History of Oligocene Erosion, Uplift and Unroofing of the
Transantarctic Mountains Deduced from Sandstone Detrital Modes in CRP-3 Drillcore, Victoria Land Basin, Antarctica
J.L. S M R L I ~ I I ~
Brilish Antarctic Survey. Natural Environment Research Council. l-Iisli Cross, Madinglcy Road - Cambrklgc CB3 OET - U K (j.smellie@bas.ac.uk)
Received 26 October 2000: a c c ~ ~ p t c ~ / in revised form 23 April 2001
Abstract - Detrital modes determined on 68 s a n d s t o n e s a m p l c s from C R P - 3 drillcore indicate a continuation of the dynamic history of uplift-rclatcd erosion and unroofing previously documented in C R P - l and CRP-212A. T h c source area is identified very strongly with the Transantarctic Mountains (TAM) Dry Valleys block in southern Victoria Land. Initial unroofing of the TAM comprised removal of much of a former capping sequence of Jurassic Kirkpatrick basalts, which preceded the formation of the Victoria Land Basin. Erosion of Beacon Supergroup outcrops took place during progressive uplift of the TAM in the Oligocene. Earliest CRP-3 Oligocene samples above 788 metres below the sea f l o o r (mbsf) were sourced
o v e r w h e l m i n g l y in B e a c o n S u p e r g r o u p s t r a t a , i n c l u d i n g a recognisable c o n t r i b u t i o n f r o m Triassic volcanogenic Lashly Formation sandstones (uppermost Victoria Group). Moving up-section, by 500 mbsf, the CRP-3 samples are depauperate quartz arenites dominantly derived from the quartzose Devonian Taylor Group. Between c. 500 and 450 mbsf, the modal parameters show a distinctive change indicating that small outcrops of basement granitoids and metamorphic rocks were also being eroded along with the remaining Beacon (mainly Taylor G r o u p ) s e q u e n c e . Apart f r o m e n i g m a t i c fluctuations in modal indices a b o v e 450 mbsf. similar to those displayed by samples in CRP-2/2A, the CRP-3 modes are essentially constant (within a broad data scatter) to the top of CRP-3. The proportion of exposed basement outcrop remained at
< 20 %. indicating negligible uplift (i.e. relative stability) throughout that period.
INTRODUCTION
The Cape Roberts Drilling Project used a sea ice platform to drill the western margin of the Victoria L a n d Basin in M c M u r d o S o u n d , t o obtain fundamental knowledge of Cenozoic palaeoclimates a n d tectonic history of the Ross S e a region. T h e background to the project, its detailed aims, methods u s e d and results s o f a r a r e summarised in C a p e Roberts Science Team (1998, 1999. 2000) Hambrey
& Wise (1998) and Barsett & Ricci (2000a, b). In this
paper, the results of a study of sandstone detrital modes on samples from CRP-3 are described and interpreted. The modes reveal a continuing dynamic history of erosional unroofing and accompanying uplift of the Transantarctic Mountains adjacent to the CRP-3 drillsite.
METHODS
S i x t y eight C R P - 3 samples were s e l e c t e d f o r modal analysis. A mean sample distribution of about one every 10 m was achieved for most of the core.
T h e upper 300 m is most poorly sampled, with a
mean sample interval of about one every 15-20 m . Twenty two sandstone samples from the Beacon S u p e r g r o u p were also analysed modally, f o r comparative purposes. Sample treatment included resin impregnation of weak samples, and staining for feldspars (method of Houghton, 1980). For each sample, 300 sand grains were counted using the Gazzi-Dickinson method ( D i c k i n s o n , 1970). I n reporting the results, all of the detrital modes are recalculated to exclude matrix (<30 pm). Counts for lithic sedimentary grains (Ls) in the CRP-3 samples were also excluded by recalculation, because they appear to be intraformational and thus do not preserve provenance information (Smellie, 2000). However, Ls forms only a tiny part of the grain population in those samples and the recalculations to exclude Ls have only a trivial effect on the modes. Grain types are described in detail by Smellie (1998). There is no obvious systematic influence of grain size on the CRP-3 sample modes, other than possibly to increase the data scatter (Cape Roberts Science Team, 2000;
see also Smellie, 1998, 2000). However, the use of the Gazzi-Dickinson method and focus on modal indices (particularly ratios) that are unaffected by grain size, should minimise that scatter (Ingersoll et
al., 1984; Smellie, 2000). As in C R P - l and CRP- 2/2A, the dominant influence o n modal variations is likely to be provenance.
