Grain-Size Analysis of Samples from Cape Roberts Core CRP-3, Victoria Land Basin, Antarctica, with Inferences about Depositional
Setting and Environn~ent
P.J. BARRKTI
Antarctic Research Centre, Victoria University o f Wellington, PO Box 600, VVelliiigton - New Zealam1 (petcr.bar-rett@vu\v.ac.nz)
Abstract
-
Grain-size analyses by sieve and S c d i s i a p h a i c picscnicd l o t 115 samples of coie from CRP-3, 12 k m oft the coast of south Victoria Land. The claka provide a useful check on visual core descriptions. The geographic setting for the strata sampled. some 790 111 of early Oligoccne nearshore marine scclimcnis with a persistent glacial influence, is reviewed, and sediment textures interpreted in that c o n t e x t . S a n d textures f r o m the CRP-3 s a m p l e s in the lower part o f the core suggest that deposition was initially primarily wave-dominated. but that at times the i n f l u e n c e of t h e waves w a s over-ridden by e p i s o d e s o f rapid sedimentation.Sedimentary cycles, recognised in the visual description of the core above 485 mbsf. show an increasing proportion of mudstone in the middle of each cycle above
330 mbsf that is interpreted to record periodic sedimentation in deeper water. Sandstone textures in the lower and upper parts of each cycle are interpreted to record departure from and return to shoreface deposition with changes in sea level. Mudstone textures above 176 mbsf indicate sedimentation below wave base. Many of the textures in both sand and mud samples show the coarse "tail" characteristic of ice-rafted debris, but others d o not, indicating ice-free periods. Many sandstones below c. 200 mbsf have virtually n o silt, but significant amounts of clay (6 to 17%) that is thought to be of post-depositional origin.
INTRODUCTION
This p a p e r presents the results of g r a i n - s i z e analyses from 1 1 5 samples of the C R P - 3 c o r e at depths ranging from 10 mbsf (metres below sea floor) to 788 mbsf, using seive and Sedigraph techniques, and provides an initial interpretation of the textures in the context of the likely environment of deposition.
T h e results are intended to provide reference data to supplement lithological descriptions in the core logs (Cape Roberts Science Team, 2000), and to help with facies interpretation. The analytical technique for the sand fraction (sieving) is simple, physical and widely practised for over a century. Thus the data acquired in this way provide a useful reference point for analyses produced b y other faster a n d m o r e sophisticated techniques, such as the Malvern laser particle size analysis system (Fielding, D u n b a r & B r y c e , this volume; Naish et al., this volume). In addition the data also provide a useful reference for grain size estimates derived from measurements taken with d o w n - h o l e l o g g i n g tools (Bucker, p e r s o n a l communication, 1999).
METHOD
Between 1 0 a n d 2 5 g of s a m p l e w e r e disaggregated by crushing gently between wooden
blocks and then stirring in distilled water and Calgon for 60 minutes in an ultrasonic bath. A microsample was checked for material not fully disaggregated. If aggregates were found the treatment was continued until disaggregation was complete. The sample was then wet-sieved into sand and mud fractions, and both fractions dried and weighed. The sand fraction (0.063- 2 mm) was then dry-sieved and a 1 g sub-sample of the m u d fraction analysed by S e d i g r a p h 5 1 0 0 . Because wet sieving invariably retains some coarse silt, dry sieving was extended to catch 4.5 and 5.0 phi fractions. The weights retained were then merged with the Sedigraph results. The analyses are reported in table 1 for each sample as frequency percent at 0.5 phi intervals for the range -1 to 10 phi (2 to 111024 mm) and the percent finer than 10 phi.
Around 116 of the samples contain more than 2%
gravel but only 7 s a m p l e s h a d m o r e t h a n 1 0 % . Because of the small sample size (typically between 1 5 a n d 3 0 g) t h e proportion of gravel c a n n o t b e reliably estimated, but the proportion is nevertheless recorded with the results.
