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Environmental impact of diamond mining on continental shelf sediments off southern Namibia
Quaternary International 92 (2002) 101–112
Environmental impact of diamond mining on continental shelf sediments off southern Namibia J. Rogersa,*, X.C. Lib b
a Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7701 South Africa Hong Kong Geological Survey, Geotechnical Engineering Office, 11/F Civil Engineering Building, 101, Princess Margaret Road, Homantin, Kowloon, Hong Kong SAR, China
Abstract After an intensive phase of prospecting using vibracores, the inner-middle shelf west-north-west of the wave-dominated Orange Delta off southern Namibia is currently being mined for marine diamonds. The diamonds were deposited by surf-zone processes in gravels during Pleistocene low sea-level stands down to depths of about 130 m. As sea level rose to its present level during the postglacial transgression, between about 18 000 and 6 000 yr BP, water depth steadily increased in the study area and drowned the gravels. Initially, quartzose fine to very fine sand of the delta-front environment was deposited on top of the gravels. Finally, prodeltaic silts and clays of the seaward prograding-feather edge of the Holocene Orange Delta were deposited on the delta-front sands. However, the actively burrowing infauna of the region was able to bioturbate the prodeltaic silt and clay into the delta-front sand. As a result, representative vibracores reveal a typical fining-upward transgressive sequence with the upper 30 cm mainly consisting of muddy sand or sandy mud. Grain-size analysis of vibracored and grab-sampled unmined surface sediment and mined surface sediment has shown that unimodal size-distributions characterise the sand-fractions of unmined surface sediment and that polymodal size-distributions characterise those of the mined surface sediment. Conservation corridors are proposed both to preserve the original transgressive sequence and to provide refuges for the benthos to recolonise the substrate after mining. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction The aim of this study was to determine the granulometric characteristics of shelf sediments seaward of the modern Orange Delta, both before mining of marine diamonds from Pleistocene lowstand gravels and after mining. The study complements an intensive marinebiological investigation. The study area lies west-northwest of the Orange Delta (Fig. 1) near the 130 m isobath. This represents the approximate position of the coastline during the Last Glacial Maximum (Rogers, 1977; Bremner et al., 1986, 1990; Rogers and Bremner, 1991). It is therefore not surprising that drowned coastline deposits of terrigenous gravel and coarse sand, deposited in high-energy surf-zone and foreshore environments, are found in these depths. Because the extensive catchment of the Orange River includes diamondiferous Cretaceous
*Corresponding author. E-mail address: [email protected] (J. Rogers).
kimberlites, it is expected that diamonds are found in these drowned gravels and sands. Walther’s Law of Facies (Walther, 1894 in Middleton, 1973) states that sediments from identifiable depositional environments, i.e. sedimentary facies, that lie beside one another in nature, can, by the lateral shifting through geological time of these depositional environments, end up being deposited on top of one another in a vertical stratigraphic sequence. The relevant depositional environments in the present study are the highenergy foreshore and surf-zone environments. They are succeeded seawards by the lower-energy delta-front environment, above wave base, and the very low-energy prodelta environment below wave base, normally at about 40 m (Bremner et al., 1990; Rogers and Bremner, 1991). During any lowstand, the four environments discussed above lay either along the modern 130-m isobath (foreshore and surf-zone) or further seaward between the modern 130- and 170-m isobaths (delta front) or beyond the modern 170-m isobath (prodelta). With the postglacial rise in sea level between 18 000 and 6 000 yr
1040-6182/02/$ - see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 1 ) 0 0 1 1 8 - 5
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Fig. 1. Map of the continental shelf of southern Namibia showing the location of the northern and southern areas and the mouth of the Orange River. The diamond-mining towns of Oranjemund in Namibia and Alexander Bay in South Africa are also shown. De Beers Marine (Pty) Ltd. is mining marine diamonds from Pleistocene lowstand-beach gravels on the middle shelf, immediately seaward of the wave-dominated Orange Delta, shown by the seaward bulge of the 50- and 100-m isobaths.
BP, the study area would have been drowned by increasing depths of water. The mean energy levels in the study area, therefore, would have steadily declined. In addition, due to the landward shift of the depositional environments, the high-energy foreshore and surf-zone gravel and sand facies would have been overlain by the lower-energy delta-front sand facies and, finally, by the very low-energy prodeltaic mud facies. This produced a characteristic fining-upward transgressive sequence. Once sea level stabilised at its Holocene highstand position, at about 6 000 yr BP, another factor came into play. Instead of the depositional environments moving landwards, they now started moving seawards as the Orange Delta constantly received sediment from its extensive catchment, draining westwards across most of South Africa, and began to prograde seawards. At present, this means that the mud of the Holocene lowenergy prodeltaic facies is propagating seawards over the study area and, in time, the delta-front sand facies may be deposited over the prodeltaic mud facies to form a characteristic coarsening-upward prograding-delta sequence above the fining-upward transgressive sequence. The study area is thus expected to show evidence mainly of Late Pleistocene to Early Holocene transgression and also of Late Holocene progradation. Recent detailed studies of the seafloor between the 110- and 130-m isobaths by marine geologists from De Beers (McMillan, personal communication, 1999), particularly, direct observation via the JAGO research submersible, have shown that either oscillatory swelldriven currents or unidirectional wind-driven currents are capable of reworking the upper metre of the
sediments. A nepheloid layer of suspended silt and clay is also frequently present, like that studied by Zoutendyk and Duvenhage (1989) on the inner Agulhas Bank, east of Cape Agulhas. An additional benefit of the direct observations from the submersible was that isolated pillars of unmined sediment were found within the mined-out areas.
2. Sampling strategy and laboratory procedures Vibracores were obtained from the study area to study the unmined Late Pleistocene and Holocene stratigraphy. Within the broader study area, six sampling areas were identified, areas 1–4 in the northern area (Figs. 1 and 2) and areas 5 and 6 in the southern area (Figs. 1 and 3). Vibracores were made available by De Beers Marine (Pty) Ltd. from areas 2 to 5. Subsequently, two sampling cruises were made to areas 1–6 in June 1994 and in February 1995. An attempt was made to recover 10 samples from each area in June 1994 and 6 samples from each area in February 1995. The relatively undisturbed surface of the grab sample was accessed inside the closed grab via an opening covered with a hinged lid. Short cores o50 cm were taken and then capped in June 1994 and scoop samples were taken in February 1995 and stored in 600-ml polypropylene jars. Marine sediment subsamples were freed of salt by the process of osmosis across a membrane of cellophane. Each subsample was suspended overnight within the dialysis tubing in a large bucket of slowly overflowing tap water. The silt and clay fraction were then separated
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Fig. 2. Map of the northern area, within which are four areas, 1–4 that were intensively grab-sampled in June 1994 (triangles) and February 1995 (circles). Pre-mining, diamond-prospecting vibracores were recovered in area 2 (VC495), area 3 (VC471) and area 4 (VC454). Mined-out areas are shown using diagonal shading.
Fig. 3. Map of the southern area, within which are two areas (5 and 6) that were intensively grab-sampled in June 1994 (triangles) and February 1995 (circles). A pre-mining, diamond-prospecting vibracore VC125 was recovered in area 5. Mined-out areas are shown using diagonal shading. Note the difference in scale between Figs. 2 and 3.
from the sand and gravel-fraction by wet-sieving the subsample through a 63-mm sieve. The mass of mud was then determined by the Andreasen-pipette method in which an aliquot is extracted with a calibrated 25-ml pipette from a 1-l stirred undispersed suspension in a graduated cylinder and transferred to a pre-weighed glass beaker. After drying at 1051C, the mass difference is multiplied by a ‘‘pipetting factor’’ to obtain the mass of mud. Each coarse fraction was dried, dry-sieved through a 2-mm sieve to separate the sand and gravel-fractions, and then weighed to obtain the mass of sand and gravel. The proportions of gravel, sand and mud were then calculated and a texture assigned using the method of Folk (1954), e.g. sandy mud or muddy sand. A binocular microscope was used both to identify the components of
the gravel- and sand-fractions and as a means of controlling the quality of the laboratory procedures. A sample splitter was then used to split each sandfraction to obtain a statistically random split of about 2–3 g for settling in a settling column. The accumulating mass of sand was weighed at 3-s intervals on a perspex disc suspended from an electronic balance. The data are then processed via an online computer. Although three curvesFfrequency, arithmetic cumulative and probability cumulativeFwere produced for each split, the frequency curves were more informative than the cumulative curves. A spreadsheet of the data (Tables 1 and 2) was created in order to produce ternary diagrams from the gravel–sand–mud data of each area (Figs. 4–9). Data from the settled sand-fractions, broken down into the
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phi-fractions or Wentworth grades (Wentworth, 1922) including very fine sand (63–125 mm), fine sand (125–250 mm), medium sand (250–500 mm), coarse sand
(500–1000 mm) and very coarse sand (1000–2000 mm) listed in Tables 1 and 2 were used for grain-size histograms by Rogers and Li (1996).