RESULTS OF THE DETRITAL MODES
T h e C R P - 3 samples arc q ~ ~ a r t z o f e l d s p a t l ~ i c sandstones similar to sandstones in CRP-land CRP- 2/2A. Most are s~ibarkoses ( s e n s u Pettijohn et al., 1973). A smaller proportion are arkoses, and several s a m p l e s , mainly between 500 and 6 0 0 mbsf, a r e quartz arenites (Tab. 1). I n addition, a few samples have mud matrix exceeding 15 % and are arkosic wackes; all are situated above 330 mbsf (see Tab. 1 in Wise et al., this volume). The mean value f o r quartz (Q) and feldspar (F) is 92.3 % (Q+F), and 32
% of samples (22 samples) have 2 95 % Q+F. These values are significantly higher than for any previous Cape Roberts Project modes (cf. Smellie, 1998, 2000).
Q and F vary antithetically. Q values show a gradual down-core increase to 98 % at 500 mbsf, falling more steeply t o 5 6 % at 7 0 0 mbsf before r e c o v e r i n g slightly, to about 7 0 % at the base of the c o r e (Fig. 1). Ratios of rounded to angular quartz grains (QrIQa) fluctuate widely (0.04-1.3) and the values are commonly higher than observed in CRP-1 and CRP- 2/2A (Smellie, 1998, 2000). The ratios are highest at 525 mbsf, falling sharply thereafter to much lower values (0.08-0.1) at the base of the core. In most of the samples, plagioclase (P) is more abundant than alkali feldspar (K). K exceeds P in samples between 400 and 620 mbsf, corresponding generally to those s a m p l e s poorest in F and richest in Q . P and K abundances apparently show no covariation, except for the section between 500 and 650 mbsf, where the indices broadly vary sympathetically. K/Q ratios are low (mainly < 0.1; Fig. 1) and are similar to those encountered in the basal 300 m of CRP-2/2A. They broadly vary antithetically with Qr/Qa ratios, as also observed in CRP-2/2A, and show a significant steady increase in samples below 500 mbsf.
Pyroxene is the only other common framework- grain constituent, with abundances generally varying between 1 and 6 % (mean: 4 . 1 %). H i g h e s t abundances (9-12 %) occur in samples above 1 7 0 mbsf, diminishing to minimum values (< 1 %) at 500 mbsf, then rising to about 6 % at the base of the core (Fig. 1).
Minor framework minerals include hornblende (pale green, pale brown), brown biotite, muscovite, o p a q u e o x i d e a n d coal, with a c o m b i n e d m o d e between 0 . 3 and 7 % (mean: 0 . 9 %). Of t h e s e , opaque grains ( o p a q u e o x i d e a n d c o a l ) a r e commonest, as in CRP-2/2A samples (Smellie, 2000).
T h e two o p a q u e grain types c a n o f t e n b e distinguished empirically: opaque oxide commonly occurs as an interstitial phase within dolerite grains, as fine granular aggregates or as individual small
grains wit11 angular shapes, whereas coal typically occurs as larger, single grains showing roundin;!,, There arc n o clear clown-core variations for the minor framework minerals and all are persistent ((';il>e Robcrts Science Team, 2000a). However, below ;iboul 400 mbsf', biotitc occurs mainly as small inclusions within cliiartz (note: only sand-size biotite grains o r inclusions were counted for the modes; most hiolite is sub-sand grade and only its presence is recorded i n table 1 , listed as an accessory phase). By contriisl,
~ n ~ i s c o v i t e occurs more commonly as free grains.
Accessory minerals include zircon, bioclastic ;iiul possible inorganic carbonate, sphene, garnet, cpiilotc (includiiig clinozoisite) and possible celadonite ( o n e occurrence; Tab. 1).