RESULTS
The results are presented in appendix 1 (frequency percent) and appendix 2 (summary statistics). They
246 P.J. Barrett
:ire a l s o s u m n ~ a r i s e d as percent sand again t h e lilhologic log in figure 1, along with data from lascr- based grain size analysis (Fielding, Uunbar & Bryce, lhis volume), which is closely comparable. Appendix 2 includes a column for the i'acics designation f o r each sample, based on the visual core description. As i n CRP-212A (Barrett & Anderson, 2000) the size frequency distributions fall into 5 main types (facies after Powell et al., 2000 and Fielding et al., 2000, only slightly modified in Powell, this volume, f o r CRP-3). These are m u d s t o ~ ~ e (facies 1 ) with less than 0 % sand, sandy mudstone (facies 2) - mudstones t y p i c a l l y with 2 0 to 4 0 % s a n d , poorly s o r t e d s a n d s t o n e (facies 3 ) - broad sand m o d e w i t h considerable mud, well sorted sandstone (facies 4 and 5 ) - well-defined sand mode and little mud (but see data from analyses), and diamictite, (facies 6 and 7 - a wide range of sizes from pebbles to clay with a broad mode in the sand range). Conglomerate is also well represented in the lower part of the core, but cannot be meaningfully measured with a sample size of 20s. The proportions of each facies in the core for t h r e e i n t e r v a l s based on s e q u e n c e s t r a t i g r a p h i c c h a r a c t e r i s t i c s ( F i e l d i n g , N a i s h & Woolfe, t h i s volume) are summarised in table 1 to indicate t h e relative importance of each facies preserved. The table also shows the broad up-core changes from sand and c o n g l o m e r a t e i n t h e l o w e r p a r t of t h e c o r e t o mudstone, diamictite and sandstone in the upper part.
F o r m o s t of t h e s a m p l e s t h e v i s u a l c o r e d e s c r i p t i o n i s c o n f i r m e d by g r a i n - s i z e a n a l y s i s , discriminating the basic sediment types of mudstone, sandy mudstone, muddy sandstone and diamictite.
However, there are three intervals that were described as sandstone in the visual core description, and yet analyses showed them to be mudstone. These results, which should be taken into account in further detailed studies, are briefly described below:
T h e interval from 17 to 2 4 mbsf in unit 1 . 2 i s described in the Initial Report as sandstone of facies 3, but 2 samples analyse as mudstone with 19 and 27% sand, typical of facies 1 or 2.
T h e interval from 3 9 to 5 1 mbsf in unit 1 . 2 i s described as sandstone of facies 3, but 3 samples analyse as mudstone with 9 to 17% sand, typical of facies 1.
The interval from 110 to 114 mbsf in unit 2.2 is identified as sandstone of facies 3, but 3 samples analyse as m u d s t o n e with 2 1 to 2 5 % s a n d , m o r e typical of facies 1 or 2
The coarser-grained reporting of these sediments reflects the difficulty in estimating mean size in sandy mudstones, especially those in glacial sediments with a significant coarse sandy "tail".
A n o t h e r d i f f e r e n c e b e t w e e n t h e v i s u a l c o r e description and the results of the textural analyses is found in the core below 580 mbsf. Most of the core is designated as sandstone of facies 3 (poorly sorted muddy sandstone) with some intervals of sandstone of
Unit Depth Particle Size Sequences
( . l l I , ~ f )
% Sand
Fig. 1 - Lithologic log for the CRP-3 c o r e , showing unit and sequence boundaries. and variations in percent sand from sieve- Sedigraph analyses. Comparative data for percent sand determined from s m a l l ( > l g ) samples b y M a l v e r n ( l a s e r ) s i z e analysis (Fielding. Dunbar & Bryce, this volume).
facies 4 or 5 (moderately to well sorted sandstone).
However a comparison of analytical data from this interval indicates that t h e s e f a c i e s g r o u p s have a considerable range in mud content with a high degree
Grain-Size Analysis of S a m p l e s from C a p e Roberts C o r e CRP-3 247
Tih. 1 - Siinimary of facies abundance in CRP-3 core (data I'roin l'owell c l a l , , this volunn.').
