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Oligocene glaciation in Prydz Bay, ODP Site 1166, Antarctica
Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 101^111 www.elsevier.com/locate/palaeo
Implications of quartz grain microtextures for onset Eocene/Oligocene glaciation in Prydz Bay, ODP Site 1166, Antarctica Kari Strand a; , Sandra Passchier b , Jari Na«si c b
a Thule Institute, P.O.Box 7300, FIN-90014 University of Oulu, Oulu, Finland Department of Geological Sciences and Byrd Polar Research Center, Ohio State University, 155 S. Oval, Columbus, OH 43210, USA c Department of Process and Environmental Engineering, P.O.Box 4300, FIN-90014 University of Oulu, Oulu, Finland
Received 4 March 2002; accepted 3 March 2003
Abstract This paper presents the results of the scanning electron microscopic (SEM) analysis of quartz grains from a selection of samples at Site 1166. Ocean Drilling Program Leg 188 drilled Site 1166 on the Prydz Bay continental shelf, Antarctica, to document onset and fluctuations of East-Antarctic glaciation. This site recovered Upper Pliocene^Holocene glacial sediments directly above Cretaceous through Lower Oligocene sediments recording the transition from preglacial to early glacial conditions. SEM analysis of quartz grains at Site 1166 was used to characterize the glacial and preglacial sediments by their diagnostic textures. Angular edges, edge abrasion as well as arcuate to straight steps, are the most frequent features in glacial deposits. The highest frequency of grains with round edges is present in Middle^Late Eocene fluvio^deltaic sands. However, angular outlines, fractured plates with subparallel linear fractures and edge abrasion indicating glacier influence are also present. Preglacial carbonaceous mudstone and laminated gray claystone show distinctive high relief quartz grains and some chemical weathering on grain surfaces. The results of the microtextural analysis of quartz grains are used to verify some critical periods of ice sheet evolution, such as the transition from the East Antarctic preglacial to glacial conditions on the continental shelf from Middle/Late Eocene to Late Eocene/Early Oligocene time. ; 2003 Elsevier B.V. All rights reserved. Keywords: glacial sedimentation; quartz sand; scanning electron microscope; surface texture; Eocene/Oligocene glaciation; ice sheet evolution; Antarctica
1. Introduction The Antarctic ice sheet and the ocean surrounding it are key components in the global cli-
* Corresponding author. Fax: +358-8-5533556. E-mail address: kari.strand@oulu.¢ (K. Strand).
mate system throughout the Cenozoic. After a gradual cooling starting about 50 Ma a worldwide decrease in N18 O at 33.7 Ma is attributed mainly to ice volume increase (Zachos et al., 2001). In Antarctica, glaciation commenced with small ephemeral ice sheets during the Late Eocene, responding to a change in ocean circulation and global cooling following the separation of Austra-
0031-0182 / 03 / $ ^ see front matter ; 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00396-1
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lia and Antarctica (Barker et al., 1998). The size of the Early Oligocene ice sheet is uncertain, but major ice expansions have been inferred at the Eocene/Oligocene boundary coeval with the earliest Oligocene Oi-1 global shift in benthic foraminiferal N18 O values (Zachos et al., 2001). Prydz Bay is at the downstream end of the Lambert Glacier drainage system that originates in the Gamburtsev Mountains of central East Antarcti-
ca. Numerical models suggest that Prydz Bay would have been where ice ¢rst reached the Antarctic margin upon ice sheet inception (Cooper et al., 2001). Because of its large size in relation to the total drainage from East Antarctica (about 20%), the Lambert Glacier^Amery Ice Shelf system in Prydz Bay is a potential indicator of processes operating in the East Antarctic interior (Hambrey et al., 1991; O’Brien et al., 2001).
Fig. 1. (A) Overview map of the primary Leg 188 drill sites with respect to port of origin (Fremantle) and destination (Hobart). (B) Map of the East Antarctic coastline between 50 and 90‡E, showing the location of Prydz Bay, Mac. Robertson Land, Antarctic stations, Leg 119 drill sites (light circles), and Leg 188 drill sites (dark circles).
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Unit
Samples taken clay
1R
Ia
2R
silt
Interpretation and Description
103
Age (Ma)
sand Interglacial Hemipelagic
Subunit Ia: Poorly sorted diatom-bearing sandy silty clay with scattered clasts
Quarternary (0.0 - 0.66)
3R 4R 5R 6R 7R
Ib
8R
Glacial/ Glaciomarine Subunits Ib and Id: Diamict and poorly sorted clayey sandy silt with dispersed clasts
9R 10R
Late Pliocene (1.9 - 2.3)
11R 12R 13R
Ic
14R
Id
Subunit Ic: Interbedded poorly sorted sandy silt(stone) with lonestones and diatom (and diatom-bearing) clayey silt(stone) with dispersed granules and sand grains
15R
mbsf
16R
II
Proglacial
17R 18R
Unit II: Claystone and diatom-bearing claystone with decimeter-sized sand beds and rare dispersed clasts
19R
Late Eocene (32 - 37)
(Diatoms (33 - 37), Pollen (34 - 39))
20R 21R 22R 23R
Late Pliocene (2.3 - 3.2)
III
24R
Unit III: Massive and deformed matrix supported coarse to very coarse sand with abundant granule-sized clasts
Fluvial/ Deltaic
25R 26R 27R 28R 29R 30R 31R
IV
32R
Lagoonal
Unit IV: Homogeneous, highly carbonaceous black clay and planar laminated very fine sandy silt and organic-rich detritus
33R
Cretaceous Turonian (86 - 91)
34R 35R 36R 37R
V
Unit V: Gray claystone with very thin planar laminations
38R 39R 40R
Fig. 2. Schematic section of Site 1166 showing lithostratigraphic units, sediment types, and interpretation of core. Sample locations marked as arrows.
With its long history of sedimentation, Prydz Bay is regarded as a key area to investigate the transition from greenhouse to icehouse conditions and the long-term record of Antarctic glaciation (Barker et al., 1998).
Three sites were drilled on Leg 188, with one each on the Prydz Bay continental shelf, slope, and rise (Fig. 1). The preliminary results by O’Brien et al. (2001) from Site 1166 on the shelf document signi¢cant changes in depositional en-
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vironments and climatic conditions of the Prydz Bay region from the Late Cretaceous (Turonian) to the Early Oligocene and into the Neogene. Due to these drastic changes the sedimentary section at Site 1166 comprises a diverse suite of strata that from the top down include glacial, early glacial and preglacial sediments. Consequently, this site was the most suitable for a study of changes in sand grain microtextures. The ages range from Holocene at the sea£oor to Late Cretaceous at the bottom of the hole, with many disconformities throughout (O’Brien et al., 2001). A study of the changes in sand grain microtextures and their frequencies in the Prydz Bay continental shelf sediments drilled at Site 1166 can provide important information on the arrival of glaciers in Prydz Bay and the changes in paleoenvironments associated with cooling at the Eocene/Oligocene boundary.