Table 1 Particle-size analysis of shelf sediments in the northern research area Date
Sample
Gravel
Sand
Mud
Texture
vcS
June ’94
1.1 1.2 1.3 1.5 1.6 1.10 1.11
0.17 0.33 0.00 0.87 0.26 0.00 0.01
77.95 94.81 86.47 80.98 85.00 85.57 81.34
21.88 4.85 13.53 17.95 14.74 14.43 17.99
(g)mS (g)(m)S mS (g)mS (g)mS mS (g)mS
0.87 8.79 21.19 2.14 0.88 0.45 1.05
1.61 2.42 3.29 2.22 6.33 3.77 0.89
Feb ’95
1.1 1.5 1.11 1.14 1.15 1.17
5.29 35.70 1.50 0.67 21.05 1.95
87.82 59.72 91.56 91.27 70.89 91.07
6.91 4.58 1.50 8.06 8.06 6.99
g(m)S g(m)S (g)S (g)(m)S g(m)S (g)(m)S
3.91 7.52 0.50 0.72 0.92 0.56
June ’94
2.1 2.2 2.3 2.4 2.5 2.6
0.12 0.00 0.21 0.00 0.00 0.15
69.06 74.42 72.28 65.26 24.37 54.38
30.82 25.58 27.51 34.74 75.63 45.47
(g)mS mS (g)mS mS sM (g)mS
Feb ’95
2.2 2.3 2.4 2.5 2.6 2.7
3.12 0.99 0.31 1.13 0.20 0.14
56.54 69.32 77.32 68.73 65.65 74.62
40.34 29.69 22.37 30.14 34.15 25.24
June ’94
3.1 3.3 3.5 3.6 3.8 3.9 3.10
6.67 10.86 54.42 0.20 1.06 0.44 0.11
44.95 40.17 25.78 30.62 49.17 32.36 43.57
Feb ’95
3.1 3.2 3.3 3.4 3.5 3.7
8.69 28.77 41.34 16.84 39.30 16.59
June ’94
4.1 4.2 4.3 4.4 4.5 4.1 4.2 4.6 4.8 4.9 4.14 4.15 4.16
Feb ’95
cS
mS
fS
vfS
2.87 3.02 5.79 2.92 9.12 4.96 3.06
39.51 48.64 38.73 41.37 45.99 51.95 51.03
33.10 31.94 17.48 32.34 22.69 24.46 25.31
16.42 11.66 11.13 3.19 7.34 8.83
7.03 3.21 15.03 8.15 4.80 14.25
32.85 21.94 35.90 42.68 32.44 39.75
27.61 15.39 29.00 36.53 25.39 27.86
6.43 5.54 4.04 10.88 0.24 4.16
1.50 1.41 1.33 1.38 0.10 1.07
3.32 3.36 2.96 6.70 0.74 1.95
33.11 38.71 31.67 36.45 9.93 23.39
24.70 25.41 32.29 9.85 13.37 23.81
(g)mS (g)mS (g)mS (g)mS (g)mS (g)mS
1.11 1.26 0.47 0.53 0.30 1.87
4.07 1.51 2.38 2.39 1.65 6.38
4.10 2.79 6.47 2.87 2.15 10.17
21.05 35.74 44.18 34.89 29.48 32.92
26.21 28.02 23.52 28.05 32.07 23.28
48.38 48.98 19.80 69.18 49.77 67.20 56.32
gmS gmS sG (g)sM (g)sM (g)sM (g)sM
4.47 7.10 6.06 8.70 4.14
7.69 9.13 8.34 0.56 8.63
5.17 8.19 3.50 3.56 8.34
12.91 6.33 4.30 9.28 11.04
14.72 1.62 3.58 8.53 17.01
4.79
3.32
7.53
12.98
14.96
29.85 12.26 17.82 9.93 10.73 6.44
61.46 58.97 40.84 73.03 49.97 76.97
gsM gsM mG gsM mG gsM
6.07 2.11 0.26 0.51 0.47 1.56
16.65 8.34 10.22 3.94 6.92 1.81
6.36 1.29 6.05 2.97 2.70 1.81
0.05 0.31 0.75 1.67 0.40 0.81
0.72 0.21 0.54 0.84 0.24 0.45
0.08 10.20 1.99 1.20 17.56
28.35 36.04 42.05 40.75 28.25
71.57 53.76 55.96 58.05 54.19
(g)sM gsM (g)sM (g)sM gsM
0.11
1.19
3.11
12.02
11.92
2.28 1.17 2.61
5.30 3.33 8.66
5.90 4.52 8.18
12.32 13.92 6.45
16.26 17.81 2.35
39.41 17.62 14.86 24.61 38.57 33.88 13.15 28.83
35.84 14.12 28.79 36.16 33.94 25.33 57.92 55.59
24.75 68.26 56.35 39.23 27.49 40.79 28.93 15.58
sG gsM gsM gsM sG mG gmS gmS
2.75 0.26 1.87 6.42 1.57 1.49 8.29 10.65
19.91 1.38 5.42 17.90 6.54 3.14 20.73 36.51
11.19 3.59 7.77 7.92 16.49 6.37 21.99 7.28
0.96 6.33 11.67 2.85 7.58 9.09 4.91 0.53
1.03 2.56 2.06 1.07 1.76 5.24 2.00 0.6
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Table 2 Particle-size analysis of shelf sediments in the southern research area Date
Sample
Gravel
Sand
Mud
Texture
vcS
cS
mS
fS
vfS
June ’94
5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9
0.10 0.19 0.33 1.31 0.00 0.00 1.07 0.00 0.07
72.55 64.14 67.36 70.18 45.97 55.28 57.28 59.86 40.78
27.35 35.67 32.31 28.51 54.03 44.72 41.65 40.14 59.15
(g)mS (g)mS (g)mS (g)mS sM mS (g)mS mS (g)sM
6.14 0.18 1.83 7.00 0.08 0.22 1.95 0.28 0.09
10.69 2.01 2.72 0.81 0.80 1.70 3.56 2.22 0.50
30.89 16.13 33.86 6.66 10.72 9.50 3.87 7.71 6.60
21.21 28.06 28.96 30.77 27.14 31.62 26.38 28.94 21.35
3.64 17.76 0.00 24.95 7.24 11.94 21.52 20.72 12.24
Feb ’95
5.2 5.3 5.5 5.6 5.9 5.11
0.36 0.89 1.48 0.06 0.11 0.70
59.99 80.40 66.39 60.08 78.55 39.34
39.65 18.71 32.13 39.86 21.34 59.96
(g)mS (g)mS (g)mS (g)mS (g)mS (g)sM
0.46 0.82 0.87 0.27 0.49 0.12
3.19 13.08 9.67 0.99 3.50 2.64
19.30 33.20 24.45 17.95 27.68 7.53
22.68 26.72 22.91 27.56 36.81 18.03
14.36 6.58 8.49 13.31 10.07 11.02
June ’94
6.1 6.5 6.7 6.10
4.31 0.54 0.12 0.00
48.53 47.14 53.35 28.73
47.16 52.32 46.53 71.27
(g)mS (g)sM (g)mS sM
3.03 2.28 0.49
9.65 0.65 0.59
5.41 5.41 3.46
22.68 25.70 23.55
7.76 13.11 25.25
Feb ’95
6.1 6.2 6.3 6.4 6.5 6.6
8.66 0.25 8.38 0.32 7.55 55.26
49.22 48.43 77.13 46.99 25.57 10.76
42.12 51.32 14.49 52.69 66.88 33.98
gmS (g)sM gmS (g)sM gsM mG
1.96 0.27 1.03 0.73 0.99 2.25
12.02 2.05 6.64 2.37 4.25 5.27
21.08 5.24 33.42 7.92 6.73 2.09
12.09 25.55 28.38 25.37 8.63 0.82
2.07 15.32 7.66 10.60 4.97 0.33
3. General state of the local shallow-marine environment The sediments will be described area by area, initially from areas 1 to 4 in the northern area (Figs. 2, 4–7 and Table 1) and from areas 5 and 6 in the southern area (Figs. 3, 8 and 9 and Table 2). The data from each area will be compared internally, i.e. within each set, especially looking for evidence of unmined and mined sediments, and between the sets taken on different cruises. Whereever available, data from the vibracores will also be incorporated, as these show the original unmined stratification. All the sediments are terrigenous, so that the gravel is composed mainly of pebbles and cobbles of rocks from the catchment of the Orange River. The sand-fraction is predominantly quartzose with minor amounts of benthic foraminifera and shell fragments, whereas the mud fraction consists of quartzose silt, clay minerals and organic matter. 3.1. Northern area In the northern area (Figs. 1 and 2), sediments from areas 1, 3 and 4 (Figs. 4, 6 and 7) were studied using the unmined area 2 as a control (Fig. 5). In vibracore VC495 from area 2 (Figs. 2 and 5), the upper 17 cm consisted of
slightly gravelly, muddy sand overlying a muddy sandy gravel. Slightly gravelly muddy sand was found in all the grab samples taken in area 2 in June 1994 and in February 1995 (Fig. 5). All the sand-fractions are dominated by fine and very fine sand (2–4j or 250–63 mm) with a coarse tail of medium to coarse sand (0–2j or 1000–250 mm), the frequency curves being unimodal with a coarse tail (Fig. 5). No vibracore was available from area 1, but it lies close to area 2 from which vibracore VC495 was recovered (Figs. 2 and 4). The June 1994 grab samples of seafloor sediment, unmined at the time, were slightly gravelly muddy sands (Fig. 4), just like the surface sediments in vibracore VC495 in area 2 (Fig. 5). Focussing on the sand-fractions, fine sand and very fine sand again predominate in these unmined sediments, medium to very coarse sand forming a minor coarse tail. The frequency curves are typically unimodal with a coarse tail (Fig. 4). Out of the area 1 grab samples recovered in February 1995, samples 1.1, 1.5, 1.11, 1.14 and 1.17 are from mined areas, whereas sample 1.15 is from an unmined area. However, this does not tally with the textural data (Fig. 4 and Table 1) as sample 1.15 has a substantial gravel content (21%) and, along with sample 1.5, is one
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Fig. 4. Area 1 in the northern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of unmined sediment recovered in June 1994 (sample 1.11) and of mainly mined sediment in February 1995 (sample 1.17).
