Mineralogy of Sediments from CRP-3, Victoria Land Basin, Antarctica, as Revealed by X-Ray Diffraction
Institut t'iir Geophysik und Geologic. Talstrassc 35. ll-O-H03 Leip/.i; - Germany
Abstract - T h e miiieralogy of the lower Oligocene (to possibly upper Eoccne) sediments of core CRP-3 drilled on the continental shelf o f McMurdo Sound in the Ross Sea. Antarctica. has been examined by the X-ray diffraction method. Quartz.
p l i ~ ~ i o c l a s e . K-feldspar and pyroxene are the most important non-clay minerals.
Amphibole occurs i n minor amounts. T h e composition of the sediments points exclusively to an origin in the Transantarctic Mountains for ihc clctrital components.
There, the plutonic and metamorphic rocks of the basement. the sccliments of the Beacon Supergroup and the volcanic rocks of the Ferrar S u p e r g r o ~ ~ p could serve as possible source lithologies. The distribution of the detrital minerals reflects a long-
term history of successive erosion and beginning valley incision. During the deposition of the lowest part of the Cenozoic sediments of CRP-3. the majority of detrital minerals was probably derived from the sediments of the upper Beacon Supergroup (Victoria Group) and the Ferrar Supergroup. as indicated by the high quartz and pyroxene concentrations. Only a very minor proportion probably was contributed by basement rocks.
From c. 620 to c. 420 mbsf the sandstone-dominated Taylor Group of the Beacon Supergroup probably acted as the main source rock and was responsible for maximum quartz concentrations and the strongly lowered feldspar and pyroxene amounts in the CRP-3 core. Above c. 420 mbsf the erosion in the valleys cutting through the Transantarctic mountains reached the level of the basement rocks and therefore the amount of basement-derived minerals like K-feldspar and amphibole in the CRP-3 sediments became more important.
INTRODUCTION
Major objectives of the international Cape Roberts Project ( C R P ) are to reconstruct the C e n o z o i c Antarctic c l i m a t e , to study the d y n a m i c s of the Antarctic i c e masses, and to enlighten the uplift history of the Transantarctic Mountains (Cape Roberts Science Team, 1998, 1999, 2000). These goals are a p p r o a c h e d by studying Q u a t e r n a r y t o lower Oligocene or possibly upper Eocene sediments from drill cores recovered on the Antarctic continental shelf in Ross Sea. The sedinients are investigated with a l a r g e variety of different s e d i m e n t o l o g i c a l , g e o c h e m i c a l , petrological, palaeontological and geophysical methods (Hambrey & Wise, 1998; Barrett
U
& Ricci, 2000a, b ) . Our paper contributes to the
p r o j e c t by presenting initial results on the mineralogical composition of the Cenozoic sediments recovered in the upper 790 mbsf (metres below sea floor) of the CRP-3 drill core. The bulk mineralogy w a s analysed by X-ray diffraction ( X R D ) . T h i s technique has been successfully used already for drill c o r e s C R P - 1 and C R P - 2 / 2 A in o r d e r to identify s o u r c e rocks of the sediments and t o d o c u m e n t temporal changes in the source area (Ehrn~ann, 1998a;
Neumann & Ehrmann. 2000).
CRP-3 was drilled as the last core of the Cape Roberts Project from October to December 1999. The
drillsite was situated in the Victoria Land Basin on the continental Antarctic shelf of McMurdo Sound in the Ross Sea ( F i g 1)). After the previous drilling campaigns of CRP-1 and CRP-2/2A in 1997 and 1998 (Cape Roberts Science Team, 1998. 1999). it was intended to recover sediments older than early
m.
1 - Location of the drillsites CRP-l. CRP-212A and CRP-3 on the continental shelf of McMurdo Sound in Ross Sea. Antarctica.and generalised aeology of the hinterland (after Warren. 1969).
"Coi-responding author (in~ie~~manii@i-z.uni-leipzig.de)
O l i g o c e n e . It was expected that these sediments should d o c ~ ~ m e n t the change from a warm climate i n I'ocene to a cooler climate a n d an ice-covered Antarctica in Oligocenc time. The expectations were only partly met (Cape Roberts Science Team, 2000).
( ' R P - 3 reached a depth of 939.40 mbsf and is the deepest bedrock clrillhole of the Antarctic continent.
Nearly all of the cored sediments above 823 mbsf a r e of early Oligocene age. Only the lowermost sediments of this interval may possibly be assumed as E o c e n e . T h e sediments c o m p r i s e mainly c o n - g l o m e r a t e s , diamictites, s a n d s t o n e s and rarely siltstones and muclstones (Fig. 2). All these sediments were deposited i n a glacially influenced environment.