2. Research area and lithostratigraphy of Site 1166 Prydz Bay is an embayment along the Antarctic margin between 66‡E and 79‡E (Fig. 1). On the southwestern side, it is bounded by the Amery Ice Shelf. The ice reaches thicknesses of 2500 m in the Southern Prince Charles Mountains and thins to around 400 m at the seaward edge of the Amery Ice Shelf (Budd et al., 1982). ODP Leg 188 drilled Site 1166 on the Prydz Bay continental shelf on the southwestern £ank of Four Ladies Bank, about 40 km southwest of ODP Site 742 of Leg 119. Site 1166 recovered Neogene diamictons and glacial clays unconformably overlying Early Oligocene/Late Eocene claystones. Below these are Paleogene sediments which record the transition from limited ice-cover in East Antarctica to the East-Antarctic ice sheet conditions (O’Brien et al., 2001; Cooper et al., 2001). The sedimentary deposits of ODP Site 1166 (Fig. 2) are divided into ¢ve lithostratigraphic units (O’Brien et al., 2001). Diatoms and palynomorphs provide the primary biostratigraphic age control. Lithostratigraphic Unit I consists of Upper Pliocene^Holocene diamicts (Fig. 3A) and few interbeds of diatomaceous claystones. The diamicts down to 94 m below sea£oor (bsf)
1166A-5R-1, 18-33 cm
A
2 cm
1166A-17R-3, 25-40 cm
B
2 cm
1166A-28R-1, 128-143 cm
C
2 cm
Fig. 3. Typical glacial sediment types of Site 1166. (A) Massive diamict (Unit I). (B) Sand with few dispersed clasts (Unit II). (C) Massive and deformed sand with abundant granule-sized clasts (Unit III).
are gravel-rich and structureless and contain reworked planktonic foraminifera, but below 94 m bsf strati¢cation exists locally. The uppermost sediment (Subunit IA, 0.0^2.74 m bsf) is a biogenic-rich clay interval with some pebble-sized clasts. Below this are two intervals of predominantly massive diamicts (Subunits IB, 2.79^ 106.36 m bsf, and ID, 123.0^135.41 m bsf) and one interval of dark gray sandy silt with lonestones and greenish gray diatom-bearing clayey silt with dispersed granules (Subunit IC, 113.30^ 117.22 m bsf). The contact between Units I and II is a major unconformity that was recovered at 135.63 m bsf. Unit II (135.63^156.62 m bsf) consists of an Upper Eocene to Lower Oligocene (33^ 37 Ma) diatom-bearing claystone with thin interbeds of sands and rare lonestones (Fig. 3B). The bottom of Unit II has rhythmically interbedded centimeter-thick sand and dark claystone. The contact between lithostratigraphic Units II and III is abrupt. Unit III (156.62^267.17 m bsf) consists of massive and deformed sands with a silty clay matrix (25^30 wt%). Diatoms were not recovered, but several specimens of pollen, spores, dino£agellates, and wood fragments were noted in the lower intervals. Unit III is of Middle^Late Eocene age (Truswell and Macphail, 2001) and the sands have a uniform fabric, are poorly
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sorted, and lack internal structures. The lower part of Unit III is deformed and folded by softsediment deformation of sandy beds and includes black organic-rich material with pieces of wood (Fig. 3C). The contact between lithostratigraphic Units III and IV was not recovered. Unit IV (276.44^314.91 m bsf) comprises black, highly carbonaceous clay and ¢ne sandy silt with organic-rich laminae and rare to moderate bioturbation. The sandy silt contains abundant mica and some pyrite. Unit V (342.80^342.96 m bsf) con-
105
sists of short interval of ¢nely laminated gray claystone. Palynomorphs indicate a Late Cretaceous Turonian age (86^91 Ma) for the sediments in Units IV and V (Truswell and Macphail, 2001).
3. SEM analysis A total of 19 representative samples covering all lithostratigraphic units were selected for detailed scanning electron microscopic (SEM) mi-
Table 1 Data for sampled cores from ODP Site 1166, frequency (%) of occurrence Surface textures
Morphological textures Angular outline Rounded outline Low relief Medium relief High relief Mechanical textures Large conchoidal fractures Small conchoidal fractures Arcuate steps Straight steps Crescentic gougles Imbricated blocks Large breakage blocks Fractured plates Subparallel linear fractures Curved grooves Straight grooves V-shaped percussion cracks Meandering ridges Edge abrasion Surface abrasion Deep troughs Chemical textures Preweathered surfaces Weathered surfaces Small precipitation features Large precipitation features Adhering particles New growth
Sample number 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
60 40 13 53 33
57 43 14 61 25
65 35 12 69 19
57 43 14 61 25
72 28 4 68 28
56 44 8 80 12
56 44 11 74 15
60 40 10 73 17
80 20 4 76 20
54 46 8 88 8
25 75 29 63 8
63 38 4 79 17
52 48 8 72 20
30 70 13 73 13
56 44 28 52 20
40 60 8 72 20
87 13 7 57 37
54 46 8 58 35
45 55 35 60 5
7 33 60 47 20 20 47 80 27 47 53 7 27 73 33 20
11 21 32 39 21 39 43 43 21 11 7 4 11 75 43 4
27 35 42 58 31 12 19 42 46 0 12 8 0 77 54 12
11 21 32 39 21 39 43 43 21 11 7 4 11 75 43 4
4 16 12 12 4 4 20 56 36 4 16 0 8 52 16 0
0 4 16 8 4 16 4 48 36 28 24 0 4 64 20 0
11 33 19 19 22 15 11 48 37 19 7 0 0 74 33 0
7 27 47 50 20 7 7 40 27 10 10 0 0 60 27 3
16 16 40 40 24 28 28 64 36 16 16 4 4 56 20 0
4 21 29 50 33 17 8 33 33 17 17 4 0 79 46 4
8 17 21 29 4 8 63 29 25 29 25 4 21 67 54 13
4 13 54 42 33 8 8 42 17 8 4 4 8 54 50 0
0 16 32 24 16 16 8 40 36 24 16 0 4 60 40 0
7 33 23 13 17 20 20 30 43 10 13 17 7 83 77 0
16 8 32 32 4 20 20 60 36 28 36 12 16 68 36 4
4 12 24 32 24 16 20 44 32 0 0 4 8 80 68 0
13 17 20 20 10 40 40 43 10 3 7 0 0 40 10 0
8 23 12 23 19 38 42 54 15 0 4 8 0 77 46 0
20 20 45 45 25 40 30 70 55 20 50 20 10 75 50 5
7 33 73 7 53 0
32 32 61 11 18 4
15 31 31 19 23 0
32 32 61 11 18 4
12 32 80 0 8 0
36 32 40 16 8 0
37 19 37 7 0 4
23 27 70 0 7 0
28 28 52 16 12 4
25 46 25 21 8 0
21 63 46 17 8 0
4 29 83 8 8 0
28 36 76 8 4 0
17 40 47 7 20 3
16 32 64 12 16 0
16 52 84 0 20 0
0 30 50 20 33 3
12 19 35 12 23 4
30 55 40 15 25 0
Sample number indexes and depths expressed as meters below sea£oor (m bsf) from ODP Leg 188, Hole 1166A; 1 = 188-1166A1R-CC, 4^6 cm (2.75 m bsf); 2 = 3R-1, 92^96 cm (20.29 m bsf); 3 = 5R-1, 69^72 cm (38.905 m bsf); 4 = 8R-1, 16^17 cm (65.565 m bsf); 5 = 9R-1, 66^70 cm (75.68 m bsf); 6 = 10R-1, 13^17 cm (84.55 m bsf); 7 = 11R-1, 49^51 cm (94.50 m bsf); 8 = 12R-1, 39^ 43 cm (104.91 m bsf); 9 = 13R-2, 100^104 cm (115.82 m bsf); 10 = 14R-2, 52^55 cm (125.04 m bsf); 11 = 17RI-4, 39^42 cm (156.31 m bsf); 12 = 017RII-4, 86^89 cm (156.78 m bsf); 13 = 18R-1, 23^26 cm (161.25 m bsf); 14 = 22R-1, 12^13 cm (199.63 m bsf); 15 = 25R-1, 53^54 cm (288.84 m bsf); 16 = 26R-1, 53^54 cm (238.44 m bsf); 17 = 32R-2, 34^38 cm (297.16 m bsf); 18 = 34R1, 34^38 cm (314.26 m bsf); 19 = 37R-CC, 6^10 cm (342.88 m bsf).