4 3
of overlap (Tab. 2). All of the well-sorted sandstones (facies 5) were over 87% sand, but so were a third of those recorded as muddy sandstones (facies 3). A further complexity discussed below is the likelihood that a significant proportion of the clay fraction in most o f these samples is post-depositional. Further studies of this interval should take these analytical data into account.
l l
2.8 to 157.22 mbsf (sequences 1-3)
INTERPRETATION OF SEDIMENT TEXTURE IN SAMPLES FROM CRP-3
0 0 Thickness (m)
1
8 01
710 6 757.22 /D 329.96 mbsf (sequences 4-13)
47 10
In this section the textural characteristics of s a m p l e s from C R P - 3 a r e interpreted in terms of s e d i m e n t accumulation off a subsiding wave- dominated coast, with modern coastal sedimentation north of Wellington, New Zealand, as an example.
T h i s interpretation i s predicated on a palaeogeographic model for the early Oligocene of t h e region developed from both regional geological considerations and information from the Cape Roberts cores. Interpretations are made mainly on differences in sorting of the sand mode (symmetrical and well- sorted is interpreted as wave-influenced) and on the presencelabsence of a coarse sand "tail" (interpreted as ice-rafted debris). In addition, a post-depositional inference is made f o r the presence in many well- sorted sands of a pure clay textural component, which is interpreted as diagenetic in origin.
2 2
14 8 2
1 Thickness (in)
1
4 01
2 6Tab. 2 - C o n ~ p a r i s o n of percent mud in samples from facies 3 (poorly sorted muddy sandstone) and facies 4 and 5 (moderately and well sorted sandstone) for the interval below 580 mbsf.
4 3
15 0 1 6 2
0 0 329 96 to 823.11 mbsf (sequence 14-23 and below)
Percent
1
5 21
51 l
3 4 7 0
0 Thickness (in)
1
6OLIGOCENE PALAEOGEOGRAPHY l
l 14
3 2
Percent
1
2 31
150 0
Facies 3 ( 19 samples) Facies 4 & 5 (8 samples)
A number of features suggest that the e n t i r e thickness of Cenozoic strata cored in CRP-3 above c . 7 9 0 mbsf accumulated rapidly on a relatively shallow marine shelf setting within a few kilometres of the Oligocene coast. Marine microfossils. notably diatoms, foraminifera and marine palynoinorphs, occur in modest numbers in the upper 330 m of the core, and marine palynomorphs are found in several deeper horizons, including one at 781 mbsf (Cape Roberts Science Team, 2000), supporting the view that the entire section above this is marine. T h e appearance of the muddy facies in the upper part of the c o r e suggests that subsidence only slightly outpaced sedimentation.
T h e geography was most likely little different from today, with a straight coast trending north-south, and high mountains to the west. The existence of mountains at this time is inferred from the granitic clasts scattered throughout the core, indicating that they had by then been incised deeply e n o u g h t o expose the basement that still forms the foothills just beyond Cape Roberts 10 km to the west. The higher slopes of the mountains comprised then, as they d o today, relatively soft flat-lying Beacon Supergroup strata, almost a kilometre of quartz sandstone overlain by a similar thickness of Permian-Triassic feldspathic sandstone and mudstone, readily eroded to feed the new offshore basin. The strata included Permian coal beds whose fragn~ents can be found throughout the Oligocene CRP core. Magnetostratigraphy suggests an average sediment accumulation rate of about 600 m1m.y. for the middle (sandstone-dominated) section of the core (magnetozone N1, interpreted as (21311, Florindo et al., this volume). However. one should k e e p in mind that the entire core includes many disconformities, and that the sedimentation rate for some if not all intervals could well be much higher.
Seismic data and the likely proximity of a coast to the west suggests that the strata cored by CRP-3 accumulated as a tabular body extending seaward
9 0 1 3 6
1 1
2 9 19
1 3 6 2 0 l 2 4 1
Percent
I
1Rmge ill
peicenf mud 1 to 26%
4 to 26%
2
I
2 7 1 4 1 4 93 2
Aveiage
15%- 12%
4 2
Standaid deviation
6%
7%
from the shoreline. Seismic sections parallel to the depositional strike show stratification to bc broadly parallel and with relatively minor channelling. the largest being one around 2 km long and 70 111 deep (Henrys et al., 2000). Seismic dip sections also show roughly parallel stratification seaward of CRP-3, but
110 information is available in a landward direction.
