Clay minerals in Southeast Indian Ocean sediments, transport mechanisms and depositional environments

Marine Geology, 25 (1977) 149--174 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

CLAY MINERALS IN SOUTHEAST INDIAN OCEAN SEDIMENTS, TRANSPORT MECHANISMS AND DEPOSITIONAL ENVIRONMENTS

KEVIN C. MORIARTY

School of Earth Sciences, Flinders University of South Australia, Bedford Park, S.A. 5042 (Australia) (Received March 28, 1977)

ABSTRACT

Moriarty, K.C., 1977. Clay minerals in Southeast Indian Ocean sediments, transport mechanisms and depositional environments. Mar. Geol., 25: 149--174. The clay mineralogy of the sediments of the Southeast Indian Ocean and Southwest Pacific Basin has been analysed by using chemical analyses and X-ray diffraction on 245 samples. The Wilkes Land sector of Antarctica supplies two different assemblages of clays to the South Indian Basin. The assemblage from western Wilkes Land is characterised by iron-bearing mica, amphibole and montmorillonite, while the eastern part is characterised by alkali felspar, quartz, muscovite and interstratified smectite. Both areas are the source of plagioclase, and the mineralogy is believed to reflect the change from a granitic to mafic shield from East to West Wilkes Land. The smectites are probably reworked from Tertiary strata on the continental shelf and rise. Minor kaolin is associated with the montmorillonite. Chlorite is derived from Victoria Land metasediments and transported by the west-flowing bottom waters to the Ad~lie continental rise. Abundant iron-bearing illite from western Wilkes Land must have been deposited on the continental shelf before the formation of the icecap. Eolian processes are believed to be the main supplier of kaolin, smectite and quartz to the South Australian basin. South Australia is the source area for abundant interstratified smectite. New Zealand supplies a dominantly mechanically weathered assemblage of muscovite, chlorite, amphibole and plagioclase to the surrounding basins. This assemblage is probably glacial eroded rock flour, from South Island.

INTRODUCTION T h e c l a y m i n e r a l o g y in d e e p o c e a n s e d i m e n t s o f h i g h l a t i t u d e s is p o t e n t i a l l y a p o w e r f u l t o o l in assessing t h e c h a n g i n g e n v i r o n m e n t s on n e a r b y c o n t i n e n t s b e c a u s e t h e m a g n i t u d e o f c l i m a t i c v a r i a t i o n s is high a n d e r o s i o n a l p r o c e s s e s p r o n o u n c e d . C l a y m i n e r a l s are s e n s i t i v e i n d i c a t o r s of t h e i r e n v i r o n m e n t o f formation and their composition could be used to show climatic variations w h i c h d o n o t a f f e c t o t h e r size f r a c t i o n s , e.g., g l a c i a t i o n s are a s s u m e d f r o m t h e a p p e a r a n c e o f i c e - r a f t e d f r a g m e n t s in a c o r e , b u t c l a y s t r a n s p o r t e d b y b o t t o m c u r r e n t s m a y i n d i c a t e glacial e r o s i o n in t h e s o u r c e a r e a l o n g b e f o r e the appearance of ice-rafted debris (Gostin and Moriarty, 1975). Off the

150

eastern Wilkes Land coast, Chapuis (1974) attempted to use diffraction analysis of the < 37 pm fraction to detect climatic change, but did not achieve convincing results because he did not know the patterns of mineral occurrence in the surface sediments of the surrounding area. Many complications may arise when using clay minerals to interpret past climatic events, especially where less severe climatic changes occur. The time lag between erosion at source and final sedimentation may be long, perhaps several million years in the case of reworked sediments. Another complication is the persistence of clay minerals in softs from one climate regime to another (Tedrow and Douglas, 1964; Carroll, 1 970) and this is to be expected in regions which alternate between humid and arid weathering. Soils formed in interglacial periods in arctic soils can be eroded unchanged in the glacial period following, thereby mixed with glacial " r o c k flour" and loess. Such clays are found in Arctic Ocean sediments which contain kaolinite and chlorite (McDougall and Harris, 1969; Carroll, 1970; Darby, 1975). Although studies over large oceanic areas (Biscaye, 1965) show that the major clay mineral abundance patterns can be explained by the interaction of continental sources and ocean distribution mechanisms, alteration and authigenesis are advanced by some authors (Berry and Johns, 1966) as major mechanisms. Clay mineral studies which include chemical analyses are sparse, but McDougall and Harris (1969) and Darby (1975) suggest no alteration and little authigenic formation in sediments with high terrigenous accumulation rates. Chemical analyses provide not only guides to compositional variations, b u t aid in absolute quantification of clay minerals. Environmental interpretations will be greatly facilitated by a knowledge of the variation in composition of similar clay minerals, for instance, the potassium c o n t e n t of micas, or iron content of smectites. These compositional variations affect X-ray diffraction structure factors, and abundances of minerals estimated from diffractograms must be qualified by reference to the composition of the species. Both chemical analyses and X-ray diffraction results have been used to construct the clay mineral distribution maps for this paper. PHYSIOGRAPHY AND PHYSICAL OCEANOGRAPHY Fig.1 shows the station locations superimposed on the generalised physiography which is based on the b a t h y m e t r y of Hayes and Connolly (1972). The location of the Antarctic Convergence occurs close to the mid-ocean ridge crest at a b o u t 50 ° S in the mid and western parts of the area. Central regions are occupied by the Circum-Antarctic Current which flows eastward, although a small westward c o m p o n e n t may occur over the ride crest (CaUahan, 1971). Close to the continental slopes of the continents, surfacewater flow is westward (Gordon, 1972). Deep-current directions and intensities have been measured by Callahan (1971), Eittreim et al. (1972), and long-term current activities inferred by Watkins and Kennett (1972) and Kennett and Watkins (1976). Average current effects over longer time intervals are important for clay mineral studies. General features of b o t t o m