The facies range from distal to proximal glacimarine including iceberg influence and i c e marginal sediments. Fluvial and deltaic sediments are also common. Evidence Sor glacial processes was found throughout the Cenozoic interval of CRP-3 (Cape Roberts Science Team, 2000).
T h e underlying sediments of C R P - 3 . f r o m 823 mbsf to the final depth, have a preliminarily specified age of mid-Devonian and were assigned to the Beacon Supergroup. Mid-Devonian sandstones of the Beacon Supergroup crop out westward of the drillsite in the Transantarctic Mountains in an altitude of about 2 km. The recovery of Beacon Sandstones in s u c h a d e e p level on the M c M u r d o shelf is an impressive evidence for the tectonic activity during the generation and opening of the West Antarctic Rift System. Thus. also the CRP-3 core did not penetrate sediments of a preglacial Cenozoic climate that is expected to have existed in Palaeocene and most of Eocene time.
T h e deepest sample investigated for this paper c o m e s from about 790 mbsf. T h e c o n g l o m e r a t e s between 790 and 823 mbsf and the Beacon sandstone are not treated. We concentrate on presenting and discussing the downhole distribution of the main sediment components only. Details on the composition and distribution of the clay mineral assemblages and of t h e heavy mineral f r a c t i o n a r e presented i n separate papers (Ehrmann. this volume; Neumann, this volume; Setti et al., this volunle). The composition of the s a n d and gravel f r a c t i o n s is h i g h l i g h t e d b y S n ~ e l l i e (this volume) and Sandroni & Talarico (this volume).
METHODS
Ninety six s e d i m e n t s a m p l e s taken in a l m o s t constant intervals of about 7 to 10 m from the CRP-3 c o r e w e r e analysed for this s t u d y i n o r d e r t o investigate the mineralogical c o m p o s i t i o n of the sediments by the X-ray diffraction method. About 10 c m 3 of bulk sediment were f r e e z e - d r i e d , and t h e gravel fraction (>2 mm) was removed by sieving. One aliquot of each sample was used further for studying heavy minerals (Neumann, this volume) and clay minerals (Ehrmann, this volume). T h e other aliquot
was inccI~;inic;~lly gro~~ncl. For the XRD measurci11ciil and analysis, i l was mixed with an internal standa~~(l o l corundum (a-AI,O,) at a sample/standard ratio ol' 5 : 1 .
R a n d o m powder mounts were X-rayed I r o n .3 to 0 0 " 2 0 with a s t e p size of 0.02O 2 @ a n d ;I
measiiring time of I second per step. The equipment consisted of a Philips generator PW I i ^ O , i i
goniomcter PW 3020 with a n automatic divcrgciu'c slit, an electronic control PW 3710, and an tinloiiii~ir sample changer PW 1775. C o K a radiation ( 4 0 m\/, 40 mA) was used. The diffractograms were evaluated using the "MacDiff"' software (Petschick, freeware).
The diffraction patterns were calibrated agiiinst (lir
~ o s i t i o n of the d(012) peak of the corundum sliii~~lard at 3.479
A
before being analysed.T h e peak heights and the peak a r e a s of {In'
individual minerals were measured after subtraction ol' background counts and were set in relation to tliosr of the corundum standard. In this way, we obtained relative abundances of the individual minerals.
The abundance of quartz is presented as the ratio between the d(100) quartz peak height at 4.26 A ancl the d(012) corundum peak height at 3 . 4 7 9 A . WC refrained from using the higher d(101) quartz peak a1 3.343
A,
because this peak may be influenced b y other mineral peaks, mainly by illite/snuscovite.The abundance of total feldspar is expressed as the ratio between the combined areas of the fcklspiir p e a k s at 3 . 2 4
A,
3 . 2 1A
and 3.18A
a n d tlic corundum peak area at 3.479A.
K-feldspar/sta~iiitii~tl ratios are based on the 3.24A
K-feldspar peak licighl.Plagioclase/standard ratios are based on t h e 3.18
A
plagioclase peak height.
Pyroxenes were identified by their main reflections forming a typical peak triplet at 3.0, 2.95 and 2.90
A.
A distinction of i n d i v i d u a l pyroxenes w a s not possible. T h e c h a r a c t e r i s t i c triple peak in s o m e samples was disturbed by other minerals, mainly calcite; these peak areas had to be corrected manually or graphically by the software.
The abundance of amphiboles is presented as the ratio of the peak a r e a at 8 . 4 - 8 . 5
A
and that of corundum. Hornblendes, tremolites. actinolites and riebeckites have basal reflections at about this cl- value. A d i s t i n c t i o n of individual ainphiboles.however, was not possible.
Zeolites of the heulandite-clinoptilolite group were identified by their d - s p a c i n g s between 8 . 9 5 and 9 . 0 6
A .
