Chapter C2 Glaucony From The Margin off Northwestern Spain

249

Chapter C2 GLAUCONY FROM THE MARGIN OFF NORTHWESTERN SPAIN by G.S. Odin and M. Lamboy INTRODUCTION

Glaucony from northwest Spanish continental margin has been described by Lamboy (1967). This study of superficial sediments is of interest because detailed morphological investigation was carried out using high magnification. Evolutionary processes have affected several sorts of substrate; this is illustrated in the French literature (Lamboy, 1968; 1975; 1976; Lamboy and Odin, 1974; Odin, 1972a; 1975a; Odin and Lamboy, 1975). The mineralogy of the Spanish green grains was first studied by Caillkre and Lamboy (1970a, b) and revised by Lamboy and Odin (1975). The latter authors also published a review of the morphological features and a discussion of the geological significance of the facies in the area. The present chapter is mainly intended to summarize the large volume of available data which has allowed the formation mechanism and geological significance of glaucony facies to be understood. The study covers about 550 sediment samples collected off Galice Province (northwestern Spain) from depths ranging between -50 m and -1000 m. From a granulometric point of view, the dredged sediments are clayey sands with a clay-size fraction content generally lower than 25 percent (Fig. 1). The sandsize fraction at the sea-bottom is characterized by the presence of carbonate. Figure 1 shows the proportion of carbonate in the sand-size fraction. This carbonate is bioclastic and is particularly abundant at depths shallower than -100 m. The rest of the sand is composed of quartz and glaucony. The clay-size fraction is remarkably homogeneous from a mineralogical point of view; it is mainly formed of illite with a small proportion of chlorite and kaolinite. This composition is similar to that of the clay found in the coastal river sediments. Finally, an interesting characteristic of the sedimentary cover in the area is the presence of green grain-bearing nodular phosphate, briefly quoted by Collet (1908) and studied in detail by Lamboy (1976). DISTRIBUTION OF GLAUCONY

Figure 2 shows the distribution of magnetic grains separated from washed sediments. The measured contents attain 50% or more on the deepest portion of the shelf down to depths of around -200 m. To the north, the magnetic grains contain smooth, brown particles which were initially considered to be authigenic 'berthierine' (Caillkre and Lamboy, 1970b; Lamboy, 1976). Elsewhere, the magnetic grains represent typical glaucony with depth distribution similar to

250

CAR EON ATE

0

<20%

20-50%

,

50-80%

50km

> 80%

Figure 1. Distribution of the clay-size fraction (< 65 pm) and carbonate fraction (> 65 pm) off NW Spain.The area under study comprises the deeper half of the shelf and the slope. Dots in the bottom figure represent nodular phosphate. (Reproduced from Lamboy, 1976)

25 1

GLAUCONY

0 0

0-1096 10-20%

20-50x

> 50%

Figure 2. Magnetic grain (mostly glaucony) content in superficial sediments off northwestern Spain. All samples deeper than 50 m contain green or partly green grains in a proportion above 1%. (Reproduced from Lamboy, 1976)

that observed on the west African Atlantic margin. The distributions of glaucony and clay are unrelated (compare Fig. 1 top and Fig. 2). From a granulometric point of view, separate analyses have been undertaken for the quartz fraction and the glauconitic fraction. This study has shown that there is no relation between the grain-size and mode of glaucony (mode between 0.6 mm and 0.18 mm) and that of quartz. Glauconitic grains are often coarser than quartz grains; but the difference in size between glaucony and quartz varies greatly from one site to another (Lamboy, 1976). In short, quartz and clay represent a detrital component, and glaucony has an independent distribution showing its in situ authigenic nature. There is, however, a clear, inverse relationship between the abundance of carbonate and that of glaucony (compare Fig. 1 bottom to Fig. 2). This relation is very significant since the glauconitization process mostly occurs at the expense of carbonate bioclasts, as will be shown below. Where glaucony is abundant, carbonate bioclasts have disappeared. This dependance is corroborated by the distribution of the combination: shelly remnants/green grains with intermediate forms, on the western side of the Spanish margin. Figure 3 considers four habits: 1) white, shelly debris, more or less perforated, 2) partly green shelly remnants, 3) olive-green grains, 4) dark-green grains.

252 present sea-level

1 Om

~

100

1

I

2m

3

0

4

0

2a

30(

Figure 3. Relationship between the distribution of carbonate bioclasts and that of glauconitic grains off northwestern Spain. Cross section of the continental shelf. 1) darkgreen grains: 2) olive-green grains: 3) partly green bioclasts; 4) white to grey bioclasts many of them may show borings with green clay inside. (Modified from Lamboy, 1976)

MORPHOLOGICAL FEATURES

The verdissement of echinoderm fragments On the Spanish shelf, various sorts of substrate shelter the glauconitization process. The glauconitic grains resulting from the genesis of green marine clay within a fragment of echinoderm skeleton are easily identified by their distinctly reticulated structure nicely illustrated by Bignot (1976). Photomicrograph 1 in Figure 4 shows this typical habit locally observed in green grains frequently as much as 1.5 to 2.0 mm in diameter. A green grain slightly more evolved than the one in Photo.1 is pictured in Photo.2; cracks appear in locally swollen areas. An even more evolved grain (Photo.3, Fig. 4) shows a large portion of the particle with deep cracks; this grain has been broken in order to show that the reticulated structure has disappeared from the main portion of the grain except for the lower left-hand comer. A detail of a cracked zone of the grain from Photo.2 is enlarged in Photo.4 (Fig. 4). This picture clearly demonstrates that cracks characterize an area which is in reliefon the exterior of the grain. This verruca-like formation essentially represents a growth feature. These observations negate the shrinkage fissure interpretation, often quoted in past literature and more recently by Morton et al. (1984), supplemented by the hypothesis of a dessication (dehydration) process. The cracking of green grains of the glaucony facies results from slow or zero growth at the grain surface at a time when the inside is expanding (see Chapter C1, p. 232). Details of the internal arrangements of the first stage of evolution of the echinoderm skeleton fragments are shown in Photo.5 (Fig. 4). The reticulated carbonate stereom is clearly shown with its smooth surface. The enlarged view in Photo.6 (Fig. 4) shows that the voids in the stereom represent a volume very similar to that of the carbonate skeleton itself. However, photomicrographs 5 and 6 demonstrate that the green marine clay mineral first develops in the pores

