. All five divisions are represented in this work; BB 1, BB 2, TB !. TB 7 and probably also TB 6 are from Whitaker's division a, TB 2 is from division b, TB 8 from division c, TB 3 and TB 4 from division d and TB 5 from division e
ANALYTICAL METHODS
Size fractions for mineralogical analysis were separated from 200-g subsamples, which were treated where necessary to remove organic matter and calcium carbonate, and then dispersed in 0.05% sodium hexametaphosphate solution by ultrasonic agitation. Sand fractions were removed with a 50-p sieve, and the clays ( < 2 / 0 and three silt fractions (2-5 p, 5-20/1, 20-50 kt) were separated by repeated sedimentation in d lute suspension. Particle size distribution was also determined in 10-g sub-samples by the pipette sampling technique. The detailed particle size distribution of sand fractions from the most sandy sediments was determined on Sediment. GeoL, 3 (1969)
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
21
200-g sub-samples by removing the silt and clay and then dry-sieving at 1/4 ~o intervals. The fine sands (50-250 ~), coarse silts and medium silts were separated into light and heavy fractions with bromoform, and analysed mineralogically with a petrological microscope. The clays, fine silts and medium silts were analysed by X-ray diffractometry of lightly compressed powders and oriented aggregates. Quartz and feldspar in the silts were determined by the bisulphate fusion method of KIELY and JACKSON (1964), but the amounts of quartz in samples containing clinoptilolite were estimated from the X-ray diffraction traces because clinoptilolite is not completely destroyed by fusion with bisulphate. Jarosite was determined from the sulphate content of nitric acid digests, and organic C by a modification of SHAW'S 0959) method. The morphology of clay and some silt minerals was studied by electron microscopy. RESULTS
Thanet Beds (including the Bullhead Bed) Table I gives the particle-size distribution of all the Palaeogene deposits studied. The Thanet Beds are composed mainly of fine sand and clay, but coarse silt is an impoitant constituent of some horizons (TB 2, TB 6). Most of the large flint nodules were removed from the Bullhead Bed samples before analysis, but some of the smaller fragments remain; these are the main constituent of the coarse sand fractions in BB 1 and BB 2. The fine sand and coarse silt fractions from all horizons of the Thanet Beds are composed mainly of quartz, flint, glauconite and alkali feldspars. Glauconite and flint fragments are both more abundant in lower horizons, especially in WHITAKER'S(1872) division a, than in higher parts of the Thanet Beds. Heavy mineral suites from the fine sands and coarse silts contain mainly iron ores, zircon, garnet, epidote, tourmaline, staurolite, kyanite, biotite, rutile and collophane. Clay fractions from the Thanet Beds contain 65-95% montmorillonite and 5-25% mica, but no kaolinite. Vermiculite and interstratified mica/chlorite occur in clay from the Bullhead Bed at Mockbeggar (BB l) and chlorite, pyrites and calcite in the blue-grey Thanet Beds at Bishopstone Point (TB 4); sample TB 4 also contains more organic C (0.3%) than the other Thanet Beds samples. Clay from the Bullhead Bed at Upnor (BB 2) contains euhedral apatite crystals, jarosite and clinoptilolite, and clays from the lowest Thanet Beds at Pegwell Bay (TB l, TB 2) also contain a little clinoptilolite. Low-temperature tridymite occurs in Whitaker's division d both at Pegwell Bay (TB 3) and at Bishopstone Point (TB 4). Small amounts of quartz and feldspar occur in clay fractions from all parts of the Thanet Beds. The montmorillonite forms short, thin laths, typically 0.5 # long and 0.05 ~ wide, which occur either singly or in aggregates; some of the aggregates show a symmetrical arrangement of laths (Plate IC). Morphologically similar montSediment. Geol., 3 (1969) 17-33
~J
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PALAEOGENESEDIMENTSIN NORTHEASTKENT
23
morillonite is common in the Upper Chalk of southeast England (WEIRand CATT, 1965). Many of the mica flakes have groups of parallel laths projecting from their edges (Plate IC); these are commonly also symmetrically arranged at angles of 120 ° to one another, but are generally thicker than the montmorillonite laths. Similar projecting laths were observed on mica flakes in the Upper Chalk (WEIR and CATT, 1965) and in the Bunter Sandstone of Germany (BEUTELSPACHERand VAN DER MAREL, 1968, plate 258); they also occur on kaolinite in the Berea and Roubidoux Sandstones of North America (REx, 1966). Rex showed that the laths in the American sandstones are authigenic overgrowths of mica or related layer lattice minerals in epitaxial relationship to authigenic kaolinite, and by analogy with his work the laths associated with the Thanet Beds micas are also probably authigenic overgrowths or outgrowths on detrital flakes. All the minerals that occur in the clay fractions are also important constituents of fine silts from the same samples, but quartz, feldspar and (where they occur) clinoptilolite, low-tridymite and jarosite are usually more abundant in the fine silts than in the clays. In contrast, mica and montmorillonite are less c o m m o n in the silts than in clay fractions. However, the fine silts comprise much smaller proportions of the total sediment than do the clay fractions (Table I). Consequently, the percentages of fine silt-sized quartz and feldspar in the whole sediment are approximately the same as those of clay-sized quartz and feldspar. The montmoriqonite occurs only as aggregates of clay particles, which are present because the samples were imperfectly disaggregated. A disordeied crystalline form of silica that most closely resembles low-temperature tridymite (BRowN, 1961, p.468) comprises approximately 10% of the fine silt from sample TB 3 and 30% of the same size fraction from TB 4. It is recognised in electron micrographs as irregular aggregates of spheres and short rods (Plate IC); the diameter of the spheres and width of the rods are both approximately 0.05 #. MIZUTANI (1966) produced a mineral with similar morphology and powder diffraction characteristics during laboratory crystallisation of quartz from amorphous silica; this suggests that the low-tridymite in the Thanet Beds crystallised authigenically from amorphous silica (e.g., opal) or silica-bearing solutions, Jarosite occurs in silt fractions from several of the Palaeogene deposits (Table II), but is most abundant in the highest parts of the Thanet Beds at U p n o r PLATE I A. Optical micrograph of clinoptilolite crystals from the coarse silt (20-50/1) of the Thanet Beds at Upper Upnor (TB 7). B. Optical micrograph of jarosite crystals from the medium silt (5-20/z) of the Thanet Beds at Lower Upnor (TB 8). C. Electron micrograph showing bladed outgrowths on mica (a), low-temperature tridymite (b) and a background of montmorillonite laths from the clay of the Thanet Beds at Bishopstone Point (TB 4). D. Electron micrograph of small euhedral and subhedral kaolinite crystals from the clay of the Woolwich Sands at Lower Upnor (WS 2). Sediment. Geol., 3 (1969) 17-33
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Bagshot Sands, Sheppey Claygate Beds, Sheppey Claygate Beds, Sheppey London Clay, East End, Sheppey London Clay, Warden Point, Sheppey London Clay, Herne Bay London Clay, Shelford London Clay, Basement Bed, Shelford Oldhaven Beds, Shelford Oldhaven Beds, Herne Bay Woolwich Sands, Lower Upnor Woolwich Sands, Lower Upnor Woolwich Sands, Bishopstone Point Thanet Beds, Lower Upnor Thanet Beds, Upper Upnor Thanet Beds, Mockbeggar Thanet Beds, Bishopstone Point Thanet Beds, Bishopstone Point Thanet Beds, Pegwell Bay Thanet Beds, Pegwell Bay Thanet Beds, Pegwell Bay Bullhead Bed, Upper Upnor Bullhead Bed, Mockbeggar
Sample
BS CB CB LC LC LC LC LC OB OB WS WS WS TB TB TB TB TB TB TB TB BB BB
1 2 1 5 4 3 2 1 2 1 3 2 1 8 7 6 5 4 3 2 1 2 I
0.0 0.0 0.0 0.0 0.2 0.2 0.2 2.6 0.6 0.2 18.9 0.6 0.3 0.4 0.3 0.0 1.5 0.2 0.0 0.0 0.0 18.9 16.9
79.6 58.5 69.5 48.2 8.l 0.5 3.8 47.4 95.9 92.5 12.4 90.1 76.0 82.1 55.2 52.8 77.8 71.8 61.2 13.3 59.8 34.4 19.3
5.0 7.6 6.4 8.3 14.7 14.1 7.5 3.7 0.8 3.8 7.0 0.1 3.1 1.8 5.1 23.0 2.6 4.0 12.1 42.4 13.4 5.8 13.1
Silt 2 0 - 5 0 It
> 2 5 0 It
5 0 - 2 5 0 tt
Sand
PARTICLE-SIZE DISTRIBUTION OF PALAEOGENE SEDIMENTS OF NORTHEAST KENT
TABLE I
1.2 4.0 2.5 7.7 17.7 11.5 13.6 6.4 0.2 0.5 14.2 0.8 4.7 2.8 11.0 5.8 3.4 5.2 5.0 13.3 11.9 3.4 7.9
5 - 2 0 It
1.2 2.9 2.0 5.0 8.1 11.2 11.3 4.3 0.1 0.9 4.8 0.3 1.2 1.2 5.9 3.2 1.2 4.0 2.0 4.4 6.5 6.0 4.4
2 - 5 I~
I 1.2 26.4 19.7 29.2 53.1 62.5 65.4 34.7 1.7 2.1 43.4 7.9 14.7 10.9 23.1 16.2 15.0 16.2 19.7 26.6 8.4 31.8 38.9
< 2 ~t
Clay
98.2 99.4 100.1 98.4 101.9 100.0 101.8 99.1 99.3 100.0 100.7 99.8 100.0 99.2 100.6 101.0 101.5 101.4 100.0 100.0 100.0 100.3 100.5
Total
25
PALAEOGENE SEDIMENTS1N NORTHEASTKENT TABLE II .1AROSITE CONTENTS OF PALAEOGENE SEDIMENTS OF NORTHEAST KENT (~/o)
Sample
BB 2
TB 6
TB 8
WS 1
WS 2
CB ]
BS 1
Fine silt Medium silt Coarse silt Whole sample
13 4 0 0.9
6 4 0 0.4
61 19 3 1.3
22 4 4 0.6
5 1 <1 <0.1
2 0 0 <0.1
1 0 0 <0.1
(TB 8). In most samples it forms yellow, strongly birefringent aggregates, the individual crystals of which are < 3 # across, but the medium silt of TB 8 contains single crystals as large as 12/~ across (Plate IB). Most of these are thin plates with strongly developed pinacoids (0001) and weaker ditrigonal prismatic - - probably { 1120} and {2110} - - and bipyramidal { 1011 } faces, but a few have a rhombohedral outline because the bipyramidal faces are developed almost to the exclusion of the pinacoids. The amounts of sulphate and iron extracted by nitric acid from the fine silt of TB 8 are both equivalent to 61°/0 jarosite; the nitric acid extractable potassium is equivalent to 35.4% jaiosite and extractable sodium to 36.6°/0 natrojarosite. The composition of this extract therefore suggests that the jarosite is neither a pure potassium nor a pure sodium jarosite, and that a little potassium and/or sodium was extracted from minerals other than jarosite. Most of the medium silt fractions from the Thanet Beds are composed of quartz with minor amounts of feldspar, muscovite, glauconite and flint fragments. The amounts of heavy minerals are less than in the coarse silts, but the mineral suites are approximately the same as those of the coarse silt and fine sand fractions. X-ray diffractometer traces of all the medium silts indicate the presence of as much as 10°/0 montrnorillonite. This may occur part!y in aggregates of clay-sized particles similar to those in the fine silts. However, it probably also occurs partly in glauconite grains, because a diffractometer trace of disaggregated glauconite pellets picked by hand f r o m the fine sand of TB 1 showed that they contain approximately equal amounts of mica and montmorillonite. Clinoptilolite is an important constituent of medium and fine silts f r o m TABLE III CLINOPTILOLrrE CONTENTS OF PALAEOGENE SEDIMENTS OF NORTHEAST KENT (~o)