of the most gravel-rich samples, whereas sample 1.14 has the least gravel (0.7%) and is more likely to consist of unmined sediment. The sand-fractions (Fig. 4), appear to be similar, but the coarse tail forms a minor mode, except in sample 1.14. So the frequency curves also suggest that sample 1.14, rather than sample 1.15 is from the unmined sediment. The top 5 cm of vibracore VC454 (Fig. 6) from area 3 again consists of slightly gravelly, muddy sand, but contains subequal amounts of mud and sand, probably because area 3 is closer to the source of the mud, the Orange Delta (Figs. 1 and 2). A gravelly muddy sand underlies the familiar surficial sediment, but, in this case, is in turn underlain by a mud layer, probably representing the Eocene basement (A. Fourie, personal communication, 1999) at the base of vibracore below a depth of only 10 cm. In other words, a source of mud lies immediately below the orebody (the potentially diamondferous gravelly sediment). The grab samples taken in June 1994 in area 3 were all from unmined sediment and not from mined sediment as planned. However, the data in Table 1 show that,
although four samples (3.6, 3.8, 3.9 and 3.10) have the texture of unmined sediment, three (3.1, 3.3 and 3.5) have substantial gravel contents and are more typical of mined sediment. Sample 3.5, for example, contains over 54% gravel. The sand-fractions for these samples (Fig. 6) are typical of mined sediment in that a wide rage of Wentworth grades are present, commonly in subequal amounts or with major modes of coarse sand, so the surveyed positions indicating samples from unmined sediment do not tally with the sedimentological data. In contrast, all but one sample (3.5) of the February 1995 grab samples were from mined sediments. However, like all the other samples of that cruise, sample 3.5 contains 39% gravel (Table 1), the second highest amount in the set. This assessment also holds for all the sand-fractions from the February 1995 cruise, all having major modes of coarse sand (Fig. 6). Area 4 is close to area 3 (Fig 2), but the top 6 cm of vibracore VC471 is a slightly gravelly sandy mud (Fig. 7), rather than the slightly gravelly muddy sand encountered in areas 1–3. It is underlain
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Fig. 5. Area 2 in the northern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of unmined sediment recovered both in June 1994 (sample 2.3) and in February 1995 (sample 2.4). Gravel–sand–mud bar graphs illustrate the texture of three subsamples of the 23-cm long pre-mining vibracore VC495.
by 24 cm of gravelly muddy sand followed by at least 20 cm of mud or sandy mud. Of the June 1994 grab samples, four samples (4.1, 4.3, 4.4 and 4.5) were from unmined sediment and only sample 4.2 was from mined sediment. The textural results (Fig. 7 and Table 1) tally with this, except for 4.5, which had a high gravel content (17.6%), which suggests mined sediment. Upon studying the sand-fractions (Fig. 7), samples 4.1 and, especially, 4.4 resemble unmined sediment, whereas 4.3 and 4.5 are biomodal and resemble the mined sediment.
Turning to the February 1995 grab samples, the textural data (Fig. 7) suggest that all are from the mined sediment. The mined status of the February 1995 samples is confirmed by the poor sorting and coarse modes in the sand-fractions (Fig. 7). 3.2. Southern area In the southern area (Figs. 1 and 3), vibracore VC125 (Fig. 8) in area 5 has a mere 5 cm of slightly gravelly muddy sand at the surface, underlain by 18 cm of similar
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Fig. 6. Area 3 in the northern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of mainly mined sediment recovered both in June 1994 (sample 3.3) and in February 1995 (sample 3.4). Gravel–sand–mud bar graphs illustrate the texture of four subsamples of the 25-cm-long pre-mining vibracore VC454.
sediment with a higher gravel content, underlain, in turn, by at least 5 cm of mud (perhaps representing the Eocene basement), a pattern now becoming familiar. Several of the grab samples taken from area 5 in June 1994 were from unmined sediment, with the exception of sample 5.3 and possibly of sample 5.2 (Table 2). The gravel–sand–mud data (Fig. 8 and Table 2) seem to support an unmined status for all the samples, including samples 5.2 and 5.3. The sand-fractions (Fig. 8) are equivocal, in that only samples 5.3 and 5.4 are unimodal, being the normal indicators of unmined
sediment. The February 1995 grab samples were all from unmined sediments and this contention is supported by the gravel–sand–mud data (Fig. 8 and Table 2). However, sample 6.2 is unimodal and much of the sand is coarse. In area 6 (Figs. 1 and 3), samples 6.1 and 6.5 of the June 1994 grab samples (Fig. 9 and Table 2) are either mined or unmined, whereas sample 6.7 is unmined sediment and sample 6.10 is mined sediment. The gravel–sand–mud data (Fig. 9 and Table 2) suggest that sample 6.1 is mined, with over 4% gravel, whereas the
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Fig. 7. Area 4 in the northern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of both mined and unmined sediment, recovered in June 1994 (sample 4.3) and of mainly mined sediment in February 1995 (sample 4.15). Gravel–sand–mud bar graphs illustrate the texture of five subsamples of the 50-cm long pre-mining vibracore VC471.
other three samples (6.5, 6.7 and 6.10) appear to be unmined sediment. The sand-fractions (Fig. 9) refine the situation further, by confirming sample 6.7 as unmined sediment, while assigning sample 6.1 to mined sediment and sample 6.5, tentatively, to unmined sediment. Samples 6.1, 6.3 and 6.5 of the February 1995 suite are from mined sediment, whereas samples 6.2, 6.4 and 6.6 are from unmined sediment. The gravel–sand–mud data (Fig. 9 and Table 2) corroborate this, with the exception of sample 6.6, which has the highest gravel content (55.3%) and is much more likely to be mined
sediment. The sand-fractions (Fig. 9) confirm that samples 6.1, 6.3 and especially 6.5 are mined sediment, but also suggest that sample 6.4 and especially the very coarse sample 6.6 are also probably mined sediment. Only sample 6.2 resembles unmined sediment.