253

Figure 4. Verdissement process of fragments of echinoderm skeleton (Scanning Electron Microscope). The marine green clay first fills the pores of the stereom fragments. The carbonate stereom later dissolves and the voids are filled with neoformed green clay. Finally, a recrystallization process destroys the original reticulated texture. (According to Odin and Lamboy, 1974) Scale bars: 100 pm (Photo. 1,2,3,4,5) or 10 pm (Photo. 6.7) or 2 pm (Photo. 8,9).

within the carbonate fragment and does not replace the stereom. During a second stage, the carbonate stereom is dissolved as shown in Photo.7 (Fig. 4),

254 where some carbonate remnants, not totally dissolved, are still visible. The nanostructure of the clay mineral is shown enlarged in Photo.8 (Fig. 4). The globules and caterpillar arrangement are characteristic of an early stage in the evolution of the green clay. At a later stage, as, for example, the one depicted in the upper right interior of the grain shown in Photo.3, better shaped automorphic figures, like the rosettes illustrated in Photo.9 (Fig. 4), may be observed. At this stage, the grains become homogeneous, and the initial porosity disappears due to filling of the voids with neoformed clay minerals. The reticulated structure disappears following a general recrystallization process in the interior of the grain, but the external reticulated habit partly remains between verrucous areas. In summary, all grains observed above are light-green or olive-green in colour. During the first stage of evolution, glauconitic minerals grow de novo in the pores of the carbonate substrate; secondly, the substrate is dissolved and glauconitic minerals can grow in the newly acquired porosity; thirdly, the glauconitic minerals undergo a general recrystallization process which obliterates the initial reticulated structure more or less completely.

The verdissement of bored shell fragments The most abundant initial substrate for glauconitization identified off northwestern Spain was shell fragments. These shell fragments, usually 0.2 to 2.0 mm in diameter, are generally not directly glauconitized. The first stage in the evolution of the carbonate particle is the formation of a porosity which is produced by boring micro-organisms. Photomicrograph 1 in Figure 5 is a view of a section of a 5 mm long shell fragment. From the interior to the exterior, the grey coloured centre is made of well-preserved carbonate; the whitish portion represents partly dissolved carbonate; and the external black filaments are filled with dark-green glaucony. Photo.2 in Figure 5 shows the external surface of the bioclast. Borings parallel or perpendicular to the surface are now filed with dark-green clay. Such perforated grains have been submitted to dilute acid solution to remove the carbonate. Photo.3 (Fig. 5) was obtained from a carbonate free grain after removal of the above portion in order to see the arrangement of the filaments within the grain. Note that the external zone of the 2 mm long decarbonated grain is much richer in clayey fiaments and forms a nearly continuous film compared to the interior where few filaments are present. An enlarged view of the external zone is shown in Photo.4 (Fig. 5). An enlarged view of a filament from the interior is pictured in Photo.5 (Fig. 5). The section of one filament is enlarged in photomicrograph 6 (Fig. 5); the contorted blades, 1 to 2 pm long, are typical of a clay mineral. The clayey filaments were found in all glauconitized carbonate substrates from the Spanish sediments. For example, a few echinodermal skeleton fragments were bored by

-+-+-+

Figure 5. Verdissement process of bored shell fragments. Optic microscopy (Photo.1, 2) and scanning electron microscopy (Photo3 to 13). Scale bars show 100 pm (Photo. 2,3,4, 10) or 10 pm (Photo. 5, 7,8,11) or 2 pm (Photo. 6 , 9 , 12).

256 micro-organisms (Photo.7 in Fig. 5). Borings are not always homogeneously filled with green clay, and some clayey filaments of the decarbonated grains show a composite section (Photo.8 in Fig. 5). An idiomorphic nanostructure resembling rosettes can be seen in the enlarged section in Photo.9 (Fig. 5). All pictures mentioned above represent comparatively early stages of evolution where the recrystallization processes have still not modified the organization of the primary green clay fillings. However, a highly-evolved stage of the same process has been identified in many grains after observation in thin section. These grains are black-green to blue-green in colour and comparatively dense; they display a zonation with a lighter and soft central part. Following ultrasonic treatment, this soft heart of the grain is removed (Photo.10 in Fig. 5). In an enlarged view of the interior of a dark-green grain submitted to ultrasonic treatment (Photo. 11 in Fig. 5), no trace of the filaments in the cortical zone (i.e. the previous densely bored part of the carbonate grain) can be seen. The nanostructure is lamellar and characteristic of a highly-evolved stage of glauconitization (Photo. 12 in Fig. 5). Proof that these dark blue-green grains result from the evolution of bored carbonate particles has been observed in some thin sections where filament ghosts are present in the soft centre of the grain. In summary, the densely bored shell fragments constitute a substrate physically equivalent to echinoderm fragments with similar porosity. Green clays first develop by filling the pores. Later on, the rest of the carbonate is dissolved and replaced by green clay. Finally, recrystallization occurs which mostly obliterates the initial filament arrangement. The phenomenon is more efficient on the outside than in the interior of the comparatively large grains, within which ghosts of filaments may be found in the heart of the particles at a very late stage of evolution. Off northwestern Spain, glauconitization of bored shell fragments is predominant.