Sample
BB 2
TB 1
TB 2
TB 3
TB 7
Clay Fine silt Medium silt Coarse silt Whole sample
2 20 15 <1 2.4
5 35 57 3 9.9
<1 15 25 0 4.0
0 0 <1 0 < 0.1
<1 50 90 10 13.4 Sediment. Geol., 3 (1969) 17-33
26
A. H. WEIR AND J, A. CA-FT
WHITAKER'S (1872) division a of the Thanet Beds both at Pegwell Bay and at Upnor (Table liD, but it is not present in equivalent beds at Mockbeggar, approximately half way between Pegwell Bay and Upnor. It occurs mainly as colourless, anhedral grains and aggregates (Plate 1A), which have a mean refractive index near to 1.488 and birefringence < 0.005. However, a few euhedral platy crystats occur in the medium silts of TB 1 and TB 7. The plates are flattened parallel to {010}, and are either eight-sided (Plate 1A) or six-sided, probably indicating the development of {001 }, {201} and {201 } forms with or without { 100}. Many of the plates are probably not uniform in composition, because the birefringence (Nv -Nx) commonly increases from < 0.002 in central parts of the crystals to > 0.006 at the margins. Woolwich Beds The mineralogical composition of the marine Woolwich Beds at Bishopstone Point (WS 1) is exactly the same as that o f the underlying Thanet Beds at the same locality (TB 5); this emphasises the difficulty experienced by many workers of distinguishing the two deposits in the field. However, the non-marine Woolwich Beds at Upnor (WS 2, WS 3) contain a slightly different mineral assemblage. The sand and coarse silt fractions yield a more restricted suite of detrital heavy minerals, which is dominated by iron ores (main!y magnetite and limonite), zircon, tourmaline futile, staurolite and kyanite, and contains little or no epidote and garnet. Also the clay fractions contain much kaolinite, a mineral that is not present in either the Thanet Beds or the marine Woolwich Beds at Bishopstone Point. In these respects the Woolwich Beds at Upnor are similar to the Reading Beds (HODGSON et al., 1967), which are probably the lateral equivalent of the Wooiwich Beds in areas west of London. The Woolwich Beds kaolinite (Plate ID) is morphologically similar to kaolinite from the Reading Beds in that it forms small, subhedral (hexagonal), platy crystals. The detailed particle size distribution of sand fractions (Fig.2) also indicates that there is a closer relationship between the marine Woolwich Beds ot northeast Kent and the Thanet Beds than between the marine and non-marine Woolwich Beds. The two most sandy Thanet Beds samples (TB 5 and TB 8) both contain well-sorted sand with maxima at 3.125 tp (115 kt). The curve for the marine Woolwich Beds sample (WS 1) is almost the same as that for TB 5, but the sand from the non-marine Woolwich Beds (WS 2) is slightly coarser Imaximum at 2.875 q~, 140/~) and not quite so well-sorted. Some of the non-marine Woolwich Beds at Upnor contain large amounts of non-detrital material; for example, WS 3 contains 27% CaCO3 (mainly shells of Corbicula), organic matter equivalent to 3.3% organic C, and much sand-sized gypsum, pyrites and collophane.
Sediment. Geol.. 3 (1969) 17
27
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
0
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Fig. 2. Particle-size distribution of sand fractions ( > 50 #) from selected Palaeogene samples, based on weight percentages at ¼ ¢0 intervals.
Oldhaven Beds The Oldhaven Beds are composed mainly of sand that is mineralogically similar to the Thanet Beds sand, but slightly finer (Fig.2). The small amounts of coarse silt are characterised by an unusual abundance (9%) of zircon, and the clay fractions contain montmorillonite with small amounts of mica, kaolinite and (in OB 2) goethite. Many of the clay micas have lath-shaped outgrowths similar to those occurring in the Thanet Beds. At both Herne Bay and Shelford quarry the Oldhaven Beds yield small concretionary rosettes of sandy gypsum crystals. London Clay The London Clay consists of blue-grey and brown silty clays that lack appreciable quantities of sand except in the lowest 1-2 ft. (the London Clay Basement Bed) and highest 20-30 ft. The average composition of the clay fractions Sediment. Geol., 3 (1969) 17-33
28
A. H, WEIR AND J. A. ( A f f
is 60% montmorillonite, 25% mica, 5-10% kaolinite and 2-5% chlorite. However, the clay from the Basement Bed contains a little quartz but no chlorite; quartz also occurs in the clay fractions of samples LC 2 and LC 3, and tow-tridymite m LC 3. Almost all the montmorillonite in the Basement Bed occurs in lath-shaped particles. In LC 2 these are accompanied by shapeless aggregates of montmorillonite composed of very small platy particles, and at higher horizons the laths are progressively replaced by these aggregates. The detrital sand and coarse silt minerals in the lower parts of the LondoJa Clay (LC 1, LC 2) are similar to those in the Oldhaven and Thanet Beds, except for the addition of chlorite, muscovite and deep-green biotite. However, the heavy mineral assemblages in the higher beds of the London Clay are similar to those in the overlying Claygate and Bagshot Beds; zircon, tourmaline, kyanite and staurolite are less common than in the lower parts, whereas the amounts of garnet, epidote and amphiboles are greatly increased. The main non-detrital sand and silt minerals are glauconite, calcite, pyrites and at some horizons (LC 4, LC 5) siderite. The amount of organic C in the Basement Bed (0.1%) is less than in other parts of the London Clay (0,3-0.6%). The effects of weathering can often be traced to unusually great depths in the London Clay. Much of this weathering probably occurred during Tertiary or Early Quaternary times, because deep weathering profiles occur beneath thick covers of Quaternary deposits as well as at the surface. For example, the London Clay forming the overburden in Shelford sand quarry is weathered to a depth of approximately 20 ft., below 8 ft. of clayey and sandy river terrace gravels. In the field the most obvious effect of weathering of the clay is a colour change from dark grey (5Y 3/t of the Munsell Color Chart) to yellowish-brown (commonly 10 YR hues). The most strongly weathered horizons (e.g,, the highest 6-8 ft. of the London Clay at Shelford) are brown throughout, and contain many small selenite crystals and nodular segregations ofjarosite. Below these layers there is a variable thickness of clay (12 ft. at Shelford), in which the brown colour is restricted to parts of the sediment adjacent to structural faces; jarosite is less common in these horizons, and the selenite forms large, twinned crystals. Selenite crystals also occur in even lower horizons, where the clay is grey throughout. Amounts of jar osite in the weathered London Clay are not given in Table II, because it occurs so irregularly that percentages cannot be accurately estimated. The deep-green biotite in the sand and coarse silt fractions is also altered in weathered (brown) parts of the London Clay. The altered mica is pale yellow-brown, and the basal sections shown by cleavage flakes are slightly more pleochroic and have greater birefringence than those of the green, unaltered biotite. The optic axial angle is also slightly greater ir~ the weathered form than the unweathered. Most of the altered flakes are almost: uniformly yellow-brown throughout, but a few are only partly weathered and show either irregular brown patches or well-marked brown, strongly birefringent margins around green unaltered cores. Sediment. Geol., ~ ~1969) 17-3~
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
29
Claygate and Bagshot Beds The fine sand and coarse silt fractions from these beds contain 70-85°,/o quartz, 9-13% feldspar and small amounts of glauconite, muscovite and flint fragments. The heavy minerals are mainly iron ores (limonite, haematite, magnetite and ilmenite) with subsidiary epidote, clinozoisite, garnet, hornblende, zircon and rutile, and rare tourmaline, chlorite, anatase, apatite, kyanite and staurolite. The clay fractions contain 80-90~o lath-shaped montmorillonite and small amounts of mica (with lath-shaped overgrowths as in the Thanet Beds) and kaolinite. The Bagshot Beds contain more sand than the Claygate Beds, but cannot be distinguished from them on other mineralogical grounds. DISCUSSION AND CONCLUSIONS
The differences in mineralogical composition between detritalcomponents from the major subdivisions of the Palaeogene sediments in northeast Kent suggest that the detritus was derived from at least three sources. A small proportion represents the washed and sorted insoluble residue of the Chalk; this is mainly flint fragments, but may also include some of the montmorillonite and mica in the clay and fine silt fractions. It is most abundant in the oldest Thanet Beds, but small amounts of flint occur in all the sediments studied. The Bull head Bed contains some material (e.g., unworn flint nodules, collophane and clay-sized apatite euhedra) that was possibly derived by dissolution of the immediately subjacent Chalk, and was not transported, sorted or intensely weathered. However, this bed is not an uncontaminated Chalk residuum, because it also contains much detrital and nondetrital material that is mineralogicaUy similar to higher parts of the Thanet Beds. If all the nodules and coarse fragments ( > 250/~) of flint are excluded from the Bullhead Bed, the remaining matrix contains much more clay than any other part of the Thanet Beds. This excess clay may be derived by dissolution of the subjacent Chalk, the insoluble residues of which probably contain much more clay than silt or fine sartd (WEIR and CATT, 1965) ; alternatively it may be an illuvial accumulation of clay washed down from higher horizons of the Thanet Beds. However, most of the detritus in the Palaeogene beds was derived from sources other than the Chalk, because flint, which is by far the most important constituent of insoluble residues from the Upper Chalk of southeast England, comprises < 5% of the detritus in all the Palaeogene sediments except the lowest Thanet Beds. Throughout most of the Palaeogene the shoreline, particularly to the west (CURRY, 1966), lay well within the areas originally covered by a great thickness of Chalk. The lack of abundant Chalk-derived detritus throughout the succession therefore suggests that the Chalk cover had been largely removed from land areas adjacent to the Palaeogene sea before the Thanetian transgression. The provenance of the detritus not derived from the Chalk cannot be inferred with certainty, but the mineralogical composition of the sand and coarse silt Sediment. Geol., 3 (1969) 17-33