4. Impact of mining on seafloor sediments It is difficult to compare the impact of natural marine processes on seafloor sediments to the impact caused by
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Fig. 8. Area 5 in the southern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of mined and unmined sediment recovered in June 1994 (sample 5.8b) and February 1995 (sample 5.11). Gravel–sand–mud bar graphs illustrate the texture of three subsamples of the 32-cm long pre-mining vibracore VC125.
mining activities, because the natural environment varies from fairweather to storm conditions. During storms, it can be expected that resuspension of the top few millimetres of mud will occur naturally and, equally naturally, it will be redeposited when the storm subsides. Similarly, having shown that the full range of particle sizes is present in the naturally stratified sediment, from gravel to clay, it is obvious that the gravel and sand will have much higher settling velocities than the silt and clay of the mud fraction. Therefore, silt and clay are
more likely to be transported beyond any area of active mining before redeposition, whereas gravel and sand will be redeposited within the mining area. However, this study shows, quite clearly, that the natural finingupward sequence left behind in the wake of the postglacial transgression is completely reorganised, so that the natural stratification is irrevocably altered. The sediment has a polymodal texture, probably because the sediment is separated into three fractions, cobbles, pebbles and tailings (sand, silt and clay) on the mining
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Fig. 9. Area 6 in the southern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of mined and unmined sediment recovered in June 1994 (sample 6.1b) and February 1995 (sample 6.5b).
vessel and then discarded over the side via a conveyor belt (cobbles and pebbles) and the tailings pipe. The main impact for the marine benthos is that the normal patchiness of the benthos will be increased by mining activity. The substrate for the benthos becomes texturally more heterogeneous instead of more homogeneous, as shown by the particle-size analysis of vibracores in mined-out area. Episodic trawling operations may also affect the seafloor sediment but, unlike diamond mining, possesses a ploughing effect. The impact on the benthos has been discussed in a report by Field et al. (1996) and in three MSc. project studies of the Department of Zoology at the University of Cape Town (Savage, 1996; Savage et al., 2001; Van der Merwe, 1996; Winckler, 1999). Van der Merwe (1996) showed, using multivariate statistics, that the unmined areas were characterised by o10% gravel and the mined areas by >10% gravel. However, these studies were equivocal on the long-term impact of diamond mining on the benthos, in that natural disturbances, for example the three biologically hostile, anoxic events detected in the bottom water of the study
area during January–March 1996 (Shillington, 1996), also played a significant role in the middle shelf off southern Namibia. Such anoxic events were also discussed in broader terms by Dingle and Nelson (1993). If conservation corridors were preserved, the benthos in the corridors could spread out into the mined area to accelerate the recolonising process. The impact of the reorganisation of the substrate on the benthos will obviously be confined to mining areas, which, over time, may expand to cover greater areas of the diamondiferous continental shelf off both Namibia and South Africa, especially now that exploration licenses for seabed depths of 200–500 m have been awarded and are being actively prospected. In other words, although the immediate impact is local, the nature of mining activity is to expand until all ore bodies have been exploited eventually to exert a more regional effect, in this case, potentially affecting an entire continental shelf. The redeposition of resuspended mud beyond active mining areas is likely to be only of local extent. The reorganisation of the natural stratification on the seabed in the middle shelf as revealed in the vibracores is
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patently irreversible. The fining-upward transgressive sequence formed over ‘long’ periods of time is destroyed immediately, once mining has passed through a particular area and will not be recreated until the next cycle of sea-level change. In contrast, when diamondiferous gravel-lag deposits lie in bedrock depressions on the rocky bottom above wave base, as is the case on the inner shelf at depths of below 40 m off Namaqualand, mining has much less impact than episodic storms in the transportation of gravel by natural processes. The impact of redeposition of resuspended mud through mining can be expected to be of short duration and can be likened to resuspension and redeposition during and after natural storms, i.e. much o1 yr, and probably over weeks if not days.
5. Conclusions This study of seafloor sediments on the inner shelf, west-north-west of the wave-dominated Orange Delta, has shown that slightly gravelly (o10%), unimodal, fine to very fine quartzose muddy sand, with a coarse tail is typical of unmined Holocene sediment on the seabed. In contrast, after diamond-mining operations have been concluded, the surficial sediment in mined-out areas is typically gravelley (>10%), polymodal coarse muddy sand. The marine benthos have to adapt to these changed circumstances, but the sediments are also affected by periodic storms that form a nepheloid layer of suspended silt and clay. In addition, several times per year, anoxic bottom water is advected onto the inner shelf off southern Namibia from the outer shelf off northern Namibia (Dingle and Nelson, 1993).
Acknowledgements De Beers Marine (Pty) Ltd. is thanked for logistical support, both in providing the vessel, ship-time and marine surveyors for position-fixing, as well as giving permission to publish this paper. We specifically wish to thank Mr. Ted Mills for his technical support at sea and our marine-biological colleagues, Professor John Field, Dr. Patti Wickens, Mrs. Heidi Winckler, Ms. Candida Savage and Ms. Karen van der Merwe for their assistance during the shipboard and laboratory phases of this investigation. The officers and crew of the Pentow Salvor, formerly the Rockfish, are thanked for their assistance at sea, Dr. Ian McMillan, Mr. Andre Fourie and Mr. Andre Goosen of De Beers Marine are thanked for their assistance with this manuscript including the draughting of Figs. 1 and 3. In addition, I wish to thank my referees, Prof. Allan Chivas, Dr. Wyss Yim, and Dr. Peter Ramsay, for their constructive comments.
Dr. Giulio Viola and Mrs. Ann Westoby aided the production of Figs. 4–9. References Bremner, J.M., Rogers, J., Birch, G.F., 1986. Surficial sediments of the continental margin of South West Africa/Namibia. Map Geological Survey of Namibia Marine Geoscience Series 16 maps on 4 sheets. Bremner, J.M., Rogers, J., Willis, J.P., 1990. Sedimentological aspects of the 1988 Orange River floods. Transactions of Royal Society of South Africa 47, 247–294. Dingle, R.V., Nelson, G., 1993. Sea bottom temperature, salinity and dissolved oxygen on the continental margin off South-Western Africa. South African Journal of Marine Science 13, 33–49. Field, J.G., Parkins, C.A., Winkler, H., Savage, C., van der Merwe, K., 1996. Specialist study #9. Impact on benthic communities. In: Impacts of Deep Sea Diamond Mining, in the Atlantic 1 Mining Licence Area in Namibia, on the Natural Systems of the Marine Environment. Environmental Evaluation Unit Report No. 11/96/ 158, University of Cape Town. Prepared for De Beers Marine (Pty) Ltd., pp. 241–281. Folk, R.L., 1954. Distinction between grain size and mineral composition in a sedimentary-rock nomenclature. Journal of Geology 62, 345–351. Middleton, G.V., 1973. Johannes Walther’s Law of the Correlation of Facies. Bulletin of Geological Society of America 84, 979–988. Rogers, J., 1977. Sedimentation on the continental margin off the Orange River and the Namib Desert. Ph.D. Thesis, Department of Geological Sciences, University of Cape Town, 212pp. Rogers, J., Bremner, J.M., 1991. The Benguela Ecosystem. Part VII. Marine-geological aspects. Oceanography Marine Biology Review 29, 1–85. Rogers, J., Li, X., 1996. Specialist Study #8: Sediment reorganization. In: Impacts of Deep Sea Diamond Mining, in the Atlantic 1 Mining Licence Area in Namibia, on the Natural Systems of the Marine Environment. Environmental Evaluation Unit Report No. 11/96/ 158, University of Cape Town. Prepared for De Beers Marine (Pty) Ltd., pp. 165–240. Savage, C., 1996. Multivariate analysis of the impact of offshore marine mining on the benthic macrofauna off the west coast of Southern Africa. MSc. Thesis, Department of Zoology, University of Cape Town, 190pp. Savage, C., Field, J.G., Warwick, R.M., 2001. Comparative metaanalysis of the impact of offshore marine mining on macrobenthic communities versus organic pollution studies. Marine Ecology Progress Series 221, 265–275. Shillington, F., 1996. Specialist study #3: measurements of bottom temperature and oxygen. In: Impacts of Deep Sea Diamond Mining, in the Atlantic 1 Mining Licence Area in Namibia, on the Natural Systems of the Marine Environment. Environmental Evaluation Unit Report No. 11/96/158, University of Cape Town. Prepared for De Beers Marine (Pty) Ltd., pp. 103–116. Van der Merwe, K., 1996. Assessing the rate of recover of benthic macrofauna after marine mining off the Namibian coast. MSc. Thesis, Department of Zoology, University of Cape Town, 179pp. Wentworth, C.K., 1922. A scale of grade and class terms for clastic sediments. Journal of Geology 30, 377–392. Winckler, H., 1999. The application of univariate and distributional analyses to assess the impacts of diamond mining on marine macrofauna off the Namibian coast. Unpublished MSc. Thesis, Department of Zoology, University of Cape Town, 225pp. Zoutendyk, P., Duvenhage, I.R., 1989. Composition and biological implications of a nepheloid layer over the inner Agulhas Bank near Mossel Bay, South Africa. Transactions of the Royal Society of South Africa 47, 187–216.