H e u l a n d i t e a n d clinoptilolite can be distinguished by their d-spacings. Whereas a peak at 8.97A
is significant for heulandite, a 8.95A
peak is characteristic for clinoptilolite.Opal-CT is characterized by peaks at 4.05 and 4 . 1 1
A
and b y t h e d ( 1 0 1 ) lattice of tridymite at 4.32A.
Because quartz is present in high amounts in all analysed samples, the tridymite peak can be seen, if at all, only as a shoulder of the d(100) quartz peak at 4 . 2 6A.
Because plagioclase is present in high amounts, the 4.05A
peak of opal-CT is superimposed by plagioclase. Therefore, only the 4.1 1A
peak couldb e used l'or identifying opal-CT. Thus. the data on tlie a n a l y s e s (Ell s w a n n . thi S v o l u m e : N e u m a n n . t h i s
occursi~ncc of opal-CT are somewhat weak. v o l u m e ) . All raw data are stored in the daka bank o f T h e p r e s e n t e d d a t a on t h e s a n d c o n t e n t of (lie tlie A l f r e d W e g c n c r Institute f o r Polar a n d M a r i n e secliirn-nis w e r e a b y - p r o d u c t g a i n e d h y s i c v i n g Rcsearcli i n Bremerhaven, (icrmany, and are available aliquots ol' o u r s a m p l e s f o r clay and heavy mineral al www.pangaea.de.
sand [%] pyroxenelstandard zeolitelstandard
quartzlstandard amphibolelstandard opal-CTIstandard
Fig. 2 - Sand content (63 pin - 2 mm). abundance of quartz, pyroxene. amphibole. ~ e o l i t e and opal-CT in the Cenozoic CRP-3 sediments.
L i t h o l o ~ y simplified after Cape Robei-ts Science Team (2000).
RESULTS throughout the core. Amphiboles arc present i n m i n o r amounts, but not in all samples. Zeolites and opt11 ("I' The XRD measurements allow the identification o f are recorded as traces in isolated samples of C ' R P . 3 . four main minerals or mineral groups i n the CRP-3
s a m p l e s ( F i g s . 2, 3, 4 ) . Q u a r t z , p l a g i o c l a s e , K - Carbonate
fcldspar and pyroxenes are the most important non- Carbonate, mainly calcite, was detected in v:iriablr c l a y m i n e r a l s a n d o c c u r in v a r i o u s a m o u n t s amounts in many of the investigated CRP-3 s;iiii~)li.:s.
feldsparistandard K-feldsparistandard plagioclaseistandard K-feldsparlplagioclase quartzlfeldspar
Fig. 3 - Abundance of total feldspar. K-feldspar and plagioclase. K-feldspar/plagioclase ratio and quartzlfeldspar ratio in the Cenozoic CRP-3 sedirnents. Lithology simplified after Cape Roberts Science Team (2000).
However, they w e r e not qiiantificd and tire not discussed in this paper, because tlie results of chemical carbonate analyses from our sample set are reportctl by Dietrich et al. (this volume).
Clay minerals
N o quantitative data o n the distribution of clay minerals were produced for this study. because details on the composition of tlie clay fraction are published in special papers (Ehrmann, this volume; Setti et al..
t h i s volume). However, clay minerals a r e ma.jor components of the CRP-3 sediments and, therefore.
some general trends obvious from this study deserve mentioning. The main clay minerals present in CRP-3 sediments are illite, chlorite and smectite. Illite and c h l o r i t e govern the clay mineral spectrum of the upper c. 9 0 m . In this interval smectite occurs in small amounts only. Between 90 and 150 mbsf illile a n d smectite are the main clay minerals, whereas c h l o r i t e plays a m i n o r r o l e . Below 1 5 0 m b s f , in contrast, smectite is the dominant clay mineral, and all other clay minerals are totally absent or present in trace amounts only.
011iirt/.
Q i i t i r t ~ occurs i n high amounts i n tlie Ccnoxoic scdiments of ('RP-:<. The quart/. content increases from the sea I'loor to 4 2 0 mbsf as indicated b y (~iiartx/stti~i(ltirt.l rtitios i n c r e a s i n g f r o m c. I to 3 . 5 (Figs, 2, 4 ) . Between 420 a n d 620 nibsf the quartz content reaches its maximum with quartzlstandard at aroinid 4. Below the maximum, the quartzlstandard ratios slowly decrease to c. 2.5 at the bottom of the investigated c o r e interval. Except for t h e d e p t h interval 0 - 1 4 0 mbsf the ratio trend is s t r o n g l y fluctuating.
