The Pretoria Saltpan: a 200,000 year Southern African lacustrine sequence

Palaeogeography, Palaeoclimatology, Palaeoecology, 101 ( 1993): 317-337

3 [7

Elsevier Science Publishers B.V., Amsterdam

The Pretoria Saltpan: a 200,000 year Southern African lacustrine sequence T.C. Partridge a, S.J. Kerr b, S.E. Metcalfe c, L. Scott a, A.S.

Talma ° and J.C. Vogel e

aTransvaal Museum, P.O. Box 413, Pretoria 0001, South Ajrica bDepartment of Geology, University of the Witwatersrand, P.O. WITS, 2050, South A[rica CSchool of Geography and Earth Resources, The University of Hull, Hull, HU6 7RX, UK dDepartment of Botany and Genetics, University of the Orange Free State, P.O. Box 339, Bloem/bntein 9300, South Africa eEMATEK, CSIR, P.O. Box 395, Pretoria 0001, South Africa (Received and accepted December I l, 1992)

ABSTRACT Partridge, T.C., Kerr, S.J., Metcalfe, S.E., Scott, L., Talma, A.S. and Vogel, J.C., 1993. The Pretoria Saltpan: a 200,000 year Southern African lacustrine sequence. Palaeogeogr., Palaeoclimatol., Palaeoecol., 101: 317-337. The Pretoria Saltpan is a circular crater 1130 m in diameter and is situated some 40 km N of Pretoria (lat. 25 34'30"/1ong. 28°04'59"E). A recent tube sampling and core drilling programme has revealed an infilling consisting of some 90 m of fine lacustrine sediments (chiefly organic muds, underlain below 30 m depth by micrites) which rest upon a further 61 m of coarse clastic debris. Granite bedrock was encountered at - 151 m. Broad sedimentary zones correspond with major phases in the evolution of the crater lake. Superimposed cyclical patterns of accumulation reflect environmental changes on millenial to seasonal timescales. 14C age determinations on algal debris from the upper 20 m of the core indicate a mean rate of sedimentation of about 1 m/2000 yr, suggesting that the lacustrine sequence may span almost 200,000 yr. Over this period major environmental changes are apparent from sedimentological, chemical, mineralogical and isotopic analyses of the core and studies of the pollen spectra and diatom assemblages present within it. This long continental sequence is therefore providing a high-resolution palaeoenvironmental record for southern mid-latitudes over much the same period as is covered by the Vostok ice-core in Antarctica.

Introduction Located some 40 k m N N W of Pretoria in the T r a n s v a a l Province o f South Africa, the Pretoria S a l t p a n was first described by Jeppe (1868). A detailed a c c o u n t of its surface form was given by the British writer, A n t h o n y T r o l l o p e (1878), b u t it was n o t until 1922 that the results of a c o m p r e h e n sive geological study were published by P.A. W a g n e r (1922). W a g n e r presented considerable evidence in s u p p o r t of a cryptovolcanic origin for the crater, a n i n t e r p r e t a t i o n s u p p o r t e d by the s u b s e q u e n t work of F e u c h t w a n g e r (1973) and, to some extent, by F u d a l i et al. (1973). C o n t r a r y views were expressed by R o h l e d e r (1933), L e o n a r d (1946) a n d M i l t o n a n d Naeser (1971), who p o i n t e d to the presence of m a n y features suggestive of 0031-0182/93/$06.00

meteorite impact. It was widely agreed, however, that firm conclusions regarding the origin of the crater would be possible only once core drilling t h r o u g h the crater filling into the u n d e r l y i n g bedrock had been carried out. The first such a t t e m p t was made by the Pratley M a n u f a c t u r i n g a n d E n g i n e e r i n g C o m p a n y in 1973, b u t technical difficulties forced the a b a n d o n m e n t of the hole at a depth of 172 m. The rotary m e t h o d s used in this a t t e m p t failed to provide a sufficiently u n d i s t u r b e d core to permit useful scientific conclusions. However, the presence of a b u n d a n t fossil pollen in some of the samples recovered suggested that the crater infilling might be an i m p o r t a n t source of p a l a e o e n v i r o n m e n t a l i n f o r m a tion. The second a t t e m p t was made late in 1988 a n d early in 1989 u n d e r the direction of T.C.