The verdissement within the foraminiferal tests Glauconitization of tests of micro-organisms is widespread in the superficial sediments collected from northwestern Spain. Infillings of foraminiferal tests are the most abundant. They are generally similar to those described elsewhere by several authors (Murray and Renard, 1891, pp. 378-391; Collet, 1908; Caspari, 1910; Wermund, 1961; Ehlmann et al., 1963; Bjerkli and Ostmo-Saeter, 1973). The microscope allows numerous miliolids, of which the test is filled with yellowish to dark-green clay (Photo. 1 in Fig. 6) to be observed. It may be noted that the test is well preserved when the first clay minerals are formed within it; but when the clay minerals are dark-green, the carbonate test is dissolved. Photomicrograph 2 (Fig. 6) shows the infilling of an Elphidium; the clay entirely fills the test. One test has been opened with pliers (Photo.3 in Fig. 6) and the nanostructure of the infilling is pictured in Photo.4 (Fig. 6). Blades typical of a clay mineral are present. Cracks appear at the surface of the lightgreen coloured infilling (Photo.5 in Fig. 6 ) when the test has been dissolved in the sediment. Later on, the infilling becomes dark-green with cracks deeper

257

Figure 6. Verdissement within the tests of foraminifers. Optic microscopy (Photo.1); scanning electron microscopy (Photo.2 to 6) Scale bars show 100 pm (Photo.l,2,3,5) or 10 pm (PhOt0.4).

than previously. Photomicrograph 6 (scale similar to Photo.2, Fig. 6 ) shows a fragment of Elphidium infilling after natural dissolution of the test, cracks are present. By this process, the grain loses its original form. As a result, the initial substrates of the most evolved grains are difficult to identify especially when the cracks are deep enough for the grain to be broken in two. In short, verdissement within foraminifera1 tests mainly consists of filling chambers with authigenic green clay. The carbonate test itself is dissolved allowing the green grain to evolve freely into the common dark-green cracked grain. Therefore, the role of the test is to create the favourable physico-chemical microenvironment for starting the verdissement.

The verdissement of detrital mica flakes The evolution of mica flakes into 'vermicular pellets' (Triplehorn, 1966) or 'accordion-like grains' (Odin, 1972a) is one of the famous examples quoted in literature (see below) as supporting the hypothesis of a transformation process.

25 8 This hypothesis assumes that evolution from denital mica (or from another 2: 1 phyllosilicate) into glauconitic mica occurs following a restricted and progressive cation exchange process (Fe for A1 especially) within a perennial minera architecture. This hypothesis has been proposed by Galliher (1935) and was beleived to apply to all the glaucony collected from the Quaternary sediments in Monterey Bay (California). However, Hein et al. (1974) showed that, in Monterey Bay, a considerable proportion of the glauconitic green grains did not result from the evolution of detrital mica flakes. The transformation hypothesis was later generalized and improved by Burst (1958a, b), Hower (1961), Seed (1965), Triplehorn (1966), to explain the genesis of all glauconitic minerals as well as most other clay minerals evolving on the surface of the earth. Detailed scanning electron microscope study of the verdissement of mica flakes has shown the transformation hypothesis to be invalid for this example (Odin, 1972a), leading to some doubt on the general validity of this theory widely accepted in the sixties and seventies. Odin (1972a) identifies three preliminary reasons why the transformation process could not have taken place. The first one alone is conclusive and concerns the question of the mass of material involved. The densities of biotite flakes and glauconitic accordion-like pellets are similar (between 2.5 and 3 g/cm3). However, in a sediment, detrital mica flakes are usually thin compared to their diameter. In contrast to this, green vermicular pellets, which we prefer to call accordion-like grains in order to emphasize the presence of cleavages perpendicular to their longitudinal axis, are usually thicker than they are wide. In northwestern Spain sediments, the glauconitized biotite flakes present at depths shallower than -90 m are often wide (more than 1 mm in diameter) and thin. Deeper, true accordion-like green grains were observed, their diameter being between 0.1 mm and 0.4 mm and their thickness often more than 1 mm. Therefore, the wide flakes appear much less prone to glauconitization than the small flakes. Photomicrographs 1 and 2 (Fig. 7) show some examples of accordion-like grains found in the Spanish sediments. These grains are all several tens of times heavier than were the initial mica flakes from which they formed since the diameter has remained the same but they have grown in thickness. Galliher himself (1935) noticed that his final 'vermicular pellets' evolved from biotite flakes 10 to 20 times thinner. It is clear, therefore, that material from the biotite cannot produce all material from the glauconitic clay by a simple chemical exchange or re-arrangement of octahedral cations as implied in the transformation hypothesis. Secondly, transformation is unlikely to occur because the chemical composition and mineral nature of the initial substrate and that of the resulting authigenic clay are in no way connected. Off northwestern Spain and in other formations of various ages, both biotite and muscovite have been observed as the initial substrate of the accordion-like glauconitic grains. Consequently, the presence or absence of iron in the initial mica is not a fundamental need for the formation of glauconitic minerals. In addition, it has been shown previously

259

Figure 7. Verdissement of mica flakes. Optic microscopy by A. Matter (Photo.1); SEM by G.S. Odin in Dijon (Photo.2 to 7). All pictures from Spanish sediments collected by M. Lamboy. The accordion-like green grains are formed by de novo crystal growths of green marine clay within the cleavage planes. Scale bars show 100 pm (Photo. 1 , 2 , 3 ) or 5 pm (Photo. 4,5,6,7). (According to Odin, 1972a; 1974; Lamboy and Odin, 1974)