Felclspars
The concentration pattern of total feldspar in the c o r e a b o v e 790 mhsf can h e divided into t h r e e s e c t i o n s (Figs. 3, 4 ) . T h e r e is a trend of- feldsparlstandard ratios decreasing from c. 4 to l between 0 and 420 mbsf. From 420 to 620 mbsf the feldsparlstandard ratios a r e q u i t e low and r a n g e constantly around 1 . Finally, between 620 and 790 mbsf the feldspars reach a high concentration level with feldsparlstandard ratios around 3 and with a n
sand [%] feldsparlstandard plagioclaselstandard quartzlfeldspar amphibolelstandard
quartzlstandard K-feldsparlstandard K-feldsparlplagioclase pyroxenelstandard
Fig. 4 - Smoothed (5-point-average) abundance of bulk mineralogy in the Cenozoic CRP-3 sediments (The outliers in quartzlfeldspar at 246.03 mbsf and in amphibolelstandard at 330.97 mbsf had been removed prior to the calculations).
increase at the beginning and ;I clccrcase at the end o f [lie interval.
T h e described pattern o f total feldspar content is ni:iinly caused b y the plagiochisc abundance.
However. the K-feldspar abundance curve shows a similar trend to the plagioclase curve (Figs. 3. 4 ) . The riitio o f K-feldsparlpl~ioclase is quite constant and fluctuates only between 0.5 and I above 420 nibs1 and below 660 mbsf (Figs. 3 . 4 ) . K-feldspar is often more substantial than plagioclase in the interval from 420-660 mbsf. as expressed by K-fcldsparlplagioclase ratios >l (Figs. 3. 4 ) .
T h e quartz/feldspar ratio curve combines a n d accentuates the trends o f the quart;/ and feldspar curves (Figs. 3, 4 ) . Because o f increasing quartz and decreasing feldspar the q~~artz/t'elclspar ratio increases slightly f r o m 0 - 4 2 0 n i b s f . Due to the feldspar m i n i n ~ u m and the quartz maximum at 420-620 m b s f the quartzlfeldspar ratio reaches maximum values f r o m 4 up to 14 in that interval. From 620 to 7 9 0 nibsf the ratio is quite low. constantly between 1 and 2. with a slightly increasing trend below 720 m b s f (Fig. 4 ) .
Pyroxenes
Similar to the curves o f feldspar. the distribution pattern o f pyroxene can be subdivided into three main sections ( F i g s . 2. 4 ) . From 0 to 420 m b s f t h e pyroxenelstandard ratios decrease from > l to 0 . 3 . Between 420 and 620 m b s f the ratios are relatively constant around 0.2. The lower interval from 620 to 790 m b s f is characterized by generally enhanced pyroxene contents, but with pyroxenelstandard ratios fluctuating b e t w e e n 0 and > 1 . 6 . T h e paper b y Neumann (this volume) discusses the pyroxene record in the heavy mineral fraction.
Amphiboles
Amphiboles do not occur in all samples o f the CRP-3 core in detectable amounts (Figs. 2, 4 ) . In the uppermost part o f CRP-3, between 0 and 130 i n b s f , the a~nphibolelstandard ratios reach up to 0.05. Apart f r o m one strong m a x i m u m o f a single sample at 330 mbsf. below 130 mbsf the amphiboles show only an incomplete occurrence and decrease generally t o c. 0.01 - 0.02. Larger gaps in the amphibole record can be observed at 540-460 m b s f . 400-350 m b s f and 300-260 m b s f . Amphiboles in the heavy mineral fraction are treated in a paper presented by Neuniann (this volume).
Zeolites
Minerals o f the heulandite-clinoptilolite group were detected in low concentrations in some o f the investigated samples ( F i g . 2 ) . E x c e p t for o n e maximum at 357.80 mbsf with a zeolitelstandard ratio o f about 0 . 5 5 , t h e samples have ratios <0.2.
Noteworthy, significant concentrations o f zeolites occur only in the upper part o f CRP-3, between 75
and 300 i n h s f . In 19 o f the 29 z e o l i t c - c o i i t ; i i ~ i i i i ~ ~ samples the /.colitc consists o f lieulandite. O n l y H i isolated s;iinplc~s contain traces of dinoptilolite.
OpaI-CT
Opal-(''1' is confined mainly t o some isolatrd samples (1;ig. 2 ) in the intervals 50-250 m b s f . .\l0 420 mhsl' and 550-650 nibsf. Opal-CTIstandaid I-iitios i n these samples average around 0.02. A m a x i m u m value ol' 0 . 1 is rciichecl in a solitary s a m p l e ; i t
560 mbsf.