Lithic grains are a minor detrital component i n ; i l l
samples and usually forni < 5 % of the mode (mean 2.7 %). They are dominated by fine volcanic types (Lv), typically forming > 90-100 % of t h e litliic population. Above 500 mbsf, most samples have <
3.5 % Lv, whereas below 500 mbsf, the proportion of L v increases gradually t o about 5 %, w i t h some samples reaching 7-9 %. Four types were separately distinguished: (i) quartz-feldspar aggregates with graphic or myrmekitic textures; (ii) monominer* a l' ic or bimineralic quartz-feldspar aggregates w i t h fine patchy (mosaic-like) or "snowflake" textures; (iii) fine lathy-textured grains formed mainly of decussatc plagioclase, rarely with altered pyroxene andlor altered glass(?); and (iv) grains with feathery-texturcil plagioclase and/or pyroxene, sometimes with rod-like or dendritic opaque oxide, smectite(?)-altered glass or prismatic pyroxene. Graphic-textured (type (i) grains show relatively high abundances at four positions i n the c o r e : 0-250, 3 8 0 - 4 4 0 , a b o u t 5 0 0 a n d below 580 mbsf (Fig. 1). Lathy-textured (type (iii) grains also show a weak peak at 380-400 mbsf, but no other significant variations with depth. Feathery-textured (type (iv) grains are more common above 440 mbsf and almost non-existent below that depth. By contrast, the quartzo-feldspathic mosaic grains (type (ii) show a striking increase below c. 550 mbsf (Fig. 1). It is principally the variation in the type (ii) grains that is responsible for the overall down-core increase in Lv noted above. Other lithic grains are uncommon. I n addition to mudrocks, t h e r e a r e also sporadic occurrences of fine quartz-mica tectonite (Lm) and polycrystalline quartz (Qp). Dolerite and granitoid grains are also present but, because of the Gazzi- Dickinson methodology used, they are not counted as such (cf. Dickinson, 1970).
PROVENANCE
All samples contain the s a m e grain types, and none are diagnostic of individual parts of the section.
Moreover, despite the presence of significant down- core variations in modal abundances, the data show
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n o distinctive (especially clear, step-like) variations that can be used as a basis for identifying pctrof'acics (c/: CRP- 1 and CRP-212A; Smellie, 1998, 2000). The grain types are identical to those identified in CRP-1 and CRP-2/2A.
MINERAL GRAINS
Mineral grain types in CRP-1, CRP-2/2A and CRP-3 samples are alike. The same provenance is interpreted f o r C R P - 3 and inferred to be the 'I'ransantarctic Mountains (TAM), comprising coarse- grained plutonic rocks (Cambro-Ordovician Granite Harbour Intrusive Complex), quartzose sandstones of the Devonian-Triassic Beacon Supergroup, Jurassic dolerites and Kirkpatrick basalts, and minor metamorphic rocks (Upper Proterozoic metamorphic basement). T h e mineral grains a r e dominated by quartz and feldspar. Both are overwhelmingly derived from Beacon sandstone and pre-Beacon basement outcrops, but t h e h i g h to very high Q r / Q a ratios strongly indicate an important to locally dominant Beacon sandstone s o u r c e (Smellie, 1998, 2000).
Minerals derived from a possible basement source (e.g. hornblende, micas, garnet, zircon) are persistent d o w n - c o r e , a l t h o u g h p r e s e n t in s m a l l quantities (<< 1 % ) . T h e p r e s e n c e of small quantities of basement ( g r a n i t o i d and m e t a m o r p h i c ) clasts suggested to Cape Roberts Team (2000) that basement outcrops were exposed and being eroded right to the base of the core. However, it may be significant that all of the "basement mineral grains" are also present as detritus in Beacon sandstones. and the basement c l a s t s identified b y C a p e R o b e r t s S c i e n c e T e a m (2000a) are mainly angular quartz andlor feldspar grains of granule grade, together with rarer pebbles.
Clasts like t h o s e a r e a l s o known within B e a c o n conglomerates, particularly in the S t Johns Range (Turnbull et al., 1994). An origin by recycling from a Beacon source is possible for some or all of that debris.
