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The petrography of the Spilsby Sandstone
P l{ 0 C E E DIN G S o ~·
THE
GEOLOGISTS' ASSOCIATION. THE
PETROGRAPHY OF THE SANDSTONE. By F. T.
I:-;G HA ~ I .
Pn .D. ,
SPILSBY
x .n.c.s.
[Ilead [ anu arv 6th, ' 'J28 .] (Received M arch 30th, 1927.]
I.
INTRODUCTION.
THE
Spilsby Sandstone forms the basement bed of th e Lin colnshire Wolds. The hill s, although only attaining a maximum height of ab out 500 ft. , with an average of a ppro ximately 3 0 0 ft., form a pronou nced feature, sin ce t o the west of this esca rpme n t lies the broad flat plain of the Kimmerid ge and Oxford Clays. at a n average lev el of 1 0 0 feet. Numerous referen ces in geological lit erature on th e beds un derl yin g the Chalk of this a rea have b een made, but no acco un ts of a ny detailed petrological examinat ion of th em h ave been publish ed . * An excellen t description of these st ra ta has been given by P rofessor Judd (7.t p . 227] and the st ra tigraphical relationship of the Spilsby Sa nds tone an d the succeeding beds, with th ose of simila r age in ot he r localiti es, wa s discussed by him [8 , p. 326). The a rea h as also been dealt with in det ail in th e correspon ding Mem oirs of th e Geological Survey [23, p. 82; 9. p. 13 ; 24, p. I06]. Previous to th ese Survey publications a description of several ex pos ures had been furnish ed by Keepin g [11 , p. 239] . and later important work on thi s a rea was ca rr ied out by Lamplugh [17. pp. 18T a nd 455 ; 12 p. 179J. who correlated these b eds with their E uro pea n equivalents in minute detail. Briefly, the successio n of beds in thi s di strict is as follows: Cha lk. Red Chalk. Carstone. Tealbv Limest one. Tealby Clay. Claxby Ironston e. Spilsby Sandst one. Kimmeridge Clay , ., Since the abo ve was written the following pa per ha s appe ared Verry an d Car ter , Ycr ksh. G.oJ. Soc. t Fo r list of references see p. I C•• PROC. GEOL.
Ass oc. ,
V OL .
XL.,
PART 1. 1929.
z
F. T. INGHAM,
The Spilsby Sandstone succeeds the Kimmeridge Clay, unconformably, since the upper portions of the clay are overlapped towards the north; fragments which are believed by some authorities to be derived Kimmeridge fossils have been found at the base of the sandstone, The outcrop of the Spilsby Sandstone extends in a south-south-easterly direction from Elsham to East Keal, a direct distance apart of about 38 miles, but in the southern portion the outcrop widens owing to the topography and the low dip of the bed (approximately 1°), and is divided into two portions by the valley of the Steeping River. I The thickness of the sandstone varies from nearly 50 feet in the south to about 10 feet in the north. A boring at Skegness indicates that this bed thins also towards the south-east (19 feet being recorded), the strata being apparently a shore-deposit. The Spilsby Sandstone is normally succeeded by the Tealby Series, but in the northern portion, owing to the Cretaceous overlap, it is overlaid by the Chalk. The exposures, unfortunately, are neither so numerous nor so good as might be expected from the large outcrop. This is partially due to the covering of the sandstone and the succeeding beds by Glacial Drift in several areas; moreover, many of the exposures described by the Geological Survey are now overgrown. Nearly forty samples were collected personally from most exposures, unless these occurred in close proximity. At the commencement of the work samples were taken at various horizons in the same exposure, but the results of examination of these did not justify a continuation of this procedure. Some of the loose sands showed contamination due to land slide and wind action; one is composed almost entirely of glacial material (p. II). II.
PHYSICAL AND MICROSCOPICAL CHARACTERS.
In its unweathered form the Spilsby Sandstone is best described as a grey calcareous grit. It frequently contains fossil remains, Pecten being especially common. The majority of the sand grains (which are predominantly quartz) are fairly angular and moderately coarse in grade. Pebbles are not of infrequent occurrence, but they tend to be segregated in particular bands, often where current-bedding is exhibited, and are not usually distributed evenly throughout the strata. The percentage of calcium carbonate present in normal specimens was found to vary from about 23% (West Keal) to about 37% (Elsham) by weight. These percentages would correspond approximately to from 22'10 to 36% by volume. Professor V. C. HUng kindly calculated the pore space from the mechanical analysis of the Elsham sample, the result obtained being 32% t? 33%. Th~re wo~ld thus be more than enough of this matenal to fill the interstices of the sandstone should the
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
3
grains of quartz be in contact. The calcite matrix, since no signs of later expansion are present, is, therefore, evidently primary and was deposited contemporaneously with the quartz. By the action of weathering the whole or the greater portion of the calcite cement has been removed, the majority of the exposures now being composed of a friable sandstone or loose sand. usually greenish-yellow in colour, but grey and white varieties are also common. In some localities the sandstone is iron-stained, a deep reddish-brown colour resulting. A few typical samples were graded mechanically. A large proportion of the material consists of medium sand (0.25 mm. to 0.5 mm. diameter). Silt and clay grades, although of fair abundance in the succeeding bed (the Claxby Ironstone), are usually absent in unweathered specimens, but occasionally a little of these materials occurs in thin layers. Occasional grains of heavy residue minerals attain a larger size than is usual in sands, grains with a length of over I mm. having been observed. Thin sections of the hard unweathered rock show that the sandstone consists chiefly of quartz grains set in a matrix of calcite; grains of glauconite and of phosphatic material are much less abundant. Occasionally fragments of felspar are observable, the species represented being orthoclase, microcline, and albite. The quartz grains exhibit much variation in size and form. Although the majority are subangular, both well rounded and sharply angular types occur. The calcite matrix is composed of fairly large crystals of different optical orientation with rather irregular boundaries. Even though each of these crystals usually encloses several grains of quartz, "lustre mottling," though occasionally seen, is not usually exhibited. Calcite is also frequently present as organic remains and very rarely forms minute secondary veins traversing the sandstone. In strongly iron-stained specimens, the quartz grains are coated and cemented by limonite, which has probably originated through the replacement of the calcite by siderite, through the action of percolating waters rich in iron and the subsequent oxidation and hydration of this carbonate. The original source of the iron is presumably the overlying Claxby Ironstone, and it is interesting to note that the ferruginous varieties of the Spilsby Sandstone appear to be more frequent in the more northern part of the area where the Ironstone is better developed. The majority of the pebbles occurring in the Spilsby Sandstone are usually described under the name of "lydites," but this general term is more particularly applied to the darker coloured varieties. Most of the pebbles are well rounded and often exhibit a good polish. Generally they do not attain a size larger than 15 mms. Variation in colour is shown, the greater
F. T. INGHAM,
4
proportion being black or dark brown; some have, however, a reddish tinge and others vary from grey to almost white. Pebbles of different colours and appearance were selected and sectioned, practically all proving to be normal cherts. The usual aggregate polarisation of chalcedonic chert was observed, although much variation in texture was disclosed, in some cases even in the same pebble; the majority showed a very fine texture. The variation in colour was found to be largely due to staining by different forms of iron oxide, and the discolouration is more pronounced on the outer portions of the pebbles. Two pebbles exhibited an interesting feature in the presence of minute sections, many of which were diamond-shaped in a matrix of cryptocrystalline chalcedony. The shape of these inclusions is highly suggestive of the rhombic sections of dolomite, but they are composed of silica and the rock would appear to have been formed by the silicification of a dolomitic or magnesian limestone. On the border of the pebble the rhombic shapes arc more clearly marked since their boundaries consist of a black material, which has the appearance of oxide of iron. This suggests that partial replacement of dolomite by siderite took place on the outer portion of these pebbles before complete silicification had taken place. Pebbles of vein-quartz are of frequent occurrence and these exhibit no features of special interest. Phosphatic pebbles and fragments are of wide-spread distribution, although more plentiful in the lower portion of the deposit. In appearance they are almost black, and devoid of lustre. Thin sections show that they consist of a brown isotropic medium, in which grains of quartz occur. Occasionally glauconite and chert fragments are included in them. This phosphatic material is very similar to that of some of the phosphorite grains in the Ludlow Bone bed, and-except as regards size-the coprolites of the Cambridge Greensand. It is probably the rolled excrement of fishes, and, in some cases, possibly, reptiles. In a few instances bone structure has been observed. III.
MINERAL
CONSTITUENTS: THEIR OCCURRENCE AND CHARACTER. Under this heading, material which would pass through a 3o-mesh sieve is described. The samples, after preliminary cleaning, where necessary, with dilute hydrochloric acid, were treated with bromoform. The heavy residues thus obtained were separated into three or four portions by the electromagnet. The weight of the heavy residue compared with that of the light fraction shows much variation. This is, however, largely due to· the glauconite present, a varying proportion of which often sank with the heavy residue. In some cases further separation by methylene iodide was rendered necessary. The average heavy residue, after making allowance for the glauconite, is estimated to be approximately 0.05%.
PROC. GEOL.
Asso c.
A .-
VOL.
XL.
PLATE 1.
(1 9 2 9) '
S P ILS B Y SANDSTONE. T E ALB Y , LI N CS .
To show q uartz grains ce mented b y second a ry Limon ite [O .L. X 30 .1
, "1',
'1:'. ....._1
]. 1
;~ ~ ':e>-~ 1
FIG . 2 .
SPILSBY S AN DS T ONE, E L SH AM,
To show secondary calcite veins t raversing rock.
LIN CS .
[O .L. X 30. } Ph_its by F .T .I. [To f ace p. 4.