DISCUSSION
The Ceno~.oic clastic sediments o f the CRP-3 cow were deposited in glacimarine and fluvial setti11.g~
( C a p e Roberts Science T e a m , 2 0 0 0 ) . T h u s . tin-
~nineralogical composition is mainly controlled by composition o f the source rocks in the hinterland.
Analysing the bulk mineralogy o f the sediments by X R D should therefore help i n i d e n t i f y i n g sourcc areas. weathering conditions and transport paths.
Downcore changes in the distribution o f individual clastic minerals are o f special interest, because thcy could indicate reorganizations o f the climate and the sediment delivery. and could help in reconstriictin~
the uplift history o f the Transantarctic Mountains. 1 1 1
general. the composition o f the CRP-3 sediments is quite similar to those recovered at CRP-1 and CRP- 212A (Ehrmann. 1998a; Neumann & El~rmann. 2000).
Forn~er investigations o f drill cores CRP-1 a n d CRP-212A identified source areas in the Transantarctic Mountains to the west and in the region o f the Ross Ice Shelf in the south (e.g. Ehrmann, 1998a; Smellie.
1998. 2000: Talarico & Sandroni. 1998: Neumann &
Ehrmann, 2000: Polozek. 2000). T h e geology of the Transantarctic Mountains (Fig. 1 ; e.g. Warren. 1969) is characterized by a crystalline basement consisting o f late Precambrian to early Palaeozoic granites and mainly anlphibolite-grade metamorphic rocks. T h e basement is overlain by sedimentary rocks. mainly sandstones. o f the Devonian t o Triassic Beacon Supergroup. Both basement rocks and sedimentary rocks are intruded by sills and dykes o f the Jurassic Ferrar Dolerite. In contrast, the region o f the present- day Ross Ice Shelf is characterized by large outcrops o f volcanic rocks o f the Cenozoic McMurdo Volcanic G r o u p . A l s o for the C R P - 3 sediments a m u l t i - component source in the Transantarctic Mountains could be identified. already by preliminary analyses o f the gravel and sand composition (Cape Roberts Science Team. 2000). In contrast to CRP-1 and CRP- 2 / 2 A , however. the volcanic rocks o f the McMurdo Volcanic Group did not act as a source. Changes in the mineralogy o f the CRP-3 sediments as revealed by the XRD analyses can be interpreted as the result o f changing successive erosion and incision o f valleys in the Transantarctic Mountains.
Mincr;ilo:q ol' Sediments from ('RP-3 529 Tin' t1u;irtz distribution in the CRP-3 core shows a
rough ~ , o n ' ~ l a t i o n to the sand content. I n genei'iil. a high timoiint of sand in the sediments corrcl;itcs to a h i g h t1ii;irtz c o n t e n t ( F i g . 2 ) . Althoitgh both pal-amctcrs show the same main trend. they notably differ in detail. The amplitudes in the fluctuations of the sand are very different from those of the quartz content tliro~ighout the core (Fig. 2). This implies that besides transport processes there is an additional control for [he distribution pattern of quartz. probably the source area.
Quartz is a c o n s t i t u e n t of most r o c k s of the Transaiitarctic Mountains and the main component of the sandstones of the Beacon Supergroup. Because of t h e high qiiartz amounts from 620 to 420 mbsf we can assume the main sediment source for this interval in the Beacon Sandstones. This is supported by a distinctly enhanced proportion of rounded quartz grains (Smellie, this volume). The influence of the B e a c o n S a n d s t o n e s o u r c e is o b v i o u s l y l e s s pronounced below 6 2 0 mbsf and above 4 2 0 mbsf.
T h e lower quartz content below 620 mbsf may be a result of the erosion of a mixed source consisting of the quartz-rich Beacon Supergroup sediments and the q u a r t z - p o o r but pyroxene-rich Ferrar Supergroup rocks. Decreasing quartz abundances above 420 mbsf c a n be interpreted as a result of t h e progressing e r o s i o n c u t t i n g t h r o u g h t h e B e a c o n S u p e r g r o u p sediments into the basement rocks. The quartz content further decreases in the upper Oligocene and lower M i o c e n e s e d i m e n t s of c o r e C R P - 2 / 2 A a n d thus indicates an ongoing incision of the valleys (Neumann
& Ehrn~ann, 2000).
I n g e n e r a l , t h e total f e l d s p a r c o n t e n t . which comprises the records of K-feldspar and plagioclase, s h o w s a rough inverse correlation to both quartz content and sand concentration in the CRP-3 core (Fig. 4). Feldspars are less stable than quartz against weathering, transport and diagenesis, because of their s o f t n e s s . c l e a v a g e a n d c h e m i c a l c o m p o s i t i o n . Therefore they prevail c o n ~ m o n l y in finer grained sedirnents, predominantly in coarse siltstones ( e . g . Blatt, 1992). However, like in CRP-2/2A, maximum f e l d s p a r a m o u n t s in C R P - 3 a r e n o t c o n f i n e d to siltstones, but occur also in fine to medium grained sandstones and conglomerates (e.g. 722.92, 407.09, 330.97. 262.70 and 227.90 mbsf). This means that in addition to weathering and transport, the composition of the source area influences the feldspar content of the sediments.