Garnet, hornblende, biotite and muscovite may also be derived from metamorphic andlor granitoid (basement) s o u r c e s . B e l o w 3 7 5 m b s f , biotite is present almost entirely as tiny crystals within quartz (and hence has a likely plutonic origin), whereas muscovite occurs mainly as separate grains. However, garnet and micas are also present and locally common detrital components of the Beacon Supergroup (Taylor and Victoria groups for garnet, Victoria Group for micas), t o g e t h e r w i t h trace a m o u n t s of zircon (Korsch, 1973; B a r r e t t e t al., 1 9 8 6 ; unpublished information of the author).
In the absence of grains sourced in the McMurdo Volcanic Group (see lithic grains. below), pyroxene is almost entirely d e r i v e d f r o m Ferrar S u p e r g r o u p (probably mainly Ferrar dolerite; Kirkpatrlck basalt may also be a m i n o r s o u r c e ) , and is thus a good proxy for the modal contribution from that rock group
(cf.
S m e l l i e , 1998, 2000). Fcrrar c o n t r i b u t i o n i s highest within the topmost 170 mbsf (Fig. 1 ), wlirrc clasts of Ferrar doleritc arc also c o m m o n ((.'apts Roberts Science Team. 2000; Sandroni & Tiiliiriro, this volume). Modal values diminish below, hclwccn170 and 400 mbsf, reflecting diminished ( I o l i . ~ r i ~ r input, and are extremely low between 4 0 0 inul c, 600 mbsf before recoverinflo higher values (si mi l i i r
to those between 170 and 400 mbsf) b e t w e e n 0 2 0 mbsf and the base of tlie core. These v a r i a l i o n s suggest that tlie proportion of Ferrar doleritc i n tin- source terrain varied at different times d u r i n g t 1 1 ( ~ depositional period represented by C R P - 3 , o r else sediment transport paths varied through time (tiippiiip different areas with differing proportions o f l:crr;ir dolerite).
LITHIC GRAINS
In contrast to previous CRP modal investigations.
CRP-3 samples lack the alkalic volcanic lithic grains and ferromagnesian minerals characteristic o f t l u ' McMurdo Volcanic Group (Smellie, 1998, 2000). The presence of fresh glass (brown, green and colourless).
reported by Cape Roberts Science Team (2000) and interpreted as possible input from distal active alkalic volcanism in northern Victoria Land. is unconfirmed for the samples examined for detrital modes. Glass grains analysed by Ponipilio et al. (this volume) have tholeiitic compositions and a likely Kirkpatrick basalt provenance.
Most of the lithic grains are volcanic-derived (Lv).
Type (i) g r a p h i c - o r myrmekite-textured g r a i n s resemble mesostasis derived from Ferrar dolerites (less commonly Kirkpatrick basalt), granitoid pinions or possibly grains recycled from the granitoid-so~~rccd Beacon Supergroup; type (ii) quartzo-feldspathic aggregates are typical of recrystallised evolved lavas;
t y p e (iii) lathy-textured plagioclase-rich g r a i n s resemble relatively unevolved lavas; and type (iv) feathery-textured grains with rod- or dendritic opaques a r e f r a g m e n t s of basaltic lavas. T h e m o s t likely s o u r c e f o r types (iii) and (iv) basalt l a v a s is the Kirkpatrick Basalt Group (see also Pompilio et al., this volume), although similar textures occur more rarely within some Ferrar dolerites (cf. Elliot et al., 1 9 9 5 ) . T h e s o u r c e of t h e t y p e (ii) g r a i n s is less obvious. There are no known outcrops of evolved lavas in the TAM in south Victoria Land. However.
fragments of identical lavas are common within parts of the Victoria G r o u p (Lashly formation) and an origin by recycling through erosion of that source is a plausible explanation for type (ii) Lv grains. Similar fragments are also common in the Lower Jurassic Hanson Formation in the central TAM (Elliot. 1996), although arguments presented later suggest that the central TAM may be too distant a source area to have contributed significantly to CRP-3 sandstones. In a d d i t i o n , g r a i n s of p o l y c r y s t a l l i n e q u a r t z ( Q p ) , metamorphic grains (Lm), dolerite and granitoid were
History of Oligoccnc Erosion, Uplil'! aiid Dnroot'ing 485
KIQ Pyroxene Lithic volcanic grains (1) Lithic volcanic grains (2)
0 0 0 1 0 2 0 3 0 4 0 5 0 5 10 15 20 0 1 ? 3 4 5 0 2 4 6 8
Fig. 1 - Down-core distribution of selected modal indices for sandstone samples in CRP-3. All values plotted are percentages. Also superimposed are mean values determined for representative groups of samples from the different provenance types (Beacon Supergroup (Taylor and Victoria groups: unpublished modal data of the author) and basement (mainly granitoids: calculated from Talarico & Sandroni.