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

318

Partridge, using a combination of tube-sampling and rotary drilling techniques. The borehole penetrated some 90 m of predominantly fine-grained lacustrine sediments, followed by some 61 m of sand and boulders. Strong artesian flows and the heterogeneous nature of the material hampered the progress of the drill-string through this material, and even greater difficulty was experienced in advancing casing to support the hole. Drilling was temporarily abandoned at - 145 m when the casing sheared. Drilling could be resumed only following stabilization of the base of the hole by grout injection. Fortunately, conditions improved rapidly thereafter and fractured granite bedrock was penetrated from - 151 m onwards. The borehole was finally terminated-in sound rock at a depth of 200 m. Despite the difficult drilling conditions encountered below 90 m, recovery was excellent throughout the drilling project. This is greatly facilitating the programme of sampling and analysis which is currently in progress. We present here a report on the progress achieved to date with our studies.

Geological and hydrological setting The Pretoria Saltpan is an impressive nearcircular feature with a rim-to-rim diameter of some 1130 m; the maximum rim elevation above the crater floor is 119 m whereas its height above the surrounding plain averages no more than about 60 m. The crater is formed almost exclusively in Nebo granite of the Bushveld Complex (age 2.05 Ga); basic intrusives are, however, locally present in the granite exposed in the walls and rim. The latter also includes outliers of Mesozoic Karoo grit beneath fragmental granite breccia. The principal materials present in the vicinity of the crater are shown in Fig. 1. The present crater floor is slightly below the level of the local water table; this has given rise to a shallow central lake some 7500 m 2 in area and no more than 3 m deep. The lake is rich in dissolved carbonates and bicarbonates, chiefly of sodium, which led to its exploitation as a source of soda brines from 1912 until 1956 (Levin, 1991). The present body of water is largely a result of excavations made during this period; prior to this

T.C. P A R T R I D G E ET AL.

the floor of the crater was occupied by a shallow seasonal pool. The limnology of the present lake has been extensively documented by Ashton and Schoeman (1983), who have estimated evaporation from its surface at 175,000 m3/yr, which is balanced by replenishment of about 48,000 m 3 received directly from rainfall, 100,000 m3/yr from runoff from the crater walls (representing 18% of rainfall) and 26,000 m3/yr from artesian flow, chiefly from the 1973 borehole sunk through the crater-fill sediments. The lake water is saturated with respect to trona (NaHCO3.Na/CO3.2HzO) and halite (NaC1), with the latter predominating; crystals of gaylussite (Na2CO3.CaCO3.5H/O) are present in the upper sediments. Ashton and Schoeman have demonstrated the presence of a notable ionic gradient in the lake from around 59,000 mg/1 at surface to nearly 300,000 mg/1 at 2.75 m depth. The pH of the brine varies from 9.2 to 10.4. Both of the boreholes sunk through the lacustrine sediments are artesian and derive their flows from the sandy aquifer between - 9 0 m and -151 m. Total dissolved solids in this groundwater average around 3400 mg/1, with major constituents in similar proportion both to the present lake and to nearby water wells in the Nebo Granite. The composition of the lake is thus consistent with evaporative enrichment of local granitic groundwater, the chemistry of which has been the subject of comprehensive analyses by Bond (1946). The distribution of bedrock outcrops and of unconsolidated materials in the vicinity of the crater are shown in Fig. 1. Figure 2 is a section drawn through the 1988/89 borehole; the subsurface relationships indicated in this section have been extrapolated away from the borehole on the basis of surface distributions and o f a gravimetric profile, coincident with the section line, recorded by Fudali et al. (1973). Attention is drawn to the presence of dykes and sills of trachyte porphyry and lamprophyre in the northeastern quadrant of the crater wall, and of a small vein and isolated boulders of carbonatite. It is these features, the caldera-like structure of the crater, and the belief that its soda brines were of volcanic origin that prompted Wagner (1922), Feuchtwanger (1973) and others to invoke a cryptovolcanic origin for