151

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currents are described by Gordon (1972) and those relevant here are: (1) Antarctic Bottom Water (AABW) is produced in the Ross Sea and travels westward along the continental rise of Wilkes Land where it mixes with similar water produced o f f the Ad~lie coast. The AABW then spills over the continental shelf at about 140 ° E longitude (off the Ad~lie coast), travels to the west along the continental rise and turns clockwise around the South Indian Basin (SI Basin) to flow north and east through the Tasman Fracture Zone into the South Tasman Basin. (2) Circum-Polar deep Water (CPW) flows eastward through the South Australia Basin (SA Basin) then south of the South Tasman Rise and northward in the Tasman Basin.

152 METHODS

A total of 245 samples of less than 2 pm clay from "Eltanin" cores were analysed by X-rays, using the analytical methods described in Moriarty (in prep., a). Most samples were taken from the top 5 cm of trigger cores except for very few from as much as 15 cm, and some piston cores were sampled where trigger cores were not available. The sediments were treated with acetic acid to remove carbonates and the clay fraction separated by centrifuge. Several treatments were given to all clay samples and these will be discussed here. The object of the treatments was to facilitate the identification of basic lattice characteristics and compositions of the minerals by removing oxides and hydroxides which occur in both free association and interlayered with the mineral lattices. The interlayer material added to the lattices during transport or after sedimentation is assumed to be removed easily, whereas that which is fixed in primary formation of the mineral should be relatively unaffected by these treatments. An example of this might be the removal of aluminium or magnesium hydroxy interlayers in smectite, but the preservation of such layers in well-crystallised chlorites through the treatments. The clay was washed and stirred in 0.5N NaOH for one hour at 80 ° C to remove opaline silica and aluminium hydroxides, then treated twice with sodium dithionite buffered with sodium bicarbonate-citrate to remove iron oxides (Mehra and Jackson, 1960). The supernatants were analysed by atomic absorption for Si, A1, Fe and Mn. The results are discussed in another paper (Moriarty, in prep., c); however, it should be noted that dissolution does not affect the interpretation here, except that gibbsite mineral will be dissolved. QUANTIFICATION

Diffractograms were produced for 227 stations. The remaining 18 were sited in areas of very low or negative clay accumulation and did not yield enough material. Silicate analyses of 129 stations represent most areas, although sediments with low clay contents, mainly on the south ridge flanks, are poorly covered. Contours on the clay mineral abundance maps are produced with reference to stations which did not yield silicate analyses but did have enough clay for XRD. The physiography was used to suggest locations of contours at basin margins. Quantification methods are complex and will be discussed only briefly here. A full description is the subject of another paper (Moriarty, in prep., a). XRD was used to identify the major clay mineral groups in several type areas for which good chemical analyses had been obtained. Each mineral was a~igned a chemical composition taken from the literature or analyses available to the author, and abundances of that mineral calculated from the chemical analyses on each sample. If a good fit to the analysis could not be obtained, the compositions were varied according to the data available from the diffraction characteristics. The effect of mineral assemblages from two or more type areas was also used to check the