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
5
The minerals identified are the following:CUBIC. Gamet, Magnetite, Pleonaste, Sphalerite. TETRAGONAL. Anatase, Rutile, Zircon. HEXAGONAL AND RHOMBOHEDRAL. Apatite, Calcite, Ilmenite, Quartz, Tourmaline. ORTHORHOMBIC. Andalusite, Brookite, Hypersthene, Staurolite. MONOCLINIC. Biotite, Chlorite, Epidote, Hornblende, Monazite, Orthoclase. TRICLINIC. Albite, Kyanite, Microcline. AUTHIGENIC MINERALS AND AGGREGATES. Chert, Glauconite, Limonite, Phosphatic material. The following notes describe the chief features of these minerals, those of the light residues (S.G.-z.8) being described first, and the minerals of each portion being considered in the order mentioned in the above table. Qua r t z. This mineral, which forms the bulk of the light residue, is usually subangular or well-rounded in form; sharply angular grains do, however, occasionally occur. Generally, the grains are clear, but often a thin film of iron staining is present, and, less commonly, staining with glauconitic material is exhibited before treatment with acid. The majority of grains contain minute inclusions, often in the form of indeterminable black dust, which occurs both irregularly and arranged in rows or streams. Such types are stated to be characteristic of the quartz of gneisses and the younger schistose rocks [14, p. 148J. Occasional inclusions of zircon, chlorite, tourmaline, and rutile are present, and in a few isolated cases crystals of the latter mineral are comparatively large. Many of the quartz grains, especially the larger ones, show undulose extinction or "mosaic" structure between crossed nicols. Or tho c l a s e is of fairly frequent occurrence in the light residue and is the most abundant felspar. The grains are usually of a platy character with the edges well rounded. .Occasional cleavage fragments are present. The greater portion of the mineral is distinctly turbid, a cloudiness due to partial alteration to kaolin being present. Such alteration is more frequent in Orthoclase than in the other species of felspar occurring in the sandstone. M i c roc lin e is the next species in order of abundance and is present in all samples. It is usually well rounded and is characterised by its fresh and clear appearance. A I bit e. This species appears to be the only variety of plagioclase present, since all the grains exhibiting lamellar twinning have a refractive index below that of Canada balsam. Many fragments are platy owing to fracture along cleavages, but the edges are usually rounded. The majority have a fresh appear-
6
F. T. INGHAM,
ance, although occasionally a superficial cloudiness has been noted. Chert fragments are of frequent occurrence, but these exhibit no features of special interest. G I a u con i t e, in the loose sands, is next in order of abundance to quartz. The majority of the grains show the usual form of foraminiferal casts, re-entrant angles being frequently exhibited, although many have a rounded appearance. The larger grains usually only transmit light on their borders, and occasionally the green colour can only be observed even on the edges when convergent light is used. Gar net is characteristic in all samples; although common in the majority, it shows a rather wide range in its relative proportions to the mineral assemblage. At least two distinct types are recognisable: one is of a distinct pink colour; the other, practically colourless. In some of the strongly coloured grains a mauve tinge is observable. It is difficult to state whether these constitute a third variety or whether the slight colour difference is a function of the thickness. All the garnet is fairly magnetic, and both the coloured and colourless crystals are obtained together in the same magnetic separation. The grains are usually of moderate size, but one grain having a length of 1.1 mm. has been observed. Irregularity of form, often with marked conchoidal fracture, is usually shown; no euhedral crystals have been observed. Frequently strong pitting is observable on the surface due to the poor development of cleavage. A few grains, especially of the colourless type, from the same reason, present an almost square outline. These, from their shape, might be mistaken for ruby spinel, but their magnetic properties assist in their identification. Inclusions are fairly frequent, chloritic material being common. Minute cavities having a rhombic section are frequent in some grains. Occasionally optical anomalies are shown. 1\1 a g net i t e. This mineral is present in all samples, but it is always subordinate in amount to ilmenite. The grains are generally subangular. By reflected light they are black and generally lacking in lustre. Occasionally rounded octahedra occur. Many of the fragments show a superficial alteration to limonite. S P h a I e r it e is extremely rare, having been observed as isolated grains in only a few samples. The grains are yellow in colour, exhibit a high refractive index, and are isotropic. Irregularity in form, the result of its perfect dodecahedral cleavage, assists in the identification of the mineral. It is similar in appearance to that of the Middle Lias of Yorkshire [21, p. 75]. S pin e I (Pleonaste) is of rare occurrence, but occasional grains are present in several samples. These show the characteristic blue-green colour and isotropism, and are usually well
THE PETROGRAPHY OF THE SPILSBY SA:-.lDSTONE.
7
rounded. The mineral is of similar type to that recorded by Professor Boswell [2, p. 246J and Dr. Neaverson [16, p. 240J, from the Inferior Oolite and the Portland Sand respectively, but it attains larger dimensions (1.40 mm. diameter). A nat a s e is one of the scarcer constituents of the heavy residue, but occurs in small quantities in nearly all samples. The grains frequently show euhedral crystals of tabular habit with edges bevelled by the pyramid, the average size being about 0.15 mm. square. Crystals of pyramidal habit are extremely rare, and these present some indications of rounding. Rounded and irregular grains are as frequent as euhedral crystals, and such types attain a size of about 0.3 mm. It is probable that the round grains are detrital and not authigenic. The colour is rather variable, ranging from almost colourless to shades of yellow and blue. Occasionally grains which are parti-coloured occur, a mixture of yellow and blue being the most common. Rut i I e is abundant in nearly all samples, and the grains. although varying in size, are on the whole fairly large (up to 0.56 mm.). The colour, as usual, varies from deep brown and semi-opaque through fox-red to orange and yellow. Well crystallised stumpy prisms, acicular crystals, geniculate twins, arrow-head twins, and lamellar twins are all represented. Irregular and rounded grains as well as crystal aggregates are of common occurrence. Distinct pleochroism is usually exhibited, and traces of the cleavages, especially that parallel to [IIIJ, are often shown. Some of the rutile appears to be ferriferous since it is affected by the electro-magnet. Z i rca n occurs in all samples in fair abundance, and exhibits great variation in size and shade of colour. The grains can conveniently be grouped into three main types. The commonest of these is colourless and water clear. Occasional fine euhedrons occur. One of the most interesting grains, from a mineralogical standpoint, is an excellent geniculate twin. Both stumpy and acicular prismatic types (to o.ag mm.) are present, but the majority show varying degrees of rounding. The second type, occasionally colourless, is usually slightly grey or yellow and is frequently dusky with inclusions. It is characterised by zoning, especially in the centre of the crystal. This variety practically always shows prismatic shape, both stumpy and needle-like crystals terminated by pyramids being present. Fractured crystals are present, and slight rounding of the edges, especially at the terminals, is common. The third type, although the rarest, is persistent and is distinguished by its purple colour. The depth of colour is variable, but in the majority of cases is quite pronounced, the pleochroism in shades of the body colour being distinct. Although traces of prism and pyramid faces are occasionally seen, no euhedral
8
F. T. INGHAM,
crystals have been observed; the majority of these grains show pronounced rounding. The greater number are of small size (0.15 mm.) but some attain a length of 0.35 mm. A pat i t e is of rare occurrence in the samples studied. This may be due to some extent to the fact that the majority of these were treated with hydrochloric acid before examination and the mineral was possibly dissolved. In natural samples, however, it is usually absent, and, where present, is always comparatively rare. The grains are colourless to grey, usually small (about 0.1 mm.) and occur as well-rounded grains or more rarely partially eroded prisms. I I men i t e, with its alteration product, leucoxene, is the dominant mineral of the heavy residues in all samples. It is usually lustrous, black, and of irregular appearance. Many of the grains are largely altered to leucoxene and several exhibit secondary growths of minute crystals of rutile and anatase. To u r m a lin e is one of the more abundant heavy minerals and occurs in a variety of colours and forms. The colours represented include grey, blue, purple, green, brown and yellow, and many gradations of shades between these are present. Rarely, parti-coloured grains occur and colour-zoning is occasionally exhibited, blue with brown being the most common. A very deep blue variety of comparative rarity usually yields a perfect uniaxial interference figure. This property has been noted in a similar type by Dr. Neaverson [16, p. 249]. One colour variety worthy of note is tinted a pale pink and shows the peculiar pleochroism from red to brownish orange. This type, usually wellrounded, although rarer than most shades, is of constant occurrence. The majority of the tourmaline shows normal pleochroism and absorption. The form exhibited by this mineral varies from prismatic crystals (to 0-4 mm.), frequently showing rhombohedral terminations but often fractured, to almost perfect spheres. Varieties exhibiting this extreme rounding have probably survived previous cycles of deposition, but their size (to 0.3 mm.) is comparatively large. Some of the more irregularly shaped grains attain a size of 0.65 mm. And a Ius i t e is one of the rarest minerals of the sandstone, two or three grains only having been identified. In the case of one grain of rounded appearance, which exhibited pleochroism from green to pale red, recourse to refractive index liquids had to be made in order to differentiate it from hypersthene. B roo kit e is the least common oxide of titanium present. It usually occurs as irregular platy fragments of a greenish yellow colour. On" grain of euhedral shape has been observed and occasional.y irregular forms of dirty appearance are present. In the platy fragments striations parallel to the vertical axis can often be seen in reflected light. Owing to the strong axial dis,
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
9
persion the grains give no definite extinction between crossed nicols, the variation in colour on rotation being striking [4, p. 20J. The anomalous interference figure is given by the platy grains in ordinary light. R y per s the n e. This mineral is a rare accessory, but isolated grains are present in a few samples. The fragments are tinted reddish brown and display the characteristic pleochroism (brownish red X, green Z). Cleavage is generally shown and iron oxide inclusions are common. S tau r 0 lit e. This is one of the most abundant and characteristic minerals of the heavy residue. The grains show much variation in size and shape. Some of these have a maximum length of about 0.74 mm. ; the majority are not usually more than 0.45 mm., and from this they vary to minute frrgments. As regards form, a small number are euhedral, many show traces of crystal boundaries, and, being often basal, give a good biaxial interference figure. The majority of the grains are irregular and angular, but very occasionally rounding to a pronounced degree is observable. Fragments which exhibit ragged edges, due to the cleavage parallel to (no) being well-marked (possibly developed in the course of attrition), are of common occurrence. A rarer variety is one showing a distinct cloudiness due to minute black inclusions. These inclusions, generally occurring in irregular or rounded grains, sometimes show a tendency, more or less marked, to be regularly arranged. A grain of this type (PI. 3, row 3) exhibits a symmetrical arrangement similar to that observable in chiastolite. It is believed that such a variety has not previously been recorded from British sediments. The colour shows a range from brown to orange and yellow, and in minute flakes the colour is hardly perceptible. Pleochroism is well marked in the majority of grains. B i 0 tit e has been observed in one sample only, where it occurs as a basal cleavage flake yielding a pseudo-uniaxial interference figure. Chlo r it e is one of the rarest constituents of the Spilsby Sandstone, having been noted in only a few slides. There it occurs infrequently in flakes of a pale bluish-green colour. The birefringence is low and the fragments yield a biaxial interference figure. E p i dot e. Grains of this mineral are rare and usually show pronounced rounding. The colour varies from almost colourless to yellow, the latter often exhibiting a greenish tinge. The size, compared with that of the majority of the other minerals present, is small, the average diameter being below 0.1 mm. Some of the grains show the typical figure with one optic axis emergent, others a partial biaxial figure with large optic axial angle. R a r n b len d e is of rarest occurrence, only very few grains having been identified. One grain showing pleochroism from
F. T. INGHAM,
10
yellow-green to blue-green bears great resemblance to arfvedsonite. Although such alkali types are characteristic of pollution (IV.) no evidence in support of contamination in the sample could be found. It was, therefore, concluded that the species is a rare accessory of the sandstone. As the mineral has been observed by Dr. Rastall [19, p. 24] in the Carstone of Norfolk, which is believed [Strahan, 22, P.486J to consist of the material of the Spilsby Sandstone and other Neocomian beds, such a conclusion does not appear to be unlikely. M 0 n a zit e. This mineral is very difficult to distinguish from epidote in the heavy residues, owing to the grains being small, of the same colour and practically always rounded, no marked difference of refractive index and birefringence being observable. One grain, however, shows a perfect biaxial figure. with small optic axial angle, of positive sign. The grain yielded weak absorption bands when tested with a spectroscopic eyepiece attachment. NORTH.