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100 µm A. Sample 1166A-1R-CC, 4-6 cm
20 µm B. Sample 1166A-1R-CC, 4-6 cm
300 µm C. Sample 1166A-5R-1, 69-72 cm
100 µm 30 µm 300 µm E. Sample 1166A-13R-2, 100-104 cm F. Sample 1166A-13R-2, 100-104 cm G. Sample 1166A-17R-4, 39-42 cm
100 µm D. Sample 1166A-8R-1, 15-17 cm
70 µm H. Sample 1166A-18R-1, 23-26 cm
Fig. 4. Selected SEM micrographs of grains from Site 1166 (Unit I^II) showing typical morphological and mechanical textures. (A) Arcuate steps and grooves (1) with slight rounding of grain edges. (B) Straight and arcuate steps. (C) Angular outline and straight steps (1). (D) Conchoidal fractures (1) and arcuate steps (2). (E) Angular outline and straight fractures (1) and steps (2). (F) Linear to conchoidal fractures (1) and edge abrasion (2). (G) Edge abrasion. (H) Edge abrasion and arcuate steps.
crotextural analysis of quartz grain surfaces (Table 1; Fig. 2). Samples (4^10 g) were ¢rst dried at 90‡C and weighed. The samples were then suspended and wet sieved in fractions 0.6, 0.25, 0.125, and 0.06 mm, dried and weighed. After light microscope control part of the samples were washed 20 min with SnCl (5 m%) to remove iron, were cleaned in boiling HCl for 10 min, and then washed with water twice (Krinsley and Doornkamp, 1973; Helland and Holmes, 1997). From each sample a total amount of 20^30 quartz grains were randomly selected from the fraction 0.250^0.6 mm and counted by using a JEOL, JSM-6400 SEM with EDS or a JEOL, JSM6300 FESEM at 12 kV at the Institute of Electron Optics, University of Oulu, Finland. A range of 27 di¡erent types of microtextures including angular to rounded outlines, surface reliefs, and mechanical textures such as conchoidal fractures, crescentic gouges, edge and surface abrasion features derived from the work of Mahaney et al. (1996) and Helland and Holmes (1997) was used to characterize glacial and non-glacial sediments by their diagnostic textures of sand grains (Figs. 4 and 5).
4. Results of SEM analysis Angular microfeatures, edge abrasion as well as arcuate to straight steps are the most frequent and relatively constant in Upper Pliocene^Holocene sediments (Unit I; Fig. 4A^F). In Unit I 50^ 80% of the grains are angular, vs. only 20^60% of the grains in Units II and III. The frequency of large breakage blocks ranges in a wide spectrum (10^40%) in the diamicts of Unit I as the sands of Unit III show constant values around 20% (Fig. 6). In the sediments of Unit I and II the frequency of angular outlines correlates with the frequency of medium relief, but anticorrelation is present in the sands of Unit III (Fig. 7). The highest frequency of grains with rounded outlines is present in the sands of Unit III. However, considerable numbers of grains with angular outlines, fractured plates, crescentic gouges and surface or edge abrasion are also present (Fig. 5A^D). The Late Cretaceous (Turonian) carbonaceous mudstones (Unit IV) have preserved high surface relief grains with angular outlines and imbricated blocks (Fig. 5E^H). Carbonaceous mudstones of Unit IV and underlying parallel laminated gray
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200 µm A. Sample 1166A-22R-1, 12-13 cm
200 µm E. Sample 1166A-32R-2, 34-38 cm
300 µm
300 µm B. Sample 1166A-22R-1, 12-13 cm
100 µm F. Sample 1166A-32R-2, 34-38 cm
C. Sample 1166A-22R-1, 12-13 cm
200 µm G. Sample 1166A-34R-1, 34-38 cm
107
200 µm D. Sample 1166A-26R-1, 52-54 cm
100 µm H. Sample 1166A-34R-1, 34-38 cm
Fig. 5. Selected SEM micrographs of grains from Site 1166 (Unit III^IV) showing typical morphological and mechanical textures. (A) Angular outline, fractured plates with straight steps (1) and subparallel linear fractures (2). (B) Rounded outline and surface abrasion with some V-shaped cracks (1). (C) Rounded outline with later imbricated blocks (1). (D) Fractured plates, subparallel linear fractures (1) and edge abrasion (2). (E) Low relief with rounded outlines. (F) Angular outline and high relief, typically preserved in clay-rich intervals. (G) Medium relief with chemically weathered surfaces. (H) Chemically weathered surfaces with adhering particles.
claystone of Unit V show chemically altered grain surfaces. Few V-shaped percussion cracks are present throughout the hole, except for Unit V where they are present on 20% of the grain surfaces. In general, straight and curved grooves correlate well with surface abrasion. When surface abrasion is present at higher frequencies, then frequencies of straight or arcuate steps are lower and vice versa (Fig. 6).
5. Discussion Analysis of quartz grain surface textures is a powerful tool when distinguishing sand grains originating from a glacial depositional environment from those a¡ected by subaqueous sedimentary processes. Glacial crushing and abrasion produces a suite of surface textures of which crescentic gouges, arcuate steps, conchoidal fractures, subparallel linear fractures, angularity and high relief are particularly diagnostic (Margolis and Kennett, 1971; Williams and Morgan, 1993;
Mahaney and Kalm, 1995; Mahaney et al., 1996). Grain to grain collisions in subaqueous environments produce V-shaped and irregular impact pits, large breakages, as well as a variety of chemical textures, such as solution pits and channels, etch patterns and crystalline overgrowths (Manker and Ponder, 1978; Georgiev and Sto¡ers, 1980; Linde and Mycielska-Dowgallo, 1980; Manickam and Barbaroux, 1987; Williams and Thomas, 1989; Passchier et al., 1997). The sediments in Unit I are predominantly diamicts and the surface textures of the sand grains indicate a history of crushing at the base of a glacier (Fig. 4A^D). Stratigraphic changes in the abundance of angular vs. rounded grains (Fig. 7) are caused by £uctuations of the ice margin. In proglacial glaciomarine environments current reworking causes edge abrasion resulting in a lower number of angular grains. In ice-contact environments angular outline and glacial textures are preserved. The structureless, gravel-rich diamicts in Subunits IB and ID are the result of subglacial deposition by grounded ice or ice-proximal sedi-
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
1 Angular outline 2 Rounded outline 3 Low relief 4 Medium relief 5 High relief 6 Large conchoidal fractures 7 Small conchoidal fractures 8 Arcuate steps 9 Straight steps 10 Crescentic gougles 11 Imbricated blocks 12 Large breakage blocks 13 Fractured plates Sub-parallel linear fractures 14 15 Curved grooves 16 Straight grooves V-shaped percussion cracks 17 18 Meandering ridges 19 Edge abrasion 20 Surface abrasion 21 Deep troughs 22 Prew eathered surfaces 23 Weathered surfaces Small precipitation features 24 Large precipitation features 25 26 Adhering particles 27 New growth 0
50 100 Frequency (%) 001R
005R
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 0
50 100 Frequency (%) 013R
017R I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 0 50 100 Frequency (%) 022R
026R
0
50 100 Frequency (%) 032R
037R
Fig. 6. Frequency of surface textures from selected samples of Site 1166.
mentation. Diamicts between 94 and 110 m bsf in Subunit IB are strati¢ed and contain a relatively low number of angular grains. The sediments probably originated in a glaciomarine depositional environment with current reworking, perhaps underneath an ice-shelf. The sandy silt with shell fragments and marine microfossils (Subunit IC) is a distal glaciomarine unit. The sand grains in Subunit IC are ice-rafted debris and 80% of the grains are angular. These sand grains escaped marine reworking since they melted out of icebergs directly released by glaciers. Unit II with an abrupt unconformity as an upper contact is a strati¢ed marine sequence that records ice rafting of pebbles. The sand grain surface textures also suggest that these sediments were deposited during glacial conditions. Although a shift in the stratigraphic distributions of angular grains is visible at the Paleogene^Neo-
gene unconformity (Fig. 7), more than 50% of the grains in the samples from Cores 17R and 18R are angular and almost 20% has a high relief. The high abundance of grains with glacial textures suggests that the depositional environment was proximal to a glacial source. At Site 742 of Leg 119, 40 km eastward, similar or slightly youngerage sediments were interpreted as proximal glacially-in£uenced proglacial or subglacial deposits (Barron and Larsen, 1991), which is in agreement with the interpretation of Unit II presented here. The coarse-grained sands in Unit III are interpreted as a record of deposition on an alluvial plain or delta, and the deformed beds record some reworking of material from underlying organic-rich horizons. The presence of rounded grains with a plethora of V-shaped percussion cracks is compatible with an alluvial origin for the sediments. However, the relative abundance
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Ia
50
Ib
100 Ic Id II
Depth (mbsf)
150
200
III
250
IV
300
V
350 0
20
40
60
80
100
Angular outline (%) Medium relief (%) High relief (%)
Fig. 7. Relief vs. angular outlines plot of frequency of morphological surface textures for correlation between samples of Site 1166.