PRETORIA SALTPAN: 200.000 YEAR SOUTHERN AFRICAN LACUSTRINE SEQUENCE

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Fig. I. Geology and surficial deposits of the Pretoria Saltpan.

the Pretoria Saltpan. However, Milton and Naeser (1971) reported fission track ages for zircon and apatite crystals in the carbonatites of between 0.6 and 1.9 Ga, and biotite from one of the lamprophyre dykes gave a K/Ar age of 1.36 Ga (Partridge et al., 1990). These ages are clearly incompatible with the youthful morphology of the crater and the fact that fragmental granite breccia overlies Mesozoic Karoo grits along sections of the crater rim. It may therefore be concluded that the association of the crater with these old intrusive rocks is fortuitous. Intrusives of similar age are well documented within the surrounding area. Of importance from the point of view of palaeoenvironmental interpretation of the crater sediments is the presence of an unbroken, raised rim around the crater, and its occurrence within a uniform granite lithology (with the exception of the very small occurrences of intrusive rocks and Karoo grit referred to previously). These factors have ensured that, since its formation, the crater has functioned as a closed system, from a surface hydrological viewpoint, and its inner surface may

be regarded as a relatively homogeneous source of clastic sediment. Origin of the crater Since the crater can be shown to be unrelated, genetically, to the minor occurrences of intrusive rocks within it, and since the chemistry of the crater brine is consistent with evaporative concentration of local granitic groundwater, the principal arguments for a volcanic origin fall away. Decisive in its indication of an impact origin has been an abundance of features preserved in the zone of sand and boulders penetrated by the borehole between 90 and 151 m. These include planar deformation features in quartz and feldspar grains, indicative of shock metamorphism at impact pressures up to 30 GPa, diaplectic quartz glass formed by recrystallization of fused target rock, isotropisation of quartz and feldspar, and the presence of melt-breccia fragments; some of the impact glasses contain opaque metal spherules (Partridge et al., 1991; Reimold et al., 1991). In fact, this clastic

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PRETORIA SALTPAN: 200,000 YEAR SOUTHERN AFRICAN LACUSTRINE SEQUENCE

zone can be classified as a suevitic impact breccia. Fission track analysis of the impact glasses is currently in progress in an attempt to approximate the date of the impact. Infra-red stimulated luminescence dating of clastic particles from the lower levels of the lacustrine sediments may also throw light on age of the crater; in the meanwhile, estimates must be based on extrapolation of the mean sedimentation rate determined from 14C dating of the upper 20 m of the core, which suggest a figure of about 200,000 years (vide infra).

Sedimentology of the lacustrine deposits The lacustrine deposits of the Pretoria Saltpan, extending from the floor of the lake to a depth of 89.95 m, can be divided naturally into a number of units. The uppermost 34 m consists of terrigenous muds and evaporites (the latter occurring both as fine grained disseminations and as diagenetic crystals); halite (NaC1) is restricted to this unit, while trona and gaylussite are confined to its uppermost 25 m and 15 m, respectively. Below 30 m to the base of the lacustrine sequence the nonclastic component (i.e. biological debris and chemical precipitates) is dominated by calcium carbonate. Between 34 m and 56 m is a transitional unit of carbonates and muds, below which a welldeveloped limestone-rich sequence is present. Viewed as a whole, the systematic change in the mineralogy of the chemical sediments conforms to a classic evaporitic sequence. Because of high HCO3 + CO3/Ca + Mg mole ratios the lake brine probably ew31ved through rapid precipitation of calcium (and some magnesium) carbonate, after which evaporative concentration of the remaining dissolved minerals took place (Ashton and Schoeman, 1983). Bioturbated sediments have been recognized down to a depth of 65 m. The entire lacustrine sequence (Fig. 3) contains interbeds of gravelly and sandy mass-flow deposits which decrease in frequency upwards. Detailed analysis has revealed a number of facies which define sedimentological cycles; these cycles are most apparent in the limestone-dominated lower part of the sequence (Partridge et al., 1991). Figure 3 illustrates fluctuations within the principal sedimentary components of the lacustrine