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Garnet Magnetite Pleonaste .. Sphalerite .. Anatase Rutile Zircon Apatite Ilmenite Tourmaline Andalusite Brookite .. Hypersthene Staurolite Biotite Chlorite Epidote Hornblende Monazite Kyanite ..
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7 7 7 7 7 7 7 6 6 7 5 FIG. 3· TABLE SHOWING DISTRIBUTION OF HEAVY MINERALS IN TYPICAL LOCALITIES. very scarce. rare. exceedingly rare 2 3 very frequent frequent. 6 scarce. 5 4 dominant. very abundant 8 abundant. 9 7 ultra dominant. 10 7
7
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8
TIlE PETROGRAPHY OF THE SPILSBY
SAKDSTO~E.
II
K y ani t e is one of the most abundant and characteristic minerals of the heavy residue. It is common in all samples, usually exhibiting a pronounced platy habit, the crystals lying on the (100) cleavage plane and yielding a good biaxial figure of negative sign. Frequently the cleavage parallel to (010) is developed, grains showing cleavage traces and resultant rectangular forms being common. The parting parallel to (oor) is also occasionally present. Rarely there occur grains which show a pronounced curvature. Although of infrequent occurrence, this type is of such a distinctive nature that the precise source may ultimately be traced. The average size of kyanite grains are larger than that of tll{; other accessories present, a length of from 0.5 mm. to over La mm. often being shown. The mineral is usually colourless, but rarely a blue colour is exhibited, this being generally of a patchy nature. Occasionally grains occur having a grey appearance due to very minute inclusions. When the residue has not been treated with acid, or when the treatment has not been prolonged, a marked green staining is often shown. This staining, varying considerably in intensity and in extent, appears to be due to glauconitic material. The general mineral assemblage shows no marked variation either laterally or vertically throughout the bed. The distribution of the heavy minerals can best be realised by reference to table p. ro. IV.
CONTAMINATION FROM RESULTS OF GLACIATION AND GRAVITY.
It may have been noticed that samples collected from several localities have not been mentioned in the previous account. The reason for this is that many of the loose sands examined showed signs of contamination, and only specimens of compact sandstones, or sands of recognisable purity, have been described. The contamination is due to a variety of causes, the principal being the effects of glaciation, but in some cases" wash" from the succeeding beds, wind action, and landslips have played a considerable part. The degree of pollution naturally varies to a great extent, and it is sometimes extremely difficult, where only a few grains are questionable, to decide whether to consider them' as original materials of the sandstone or to regard them as proofs of slight contamination. No doubt, however, exists when the percentage proportion of heavy residue is markedly in excess of normal samples. A sample from ncar Caskill's Farm, Barnetby, on the mapped outcrop of the Spilsby Sandstone, proved on examination to have a mixed composition distinctly dissimilar to that of the usual types; the heavy residue being 0.144% and the mineral assemblage being very different. The heavy minerals identified include
,12
F. T. INGHAM,
the following in relative order of abundance :-Ilmenite, zircon, garnet, tourmaline, rutile, epidote, hornblende, augite, arfvedsonite, hypersthene, magnetite, staurolite, kyanite, anatase, bronzite. chloritoid, brookite, andalusite, monazite, muscovite, sillimanite, and glaucophane. Characteristic features of this sample are the large number of species present and the marked abundance of amphiboles, epidote and garnet, whilst the scarcity of staurolite and kyanite is in strong contrast to typical Spilsby Sandstone residues. Comparison was made with the heavy residues of the North Sea Drift described by Professor Boswell [1, p. 79]. Although a slight difference in relative proportions occurs and more species have been determined from the latter (due to some extent to the larger number of samples examined), the affinities are so striking that the sand undoubtedly consists mainly, if not entirely, of glacial material. The other forms of contamination result chiefly from effects of gravity causing material from the higher beds to be added to the unconsolidated Spilsby Sandstone. The criterion of this pollution is the high proportional amount of heavy residue, samples from the following localities yielding the following amounts : Audleby 0.44%, Fonaby 0.46% and Nettleton 0.539%. Such abundant heavy residues are due, not to an increased number of mineral species present, but to the addition of numerous spherical limonitic grains of lustrous appearance. The source of this material is undoubtedly the Claxby ironstone, where pellets and grains of this character are a distirictive feature. Samples which showed signs of contamination besides those mentioned above were obtained from the following localities : Claxby, Caistor, South Willingham, Tealby (west of the Church), and Hareby.
V. POSSIBLE SOURCE OF DETRITAL MATERIALS. From stratigraphical and lithological grounds alike, most authorities consider the Spilsby Sandstone and other " Neocomian " beds of Lincolnshire (together with the Speeton Series of Yorkshire) to have been laid down in ·the Anglo-Germanic basin as distinct from the Anglo-Parisian basin of southern England, [17, 12, p. 179; 10, pp. 283 and 300J, an uplift in Upper Jurassic times having occurred. Evidence as to the exact age of these deposits is not precise, since correlation by palceontological methods with the better known shallow water types is difficult, the fossils of the deeper northern sea having different affinities. The generally accepted view [5, p. 240J is that the whole, or the greater portion, of the Spilsby Sandstone is equivalent in age to the Purbeckian. but different opinions have been held as to the age of the lower part of the deposit. A t the base of the strata occurs a nodule bed,
TH E PETROGR APHY OF THE S P I LSBY SAl'DSTONE .