30
A.H.
WEIR
A N D J. A. C A I T
fractions suggests that during the Palaeogene at least two types of source rocks other than the Chalk were involved. There are no significant mineralogical changes in the lower parts of the succession in northeast Kent (the Thanet, Woolwich and Oldhaven Beds and lower London Clay), but in areas closer to London the non-marine Woolwich Beds and highest parts of the succession (upper London Clay, Claygate and Bagshot Beds) both contain more restricted detrital mineral suites than that typical of northeast Kent. The non-marine Woolwich Beds assemblage is clearly related to that of the Reading Beds, and is largely composed of minerals derived ultimately from granites (zircon, tourmaline, rutile and kaolinite) and metamorphic rocks (staurolite, kyanite) similar to those in the Armorica~ massifs lying to the south and west. In contrast, the upper parts of the London Clay and the Claygate and Bagshot Beds are mainly composed of material derived from metamorphic rocks bearing garnet and minerals of the amphibole and epidote groups. The detritus of the Thanet Beds, marine Woolwich Beds, Oldhaven Beds and lower parts of the London Clay probably contains material from both garnetepidote-amphibolite metamorphics and Armorican sources. Two main morphological types of montmorillonite occur in the Palaeogene deposits. The sandier beds contain only laths and aggregates of laths; these also occur in the lower parts of the London Clay, but are largely replaced by anhedral flakes in higher parts of the London Clay. This difference indicates either that the montmorillonite was derived from two sources or that the laths formed in the sandy, well-drained beds by the action of interpore solutions on detrital clays. The Upper Chalk is a possible source of detrital montmorillonite laths, and does contain stellate aggregates similar to those in the Palaeogene. However, too little is known at present either of the composition and morphology of clays in sediments older than the Chalk or of the changes clays suffer during transportation, sedimentation and diagenesis, to eliminate other sources of montmoriUonite or the possibility of its diagenetic recrystallisation in the sandy deposits. The composition of non-detrital components in the sediments indicates to some extent the conditions in which the detritus was deposited. Palaeontologica] evidence shows that the Thanet Beds, Woolwich Beds in northeast Kent, Oldhaven Beds and London Clay are all marine deposits (CURRY, 1965), and this is confirmed by the glauconite they contain. Glauconite is absent in the Woolwich Beds at Upnor, but does occur in the unfossiliferous Claygate and Bagshot Beds. The only non-marine deposits in the Palaeogene east of the Medway valley are therefore the Woolwich Beds of the Medway area. Parts of the Thanet Beds (WRITAKER'S, 1872, division d, represented by TB 4), some of the non-marine Woolwich Beds (WS 3) and most of the London Clay (LC 2-LC 5) were probably deposited in reducing conditions, because they contain pyrites, siderite and more organic matter than other beds in the Palaeogene. The clay fractions in these reduced sediments are similar to clays in the other (oxidised) sediments, except that, apart from the non-marine sample WS 3, they all contain a little chlorite, This Sediment. Geol., 3 ( t 9 6 9 ) t Z . '. ~.
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
31
suggests that the clay-sized chlorite is not truly detrital in origin, but was formed in the sediment during or after deposition as a result of the reducing conditions. The clinoptilolite in the lowest Thanet Beds at Pegwell Bay and Upnor is probably not detrital in origin, because it commonly forms fresh, euhedral crystals, and because in a few feet of beds it is unusually abundant for such a rarely observed mineral. Zeolites of the clinoptilolite-heulandite group often form by alteration of glassy and other volcanic materials; for example, in the folded volcanic tufts and claystones of the John Day Formation of Oregon clinoptilolite was probably formed at a depth of burial between 1,250 and 4,000 ft. from alkaline solutions bearing silica and alkali metals derived from volcanic glass (HAY, 1963). However, the Thanet Beds are unusual (but not unique) among clinoptilolite-bearing sediments in that they contain no recognisable volcanic materials; HAY (1966, pp.22-30) listed only four definite identifications of clinoptilolite-heulandite in marine sediments not containing abundant volcanic materials. The nearest known Tertiary volcanic centres of the northern Atlantic (Thulean) province were at least 600 km from eastern Kent. Ash might have been blown this distance, and after deposition might have been completely altered, so as to leave no recognisable trace of volcanic material. However, the silica and alkalis needed for post-depositional zeolite formation might alternatively have been derived from non-volcanic detrital components of the sediment. In the John Day Formation clinoptilolite is accompanied by several other authigenic minerals, including mica (celadonite) and opaline silica, and somewhat similar minerals of probable authigenic origin also occur in the marine Palaeogene deposits, particularly the Thanet Beds. First, the laths of layer silicates on detrital mica flakes in clays from the Thanet, Oldhaven and Bagshot Beds indicate possible post-depositional growth of mica. Second, opal does not occur in any of the deposits, but some parts of the Thanet Beds contain low-temperature tridymite, which probably also formed after deposition from opaline silica or solutions containing silica. However, in other ways the authigenic mineral suite of the Thanet Beds is not strictly comparable with that of the John Day Formation. In particular, the John Day Formation contains some authigenic minerals (montmorillonite, orthoclase), which are probably only detrital components of the Thanet Beds; also, whereas celadonite is restricted to clinoptilolite-bearing horizons of the John Day Formation, the layer silicate overgrowths occur in many parts of the Palaeogene succession, not containing authigenic zeolites. Nevertheless, solutions containing silica and alkali metals probably affected most of the permeable Palaeogene sediments at some time after deposition. The widespread activity of such solutions suggests that the silica and alkalis were leached from common detrital minerals rather than derived by alteration of volcanic glass, because the nearest volcanic centres were too distant to provide an almost continuous supply of wind-b!own ash throughout the Palaeogene. Therefore, we cannot disprove that the clinoptilolite formed by alteration of volcanic material, but suspect that solutions conSediment. Geol., 3 (1969) 17-33
32
A . H . WEIR AND .I.A. ('ATi
taining alkalis and silica derived from common detrital minerals provided ~ geochemical environment in some ways resembling that of the John Day Formation. The detrital fractions of the Kentish Palaeogene deposits are by ~, means mineralogically unusual, and if the clinoptilolite did form by dissolution of common detrital minerals, why is it not recorded more frequently in similar beds? In the Thanet Beds clinoptilolite is mainly confined to size fractions (fine and medium silts) not usually analysed in conventional mineralogical studies of sandy sediments, and this may explain its apparent rarity. During weathering of the London Clay the main processes causing changes in its mineralogical composition were oxidation of pyrites and leaching by the sulphuric acid produced during this oxidation. The colour change from grey to. brown was caused by the formation of iron oxides from the pyrites, and the gypsum was formed by reaction between sulphuric acid and calcium carbonate in the clay. Changes in the green biotite flakes might also have been caused by sulphuric acid leaching. RIMSAITE (1967) showed that natural weathering of biotites involving removal of potassium and oxidation of iron causes similar changes in colour and other optical properties to those seen in the London Clay biotite. Jarosite forming nodular concretions in the weathered London Clay probably crystallised from solutions carrying sulphate ions, potassium leached from the biotite (and possibly other layer silicate minerals), and iron derived by acid dissolution of oxides or siderite. Sulphate-rich solutions descending from the weathered clay also might have caused formation of gypsum crystals in the underlying Oldhaven Beds. ACKNOWLEDGEMENTS
We thank Mr. R. D. Green for help in collecting samples, Mr. R. D. Woods for electron micrographs, Mrs. M. Thomas for optical micrographs and Mr. E. M. Thomson for drawing the diagrams. REFERENCES BAKER, H. A., 1920. Quartzite pebbles of the Oldhaven Beds of the southern part of the L o n d o n Basin. Geol. Mag., 57:62-70. BEUTELSPACHER, H. and VAN DER MAREL, H. W., 1968. Atlas of Electron Microscopy of Clal Minerals and their Admixtures. Elsevier, Amsterdam, 333 pp. BOSWELL, P. G. H., 1915. The stratigraphy and petrology of the Lower Eocene deposits of the north-eastern part of the L o n d o n Basin; Quart. J. Geol. Soc. London, 71 : 536-591. BOSWELL, P. G. H., 1923. The petrography of the Cretaceous a n d Tertiary outliers of the west of England. Quart. J. Geol. Soc. London, 79:205-230. BROWN, G., 1961. Other minerals. In: G. BROWS (Editor), The X-Ray Identification and Crystal Structures of Clay Minerals. Mineral. Society, London, pp. 467-488. CURRY, D., 1965. The Palaeogene beds of southeast England. Proc. Geologists' Assoc. Engl, 76:151-173. CURRY, D., 1966. Problems of correlation in the Anglo-Paris-Belgian Basin. Proc. Geologists' Assoc. Engl., 77:437-467.