T h e scenario proposed here is very like that of DC S a n t i s and Barrett ( 1 9 9 8 , F i g . 1) for the early Miocene strata in CRP-1, though by then the climate was much colder and influenced by more extensive a n d a t times g r o u n d e d i c e . In the milder early Oligocene times, waves were most likely to have been t h e strongest p h y s i c a l i n f l u e n c e on s e d i m e n t deposition offshore. Tidal currents were probably quite limited in their strength, assuming that the past tidal range was as small as that of today (maximum spring tide of c. 1.0 m, Cape Roberts Science Team, 2000).
Coastal climate was cool temperate, inferred from terrestrial palynomorphs in samples throughout most of the sequence, consistently in the upper 400 ~n of the core and at several further levels down to 781 mbsf. The palynomorphs represent a low-diversity woody vegetation (Raine & Askin, this volume).
Nevertheless glaciers from local ice caps or inland ice sheets were discharging ice into the sea from time to time, attested by out-sized and striated clasts at a number of levels throughout the core above 775 mbsf (Atkins, this volume). They might have grounded as far offshore as the CRP-3 site, but if they did then no lithological evidence remains (Powell et al., this volume). Evidence of grounded ice has been well e s t a b l i s h e d in t h e y o u n g e r l a t e Oligocene-early M i o c e n e strata of C R P - 2 / 2 A , w h e r e it typically formed the ice-scoured glacial surface of erosion at the base of the many depositional sequences (Fielding et al., 2000). Advance and retreat of a grounded ice margin had far less of an influence in the deposition of strata cored by CRP-3. This is especially so for strata below 330 mbsf, which are described simply in terms of a conglomerate-sandstone motif B that does not involve the direct influence of ice (Fielding, Naish and Woolfe, this volume).
BASIS FOR THE INTERPRETATION
The textural features of the main sediment types recognised from a review of the analytical results are discussed i n the c o n t e x t of a nearshore, wave- dominated, micro-tidal setting on a wave-graded shelf with a supply of mixed sediment. The concept of the wave-graded shelf was revived by Swift (1970), and used in many facies-oriented studies in the 1980's (summarised in Elliott, 1986). It has been included in the Swift and Thorne (1991) model for continental shelf sedimentation, a n d i s implicit in the use of textural variation f o r i d e n t i f y i n g s e q u e n c e s i n sequence stratigraphic analysis (Fielding et al., 2000).
In CRP-2A it is also the basis for linking the advance
and retreat of the ice ~iiargiii with tlie fall a n d risc i ~ i
sea level din-ing the deposition o f cycles 9 , 10 ;iinl 1 l whose chronology has provided the firs1 ilirrct evidence for orbital forcingof the ancient Ani:iirli(.
ice sheet at Milankovitch frequencies (Naish (-1 i l l , ,
2001).
In brief, a mixture of sand and mud is supplied by rivers to the coast, which in this wave-dominatc(1 setting separates into mud largely carried offshore i n suspension, and sand that is moved parallel to tin' coast by longshore drift and tidal c u r r e n t s . T h i s movement will be assisted by sediment r e s ~ ~ s p c n s i o n through wave action, and further assisted by tidzil currents. Sorting is best on the shore face itself where oscillatory water movement from waves r e s u l t s i n highest flow intensities. Variation in texture in this setting can therefore be considered as a spectrum or continuous variation from offshore mud through siunly mud to m u d d y shoreface sand to w e l l sorted f o r e s h o r e s a n d ( F i g . 2a). A process-oriented discussion of the wave-grading c o n c e p t , a n d its application to modern nearshore sediments can he found in Dunbar et al. (1997).
A modern example of sedimentation on a wavc- graded coast has been provided by a coastal transcct from sandy beach to offshore mud at PekaPeka. north of Wellington (Perrett, 1990). Since the post-glaciiil rise in sea level around 6000 years ago sediment lias been accumulating from beach to mid-shelf depths (c. 5 0 m) as a seaward-dipping sheet. The sediment c o m e s f r o m rivers e r o d i n g t h e m o u n t a i n s of the southern North Island, and depositing just seaward ol' the c o a s t , t h e coarser s a n d y s e d i m e n t being distributed alongshore by wave- and tidally generated currents. At the same time this sediment i s gradecl normal t o the coast, along with the f i n e r muddy sediment, by low to moderate energy waves (average significant wave height is about 1.3 m and mean zero crossing period between 4 and 5 seconds, Harris, 1990), with the finest sediment carried seaward to s e t t l e o u t f r o m suspension. F i g u r e 2 b s h o w s the changing textural pattern in histograms of sea floor samples from offshore mud through sandy mud to muddy sand and well-sorted beach sand at PekaPeka.