of angular grains (30^50%) and the presence of grains with features that only develop in a subglacial environment (Fig. 5A) suggest that the sedimentary system is also sourced by a glacier. No sequence comparable to the homogeneous coarse sands was recovered at Site 742 suggesting that the deposit is a local phenomenon. The coarse sands may record a proglacial outwash system or a braided delta in a more ice-distal setting, perhaps with glaciers situated on land. Sand grains in Unit IV display large breakage blocks indicating grain to grain impact, as well as dulled surfaces and euhedral (angular) outlines indicating dissolution and recrystallization (Fig. 5E,F). These textures are compatible with deposition in a restricted marine or lagoonal environ-
109
ment (cf. Passchier et al., 1997; O’Brien et al., 2001). None of the sand grain surfaces shows indications of glacial activity. A similar-looking sequence of carbonaceous material was drilled in the bottom 2 m at Site 742 and was interpreted as £uvial or possibly lacustrine based on lack of marine microfossils (Barron and Larsen, 1991). The claystone of Unit V has grain surface textures indicating subaqueous transport and chemical alteration. Unit V correlates lithologically and in seismic stratigraphy with Site 741 of Leg 119, 110 km away, from which Lower Cretaceous gray claystone was recovered (Barron and Larsen, 1991). The lack of glacial textures suggests that the sediments of Units IV and V were deposited prior to the initiation of Antarctic glaciation. Site 1166 provides a record of cooling and ice sheet development in Prydz Bay from the Middle^ Late Eocene. The record from the Cape Roberts drilling Project (CRP) in the Ross Sea, on the Paci¢c side of the Antarctic margin, produced a record of progressive cooling to full-glacial conditions between 34 and 17 Ma. CRP results indicate that progressive cooling was already on its way by 34 Ma when temperate glaciers fed from an inland ice sheet released icebergs into the Ross Sea (Davey et al., 2001). The record at Site 1166 is in agreement with these results and, in addition, it provides important information about the initial stages of Antarctic glaciation during the Eocene/ Oligocene transition. The glacial signature of sand grains in the £uvio^deltaic sands of Unit III indicates that glaciers were present in the Prydz Bay area in Middle^Late Eocene time. The glaciomarine sequence of Unit II signals the arrival of glaciers at sea level and the development of the Lambert Glacier^Amery Ice Shelf drainage system draining the East Antarctic ice sheet 37^33 Ma. Major ice sheet expansion in Prydz Bay is recorded in Early Oligocene to Late Miocene sigmoidal sequences and topset beds, which prograded the Prydz Bay continental shelf about 30^40 km seaward (Cooper et al., 2001).
6. Summary and concluding remarks The present study of changes in sand grain mi-
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crotextures records glacial development since the Middle^Late Eocene. The distinguished microtextures identi¢ed as typical of glacial deposits include subparallel linear fractures, conchoidal fractures, arcuate steps, grain angularity, straight 1. Braided alluvium (Unit III) and preceding lagoonal setting (Unit IV) during late Cretaceous (Turonian)
2. Middle to late Eocene braided alluvium and ice-sheet appearance (Unit III)
3. Lowstand and late Eocene to early Oligocene erosion and ice-sheet propagation (upper part of Unit III)
4. Transgressions and ice-sheet development (Unit II) during late Oligocene
5. Plio-Pleistocene ice-sheet development (Unit I)
grooves, and crescentic gouges. Upper Cretaceous ¢nely laminated claystone (Unit V) and carbonaceous siltstone and mudstone (Unit IV) lack glacial microtextures. These sediments predate the initiation of Antarctic glaciation (Fig. 8). Part of the sand grains of Middle^Upper Eocene £uvio^ deltaic sands (Unit III) show evidence of a distal glacial source. The Upper Eocene/Lower Oligocene diatomaceous claystones with interbedded sand and few lonestones (Unit II) were also deposited during early glacial conditions, and signal the arrival of glaciers at sea level in Prydz Bay. The Neogene sediments of Unit I show evidence of glaciation at sea level throughout. The results of this study show that glaciation in the Prydz Bay basin started as early as the Middle^Late Eocene and that glaciers draining an inland ice sheet reached sea level by 37^33 Ma. These results are in agreement with the presence of outlet glaciers in the Ross Sea by 34 Ma (Davey et al., 2001) and the worldwide decrease in N18 O 33.7 Ma (Zachos et al., 2001).
Acknowledgements We would like to thank the Ocean Drilling Program for providing samples for this study, and especially the colleagues from the Leg 188 shipboard sedimentologists group, and co-chief scientists Phil O’Brien and Alan Cooper for helpful discussions regarding the sediment interpretations. We thank the sta¡ of the Institute of Electron Optics, especially Raija Peura, for help with the SEM/EDS and FESEM work. Helpful com-
Fig. 8. Summary diagrams for the depositional setting of the Prydz Bay region. (1) Preglacial setting with typically rounded and surface weathered sand grains and no crushing. (2) Initial glaciers and sand grains show some fractured plates and fractures after crushing because of ice behavior. (3) Ice sheet propagation and sand grains showing distinctively fractured surfaces after glacial crushing. (4) Development of ice sheet with marine transgressions with sandy portions recording angular grains and fractures after glacial crushing. (5) Full glaciation with episodes when ice sheets on the over-deepened continental shelf and sand grains in diamicts show range angular grains and fractures after glacial crushing. (From: O’Brien et al., 2001).
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ments and improvements by the reviewers M.A. Holmes and W.C. Mahaney and by the associate editor Phil E. O’Brian are appreciated. This study was supported by the Academy of Finland (Project No. 49039) and the University of Oulu/Thule Institute providing a grant for global change studies.
References Barker, P.F., Barrett, P.J., Camerlenghi, A., Cooper, A.K., Davey, F.J., Domack, E., Escutia, C., Kristo¡ersen, Y., O’Brien, P.E., 1998. Ice sheet history from Antarctic continental margin sediments: The ANTOSTRAT approach. Terra Antarct. 5, 737^760. Barron, J., Larsen, B., 1991. Proc. ODP, Sci. Results 119. Ocean Drilling Program, College Station, TX. Budd, W., Cory, M.J., Jacka, T.H., 1982. Results from the Amery Ice Shelf Project. Ann. Glaciol. 3, 36^41. Cooper, A.K., O’Brien, P.E., ODP Leg 188 Shipboard Scienti¢c Party, 2001. Early stages of East Antarctic glaciation ^ Insights from drilling and seismic re£ection data in the Prydz Bay Region. In: Florindo, F., Cooper, A.K. (Eds.), The Geologic Record of the Antarctic Ice Sheet from Drilling, Coring and Seismic Studies. Quaderni di Geo¢sica 16, pp. 41^42. Davey, F.J., Barrett, P.J., Cita, M.B., van der Meer, J.J.M., Tessensohn, F., Thomson, M.R.A., Webb, P.-N., Woolfe, K.J., 2001. Drilling for Antarctic Cenozoic climate and tectonic history at Cape Roberts, Southwestern Ross Sea. EOS Trans. Am. Geophys. Union 82, 585^590. Georgiev, V.M., Sto¡ers, P., 1980. Surface textures of quartz grains from Late Pleistocene to Holocene sediments of the Persian Gulf/Gulf of Oman ^ An application of the scanning electron microscope. Mar. Geol. 36, 85^96. Hambrey, M.J.L., Ehrmann, W.U., Larsen B., 1991. Cenozoic glacial record of the Prydz Bay continental shelf, East Antarctica. In: Barron, J., Larsen, B., Proc. ODP, Sci. Results 119. Ocean Drilling Program, College Station, TX, pp. 77^ 132. Helland, P.E., Holmes, M.A., 1997. Surface textural analysis of quartz sand grains from ODP Site 918 o¡ the southeast coast of Greenland suggests glaciation of southern Green-