Sediment. Geol., 3 (1969) 17 3~
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
33
DAVIES, G. M., 1915. Detrital andalusite in Cretaceous and Eocene sands. Mineral. Mag., 17: 218-220. DAVIES, G. M., 1916. The rocks and minerals of the Croydon regional survey area. Proc. Trans. Croydon Nat. Hist. Sci. Soc. 1915-1916: 53-96. DINES, H. G., HOLMES, S. C. A. and ROBBIE, J. A., 1954. Geology of the country around Chatham. Geol. Surv. Gt. Brit., Mere. Geol. Surv. Gt. Brit., Engl. Wales, 157 pp. GARDINER, M. 1., 1888. The greensand bed at the base of the Thanet Sand. Quart. J. Geol. Soc. London, 44: 755-760. GROVES, A. W., 1928. Eocene and Pliocene outliers between Chipstead and Headley, Surrey. Proc. Geologists' Assoc. Engl., 39:471-485. GURR, P. R., 1962. A new fish fauna from the Woolwich B o t t o m Bed (Sparnacian) of Herne Bay, Kent. Proc. Geologists' Assoc. Engl., 73:419-447. HAY, R. L., 1963. Stratigraphy and zeolitic diagenesis of the J o h n Day F o r m a t i o n of Oregon. Univ. Calif. (Berkeley) Publ. Geol. Sci., 42:199-262. HAY, R. L., 1966. Zeolites and zeolitic reactions in sedimentary rocks. Geol. Soc. Am., Spec. Papers, 85 : 130 pp. HODGSON, J. M., CATT, J. A. and WEIR, A. H., 1967. The origin and development of claywith-flints and associated soil horizons on the South Downs. J. Soil Sci., 18:85-102. KIELY, P. V. and JACKSON, M. L., 1964. Selective dissolution of micas from potassium feldspars by sodium pyrosulfate fusion of soils and sediments. Am. Mineralogist, 49:1648-1659. MIZUTANI, S., 1966. Transformation of silica under hydrothermal conditions. J. Earth Sci., Nagoya Univ., 14: 56-88. PITCHER, W. S., PEAKE, N. B., CARRECK, J. N., KIRKALDY, J. F., HESTER, S. W. and HANCOCK, J. M., 1958. Geologists' Association Guides, 30. The London Region. Benham, Colchester, 41 pp. REX, R. W., 1966. Authigenic kaolinite and mica as evidence for phase equilibria at low temperatures. Clays Clay Minerals, Proc. Natl. Conf. Clays Clay Minerals, 13th, 1964, 25 : 95-104. RIMSAITE, J., 1967. Biotites intermediate between dioctahedral and trioctahedral micas. Clays Clay Minerals, Proc. Natl. Conf. Clays Clay Minerals, 15th, 1966, 27:375-393. SHAW, K., 1959. Determination of organic carbon in soil and plant material. J. Soil Sci.~ 10:316-326. SMART, J. G. O., BlSSON, G. and WORSSAM, B. C., 1966. Geology of the country around Canterbury and Folkestone. Geol. Surv. Gt. Brit., Mem. Geol. Surv. Gt. Brit., Engl. Wales, 337 pp. STINTON, F. C., 1965. Field meeting in the Lower L o n d o n Tertiaries of Kent. Proc. Geologists' Assoc. Engl., 76:175-178. WALDER, P. S., 1964. Mineralogy of the Eocene sediments in the Isle of Wight. Proc. Geologists' Assoc. Engl., 75:291-394. WEIR, A. H. and CATT, J. A., 1965. The mineralogy of some Upper Chalk samples from the Arundel area, Sussex. Clay Minerals, 6:97-110. WHITE, H. J. O., 1928. The geology of the country near Ramsgate and Dover. Geol. Surv. Gt. Brit., Mere. Geol. Surv. Gt. Brit., Engl. Wales, 98 pp. WHITAKER, W., 1872. The geology of the L o n d o n Basin. Geol. Surv. Gt. Brit., Mere. Geol. Surv. Gt. Brit., Engl. Wales, 1872:619 pp.
Sediment. Geol., 3 (1969) 17-33
THE MINERALOGY OF PALAEOGENE SEDIMENTS IN NORTHEAST KENT (GREAT BRITAIN) A. H. WEIR ANDJ. A. CATT Pedology Department, Rothamsted Experimental Station, Harpenden, Herts. (Great Britain) (Received May 3, 1968) (Resubmitted August 28, 1968) SUMMARY The Palaeogene deposits of northeast Kent are approximately 700 ft. thick and consist of unconsolidated clays and loamy sands, which are mainly marine in origin. The mineralogical composition of fine sand (50-250 p), silt (2-5 p, 5-20 p and 20-50 #) and clay ( < 2 p) fractions from samples representing the main subdivisions of the succession is described, and possible sources of the detritus and mode of origin of some non-detrital minerals are discussed. The detritus was derived ultimately from three main sources: (a) the Chalk; (b) metamorphic rocks (containing staurolite and kyanite) and granites, possibly of Armorican massifs; (c) metamorphic rocks containing garnet, epidote and amphiboles. Clay fractions in most of the deposits are composed of montmorillonite with subsidiary mica, but kaolinite occurs in beds containing abundant detritus from possible Armorican sources. The main non-detrital minerals are glauconite, jarosite, pyrites, low-temperature tridymite and clinoptilolite, and detrital clay micas in the sandy sediments have authigenic growths of a layer silicate mineral. The main clay-rich subdivision of the succession (the London Clay) is weathered to great depths; mineralogical effects of this weathering include oxidation of pyrites, alteration of detrital biotite and formation of selenite and jarosite. INTRODUCTION The Palaeogene deposits of the London Tertiary Basin cover large areas both north and south of the estuary and valley of the River Thames as far west as Hungerford, but this paper concerns only those exposed on or near the north Kent coast from the River Medway eastwards to Thanet. These are divisible into six main lithological units, which are (in ascending order) the Thanet Beds, Woolwich Beds, Oldhaven Beds, London Clay, Claygate Beds and Lower Bagshot Beds. The Thanet Beds are separated from the underlying Senonian Chalk by the Bullhead Bed, a thin deposit of glauconitic sandy clay containing abundant flint nodules. This comparatively local succession within the Anglo-Paris-Belgian Basin, which Sediment. Geol., 3 (1969) 17-33