13
and some of the constit uents of th is have been regarded as rolled casts of fossils derived from the Kimmeridge Clay and Portlandian. Thus Jukes-Browne [10 , p . 283J st ates :-" The beds represented by deri ved fossils were probably of the age of the P ortl and sands, and there is no pro of that an y equiva lent of the U pper Portland Beds was ever formed in the northern area " (York shire and Lin colnshire). Lamplugh , on the othe r hand , whil st agreeing that the upperm ost portion of th e Kimmeridge Clay may have been removed by local erosion , does not consider th e nodules are casts of derived fossils , but merely phosp ha tic concretions. The bearing of the petrography of th e sandstone on th e point will be discussed later. Th e land which could furni sh th e detritus at this period in clud ed the whole of Grea t Brit ain and Ireland, with the excepti on of the two basins of deposit ion. The western dra inage part of the area of the Anglo-Germanic basin probably consis ted of Scotl and , Ireland, Wales, Central England, and part of E ast Anglia : whilst a portion of south Wales, south-eastern En gland, and the Armorican ar ea would probably drain into the Anglo Parisian basin. Th e land mass whi ch could yield th e mat erial of th e Spilsby Sandstone would thus include the met am orphic and pr e-Cambrian rocks of Scotl and , th e Carboniferous strata of northern England and Ireland. th e Welsh Palreozoic rocks, th e Triassic and Jurassic beds of Centra l En gland . and probabl y Palreozoic rocks under E ast Anglia. The comparat ive coarseness of the sandsto ne sugges ts, at first sight , a possible deri vation from th e Millstone Grit . This hasbeen st udied in det ail in Yorkshire [6, p . 251]. Alt hough chere are ma ny points of resembl an ce with thi s deposit , such as th e typ es of quartz grain s, th e presence of fresh microcline and t he common occurrence of zircon , garnet, ilm enite, t ourmaline and rutile, there is an outstanding differen ce in the ab sence of both stauro lite and kyanite from th e Carboniferous strata. It would thus be imp ossibl e for this sediment alone t o have furn ished th e' mat erial of the Spilsby Sandst one. The original kn own sources of th e latter tw o minerals. as regards British deposit s, are rest ricte d to tw o areas : (i), th e Armorican mass of which Brittany is the remn ant ; an d (ii), th e Scottish Highlands. Moreover , it is difficult to consi der t he materials as having sur vived any pr evious erosion cycles, for th e kyanite grains are of fresh appearance and of large size, having a length up t o 1. 2 mm . ; the staurolite also occas ionally shows euhedral shape an d com monly traces of crystal bou ndaries which , unless protected by a "host," would have disappeared duri ng these cycles. The Arm orican mass is believed t o be the source of the staurolite and kyanite occur ring in th e Portland Sand of Dorset (13, p . 87 : 16, p . 254J, but in that area the grains are of distinctly sm all er size. Further, an uplift of th e Charnian
F. T. INGHAM,
axis [25, p. 372 and p. xixJ, separating the Anglo-Germanic basin from the Anglo-Parisian basin, occurred before the Purbeckian [18, pp. 142 and 144J, early in Portlandian, or even in late Kimmeridgian times. It is thus most probable that the Armorican area drained entirely into the nearer and more southerly sea. It would appear from the large size of the heavy mineral grains that the material has been brought from no great distance. It is possible that an area of pre-Palseozoic and Palseozoic rocks now covered with later sediments formed the mass from which part; at least, of the Spilsby Sandstone was derived. As. however, no direct evidence of kyanite and staurolite-bearing rocks has been obtained from deep borings, the matter must remain problematical. The conclusion is, therefore, drawn that the kyanitc and staurolite were furnished by the Scottish Highlands. Confirmatory evidence of a northerly origin is yielded by the presence of purple zircons, similar in type to those described by Mackie [15, p. 184J from the Archzean gneisses of the north of Scotland. This mineral and also microcline may have been derived from the Millstone Grit, but occasional crystals of the former, showing little signs of attrition, suggest the possibility of direct derivation, of at least some portion, from the Lewisian. A similar origin, from either or both, is possible in the case of quartz, grains of which show undulose extinction and inclusions of the types which are found in granitoid gneisses and schists. Hypersthene is also present in the Lewisian gneisses of Sutherland. Monazite similarly occurs both in the Millstone Grit and in the granites of northern Scotland. Since the garnet present is not all of one type, it has probably been derived from various sources. Occasional pink grains (showing the marked angularity and pitting due to the development of the dodecahedral cleavage) greatly resemble those described by Bosworth [3, p. 57J from Carboniferous sandstones of Scotland. A similar variety has also been noted in the Millstone Grit of Yorkshire [6, p. 265J ; thus, grains showing deep pits with apparent rhombic sections have probably been derived from this last formation. The colourless type is similar to, but attains a larger size than that of the Portlandian [16, p. 248J and most likely came from the same source. The majority of the Chert pebbles have marked affinities to the cherts of the Carboniferous Limestone and probably owe their origin to these beds in Yorkshire. From the above evidence it is concluded that the Spilshy Sandstone is composed of material which had a northern origin, and in which Scottish rocks played no small part. The Millstone Grit of northern England may also have contributed, but
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
IS
it is difficult to say to what extent, since it, too, contains much Scottish material. At first sight a northern origin does not appear to fit in with the fact that the Speeton Beds of Yorkshire, consisting almost entirely of clays, lie to the north of the Spilsby Sandstone, obviously of such grade they must have been deposited either in a deeper sea or farther from the shore-line. The explanation of this apparent anomaly appears to be as follows. The marked resemblance in mineral composition of the bed under consideration to that of the Portland Sand of Buckinghamshire has previously been mentioned, and a probable northern origin has been allotted to this material [16, jY.. 254J. It is the writer's belief, after examination and comparison of the heavy minerals of these two beds, that the detritus in both cases was furnished by the same drainage area. Mention has previously been made of the general uplift of the British Isles during the Upper Jurassic period and to Dr. Rastall's belief in an additional local elevation, which was a " posthumous" continuation of the Charnwood axis in late Jurassic times, forming a ridge from Charnwood Forest in a south-easterly direction through Bedfordshire. A later paper [20, p. 190J by the same author brings forward evidence of the existence of a second axis of uplift, parallel to the former from Nuneaton to Leighton Buzzard. The larger grain size of the Spilsby Sandstone compared with the Portland Sand is probably due to the increased elevation of the Scottish land area and consequent rejuvenation of the rivers. If the geographical evolution from the Kimmeridge period be considered, it will be seen that the rivers, draining the north and bringing the detritus which formed the Portland Sand, would gradually have their outlet to the south blocked by the ridge, mentioned above, during the process of development. The rivers would then tend to flow at right angles to the axis of uplift, namely, in a north-easterly direction into the Anglo-Germanic basin, which, by the effect of this local uplift and also the general uplift which was evolving at the time, had become separated from the southern (Anglo-Parisian) sea. This hypothesis would account for the fact that the Spilsby Sandstone thins both towards the north and to the east, and also for the presence of clays at Speeton, which would be at some considerable distance from the river mouth discharging the sandy material. VI. SUMMARY. The chief points of interest revealed by the study of the Spilsby Sandstone are the following : (i.) The deposit varies from a calcareous grit to an unconsolidated sandy material, the proportion of medium sand present being high. (ii.) Some of the loose sands exhibit evidence of contamina,
16
F. T. INGHAM,
tion, detritus from the Drift and Claxby Ironstone having been added. (iii.) Heavy residues show a general abundance of ilmenite, kyanite, staurolite, zircon and garnet, and the less common occurrence of pleonaste, hypersthene, [epidote, and monazite, with little variation throughout the area. This assemblage bears a marked resemblance to that of the Portland Sands of Buckinghamshire. The large size (just over I mm.) of some of the grains of the Spilsby Sandstone is, however, distinctive. (iv.) The examination and comparison of the minerals and pebbles occurring in the bed indicate a northern origin (in part Scottish) for the material. (v.) From a consideration of the constituents found, in conjunction with the pa1
VII. 1.
2.
c' 4. 5. 6. 7. 8.
9. 10.
II. 12.
BIBLIOGRAPHY.
BOSWELL, P. G. H. The Petrology of the North Sea Drift and Upper Glacial Brick Earths in East Anglia. Proc. Geol. Assoc., xxvii. (1916). ---The Petrography of the Sands of the Upper Lias and Lower Inferior Oolite in the West of England. Geol, Mag., lxi., (19 2 4) ' BOSWORTH, T. O. The Heavy Minerals in the Sandstones of the Scottish Carboniferous Rocks. Proc, Geol, Assoc., xxiv. (1913). BRAMMALL, A., and HARWOOD, H. F. The Occurrence of Rutile, Brookite, and Anatase on Dartmoor. Min. Mag., xx. (1923). DAVIES, A. M. Geology of the British Isles (Handbuch der regional Geologie). GILLIGAN, A. The Petrography of the Millstone Grit of Yorkshire. Quart. Journ. Geol, Soc., lxxv. (1919). JUDD. J. W. On the Strata which form the base of the Lincolnshire Wolds. Quart. Journ. Geol, Soc., xxiii. (1867). Additional Observations on the Neocomian Strata of Yorkshire and Lincolnshire, with notes on their relations to the beds of the same age throughout Northern Europe. Quart. Journ. Geoi, Soc., xxvi. (1870). JUKES-BROWN, A. J. Explanation of Sheet 84. Mem. Ceol. Suru., 188 7. The Building of the British Isles, 19II. KEEPING, H. On Some Sections of Lincolnshire Neocomian. Quart. [ourn. Geol: Soc., xxxviii. (1882). LAMPLUGH, G. \V. On the Speeton Series in Yorkshire and Lincolnshire. Quart. Journ. Geol, Soc., Iii. (1896).
PROC. GEOL.
Assoc.
FIG . I .
VOL .
XL.
SI' IL SBY SA ~ DSTO:-lE. S I' I LS B Y , LI N CS .
T o show orga nic rema ins .
F IG .
J.
PLATE 2~
(1929) .
[O .L X 30.]
SP I LSBY SAN DS TONE , C LA XKY, LI N CS .
T o show presen ce of Microclin e,
[
X 35·] Photomics by F.T . I. ~ To
f au p.
16.
PROC. GEOL.
Assoc.,
VOL.
XL.
PLATE
3.
HEAVY MINERAL GRAINS ISOLATED FROM SPILSBY SANDSTONE.
[For explanation see p. 170J
[To face p. I6.
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
13·
LATTER, M. 1°.
17
The Petrography of the Portland Sand of Dorset.
Proo. Geol. Assoc., xxxvii. (1926). The Sands and Sandstones of Eastern Moray (1896).
14· MACKIE, W.
Trans. Edin. Geol, Soc., vii. (1897).
18.
The Source of the Purple Zircons in the Sedimentary Rocks of Scotland. Geol, Mag., lix. (1922). NEAVERSON, E. The Petrography of the Upper Kimmeridge Clay and Portland Sand in Dorset, Wiltshire, Oxfordshire and Buckinghamshire. Proc. Geoi. Assoc., xxxvi. (1925). PAVLOW, A., and LAMPLUGH, G. W. Argiles de Speeton et leur equivalents. Butt. de la Soc. Imp. Nai., Moscow, vol. v., 1892. RASTALL, R. H. Cambridgeshire, Bedfordshire, and West Norfolk.
19·
--~-
IS·
16. 17·
Geology in the Field,
1910.
21.
The Mineral Composition of the Lower Greensand Strata of Eastern England. Geol, Mag., 1919. On the Tectonics of the Southern Midlands. Geol. Mag., lxii., Ig25. STEAD, ]. E. Cleveland Ironstone and Iron. Proc, Cleveland Inst.
22.
STRAHAN, A.
20.
Eng.,
1920.
Notes on the Relations of the Lincolnshire Carstone.
Quart. Journ. Geol. Soc., xlii. (1886). 23·
LSSHER, W. A. E., JUKES-BROWN, A. J., and STRAHAN, A. Explanation of Sheet 83. Mem. Geol. Suru., 1888. USSHER, W. A. E. and others. Explanation of Sheet 86. Mem, Geol. Surv., r Soo. \VATTS, W. \V. Excursion to Charnwood Forest. Proc, Geol. Assoc.., xvii., 1902.
EXPLANATION OF PLATE 3. Top row, reading from left to right : KYANITE grain with pronounced curvature. Somersby (X 70). GARNET grain with conchoidal fracture. Tealby (X 70). GARNET exhibiting rounding. Spilsby (X 75). ANATASE, tabular habit, with bipyramid faces. Spilsby (X 200).
Second row, from left to right : KYANITE, showing parting parallel to (001). Salmorby (X 65). ZIRCON, showing zonal distribution of minute inclusions. Goulceby (X 135). ZIRCON, nearly perfect geniculate twin. Spilsby (X 160). ANATASE, bipyramidal crystal, slightly rounded.