A l t h o u g h m e a n s p r i n g tidal r a n g e is 2.1 111, and shore-parallel tidal currents run at up to 40 cmlsec seaward of the surf zone, this has not affected the shore-normal textural gradient. This model is the subject of continuing study (Dunbar & Barrett (2001).
INTERPRETATION O F SAMPLES FROM CRP-3 The CRP core has been interpreted in the basis of lithofacies patterns to form a series of cycles that are best developed in the upper 330 m. Here a simple interpretation is made of each facies that also happens to be consistent with the sequence stratigraphic model of Fielding et al. (this volume). In the samples from t h e C R P - 3 c o r e the m u d s t o n e s of f a c i e s 1 a r e
(ii'iiiii-Si~c Analysis of Samples from ('api.' Roberts Core C R P 3
slioaliny wave Xeakoi
zone rone
- - - -p--
mean fair weather wave /
base (5-1 5 m)
mean storm wave sed~rneni 9"-
base (l 5-50 nil
Descriptive fades Interpretive facies
5 4 m
0 SAND SILT CLAY
30 Offshore V
20
g
10 0 --- -
--
30 0 Sandy n
7
20 - L I I
0 c
10 g
0- 5-
60 Beach
50 0 0
0
30 -5 C
20 0 10
0 --
3
9.73 mbsf Diarnictite 030 11 m
CM
20 73 SAND SILT CLAY
8
10 0Â
0 0Â
CM 3 9 . 1 5 mbsf
30 T- Mudstone
2
CM20
g
v1; 0Â
W 3 1 . 9 1 mbsf sandy o
30
0
mudstone CO v"0
7 2 . 0 3 mbsf
Muddy N-
v"
40 sandstone 7
30 U 20 0Â 10 0-
60 4 5 5 . 0 0 mbsf
Sandstone 40
-F
30 -73
2
20 g c
10 - 0Â
0 - -
4 3 3 . 6 6 mbsf
Sandstone
30 0
20 5-
10
- 1 0 1 2 3 4 5 6 7 8 9 1 0
Diameter (phi) Diameter (phi)
Fig. 2 - Facies model for coastal sedimentation, accompanied by textural data from a transect off a present day wave-dominated coast. and from a selection of CRP-3 samples. A. Model for wave-dominated nearshore coastal sedimentation (from Barrett. 1989. adapted from EIIiott. 1986). B. Histograms of modern sea floor sediment from a transect from offshore mud to beach sand off PekaPeka. 50 km north of Wellington. NZ. showing the progressive onshore increase in sand content and improved sorting of the sand mode. Water depth for each sample shown with histogram in m. C. Histograms of CRP-3 samples chosen for illustrative comparison with the sediment accumulating off Peka Peka. Note. however, the coarse tail of coarse sand in samples from 31.91. 39.15. 298 and 433 mbsf, interpreted as ice-rafted debris.
characterised by a broad size distribution mostly i n t h e m u d r a n g e with a small sand tail ( F i g . 2 c , 3 9 . 1 5 mbsf). T h e s e a r e interpreted to have been deposited from suspension with virtually no current or wave activity, and hence are considered to represent sedimentation below wave base. This is typically below 15 to 5 0 m for an open coast depending o n fetch (Elliott, 1986) - in an einbayment like the Ross Sea in the early Oligocene this depth was most likely around 20 m and unlikely to have been more than 30 m. Sandy mudstone samples typical of facies 2 show a small but distinct typically fine-grained sand mode, carried offshore either by wind or m o r e likely as nearshore sediment suspended and redeposited by density currents during storms (Fig. 2c, 31.91 mbsf).
They are presumed to represent deposition closer to t h e c o a s t and in s l i g h t l y shallow water. M u d d y s a n d s t o n e s typical of facies 3 a r e interpreted as s e d i m e n t deposited within a consistently t h o u g h weakly wave-influenced zone (Fig. 2c, 72.03 nibsf), but these too might represent storm-generated density current sedimentation. The fine to coarse moderately to w e l l s o r t e d s a n d s t o n e s of f a c i e s 4 a n d 5 a r e interpreted as foreshore and shoreface sands (Fig. 2c, 294.52 mbsf, 455.00 mbsf, 433.66 mbsfj.