18
A. H , W E I R A N D ,1. A. C ~ I ' !
is approximately 700 ft. thick, comprises the Thanetian, Sparnacian and Ypresian Stages (CURRY, 1966). The mineralogical survey reported in this paper is part of a study of the origin, distribution and weathering of loess and associated Quaternary deposils in Kent, many of which contain material derived from local Tertiary sediments in addition to far-travelled wind-blown silt. The composition of sand fractions fi'om Palaeogene beds in the London and Hampshire parts of the Anglo-Paris-Belgian Basin was described by DAVIES(1915, 1916), BOSWELL(1915, 1923), BAKER(1920), GROVES (1928), WALD~R(1964) and others, but these accounts are scarcely adequate for provenance and other studies of the Quaternary deposits. Knowledge of the composition of silt fractions (2-50/~ equivalent spherical diameter) is especially important in work involving loess, and investigation of clay fractions (<2/~) is necessaryin weathering studies. Mineralogical analysis of the Palaeogene sediments was therefore extended to include all size fractions. This full analysis also provides more information of the conditions in which the Palaeogene sediments themselves were deposited and of the ultimate source of the detritus than could be obtained from the study of sand fractions alone. L O C A T I O N OF SAMPLES
Fig. 1 shows the approximate location of samples taken from six exposures of Palaeogene beds in northeast Kent. The Bullhead Bed (sample BB2) and basal Thanet Beds (TB 7) at Upnor were sampled at Tower Hill, a locality described by PITCHER et al. (1958, pp.12-13); the highest Thanet Beds (TB 8) and Woolwich Beds samples (WS 2, WS 3) at Upnor were taken from Beacon Hill Quarry and represent beds 17, 11 and 7 of the succession described by STINTON (1965). The highest Palaeogene deposits (upper part of the London Clay, Claygate Beds and Lower Bagshot Beds) occur in eastern Kent only on the north coast of the Isle of Sheppey. The least incomplete succession in these beds is that seen at East End (Grid Reference: TR 970733), where the highest 20-30 ft. of the London Clay (represented by LC 5) are much sandier than lower horizons exposed at Sheiford and Herne Bay, and pass upwards by alternation into the fine sands and loams of the Claygate Beds. Fossils are rare and the base of the Claygate Beds is probably best drawn below a prominent bed, 4-5 ft. thick, of brown and buff fine sand (sample CB 1) containing a few clay partings. This is overlain by 20-24 ft. of fine sands and loams with numerous grey and yellow sandy clay partings; CB 2 combines several sand and clay partings from near the middle of this bed. The base of the Bagshot Beds probably occurs immediately below 5 ft. of yellow and orange sands, which are divided into two main courses by a thin yellow clay parting; BS 1 is from the upper sand course. The overlying parts of the Bagshot Beds are very disturbed. A sample (LC 4) was taken approximately 180 ft. below the top of the London Clay near Warden Point, Sheppey (TR 015728). Sediment, Geol., 3 ( 1 9 6 9 ) 17--3 ~
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
-~;~,,
LONDON
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19
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-I Fig. 1. Geographic and stratigraphic location of analysed samples from the Palaeogene deposits of northeast Kent.
The succession at Mockbeggar (BB 1, TB 6) was briefly described by DINES et al. (1954, p.80), and Shelford Quarry, where the basal London Clay (LC 1, LC 2) and Oldhaven Beds (OB 2) were sampled, was described by SMART et al. (1966, pp.193, 197, 198). The cliffs east of Herne Bay provide the best exposure of the Palaeogene deposits in Kent. The lowest visible part of the Thanet Beds is here a blue-grey, shelly, clayey sand (TB 4), which is exposed at low tide on the foreshore east of Bishopstone Point. This is overlain by approximately 40 ft. of Sediment. Geol., 3 (1969) 17-33
20
A.H.
W E I R A N D J. A. ( A I ' t
grey, yellow and blown sands and loams (TB 5) containing a thin impersisten', sandstone, which lies 30 ft. below the top of the Thanet Beds and forms a prominent ledge near the cliff base from Bishopstone Point eastwards. The overlying Wooiwich Beds (represented by WS 1) are lithologically similar to the upper parts of the Thanet Beds, but are separated from them by a pebbly clay containing a rich fish fauna (GuRR, 1962). The base of the Oldhaven Beds is marked by a more pro ~ minent pebble bed; sample OB 1 was taken approximately 12 ft. above this. WHITE (1928, pp.49-51) summarised earlier work on the Thanet Beds exposed at Pegwell Bay near Ramsgate. Here, as at Mockbeggar, the Bullhead Bed consists of 3-6 inches of glauconitic sandy clay containing many unworn flint nodules overlying 0.5-I .5 inches of partly-shattered tabular flint, which in turn rests directly on the Chalk. The lowest horizon above the Bullhead Bed is represented by sample TB 1 and consists of 8-12 inches of glauconitic sandy loam with pink streaks near the top and a few flint nodules. This is succeeded by 1.5 ft. of mottled pinkish-brown sandy loam with glauconitic streaks near the base; the intermingling of these two beds is attributed to animal burrowing. Sample TB 2 was taken 3 ft. above the base of the overlying bed, which comprises 11-12 ft. of well-bedded, buffsilts and sandy clays, and TB 3 is fiom the upper, bluish-grey part of the 55 ft. bed mentioned by WHITE (1928, p.50). WmTAKER (1872, p.56) recognised five main divisions (beds a-e) of the Thanet Beds in Kent. Confusion has been caused by some authors failing to distinguish between the Bullhead Bed and the lowest of Whitaker's divisions (bed a or the "base-bed"). The base-bed contains the Bullhead Bed plus a variable thickness (generally < 5 ft.) of clayey greensand in which flints occur only sporadically, but DINES et al. (1954, p.76) equated the whole of the base-bed with the Bullhead Bed. GARDINER'S(1888) mineralogical study of the base-bed was probably confined to sand fractions from the clayey greensand and did not include the Bullhead Bed. All five divisions are represented in this work; BB 1, BB 2, TB !. TB 7 and probably also TB 6 are from Whitaker's division a, TB 2 is from division b, TB 8 from division c, TB 3 and TB 4 from division d and TB 5 from division e
ANALYTICAL METHODS
Size fractions for mineralogical analysis were separated from 200-g subsamples, which were treated where necessary to remove organic matter and calcium carbonate, and then dispersed in 0.05% sodium hexametaphosphate solution by ultrasonic agitation. Sand fractions were removed with a 50-p sieve, and the clays ( < 2 / 0 and three silt fractions (2-5 p, 5-20/1, 20-50 kt) were separated by repeated sedimentation in d lute suspension. Particle size distribution was also determined in 10-g sub-samples by the pipette sampling technique. The detailed particle size distribution of sand fractions from the most sandy sediments was determined on Sediment. GeoL, 3 (1969)
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
21
200-g sub-samples by removing the silt and clay and then dry-sieving at 1/4 ~o intervals. The fine sands (50-250 ~), coarse silts and medium silts were separated into light and heavy fractions with bromoform, and analysed mineralogically with a petrological microscope. The clays, fine silts and medium silts were analysed by X-ray diffractometry of lightly compressed powders and oriented aggregates. Quartz and feldspar in the silts were determined by the bisulphate fusion method of KIELY and JACKSON (1964), but the amounts of quartz in samples containing clinoptilolite were estimated from the X-ray diffraction traces because clinoptilolite is not completely destroyed by fusion with bisulphate. Jarosite was determined from the sulphate content of nitric acid digests, and organic C by a modification of SHAW'S 0959) method. The morphology of clay and some silt minerals was studied by electron microscopy. RESULTS
Thanet Beds (including the Bullhead Bed) Table I gives the particle-size distribution of all the Palaeogene deposits studied. The Thanet Beds are composed mainly of fine sand and clay, but coarse silt is an impoitant constituent of some horizons (TB 2, TB 6). Most of the large flint nodules were removed from the Bullhead Bed samples before analysis, but some of the smaller fragments remain; these are the main constituent of the coarse sand fractions in BB 1 and BB 2. The fine sand and coarse silt fractions from all horizons of the Thanet Beds are composed mainly of quartz, flint, glauconite and alkali feldspars. Glauconite and flint fragments are both more abundant in lower horizons, especially in WHITAKER'S(1872) division a, than in higher parts of the Thanet Beds. Heavy mineral suites from the fine sands and coarse silts contain mainly iron ores, zircon, garnet, epidote, tourmaline, staurolite, kyanite, biotite, rutile and collophane. Clay fractions from the Thanet Beds contain 65-95% montmorillonite and 5-25% mica, but no kaolinite. Vermiculite and interstratified mica/chlorite occur in clay from the Bullhead Bed at Mockbeggar (BB l) and chlorite, pyrites and calcite in the blue-grey Thanet Beds at Bishopstone Point (TB 4); sample TB 4 also contains more organic C (0.3%) than the other Thanet Beds samples. Clay from the Bullhead Bed at Upnor (BB 2) contains euhedral apatite crystals, jarosite and clinoptilolite, and clays from the lowest Thanet Beds at Pegwell Bay (TB l, TB 2) also contain a little clinoptilolite. Low-temperature tridymite occurs in Whitaker's division d both at Pegwell Bay (TB 3) and at Bishopstone Point (TB 4). Small amounts of quartz and feldspar occur in clay fractions from all parts of the Thanet Beds. The montmorillonite forms short, thin laths, typically 0.5 # long and 0.05 ~ wide, which occur either singly or in aggregates; some of the aggregates show a symmetrical arrangement of laths (Plate IC). Morphologically similar montSediment. Geol., 3 (1969) 17-33
~J
J
PALAEOGENESEDIMENTSIN NORTHEASTKENT
23