111
land at 11 Ma. Palaeogeogr. Palaeoclimatol. Palaeoecol. 135, 109^121. Krinsley, D.H., Doornkamp, J.C., 1973. Atlas of Quartz Sand Surface Textures. Cambridge University Press, Cambridge, pp. 7^15. Linde, K., Mycielska-Dowgallo, E., 1980. Some experimentally produced microtextures on grain surfaces of quartz sand. Geogr. Ann. 62A, 171^184. Mahaney, W.C., Kalm, V., 1995. Scanning electron microscopy of Pleistocene tills in Estonia. Boreas 24, 13^29. Mahaney, W.C., Claridge, G.C., Campbell, I., 1996. Microtextures on quartz grains in tills from Antarctica. Palaeogeogr. Palaeoclimatol. Palaeoecol. 121, 89^103. Manickam, S., Barbaroux, L., 1987. Variations in the surface texture of suspended quartz grains in the Loire river: An SEM study. Sedimentology 34, 495^510. Manker, J.P., Ponder, R.D., 1978. Quartz grain surface features from £uvial environments of Northeastern Georgia. J. Sediment. Petrol. 48, 1227^1232. Margolis, S.V., Kennett, J.P., 1971. Cenozoic paleoglacial history of Antarctica recorded in subantarctic deep-sea cores. Am. J. Sci. 271, 1^36. O’Brien, P.E., Cooper, A.K., Richter, C. et al., 2001. Proc. ODP, Init. Repts. 188 [CD-ROM]. Available from Ocean Drilling Program, Texas ApM University, College Station TX 77845-9547, USA. [Online]. Available from: http:// www-odp.tamu.edu/publications/188_IR/188ir.htm. Passchier, S., Uscinowicz, S., Laban, C., 1997. Sediment supply and transport directions in the Gulf of Gdansk as observed from SEM analysis of quartz grain surface textures. Prace Panstwowego Instytutu Geologicznego 158, 1^27. Truswell, E.M., Macphail, M.K., 2001. Palynology of Cenozoic and Mesozoic sequences on the East Antarctic continental margin ^ A review. In: Florindo F., Cooper, A.K. (Eds.), The Geologic Record of the Antarctic Ice Sheet from Drilling, Coring and Seismic Studies. Quaderni di Geo¢sica 16, pp. 185^186. Williams, A.T., Morgan, P., 1993. Scanning electron microscope evidence for o¡shore^onshore sand transport at Fire Island, New York, USA. Sedimentology 40, 63^77. Williams, A.T., Thomas, M.C., 1989. Analysis of barrier island surface sediments by scanning electron microscopy. Mar. Geol. 86, 101^118. Zachos, J., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001. Trends, rhythms, and aberrations in Global Climate 65 Ma to Present. Science 292, 686^693.
PALAEO 3119 21-8-03
Implications of quartz grain microtextures for onset Eocene/Oligocene glaciation in Prydz Bay, ODP Site 1166, Antarctica Kari Strand a; , Sandra Passchier b , Jari Na«si c b
a Thule Institute, P.O.Box 7300, FIN-90014 University of Oulu, Oulu, Finland Department of Geological Sciences and Byrd Polar Research Center, Ohio State University, 155 S. Oval, Columbus, OH 43210, USA c Department of Process and Environmental Engineering, P.O.Box 4300, FIN-90014 University of Oulu, Oulu, Finland
Received 4 March 2002; accepted 3 March 2003
Abstract This paper presents the results of the scanning electron microscopic (SEM) analysis of quartz grains from a selection of samples at Site 1166. Ocean Drilling Program Leg 188 drilled Site 1166 on the Prydz Bay continental shelf, Antarctica, to document onset and fluctuations of East-Antarctic glaciation. This site recovered Upper Pliocene^Holocene glacial sediments directly above Cretaceous through Lower Oligocene sediments recording the transition from preglacial to early glacial conditions. SEM analysis of quartz grains at Site 1166 was used to characterize the glacial and preglacial sediments by their diagnostic textures. Angular edges, edge abrasion as well as arcuate to straight steps, are the most frequent features in glacial deposits. The highest frequency of grains with round edges is present in Middle^Late Eocene fluvio^deltaic sands. However, angular outlines, fractured plates with subparallel linear fractures and edge abrasion indicating glacier influence are also present. Preglacial carbonaceous mudstone and laminated gray claystone show distinctive high relief quartz grains and some chemical weathering on grain surfaces. The results of the microtextural analysis of quartz grains are used to verify some critical periods of ice sheet evolution, such as the transition from the East Antarctic preglacial to glacial conditions on the continental shelf from Middle/Late Eocene to Late Eocene/Early Oligocene time. ; 2003 Elsevier B.V. All rights reserved. Keywords: glacial sedimentation; quartz sand; scanning electron microscope; surface texture; Eocene/Oligocene glaciation; ice sheet evolution; Antarctica
1. Introduction The Antarctic ice sheet and the ocean surrounding it are key components in the global cli-
* Corresponding author. Fax: +358-8-5533556. E-mail address: kari.strand@oulu.¢ (K. Strand).
mate system throughout the Cenozoic. After a gradual cooling starting about 50 Ma a worldwide decrease in N18 O at 33.7 Ma is attributed mainly to ice volume increase (Zachos et al., 2001). In Antarctica, glaciation commenced with small ephemeral ice sheets during the Late Eocene, responding to a change in ocean circulation and global cooling following the separation of Austra-
0031-0182 / 03 / $ ^ see front matter ; 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00396-1
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lia and Antarctica (Barker et al., 1998). The size of the Early Oligocene ice sheet is uncertain, but major ice expansions have been inferred at the Eocene/Oligocene boundary coeval with the earliest Oligocene Oi-1 global shift in benthic foraminiferal N18 O values (Zachos et al., 2001). Prydz Bay is at the downstream end of the Lambert Glacier drainage system that originates in the Gamburtsev Mountains of central East Antarcti-
ca. Numerical models suggest that Prydz Bay would have been where ice ¢rst reached the Antarctic margin upon ice sheet inception (Cooper et al., 2001). Because of its large size in relation to the total drainage from East Antarctica (about 20%), the Lambert Glacier^Amery Ice Shelf system in Prydz Bay is a potential indicator of processes operating in the East Antarctic interior (Hambrey et al., 1991; O’Brien et al., 2001).
Fig. 1. (A) Overview map of the primary Leg 188 drill sites with respect to port of origin (Fremantle) and destination (Hobart). (B) Map of the East Antarctic coastline between 50 and 90‡E, showing the location of Prydz Bay, Mac. Robertson Land, Antarctic stations, Leg 119 drill sites (light circles), and Leg 188 drill sites (dark circles).
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Unit
Samples taken clay
1R
Ia
2R
silt
Interpretation and Description
103
Age (Ma)
sand Interglacial Hemipelagic
Subunit Ia: Poorly sorted diatom-bearing sandy silty clay with scattered clasts
Quarternary (0.0 - 0.66)
3R 4R 5R 6R 7R
Ib
8R
Glacial/ Glaciomarine Subunits Ib and Id: Diamict and poorly sorted clayey sandy silt with dispersed clasts
9R 10R
Late Pliocene (1.9 - 2.3)
11R 12R 13R
Ic
14R
Id
Subunit Ic: Interbedded poorly sorted sandy silt(stone) with lonestones and diatom (and diatom-bearing) clayey silt(stone) with dispersed granules and sand grains
15R
mbsf
16R
II
Proglacial
17R 18R
Unit II: Claystone and diatom-bearing claystone with decimeter-sized sand beds and rare dispersed clasts
19R
Late Eocene (32 - 37)
(Diatoms (33 - 37), Pollen (34 - 39))
20R 21R 22R 23R
Late Pliocene (2.3 - 3.2)
III
24R
Unit III: Massive and deformed matrix supported coarse to very coarse sand with abundant granule-sized clasts
Fluvial/ Deltaic
25R 26R 27R 28R 29R 30R 31R
IV
32R
Lagoonal
Unit IV: Homogeneous, highly carbonaceous black clay and planar laminated very fine sandy silt and organic-rich detritus
33R
Cretaceous Turonian (86 - 91)
34R 35R 36R 37R
V
Unit V: Gray claystone with very thin planar laminations
38R 39R 40R
Fig. 2. Schematic section of Site 1166 showing lithostratigraphic units, sediment types, and interpretation of core. Sample locations marked as arrows.