Environmental impact of diamond mining on continental shelf sediments off southern Namibia J. Rogersa,*, X.C. Lib b
a Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7701 South Africa Hong Kong Geological Survey, Geotechnical Engineering Office, 11/F Civil Engineering Building, 101, Princess Margaret Road, Homantin, Kowloon, Hong Kong SAR, China
Abstract After an intensive phase of prospecting using vibracores, the inner-middle shelf west-north-west of the wave-dominated Orange Delta off southern Namibia is currently being mined for marine diamonds. The diamonds were deposited by surf-zone processes in gravels during Pleistocene low sea-level stands down to depths of about 130 m. As sea level rose to its present level during the postglacial transgression, between about 18 000 and 6 000 yr BP, water depth steadily increased in the study area and drowned the gravels. Initially, quartzose fine to very fine sand of the delta-front environment was deposited on top of the gravels. Finally, prodeltaic silts and clays of the seaward prograding-feather edge of the Holocene Orange Delta were deposited on the delta-front sands. However, the actively burrowing infauna of the region was able to bioturbate the prodeltaic silt and clay into the delta-front sand. As a result, representative vibracores reveal a typical fining-upward transgressive sequence with the upper 30 cm mainly consisting of muddy sand or sandy mud. Grain-size analysis of vibracored and grab-sampled unmined surface sediment and mined surface sediment has shown that unimodal size-distributions characterise the sand-fractions of unmined surface sediment and that polymodal size-distributions characterise those of the mined surface sediment. Conservation corridors are proposed both to preserve the original transgressive sequence and to provide refuges for the benthos to recolonise the substrate after mining. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction The aim of this study was to determine the granulometric characteristics of shelf sediments seaward of the modern Orange Delta, both before mining of marine diamonds from Pleistocene lowstand gravels and after mining. The study complements an intensive marinebiological investigation. The study area lies west-northwest of the Orange Delta (Fig. 1) near the 130 m isobath. This represents the approximate position of the coastline during the Last Glacial Maximum (Rogers, 1977; Bremner et al., 1986, 1990; Rogers and Bremner, 1991). It is therefore not surprising that drowned coastline deposits of terrigenous gravel and coarse sand, deposited in high-energy surf-zone and foreshore environments, are found in these depths. Because the extensive catchment of the Orange River includes diamondiferous Cretaceous
*Corresponding author. E-mail address: [email protected] (J. Rogers).
kimberlites, it is expected that diamonds are found in these drowned gravels and sands. Walther’s Law of Facies (Walther, 1894 in Middleton, 1973) states that sediments from identifiable depositional environments, i.e. sedimentary facies, that lie beside one another in nature, can, by the lateral shifting through geological time of these depositional environments, end up being deposited on top of one another in a vertical stratigraphic sequence. The relevant depositional environments in the present study are the highenergy foreshore and surf-zone environments. They are succeeded seawards by the lower-energy delta-front environment, above wave base, and the very low-energy prodelta environment below wave base, normally at about 40 m (Bremner et al., 1990; Rogers and Bremner, 1991). During any lowstand, the four environments discussed above lay either along the modern 130-m isobath (foreshore and surf-zone) or further seaward between the modern 130- and 170-m isobaths (delta front) or beyond the modern 170-m isobath (prodelta). With the postglacial rise in sea level between 18 000 and 6 000 yr
1040-6182/02/$ - see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 1 ) 0 0 1 1 8 - 5
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Fig. 1. Map of the continental shelf of southern Namibia showing the location of the northern and southern areas and the mouth of the Orange River. The diamond-mining towns of Oranjemund in Namibia and Alexander Bay in South Africa are also shown. De Beers Marine (Pty) Ltd. is mining marine diamonds from Pleistocene lowstand-beach gravels on the middle shelf, immediately seaward of the wave-dominated Orange Delta, shown by the seaward bulge of the 50- and 100-m isobaths.
BP, the study area would have been drowned by increasing depths of water. The mean energy levels in the study area, therefore, would have steadily declined. In addition, due to the landward shift of the depositional environments, the high-energy foreshore and surf-zone gravel and sand facies would have been overlain by the lower-energy delta-front sand facies and, finally, by the very low-energy prodeltaic mud facies. This produced a characteristic fining-upward transgressive sequence. Once sea level stabilised at its Holocene highstand position, at about 6 000 yr BP, another factor came into play. Instead of the depositional environments moving landwards, they now started moving seawards as the Orange Delta constantly received sediment from its extensive catchment, draining westwards across most of South Africa, and began to prograde seawards. At present, this means that the mud of the Holocene lowenergy prodeltaic facies is propagating seawards over the study area and, in time, the delta-front sand facies may be deposited over the prodeltaic mud facies to form a characteristic coarsening-upward prograding-delta sequence above the fining-upward transgressive sequence. The study area is thus expected to show evidence mainly of Late Pleistocene to Early Holocene transgression and also of Late Holocene progradation. Recent detailed studies of the seafloor between the 110- and 130-m isobaths by marine geologists from De Beers (McMillan, personal communication, 1999), particularly, direct observation via the JAGO research submersible, have shown that either oscillatory swelldriven currents or unidirectional wind-driven currents are capable of reworking the upper metre of the
sediments. A nepheloid layer of suspended silt and clay is also frequently present, like that studied by Zoutendyk and Duvenhage (1989) on the inner Agulhas Bank, east of Cape Agulhas. An additional benefit of the direct observations from the submersible was that isolated pillars of unmined sediment were found within the mined-out areas.
2. Sampling strategy and laboratory procedures Vibracores were obtained from the study area to study the unmined Late Pleistocene and Holocene stratigraphy. Within the broader study area, six sampling areas were identified, areas 1–4 in the northern area (Figs. 1 and 2) and areas 5 and 6 in the southern area (Figs. 1 and 3). Vibracores were made available by De Beers Marine (Pty) Ltd. from areas 2 to 5. Subsequently, two sampling cruises were made to areas 1–6 in June 1994 and in February 1995. An attempt was made to recover 10 samples from each area in June 1994 and 6 samples from each area in February 1995. The relatively undisturbed surface of the grab sample was accessed inside the closed grab via an opening covered with a hinged lid. Short cores o50 cm were taken and then capped in June 1994 and scoop samples were taken in February 1995 and stored in 600-ml polypropylene jars. Marine sediment subsamples were freed of salt by the process of osmosis across a membrane of cellophane. Each subsample was suspended overnight within the dialysis tubing in a large bucket of slowly overflowing tap water. The silt and clay fraction were then separated
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Fig. 2. Map of the northern area, within which are four areas, 1–4 that were intensively grab-sampled in June 1994 (triangles) and February 1995 (circles). Pre-mining, diamond-prospecting vibracores were recovered in area 2 (VC495), area 3 (VC471) and area 4 (VC454). Mined-out areas are shown using diagonal shading.
Fig. 3. Map of the southern area, within which are two areas (5 and 6) that were intensively grab-sampled in June 1994 (triangles) and February 1995 (circles). A pre-mining, diamond-prospecting vibracore VC125 was recovered in area 5. Mined-out areas are shown using diagonal shading. Note the difference in scale between Figs. 2 and 3.
from the sand and gravel-fraction by wet-sieving the subsample through a 63-mm sieve. The mass of mud was then determined by the Andreasen-pipette method in which an aliquot is extracted with a calibrated 25-ml pipette from a 1-l stirred undispersed suspension in a graduated cylinder and transferred to a pre-weighed glass beaker. After drying at 1051C, the mass difference is multiplied by a ‘‘pipetting factor’’ to obtain the mass of mud. Each coarse fraction was dried, dry-sieved through a 2-mm sieve to separate the sand and gravel-fractions, and then weighed to obtain the mass of sand and gravel. The proportions of gravel, sand and mud were then calculated and a texture assigned using the method of Folk (1954), e.g. sandy mud or muddy sand. A binocular microscope was used both to identify the components of
the gravel- and sand-fractions and as a means of controlling the quality of the laboratory procedures. A sample splitter was then used to split each sandfraction to obtain a statistically random split of about 2–3 g for settling in a settling column. The accumulating mass of sand was weighed at 3-s intervals on a perspex disc suspended from an electronic balance. The data are then processed via an online computer. Although three curvesFfrequency, arithmetic cumulative and probability cumulativeFwere produced for each split, the frequency curves were more informative than the cumulative curves. A spreadsheet of the data (Tables 1 and 2) was created in order to produce ternary diagrams from the gravel–sand–mud data of each area (Figs. 4–9). Data from the settled sand-fractions, broken down into the
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phi-fractions or Wentworth grades (Wentworth, 1922) including very fine sand (63–125 mm), fine sand (125–250 mm), medium sand (250–500 mm), coarse sand
(500–1000 mm) and very coarse sand (1000–2000 mm) listed in Tables 1 and 2 were used for grain-size histograms by Rogers and Li (1996).