153

results. When a consistent composition was obtained, coefficients of intensity were derived for use on all samples in that area. These turned o u t to be fairly uniform over the area of study except for smectite and illite minerals. In the case of the smectite, the problem was overcome by using the cation exchange capacity (CEC), while the illite abundances were calculated using coefficients which varied with their crystallinity, but the results are subject to artifacts. The restraints imposed by the two sets of data, chemical and mineralogical, ensure that many major errors c o m m o n to semiquantitative X R D analyses are very much reduced. RESULTS

General The abundance of the major clay mineral groups, viz., smectite, illite, kaolin, quartz, amphibole, chlorite, plagioclase and potash felspar is presented in Figs.2--9. Figs.10--12 are maps of parameters useful in deciding clay mineral composition and show variations within some of the groups. Fig .10 shows the width of the 10-h peak at half peak height to indicate the major changes in crystallinity of illite in the southeast Indian ocean. High crystallinity is indicated by low widths and vice versa. Fig.12 shows the ratio of the cation exchange capacity of the clay to the 18-£ peak heights. High values of this ratio indicate interstratification in the smectites or poor crystallinity. The use of these ratios is described in detail in Moriarty (in prep., a). The time interval represented by the samples, and their age, are affected by the presence of reworking as well as sedimentation rates. If there is reworking, and this is most likely over most of the area of this study, the depth of the reworked layer will depend on current activity and bioturbation. Payne and Conolly (1972, p.353) measure 10--20 cm of reworking in cores from the South Indian Basin. The clay mineral assemblage to some depth in a core will therefore reflect the average clay sedimentation over a long period, the length of which will depend on the sedimentation rate. Clay mineralogy should show only gradual changes with depth in a core and Moriarty {1970) demonstrated this on trigger cores, b u t turbidity currents can sediment a new layer with a sharp contact. In order to distinguish the source area it is important to know whether the clay arrived at the sampling site by turbidity current or other means. The amount of opaline silica was measured after it was extracted from the clays and the map of its percentage abundance is shown in Fig .13. If it is assumed that a high content of opaline silica indicates pelagic sedimentation, then the map can be used to show where turbidity currents have n o t occurred recently. It is possible that turbidity currents might rework siliceous material from the continental shelf or rise, and this was taken into account when using the map. Some detailed core descriptions by Payne and Conolly (1972) were useful in the South Indian Basin. It should be noted here that low contents

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Fig.2. Smectite mineral abundance in percent calculated from cation-exchange capacity.

of opaline silica in the clay do not necessarily indicate the presence of turbidity currents. Many other factors, such as productivity variations or dissolution, could produce the effect. Fig.14 shows the percentage of A1203 entracted with the opaline silica, and, if high, indicates the presence of gibbsite mineral. Fig.15 shows the percentage of clay minerals in the salt-free sediment, but the minerals removed by the dissolution treatments are not included as clay minerals.

Clay mineral distributions 10-• minerals. These minerals are usually simply classified as iUites in studies of oceanic clays, and a single coefficient used to describe their abundance,

155

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Fig.3. Chlorite abundance in percent. Clays from stations marked " T " contained talc.

b u t the diffraction characteristics demonstrate major differences in the species present in the Southeast Indian Ocean so that both chemical and diffraction data must be used to derive realistic abundances. The diffraction characteristics alone can indicate the major types if the three maps of abundance (Fig.4), crystallinity (Fig.10), and 10/5-A ratio ( F i g . l l ) are used together. The main types present in the area are: (1) Muscovites or dioctahedral micas which are very well crystallised and show low to intermediate 10/5-h ratios (3--5) indicating variable b u t low iron contents. (2) Fe-illite which is dioctahedral mica showing very low crystallinity and a high 10/5-A ratio indicating a high iron content. (3) Illite which shows intermediate crystallinities and low 10/5-A ratios.

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It is difficult to define an area containing a typical "illite" from the maps, perhaps indicating that this mineral is a mixture of the two types above in some places, while in others it may be a detrital phase from a single source. An examination of b-parameters revealed no detectable trioctahedral micas such as biotite, but small quantities are believed to be present in the South Indian Basin. This is believed to be responsible for the higher 10/5-A ratios in the well~rystallised muscovites of the Ad~lie Rise. The 10-A minerals are very abundant close t o the continents and are a minor c o m p o n e n t only on the mid-ocean ridge. Their greatest abundance occurs in the South Indian Basin, mainly in the zone of Fe-illite in the western part. The main interest in the distribution is the qualitative changes shown by the maps of the compositional parameters. While the abundance map (Fig.4) shows

157

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features which indicate northward transport of 10-A minerals into the Australian Antarctic Discordance, the crystallinity map (Fig.10) shows that Fe-illite is carried north in a narrow zone over the ridge west. There is also evidence that it is transported to the east across the South Indian Abyssal Plain toward the Balleny Trough, and perhaps also northward to the ridge west at a b o u t 105 ° E. However, the 10/5-A ratios do n o t clearly support the latter direction of transport. It is interesting to note that the patterns Of transport do n o t follow the b a t h y m e t r y very closely, indicating that the transport mechanisms do n o t depend on the b o t t o m contours. This means that while turbidity currents could have been the initial transporters of clay to the deep basin, their effect on the patterns has been modified by later events.