With its long history of sedimentation, Prydz Bay is regarded as a key area to investigate the transition from greenhouse to icehouse conditions and the long-term record of Antarctic glaciation (Barker et al., 1998).
Three sites were drilled on Leg 188, with one each on the Prydz Bay continental shelf, slope, and rise (Fig. 1). The preliminary results by O’Brien et al. (2001) from Site 1166 on the shelf document signi¢cant changes in depositional en-
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vironments and climatic conditions of the Prydz Bay region from the Late Cretaceous (Turonian) to the Early Oligocene and into the Neogene. Due to these drastic changes the sedimentary section at Site 1166 comprises a diverse suite of strata that from the top down include glacial, early glacial and preglacial sediments. Consequently, this site was the most suitable for a study of changes in sand grain microtextures. The ages range from Holocene at the sea£oor to Late Cretaceous at the bottom of the hole, with many disconformities throughout (O’Brien et al., 2001). A study of the changes in sand grain microtextures and their frequencies in the Prydz Bay continental shelf sediments drilled at Site 1166 can provide important information on the arrival of glaciers in Prydz Bay and the changes in paleoenvironments associated with cooling at the Eocene/Oligocene boundary.
2. Research area and lithostratigraphy of Site 1166 Prydz Bay is an embayment along the Antarctic margin between 66‡E and 79‡E (Fig. 1). On the southwestern side, it is bounded by the Amery Ice Shelf. The ice reaches thicknesses of 2500 m in the Southern Prince Charles Mountains and thins to around 400 m at the seaward edge of the Amery Ice Shelf (Budd et al., 1982). ODP Leg 188 drilled Site 1166 on the Prydz Bay continental shelf on the southwestern £ank of Four Ladies Bank, about 40 km southwest of ODP Site 742 of Leg 119. Site 1166 recovered Neogene diamictons and glacial clays unconformably overlying Early Oligocene/Late Eocene claystones. Below these are Paleogene sediments which record the transition from limited ice-cover in East Antarctica to the East-Antarctic ice sheet conditions (O’Brien et al., 2001; Cooper et al., 2001). The sedimentary deposits of ODP Site 1166 (Fig. 2) are divided into ¢ve lithostratigraphic units (O’Brien et al., 2001). Diatoms and palynomorphs provide the primary biostratigraphic age control. Lithostratigraphic Unit I consists of Upper Pliocene^Holocene diamicts (Fig. 3A) and few interbeds of diatomaceous claystones. The diamicts down to 94 m below sea£oor (bsf)
1166A-5R-1, 18-33 cm
A
2 cm
1166A-17R-3, 25-40 cm
B
2 cm
1166A-28R-1, 128-143 cm
C
2 cm
Fig. 3. Typical glacial sediment types of Site 1166. (A) Massive diamict (Unit I). (B) Sand with few dispersed clasts (Unit II). (C) Massive and deformed sand with abundant granule-sized clasts (Unit III).
are gravel-rich and structureless and contain reworked planktonic foraminifera, but below 94 m bsf strati¢cation exists locally. The uppermost sediment (Subunit IA, 0.0^2.74 m bsf) is a biogenic-rich clay interval with some pebble-sized clasts. Below this are two intervals of predominantly massive diamicts (Subunits IB, 2.79^ 106.36 m bsf, and ID, 123.0^135.41 m bsf) and one interval of dark gray sandy silt with lonestones and greenish gray diatom-bearing clayey silt with dispersed granules (Subunit IC, 113.30^ 117.22 m bsf). The contact between Units I and II is a major unconformity that was recovered at 135.63 m bsf. Unit II (135.63^156.62 m bsf) consists of an Upper Eocene to Lower Oligocene (33^ 37 Ma) diatom-bearing claystone with thin interbeds of sands and rare lonestones (Fig. 3B). The bottom of Unit II has rhythmically interbedded centimeter-thick sand and dark claystone. The contact between lithostratigraphic Units II and III is abrupt. Unit III (156.62^267.17 m bsf) consists of massive and deformed sands with a silty clay matrix (25^30 wt%). Diatoms were not recovered, but several specimens of pollen, spores, dino£agellates, and wood fragments were noted in the lower intervals. Unit III is of Middle^Late Eocene age (Truswell and Macphail, 2001) and the sands have a uniform fabric, are poorly
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sorted, and lack internal structures. The lower part of Unit III is deformed and folded by softsediment deformation of sandy beds and includes black organic-rich material with pieces of wood (Fig. 3C). The contact between lithostratigraphic Units III and IV was not recovered. Unit IV (276.44^314.91 m bsf) comprises black, highly carbonaceous clay and ¢ne sandy silt with organic-rich laminae and rare to moderate bioturbation. The sandy silt contains abundant mica and some pyrite. Unit V (342.80^342.96 m bsf) con-
105
sists of short interval of ¢nely laminated gray claystone. Palynomorphs indicate a Late Cretaceous Turonian age (86^91 Ma) for the sediments in Units IV and V (Truswell and Macphail, 2001).
3. SEM analysis A total of 19 representative samples covering all lithostratigraphic units were selected for detailed scanning electron microscopic (SEM) mi-
Table 1 Data for sampled cores from ODP Site 1166, frequency (%) of occurrence Surface textures
Morphological textures Angular outline Rounded outline Low relief Medium relief High relief Mechanical textures Large conchoidal fractures Small conchoidal fractures Arcuate steps Straight steps Crescentic gougles Imbricated blocks Large breakage blocks Fractured plates Subparallel linear fractures Curved grooves Straight grooves V-shaped percussion cracks Meandering ridges Edge abrasion Surface abrasion Deep troughs Chemical textures Preweathered surfaces Weathered surfaces Small precipitation features Large precipitation features Adhering particles New growth
Sample number 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
60 40 13 53 33
57 43 14 61 25
65 35 12 69 19
57 43 14 61 25
72 28 4 68 28
56 44 8 80 12
56 44 11 74 15
60 40 10 73 17
80 20 4 76 20
54 46 8 88 8
25 75 29 63 8
63 38 4 79 17
52 48 8 72 20
30 70 13 73 13
56 44 28 52 20
40 60 8 72 20
87 13 7 57 37
54 46 8 58 35
45 55 35 60 5
7 33 60 47 20 20 47 80 27 47 53 7 27 73 33 20
11 21 32 39 21 39 43 43 21 11 7 4 11 75 43 4
27 35 42 58 31 12 19 42 46 0 12 8 0 77 54 12
11 21 32 39 21 39 43 43 21 11 7 4 11 75 43 4
4 16 12 12 4 4 20 56 36 4 16 0 8 52 16 0
0 4 16 8 4 16 4 48 36 28 24 0 4 64 20 0
11 33 19 19 22 15 11 48 37 19 7 0 0 74 33 0
7 27 47 50 20 7 7 40 27 10 10 0 0 60 27 3
16 16 40 40 24 28 28 64 36 16 16 4 4 56 20 0
4 21 29 50 33 17 8 33 33 17 17 4 0 79 46 4
8 17 21 29 4 8 63 29 25 29 25 4 21 67 54 13
4 13 54 42 33 8 8 42 17 8 4 4 8 54 50 0
0 16 32 24 16 16 8 40 36 24 16 0 4 60 40 0
7 33 23 13 17 20 20 30 43 10 13 17 7 83 77 0
16 8 32 32 4 20 20 60 36 28 36 12 16 68 36 4
4 12 24 32 24 16 20 44 32 0 0 4 8 80 68 0
13 17 20 20 10 40 40 43 10 3 7 0 0 40 10 0
8 23 12 23 19 38 42 54 15 0 4 8 0 77 46 0
20 20 45 45 25 40 30 70 55 20 50 20 10 75 50 5
7 33 73 7 53 0
32 32 61 11 18 4
15 31 31 19 23 0
32 32 61 11 18 4
12 32 80 0 8 0
36 32 40 16 8 0
37 19 37 7 0 4
23 27 70 0 7 0
28 28 52 16 12 4
25 46 25 21 8 0
21 63 46 17 8 0
4 29 83 8 8 0
28 36 76 8 4 0
17 40 47 7 20 3
16 32 64 12 16 0
16 52 84 0 20 0
0 30 50 20 33 3
12 19 35 12 23 4
30 55 40 15 25 0
Sample number indexes and depths expressed as meters below sea£oor (m bsf) from ODP Leg 188, Hole 1166A; 1 = 188-1166A1R-CC, 4^6 cm (2.75 m bsf); 2 = 3R-1, 92^96 cm (20.29 m bsf); 3 = 5R-1, 69^72 cm (38.905 m bsf); 4 = 8R-1, 16^17 cm (65.565 m bsf); 5 = 9R-1, 66^70 cm (75.68 m bsf); 6 = 10R-1, 13^17 cm (84.55 m bsf); 7 = 11R-1, 49^51 cm (94.50 m bsf); 8 = 12R-1, 39^ 43 cm (104.91 m bsf); 9 = 13R-2, 100^104 cm (115.82 m bsf); 10 = 14R-2, 52^55 cm (125.04 m bsf); 11 = 17RI-4, 39^42 cm (156.31 m bsf); 12 = 017RII-4, 86^89 cm (156.78 m bsf); 13 = 18R-1, 23^26 cm (161.25 m bsf); 14 = 22R-1, 12^13 cm (199.63 m bsf); 15 = 25R-1, 53^54 cm (288.84 m bsf); 16 = 26R-1, 53^54 cm (238.44 m bsf); 17 = 32R-2, 34^38 cm (297.16 m bsf); 18 = 34R1, 34^38 cm (314.26 m bsf); 19 = 37R-CC, 6^10 cm (342.88 m bsf).