Table 1 Particle-size analysis of shelf sediments in the northern research area Date
Sample
Gravel
Sand
Mud
Texture
vcS
June ’94
1.1 1.2 1.3 1.5 1.6 1.10 1.11
0.17 0.33 0.00 0.87 0.26 0.00 0.01
77.95 94.81 86.47 80.98 85.00 85.57 81.34
21.88 4.85 13.53 17.95 14.74 14.43 17.99
(g)mS (g)(m)S mS (g)mS (g)mS mS (g)mS
0.87 8.79 21.19 2.14 0.88 0.45 1.05
1.61 2.42 3.29 2.22 6.33 3.77 0.89
Feb ’95
1.1 1.5 1.11 1.14 1.15 1.17
5.29 35.70 1.50 0.67 21.05 1.95
87.82 59.72 91.56 91.27 70.89 91.07
6.91 4.58 1.50 8.06 8.06 6.99
g(m)S g(m)S (g)S (g)(m)S g(m)S (g)(m)S
3.91 7.52 0.50 0.72 0.92 0.56
June ’94
2.1 2.2 2.3 2.4 2.5 2.6
0.12 0.00 0.21 0.00 0.00 0.15
69.06 74.42 72.28 65.26 24.37 54.38
30.82 25.58 27.51 34.74 75.63 45.47
(g)mS mS (g)mS mS sM (g)mS
Feb ’95
2.2 2.3 2.4 2.5 2.6 2.7
3.12 0.99 0.31 1.13 0.20 0.14
56.54 69.32 77.32 68.73 65.65 74.62
40.34 29.69 22.37 30.14 34.15 25.24
June ’94
3.1 3.3 3.5 3.6 3.8 3.9 3.10
6.67 10.86 54.42 0.20 1.06 0.44 0.11
44.95 40.17 25.78 30.62 49.17 32.36 43.57
Feb ’95
3.1 3.2 3.3 3.4 3.5 3.7
8.69 28.77 41.34 16.84 39.30 16.59
June ’94
4.1 4.2 4.3 4.4 4.5 4.1 4.2 4.6 4.8 4.9 4.14 4.15 4.16
Feb ’95
cS
mS
fS
vfS
2.87 3.02 5.79 2.92 9.12 4.96 3.06
39.51 48.64 38.73 41.37 45.99 51.95 51.03
33.10 31.94 17.48 32.34 22.69 24.46 25.31
16.42 11.66 11.13 3.19 7.34 8.83
7.03 3.21 15.03 8.15 4.80 14.25
32.85 21.94 35.90 42.68 32.44 39.75
27.61 15.39 29.00 36.53 25.39 27.86
6.43 5.54 4.04 10.88 0.24 4.16
1.50 1.41 1.33 1.38 0.10 1.07
3.32 3.36 2.96 6.70 0.74 1.95
33.11 38.71 31.67 36.45 9.93 23.39
24.70 25.41 32.29 9.85 13.37 23.81
(g)mS (g)mS (g)mS (g)mS (g)mS (g)mS
1.11 1.26 0.47 0.53 0.30 1.87
4.07 1.51 2.38 2.39 1.65 6.38
4.10 2.79 6.47 2.87 2.15 10.17
21.05 35.74 44.18 34.89 29.48 32.92
26.21 28.02 23.52 28.05 32.07 23.28
48.38 48.98 19.80 69.18 49.77 67.20 56.32
gmS gmS sG (g)sM (g)sM (g)sM (g)sM
4.47 7.10 6.06 8.70 4.14
7.69 9.13 8.34 0.56 8.63
5.17 8.19 3.50 3.56 8.34
12.91 6.33 4.30 9.28 11.04
14.72 1.62 3.58 8.53 17.01
4.79
3.32
7.53
12.98
14.96
29.85 12.26 17.82 9.93 10.73 6.44
61.46 58.97 40.84 73.03 49.97 76.97
gsM gsM mG gsM mG gsM
6.07 2.11 0.26 0.51 0.47 1.56
16.65 8.34 10.22 3.94 6.92 1.81
6.36 1.29 6.05 2.97 2.70 1.81
0.05 0.31 0.75 1.67 0.40 0.81
0.72 0.21 0.54 0.84 0.24 0.45
0.08 10.20 1.99 1.20 17.56
28.35 36.04 42.05 40.75 28.25
71.57 53.76 55.96 58.05 54.19
(g)sM gsM (g)sM (g)sM gsM
0.11
1.19
3.11
12.02
11.92
2.28 1.17 2.61
5.30 3.33 8.66
5.90 4.52 8.18
12.32 13.92 6.45
16.26 17.81 2.35
39.41 17.62 14.86 24.61 38.57 33.88 13.15 28.83
35.84 14.12 28.79 36.16 33.94 25.33 57.92 55.59
24.75 68.26 56.35 39.23 27.49 40.79 28.93 15.58
sG gsM gsM gsM sG mG gmS gmS
2.75 0.26 1.87 6.42 1.57 1.49 8.29 10.65
19.91 1.38 5.42 17.90 6.54 3.14 20.73 36.51
11.19 3.59 7.77 7.92 16.49 6.37 21.99 7.28
0.96 6.33 11.67 2.85 7.58 9.09 4.91 0.53
1.03 2.56 2.06 1.07 1.76 5.24 2.00 0.6
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Table 2 Particle-size analysis of shelf sediments in the southern research area Date
Sample
Gravel
Sand
Mud
Texture
vcS
cS
mS
fS
vfS
June ’94
5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9
0.10 0.19 0.33 1.31 0.00 0.00 1.07 0.00 0.07
72.55 64.14 67.36 70.18 45.97 55.28 57.28 59.86 40.78
27.35 35.67 32.31 28.51 54.03 44.72 41.65 40.14 59.15
(g)mS (g)mS (g)mS (g)mS sM mS (g)mS mS (g)sM
6.14 0.18 1.83 7.00 0.08 0.22 1.95 0.28 0.09
10.69 2.01 2.72 0.81 0.80 1.70 3.56 2.22 0.50
30.89 16.13 33.86 6.66 10.72 9.50 3.87 7.71 6.60
21.21 28.06 28.96 30.77 27.14 31.62 26.38 28.94 21.35
3.64 17.76 0.00 24.95 7.24 11.94 21.52 20.72 12.24
Feb ’95
5.2 5.3 5.5 5.6 5.9 5.11
0.36 0.89 1.48 0.06 0.11 0.70
59.99 80.40 66.39 60.08 78.55 39.34
39.65 18.71 32.13 39.86 21.34 59.96
(g)mS (g)mS (g)mS (g)mS (g)mS (g)sM
0.46 0.82 0.87 0.27 0.49 0.12
3.19 13.08 9.67 0.99 3.50 2.64
19.30 33.20 24.45 17.95 27.68 7.53
22.68 26.72 22.91 27.56 36.81 18.03
14.36 6.58 8.49 13.31 10.07 11.02
June ’94
6.1 6.5 6.7 6.10
4.31 0.54 0.12 0.00
48.53 47.14 53.35 28.73
47.16 52.32 46.53 71.27
(g)mS (g)sM (g)mS sM
3.03 2.28 0.49
9.65 0.65 0.59
5.41 5.41 3.46
22.68 25.70 23.55
7.76 13.11 25.25
Feb ’95
6.1 6.2 6.3 6.4 6.5 6.6
8.66 0.25 8.38 0.32 7.55 55.26
49.22 48.43 77.13 46.99 25.57 10.76
42.12 51.32 14.49 52.69 66.88 33.98
gmS (g)sM gmS (g)sM gsM mG
1.96 0.27 1.03 0.73 0.99 2.25
12.02 2.05 6.64 2.37 4.25 5.27
21.08 5.24 33.42 7.92 6.73 2.09
12.09 25.55 28.38 25.37 8.63 0.82
2.07 15.32 7.66 10.60 4.97 0.33
3. General state of the local shallow-marine environment The sediments will be described area by area, initially from areas 1 to 4 in the northern area (Figs. 2, 4–7 and Table 1) and from areas 5 and 6 in the southern area (Figs. 3, 8 and 9 and Table 2). The data from each area will be compared internally, i.e. within each set, especially looking for evidence of unmined and mined sediments, and between the sets taken on different cruises. Whereever available, data from the vibracores will also be incorporated, as these show the original unmined stratification. All the sediments are terrigenous, so that the gravel is composed mainly of pebbles and cobbles of rocks from the catchment of the Orange River. The sand-fraction is predominantly quartzose with minor amounts of benthic foraminifera and shell fragments, whereas the mud fraction consists of quartzose silt, clay minerals and organic matter. 3.1. Northern area In the northern area (Figs. 1 and 2), sediments from areas 1, 3 and 4 (Figs. 4, 6 and 7) were studied using the unmined area 2 as a control (Fig. 5). In vibracore VC495 from area 2 (Figs. 2 and 5), the upper 17 cm consisted of
slightly gravelly, muddy sand overlying a muddy sandy gravel. Slightly gravelly muddy sand was found in all the grab samples taken in area 2 in June 1994 and in February 1995 (Fig. 5). All the sand-fractions are dominated by fine and very fine sand (2–4j or 250–63 mm) with a coarse tail of medium to coarse sand (0–2j or 1000–250 mm), the frequency curves being unimodal with a coarse tail (Fig. 5). No vibracore was available from area 1, but it lies close to area 2 from which vibracore VC495 was recovered (Figs. 2 and 4). The June 1994 grab samples of seafloor sediment, unmined at the time, were slightly gravelly muddy sands (Fig. 4), just like the surface sediments in vibracore VC495 in area 2 (Fig. 5). Focussing on the sand-fractions, fine sand and very fine sand again predominate in these unmined sediments, medium to very coarse sand forming a minor coarse tail. The frequency curves are typically unimodal with a coarse tail (Fig. 4). Out of the area 1 grab samples recovered in February 1995, samples 1.1, 1.5, 1.11, 1.14 and 1.17 are from mined areas, whereas sample 1.15 is from an unmined area. However, this does not tally with the textural data (Fig. 4 and Table 1) as sample 1.15 has a substantial gravel content (21%) and, along with sample 1.5, is one
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Fig. 4. Area 1 in the northern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of unmined sediment recovered in June 1994 (sample 1.11) and of mainly mined sediment in February 1995 (sample 1.17).