K-feldspar c o m e s main1 y f r o m c r y s t a l l i n e basement rocks (Barrett et al., 1986; George, 1989;
Hambrey & Wise, 1998: Smellie, 2000). The presence o f K - f e l d s p a r in t h e l o w e r m o s t p a r t of t h e c o r e documents that the basement was exposed and active as sediment source since early Oligocene or even late E o c e n e . T h e o c c u r r e n c e of s m a l l g r a n i t o i d a n d metamorphic clasts ( C a p e Roberts S c i e n c e Team, 2000) points into the same direction. The K-feldspar
source seems to he almost exhausted between c. 6 2 0 itiul 400 mhsf. Above c. 400 mhsf ;I slight increase of.
K-feklspar amount ( F i g . 4 ) indicates that t h e basement source became more important. even if the Beacon Siipci~group still co~itributed large amounts o f detritus o f the CRP-3 scdimcnts. This becomes also evident from the sandstone dcti~it;il modes (Smellie, this volume).
Plcigioclase felclspars of CRP-3 can be mainly derived from sources such as thc basement rocks, the Fcrrar Supergroup, the sandstones of the B e a c o n Supergroup and the volcanic rocks ol' the McMurdo Volcanic Group (Barrett et al.. 1986: George. 1989;
Hambrey & Wise. 1998: Smellie. 2000). However, n o volcanic rocks of the McMurdo Volcanic Group were detected in the sand and gravel fractions of the CRP- 3 s e d i m e n t s ( C a p e Roberts Science Team, 2 0 0 0 ; Smellie. this volume). Another possible source for the plagioclase could be seen in the 48 Ma old alkaline v o l c a n i s m of northern Victoria Land, w h i c h i s recorded by volcanic glass shards o c c ~ ~ r r i n g in t h e Cenozoic interval of the CRP-3 core (Cape Roberts Science Team. 2000).
T h e most striking feature in the distribution o f plagioclase is its very low concentration between 620 and 420 mbsf coinciding with a higher and stronger fluctuating K-feldspar/plagioclase ratio (Figs. 3, 4).
The strongly reduced plagioclase input in this interval may be explained by a sediment source in the Beacon Sandstones as documented by maximum sand a n d q u a r t z c o n t e n t s . T h e s t e a d y increasing f e l d s p a r abundance above 420 mbsf may be explained by a n enhanced erosion of the either basement rocks o r dolerites of the Ferrar Supergroup.
High abundances of pyroxenes generally indicate a volcanic source. In CRP-3, the Ferrar Supergroup is a very likely s o u r c e ( P o l o z e k , 2 0 0 0 ; S m e l l i e , t h i s v o l u m e ) . T h e M c M u r d o Volcanic G r o u p c a n b e excluded as a source (see above). Granite Harbour I n t r u s i v e C o m p l e x a n d Koettlitz G r o u p c o u l d c o n t r i b u t e o n l y m i n o r a m o u n t s of p y r o x e n e . Unfortunately, individual pyroxene group minerals are not distinguishable by XRD. Therefore the pyroxenes c o u l d not b e a t t r i b u t e d t o a specific s o u r c e . I n general, pyroxenes are unstable against weathering and alteration. Thus. the distribution pattern of the pyroxene group minerals may be a combined result of changing sources and varying climate and transport conditions.
T h e slightly enhanced pyroxene content at 750- 6 2 0 mbsf m a y i n d i c a t e t h e erosion of p y r o x e n e - bearing rocks. A possible source for the pyroxene in this p a r t of t h e c o r e is t h e u p p e r p a r t of t h e s t r a t i g r a p h i c s e q u e n c e i n t h e T r a n s a n t a r c t i c M o u n t a i n s , i n c l u d i n g t h e V i c t o r i a - G r o u p of t h e Beacon Supergsoup and the Ferrar Supergroup with Ferrar Dolerite and Kirkpatrick Basalt. Thus, abundant clasts of Ferrar Dolerite are reported from this part of the core (Cape Roberts Science Team, 2000).
510 M. Neumann & W. Ehrmann The conspicuously low pyroxene abundances at
620-420 mbsf coincide with high quartz content and low feldspar content. Such a mineral assemblage may he explained by the erosion of the older part of the Beacon Supergroup, the Taylor G r o u p , which is almost void of Ferrar Dolerites and hence pyroxcne- free. This interpretation is in accordance with the findings by Smellie (this volume) based on investigations of sandstone detrital modes. Only very low occurrences of dolerite clasts were reported from this part of the core (Cape Roberts Science Team, 2000).