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100 µm A. Sample 1166A-1R-CC, 4-6 cm
20 µm B. Sample 1166A-1R-CC, 4-6 cm
300 µm C. Sample 1166A-5R-1, 69-72 cm
100 µm 30 µm 300 µm E. Sample 1166A-13R-2, 100-104 cm F. Sample 1166A-13R-2, 100-104 cm G. Sample 1166A-17R-4, 39-42 cm
100 µm D. Sample 1166A-8R-1, 15-17 cm
70 µm H. Sample 1166A-18R-1, 23-26 cm
Fig. 4. Selected SEM micrographs of grains from Site 1166 (Unit I^II) showing typical morphological and mechanical textures. (A) Arcuate steps and grooves (1) with slight rounding of grain edges. (B) Straight and arcuate steps. (C) Angular outline and straight steps (1). (D) Conchoidal fractures (1) and arcuate steps (2). (E) Angular outline and straight fractures (1) and steps (2). (F) Linear to conchoidal fractures (1) and edge abrasion (2). (G) Edge abrasion. (H) Edge abrasion and arcuate steps.
crotextural analysis of quartz grain surfaces (Table 1; Fig. 2). Samples (4^10 g) were ¢rst dried at 90‡C and weighed. The samples were then suspended and wet sieved in fractions 0.6, 0.25, 0.125, and 0.06 mm, dried and weighed. After light microscope control part of the samples were washed 20 min with SnCl (5 m%) to remove iron, were cleaned in boiling HCl for 10 min, and then washed with water twice (Krinsley and Doornkamp, 1973; Helland and Holmes, 1997). From each sample a total amount of 20^30 quartz grains were randomly selected from the fraction 0.250^0.6 mm and counted by using a JEOL, JSM-6400 SEM with EDS or a JEOL, JSM6300 FESEM at 12 kV at the Institute of Electron Optics, University of Oulu, Finland. A range of 27 di¡erent types of microtextures including angular to rounded outlines, surface reliefs, and mechanical textures such as conchoidal fractures, crescentic gouges, edge and surface abrasion features derived from the work of Mahaney et al. (1996) and Helland and Holmes (1997) was used to characterize glacial and non-glacial sediments by their diagnostic textures of sand grains (Figs. 4 and 5).
4. Results of SEM analysis Angular microfeatures, edge abrasion as well as arcuate to straight steps are the most frequent and relatively constant in Upper Pliocene^Holocene sediments (Unit I; Fig. 4A^F). In Unit I 50^ 80% of the grains are angular, vs. only 20^60% of the grains in Units II and III. The frequency of large breakage blocks ranges in a wide spectrum (10^40%) in the diamicts of Unit I as the sands of Unit III show constant values around 20% (Fig. 6). In the sediments of Unit I and II the frequency of angular outlines correlates with the frequency of medium relief, but anticorrelation is present in the sands of Unit III (Fig. 7). The highest frequency of grains with rounded outlines is present in the sands of Unit III. However, considerable numbers of grains with angular outlines, fractured plates, crescentic gouges and surface or edge abrasion are also present (Fig. 5A^D). The Late Cretaceous (Turonian) carbonaceous mudstones (Unit IV) have preserved high surface relief grains with angular outlines and imbricated blocks (Fig. 5E^H). Carbonaceous mudstones of Unit IV and underlying parallel laminated gray
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200 µm A. Sample 1166A-22R-1, 12-13 cm
200 µm E. Sample 1166A-32R-2, 34-38 cm
300 µm
300 µm B. Sample 1166A-22R-1, 12-13 cm
100 µm F. Sample 1166A-32R-2, 34-38 cm
C. Sample 1166A-22R-1, 12-13 cm
200 µm G. Sample 1166A-34R-1, 34-38 cm
107
200 µm D. Sample 1166A-26R-1, 52-54 cm
100 µm H. Sample 1166A-34R-1, 34-38 cm
Fig. 5. Selected SEM micrographs of grains from Site 1166 (Unit III^IV) showing typical morphological and mechanical textures. (A) Angular outline, fractured plates with straight steps (1) and subparallel linear fractures (2). (B) Rounded outline and surface abrasion with some V-shaped cracks (1). (C) Rounded outline with later imbricated blocks (1). (D) Fractured plates, subparallel linear fractures (1) and edge abrasion (2). (E) Low relief with rounded outlines. (F) Angular outline and high relief, typically preserved in clay-rich intervals. (G) Medium relief with chemically weathered surfaces. (H) Chemically weathered surfaces with adhering particles.
claystone of Unit V show chemically altered grain surfaces. Few V-shaped percussion cracks are present throughout the hole, except for Unit V where they are present on 20% of the grain surfaces. In general, straight and curved grooves correlate well with surface abrasion. When surface abrasion is present at higher frequencies, then frequencies of straight or arcuate steps are lower and vice versa (Fig. 6).
5. Discussion Analysis of quartz grain surface textures is a powerful tool when distinguishing sand grains originating from a glacial depositional environment from those a¡ected by subaqueous sedimentary processes. Glacial crushing and abrasion produces a suite of surface textures of which crescentic gouges, arcuate steps, conchoidal fractures, subparallel linear fractures, angularity and high relief are particularly diagnostic (Margolis and Kennett, 1971; Williams and Morgan, 1993;
Mahaney and Kalm, 1995; Mahaney et al., 1996). Grain to grain collisions in subaqueous environments produce V-shaped and irregular impact pits, large breakages, as well as a variety of chemical textures, such as solution pits and channels, etch patterns and crystalline overgrowths (Manker and Ponder, 1978; Georgiev and Sto¡ers, 1980; Linde and Mycielska-Dowgallo, 1980; Manickam and Barbaroux, 1987; Williams and Thomas, 1989; Passchier et al., 1997). The sediments in Unit I are predominantly diamicts and the surface textures of the sand grains indicate a history of crushing at the base of a glacier (Fig. 4A^D). Stratigraphic changes in the abundance of angular vs. rounded grains (Fig. 7) are caused by £uctuations of the ice margin. In proglacial glaciomarine environments current reworking causes edge abrasion resulting in a lower number of angular grains. In ice-contact environments angular outline and glacial textures are preserved. The structureless, gravel-rich diamicts in Subunits IB and ID are the result of subglacial deposition by grounded ice or ice-proximal sedi-
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
1 Angular outline 2 Rounded outline 3 Low relief 4 Medium relief 5 High relief 6 Large conchoidal fractures 7 Small conchoidal fractures 8 Arcuate steps 9 Straight steps 10 Crescentic gougles 11 Imbricated blocks 12 Large breakage blocks 13 Fractured plates Sub-parallel linear fractures 14 15 Curved grooves 16 Straight grooves V-shaped percussion cracks 17 18 Meandering ridges 19 Edge abrasion 20 Surface abrasion 21 Deep troughs 22 Prew eathered surfaces 23 Weathered surfaces Small precipitation features 24 Large precipitation features 25 26 Adhering particles 27 New growth 0
50 100 Frequency (%) 001R
005R
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 0
50 100 Frequency (%) 013R
017R I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 0 50 100 Frequency (%) 022R
026R
0
50 100 Frequency (%) 032R
037R
Fig. 6. Frequency of surface textures from selected samples of Site 1166.