260 (Chapter B3) that, in a different environment, the resulting authigenic clay is not a glauconitic mineral, but phyllite V which does not have a 2: 1-type mineral architecture. Therefore, the nature of the resulting authigenic mineral is not influenced by the nature of the substrate; a simple octahedral cation exchange or re-arrangement cannot account for these observations. There is a third reason why the transformation process is an unlikely explanation for the verdissement of mica flakes. Even when evolution of the mica into an accordion-like grain is nearly complete, mica remains are preserved in the cleavages of the broken grain. Therefore, the cations of the initial micaceous substrate are not only quantitatively insufficient to form the final glauconitic material, but sometimes they are not used at all. The cleavage surfaces perpendicular to the longitudinal axis of accordionlike grains were observed using a scanning electron microscope. In Figure 3, photo.3 shows a broken mica flake. Locally, the surface is flat and represents the unaltered or little altered biotite mica (Photo.4 in Fig. 7). Small blades begin to develop on this surface by a crystal growth process. Photomicrograph 5 (Fig. 7) testifies to the multiplication of these blades from which a box-work fabric results. Upon this structure, rosettes develop unrelated to the micaceous substrate (Photo.6 in Fig. 7). The relationship between the mica substrate and the neoformed green clay is shown in Photo.7 (Fig. 7). In this picture, unaltered mica is visible in a fissure while in the open cleavage between two mica layers the green clay is growing into authigenic figures. The material grown in the cleavage leads to an opening of the accordion and the thickening of the initially thin flake. In summary, the transformation theory developped after observation of the evolution of biotite mica flakes into glauconitic, accordion-like grains is not c o n f i i e d by more detailed study. Mica flakes must only be considered as physical substrates within which cleavage planes delimit lamellar pores where green marine clay may grow as in the various types of bioclast pores studied above. The material from the detrital flakes, cations or groups of cations, does not significantly contribute to the formation of the authigenic clay and a fortiori cannot be suspected of being a prerequisite chemical or crystallographical precursor to the formation of glauconitic minerals.

The verdissement of quartz grains The verdissement phenomenon in quartz grains has long been known (Cayeux, 1916). A number of quartz grains collected from superficial sediments from the Spanish continental shelf and upper slope are green coloured. It has been difficult to obtain good pictures from them, but some are available for a similar phenomenon observed in Albian sandstones from Boulonnais, Paris Basin, France (Odin, 1975a; and Fig. 8). Photomicrograph 1 in Figure 8 was obtained from a thin section of a rounded quartz grain, 2 to 3 mm in diameter. A fissure in this quartz has been invaded by green clay, grey in the photomicrograph. A more highly fissured quartz grain is shown in Photo.2 (Fig. 8). Note that the fissures are more or

261

Figure 8. Verdissement of quartz grains. Optic microscopy of an Albian sandstone: Photo.1-3; scanning electron microscopy of green quartz collected from Spanish sediments: Photo.4-6 (according to Odin, 1975a). The glauconitic minerals are formed in the fissures and rarely replace the silica. Scale bars show 100 pm @hoto.l,2,3) or 5 pm @hOt0.4,5,6).

less parallel in the form of a net, which is green coloured in thin section. In Photo.3 (Fig. S), a broken, fissured, and dissolved quartz grain can be seen to the left, and, to the right, a dark-grey rounded particle of a very similar size and form. In thin section this grey grain is green coloured; this has been interpreted as the ghost of an initial quartz grain because numerous small remnants of quartz are present; those remnants are more or less parallel (NS in the photomicrograph) and this orientation most likely represents the orientation of the previous fissures along which glauconitization proceeded. Quartz grains similar to that in Photo.2 are not rare in the Spanish deposit; one of these was broken for scanning electron microscopic study. Photo.4 (Fig. 8) illustrates a

262 quartz (Q) fissure which has been opened, on which a film of green clay is present. In Photo.5 (Fig. 8) to the left, the green clay shows a xenomorphous surface following the form of the fragment of quartz which has been removed. To the right, more or less automorphic figures, also shown in Photo.6 (Fig. €9, are very similar to some of those observed in the opened accordion-like grains illustrated in Figure 7. In summary, fractures in a quartz grain may harbour the genesis of green marine clay, which seems to develop at the expense of the siliceous material. Quartz appears less favourable than biotite for green clay genesis because the green clay only replaces the progressively 'dissolved quartz grain, which retains its volume and shape. This process of replacement is not specific to quartz and was also observed for certain carbonate substrates as shown below.

The verdissement of non-bored shell fragments Mollusc shell fragments are sometimes characterized by a well-shaped structure showing prisms or crystallographically oriented layers. A similar arrangement has been observed in magnetic green grains from the Spanish shelf and in several ancient sediments such as the Lutetian glauconie grossi2re from the Paris Basin (Odin, 1969; 1971). The structure of these green grains obviously results from the delicate replacement of the carbonate structure as illustrated below. This mechanism appears different from the one illustrated in the four paragraphs above. The most typical green grains are flat and green coloured, and may reach large dimensions. In some cases, the original external ornamentation of the shell (such as ribs or striae) is still preserved (Photo.3 in Fig. 9). The enlarged view of the green grain in the upper left-hand corner of Photo.3 (Photo.4 in Fig. 9) shows a nice zebra-structure similar to that of a carbonate shell fragment (Bignot, 1974). A thin section of this sort of green grain (Photo.2 in Fig. 9) displays layers alternatively oriented in different directions (crossed nicols observation). At a later stage of evolution, a recrystallization process occurs concomitantly with crystal growth; cracks develop and the grain becomes thicker. Photo.1 (Fig. 9) illustrates this stage of evolution where one side of the grain exhibits the initial zebra-structure and the other a verrucous, cracked aspect. The cracked side is convex but the side showing the zebra-structure preserved is generally concave. This observation again proves that the cracked side actually represents later growth which followed the replacement phenomenon. The scanning electron microscope allowed more detailed pictures showing the relations between the layers to be obtained (Photo.5 in Fig. 9). Photo.6 (Fig. 9) illustrates the various alternative orientations of the successive layers of green material. An enlarged view (Photo.7 in Fig. 9) displays a very particular arrangement of the clay minerals which are actually oriented and do not form idiomorphic figures.

263

Figure 9. Verdissement of non-bored shell fragments. Optic views (Photo.l,3,4); thin section (Photo.2, courtesy of A. Matter); SEM (Photo. 5, 6,7). All pictures illustrate Lutetian green grains from the Paris Basin; similar green grains with a zebra-structure are present on the shelf off northwestern Spain. (Partly according to Odin, 1969; 1971) Scale bars show 500 pm (Photo. 1 , 2 , 3 , 4 ) or 10 pm (Photo. 5,6,7).