1998). Note that detritus derived from Ferrar dolerite will have no effect on these ratios but is the likely primary control on the pyroxene abundances shown. Curves drawn in each diagram are based on 5-point runningaaverages. See Tab.1 for explanation of modal abbreviations.
Lithic volcanic grain types (i-iv). and their provenance interpretation, are described in the text.
noted above, although dolerite and granitoid are too DISCUSSION coarse-grained to be included in the modes. Together,
they have a persistent down-core distribution (Cape EROSIONAL HISTORY AND LOCATION OF THE Roberts Science Team, 2000). Only dolerite grains SOURCE TERAIN
s h o w a few obvious zones of greater abundance,
located mainly within the upper 180 mbsf, but also at It is possible to estimate the relative contributions 370-400 and 480-540 mbsf (see also Sandroni & from the different lithological provenance "types" (i.e.
Talarico, this volume). Q p and Lm also occur as a granitoid and metamorphic basement, Beacon, Ferrar detrital component in Beacon sandstones and need not dolerite, Kirkpatrick basalt) by comparing the indicate a basement provenance. different modal contributions of those sources (cf.
Smellie, 2000). QrIQa ratios are a particularly useful discriminant. Because of the short distance between the TAM source terrain and the CRP-3 drillsite (30- 100 km), angular quartz grains are unlikely to become significantly rounded by normal fluvial or glacial irocesses. Very short transport distances for CRP-3 sediments were also inferred by Atkins (this volume), based on clast shapes and surface features. Thus, rounded-well rounded quartz grains will be derived exclusively by erosion from Beacon sandstones.
Moreover, only Beacon sandstones and basement granitoids will contribute angular quartz. In addition, there are significant differences in QrIQa ratios for different parts of the Beacon Supergroup: high QrIQa ratios are characteristic of the Devonian Taylor Group, with much lower ratios for Victoria Group sandstones (Fig. 1). Similarly, KIQ ratios are very low in the Taylor Group, higher in the Victoria Group and very high in basement granitoids (Fig. 1); neither Ferrar dolerites nor Kirkpatrick basalts contribute to KIQ ratios. Smellie (2000) used a combination of QrIQa and KIQ ratios to distinguish between Beacon and basement sources: basement sources have very low QrIQa ratios combined with high KIQ, whereas the converse is true for Beacon sources. In figure 1, m o d a l d a t a f o r t h e d i f f e r e n t s o u r c e s a r e superimposed for comparison with CRP-3 samples.
For Q r I Q a and KIQ r a t i o s , t h e r e i s e x c e l l e n t antithetic agreement. In particular, from the base of the c o r e up to c. 5 0 0 inbsf, the d o m i n a n t source apparently changes from dominantly Victoria Group to dominantly Taylor Group. This is also consistent with the observed decrease in evolved volcanic lithic grains from 800 to 500 mbsf, a trend that a basement source cannot emulate: the sole k n o w n source of evolved-Lv detritus ( L v t y p e ( i i ) , above) is b y recycling from the upper Victoria Group. Above 500 mbsf, observed trends of (diminishing) QrIQa and (increasing) KIQ ratios either indicate an increasing influx of Victoria Group detritus o r else detritus derived from basement rocks. However, the much lower and roughly unchanging proportion of evolved- Lv grains above 500 mbsf suggests that the trends are much more likely to be an effect of contributions from a basement source.