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Zones of high mica crystallinity occur on the southern flanks of the ridge in areas of high to very high opaline silica abundance (Fig.13), and these probably indicate areas which only ice-rafted debris reaches. To show the sedimentation rates o f clay are low in these areas, the data of Kennett and Watkins (1976) can be combined with the clay contents of Fig.15. A continuous zone of high crystallinity mica also occurs on the north flank of the ridge (Fig.10), but the abundance pattern is complex here, and ice-rafting is not likely to be significant at these latitudes (40--47°S) which are north of the convergence (Watkins et al., 1975). The 10-~, minerals o f the South Australian Basin are abundant but of curiously high crystal!inities, considering that these minerals are probably derived from Australian soils which generally exhibit low crystallinities (J.

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Pickering, pers. comm.). Lower crystallinities occur southwest of Australia, and the 10/5-£ ratios are l o w to intermediate, suggesting that this is an area where soil illites are sedimenting. The areas around Tasmania and southeastern Australia show high crystallinities combined with very low 10/5-A ratios, indicating muscovite micas.

Chlorite. Moriarty (in prep., a) assessed the evidence for variation in the composition of the chlorites in these samples and suggests that the iron content is l o w and relatively constant and there is no great variation in magnesium content. Chlorite (Fig.3) occurs most abundantly in the Solander Trough, Emerald Basin and Southwest Pacific Basin. A zone of welldefined chlorite abundance occurs north of the Ad61ie coast, and can be

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related to a continental source, but the zone west of the Australian Antarctic Discordance is not related to an obvious continental source. The occurrence of chlorite is clearly connected with that of muscovite of high crystaUinity.

Amphibole. There are two distinct areas o f amphibole abundance (Fig.7), one coinciding with the Fe.illitedistribution pattern in the western South Indian Basin and the other with the chlorite zone around South Island, N e w Zealand and in the Southwest Pacific Basin. It was not detectable in any samples ~ o m the sediments around the Australian continent. The abundances are arbitrary since there was no way of estimating a coefficient of intensity and the percentages are purely relative measures only.

161

ALKALI

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Fig.9. Alkali felspar abundance in percent.

Quartz. The pattern of occurrence of quartz (Fig.6) is patchy and appears to coincide with the chlorite pattern north of the Ad61ie coast, b u t n o t in the Southwest Pacific Basin and adjacent areas. It is relatively abundant around southwest Australia and in the Tasman Basin.

Plagioclase. This mineral is relatively abundant in most of the sediments o f f the Antarctic coast, in the Emerald and Southwest Pacific Basins and close to South Island, New Zealand (Fig.8). The distribution is therefore related to the occurrence of mechanically weathered products such as chlorite and muscovite and this is emphasised by its low abundance in sediments around the Australian continental margin. However, the area over the mid-ocean ridge, to the west of the Australian Antarctic Discordance is n o t related to

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any of these, and instead, correlates with a zone of high smectite abundance (Fig.2). This smectite has a high iron c o n t e n t and may be described as nontronite (Moriarty, in prep., b).

Alkali felspar. This mineral is n o t a b u n d a n t (Fig.9), but its distribution correlates fairly well with quartz in the South Indian Basin and on the mid-ocean ridge. Its abundance is very low, or zero, in the sediments of the South Australian, Emerald and Southwest Pacific Basins, but higher in t h e South Tasman Basin. It is also very low in abundance in the Balleny Trough which agrees with the quartz results. Kaolinite. The pattern of major occurrence of kaolinite shows a clear relation-

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ship to Australian sources, especially southwest Australia (Fig.5), however a low but very significant percentage occurs in the South Indian Basin. It is difficult to detect in the latter area because of the abundant chlorite, but inspection of the 3.5-A peaks showed it to be most abundant on the western and central parts of the abyssal plain. The quantities in Fig.5 should be taken only as a guide to occurrence in this area. Gibbsite. This mineral is n o t a b u n d a n t in the clay, but it shows a distribution which is generally similar to kaolinite if the higher abundances of A1203 (Fig. 14) are considered to be caused by dissolution of this mineral.