Third roto, from left to right : STAUROLITE, showing sharp teeth-like projections produced by attrition. Tealby, West (X 200). STAUROLITE, cleavage flake showing traces of 101 and ITO faces and inclusions of quartz. Claxby (X 500). STAUROLITE crystal fragment with regularly arranged inclusions. Tealby, West (X 200).
Fourth row, reading from left to right : BROOKITE, typical platy fragment. Halton Holgate, West (X 175). BROOKITE, grain showing typical striations. Halton Holgate, East (X 200).
Bottom row, from left to right : RUTILE, stumpy crystal showing traces of cleavage. Tealby, East (X 84). RUTILE, irregular striated grain. Elsham (X 84)' RUTILE, euhedral crystal of prismatic habit. Halton Holgate (X ISO). RUTILE. twinned on (301). East Keal (X 135). PROC. GEOL. Assoc., VOL. XL., PART I, 1929. 2
THE
GEOLOGISTS' ASSOCIATION. THE
PETROGRAPHY OF THE SANDSTONE. By F. T.
I:-;G HA ~ I .
Pn .D. ,
SPILSBY
x .n.c.s.
[Ilead [ anu arv 6th, ' 'J28 .] (Received M arch 30th, 1927.]
I.
INTRODUCTION.
THE
Spilsby Sandstone forms the basement bed of th e Lin colnshire Wolds. The hill s, although only attaining a maximum height of ab out 500 ft. , with an average of a ppro ximately 3 0 0 ft., form a pronou nced feature, sin ce t o the west of this esca rpme n t lies the broad flat plain of the Kimmerid ge and Oxford Clays. at a n average lev el of 1 0 0 feet. Numerous referen ces in geological lit erature on th e beds un derl yin g the Chalk of this a rea have b een made, but no acco un ts of a ny detailed petrological examinat ion of th em h ave been publish ed . * An excellen t description of these st ra ta has been given by P rofessor Judd (7.t p . 227] and the st ra tigraphical relationship of the Spilsby Sa nds tone an d the succeeding beds, with th ose of simila r age in ot he r localiti es, wa s discussed by him [8 , p. 326). The a rea h as also been dealt with in det ail in th e correspon ding Mem oirs of th e Geological Survey [23, p. 82; 9. p. 13 ; 24, p. I06]. Previous to th ese Survey publications a description of several ex pos ures had been furnish ed by Keepin g [11 , p. 239] . and later important work on thi s a rea was ca rr ied out by Lamplugh [17. pp. 18T a nd 455 ; 12 p. 179J. who correlated these b eds with their E uro pea n equivalents in minute detail. Briefly, the successio n of beds in thi s di strict is as follows: Cha lk. Red Chalk. Carstone. Tealbv Limest one. Tealby Clay. Claxby Ironston e. Spilsby Sandst one. Kimmeridge Clay , ., Since the abo ve was written the following pa per ha s appe ared Verry an d Car ter , Ycr ksh. G.oJ. Soc. t Fo r list of references see p. I C•• PROC. GEOL.
Ass oc. ,
V OL .
XL.,
PART 1. 1929.
z
F. T. INGHAM,
The Spilsby Sandstone succeeds the Kimmeridge Clay, unconformably, since the upper portions of the clay are overlapped towards the north; fragments which are believed by some authorities to be derived Kimmeridge fossils have been found at the base of the sandstone, The outcrop of the Spilsby Sandstone extends in a south-south-easterly direction from Elsham to East Keal, a direct distance apart of about 38 miles, but in the southern portion the outcrop widens owing to the topography and the low dip of the bed (approximately 1°), and is divided into two portions by the valley of the Steeping River. I The thickness of the sandstone varies from nearly 50 feet in the south to about 10 feet in the north. A boring at Skegness indicates that this bed thins also towards the south-east (19 feet being recorded), the strata being apparently a shore-deposit. The Spilsby Sandstone is normally succeeded by the Tealby Series, but in the northern portion, owing to the Cretaceous overlap, it is overlaid by the Chalk. The exposures, unfortunately, are neither so numerous nor so good as might be expected from the large outcrop. This is partially due to the covering of the sandstone and the succeeding beds by Glacial Drift in several areas; moreover, many of the exposures described by the Geological Survey are now overgrown. Nearly forty samples were collected personally from most exposures, unless these occurred in close proximity. At the commencement of the work samples were taken at various horizons in the same exposure, but the results of examination of these did not justify a continuation of this procedure. Some of the loose sands showed contamination due to land slide and wind action; one is composed almost entirely of glacial material (p. II). II.
PHYSICAL AND MICROSCOPICAL CHARACTERS.
In its unweathered form the Spilsby Sandstone is best described as a grey calcareous grit. It frequently contains fossil remains, Pecten being especially common. The majority of the sand grains (which are predominantly quartz) are fairly angular and moderately coarse in grade. Pebbles are not of infrequent occurrence, but they tend to be segregated in particular bands, often where current-bedding is exhibited, and are not usually distributed evenly throughout the strata. The percentage of calcium carbonate present in normal specimens was found to vary from about 23% (West Keal) to about 37% (Elsham) by weight. These percentages would correspond approximately to from 22'10 to 36% by volume. Professor V. C. HUng kindly calculated the pore space from the mechanical analysis of the Elsham sample, the result obtained being 32% t? 33%. Th~re wo~ld thus be more than enough of this matenal to fill the interstices of the sandstone should the
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
3
grains of quartz be in contact. The calcite matrix, since no signs of later expansion are present, is, therefore, evidently primary and was deposited contemporaneously with the quartz. By the action of weathering the whole or the greater portion of the calcite cement has been removed, the majority of the exposures now being composed of a friable sandstone or loose sand. usually greenish-yellow in colour, but grey and white varieties are also common. In some localities the sandstone is iron-stained, a deep reddish-brown colour resulting. A few typical samples were graded mechanically. A large proportion of the material consists of medium sand (0.25 mm. to 0.5 mm. diameter). Silt and clay grades, although of fair abundance in the succeeding bed (the Claxby Ironstone), are usually absent in unweathered specimens, but occasionally a little of these materials occurs in thin layers. Occasional grains of heavy residue minerals attain a larger size than is usual in sands, grains with a length of over I mm. having been observed. Thin sections of the hard unweathered rock show that the sandstone consists chiefly of quartz grains set in a matrix of calcite; grains of glauconite and of phosphatic material are much less abundant. Occasionally fragments of felspar are observable, the species represented being orthoclase, microcline, and albite. The quartz grains exhibit much variation in size and form. Although the majority are subangular, both well rounded and sharply angular types occur. The calcite matrix is composed of fairly large crystals of different optical orientation with rather irregular boundaries. Even though each of these crystals usually encloses several grains of quartz, "lustre mottling," though occasionally seen, is not usually exhibited. Calcite is also frequently present as organic remains and very rarely forms minute secondary veins traversing the sandstone. In strongly iron-stained specimens, the quartz grains are coated and cemented by limonite, which has probably originated through the replacement of the calcite by siderite, through the action of percolating waters rich in iron and the subsequent oxidation and hydration of this carbonate. The original source of the iron is presumably the overlying Claxby Ironstone, and it is interesting to note that the ferruginous varieties of the Spilsby Sandstone appear to be more frequent in the more northern part of the area where the Ironstone is better developed. The majority of the pebbles occurring in the Spilsby Sandstone are usually described under the name of "lydites," but this general term is more particularly applied to the darker coloured varieties. Most of the pebbles are well rounded and often exhibit a good polish. Generally they do not attain a size larger than 15 mms. Variation in colour is shown, the greater
F. T. INGHAM,
4
proportion being black or dark brown; some have, however, a reddish tinge and others vary from grey to almost white. Pebbles of different colours and appearance were selected and sectioned, practically all proving to be normal cherts. The usual aggregate polarisation of chalcedonic chert was observed, although much variation in texture was disclosed, in some cases even in the same pebble; the majority showed a very fine texture. The variation in colour was found to be largely due to staining by different forms of iron oxide, and the discolouration is more pronounced on the outer portions of the pebbles. Two pebbles exhibited an interesting feature in the presence of minute sections, many of which were diamond-shaped in a matrix of cryptocrystalline chalcedony. The shape of these inclusions is highly suggestive of the rhombic sections of dolomite, but they are composed of silica and the rock would appear to have been formed by the silicification of a dolomitic or magnesian limestone. On the border of the pebble the rhombic shapes arc more clearly marked since their boundaries consist of a black material, which has the appearance of oxide of iron. This suggests that partial replacement of dolomite by siderite took place on the outer portion of these pebbles before complete silicification had taken place. Pebbles of vein-quartz are of frequent occurrence and these exhibit no features of special interest. Phosphatic pebbles and fragments are of wide-spread distribution, although more plentiful in the lower portion of the deposit. In appearance they are almost black, and devoid of lustre. Thin sections show that they consist of a brown isotropic medium, in which grains of quartz occur. Occasionally glauconite and chert fragments are included in them. This phosphatic material is very similar to that of some of the phosphorite grains in the Ludlow Bone bed, and-except as regards size-the coprolites of the Cambridge Greensand. It is probably the rolled excrement of fishes, and, in some cases, possibly, reptiles. In a few instances bone structure has been observed. III.
MINERAL
CONSTITUENTS: THEIR OCCURRENCE AND CHARACTER. Under this heading, material which would pass through a 3o-mesh sieve is described. The samples, after preliminary cleaning, where necessary, with dilute hydrochloric acid, were treated with bromoform. The heavy residues thus obtained were separated into three or four portions by the electromagnet. The weight of the heavy residue compared with that of the light fraction shows much variation. This is, however, largely due to· the glauconite present, a varying proportion of which often sank with the heavy residue. In some cases further separation by methylene iodide was rendered necessary. The average heavy residue, after making allowance for the glauconite, is estimated to be approximately 0.05%.
PROC. GEOL.
Asso c.
A .-
VOL.
XL.
PLATE 1.
(1 9 2 9) '
S P ILS B Y SANDSTONE. T E ALB Y , LI N CS .