321

sequence with depth. Attention is drawn to the virtual absence of organic carbon (chiefly represented by algal debris) from 6.0 to 11.5 m and from 33.5 to 77.0 m. Both of these zones coincide generally with intervals within which pollen is very poorly preserved or absent (vide infra). The upper interval can be attributed logically to a fall in the lake level and oxidation of sediments in the vadose zone during one or more xeric episodes. The depth of the lower zone, which contains diatoms, is too great to permit a similar explanation, but localized (and hitherto unexplained) throughflow of oxygenated water may have been responsible for the destruction of a major part of the organic component; this possibility is favoured by the absence, throughout this interval, of sulphur (which is otherwise present in significant amount in the anaerobic zone below 20 m depth).

Dating of the sediments Although several methods (including infra-red stimulated luminescence and ionium dating) are currently being investigated in an effort to provide a chronology for the deeper levels of the core, the only dates currently available have been provided by radiocarbon analyses performed on several samples from the uppermost 20 m. The organic component consists mainly of dead algae, together with some plant debris, which accumulated together with the non-organic sediment. It should thus provide trustworthy radiocarbon dates. Dates obtained from the insoluble organic material in the sediment show a regular increase in age to 39,900 yr B.P. at 19,7 m depth (Fig. 4). The rates of sedimentation from 0 to 6 m and from 12 to 20 m are similar at about 90cm/1000 yr, while between 6 and 12 m sedimentation was much slower at about 30 cm/1000 yr. This intermediate interval, dating from about 7500 to 29,500 yr B.P., corresponds with the section containing low organic carbon and no pollen, and may reflect more arid conditions with frequent drying of the lake. Extrapolation of the average accumulation rate of 20 m in 40,000 yr would suggest that the lacustrine sequence could have started accumulating about 200,000 years ago. In addition to dating of the organic matter, the

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PRETORI~ SALTPAN: 200,000 YEAR SOUTHERN AFRICAN LACUSTRINE SEQUENCE

AGE 10 I

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sodium carbonate was measured separately (Pta5168) from the remaining sodium/calcium carbonate (Pta-5167). The latter fraction contained slightly less radiocarbon, indicating the presence of some older CaCO 3 in the sediment. The sample at 1605 cm depth was washed until the alkalinity was reduced to pH 7.5. The remaining CaCO3 in the sediment was then analyzed. The apparent age of 16,300 yr (Pta-5521) is somewhat higher than that of the samples from 1236 cm (Pta-5223) and 1970cm (Pta-5176), but is again much younger than that of the organic matter, showing that the carbonate at this depth is a later addition to the deposit. These findings must be taken into account in the interpretation of the sediments: the higher radiocarbon content of the carbonate fractions is due either to the presence of secondary carbonate deposition in the sediment or is caused by extensive isotope exchange between primary carbonates and the aqueous solution since deposition.

Fig. 4. ~'*C dates for the upper part of the Pretoria Saltpan lacustrine sequence, plotted against depth.

14C content of the carbonate fraction in several samples was also measured (Table 1). The carbonate in these (dried) samples includes sodium carbonate that is dissolved in the brine saturating the sediment. In all the cases the apparent age of the carbonate is much younger than the age of the organics, indicating that a substantial portion of this carbonate was introduced into the deposit from younger groundwater at a later stage. In the case of the sample at 612 cm the readily soluble

Chemical variations within the sediments and their pore water

Pore water from selected core samples was distilled off for stable isotope analysis. Soluble salts from the sediment were then extracted by the addition of distilled water to the dried material and decanting of the supernatant after settling. Chemical analysis of these solutions permitted the calculation of concentrations of ions dissolved in the pore water of the sediment. The uppermost 35

TABLE I Isotopic data for 14C samples

Depth (cm)

Organic matter

Carbonate

Anal. no. 613C Pta0

14C (pmC)

245 388 50(1 618

5240 5228 5222 5770

- 17.3 - 16,8 17,0 20,2

75.90_+0.57 58.68_+0.39 57.32 _+0.45 41.19__+0.40

1233 40 160(I 10 1966 74

5225 5559 5180

- 18,2 - 13,7 - 19,0

237 382 492 612

2.13_+0.19 1.79-}-0.17 0.71 +__0.14

Age (yr B.P.) 2330-+50 4420_+50 4600_+ 60 7200__+80 31,000_+70 32,500_+750 39,900_+ 1600

Anal. no. c~13C Pta0

~*C (pmC)

Age lyr B.P.)