The low KIQ ratios (mostly < 0.08) compared to mean values for KIQ in granitoid basement (0.84) imply that the proportion of detritus contributed by basement was s m a l l , s u g g e s t i n g o n l y a s m a l l basement outcrop. Antithetic fluctuations in QrIQa and KIQ ratios can also be interpreted to suggest that different periods experienced inputs of sediment with different source-proportions, very s i m i l a r to t h e variable modes for those ratios observed in CRP-212A (Smellie, 2000). Applying a simple lever rule to the modal values suggests that the proportion of detritus derived from basement outcrops generally fluctuated between about 5 and 2 0 % in alternate periods of reduced and enhanced influx, respectively. Although
evidence for a ma.jor influence of a Victori;~ ( i i . 0 1 1 1 )
source is lacking above 500 mbsf, the sponulir hiit persistent presence of Permo-Triassic palynof'ossil~.
coal debris and evolved-Lv
(cf.
Cape Roberts Sciciicr 'rc;im. 2000a) indicates that Victoria G r o u p str;it;i continued to be exposed, but that they formed only n small part of the outcrop being eroded.Lithic grains derived from the Kirkpatrick biisiilts (L,v lypcs (iii) and (iv), above) are slightly commoii~-r (c. 2 % ) above c. 430 mbsf, in a section of the corr in which Kirkpatrick basalt clasts a r c a l s o conspicuous (Cape Roberts Science Team, 2000;i;
Sandroni & Talarico, this volume). They become scarcer below that depth. However, they always f'orm a minor part of the grain population (range of modiil values: 0-2.3 %). The general paucity of detritus from that source suggests that the Kirkpatrick basalt i f outcrop was largely removed at some time prior to the depositional period represented within CRP-3.
The modes for samples within CRP-3, parlicukirly those obtained c l o s e to 5 0 0 mbsf. i n d i c a t e ;I
dominance of sand-grade sediment derived from ;L
source dominated by the Taylor Group of the Beacon Supergroup. This implies that strata of t h e Taylor Group formed a major part of the source terrain. This conclusion provides a strong clue about the locution of the source area that fed the Oligocene Victoria Land Basin. The TAM are divided into several crustal blocks on a r a n g e of s c a l e s , which probably experienced different rates and periods of uplift (ç,i,'
Wrenn & Webb, 1982; Stump & Fitzgerald. 1992;
Huybrechts, 1993; Fitzgerald, 1994; Busetti et al., 1999; van del- Wateren et al., 1999). Major criistal lineaments probably exist at the sites of the Mackay and Ferrar glaciers. The Dry Valleys block in southern Victoria L a n d is t h e major o u t c r o p a r e a of the Devonian Taylor Group, which reaches thicknesses of 750-1 1 0 0 m ( G u n n & Warren, 1962; McElroy &
Rose, 1987; Allibone et al., 1991; Isaac et al., 1996).
The Taylor Group thins rapidly to the north, south and west (0-300 m in the central TAM, absent i n northern Victoria Land; Barrett et al., 1986; Collinson e t a l . , 1986). M o r e o v e r , outcrops in t h e Convoy Range, north of the Mackay Glacier, are dominated by F e r r a r d o l e r i t e ( P o c k n a l l e t al., 1 9 9 4 ) . Any significant sediment contribution derived from that source would thus have a dominant dolerite signature, which is not o b v i o u s in C R P - 3 s a m p l e s (except possibly the very high Ferrar-derived pyroxene input above 170 mbsf). Absence of lithologically distinctive metagreywacke and carbonate clasts and grains in CRP-3 sandstones is also a source terrain indicator.
These are relatively uncommon lithologies in Victoria Land but are thickly developed and well exposed in the central TAM (Archaeocyathid-bearing Shackleton Limestone (c. 8 km thick; Cambrian; Byrd Group) and thermally metamorphosed greywacke and argillite of t h e G o l d i e F o r m a t i o n ( c . 7 k m t h i c k ; l a t e Precambrian; Beardmore Group); Laird et al., 197 1).
Finally. the virtual absence of low-pi';i(lc metamorphic clasts mid lithic grains in CRP-3 (and ('RP-212A;) samples s u g g e s t that the Protcrozoic low-gsaclc n~etaniorphic terrain of the Skelton Group. which is restricted 1 0 the area between the I'ei~ar and Skclton glaciers, was also not contributing significant clctri[us to the ('RP-3 sequence.