To show q uartz grains ce mented b y second a ry Limon ite [O .L. X 30 .1
, "1',
'1:'. ....._1
]. 1
;~ ~ ':e>-~ 1
FIG . 2 .
SPILSBY S AN DS T ONE, E L SH AM,
To show secondary calcite veins t raversing rock.
LIN CS .
[O .L. X 30. } Ph_its by F .T .I. [To f ace p. 4.
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
5
The minerals identified are the following:CUBIC. Gamet, Magnetite, Pleonaste, Sphalerite. TETRAGONAL. Anatase, Rutile, Zircon. HEXAGONAL AND RHOMBOHEDRAL. Apatite, Calcite, Ilmenite, Quartz, Tourmaline. ORTHORHOMBIC. Andalusite, Brookite, Hypersthene, Staurolite. MONOCLINIC. Biotite, Chlorite, Epidote, Hornblende, Monazite, Orthoclase. TRICLINIC. Albite, Kyanite, Microcline. AUTHIGENIC MINERALS AND AGGREGATES. Chert, Glauconite, Limonite, Phosphatic material. The following notes describe the chief features of these minerals, those of the light residues (S.G.-z.8) being described first, and the minerals of each portion being considered in the order mentioned in the above table. Qua r t z. This mineral, which forms the bulk of the light residue, is usually subangular or well-rounded in form; sharply angular grains do, however, occasionally occur. Generally, the grains are clear, but often a thin film of iron staining is present, and, less commonly, staining with glauconitic material is exhibited before treatment with acid. The majority of grains contain minute inclusions, often in the form of indeterminable black dust, which occurs both irregularly and arranged in rows or streams. Such types are stated to be characteristic of the quartz of gneisses and the younger schistose rocks [14, p. 148J. Occasional inclusions of zircon, chlorite, tourmaline, and rutile are present, and in a few isolated cases crystals of the latter mineral are comparatively large. Many of the quartz grains, especially the larger ones, show undulose extinction or "mosaic" structure between crossed nicols. Or tho c l a s e is of fairly frequent occurrence in the light residue and is the most abundant felspar. The grains are usually of a platy character with the edges well rounded. .Occasional cleavage fragments are present. The greater portion of the mineral is distinctly turbid, a cloudiness due to partial alteration to kaolin being present. Such alteration is more frequent in Orthoclase than in the other species of felspar occurring in the sandstone. M i c roc lin e is the next species in order of abundance and is present in all samples. It is usually well rounded and is characterised by its fresh and clear appearance. A I bit e. This species appears to be the only variety of plagioclase present, since all the grains exhibiting lamellar twinning have a refractive index below that of Canada balsam. Many fragments are platy owing to fracture along cleavages, but the edges are usually rounded. The majority have a fresh appear-
6
F. T. INGHAM,
ance, although occasionally a superficial cloudiness has been noted. Chert fragments are of frequent occurrence, but these exhibit no features of special interest. G I a u con i t e, in the loose sands, is next in order of abundance to quartz. The majority of the grains show the usual form of foraminiferal casts, re-entrant angles being frequently exhibited, although many have a rounded appearance. The larger grains usually only transmit light on their borders, and occasionally the green colour can only be observed even on the edges when convergent light is used. Gar net is characteristic in all samples; although common in the majority, it shows a rather wide range in its relative proportions to the mineral assemblage. At least two distinct types are recognisable: one is of a distinct pink colour; the other, practically colourless. In some of the strongly coloured grains a mauve tinge is observable. It is difficult to state whether these constitute a third variety or whether the slight colour difference is a function of the thickness. All the garnet is fairly magnetic, and both the coloured and colourless crystals are obtained together in the same magnetic separation. The grains are usually of moderate size, but one grain having a length of 1.1 mm. has been observed. Irregularity of form, often with marked conchoidal fracture, is usually shown; no euhedral crystals have been observed. Frequently strong pitting is observable on the surface due to the poor development of cleavage. A few grains, especially of the colourless type, from the same reason, present an almost square outline. These, from their shape, might be mistaken for ruby spinel, but their magnetic properties assist in their identification. Inclusions are fairly frequent, chloritic material being common. Minute cavities having a rhombic section are frequent in some grains. Occasionally optical anomalies are shown. 1\1 a g net i t e. This mineral is present in all samples, but it is always subordinate in amount to ilmenite. The grains are generally subangular. By reflected light they are black and generally lacking in lustre. Occasionally rounded octahedra occur. Many of the fragments show a superficial alteration to limonite. S P h a I e r it e is extremely rare, having been observed as isolated grains in only a few samples. The grains are yellow in colour, exhibit a high refractive index, and are isotropic. Irregularity in form, the result of its perfect dodecahedral cleavage, assists in the identification of the mineral. It is similar in appearance to that of the Middle Lias of Yorkshire [21, p. 75]. S pin e I (Pleonaste) is of rare occurrence, but occasional grains are present in several samples. These show the characteristic blue-green colour and isotropism, and are usually well
THE PETROGRAPHY OF THE SPILSBY SA:-.lDSTONE.
7
rounded. The mineral is of similar type to that recorded by Professor Boswell [2, p. 246J and Dr. Neaverson [16, p. 240J, from the Inferior Oolite and the Portland Sand respectively, but it attains larger dimensions (1.40 mm. diameter). A nat a s e is one of the scarcer constituents of the heavy residue, but occurs in small quantities in nearly all samples. The grains frequently show euhedral crystals of tabular habit with edges bevelled by the pyramid, the average size being about 0.15 mm. square. Crystals of pyramidal habit are extremely rare, and these present some indications of rounding. Rounded and irregular grains are as frequent as euhedral crystals, and such types attain a size of about 0.3 mm. It is probable that the round grains are detrital and not authigenic. The colour is rather variable, ranging from almost colourless to shades of yellow and blue. Occasionally grains which are parti-coloured occur, a mixture of yellow and blue being the most common. Rut i I e is abundant in nearly all samples, and the grains. although varying in size, are on the whole fairly large (up to 0.56 mm.). The colour, as usual, varies from deep brown and semi-opaque through fox-red to orange and yellow. Well crystallised stumpy prisms, acicular crystals, geniculate twins, arrow-head twins, and lamellar twins are all represented. Irregular and rounded grains as well as crystal aggregates are of common occurrence. Distinct pleochroism is usually exhibited, and traces of the cleavages, especially that parallel to [IIIJ, are often shown. Some of the rutile appears to be ferriferous since it is affected by the electro-magnet. Z i rca n occurs in all samples in fair abundance, and exhibits great variation in size and shade of colour. The grains can conveniently be grouped into three main types. The commonest of these is colourless and water clear. Occasional fine euhedrons occur. One of the most interesting grains, from a mineralogical standpoint, is an excellent geniculate twin. Both stumpy and acicular prismatic types (to o.ag mm.) are present, but the majority show varying degrees of rounding. The second type, occasionally colourless, is usually slightly grey or yellow and is frequently dusky with inclusions. It is characterised by zoning, especially in the centre of the crystal. This variety practically always shows prismatic shape, both stumpy and needle-like crystals terminated by pyramids being present. Fractured crystals are present, and slight rounding of the edges, especially at the terminals, is common. The third type, although the rarest, is persistent and is distinguished by its purple colour. The depth of colour is variable, but in the majority of cases is quite pronounced, the pleochroism in shades of the body colour being distinct. Although traces of prism and pyramid faces are occasionally seen, no euhedral
8
F. T. INGHAM,
crystals have been observed; the majority of these grains show pronounced rounding. The greater number are of small size (0.15 mm.) but some attain a length of 0.35 mm. A pat i t e is of rare occurrence in the samples studied. This may be due to some extent to the fact that the majority of these were treated with hydrochloric acid before examination and the mineral was possibly dissolved. In natural samples, however, it is usually absent, and, where present, is always comparatively rare. The grains are colourless to grey, usually small (about 0.1 mm.) and occur as well-rounded grains or more rarely partially eroded prisms. I I men i t e, with its alteration product, leucoxene, is the dominant mineral of the heavy residues in all samples. It is usually lustrous, black, and of irregular appearance. Many of the grains are largely altered to leucoxene and several exhibit secondary growths of minute crystals of rutile and anatase. To u r m a lin e is one of the more abundant heavy minerals and occurs in a variety of colours and forms. The colours represented include grey, blue, purple, green, brown and yellow, and many gradations of shades between these are present. Rarely, parti-coloured grains occur and colour-zoning is occasionally exhibited, blue with brown being the most common. A very deep blue variety of comparative rarity usually yields a perfect uniaxial interference figure. This property has been noted in a similar type by Dr. Neaverson [16, p. 249]. One colour variety worthy of note is tinted a pale pink and shows the peculiar pleochroism from red to brownish orange. This type, usually wellrounded, although rarer than most shades, is of constant occurrence. The majority of the tourmaline shows normal pleochroism and absorption. The form exhibited by this mineral varies from prismatic crystals (to 0-4 mm.), frequently showing rhombohedral terminations but often fractured, to almost perfect spheres. Varieties exhibiting this extreme rounding have probably survived previous cycles of deposition, but their size (to 0.3 mm.) is comparatively large. Some of the more irregularly shaped grains attain a size of 0.65 mm. And a Ius i t e is one of the rarest minerals of the sandstone, two or three grains only having been identified. In the case of one grain of rounded appearance, which exhibited pleochroism from green to pale red, recourse to refractive index liquids had to be made in order to differentiate it from hypersthene. B roo kit e is the least common oxide of titanium present. It usually occurs as irregular platy fragments of a greenish yellow colour. On" grain of euhedral shape has been observed and occasional.y irregular forms of dirty appearance are present. In the platy fragments striations parallel to the vertical axis can often be seen in reflected light. Owing to the strong axial dis,
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
9
persion the grains give no definite extinction between crossed nicols, the variation in colour on rotation being striking [4, p. 20J. The anomalous interference figure is given by the platy grains in ordinary light. R y per s the n e. This mineral is a rare accessory, but isolated grains are present in a few samples. The fragments are tinted reddish brown and display the characteristic pleochroism (brownish red X, green Z). Cleavage is generally shown and iron oxide inclusions are common. S tau r 0 lit e. This is one of the most abundant and characteristic minerals of the heavy residue. The grains show much variation in size and shape. Some of these have a maximum length of about 0.74 mm. ; the majority are not usually more than 0.45 mm., and from this they vary to minute frrgments. As regards form, a small number are euhedral, many show traces of crystal boundaries, and, being often basal, give a good biaxial interference figure. The majority of the grains are irregular and angular, but very occasionally rounding to a pronounced degree is observable. Fragments which exhibit ragged edges, due to the cleavage parallel to (no) being well-marked (possibly developed in the course of attrition), are of common occurrence. A rarer variety is one showing a distinct cloudiness due to minute black inclusions. These inclusions, generally occurring in irregular or rounded grains, sometimes show a tendency, more or less marked, to be regularly arranged. A grain of this type (PI. 3, row 3) exhibits a symmetrical arrangement similar to that observable in chiastolite. It is believed that such a variety has not previously been recorded from British sediments. The colour shows a range from brown to orange and yellow, and in minute flakes the colour is hardly perceptible. Pleochroism is well marked in the majority of grains. B i 0 tit e has been observed in one sample only, where it occurs as a basal cleavage flake yielding a pseudo-uniaxial interference figure. Chlo r it e is one of the rarest constituents of the Spilsby Sandstone, having been noted in only a few slides. There it occurs infrequently in flakes of a pale bluish-green colour. The birefringence is low and the fragments yield a biaxial interference figure. E p i dot e. Grains of this mineral are rare and usually show pronounced rounding. The colour varies from almost colourless to yellow, the latter often exhibiting a greenish tinge. The size, compared with that of the majority of the other minerals present, is small, the average diameter being below 0.1 mm. Some of the grains show the typical figure with one optic axis emergent, others a partial biaxial figure with large optic axial angle. R a r n b len d e is of rarest occurrence, only very few grains having been identified. One grain showing pleochroism from
F. T. INGHAM,
10
yellow-green to blue-green bears great resemblance to arfvedsonite. Although such alkali types are characteristic of pollution (IV.) no evidence in support of contamination in the sample could be found. It was, therefore, concluded that the species is a rare accessory of the sandstone. As the mineral has been observed by Dr. Rastall [19, p. 24] in the Carstone of Norfolk, which is believed [Strahan, 22, P.486J to consist of the material of the Spilsby Sandstone and other Neocomian beds, such a conclusion does not appear to be unlikely. M 0 n a zit e. This mineral is very difficult to distinguish from epidote in the heavy residues, owing to the grains being small, of the same colour and practically always rounded, no marked difference of refractive index and birefringence being observable. One grain, however, shows a perfect biaxial figure. with small optic axial angle, of positive sign. The grain yielded weak absorption bands when tested with a spectroscopic eyepiece attachment. NORTH.