Chemical fraction

5219 5167 5168 5223 5521 5126

81.82_+0.46 77.00_+0.60 83.22+0.61 73.25_+0.56 14.11+0.68 31.76_+0.37

216(t--+45 2670_+6(/ 1990_+60 3040+_60 16,300_+390 9800_+90

Ca/NaCO3 Ca/Na2CO3 Na2CO 3 Ca/Na2CO ~ CaCO 3 Ca/Na2CO 3

+9.2 + 10.6 +7.0 +8.3 +127.0 + 11.2

324

T.C. PARTRIDGEET AL.

m contain Na, COa and CI only, in concentrations of 20-200 g/l, comparable to the levels found in the overlying saline lake (Ashton and Schoeman, 1983). At greater depth the salinity decreases dramatically to levels not exceeding a few thousand mg/1 (Fig. 5), and significant concentrations of SO4, Ca and Mg appear, approaching those within the local groundwater. The sediments of the upper saline region are characterised also

C03 o o

CONTENT OF PORE WATER IN g/I lOO I

2oo

,I

by the presence of trona, halite and gaylussite detected by XRD analysis. The reconstructed porewater concentrations suggest that there was sufficient water present to maintain all of the measured Na and C1 in solution. The halite detected in the dried sediment (Fig. 3) was therefore practically all precipitated during the drying of the samples and is not part of the mineral assemblage of the core.

Cl CONTENT OF PORE WATER

IN 0

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Fig. 5. Chemistry of pore water and precipitates within the Pretoria Saltpan lacustrine sequence.

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PRETORIA SALTPAN: 200,000 YEAR SOUTHERN AFRICAN LACUSTR1NF. SEQUENCE

The amount of carbonate in solution was calculated from the alkalinity and values of excess Na (i.e. the amount not associated with the halite). In the upper part of the column this ranged from 1 to 6 equivalents per liter in the pore water. The pH of the pore water in this upper zone was 10 and higher, consistent with a Na2COa solution. Preliminary calculations suggest that only in the depth range 6 8 m would the pore water have been saturated with Na2CO a and could some of this carbonate (4 eq/1 solubility) have been presenl as crystals in the deposit. It is not clear at this stage what the exact precipitation requirements for the other carbonates (trona and gaylussite) are. I1 does appear, however, that a substantial part of the carbonates found in the dried samples may have been dissolved in the pore water and not have been precipitated within the sediment. The high solubility of the carbonate and its subsequent high mobility is consistent with its high ~4C content. The lower salt content of pore water above 6 m in the column may be due to its removal by salt exploitation between 1912 and 1956.

Palaeoclimatic reconstruction from the sediments Fluctuations in the principal constituents of the lacustrine sequence are believed to have occurred chiefly in response to changing climatic moisture regimes. Because of the isolation--in sedimentological t e r m s - - o f the crater, and its apparent operation as a closed system (in respect of surface hydrology) within an effectively uniform bedrock lithology, it is possible to link these fluctuations to changes in precipitation and evaporation. In short, it is likely that the sedimentary record can be interpreted in terms of a local index of aridity. As a generalized working hypothesis we have assumed that under xeric conditions smaller amounts of clastic sediment (in particular of clay and silt produced by chemical decomposition of the granite) would be derived from the crater walls. At the same time the salinity of the lake would increase; in a chloride-dominated lake like the Saltpan such an increase in ionic concentration causes a reduction in the abundance of blue-green algae which are the main contributors to the organic carbon content of the sediments, although