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Talc. The presence of a diffraction peak corresponding to talc is indicated on Fig.3. Most of the sites are located on the mid-ocean ridge, in areas of low or negative accumulation rate of clays. Smectite. The chemical analyses and cation-exchange capacities show that the smectite in the southeast Indian Ocean consists of several types (Moriarty, in prep., a). Iron contents in the clay (Moriaty, in prep., b) correlate well with the smectite distribution on the Southeast Indian Ridge and indicate an iron rich or nontronitic composition in that area, which also shows the highest abundances (Fig,2) in the study area. The crystallinity of the smectite is also the highest, as shown by Fig.12, and it always displays sharp peaks on

165

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Fig.13. Percent opaline silica removed from clay fraction by sodium hydroxide.

the diffractograms. The same association of properties apply to the smectite of the Solander Trough, Emerald Basin, Southwest Pacific Basin and the South Tasman Basin. A fairly well-crystallised smectite occurs on the South Indian Abyssal Plain, but its iron c o n t e n t is not high and it is probably a montmorillonite. It mixes with an interstratified smectite which is most a b u n d a n t on the Ad~lie Rise (Fig.12). There are two main occurrences of interstratified smectite in the northern basins (Fig.12), one is an area of low abundance (Fig.2) west of New Zealand, and the other is an area of higher abundance south of South Australia. Although it is n o t quite so obvious on the maps, the basins around the southeast of Australia show a better crystallised smectite which in places is very abundant. This is most obvious to the east of Bass Strait.

165

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D ISCUSSION The similarities or contrasts in the patterns of abundance of the clay minerals can be used to indicate sources and the mechanisms b y which they were brought to the depositional site. There are four fairly well-defined geographical provinces in the southeast Indian Ocean, viz. the South Indian Basin, the South Australim~ Basin, t h e Southeast Indian Ridge and the set of basins west and south of N e w Zealand. Since each of these has a dominant source area close by, they will be considered separately because the suite of clay minerals from each source is distinct.

167

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South Indian Basin

In terms of conditions for formation and weathering, the clay mineral assemblage can be divided into mechanically and chemically weathered components. Those which show predominantly mechanical weathering are chlorite, muscovite, amphibole and probably plagioclase and alkali felspar. The Fe-illite indicates mild leaching while the montmorillonite, kaolinite and interstratified smectite are the products of intensive weathering. The abundance of the latter minerals is not high, but their presence is significant for the reconstruction of paleoc!imates on Wilkes Land. The geographical associations of the mechanically eroded assemblages are important because they can be expected to indicate the composition of the

168

rocks in the source area. The extensive ice erosion is the obvious mechanism of formation, while transport to the continental shelf would be mainly by glaciers and ice-rafting. The abundance maps, or maps showing compositional parameters can identify the source area if the transport mechanisms are known. In the case of the South Indian Abyssal Plain, turbidity flows are a major mechanism (Payne and ConoUy, 1972) and ice-rafting a minor one if the low sedimentation rate of ice-rafted debris on the south flank of the ridge is an indication of the rate closer to the continent. This is reasonable since most melting occurs in the warmer waters over the mid-ocean ridge. Clay-sized particles are also transported by b o t t o m currents and this is a major mechanism on the continental rise of Wilkes Land where contourite sedimentation is c o m m o n (Payne and Conolly, 1972). The clay mineral abundance contours in the South Indian Basin are closely controlled by the b a t h y m e t r y and outline of the abyssal plain. This is n o t an artifact of the contouring, as sufficient sites exist along several sections of the basin to confirm this. Even in the Australian Antarctic Discordance Zone, the apparent northward transport through the zone is n o t very significant and is only obvious through examination of the clay mineral compositions rather than abundances. This evidence suggests that b o t t o m currents are n o t effective over long distances in transporting clay. The clay content of the sediments (Fig.15), also supports this when considered with the fact that the sediment accumulation rates are very low on the southern flanks of the ridge (Scott et al., 1972; Moriarty, in prep., b). Very little clay is transported for more than a b o u t 200 km over the margins of the abyssal plain. B o t t o m currents probably mix the clays over this distance as they move over the surface of the plain, but the residence time as suspended load is small and the clay c o n t e n t of the sediment must be high for significant transport to occur. The overall implication is that the clay in much o f the abyssal sediments of the South Indian Basin was n o t transported from the continent by b o t t o m currents as a primary mechanism. Some areas of higher velocity or turbulence may be exceptions to this and the continental rise is the most obvious. The major mechanism for the delivery of clay must be turbidity currents transporting material from the continental shelf, and if allowance is made for the mixing effect of transport by b o t t o m currents which are parallel to the shelf, the clay mineralogy on the abyssal plain opposite the shelf will be characteristic of the sediments on the shelf. There is one modification necessary here, since the extensive submarine canyon system described by Payne and Connolly {1972) has been eroded into older sediments, so the clay mineral assemblage reaching the abyssal plain may contain some components from these. In fact, it is likely that the small quantity of kaolinite, and possibly the minor amounts of well-crystallised montmorillonite found on the plain come from this source, or have been reworked on the shelf from older exposed strata. Neither these two minerals, nor the very abundant Fe-illite, are detectable in the clays on the southern flank of the South Indian Ridge.