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7 7 7 7 7 7 7 6 6 7 5 FIG. 3· TABLE SHOWING DISTRIBUTION OF HEAVY MINERALS IN TYPICAL LOCALITIES. very scarce. rare. exceedingly rare 2 3 very frequent frequent. 6 scarce. 5 4 dominant. very abundant 8 abundant. 9 7 ultra dominant. 10 7
7
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TIlE PETROGRAPHY OF THE SPILSBY
SAKDSTO~E.
II
K y ani t e is one of the most abundant and characteristic minerals of the heavy residue. It is common in all samples, usually exhibiting a pronounced platy habit, the crystals lying on the (100) cleavage plane and yielding a good biaxial figure of negative sign. Frequently the cleavage parallel to (010) is developed, grains showing cleavage traces and resultant rectangular forms being common. The parting parallel to (oor) is also occasionally present. Rarely there occur grains which show a pronounced curvature. Although of infrequent occurrence, this type is of such a distinctive nature that the precise source may ultimately be traced. The average size of kyanite grains are larger than that of tll{; other accessories present, a length of from 0.5 mm. to over La mm. often being shown. The mineral is usually colourless, but rarely a blue colour is exhibited, this being generally of a patchy nature. Occasionally grains occur having a grey appearance due to very minute inclusions. When the residue has not been treated with acid, or when the treatment has not been prolonged, a marked green staining is often shown. This staining, varying considerably in intensity and in extent, appears to be due to glauconitic material. The general mineral assemblage shows no marked variation either laterally or vertically throughout the bed. The distribution of the heavy minerals can best be realised by reference to table p. ro. IV.
CONTAMINATION FROM RESULTS OF GLACIATION AND GRAVITY.
It may have been noticed that samples collected from several localities have not been mentioned in the previous account. The reason for this is that many of the loose sands examined showed signs of contamination, and only specimens of compact sandstones, or sands of recognisable purity, have been described. The contamination is due to a variety of causes, the principal being the effects of glaciation, but in some cases" wash" from the succeeding beds, wind action, and landslips have played a considerable part. The degree of pollution naturally varies to a great extent, and it is sometimes extremely difficult, where only a few grains are questionable, to decide whether to consider them' as original materials of the sandstone or to regard them as proofs of slight contamination. No doubt, however, exists when the percentage proportion of heavy residue is markedly in excess of normal samples. A sample from ncar Caskill's Farm, Barnetby, on the mapped outcrop of the Spilsby Sandstone, proved on examination to have a mixed composition distinctly dissimilar to that of the usual types; the heavy residue being 0.144% and the mineral assemblage being very different. The heavy minerals identified include
,12
F. T. INGHAM,
the following in relative order of abundance :-Ilmenite, zircon, garnet, tourmaline, rutile, epidote, hornblende, augite, arfvedsonite, hypersthene, magnetite, staurolite, kyanite, anatase, bronzite. chloritoid, brookite, andalusite, monazite, muscovite, sillimanite, and glaucophane. Characteristic features of this sample are the large number of species present and the marked abundance of amphiboles, epidote and garnet, whilst the scarcity of staurolite and kyanite is in strong contrast to typical Spilsby Sandstone residues. Comparison was made with the heavy residues of the North Sea Drift described by Professor Boswell [1, p. 79]. Although a slight difference in relative proportions occurs and more species have been determined from the latter (due to some extent to the larger number of samples examined), the affinities are so striking that the sand undoubtedly consists mainly, if not entirely, of glacial material. The other forms of contamination result chiefly from effects of gravity causing material from the higher beds to be added to the unconsolidated Spilsby Sandstone. The criterion of this pollution is the high proportional amount of heavy residue, samples from the following localities yielding the following amounts : Audleby 0.44%, Fonaby 0.46% and Nettleton 0.539%. Such abundant heavy residues are due, not to an increased number of mineral species present, but to the addition of numerous spherical limonitic grains of lustrous appearance. The source of this material is undoubtedly the Claxby ironstone, where pellets and grains of this character are a distirictive feature. Samples which showed signs of contamination besides those mentioned above were obtained from the following localities : Claxby, Caistor, South Willingham, Tealby (west of the Church), and Hareby.
V. POSSIBLE SOURCE OF DETRITAL MATERIALS. From stratigraphical and lithological grounds alike, most authorities consider the Spilsby Sandstone and other " Neocomian " beds of Lincolnshire (together with the Speeton Series of Yorkshire) to have been laid down in ·the Anglo-Germanic basin as distinct from the Anglo-Parisian basin of southern England, [17, 12, p. 179; 10, pp. 283 and 300J, an uplift in Upper Jurassic times having occurred. Evidence as to the exact age of these deposits is not precise, since correlation by palceontological methods with the better known shallow water types is difficult, the fossils of the deeper northern sea having different affinities. The generally accepted view [5, p. 240J is that the whole, or the greater portion, of the Spilsby Sandstone is equivalent in age to the Purbeckian. but different opinions have been held as to the age of the lower part of the deposit. A t the base of the strata occurs a nodule bed,
TH E PETROGR APHY OF THE S P I LSBY SAl'DSTONE .