mentation. Diamicts between 94 and 110 m bsf in Subunit IB are strati¢ed and contain a relatively low number of angular grains. The sediments probably originated in a glaciomarine depositional environment with current reworking, perhaps underneath an ice-shelf. The sandy silt with shell fragments and marine microfossils (Subunit IC) is a distal glaciomarine unit. The sand grains in Subunit IC are ice-rafted debris and 80% of the grains are angular. These sand grains escaped marine reworking since they melted out of icebergs directly released by glaciers. Unit II with an abrupt unconformity as an upper contact is a strati¢ed marine sequence that records ice rafting of pebbles. The sand grain surface textures also suggest that these sediments were deposited during glacial conditions. Although a shift in the stratigraphic distributions of angular grains is visible at the Paleogene^Neo-
gene unconformity (Fig. 7), more than 50% of the grains in the samples from Cores 17R and 18R are angular and almost 20% has a high relief. The high abundance of grains with glacial textures suggests that the depositional environment was proximal to a glacial source. At Site 742 of Leg 119, 40 km eastward, similar or slightly youngerage sediments were interpreted as proximal glacially-in£uenced proglacial or subglacial deposits (Barron and Larsen, 1991), which is in agreement with the interpretation of Unit II presented here. The coarse-grained sands in Unit III are interpreted as a record of deposition on an alluvial plain or delta, and the deformed beds record some reworking of material from underlying organic-rich horizons. The presence of rounded grains with a plethora of V-shaped percussion cracks is compatible with an alluvial origin for the sediments. However, the relative abundance
PALAEO 3119 21-8-03
K. Strand et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 101^111 Lithostratigraphic Unit 0
Ia
50
Ib
100 Ic Id II
Depth (mbsf)
150
200
III
250
IV
300
V
350 0
20
40
60
80
100
Angular outline (%) Medium relief (%) High relief (%)
Fig. 7. Relief vs. angular outlines plot of frequency of morphological surface textures for correlation between samples of Site 1166.
of angular grains (30^50%) and the presence of grains with features that only develop in a subglacial environment (Fig. 5A) suggest that the sedimentary system is also sourced by a glacier. No sequence comparable to the homogeneous coarse sands was recovered at Site 742 suggesting that the deposit is a local phenomenon. The coarse sands may record a proglacial outwash system or a braided delta in a more ice-distal setting, perhaps with glaciers situated on land. Sand grains in Unit IV display large breakage blocks indicating grain to grain impact, as well as dulled surfaces and euhedral (angular) outlines indicating dissolution and recrystallization (Fig. 5E,F). These textures are compatible with deposition in a restricted marine or lagoonal environ-
109
ment (cf. Passchier et al., 1997; O’Brien et al., 2001). None of the sand grain surfaces shows indications of glacial activity. A similar-looking sequence of carbonaceous material was drilled in the bottom 2 m at Site 742 and was interpreted as £uvial or possibly lacustrine based on lack of marine microfossils (Barron and Larsen, 1991). The claystone of Unit V has grain surface textures indicating subaqueous transport and chemical alteration. Unit V correlates lithologically and in seismic stratigraphy with Site 741 of Leg 119, 110 km away, from which Lower Cretaceous gray claystone was recovered (Barron and Larsen, 1991). The lack of glacial textures suggests that the sediments of Units IV and V were deposited prior to the initiation of Antarctic glaciation. Site 1166 provides a record of cooling and ice sheet development in Prydz Bay from the Middle^ Late Eocene. The record from the Cape Roberts drilling Project (CRP) in the Ross Sea, on the Paci¢c side of the Antarctic margin, produced a record of progressive cooling to full-glacial conditions between 34 and 17 Ma. CRP results indicate that progressive cooling was already on its way by 34 Ma when temperate glaciers fed from an inland ice sheet released icebergs into the Ross Sea (Davey et al., 2001). The record at Site 1166 is in agreement with these results and, in addition, it provides important information about the initial stages of Antarctic glaciation during the Eocene/ Oligocene transition. The glacial signature of sand grains in the £uvio^deltaic sands of Unit III indicates that glaciers were present in the Prydz Bay area in Middle^Late Eocene time. The glaciomarine sequence of Unit II signals the arrival of glaciers at sea level and the development of the Lambert Glacier^Amery Ice Shelf drainage system draining the East Antarctic ice sheet 37^33 Ma. Major ice sheet expansion in Prydz Bay is recorded in Early Oligocene to Late Miocene sigmoidal sequences and topset beds, which prograded the Prydz Bay continental shelf about 30^40 km seaward (Cooper et al., 2001).
6. Summary and concluding remarks The present study of changes in sand grain mi-
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K. Strand et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 101^111
crotextures records glacial development since the Middle^Late Eocene. The distinguished microtextures identi¢ed as typical of glacial deposits include subparallel linear fractures, conchoidal fractures, arcuate steps, grain angularity, straight 1. Braided alluvium (Unit III) and preceding lagoonal setting (Unit IV) during late Cretaceous (Turonian)
2. Middle to late Eocene braided alluvium and ice-sheet appearance (Unit III)
3. Lowstand and late Eocene to early Oligocene erosion and ice-sheet propagation (upper part of Unit III)
4. Transgressions and ice-sheet development (Unit II) during late Oligocene
5. Plio-Pleistocene ice-sheet development (Unit I)
grooves, and crescentic gouges. Upper Cretaceous ¢nely laminated claystone (Unit V) and carbonaceous siltstone and mudstone (Unit IV) lack glacial microtextures. These sediments predate the initiation of Antarctic glaciation (Fig. 8). Part of the sand grains of Middle^Upper Eocene £uvio^ deltaic sands (Unit III) show evidence of a distal glacial source. The Upper Eocene/Lower Oligocene diatomaceous claystones with interbedded sand and few lonestones (Unit II) were also deposited during early glacial conditions, and signal the arrival of glaciers at sea level in Prydz Bay. The Neogene sediments of Unit I show evidence of glaciation at sea level throughout. The results of this study show that glaciation in the Prydz Bay basin started as early as the Middle^Late Eocene and that glaciers draining an inland ice sheet reached sea level by 37^33 Ma. These results are in agreement with the presence of outlet glaciers in the Ross Sea by 34 Ma (Davey et al., 2001) and the worldwide decrease in N18 O 33.7 Ma (Zachos et al., 2001).
Acknowledgements We would like to thank the Ocean Drilling Program for providing samples for this study, and especially the colleagues from the Leg 188 shipboard sedimentologists group, and co-chief scientists Phil O’Brien and Alan Cooper for helpful discussions regarding the sediment interpretations. We thank the sta¡ of the Institute of Electron Optics, especially Raija Peura, for help with the SEM/EDS and FESEM work. Helpful com-
Fig. 8. Summary diagrams for the depositional setting of the Prydz Bay region. (1) Preglacial setting with typically rounded and surface weathered sand grains and no crushing. (2) Initial glaciers and sand grains show some fractured plates and fractures after crushing because of ice behavior. (3) Ice sheet propagation and sand grains showing distinctively fractured surfaces after glacial crushing. (4) Development of ice sheet with marine transgressions with sandy portions recording angular grains and fractures after glacial crushing. (5) Full glaciation with episodes when ice sheets on the over-deepened continental shelf and sand grains in diamicts show range angular grains and fractures after glacial crushing. (From: O’Brien et al., 2001).
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ments and improvements by the reviewers M.A. Holmes and W.C. Mahaney and by the associate editor Phil E. O’Brian are appreciated. This study was supported by the Academy of Finland (Project No. 49039) and the University of Oulu/Thule Institute providing a grant for global change studies.
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