264 Lamboy (1976) has illustrated a similar process of replacement for biogenic carbonate fragments without initial zebra-structure and without borings. Again, dark-green cracked grains result from the complete evolution of the carbonate bioclasts. Such a replacement (without zebra-stucture) has also been observed in bored shell fragments and involves glauconitization of the areas between the borings. Therefore, the replacement process is common and constitutes a phenomenon apparently distinct from the filling process. In summary, magnetic green grains with a zebra-structure are easily recognizable (Triplehorn, 1966). They result from the verdissement of shell fragments and represent a replacement process. Following this process, later evolution consists of disordered crystal growth in the centre of the grain. The zebra-structure may be preserved on the surface but is usually destroyed when cracks develop.

Other morphological features

Formation of rounded grains

The foregoing six examples of verdissement describe the formation of green marine clay within a granular substrate. The resulting green grains are initially formed either by replacement or by filling pre-existing pores. Later on, crystal growth modify the form of the substrate and a cracked grain results. However, a large proportion of the dark-green grains separated from the superficial sediments of the deepest portion of the Spanish shelf have a rounded form and a smooth surface. Lamboy (1976) has shown that these rounded grains result from the evolution of cracked grains, and represent a new evolutionary stage. Two processes are able to transform the cracked grains into rounded grains. The f i s t process consists of filling the cracks in already well evolved darkgreen grains (Lamboy, 1975; 1976 p.123). To observe the presence of one or several generations of filled cracks, special preparation in oils of different indices is needed. The green clay which fills these earlier cracks is usually lighter than the rest of the grain. An easy means of revealing these concealed cracks is to submit the rounded grains to ultrasonic treatment. This removes the later green clay which is softer than that formed earlier. The second process able to produce rounded grains is the addition of what has been described as fibro-radiated rims (Collet, 1908; Cayeux, 1916). According to observations by Bailey and Atherton (1969), Zumpe (1971), or Lamboy (1976), these cortices may be either clayey or clayey and phosphatic. Photomicrograph 1 in Figure 10 displays a dark-green rounded grain, about 600 pm in diameter. This grain has been superficially broken with pliers and the external envelope can be observed (Photo.2 in Fig. 10). The cortex is more than 30 pm thick. The green clay crystals are radially arranged, and this explains the fibro-radiated structure observed in thin sections. In some grains, the cortex is composed of several layers with the external one invariably thicker and radially oriented (Photo.3 in Fig. 10). In the majority of rounded grains it is difficult to recognize the original substrate. However, sometimes ghosts of structures have been observed from

265

Figure 10. Dark-green grains made round by addition of a cortex. The phenomenon illustrated follows the evolution shown in Figure 5; it possibly occurs within the sediment after moderate burial. Scanning electron microscopy. Scale bars show 100 pm (Photo.1) or 10 pm (Photo.2,3).

which it can confidently be presumed that the original substrates were bored shelly fragments. The above observations mainly prove that the rounded appearance of dark-green grains from northwestern Spain does not result from an in situ reworking and abrasion of previously irregular grains but correspond to an addition of a later generation of green clay around evolved green grains. A physical rounding of the green grain before this addition of green clay cannot be ruled out, however. T k verdissement of large lithoclasts The complete verdissement of comparatively small sediment particles has been described above. The diameter of these particles may vary from 0.1 mm for foraminifer chambers and quartz grains, to several millimetres for bored shelly fragments. But the sediments from northwestern Spain contain much larger substrates which have partly undergone verdissement. For example, numerous boulders or large fragments of sedimentary rocks are green coloured externally. Carbonate-rich blocks are more favourable to the development of a green film than other boulders.

266

As far as size is concerned, a complete range exists from fully glauconitized small grains (0.5 mm in diameter), through larger grains (more than 2-5 mm in size) where external portions are entirely green and indurated, whereas their centres, although green, have remained soft (see Fig. 5 , p. 255), and onto centimetric or decimetric boulders and blocks with a millimetric green film present at the surface. Detailed observation on thin sections or on broken grains indicated that the green clay had developped at the surface because that part of the particle had undergone physical boring and chemical alteration. This had allowed pores to form and green clay to accumulate in them. MINERALOGICAL STUDY

X-ray diffraction study

Figure 11. X-ray diffraction patterns of decarbonated green grains. Slightly-evolved grains contain less than 6% K 2 0 (ML 210: fillings of bored shell fragments; ML 348: green echinoderm fragments with reticulated structure and cracked olive-green grains). Highlyevolved grains contain about 8.8% K20, and are dark-green in colour (ML 212: dark-green grains).

267 The facies term glaucony has been used above to designate the green grains from northwestern Spain because preliminary mineralogical results have shown glauconitic minerals to be present throughout that area. X-ray diffraction patterns are simple and easy to interpret since the pure authigenic phase can be separated by dissolution of the carbonate substrate, in contrast to the previously discussed example from the Gulf of Guinea (Chapter C1) where the clayey nature of the initial substrate prevented pure authigenic phases, especially in relatively unevolved grains, from being identified. The diagrams obtained varied, but an obvious relationship exists between the morphological evolution and the X-ray diffraction patterns as shown in Figure 11. Bored carbonate fragments were dissolved in dilute acid to obtain pure green filaments; in sample ML 210, the powder was olive-green and the diagram indicates a large proportion of glauconitic smectite. The green clay filling the fragments of echinoderm stereom is also made of very open minerals (sample 348 in Fig. 11). The diagram for sample 349 was obtained from selected, olive-green grains with many cracks. Sample 212 produced a diagram with a comparatively well-shaped peak near 10 A; this sample is predominantly composed of dark-green rounded grains. It is worth noting that, in spite of a highly-evolved morphology and of a sharp 001 diffraction peak located at 10 A, the dark-green grains give an X-ray diffraction pattern with a restricted number of diffractions. This is characteristic of a disorder in the crystallographical structure (Bentor and Kastner, 1964). GLAUCONY ML253