A feature in many CRP-3 samples of both sand and mud facies is a coarse tail of grains, normally just a few tenths of a percent in each size class, but persisting over several classes and coarser than the expected upper limit of the histogram. This is evident in 6 of the 7 samples from CRP-3 in figure 2c, but is a b s e n t in t h e P e k a P e k a s a m p l e s . T h i s tail i s interpreted as ice-rafted debris. It can be clearly seen in many samples at various levels in CRP-3, but is plainly absent from others (presence and absence are recorded in appendix 2 ) . T h e lowest s a m p l e t h a t shows an ice-rafted coarse tail is at 756 mbsf.
Diamictites form a significant proportion (19%) of the upper 157m of CRP-3, but are virtually absent below this. Texturally they are sandy mud (example in F i g . 2c) o r m u d d y sand with a variable g r a v e l content, though this cannot be reliably assessed from a 20g sample. Those in CRP-3 are considered to have formed by rain-out processes from melting i c e o r from debris flows (Powell et al., this volume). Only 3 samples were analysed - insufficient to characterise these sediments as a group. However, their texture is consistent with sedimentation from ice-rafting and suspension below wave base, with little disturbance from traction currents. These sediments may also have been reworked through redeposition b y s e d i m e n t gravity flows, which would not have changed their mixed nature.
T h e s i m p l e m o d e l o u t l i n e d a b o v e , u s e d i n conjunction with the recognition of ice-rafted debris, can explain the textural patterns of m o s t s a m p l e s from the upper 330 m . T h e upper 3 cycles, to 1 5 7 mbsf, are inferred to have been deposited mostly
shallowing. as tlie facies proportions in table .< imply.
Cycles 4- 14 include more sandstone with well-sorli.-(l fine to medium sand modes typical of nearshorc or i n a few cases even beach deposition. Below C'yrlr 14 a n d 3 3 0 inhsl'. however, the strata a r e hivgi'ly sandstone, and while many of the samples still h a v e the well-defined modes typical of the nearshore ;nul shorefacc sand above, many others have cithvr :I much broader mode or are bimodal (Fig. 3 ) . Tin-sr a r e interpreted as e i t h e r c u r r e n t - d e p o s i t e d o r redeposited (distinguished from wave-winnowcd s;iml in appendix 2), perhi'ips introduced during mi~ssivl' river discharges and buried without sufficient time l o r wave action t o be effective in grading them. Despi~r this influence the well-sorted sand textures typiciil of wave-gsading are found at a number of levels tlii'ougli the lower past of the core to a depth off 7 5 6 mhsf, implying that the depositional environment 01' ilicsi.- times was a wave-dominated coast characterised by periods of rapid sediment influx.
POSSIBLE POST-DEPOSITIONAL INFLUENCE ON TEXTURE
M o s t modern s h o r e f a c e a n d beach s a n d o n :I
wave-dominated coast has a mud content of less than 1 percent and near-shore shallow marine sands i n depths of reduced wave energy beyond the surf Lone have mud contents in the 1 to 20% range. However, for these sediments the mud is always a mixture of silt and clay in subequal proportions. Many sands from CRP-3 have a well-sorted sand mode, very little or no silt and 5 to 21% clay, quite at variance will1 t h e texture of m o d e r n s a n d i n this s e t t i n g . T h i s i n d i c a t e s that t h e c l a y c a n n o t b e of primary depositional origin, and most likely formed after tlie sand was deposited. The explanation offered here for this texture is that this clay c o m p o n e n t has been precipitated f r o m c i r c u l a t i n g p o r e w a t e r af-ter d e p o s i t i o n . T h i s e x p l a n a t i o n i s s u p p o r t e d a n d amplified by Wise et al. (this volume), who describe significant amounts of well-crystallised clays in tlie pores of many sandstones in the lower part of the C R P - 3 c o r e , a n d c o n c l u d e t h a t they f o r m e d by authigenesis.