T h e increasing pyroxene content in the upper 4 2 0 mbsf coincides with increasing feldspar and decreasing quartz abundances, which we explained as a result of erosion in the valleys of the Transantarctic Mountains reaching the basement level. However, basement rocks that represented <20% of the provenance area as postulated by Smellie (this volume) cannot explain the high pyroxene concentrations. An additional source is required and could be seen in the sills and dykes of the Ferrar Supergroup. This idea finds support in the occurrence of abundant dolerite clasts throughout this part of the core (Cape Roberts Science Team, 2000). The trend of increasing pyroxenelstandard ratios continues throughout most of the sedimentary sequence of CRP-212A (Neumann & Ehrmann, 2000).
I n the absence of detritus delivered from the M c M u r d o Volcanic G r o u p , the main s o u r c e f o r amphiboles is the pre-Devonian basement. T h e sporadic occurrence of amphiboles in the lower part of CRP-3 may indicate that small areas of basement were exposed to erosion. The distinct increase of amphibole amounts above c. 250 mbsf (Fig. 4) marks the beginning of an intensified erosion of basement rocks in incised valleys in the Transantarctic Mountains. In CRP- 1 and CRP-2/2A amphibolel standard ratios were recorded which are similar or slightly higher than those in the upper part of core C R P - 3 (Ehrmann, 1998a; Neumann & E h r m a n n , 2000). Further investigations of the amphiboles as part of the heavy mineral fraction in CRP-3 sediments are presented by Neumann (this volume).
Although the diagenesis of the CRP-3 sediments is not a major topic of this paper, some information was gained from the XRD analyses. Opal-CT can be used as an indicator of the diagenetic alteration of the sediments. O p a l - C T originates f r o m opal-A of siliceous microfossils or from volcanic glass. It is metastable and it is transformed to microcrystalline quartz under progressing diagenesis. T h e transformation is mainly influenced by host rock lithology and interstitial water chemistry, time and temperature (Riech & von R a d , 1979). Siliceous microfossils as a n opal source are present above 2 0 0 mbsf ( C a p e R o b e r t s S c i e n c e Team, 2 0 0 0 ) . Volcanic glass a s t h e other probable source was described throughout the CRP-3 core (Cape Roberts Science Team, 2000). In CRP-3 opal-CT occurs only
in trace amounts a n d only i n isolated s a m p l e s (Fig. 2). The extremely low concentrations of opal (,''I' may indicate that the physicocheniical conclilioiis l'or a n opal-CT precipitation were not realiseil in t h e sediments. Another explanation for t h e low concentrations may be that most o f the o p a - ( " ' l ' already has transformed to microcrystalline qiiiirtz. 111
CRP-212A relatively high opal-CTlstandarcl ratios OS up to 0.1 were restricted to the upper 320 in ol' tIns core comprising lower Miocene and upper Oligoccnr sediments. Within this interval a clear trend t o
downcore decreasing concentrations c o u l d hr observed. The interval 320 to 620 mbsf of C R P - 3 w:is virtually void of opal-CT (Neumann & Elir11iii1111.
2000). Therefore, the poor record of opal-C'l' i n llic lower Oligocene and possibly upper Eocene sediments of CRP-3 can be regarded as the progression of tin- diagenetic path in the sediments off Cape Roberts.
Diagenetic minerals of the heulandite g r o u p a r c restricted to the interval between 360 and 7 0 mhsl' (Fig. 2). Heulandites a r e potassium-rich marine zeolites, but are also known from hydrotherniiil formation in drusy effusive rocks or from volcanic ashes (Rofller, 1984). In marine e n v i r o n m e n t s heulandite precipitates from silica-rich interstitial water in the presence of sufficient a l u m i n i u m , alkalines and earth alkalines (Kastner & Stonecipher, 1978), which probably come from the dissolution o f volcanic feldspar components. The silica may be derived from volcanic glass or from microfossils.
Very low amounts of clinoptilolite are restricted to 1 0 isolated scattered samples. C l i n o p t i l o l i t c precipitates only under low SiIAl-ratios in the interstitial water (Kastner & Stonecipher, 1978). The far-reaching absence of clinoptilolite l e a d s to the assumption that the SilAl-ratios in the interstitial water w e r e raised. T h e absence of remarkable, amounts of clinoptilolite is an important difference to most of the Oligocene and older sediments in the Antarctic Ocean. For example, clinoptilolite was f o u n d in several Oligocene and E o c e n e c o r e s recovered by the Ocean Drilling Program (Bohrmann
& Ehrmann, 1991; Ehrmann & Mackensen, 1992).