UPLIFT AND UNROOFING HISTORY
The s e q u e n t i a l trends for m o d a l indices documented above are a clear indication of a dynamic evolving provenance, which, in the Ross Sea region.
can be interpreted in terms of TAM uplift and erosion in southern Victoria Land. At least three episodes can be identified from the CRP-3 Oligocene sequence:
Period I ( p e - C R P - 3 ) . The paucity of distinctive detritus derived from a Kirkpatrick basalt source implies that the Kirkpatrick basalt outcrop had already been largely removed prior to the Oligocene period represented by CRP-3. The period of removal of the Kirkpatrick basalt outcrop may have coincided with t h e erosion of about 3 km of Beacon Supergroup strata, down to t h e lower few hundred metres at CRP-3 (P.J. Barren, personal communication). This period of erosion is unrepresented in C R P - 3 but presumably corresponds to a major phase of early, pre-Oligocene uplift and unroofing of the TAM.
Period 2 (788 to c. 450 mbsf). From the base of the CRP-3 core to c. 500 mbsf, progressive changes in the modal indices indicate that the source terrain c h a n g e d f r o m o n e d o m i n a t e d by Victoria G r o u p strata, initially uppermost Victoria Group (Lashly Formation), to one dominated by the Taylor Group.
At c. 500 mbsf, the modal trends alter direction (e.g.
f r o m higher to l o w e r QrIQa r a t i o s ; F i g . 1 ) . T h e change corresponds to an abrupt influx of detritus from the basement outcrop, which was either exposed f o r the first time. or e l s e t h e a r e a of b a s e m e n t o u t c r o p available f o r e r o s i o n w a s significantly increased by uplift. As the change occurs within a period of rapidly varying modes caused by uplift, and the upper surface of the basement outcrop is a gently dipping homoclinal surface (Kukri Erosion Surface), the former seems more likely. The modes continue to c h a n g e rapidly u p to c. 4 5 0 m b s f , i n d i c a t i n g an increasing contribution from the basement outcrops.
In terms of simple homoclinal uplift of the TAM, a period of progressive upward movement and erosion is envisaged. Had it been abrupt uplift, exposing (for example) Victoria and Taylor groups more or less simultaneously along steep scarps, we would expect to see a more homogeneous mixture of detritus rather than the progressive modal trends observed. The up- c o r e diminution of Victoria G r o u p d e t r i t u s a l s o suggests that the Victoria Group strata were back- stripped by scarp retreat relative to the underlying
Tiiylor (iroiip strat;i. Usi~ig pyroxene modal variations as ii proxy for Fcrrar doleritc input, i t also appears that thc propor~ion o f l^cn'ar dolcrite i n the source area diminished up-core during that period. In other words, from :I source comprising Victoria G r o u p intrnded b y s h e d s of doleritc. the source terrain changecl to one ilominated by Taylor Group largely lacking clolcritc. I n the D r y Valleys block today, mapping indicates that the Victoria Group contains about 50 % by thickness of I-'errar dolerite, whereas the Taylor Group is largely free of dolerite (Isaac e t al., 1996). Conversely. the hinterland of Cape Roberts, crossed by the Mackay Glacier, contains abundant dolerite that would swamp the modes with dolerite- derived detritus, which is not observed. T h e s e observations are consistent with the modal data and also support interpretation of the TAM in southern Victoria Land as the likely CRP-3 provenance area.
The subsequent up-core change (particularly above 400 mbsf) to much higher pyroxene contents could b e correlated with the unroofing of the peneplain sill (also known as the upper sill: Turnbull et al., 1994).
The sill is a laterally very extensive and thick (300- 500 m) dolerite mainly emplaced along the Kukri Erosion Surface unconformity that separates t h e Beacon Supergroup from basement (Gunn & Warren, 1 9 6 2 ; Turnbull e t a l . , 1994: Isaac et a l . , 1 9 9 6 ) . Period 3 (above c. 450 mbsf). The modal indices also show a significant change at c. 450 nibsf. Above that depth, most of the indices show a comparatively wide scatter superimposed on only very gradual up- c o r e variations (e.g. diminution in Q , a n t i t h e t i c increase in P). The scatter can be resolved statistically into a series of weakly-defined peaks and troughs, which may have importance for uplift and erosion.