Iq ITa ;

I

Hard middle

Soft centre

./=CO.R7:

Figure 12. X-ray diffraction patterns of various portions of dark-green grains resulting from the evolution of bored carbonate fragments later surrounded with a cortex. The hard nuclei are illustrated in Fig. 5, the cortices are illustrated in Fig. 10. (According to Lamboy and Odin, 1975)

268 The crystallographic characteristics of the green clay vary not only from sediment to sediment, but, within a given sample, from one grain to another. For example, the magnetic grains from sample ML 349 included bored green grains, olive-green grains with numerous cracks, and dark-green smooth grains. Powders prepared respectively from these different grains gave diagrams very similar to the three lower patterns shown in Figure 11. The different portions of the dark-green grains with a cortex were separated using dilute acid and ultrasonic treatment. Three diagrams were obtained for sample ML 253 (Fig. 12). In contrast to the rest of the grain, the cortex is made of glauconitic minerals poorly crystallized (open). The selected hard nuclei show better shaped peaks, and all hkl peaks of the glauconitic mica are visible. However, the 112 and 112- peaks remain much smaller than the 003 peak.

This is related to their disordered structure, and possibly to the fact that the randomly oriented powder is easily re-oriented when deposited on the glass slide; this orientation increases all 001 diffraction peaks. This artifact is especially difficult to avoid for the apparently very fine-grained soft, blue-green clay removed from the centre of the grains. The resulting 001 peaks and probably the 003 ones are artificially increased for this fraction. This allows the mean thickness of the layers, which is about 10.2 A, to be observed.

Chemical study Table 1. Chemical analyses of dark-green grains collected off northwestern Spain. Potassium data are maximum values. SiOz

A1203

Fe2O3

FeO

CaO

MgO

Na20

K20

H20-

H2O'

Total

M L 212

46.4

0

27.9

3.2

1.0

4.1

0.2

8.75

( C 242A)

45.7

6.2

25.1

2.1

0.7

4.1

0.2

(8.8)

1.4

6.3

100.6

M L 253

46.9

6.5

23.1

1.6

0.8

3.8

0.2

(8.9)

1.4

6.1

99.3

47.6

4.3

24.6

1.7

0.7

3.9

0.3

(8.9)

1.4

6.1

99.5

45.4

5.8

6.4

1.6

0.7

3.5

0.3

(8.2)

1.6

6.2

99.7

48.7

7.8

19.9

2.6

0.7

3.8

0.2

(8.0)

1.8

6.2

99.8

8.05

99.6

( C 243A)

M L 276 ( G 245A)

M L 350 ( C 250A)

M L 279 (G 252 A )

The chemical composition of glaucony separated from the Spanish sediments was studied by Caillkre and Lamboy (1970b). The authors measured diverse potassium contents down to 3% in light-green grains and around 8% in dark-green grains. They also emphasized the exceptionally low alumina content (which may reach zero percent in a few samples) and the very high iron content. The purity of the separates analysed by these authors was not very good, however, especially for the light-green grains, and the range of values obtained in this preliminary study was much too high. More recent analyses

269 were undertaken on the same sediments after a better purification. From these results, it may be concluded 1) that some goethite may be present in dark-green grains; 2) that the samples thought to be aluminium-free in fact contained a measurable proportion of that element (Odin, 1975a).

For comparison with previously published data, Table 1 indicates both the number of the sediment (ML,) and the specific number of the glauconitic fraction purified later (G) for which the first author of this chapter has usually selected the size-fraction -500 pm +160 pm (A). Note that the high potassium contents must be considered as maximum values because reference materials measured together were found systematically 5% too high and because sample G 245A measured at 8.9% in 1975 was remeasured recently at 8.1%.

In summary, the purified green clays have a fairly constant chemical composition, the main variation being the potassium content, which increases when the morphology of the grains indicates a more evolved stage.

Isotopic study Stable isotopes have been measured in various green grains from Spain (Keppens et al., in press). Figure 13 suggests a trend from low 6 l 8 0 values measured for K-poor green grains toward high 6l80 values for K-rich green grains. However, sample ML 253 deviates from the regular trend shown by the other four samples. Confirmatory experiments would be of interest; however, the particular nature of the initial substrate suggests some remarks. Because the purified green grains do not contain remnants of the initial substrate dissolved by acid leaching, the measured values concern authigenic minerals alone. In spite of this, the smectite-rich (K-poor) green grains show a 6l80 value clearly lower than the K-rich grains, and, therefore, mimic a light isotope inheritance. I

ML276

23

22

i

ML212

I1

I

GSO 87

---I

ML253

ML279

1

5

I

6

1

7

4

8

K20

4

I

9

*

W0/o

Figure 13. Oxygen isotopic composition of diversely evolved glauconitic grains from northwestern Spain.(According to Keppens et al., in press)

270 Precise and definitive conclusions cannot be drawn from analysis of these sediments because the moment of glauconitization may have widely influenced the presently measured composition due to the occurrence of large fluctuations in sea-water composition during Quaternary time. Two types of glauconitic grains were analysed for K and Ar: olive-green grains made of an open clay mineral separated from the sediment ML 210, and dark-green grains of a mica-like clay mineral. Table 2 shows rather imprecise results for potassium content, probably due to sample inhomogenity. Argon content was also measured with a large error bar due to atmospheric argon contamination. This was usual 15 years ago when measurements were made by routine analysis. However, the results clearly indicate that the radiogenic argon content of the green grains was not negligible. The apparent ages calculated taking into consideration the potassium content are analytically similar for both types of glauconitic grain. This is in contrast to what was shown for the green faecal pellets from the Gulf of Guinea (Chapter C l ) , where the apparent ages were dependent on the potassium content i.e., on the stage of evolution. This is easy to understand when it is remembered that, off northwestern Spain, the initial glauconitization substrates (mainly carbonate bioclasts) do not contain radiogenic argon. Therefore, the radiogenic argon measured today is totally authigenic in both types of grain, evolved or otherwise, and comes from the decay of the potassium present in the grains. The analysed glauconies were probably formed between 5 Ma and 6 Ma ago. A corollary of this assumption is that, since that time, the measured glauconies have been "dead" i.e., they have not exchanged cations with sea-water and the system is closed. In other words, the glaucony observed on the deep part of the northwestern Spanish continental shelf is relict, and no sediments have been deposited during the last 5 Ma (except for some foraminiferal tests and a small quantity of detrital clay). This conclusion agrees with our present knowledge of the shelf where Miocene sediments underlie the soft sedimentary cover. Table 2. K-Ar analysis of two types of green grains from the sedimentary cover off the NW Spanish shelf. Data by G.S.O. in the University of Berne. (According to Odin, 1975a; Lamboy and Odin, 1975) Sample