A l s o t h e u p p e r E o c e n e sediments of t h e nearby CIROS- 1 core contained clinoptilolite ( E h r n ~ a n n , 1998b).
In CRP-2/2A, minerals of the heulandite group are mainly restricted to the lower Oligocene sediments below c. 480 mbsf. Maximum zeolitelstandard ratios reached values of 0.1 to 0.18 (Neumann & Ehrmann, 2000). In CRP-3 the uppermost sample with notable z e o l i t e c o n t e n t c o m e s f r o m a depth of 7 0 m b s f . Zeolitelstandard ratios are similar as in CRP-212A.
CONCLUSIONS
The presented data on the bulk mineralogy of the CRP-3 sediments between the seafloor and a depth of 7 9 0 mbsf a r e semiquantitative abundances of
individual minerals. Quartz, plagioclasc. K-feldspar a n d pyroxene are the most common constituents of the CRP-1 sediments above 790 mhsf. Tlicse niui~i minenils a r e present in all samples t h r o i ~ l i o ~ ~ t the core, hiit in fluctuating amounts, Ampliibolc. zcolitc and opal-CT contribute insignificantly to the hulk mineralogy. Relative changes of the abundance of individual minerals or mineral groups or between mineriils o r mineral groups give information on the sediment source as well as o n the uplift and erosional history of the adjacent Transantarctic Mountains.
All of the detected, non-diagenctic minerals point to a sediment source in the Transantarctic Mountains west of the Cape Roberts region. The plutonic and metamorphic rocks of the basement, the sedimentary rocks of the Beacon Supergroup and the volcanic rocks of the Ferrar Supergroup all contribute to the detrital components of the CRP-3 sediments. Although the abundances of individual sediment components are fluctuating, they d o not indicate a major change in the location of the source during the time documented by the CRP-3 core. However, the CRP-3 sediments document a gradual change in the lithology of the rocks eroded in the Transantarctic Mountains. T h e oldest recovered Cenozoic interval from 790 to c. 620 mbsf contains predominantly quartz, feldspar and pyroxene. We assume that the youngest stratigraphic units of the hinterland served as a source. These units a r e the upper Beacon Supergroup (Victoria Group) and the Ferrar Supergroup including the Kirkpatrick Formation. However, the occurrence of K-feldspar and of trace amounts of amphiboles indicate that probably small areas of the basement were exposed already at that time.
The sandstone-dominated Taylor Group of t h e Beacon Supergroup probably is the main source rock f o r s e d i m e n t s of t h e interval f r o m c. 6 2 0 to c. 4 2 0 m b s f c h a r a c t e r i z e d by very h i g h q u a r t z c o n c e n t r a t i o n s . T h e lower p r o p o r t i o n of F e r r a r D o l e r i t e s i l l s i n t h e Taylor G r o u p r e s u l t s i n t h e decreased pyroxene concentration of this interval.
Above c. 420 mbsf the erosion in the Transantarctic Mountains reached t h e level of the basement rocks and, therefore, the amount of the basement-derived minerals amphibole and K-feldspar b e c a m e m o r e important in the CRP-3 sedirnents, whereas the quartz c o n c e n t r a t i o n s d e c r e a s e d steadily. T h e r i s i n g concentrations of plagioclase and pyroxene require an additional source, probably the sills and dykes of the Ferrar Supergroup.
O u r XRD data confirm and extend t h e r e s u l t s f r o m analyses of the composition of the gravel and s a n d fractions (Cape Roberts Science Team, 2000;
Smellie, this volume). These studies document also a progressive shift in the source rock lithologies from sediments of the B e a c o n Supergroup to b a s e m e n t rocks caused by a progressive uplift during deposition of t h e sediments below 500 mbsf, and by a stable, reduced uplift during deposition of t h e sediments above 500 mbsf.
S o m e (li;igeeetic :ibcration of the sediments i s indicated by the occiirrence of' tracc amounts o f opal- C'l' i n sevcriil inter~iils o l ("RP-3 and by some zeolite mainly confined (o depths between 360 to 80 mbsf.
ACKNOWl~1~IXiIMl~N'~S - Thanks to the Cape Roberts C a m p Team a n d thc C'rary L a b Team a t McMurdo for discussions. s a m p l i n ~ arul a fine drilling season. All laboratory work for this study was carried out at the Alfred Wegener Institute for Polar a n d Marine Research in Brcmcrhavcn, Germany. 14. Rhodes is appreciated for technical assistance. Reviews by L. Marinoni, I. Memmi and U . Schiissler helped to improve the manuscript.
Financial support was provided b y the D e u t s c h e Forschiingsgmein.schuft.
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