For example, QrIQa and KIQ ratios, and pyroxene modes, show several peaks, which cross-correlate (in the c a s e of QrIQa and KIQ ratios, they c o r r e l a t e antithetically). Despite greater data scatter for CRP-3 s a m p l e s , the patterns broadly r e s e m b l e t h o s e previously described f o r CRP-212A s a m p l e s (particularly below c. 300 mbsf in that core; Smellie, 2000). However, the origin of the CRP-212A modal variations is still enigmatic. It remains hard to ascribe the variations to an entirely sedimentological origin e . g . by varying the transport paths between source and basin, to alternately tap lithologically different provenances). However, the peaks and troughs are superimposed on a more general pattern of only slight up-core provenance variation. T h e modes remain dominated by Beacon sandstone detritus (probably mainly Taylor Group), together with material shed from a relatively small basement outcrop representing probably < 20 % of the provenance area, and some Ferrar dolerite. The proportion of basement outcrop a p p a r e n t l y c h a n g e d little to t h e top of C R P - 3 , implying that uplift was either extremely slow or had possibly ceased during that period.
488 J.L. Smellie
CONCLUSIONS
Detrital modes were determined on 68 samples from the CRP-3 drillcore. For the Oligocene period, they reveal a continuation of the dynamic history of uplift-related e r o s i o n a n d b a c k - s t r i p p i n g o f t h e 'ransantarctic Mountains previously documented by detrital modes for younger sequences in CRP-1 and CRP-212A. T h e new d a t a a l s o give t h e c l e a r e s t indication s o far that the source region was almost certainly the Dry Valleys block of southern Victoria Land. The modal variations indicate that the earliest s e d i m e n t s w e r e derived o v e r w h e l m i n g l y from a B e a c o n s o u r c e , i n i t i a l l y Victoria G r o u p strata (including a distinctive contribution from the Triassic L a s h l y F o r m a t i o n ) , w h e r e a s by c. 5 0 0 mbsf t h e s o u r c e w a s a l m o s t e n t i r e l y Taylor G r o u p s t r a t a . A b o v e 5 0 0 m b s f , a n i m p o r t a n t contribution was introduced from a source identified as basement. The b a s e m e n t c o n t r i b u t i o n i n c r e a s e d upward t o c. 450 mbsf, but thereafter the proportion of basement outcrop being eroded remained roughly constant (at
< 20%) to the top of the core. Interpretation of the modal trends indicates that progressive rather than sudden uplift occurred during the period represented up to c. 450 mbsf, after which the remainder of the CRP-3 sequence accumulated in a period of relative stability o r o n l y s l i g h t uplift. A l t h o u g h n o t represented by samples in the CRP-3 sequence, it is also evident that a major episode of uplift and erosion of the TAM must also have preceded the formation of the Victoria Land Basin.
A C K N O W L E D G E M E N T S - I a m p a r t i c u l a r l y g r a t e f u l t o J i m C o l l i n s o n f o r B e a c o n s a n d s t o n e s a m p l e s , n u m e r o u s reprints of papers o n Beacon stratigraphy and petrology, a n d access to unpublished detrital m o d e s of B e a c o n sandstones;
t o B i l l H a m m e r a n d t h e l a t e K e n W o o l f e f o r a d d i t i o n a l r e f e r e n c e s a m p l e s o f B e a c o n s a n d s t o n e s ; t o M a s s i m o Pompilio and H a n n e s G r o b e f o r help w i t h obtaining C R P - 3 samples; and to Stewart B u s h a n d M i k e Tabecki for making a n d s t a i n i n g t h i n s e c t i o n s f r o m d i f f i c u l t c o r e m a t e r i a l . Finally. m y grateful t h a n k s a r e e x t e n d e d t o a l l involved i n t h e C a p e Roberts Project. f r o m inception to completion, f o r their invaluable h e l p a n d constructive discussions during t h e c o u r s e of t h r e e d r i l l i n g s e a s o n s a n d w o r k s h o p s , a n d to a l l n ~ e n i b e r s o f S c o t t B a s e a n d M c M u r d o S t a t i o n f o r t h e i r unstinting hospitality. Constructive c o m m e n t s o n this p a p e r b y W e r n e r E h r m a n n a n d D a v i d E l l i o t a r e a l s o g r e a t l y appreciated.
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