13
and some of the constit uents of th is have been regarded as rolled casts of fossils derived from the Kimmeridge Clay and Portlandian. Thus Jukes-Browne [10 , p . 283J st ates :-" The beds represented by deri ved fossils were probably of the age of the P ortl and sands, and there is no pro of that an y equiva lent of the U pper Portland Beds was ever formed in the northern area " (York shire and Lin colnshire). Lamplugh , on the othe r hand , whil st agreeing that the upperm ost portion of th e Kimmeridge Clay may have been removed by local erosion , does not consider th e nodules are casts of derived fossils , but merely phosp ha tic concretions. The bearing of the petrography of th e sandstone on th e point will be discussed later. Th e land which could furni sh th e detritus at this period in clud ed the whole of Grea t Brit ain and Ireland, with the excepti on of the two basins of deposit ion. The western dra inage part of the area of the Anglo-Germanic basin probably consis ted of Scotl and , Ireland, Wales, Central England, and part of E ast Anglia : whilst a portion of south Wales, south-eastern En gland, and the Armorican ar ea would probably drain into the Anglo Parisian basin. Th e land mass whi ch could yield th e mat erial of th e Spilsby Sandstone would thus include the met am orphic and pr e-Cambrian rocks of Scotl and , th e Carboniferous strata of northern England and Ireland. th e Welsh Palreozoic rocks, th e Triassic and Jurassic beds of Centra l En gland . and probabl y Palreozoic rocks under E ast Anglia. The comparat ive coarseness of the sandsto ne sugges ts, at first sight , a possible deri vation from th e Millstone Grit . This hasbeen st udied in det ail in Yorkshire [6, p . 251]. Alt hough chere are ma ny points of resembl an ce with thi s deposit , such as th e typ es of quartz grain s, th e presence of fresh microcline and t he common occurrence of zircon , garnet, ilm enite, t ourmaline and rutile, there is an outstanding differen ce in the ab sence of both stauro lite and kyanite from th e Carboniferous strata. It would thus be imp ossibl e for this sediment alone t o have furn ished th e' mat erial of the Spilsby Sandst one. The original kn own sources of th e latter tw o minerals. as regards British deposit s, are rest ricte d to tw o areas : (i), th e Armorican mass of which Brittany is the remn ant ; an d (ii), th e Scottish Highlands. Moreover , it is difficult to consi der t he materials as having sur vived any pr evious erosion cycles, for th e kyanite grains are of fresh appearance and of large size, having a length up t o 1. 2 mm . ; the staurolite also occas ionally shows euhedral shape an d com monly traces of crystal bou ndaries which , unless protected by a "host," would have disappeared duri ng these cycles. The Arm orican mass is believed t o be the source of the staurolite and kyanite occur ring in th e Portland Sand of Dorset (13, p . 87 : 16, p . 254J, but in that area the grains are of distinctly sm all er size. Further, an uplift of th e Charnian
F. T. INGHAM,
axis [25, p. 372 and p. xixJ, separating the Anglo-Germanic basin from the Anglo-Parisian basin, occurred before the Purbeckian [18, pp. 142 and 144J, early in Portlandian, or even in late Kimmeridgian times. It is thus most probable that the Armorican area drained entirely into the nearer and more southerly sea. It would appear from the large size of the heavy mineral grains that the material has been brought from no great distance. It is possible that an area of pre-Palseozoic and Palseozoic rocks now covered with later sediments formed the mass from which part; at least, of the Spilsby Sandstone was derived. As. however, no direct evidence of kyanite and staurolite-bearing rocks has been obtained from deep borings, the matter must remain problematical. The conclusion is, therefore, drawn that the kyanitc and staurolite were furnished by the Scottish Highlands. Confirmatory evidence of a northerly origin is yielded by the presence of purple zircons, similar in type to those described by Mackie [15, p. 184J from the Archzean gneisses of the north of Scotland. This mineral and also microcline may have been derived from the Millstone Grit, but occasional crystals of the former, showing little signs of attrition, suggest the possibility of direct derivation, of at least some portion, from the Lewisian. A similar origin, from either or both, is possible in the case of quartz, grains of which show undulose extinction and inclusions of the types which are found in granitoid gneisses and schists. Hypersthene is also present in the Lewisian gneisses of Sutherland. Monazite similarly occurs both in the Millstone Grit and in the granites of northern Scotland. Since the garnet present is not all of one type, it has probably been derived from various sources. Occasional pink grains (showing the marked angularity and pitting due to the development of the dodecahedral cleavage) greatly resemble those described by Bosworth [3, p. 57J from Carboniferous sandstones of Scotland. A similar variety has also been noted in the Millstone Grit of Yorkshire [6, p. 265J ; thus, grains showing deep pits with apparent rhombic sections have probably been derived from this last formation. The colourless type is similar to, but attains a larger size than that of the Portlandian [16, p. 248J and most likely came from the same source. The majority of the Chert pebbles have marked affinities to the cherts of the Carboniferous Limestone and probably owe their origin to these beds in Yorkshire. From the above evidence it is concluded that the Spilshy Sandstone is composed of material which had a northern origin, and in which Scottish rocks played no small part. The Millstone Grit of northern England may also have contributed, but
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
IS
it is difficult to say to what extent, since it, too, contains much Scottish material. At first sight a northern origin does not appear to fit in with the fact that the Speeton Beds of Yorkshire, consisting almost entirely of clays, lie to the north of the Spilsby Sandstone, obviously of such grade they must have been deposited either in a deeper sea or farther from the shore-line. The explanation of this apparent anomaly appears to be as follows. The marked resemblance in mineral composition of the bed under consideration to that of the Portland Sand of Buckinghamshire has previously been mentioned, and a probable northern origin has been allotted to this material [16, jY.. 254J. It is the writer's belief, after examination and comparison of the heavy minerals of these two beds, that the detritus in both cases was furnished by the same drainage area. Mention has previously been made of the general uplift of the British Isles during the Upper Jurassic period and to Dr. Rastall's belief in an additional local elevation, which was a " posthumous" continuation of the Charnwood axis in late Jurassic times, forming a ridge from Charnwood Forest in a south-easterly direction through Bedfordshire. A later paper [20, p. 190J by the same author brings forward evidence of the existence of a second axis of uplift, parallel to the former from Nuneaton to Leighton Buzzard. The larger grain size of the Spilsby Sandstone compared with the Portland Sand is probably due to the increased elevation of the Scottish land area and consequent rejuvenation of the rivers. If the geographical evolution from the Kimmeridge period be considered, it will be seen that the rivers, draining the north and bringing the detritus which formed the Portland Sand, would gradually have their outlet to the south blocked by the ridge, mentioned above, during the process of development. The rivers would then tend to flow at right angles to the axis of uplift, namely, in a north-easterly direction into the Anglo-Germanic basin, which, by the effect of this local uplift and also the general uplift which was evolving at the time, had become separated from the southern (Anglo-Parisian) sea. This hypothesis would account for the fact that the Spilsby Sandstone thins both towards the north and to the east, and also for the presence of clays at Speeton, which would be at some considerable distance from the river mouth discharging the sandy material. VI. SUMMARY. The chief points of interest revealed by the study of the Spilsby Sandstone are the following : (i.) The deposit varies from a calcareous grit to an unconsolidated sandy material, the proportion of medium sand present being high. (ii.) Some of the loose sands exhibit evidence of contamina,
16
F. T. INGHAM,
tion, detritus from the Drift and Claxby Ironstone having been added. (iii.) Heavy residues show a general abundance of ilmenite, kyanite, staurolite, zircon and garnet, and the less common occurrence of pleonaste, hypersthene, [epidote, and monazite, with little variation throughout the area. This assemblage bears a marked resemblance to that of the Portland Sands of Buckinghamshire. The large size (just over I mm.) of some of the grains of the Spilsby Sandstone is, however, distinctive. (iv.) The examination and comparison of the minerals and pebbles occurring in the bed indicate a northern origin (in part Scottish) for the material. (v.) From a consideration of the constituents found, in conjunction with the pa1
VII. 1.
2.
c' 4. 5. 6. 7. 8.
9. 10.
II. 12.
BIBLIOGRAPHY.
BOSWELL, P. G. H. The Petrology of the North Sea Drift and Upper Glacial Brick Earths in East Anglia. Proc. Geol. Assoc., xxvii. (1916). ---The Petrography of the Sands of the Upper Lias and Lower Inferior Oolite in the West of England. Geol, Mag., lxi., (19 2 4) ' BOSWORTH, T. O. The Heavy Minerals in the Sandstones of the Scottish Carboniferous Rocks. Proc, Geol, Assoc., xxiv. (1913). BRAMMALL, A., and HARWOOD, H. F. The Occurrence of Rutile, Brookite, and Anatase on Dartmoor. Min. Mag., xx. (1923). DAVIES, A. M. Geology of the British Isles (Handbuch der regional Geologie). GILLIGAN, A. The Petrography of the Millstone Grit of Yorkshire. Quart. Journ. Geol, Soc., lxxv. (1919). JUDD. J. W. On the Strata which form the base of the Lincolnshire Wolds. Quart. Journ. Geol, Soc., xxiii. (1867). Additional Observations on the Neocomian Strata of Yorkshire and Lincolnshire, with notes on their relations to the beds of the same age throughout Northern Europe. Quart. Journ. Geoi, Soc., xxvi. (1870). JUKES-BROWN, A. J. Explanation of Sheet 84. Mem. Ceol. Suru., 188 7. The Building of the British Isles, 19II. KEEPING, H. On Some Sections of Lincolnshire Neocomian. Quart. [ourn. Geol: Soc., xxxviii. (1882). LAMPLUGH, G. \V. On the Speeton Series in Yorkshire and Lincolnshire. Quart. Journ. Geol, Soc., Iii. (1896).
PROC. GEOL.
Assoc.
FIG . I .
VOL .
XL.
SI' IL SBY SA ~ DSTO:-lE. S I' I LS B Y , LI N CS .
T o show orga nic rema ins .
F IG .
J.
PLATE 2~
(1929) .
[O .L X 30.]
SP I LSBY SAN DS TONE , C LA XKY, LI N CS .
T o show presen ce of Microclin e,
[
X 35·] Photomics by F.T . I. ~ To
f au p.
16.
PROC. GEOL.
Assoc.,
VOL.
XL.
PLATE
3.
HEAVY MINERAL GRAINS ISOLATED FROM SPILSBY SANDSTONE.
[For explanation see p. 170J
[To face p. I6.
THE PETROGRAPHY OF THE SPILSBY SANDSTONE.
13·
LATTER, M. 1°.
17
The Petrography of the Portland Sand of Dorset.
Proo. Geol. Assoc., xxxvii. (1926). The Sands and Sandstones of Eastern Moray (1896).
14· MACKIE, W.
Trans. Edin. Geol, Soc., vii. (1897).
18.
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EXPLANATION OF PLATE 3. Top row, reading from left to right : KYANITE grain with pronounced curvature. Somersby (X 70). GARNET grain with conchoidal fracture. Tealby (X 70). GARNET exhibiting rounding. Spilsby (X 75). ANATASE, tabular habit, with bipyramid faces. Spilsby (X 200).
Second row, from left to right : KYANITE, showing parting parallel to (001). Salmorby (X 65). ZIRCON, showing zonal distribution of minute inclusions. Goulceby (X 135). ZIRCON, nearly perfect geniculate twin. Spilsby (X 160). ANATASE, bipyramidal crystal, slightly rounded.
Third roto, from left to right : STAUROLITE, showing sharp teeth-like projections produced by attrition. Tealby, West (X 200). STAUROLITE, cleavage flake showing traces of 101 and ITO faces and inclusions of quartz. Claxby (X 500). STAUROLITE crystal fragment with regularly arranged inclusions. Tealby, West (X 200).
Fourth row, reading from left to right : BROOKITE, typical platy fragment. Halton Holgate, West (X 175). BROOKITE, grain showing typical striations. Halton Holgate, East (X 200).
Bottom row, from left to right : RUTILE, stumpy crystal showing traces of cleavage. Tealby, East (X 84). RUTILE, irregular striated grain. Elsham (X 84)' RUTILE, euhedral crystal of prismatic habit. Halton Holgate (X ISO). RUTILE. twinned on (301). East Keal (X 135). PROC. GEOL. Assoc., VOL. XL., PART I, 1929. 2