F.P.

wt.

ML 210 ( - 175 m )

5.3 2 0.3

h l L 279 ( - 190 m )

8.0

0.2

W,

Atmospheric A r

Rad. Ar (n l .g-l)

Apparent age (Ma+ 2 0 )

5.6 ? 0.3

87.4

1.017

5.7 ? 1.7

0.3

62.3

1.504

5.6 ? 0.6

K20 A.A.

8.0

The conclusion proposed above is similar to what can be deduced from the glauconies from Chatham Rise (New Zealand) analysed by Cullen (1967) or the major portion of the glauconies from the south African continental shelf (Odin, 1985b).

27 1 DISCUSSION

Morphological features and glauconitization process The sediments from northwestern Spain allowed a large variety of initial glauconitization substrates to be identified. Their diverse mineralogical and chemical nature has no particular relationship to the green clay formed within them. A number of substrates are carbonate bioclasts, which usually undergo very complete evolution leading to dark-green grains with little morphological resemblance to the initial clasts. Some substrates observed off northwestern Spain are mineral debris (quartz, biotite); in this case, evolution is not finished: mineral remnants and the initial structure of these substrates can be recognized. Carbonate substrates appear specifically favourable for harbouring glauconitization, as had long been noted by earlier authors (Cayeux, 1932). This unequivocal fact was forgotten by most authors supporting the mechanism of the transformation of inherited clay into glauconitic minerals as the major process in glauconitization. Lamboy (1976) notes that the ability of carbonate constituents to harbour glauconitization is mostly due to their alterability. This is supported by the need for the presence of pores in a substrate for glauconitic minerals to grow, as discussed below. There is a clear trend in all the evolution described for the various substrates from northwestern Spain; during an initial stage, the general structure of the substrate remains unchanged; later, the substrate is destroyed, (more or less depending on its alterability); ghosts of the initial structure may still be recognized during this second stage. A third stage consists of a general deformation of the grain, which becomes cracked whatever the initial substrate. Finally, these cracked grains may further evolve into dark-green grains with the external fissures and cracks filled in.

Mineralogical features and glauconitization process Glaucony from the sedimentary cover off northwestern Spain allowed a clear relationship between morphological and mineralogical evolution to be established. The latter is similar to that described for the Gulf of Guinea in its initial stages, but progresses further (like morphological evolution). The glauconitic minerals form a continuous series from smectitic nascent components up to micaceous highly-evolved components. As already observed for the sediments from the Gulf of Guinea, the potassium content is all what is needed to reliably estimate the degree of evolution. However, with some exceptions, the above general scheme needs developing, for example, because not all substrates in a given sediment are equally favourable, the degree of glauconitization will never be uniform. Furthermore, it has been noted that not all portions of a given substrate are equally favourable for glauconitization to develop; therefore, even a single grain will display various evolutionary stages, as has been shown by X-ray diffraction. This indicates that multiple analysis of various fractions of each grain and sediment

272 will need to be undertaken in connection with each mineralogical characteristic, if the history of a sediment is to be accurately documented.

Age of the glauconitization process off northwestern Spain The sedimentological significance of the glauconitic sedimentary cover off northwestern Spain is made more complex by its long history. Geochronological and sedimentological evidence indicates that a large proportion of the glaucony collected from the shelf was formed 5 or 6 Ma ago. No trace of a burial process is evident in the glauconitic grains; however, alteration has occurred since traces of goethite have been observed on X-ray diffraction patterns resulting in an abnormally high ferric oxide content. The major portion of the glaucony observed in the sediments dredged off northwestern Spain is relict and related either to the late Miocene regressive period or to the general transgression identified at the beginning of Pliocene time on this margin (Durand, 1974). However, on the one hand, we have observed in situ reworked nummulites and other Late Eocene microfossils of which the chambers were filled with marine green clay suggesting the possibility of an inheritance. On the other hand, the episodic presence of green clay within foraminifera1 tests of Holocene age, on the outer shelf and slope, indicates that glauconitization occurred at different periods.

Mechanism of glauconitization The study of the Spanish sediments has proved that glauconitic clays were initially formed by a crystal growth mechanism and not by a transformation mechanism because 1) in the majority of cases, the green grains were obtained from non-clayey substrates; 2) in cases where an earlier micaceous substrate was utilized (biotite mica flakes), the latter would not have been able to support a transformation mechanism from a simple quantitative point of view; and 3) the same substrate was able to give different authigenic green clays in different environments (mica flakes evolve to verdine off French Guiana -see Chapter B3- and into glaucony off northwestern Spain) whereas the same green clay could be obtained from different mineralogical substrates. Therefore, the study of the Spanish sediments allows us to dissociate the nature and composition. of the substrates from the nature and composition of the authigenic green-clay. This is in conflict with the theory of transformation which postulates a mineralogical link between the inherited material and the authigenic one (Burst, 1958a; Shutov et al., 1970) including for Recent sediments (Bell and Goodell, 1967). This conclusion concerns the first stage of evolution of the substrates. During the second evolutionary stage, the initial potassium-poor marine green clay becomes potassium-rich. It would be possible to invoke a transformation process by potassium absorption for this second stage. However, the morphological and nanostructural features indicate that the initial structure is always destroyed; this whole reorganization on a nanostructural scale can only

273

be understood by assuming a recrystallization process i.e., dissolution of the previously formed clay and use of the available cations to form new microcrystals, richer in potassium. Another point supporting the crystal growth mechanism has already been observed in the glaucony from the Gulf of Guinea and is well illustrated in the verdissement of echinoderm and mollusc skeleton fragments, foraminiferal casts and biotite mica flakes; it concerns the volume increase of the initial substrate and resulting cracks at the surface of the many glauconitic grains.