morillonite is common in the Upper Chalk of southeast England (WEIRand CATT, 1965). Many of the mica flakes have groups of parallel laths projecting from their edges (Plate IC); these are commonly also symmetrically arranged at angles of 120 ° to one another, but are generally thicker than the montmorillonite laths. Similar projecting laths were observed on mica flakes in the Upper Chalk (WEIR and CATT, 1965) and in the Bunter Sandstone of Germany (BEUTELSPACHERand VAN DER MAREL, 1968, plate 258); they also occur on kaolinite in the Berea and Roubidoux Sandstones of North America (REx, 1966). Rex showed that the laths in the American sandstones are authigenic overgrowths of mica or related layer lattice minerals in epitaxial relationship to authigenic kaolinite, and by analogy with his work the laths associated with the Thanet Beds micas are also probably authigenic overgrowths or outgrowths on detrital flakes. All the minerals that occur in the clay fractions are also important constituents of fine silts from the same samples, but quartz, feldspar and (where they occur) clinoptilolite, low-tridymite and jarosite are usually more abundant in the fine silts than in the clays. In contrast, mica and montmorillonite are less c o m m o n in the silts than in clay fractions. However, the fine silts comprise much smaller proportions of the total sediment than do the clay fractions (Table I). Consequently, the percentages of fine silt-sized quartz and feldspar in the whole sediment are approximately the same as those of clay-sized quartz and feldspar. The montmoriqonite occurs only as aggregates of clay particles, which are present because the samples were imperfectly disaggregated. A disordeied crystalline form of silica that most closely resembles low-temperature tridymite (BRowN, 1961, p.468) comprises approximately 10% of the fine silt from sample TB 3 and 30% of the same size fraction from TB 4. It is recognised in electron micrographs as irregular aggregates of spheres and short rods (Plate IC); the diameter of the spheres and width of the rods are both approximately 0.05 #. MIZUTANI (1966) produced a mineral with similar morphology and powder diffraction characteristics during laboratory crystallisation of quartz from amorphous silica; this suggests that the low-tridymite in the Thanet Beds crystallised authigenically from amorphous silica (e.g., opal) or silica-bearing solutions, Jarosite occurs in silt fractions from several of the Palaeogene deposits (Table II), but is most abundant in the highest parts of the Thanet Beds at U p n o r PLATE I A. Optical micrograph of clinoptilolite crystals from the coarse silt (20-50/1) of the Thanet Beds at Upper Upnor (TB 7). B. Optical micrograph of jarosite crystals from the medium silt (5-20/z) of the Thanet Beds at Lower Upnor (TB 8). C. Electron micrograph showing bladed outgrowths on mica (a), low-temperature tridymite (b) and a background of montmorillonite laths from the clay of the Thanet Beds at Bishopstone Point (TB 4). D. Electron micrograph of small euhedral and subhedral kaolinite crystals from the clay of the Woolwich Sands at Lower Upnor (WS 2). Sediment. Geol., 3 (1969) 17-33
7~
C~
e~
Bagshot Sands, Sheppey Claygate Beds, Sheppey Claygate Beds, Sheppey London Clay, East End, Sheppey London Clay, Warden Point, Sheppey London Clay, Herne Bay London Clay, Shelford London Clay, Basement Bed, Shelford Oldhaven Beds, Shelford Oldhaven Beds, Herne Bay Woolwich Sands, Lower Upnor Woolwich Sands, Lower Upnor Woolwich Sands, Bishopstone Point Thanet Beds, Lower Upnor Thanet Beds, Upper Upnor Thanet Beds, Mockbeggar Thanet Beds, Bishopstone Point Thanet Beds, Bishopstone Point Thanet Beds, Pegwell Bay Thanet Beds, Pegwell Bay Thanet Beds, Pegwell Bay Bullhead Bed, Upper Upnor Bullhead Bed, Mockbeggar
Sample
BS CB CB LC LC LC LC LC OB OB WS WS WS TB TB TB TB TB TB TB TB BB BB
1 2 1 5 4 3 2 1 2 1 3 2 1 8 7 6 5 4 3 2 1 2 I
0.0 0.0 0.0 0.0 0.2 0.2 0.2 2.6 0.6 0.2 18.9 0.6 0.3 0.4 0.3 0.0 1.5 0.2 0.0 0.0 0.0 18.9 16.9
79.6 58.5 69.5 48.2 8.l 0.5 3.8 47.4 95.9 92.5 12.4 90.1 76.0 82.1 55.2 52.8 77.8 71.8 61.2 13.3 59.8 34.4 19.3
5.0 7.6 6.4 8.3 14.7 14.1 7.5 3.7 0.8 3.8 7.0 0.1 3.1 1.8 5.1 23.0 2.6 4.0 12.1 42.4 13.4 5.8 13.1
Silt 2 0 - 5 0 It
> 2 5 0 It
5 0 - 2 5 0 tt
Sand
PARTICLE-SIZE DISTRIBUTION OF PALAEOGENE SEDIMENTS OF NORTHEAST KENT
TABLE I
1.2 4.0 2.5 7.7 17.7 11.5 13.6 6.4 0.2 0.5 14.2 0.8 4.7 2.8 11.0 5.8 3.4 5.2 5.0 13.3 11.9 3.4 7.9
5 - 2 0 It
1.2 2.9 2.0 5.0 8.1 11.2 11.3 4.3 0.1 0.9 4.8 0.3 1.2 1.2 5.9 3.2 1.2 4.0 2.0 4.4 6.5 6.0 4.4
2 - 5 I~
I 1.2 26.4 19.7 29.2 53.1 62.5 65.4 34.7 1.7 2.1 43.4 7.9 14.7 10.9 23.1 16.2 15.0 16.2 19.7 26.6 8.4 31.8 38.9
< 2 ~t
Clay
98.2 99.4 100.1 98.4 101.9 100.0 101.8 99.1 99.3 100.0 100.7 99.8 100.0 99.2 100.6 101.0 101.5 101.4 100.0 100.0 100.0 100.3 100.5
Total
25
PALAEOGENE SEDIMENTS1N NORTHEASTKENT TABLE II .1AROSITE CONTENTS OF PALAEOGENE SEDIMENTS OF NORTHEAST KENT (~/o)
Sample
BB 2
TB 6
TB 8
WS 1
WS 2
CB ]
BS 1
Fine silt Medium silt Coarse silt Whole sample
13 4 0 0.9
6 4 0 0.4
61 19 3 1.3
22 4 4 0.6
5 1 <1 <0.1
2 0 0 <0.1
1 0 0 <0.1
(TB 8). In most samples it forms yellow, strongly birefringent aggregates, the individual crystals of which are < 3 # across, but the medium silt of TB 8 contains single crystals as large as 12/~ across (Plate IB). Most of these are thin plates with strongly developed pinacoids (0001) and weaker ditrigonal prismatic - - probably { 1120} and {2110} - - and bipyramidal { 1011 } faces, but a few have a rhombohedral outline because the bipyramidal faces are developed almost to the exclusion of the pinacoids. The amounts of sulphate and iron extracted by nitric acid from the fine silt of TB 8 are both equivalent to 61°/0 jarosite; the nitric acid extractable potassium is equivalent to 35.4% jaiosite and extractable sodium to 36.6°/0 natrojarosite. The composition of this extract therefore suggests that the jarosite is neither a pure potassium nor a pure sodium jarosite, and that a little potassium and/or sodium was extracted from minerals other than jarosite. Most of the medium silt fractions from the Thanet Beds are composed of quartz with minor amounts of feldspar, muscovite, glauconite and flint fragments. The amounts of heavy minerals are less than in the coarse silts, but the mineral suites are approximately the same as those of the coarse silt and fine sand fractions. X-ray diffractometer traces of all the medium silts indicate the presence of as much as 10°/0 montrnorillonite. This may occur part!y in aggregates of clay-sized particles similar to those in the fine silts. However, it probably also occurs partly in glauconite grains, because a diffractometer trace of disaggregated glauconite pellets picked by hand f r o m the fine sand of TB 1 showed that they contain approximately equal amounts of mica and montmorillonite. Clinoptilolite is an important constituent of medium and fine silts f r o m TABLE III CLINOPTILOLrrE CONTENTS OF PALAEOGENE SEDIMENTS OF NORTHEAST KENT (~o)
Sample
BB 2
TB 1
TB 2
TB 3
TB 7
Clay Fine silt Medium silt Coarse silt Whole sample
2 20 15 <1 2.4
5 35 57 3 9.9
<1 15 25 0 4.0
0 0 <1 0 < 0.1
<1 50 90 10 13.4 Sediment. Geol., 3 (1969) 17-33
26
A. H. WEIR AND J, A. CA-FT
WHITAKER'S (1872) division a of the Thanet Beds both at Pegwell Bay and at Upnor (Table liD, but it is not present in equivalent beds at Mockbeggar, approximately half way between Pegwell Bay and Upnor. It occurs mainly as colourless, anhedral grains and aggregates (Plate 1A), which have a mean refractive index near to 1.488 and birefringence < 0.005. However, a few euhedral platy crystats occur in the medium silts of TB 1 and TB 7. The plates are flattened parallel to {010}, and are either eight-sided (Plate 1A) or six-sided, probably indicating the development of {001 }, {201} and {201 } forms with or without { 100}. Many of the plates are probably not uniform in composition, because the birefringence (Nv -Nx) commonly increases from < 0.002 in central parts of the crystals to > 0.006 at the margins. Woolwich Beds The mineralogical composition of the marine Woolwich Beds at Bishopstone Point (WS 1) is exactly the same as that o f the underlying Thanet Beds at the same locality (TB 5); this emphasises the difficulty experienced by many workers of distinguishing the two deposits in the field. However, the non-marine Woolwich Beds at Upnor (WS 2, WS 3) contain a slightly different mineral assemblage. The sand and coarse silt fractions yield a more restricted suite of detrital heavy minerals, which is dominated by iron ores (main!y magnetite and limonite), zircon, tourmaline futile, staurolite and kyanite, and contains little or no epidote and garnet. Also the clay fractions contain much kaolinite, a mineral that is not present in either the Thanet Beds or the marine Woolwich Beds at Bishopstone Point. In these respects the Woolwich Beds at Upnor are similar to the Reading Beds (HODGSON et al., 1967), which are probably the lateral equivalent of the Wooiwich Beds in areas west of London. The Woolwich Beds kaolinite (Plate ID) is morphologically similar to kaolinite from the Reading Beds in that it forms small, subhedral (hexagonal), platy crystals. The detailed particle size distribution of sand fractions (Fig.2) also indicates that there is a closer relationship between the marine Woolwich Beds ot northeast Kent and the Thanet Beds than between the marine and non-marine Woolwich Beds. The two most sandy Thanet Beds samples (TB 5 and TB 8) both contain well-sorted sand with maxima at 3.125 tp (115 kt). The curve for the marine Woolwich Beds sample (WS 1) is almost the same as that for TB 5, but the sand from the non-marine Woolwich Beds (WS 2) is slightly coarser Imaximum at 2.875 q~, 140/~) and not quite so well-sorted. Some of the non-marine Woolwich Beds at Upnor contain large amounts of non-detrital material; for example, WS 3 contains 27% CaCO3 (mainly shells of Corbicula), organic matter equivalent to 3.3% organic C, and much sand-sized gypsum, pyrites and collophane.