CONCLUSIONS
The early Oligocene strata cored by CRP-3 to 790 mbsf were deposited in a rapidly subsiding nearshore m a r i n e e n v i r o n m e n t w i t h a p e r s i s t e n t glacial influence. Grain size analyses from these strata were c o m p a r e d w i t h t h o s e f r o m a m o d e r n n e a r s h o r e sediment prism accumulating in a wave-dominated environment in temperate New Zealand. Sand textures from the CRP-3 samples suggest that deposition was below wave b a s e , w i t h o n l y b r i e f p e r i o d s of initially primarily nearshore and that the influence of
60
680.96
mbsf756.00
mbsf 5040 Wave formed
30 sandstone
20 10 0
Current formed sandstone
520.67
mbsf713.02
mbsf-1 0 1 2 3 4 5 6
Diameter (phi) Diameter (phi)
Fig. 3 - Histograms of sandstone samples from the older sand-dominated section of CRP-3. showing typical cxamplcs of sand modes interpreted as wave-formed (bell-shaped. narrow). current-formed (bell-shaped, broader) and rapidly deposited (broad. rectangular o r bimodal). All but one of the six samples have virtually no silt. but between 7 and 16% clay. which is most likely to have been precipitated after deposition.
Barrett P.J. 1989. Sediment texture. In: P.J. Barrett (ed.), Antarctic
the waves was occasionall~ over-ridden ,ylacial l i i s f o ~ ; ~ , fro111 the CIROS-l drill hole, MrMiirdo Sound.
of rapid sedimentation. The increasing proportion of DSIR Bulletin. 245, 49-58.
mudstone above 330 mbsf records more frequent Barrett P.J. & Anderson, S. 2000. Grain-size analysis of samples sedimentation in deeper water, but sandstone textures f r o m C R P 2 / 2 A , Victoria Land Basin. Antarctica. Terra
Antartica. 7. 373-378.
s u g g e s t t h a t there are periods Cape Robens Science Team. 1998. Initial Rcport on CRP-l. Cape shoreface deposition. Many of the textures in both Roberts Project, Antarctica. Terra Antartica, 5. 1-187.
s a n d and mud s a m p l e s s h o w t h e c o a r s e "tail" Cape Roberts Science Team. 2000. Studies from the Cape Roberts characteristic of ice-rafted debris, but others do not, project, Ross Sea. Antarctica - Initial Report on CRP-3. Terra
Antartica. 7, 1-209 p with Supplement. 305 p.
indicating ice-free periods. De Santis L. & Barrett P.J. 1998. Grain-size analysis of samples The grain size analyses showed that some of the from CRP- 1 . Term Anfartica. 5. 375-382.
u p p e r parts of the c o r e w e r e significantly finer- g r a i n e d than was r e c o g n i s e d at t h e preliminary l o g g i n g stage, d e m o n s t r a t i n g t h e utility of such analyses in describing texturally complex sediments.
T h e analyses also revealed significant amounts (up to 2070) of fine-grained clay that is inferred to be of authigenic origin.
ACKNOWLEDGEMENTS - I wish to acknowledge the e x p e r t i s e , care and d e d i c a t i o n of J o Anderson, Sedimentology Technican, VUW. in processing the samples, a n d thank the School of E a r t h Sciences. VUW, for supporting the Sedimentology Laboratory. I am also grateful to Tim Naish for discussion and comment, and to Gavin Dunbar and Chris Fielding for helpful reviews.
REFERENCES
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Appendix I - Frequency percent in each size class (limits in phi units) for grain-size analyses (gravel-free) from CRP-3
Appendix 2 - Statistics for grain-size analyses (percent gravel followed by gravel-free graphic measures of Folk and proportions of sand-silt clay) from CRP-3. Percentiles used for the graphic measures for clay-rich samples were obtained by limiting clay size to 14 phi,
Interpretive elements for each sample are given on the right hand side of the table as follows:
Bottom PI-ocesses - ~ o r t e d / r a ~ i d l y d e p o s i t e c l - r e d e p o ~ i t e c i l ~ r e n t l ~ e S ~ ~ r f x e processes - ice-rafting *l110 ice rzifti~ig -/can't tell "blank" Diagenetic processes - diagenetic clays */none -1 cant tell "blank".
hterpretafion - see caption