Environment for glauconi tizat ion The most important feature emphasized by the study of the Spanish glauconies is the role of pores in the genesis of the green clays. A variety of pores are involved: chambers of microfaunal tests (50 p m to 300 pm in diameter), pores of echinodermal stereom or borings in carbonate bioclasts (all are cylindrical pores, 10 pm to 15 pm in diameter), fissures along cleavage planes in mica flakes, or fissures in quartz grains (about 1 pm in thickness). All these pores are initially filled with glauconitic minerals which preserve the initial structure of the substrate. But glauconitic minerals which intimately replace the initial carbonate substrate have also been found. It is suggested that the fundamental process involved is similar in both cases. As far as the replacement is concerned, the "pore" filled is simply a dissolved microcrystal of carbonate immediately replaced by clayey material which, consequently, takes on the form of the earlier carbonate crystal and the morphology of the bioclast. This leads to an apparent orientation of the authigenic clay particles moulding the previous carbonate nanostructure. Therefore, pores constitute the favourable microenvironment for green clay crystal growth. The size of the porous substrates is also an interesting character influencing green clay formation. The most favourable substrate appears to be formed of deposited grains 100 pm to 500 pm in size. As a function of their alterability (ability to form pores), the favourable substrates and resulting green grains will be smaller or larger. Quartz or mica appear more favourable when their size approaches 100 pm,whereas 0.5 to 1 mm large carbonate particles are usually entirely glauconitized. Larger substrates will only be partly glauconitized at the surface. The size of the deposited substrate, the portion of the substrate submitted to glauconitization, and the presence of pores are all aspects of a single phenomenon which can be interpreted in terms of confinement of the microenvironment favourable for green-clays to grow. As was the case for the sediments from the Gulf of Guinea, there are no glauconitic minerals in the free clay-size fraction of the Spanish shelf. If green clay forms in pores of the deposited grains, it is because they find a more favourable microenvironment there, more confined than between sand-sized grains for example. Inside a large boulder, this confinement will be much too closed, and the cationic exchanges will not be rapid enough for the substrate to dissolve or for the green clay to gather the necessary cations for effective crystal growth. On the other hand, sand-sized particles of the sediment allow

274

an easier equilibrium between crystal growth, concomitant cationic feeding, and substrate alteration. A semi-confined microenvironment is, therefore, a prerequisite for the formation and evolution of glauconitic minerals. This notion is fundamental to glaucony genesis; a corollary is that, if glauconitic minerals are always more abundant with a granular habit, it is because the favourable initial substrate was itself in the form of grains. This conclusion differs from that suggested by papers which suppose the first step in glauconitization to be the presence of inherited nontronite (a smectite which resembles glauconitic smectite), and the second the formation of pellets by physical agglomeration or biologic agglutination (Trauth et al., 1969). In short, in glaucony genesis, granular habit precedes the formation of green clay and not vice versa. SUMMARY

The glauconitic sediments from the continental shelf off northwestern Spain allowed various stages of the glauconitization process for various types of substrates to be observed. Verdissemen t mostly occurred within biogenic carbonate fragments which appear very favourable for harbouring green clay growth. Microfaunal tests and mineral debris (biotite mica or quartz) have also been utilized in these Spanish sediments. Verdissement mostly took place at a time near the Miocene-Pliocene boundary. The obtained green grains are essentially authigenic and dominantly result from the evolution of shelly bioclasts. It can be seen that all green clays have formed by a crystal growth process, and the transformation process theory is contradicted by the observations. The mechanism comprises two stages; the preliminary stage is largely favoured by the presence of micropores in the substrates; it consists of filling pores with potassium-poor glauconitic minerals while preserving the substrate. The second stage is conditioned by the dissolution of the substrate remnants and consists of crystal growth of a second generation of green clay followed by a recrystallization process allowing potassium-rich glauconitic minerals to grow partly from the dissolution products of the potassium-poor glauconitic minerals formed earlier. The most abundant and usual product of glauconitization is green grains. This habit results from the fact that granular substrates are initially the most favourable for allowing green clay to crystallize i.e., the green clay genesis follows the granular habit. The pores within the grains delimitate a favourable microenvironment which is characterized by isolation from open sea-water. But pores also facilitate, to a certain degree, cationic exchanges with interstitial fluids. This microenvironment is defined as semi-confined and its influence is well illustrated off northwestern Spain. Apparently, the whole evolution, including the last stage leading to potassium-rich dark-green grains, is possible in close contact with the open sea-water environment without burial diagenesis.

275 ACKNOWLEDGEMENTS

Scanning electron microscope study was undertaken by the authors at the DCpartement de GCologie, UniversitC de Dijon (in 1970-1971), and at the Laboratoire de micropalkontologie, UniversitC Pierre et Marie Curie, Paris (in 1972-1979). The microfaunal tests filled of glauconitic minerals were identified by A. Blondeau and P.A. Dupeuble. Argon isotopes were measured by the editor thanks to the kind welcome and help of J.C. Hunziker in Berne. Chemical analyses, especially for potassium, were carried out by M. Lenoble and G. Richebois. High quality photographic processing by 0. Fay is greatly acknowledged. This chapter was improved for English by Dr. Pennington.