Sediment. Geol.. 3 (1969) 17
27
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
0
Woolwich Sands 40
/" ',,
WSI • ........ •
Oldhaven Beds
T,a00~ B0d,
o
IWS2O
OB2 • . . . . .
•
,"
I TB8 : : TBS 0 - - - 4 3
/
l~\ i \~'I, ~!. \',
, ,/
& I \
/ ,:~i I, "~o I~.
"
, /
Wt
/!/
o,
/
/
/ \'
i '
/ I
i
,o? I
/
I
/ I
! ~/S
_
-~-
/ .~
O ~ _ o ~ . ~ ~ . i ~ . . U , . . ~ t ~ .
I
2"00
I
2"50
'"- '
V..
\
~,
!\
/
/
I! /
i;,,
\
/
;.."/
\
,~. ',, ',~:
,. ,". •
\
\
,;-t ',".5
\
,,' ;.
"~-Y
~ ,,
',~
/
I .-"~' ._~-"..C., m-
(".
/i ~ \ \ /i)! I , \\ \ k
i
!
?
v
i'~ ,I~
/;
/
,./
'
/\/t Ii\/
I :
r:
~ _
~
-_.,< :." "
\ ~_'~~_ ":.., ~
"
~
I
3.00
I
3'50
I
4.00
Fig. 2. Particle-size distribution of sand fractions ( > 50 #) from selected Palaeogene samples, based on weight percentages at ¼ ¢0 intervals.
Oldhaven Beds The Oldhaven Beds are composed mainly of sand that is mineralogically similar to the Thanet Beds sand, but slightly finer (Fig.2). The small amounts of coarse silt are characterised by an unusual abundance (9%) of zircon, and the clay fractions contain montmorillonite with small amounts of mica, kaolinite and (in OB 2) goethite. Many of the clay micas have lath-shaped outgrowths similar to those occurring in the Thanet Beds. At both Herne Bay and Shelford quarry the Oldhaven Beds yield small concretionary rosettes of sandy gypsum crystals. London Clay The London Clay consists of blue-grey and brown silty clays that lack appreciable quantities of sand except in the lowest 1-2 ft. (the London Clay Basement Bed) and highest 20-30 ft. The average composition of the clay fractions Sediment. Geol., 3 (1969) 17-33
28
A. H, WEIR AND J. A. ( A f f
is 60% montmorillonite, 25% mica, 5-10% kaolinite and 2-5% chlorite. However, the clay from the Basement Bed contains a little quartz but no chlorite; quartz also occurs in the clay fractions of samples LC 2 and LC 3, and tow-tridymite m LC 3. Almost all the montmorillonite in the Basement Bed occurs in lath-shaped particles. In LC 2 these are accompanied by shapeless aggregates of montmorillonite composed of very small platy particles, and at higher horizons the laths are progressively replaced by these aggregates. The detrital sand and coarse silt minerals in the lower parts of the LondoJa Clay (LC 1, LC 2) are similar to those in the Oldhaven and Thanet Beds, except for the addition of chlorite, muscovite and deep-green biotite. However, the heavy mineral assemblages in the higher beds of the London Clay are similar to those in the overlying Claygate and Bagshot Beds; zircon, tourmaline, kyanite and staurolite are less common than in the lower parts, whereas the amounts of garnet, epidote and amphiboles are greatly increased. The main non-detrital sand and silt minerals are glauconite, calcite, pyrites and at some horizons (LC 4, LC 5) siderite. The amount of organic C in the Basement Bed (0.1%) is less than in other parts of the London Clay (0,3-0.6%). The effects of weathering can often be traced to unusually great depths in the London Clay. Much of this weathering probably occurred during Tertiary or Early Quaternary times, because deep weathering profiles occur beneath thick covers of Quaternary deposits as well as at the surface. For example, the London Clay forming the overburden in Shelford sand quarry is weathered to a depth of approximately 20 ft., below 8 ft. of clayey and sandy river terrace gravels. In the field the most obvious effect of weathering of the clay is a colour change from dark grey (5Y 3/t of the Munsell Color Chart) to yellowish-brown (commonly 10 YR hues). The most strongly weathered horizons (e.g,, the highest 6-8 ft. of the London Clay at Shelford) are brown throughout, and contain many small selenite crystals and nodular segregations ofjarosite. Below these layers there is a variable thickness of clay (12 ft. at Shelford), in which the brown colour is restricted to parts of the sediment adjacent to structural faces; jarosite is less common in these horizons, and the selenite forms large, twinned crystals. Selenite crystals also occur in even lower horizons, where the clay is grey throughout. Amounts of jar osite in the weathered London Clay are not given in Table II, because it occurs so irregularly that percentages cannot be accurately estimated. The deep-green biotite in the sand and coarse silt fractions is also altered in weathered (brown) parts of the London Clay. The altered mica is pale yellow-brown, and the basal sections shown by cleavage flakes are slightly more pleochroic and have greater birefringence than those of the green, unaltered biotite. The optic axial angle is also slightly greater ir~ the weathered form than the unweathered. Most of the altered flakes are almost: uniformly yellow-brown throughout, but a few are only partly weathered and show either irregular brown patches or well-marked brown, strongly birefringent margins around green unaltered cores. Sediment. Geol., ~ ~1969) 17-3~
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
29
Claygate and Bagshot Beds The fine sand and coarse silt fractions from these beds contain 70-85°,/o quartz, 9-13% feldspar and small amounts of glauconite, muscovite and flint fragments. The heavy minerals are mainly iron ores (limonite, haematite, magnetite and ilmenite) with subsidiary epidote, clinozoisite, garnet, hornblende, zircon and rutile, and rare tourmaline, chlorite, anatase, apatite, kyanite and staurolite. The clay fractions contain 80-90~o lath-shaped montmorillonite and small amounts of mica (with lath-shaped overgrowths as in the Thanet Beds) and kaolinite. The Bagshot Beds contain more sand than the Claygate Beds, but cannot be distinguished from them on other mineralogical grounds. DISCUSSION AND CONCLUSIONS
The differences in mineralogical composition between detritalcomponents from the major subdivisions of the Palaeogene sediments in northeast Kent suggest that the detritus was derived from at least three sources. A small proportion represents the washed and sorted insoluble residue of the Chalk; this is mainly flint fragments, but may also include some of the montmorillonite and mica in the clay and fine silt fractions. It is most abundant in the oldest Thanet Beds, but small amounts of flint occur in all the sediments studied. The Bull head Bed contains some material (e.g., unworn flint nodules, collophane and clay-sized apatite euhedra) that was possibly derived by dissolution of the immediately subjacent Chalk, and was not transported, sorted or intensely weathered. However, this bed is not an uncontaminated Chalk residuum, because it also contains much detrital and nondetrital material that is mineralogicaUy similar to higher parts of the Thanet Beds. If all the nodules and coarse fragments ( > 250/~) of flint are excluded from the Bullhead Bed, the remaining matrix contains much more clay than any other part of the Thanet Beds. This excess clay may be derived by dissolution of the subjacent Chalk, the insoluble residues of which probably contain much more clay than silt or fine sartd (WEIR and CATT, 1965) ; alternatively it may be an illuvial accumulation of clay washed down from higher horizons of the Thanet Beds. However, most of the detritus in the Palaeogene beds was derived from sources other than the Chalk, because flint, which is by far the most important constituent of insoluble residues from the Upper Chalk of southeast England, comprises < 5% of the detritus in all the Palaeogene sediments except the lowest Thanet Beds. Throughout most of the Palaeogene the shoreline, particularly to the west (CURRY, 1966), lay well within the areas originally covered by a great thickness of Chalk. The lack of abundant Chalk-derived detritus throughout the succession therefore suggests that the Chalk cover had been largely removed from land areas adjacent to the Palaeogene sea before the Thanetian transgression. The provenance of the detritus not derived from the Chalk cannot be inferred with certainty, but the mineralogical composition of the sand and coarse silt Sediment. Geol., 3 (1969) 17-33
30
A.H.