169

/'~ density current aeolian t,-1 bottom current .2'/ northern limit of /c antarctic influence

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Fig.16. Clay mineral sources and transport processes. Boundaries between clay assemblages from different sources are approximate and arbitrary where gradual mixing o f clay minerals occurs, but good where abundances change rapidly. Arrows show suggested directions and position of bottom currents from sources referred to in text.

These must have been ice-rafted to their depositional site for they contain an assemblage of mica, chlorite and felspar with minor amphibole in a zone of pelagic sedimentation. If this assemblage is representative of the glacial marine sediments being deposited on the abyssal plain and continental shelf -and there seems no reason w h y the icebergs should carry a different assemblage over to the ridge -- then present ice erosion on Antarctica is acting on unweathered material. This implies that not only the kaolinite and montmorillonite, but also the very abundant Fe-illite were deposited on the continental shelf at an earlier time. This argument establishes two important facts: the mineralogical composition of the rocks of Wilkes Land and some

1Ti)

constraints on the paleoclimatic history of weathering on those rocks. Fig.16 summarises the clay mineral assemblages in terms of their source areas. Plagioclase is ubiquitous, b u t there appears to be a change from minerals typical of granitic rocks to those of more mafic type from East to West Wilkes Land. The reconstruction of the Wilkes Land sector Gondwanaland by Weissel and Hayes (1972) would place the Antarctic equivalent of the Western Australian greenstone belts in western Wilkes Land, while the equivalent of the Australian granitic shield would outcrop east of 130 ° E. The chlorite must be derived from the extensive greenschist facies in the rocks of the Trans-Antarctic Mountains and Victoria Land which outcrop mainly east of 155°E (Bushnell, 1970). The flat-lying Beacon group sandstones and volcanics do not appear to comprise a major source of minerals, the only possible correlation being with quartz and kaolinite. Erosion from the metasediments is the dominant source. The chlorite is probably transported by the Antarctic B o t t o m Water as it travels from the Ross Sea along the continental shelf and rise to a b o u t 140 ° E where it spills over into the abyssal regions. This would explain the high abundance on the Ad~lie Rise. Nearb o t t o m particulate suspended matter is dense in the region (Eittreim et al., 1972). The continental shelf of western Wilkes Land is supplying large quantities of Fe-illite to the abyssal plain. There is none detectable in the ice-rafted material, indicating that the illite was deposited on the shelf at some time in the past. If the mineral represents mild leaching conditions, possibly on biotite or ferrous micas, then its abundance indicates that large quantities of the fresh parent material have been exposed to these conditions, which must have been before the present intense glaciation. The icecap is believed to have formed in the Pliocene (Shackleton and Kennett, 1975) so the Fe-illite would be Pliocene or older. A previous glaciation phase is implied by the absence of large quantities of more intensively weathered products, and the requirement that large areas of fresh micas be exposed to form the Fe-illite. The montmorillonite and kaolinite probably represent reworking of the original weathered mantle material which would have been deposited on the shelf at the time of the first glaciation. This subject will be discussed in detail in another paper (Moriarty, 1977d). The interstratified smectite of the Ad~lie Rise is probably the same as the group " A " clays of Chapius (1974) and may represent the same time of formation as the montmorillonite of the western plain. Payne and Conolly (1972) describe the reworking of Tertiary sediments on the Ad~lie Rise, and Chapius found that the abundance of the group " A " clays increased as climate cooled. Since these cooling periods also correspond to times of increasedbottom-current activity according to the models of Gordon {1971), the change in abundance could be due to increased scour and reworking of older deposits on the rise, rather than transport from the continent.