of the most gravel-rich samples, whereas sample 1.14 has the least gravel (0.7%) and is more likely to consist of unmined sediment. The sand-fractions (Fig. 4), appear to be similar, but the coarse tail forms a minor mode, except in sample 1.14. So the frequency curves also suggest that sample 1.14, rather than sample 1.15 is from the unmined sediment. The top 5 cm of vibracore VC454 (Fig. 6) from area 3 again consists of slightly gravelly, muddy sand, but contains subequal amounts of mud and sand, probably because area 3 is closer to the source of the mud, the Orange Delta (Figs. 1 and 2). A gravelly muddy sand underlies the familiar surficial sediment, but, in this case, is in turn underlain by a mud layer, probably representing the Eocene basement (A. Fourie, personal communication, 1999) at the base of vibracore below a depth of only 10 cm. In other words, a source of mud lies immediately below the orebody (the potentially diamondferous gravelly sediment). The grab samples taken in June 1994 in area 3 were all from unmined sediment and not from mined sediment as planned. However, the data in Table 1 show that,
although four samples (3.6, 3.8, 3.9 and 3.10) have the texture of unmined sediment, three (3.1, 3.3 and 3.5) have substantial gravel contents and are more typical of mined sediment. Sample 3.5, for example, contains over 54% gravel. The sand-fractions for these samples (Fig. 6) are typical of mined sediment in that a wide rage of Wentworth grades are present, commonly in subequal amounts or with major modes of coarse sand, so the surveyed positions indicating samples from unmined sediment do not tally with the sedimentological data. In contrast, all but one sample (3.5) of the February 1995 grab samples were from mined sediments. However, like all the other samples of that cruise, sample 3.5 contains 39% gravel (Table 1), the second highest amount in the set. This assessment also holds for all the sand-fractions from the February 1995 cruise, all having major modes of coarse sand (Fig. 6). Area 4 is close to area 3 (Fig 2), but the top 6 cm of vibracore VC471 is a slightly gravelly sandy mud (Fig. 7), rather than the slightly gravelly muddy sand encountered in areas 1–3. It is underlain
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Fig. 5. Area 2 in the northern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of unmined sediment recovered both in June 1994 (sample 2.3) and in February 1995 (sample 2.4). Gravel–sand–mud bar graphs illustrate the texture of three subsamples of the 23-cm long pre-mining vibracore VC495.
by 24 cm of gravelly muddy sand followed by at least 20 cm of mud or sandy mud. Of the June 1994 grab samples, four samples (4.1, 4.3, 4.4 and 4.5) were from unmined sediment and only sample 4.2 was from mined sediment. The textural results (Fig. 7 and Table 1) tally with this, except for 4.5, which had a high gravel content (17.6%), which suggests mined sediment. Upon studying the sand-fractions (Fig. 7), samples 4.1 and, especially, 4.4 resemble unmined sediment, whereas 4.3 and 4.5 are biomodal and resemble the mined sediment.
Turning to the February 1995 grab samples, the textural data (Fig. 7) suggest that all are from the mined sediment. The mined status of the February 1995 samples is confirmed by the poor sorting and coarse modes in the sand-fractions (Fig. 7). 3.2. Southern area In the southern area (Figs. 1 and 3), vibracore VC125 (Fig. 8) in area 5 has a mere 5 cm of slightly gravelly muddy sand at the surface, underlain by 18 cm of similar
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Fig. 6. Area 3 in the northern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of mainly mined sediment recovered both in June 1994 (sample 3.3) and in February 1995 (sample 3.4). Gravel–sand–mud bar graphs illustrate the texture of four subsamples of the 25-cm-long pre-mining vibracore VC454.
sediment with a higher gravel content, underlain, in turn, by at least 5 cm of mud (perhaps representing the Eocene basement), a pattern now becoming familiar. Several of the grab samples taken from area 5 in June 1994 were from unmined sediment, with the exception of sample 5.3 and possibly of sample 5.2 (Table 2). The gravel–sand–mud data (Fig. 8 and Table 2) seem to support an unmined status for all the samples, including samples 5.2 and 5.3. The sand-fractions (Fig. 8) are equivocal, in that only samples 5.3 and 5.4 are unimodal, being the normal indicators of unmined
sediment. The February 1995 grab samples were all from unmined sediments and this contention is supported by the gravel–sand–mud data (Fig. 8 and Table 2). However, sample 6.2 is unimodal and much of the sand is coarse. In area 6 (Figs. 1 and 3), samples 6.1 and 6.5 of the June 1994 grab samples (Fig. 9 and Table 2) are either mined or unmined, whereas sample 6.7 is unmined sediment and sample 6.10 is mined sediment. The gravel–sand–mud data (Fig. 9 and Table 2) suggest that sample 6.1 is mined, with over 4% gravel, whereas the
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Fig. 7. Area 4 in the northern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of both mined and unmined sediment, recovered in June 1994 (sample 4.3) and of mainly mined sediment in February 1995 (sample 4.15). Gravel–sand–mud bar graphs illustrate the texture of five subsamples of the 50-cm long pre-mining vibracore VC471.
other three samples (6.5, 6.7 and 6.10) appear to be unmined sediment. The sand-fractions (Fig. 9) refine the situation further, by confirming sample 6.7 as unmined sediment, while assigning sample 6.1 to mined sediment and sample 6.5, tentatively, to unmined sediment. Samples 6.1, 6.3 and 6.5 of the February 1995 suite are from mined sediment, whereas samples 6.2, 6.4 and 6.6 are from unmined sediment. The gravel–sand–mud data (Fig. 9 and Table 2) corroborate this, with the exception of sample 6.6, which has the highest gravel content (55.3%) and is much more likely to be mined
sediment. The sand-fractions (Fig. 9) confirm that samples 6.1, 6.3 and especially 6.5 are mined sediment, but also suggest that sample 6.4 and especially the very coarse sample 6.6 are also probably mined sediment. Only sample 6.2 resembles unmined sediment.