WEIR
A N D J. A. C A I T
fractions suggests that during the Palaeogene at least two types of source rocks other than the Chalk were involved. There are no significant mineralogical changes in the lower parts of the succession in northeast Kent (the Thanet, Woolwich and Oldhaven Beds and lower London Clay), but in areas closer to London the non-marine Woolwich Beds and highest parts of the succession (upper London Clay, Claygate and Bagshot Beds) both contain more restricted detrital mineral suites than that typical of northeast Kent. The non-marine Woolwich Beds assemblage is clearly related to that of the Reading Beds, and is largely composed of minerals derived ultimately from granites (zircon, tourmaline, rutile and kaolinite) and metamorphic rocks (staurolite, kyanite) similar to those in the Armorica~ massifs lying to the south and west. In contrast, the upper parts of the London Clay and the Claygate and Bagshot Beds are mainly composed of material derived from metamorphic rocks bearing garnet and minerals of the amphibole and epidote groups. The detritus of the Thanet Beds, marine Woolwich Beds, Oldhaven Beds and lower parts of the London Clay probably contains material from both garnetepidote-amphibolite metamorphics and Armorican sources. Two main morphological types of montmorillonite occur in the Palaeogene deposits. The sandier beds contain only laths and aggregates of laths; these also occur in the lower parts of the London Clay, but are largely replaced by anhedral flakes in higher parts of the London Clay. This difference indicates either that the montmorillonite was derived from two sources or that the laths formed in the sandy, well-drained beds by the action of interpore solutions on detrital clays. The Upper Chalk is a possible source of detrital montmorillonite laths, and does contain stellate aggregates similar to those in the Palaeogene. However, too little is known at present either of the composition and morphology of clays in sediments older than the Chalk or of the changes clays suffer during transportation, sedimentation and diagenesis, to eliminate other sources of montmoriUonite or the possibility of its diagenetic recrystallisation in the sandy deposits. The composition of non-detrital components in the sediments indicates to some extent the conditions in which the detritus was deposited. Palaeontologica] evidence shows that the Thanet Beds, Woolwich Beds in northeast Kent, Oldhaven Beds and London Clay are all marine deposits (CURRY, 1965), and this is confirmed by the glauconite they contain. Glauconite is absent in the Woolwich Beds at Upnor, but does occur in the unfossiliferous Claygate and Bagshot Beds. The only non-marine deposits in the Palaeogene east of the Medway valley are therefore the Woolwich Beds of the Medway area. Parts of the Thanet Beds (WRITAKER'S, 1872, division d, represented by TB 4), some of the non-marine Woolwich Beds (WS 3) and most of the London Clay (LC 2-LC 5) were probably deposited in reducing conditions, because they contain pyrites, siderite and more organic matter than other beds in the Palaeogene. The clay fractions in these reduced sediments are similar to clays in the other (oxidised) sediments, except that, apart from the non-marine sample WS 3, they all contain a little chlorite, This Sediment. Geol., 3 ( t 9 6 9 ) t Z . '. ~.
PALAEOGENE SEDIMENTS IN NORTHEAST KENT
31
suggests that the clay-sized chlorite is not truly detrital in origin, but was formed in the sediment during or after deposition as a result of the reducing conditions. The clinoptilolite in the lowest Thanet Beds at Pegwell Bay and Upnor is probably not detrital in origin, because it commonly forms fresh, euhedral crystals, and because in a few feet of beds it is unusually abundant for such a rarely observed mineral. Zeolites of the clinoptilolite-heulandite group often form by alteration of glassy and other volcanic materials; for example, in the folded volcanic tufts and claystones of the John Day Formation of Oregon clinoptilolite was probably formed at a depth of burial between 1,250 and 4,000 ft. from alkaline solutions bearing silica and alkali metals derived from volcanic glass (HAY, 1963). However, the Thanet Beds are unusual (but not unique) among clinoptilolite-bearing sediments in that they contain no recognisable volcanic materials; HAY (1966, pp.22-30) listed only four definite identifications of clinoptilolite-heulandite in marine sediments not containing abundant volcanic materials. The nearest known Tertiary volcanic centres of the northern Atlantic (Thulean) province were at least 600 km from eastern Kent. Ash might have been blown this distance, and after deposition might have been completely altered, so as to leave no recognisable trace of volcanic material. However, the silica and alkalis needed for post-depositional zeolite formation might alternatively have been derived from non-volcanic detrital components of the sediment. In the John Day Formation clinoptilolite is accompanied by several other authigenic minerals, including mica (celadonite) and opaline silica, and somewhat similar minerals of probable authigenic origin also occur in the marine Palaeogene deposits, particularly the Thanet Beds. First, the laths of layer silicates on detrital mica flakes in clays from the Thanet, Oldhaven and Bagshot Beds indicate possible post-depositional growth of mica. Second, opal does not occur in any of the deposits, but some parts of the Thanet Beds contain low-temperature tridymite, which probably also formed after deposition from opaline silica or solutions containing silica. However, in other ways the authigenic mineral suite of the Thanet Beds is not strictly comparable with that of the John Day Formation. In particular, the John Day Formation contains some authigenic minerals (montmorillonite, orthoclase), which are probably only detrital components of the Thanet Beds; also, whereas celadonite is restricted to clinoptilolite-bearing horizons of the John Day Formation, the layer silicate overgrowths occur in many parts of the Palaeogene succession, not containing authigenic zeolites. Nevertheless, solutions containing silica and alkali metals probably affected most of the permeable Palaeogene sediments at some time after deposition. The widespread activity of such solutions suggests that the silica and alkalis were leached from common detrital minerals rather than derived by alteration of volcanic glass, because the nearest volcanic centres were too distant to provide an almost continuous supply of wind-b!own ash throughout the Palaeogene. Therefore, we cannot disprove that the clinoptilolite formed by alteration of volcanic material, but suspect that solutions conSediment. Geol., 3 (1969) 17-33
32
A . H . WEIR AND .I.A. ('ATi
taining alkalis and silica derived from common detrital minerals provided ~ geochemical environment in some ways resembling that of the John Day Formation. The detrital fractions of the Kentish Palaeogene deposits are by ~, means mineralogically unusual, and if the clinoptilolite did form by dissolution of common detrital minerals, why is it not recorded more frequently in similar beds? In the Thanet Beds clinoptilolite is mainly confined to size fractions (fine and medium silts) not usually analysed in conventional mineralogical studies of sandy sediments, and this may explain its apparent rarity. During weathering of the London Clay the main processes causing changes in its mineralogical composition were oxidation of pyrites and leaching by the sulphuric acid produced during this oxidation. The colour change from grey to. brown was caused by the formation of iron oxides from the pyrites, and the gypsum was formed by reaction between sulphuric acid and calcium carbonate in the clay. Changes in the green biotite flakes might also have been caused by sulphuric acid leaching. RIMSAITE (1967) showed that natural weathering of biotites involving removal of potassium and oxidation of iron causes similar changes in colour and other optical properties to those seen in the London Clay biotite. Jarosite forming nodular concretions in the weathered London Clay probably crystallised from solutions carrying sulphate ions, potassium leached from the biotite (and possibly other layer silicate minerals), and iron derived by acid dissolution of oxides or siderite. Sulphate-rich solutions descending from the weathered clay also might have caused formation of gypsum crystals in the underlying Oldhaven Beds. ACKNOWLEDGEMENTS
We thank Mr. R. D. Green for help in collecting samples, Mr. R. D. Woods for electron micrographs, Mrs. M. Thomas for optical micrographs and Mr. E. M. Thomson for drawing the diagrams. REFERENCES BAKER, H. A., 1920. Quartzite pebbles of the Oldhaven Beds of the southern part of the L o n d o n Basin. Geol. Mag., 57:62-70. BEUTELSPACHER, H. and VAN DER MAREL, H. W., 1968. Atlas of Electron Microscopy of Clal Minerals and their Admixtures. Elsevier, Amsterdam, 333 pp. BOSWELL, P. G. H., 1915. The stratigraphy and petrology of the Lower Eocene deposits of the north-eastern part of the L o n d o n Basin; Quart. J. Geol. Soc. London, 71 : 536-591. BOSWELL, P. G. H., 1923. The petrography of the Cretaceous a n d Tertiary outliers of the west of England. Quart. J. Geol. Soc. London, 79:205-230. BROWN, G., 1961. Other minerals. In: G. BROWS (Editor), The X-Ray Identification and Crystal Structures of Clay Minerals. Mineral. Society, London, pp. 467-488. CURRY, D., 1965. The Palaeogene beds of southeast England. Proc. Geologists' Assoc. Engl, 76:151-173. CURRY, D., 1966. Problems of correlation in the Anglo-Paris-Belgian Basin. Proc. Geologists' Assoc. Engl., 77:437-467.
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PALAEOGENE SEDIMENTS IN NORTHEAST KENT
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