171

South Australian Basin The clay mineral assemblages in the basin are from highly weathered sources, which is to be expected because the western and central parts of Australia are occupied by a deeply weathered peneplain. Only one large river, the Murray, which does not drain the peneplained areas, enters at a location likely to affect the distribution patterns in the South Australian Basin. Rainfall is typical of deserts over much of the region except near the coast; dust storms are c o m m o n when winds blow from the north. The clay minerals consist of kaolinite and quartz which appear to have a major source in Western Australia (Figs.5 and 6); interstratified smectite whose dominant source area appears to be South Australia (Figs.2 and 12); illite which is ubiquitous (Fig.4) and minor gibbsite (Fig.14). The patterns of abundance of kaolinite, quartz and smectite do n o t appear to be controlled by the b a t h y m e t r y or outlines of the abyssal plain, and appear to be the result of wind and water transport rather than turbidity flows. This is reasonable considering the lack of rivers to supply the continental shelf with sediments. Tasman and Emerald Basins All the clay mineral distribution maps show a marked difference in the assemblages in the western and eastern parts of the Tasman Basin. The eastern sediments contain a minimally weathered assemblage of muscovite, chlorite, amphibole and plagioclase with minor quartz and interstratified smectite, while the western sediments contain weathered clays consisting of the minerals kaolinite, montmorillonite, illite and minor quartz. The patterns of abundance and occurrence show New Zealand to be the source o f the mechanically weathered assemblage and Australia the source of the weathered assemblage. The New Zealand assemblage is mechanically eroded and probably derives from the extensive glaciation of South Island. The same assemblage exists in the Solander Trough, Emerald Basin and the Southwest Pacific Basin, except for the interstratified smectite, which may be derived from sources which have access only to the Tasman Basin. Some variations in relative abundance occur. The chlorite c o n t e n t is higher in the Emerald and Southwest Pacific Basins, and in some samples is the highest observed in the area of study. The mica c o n t e n t is very low (Fig.4) and this is surprising considering the abundance of chlorite. There are extensive greenschist facies rocks on South Island and, as mentioned above, in the Trans-Antarctic Mountains, and ice-rafting or b o t t o m currents may supply chlorite from both sources to these basins. If this is true, it suggests a comparatively small outcrop of micaceous rocks in the source areas in order to explain the low relative abundance of the mica. The micas found in the eastern Tasman Basin must be eroded from the crystalline basement rocks through which the glaciers must pass before

172

entering the sea on the western side of South Island. Alkali felspar shows a distribution pattern which suggests a source to the south of the Tasman Basin. Eittreim et al. (1971) suggest considerable transport of material from the Tasman Fracture Zone into the South Tasman Basin, and the high crystallinity of the micas with the alkali felspar may be explained by erosion of Antarctic sediments from the zone. CONCLUSIONS

(1) Ross Sea bottom water carries chlorite rich clays from the TransAntarctic Mountains along the continental shelf and rise to the Ad~lie Rise, where they are deposited and mixed with clays from the granitic shield of eastern Wilkes Land. Alkali felspar, quartz and muscovite are glacial erosion products of eastern Wilkes Land. (2) The South Indian Plain is dominated by an iron-rich illitic clay from pre-icecap weathering of mafic shield rocks of western Wilkes Land. Amphibole, plagioclase and biotite are glacial erosion products of the same area. (3) Evidence of intensive weathering on Wilkes Land is provided by the presence of montmoriUonite, interstratified smectite and minor kaolinite in the abyssal plain sediments. These are probably reworked from Tertiary-age strata on the continental shelf and rise. (4) Eolian transport from Western Australia spreads kaolinite in a southeasterly trajectory as far as the South Tasman Basin. Interstratified smectites are derived mainly from South Australia, possibly by eolian and river transport, (5) Mechanical erosion products, especially chlorite, dominate recent clay in basins west and south of New Zealand. Some Antarctic influence is seen in clays of the South Tasman Basin, probably reworked from Tasman Fracture Zone sediments by bottom waters. (6) Ice-rafting of clays from Antarctica is relatively minor compared to bottom-current input in the South Indian Basin. Local areas on the mid-ocean ridge flank and perhaps much of the crest and north flank receive ice-rafted clay. ACKNOWLEDGEMENTS

This research forms part of the author's doctoral thesis and was supported by a Flinders University Research Grant. I am very grateful to the Division of Soils, C.S.I.R.O. in Adelaide where most of the analytical work was carried out. I am especially grateful to Dr. Keith Norrish, Mr. John Pickering and Mr. John Hutton of the Division of Soils for advice and assistance at all stages of the analytical work. I thank Professor Chris yon der Botch for critically reading this manuscript and for assistance throughout the research. The cores for the study were collected on cruises 39, 47 and 50 of the USNS "Eltanin" or made available by the A n t ~ t i c core library of the Florida State University; both sources funded by the National Science Foundation.