4. Impact of mining on seafloor sediments It is difficult to compare the impact of natural marine processes on seafloor sediments to the impact caused by
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Fig. 8. Area 5 in the southern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of mined and unmined sediment recovered in June 1994 (sample 5.8b) and February 1995 (sample 5.11). Gravel–sand–mud bar graphs illustrate the texture of three subsamples of the 32-cm long pre-mining vibracore VC125.
mining activities, because the natural environment varies from fairweather to storm conditions. During storms, it can be expected that resuspension of the top few millimetres of mud will occur naturally and, equally naturally, it will be redeposited when the storm subsides. Similarly, having shown that the full range of particle sizes is present in the naturally stratified sediment, from gravel to clay, it is obvious that the gravel and sand will have much higher settling velocities than the silt and clay of the mud fraction. Therefore, silt and clay are
more likely to be transported beyond any area of active mining before redeposition, whereas gravel and sand will be redeposited within the mining area. However, this study shows, quite clearly, that the natural finingupward sequence left behind in the wake of the postglacial transgression is completely reorganised, so that the natural stratification is irrevocably altered. The sediment has a polymodal texture, probably because the sediment is separated into three fractions, cobbles, pebbles and tailings (sand, silt and clay) on the mining
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Fig. 9. Area 6 in the southern area: gravel–sand–mud ternary diagram and representative particle-size distributions of the sand-fractions of grab samples of mined and unmined sediment recovered in June 1994 (sample 6.1b) and February 1995 (sample 6.5b).
vessel and then discarded over the side via a conveyor belt (cobbles and pebbles) and the tailings pipe. The main impact for the marine benthos is that the normal patchiness of the benthos will be increased by mining activity. The substrate for the benthos becomes texturally more heterogeneous instead of more homogeneous, as shown by the particle-size analysis of vibracores in mined-out area. Episodic trawling operations may also affect the seafloor sediment but, unlike diamond mining, possesses a ploughing effect. The impact on the benthos has been discussed in a report by Field et al. (1996) and in three MSc. project studies of the Department of Zoology at the University of Cape Town (Savage, 1996; Savage et al., 2001; Van der Merwe, 1996; Winckler, 1999). Van der Merwe (1996) showed, using multivariate statistics, that the unmined areas were characterised by o10% gravel and the mined areas by >10% gravel. However, these studies were equivocal on the long-term impact of diamond mining on the benthos, in that natural disturbances, for example the three biologically hostile, anoxic events detected in the bottom water of the study
area during January–March 1996 (Shillington, 1996), also played a significant role in the middle shelf off southern Namibia. Such anoxic events were also discussed in broader terms by Dingle and Nelson (1993). If conservation corridors were preserved, the benthos in the corridors could spread out into the mined area to accelerate the recolonising process. The impact of the reorganisation of the substrate on the benthos will obviously be confined to mining areas, which, over time, may expand to cover greater areas of the diamondiferous continental shelf off both Namibia and South Africa, especially now that exploration licenses for seabed depths of 200–500 m have been awarded and are being actively prospected. In other words, although the immediate impact is local, the nature of mining activity is to expand until all ore bodies have been exploited eventually to exert a more regional effect, in this case, potentially affecting an entire continental shelf. The redeposition of resuspended mud beyond active mining areas is likely to be only of local extent. The reorganisation of the natural stratification on the seabed in the middle shelf as revealed in the vibracores is
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patently irreversible. The fining-upward transgressive sequence formed over ‘long’ periods of time is destroyed immediately, once mining has passed through a particular area and will not be recreated until the next cycle of sea-level change. In contrast, when diamondiferous gravel-lag deposits lie in bedrock depressions on the rocky bottom above wave base, as is the case on the inner shelf at depths of below 40 m off Namaqualand, mining has much less impact than episodic storms in the transportation of gravel by natural processes. The impact of redeposition of resuspended mud through mining can be expected to be of short duration and can be likened to resuspension and redeposition during and after natural storms, i.e. much o1 yr, and probably over weeks if not days.
5. Conclusions This study of seafloor sediments on the inner shelf, west-north-west of the wave-dominated Orange Delta, has shown that slightly gravelly (o10%), unimodal, fine to very fine quartzose muddy sand, with a coarse tail is typical of unmined Holocene sediment on the seabed. In contrast, after diamond-mining operations have been concluded, the surficial sediment in mined-out areas is typically gravelley (>10%), polymodal coarse muddy sand. The marine benthos have to adapt to these changed circumstances, but the sediments are also affected by periodic storms that form a nepheloid layer of suspended silt and clay. In addition, several times per year, anoxic bottom water is advected onto the inner shelf off southern Namibia from the outer shelf off northern Namibia (Dingle and Nelson, 1993).
Acknowledgements De Beers Marine (Pty) Ltd. is thanked for logistical support, both in providing the vessel, ship-time and marine surveyors for position-fixing, as well as giving permission to publish this paper. We specifically wish to thank Mr. Ted Mills for his technical support at sea and our marine-biological colleagues, Professor John Field, Dr. Patti Wickens, Mrs. Heidi Winckler, Ms. Candida Savage and Ms. Karen van der Merwe for their assistance during the shipboard and laboratory phases of this investigation. The officers and crew of the Pentow Salvor, formerly the Rockfish, are thanked for their assistance at sea, Dr. Ian McMillan, Mr. Andre Fourie and Mr. Andre Goosen of De Beers Marine are thanked for their assistance with this manuscript including the draughting of Figs. 1 and 3. In addition, I wish to thank my referees, Prof. Allan Chivas, Dr. Wyss Yim, and Dr. Peter Ramsay, for their constructive comments.
Dr. Giulio Viola and Mrs. Ann Westoby aided the production of Figs. 4–9. References Bremner, J.M., Rogers, J., Birch, G.F., 1986. Surficial sediments of the continental margin of South West Africa/Namibia. Map Geological Survey of Namibia Marine Geoscience Series 16 maps on 4 sheets. Bremner, J.M., Rogers, J., Willis, J.P., 1990. Sedimentological aspects of the 1988 Orange River floods. Transactions of Royal Society of South Africa 47, 247–294. Dingle, R.V., Nelson, G., 1993. Sea bottom temperature, salinity and dissolved oxygen on the continental margin off South-Western Africa. South African Journal of Marine Science 13, 33–49. Field, J.G., Parkins, C.A., Winkler, H., Savage, C., van der Merwe, K., 1996. Specialist study #9. Impact on benthic communities. In: Impacts of Deep Sea Diamond Mining, in the Atlantic 1 Mining Licence Area in Namibia, on the Natural Systems of the Marine Environment. Environmental Evaluation Unit Report No. 11/96/ 158, University of Cape Town. Prepared for De Beers Marine (Pty) Ltd., pp. 241–281. Folk, R.L., 1954. Distinction between grain size and mineral composition in a sedimentary-rock nomenclature. Journal of Geology 62, 345–351. Middleton, G.V., 1973. Johannes Walther’s Law of the Correlation of Facies. Bulletin of Geological Society of America 84, 979–988. Rogers, J., 1977. Sedimentation on the continental margin off the Orange River and the Namib Desert. Ph.D. Thesis, Department of Geological Sciences, University of Cape Town, 212pp. Rogers, J., Bremner, J.M., 1991. The Benguela Ecosystem. Part VII. Marine-geological aspects. Oceanography Marine Biology Review 29, 1–85. Rogers, J., Li, X., 1996. Specialist Study #8: Sediment reorganization. In: Impacts of Deep Sea Diamond Mining, in the Atlantic 1 Mining Licence Area in Namibia, on the Natural Systems of the Marine Environment. Environmental Evaluation Unit Report No. 11/96/ 158, University of Cape Town. Prepared for De Beers Marine (Pty) Ltd., pp. 165–240. Savage, C., 1996. Multivariate analysis of the impact of offshore marine mining on the benthic macrofauna off the west coast of Southern Africa. MSc. Thesis, Department of Zoology, University of Cape Town, 190pp. Savage, C., Field, J.G., Warwick, R.M., 2001. Comparative metaanalysis of the impact of offshore marine mining on macrobenthic communities versus organic pollution studies. Marine Ecology Progress Series 221, 265–275. Shillington, F., 1996. Specialist study #3: measurements of bottom temperature and oxygen. In: Impacts of Deep Sea Diamond Mining, in the Atlantic 1 Mining Licence Area in Namibia, on the Natural Systems of the Marine Environment. Environmental Evaluation Unit Report No. 11/96/158, University of Cape Town. Prepared for De Beers Marine (Pty) Ltd., pp. 103–116. Van der Merwe, K., 1996. Assessing the rate of recover of benthic macrofauna after marine mining off the Namibian coast. MSc. Thesis, Department of Zoology, University of Cape Town, 179pp. Wentworth, C.K., 1922. A scale of grade and class terms for clastic sediments. Journal of Geology 30, 377–392. Winckler, H., 1999. The application of univariate and distributional analyses to assess the impacts of diamond mining on marine macrofauna off the Namibian coast. Unpublished MSc. Thesis, Department of Zoology, University of Cape Town, 225pp. Zoutendyk, P., Duvenhage, I.R., 1989. Composition and biological implications of a nepheloid layer over the inner Agulhas Bank near Mossel Bay, South Africa. Transactions of the Royal Society of South Africa 47, 187–216.