173 REFERENCES Berry, R.W. and Johns, D.J., 1966. Mineralogy of the clay-sized fraction of some North Atlantic--Arctic ocean bottom sediments. Geol. Soc. Am. Bull., 77: 183--196. Biscaye, P.E., 1965. Mineralogy and sedimentation of Recent deep sea clays in the Atlantic Ocean and adjacent seas and oceans. Geol. Soc. Am. Bull., 76: 803--832. Bushnell, V.C., 1969/1970. Antarctic Map Folio Series, 12. Am. Geogr. Soc., Folio 12. Carroll, D., 1970. Clay minerals in Arctic ocean sea floor sediments. J. Sediment. Petrol., 40: 814--821. Chapius, R.A., 1974. Sediment Response to Climatic Change as Recorded in Deep Sea Piston Cores from the Southern Ocean. Thesis, Florida State University, Tallahassee, Fla. Cochran, J.K. and Osmond, J.K., 1976. Sedimentation patterns and accumulation rates in the Tasman basin. Deep-Sea Res., 23: 193--210. Conolly, J.R., 1968. Geology and submarine canyons, East Bass Strait. Mar. Geol., 6: 449--461. Darby, D.A., 1975. Kaolinite and other clay minerals in Arctic ocean sediments. J. Sediment. Petrol., 45(1): 272--279. Dudas, M.J. and Harward, M.E., 1971. Effect of dissolution treatment on standard and soil clays. Proc. Soil Sci. Soc. Am., 35(1): 134--140. Eittreim, S., Gordon, A.L., Ewing, M., Thorndike, E.M. and Bruckhausen, P., 1972. The nepheloid layer and observed bottom currents in the Indian--Pacific Antarctic sea. In: A.L. Gordon (Editor), Studies in Physical Oceanography -- A Tribute to Georg Wust on his 80th Birthday. Gordon and Breach, New York, N.Y. Gordon, A.L., 1971. Recent physical oceanographic studies of Antarctic Waters. In: L.O. Quam (Editor), Research in the Antarctic. Am. Assoc. Adv. Sci., Washington, D.C., 768 pp. Gordon, A.L., 1972. Physical oceanography of the Southeast Indian ocean. Am. Geophys. Union, Antarct. Res. Set., 19: 3--10. Gordon, A.L., 1972a. Spreading of Antarctic bottom waters, 2. In: A.L. Gordon (Editor), Studies in Physical Oceanography -- A Tribute to Georg Wust on his 80th Birthday. Gordon and Breach, New York, N.Y. Gostin, V.A. and Moriarty, K.C., 1975. Investigations of Tertiary clay mineral distribution around Tasmania, DSDP Leg 29. In: J.P. Kennett, R.E. Houtz et al., Initial Reports of the Deep Sea Drilling Project, 29. U.S. Govt. Printing Office, Washington, D.C. Hayes, D.E. and Connolly, J.R., 1972. Morphology of the Southeast Indian Ocean. Am. Geophys, Union, Antarct. Res. Ser., 19: 125--146. Kennett, J.P. and Watkins, N.D., 1976. Regional deep sea dynamic processes recorded by late Cenozoic sediments of the Southeast Indian Ocean. Geol. Soc. Am. Bull., 87(3): 321--339. McDougall, J.D. and Harris, R.C., 1969. The geochemistry of an Arctic watershed. Can. J. Earth. Sci., 6: 305--315. Mehra, O.P. and Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite--citrate system buffered with sodium bicarbonate. In: Swineford (Editor), 7th National Conference Clays and Clay Minerals, Clay Mineral Society, Washington, D.C., pp.317--327. Moriarty, K.C., 1970. The Clay Mineral Distribution in the Southern Ocean between Australia and Antarctica. Thesis, University of Adelaide, Adelaide, Australia. Moriarty, K.C., in prep., a. Composition of clay minerals from the Southeast Indian Ocean and high resolution quantification methods. Moriarty, K.C., in prep., b. Geochemistry and clay minerals in the southeast Indian Ocean. Moriarty, K.C., in prep., c. Sources of trace metals in amorphous minerals extracted from southeast Indian Ocean clays. Moriarty, K.C., in prep., d. Paleoclimates of Antarctica and Australia from the clay mineralogy of Deep Sea Drilling Project cores in the Southeast Indian Ocean.

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