325

aquatic and semi-aquatic plants appear to have been important at times (vide infra). These responses would tend to be reversed during mesic intervals. Models are currently being generated in an effort to derive transfer functions, based on data from sites spread across the east-west moisture gradient which characterizes southern Africa. While these initiatives are in progress it would be premature to attempt a detailed analysis of palaeoclimatic fluctuations from the available sedimentological data; the utility of the results would, in any case, be limited in the absence of a chronological framework for the older part of the succession. The potential value of the Pretoria Saltpan as a source of data on past climates of the southern mid-latitudes is indicated, by way of example, in the clear signal which emerges for the last 40,000 years. Over this interval the sedimentological evidence indicates that between 29,500 and 7500 yr B.P. conditions were more arid than today, with probable frequent drying of the lake which precluded the generation and preservation of organic matter. This contrasts with previous syntheses for southern Africa (e.g. Partridge et al., 1990), which inferred that xeric conditions were restricted to a short interval on either side of and during the Last Glacial Maximum. From the early Holocene the Saltpan data indicate increased available moisture until recent times. It is gratifying to note that, despite the presence of gaps, these trends are, to a considerable extent, supported by the biological evidence presented below.

The palynological record The present vegetation at the Saltpan crater consists of Sourish Mixed Bushveld (Acocks, 1953; Grunow, 1967) and the climate is subtropical and dry sub-humid. The analysis of exploratory samples from the 1973 core (Scott, 1988) showed that changes in palaeoenvironments are reflected in pollen from the site. Preliminary palynological data derived from the new borehole are described here and provide further evidence for such changes. A surface pollen sample (Scott, 1988), and several samples of the 1988/89 borehole core, taken at regular intervals, were processed to extract

326

T.C. PARTRIDGE ET AL.

palynomorphs using standard HC1, KOH and HF treatment and mineral separation with ZnC12solution. Good pollen yields were obtained in the 0 6, 12-34 and 72-88 m intervals. Unfortunately the 6.5-12 and the 34-72 m sections did not preserve pollen except for a single very poor recovery at the 47 m level; this distribution was confirmed by additional sampling and more rigorous preparation methods (Fig. 6). Because of the absence of pollen in these intervals reconstructions of continuous changes in the vegetation at the site will not be possible. Additional information may, however, be gained through studies of the phytoliths contained in the borehole core, which should yield evidence relating to grass cover for sections of the core without pollen. Some of the main pollen groups are expressed as percentages in Fig. 7, which indicates that grass pollen (Poaceae) generally forms the most prominent component of the available spectra. Strong representation of Cyperaceae and aquatic (mainly Typha) pollen at around the 4, 20, 34 and 85 m levels suggests that shallow-water swampy conditions prevailed locally in the crater at those times. Botryococcus algae (Fig. 8) in the core sediments presumably indicate eutrophic water with lower salt concentrations in the crater. The ratios between these values and those of the Cyperaceae

100%

75%

50%

25%

0% o

lO

20

30

40

50

60

~ Poaceae

[ ] Compositae

[]Cheno/am

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I~cyperaceae

[ ] Fynbos

Fig. 7. Summary pollen diagram of the Pretoria Saltpan lacustrine sequence.

and aquatic pollen fluctuate and may be related to subtle changes in water depth in the crater. Relatively deep, fresh water conditions are, however, tentatively associated with a lack of Cyperaceae and other aquatics; under such conditions it can be assumed that no swampy zone would have been present between the lake shore and the surrounding vegetation, and Botryococcus may also have declined in favour of algae adapted to lower trophic levels. Increases in Podocarpus pollen percentages in

Thousands

100

80

~ 6o

!,o ~ 20

20

40 Depth (m)

80

• Podocarpue [ ] other AP

120

"1

70

60

8O

Fig. 6. Pollen concentrations per gram in the upper 90 m of the Pretoria Saltpan borehole core.

:100

327

PRETORIA SALTPAN: 200,000 YEAR SOUTHERN AFRICAN LACUSTRINE SEQUENCE

% Thousands

7G

6C

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3C -~

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o

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Aquatics

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Botryococcus conc./g

Fig. 8. Percentages of Cyperaceae and aquatic pollen (left scale) and concentrations of Bot
the core samples provide evidence for relatively wet conditions, unless such increases can be ascribed to over-representation of this type due to long distance transport (Scott, 1982). Podocarpus pollen peaks are especially evident during certain episodes before 40,000 yr B.P. (Fig. 7). High proportions of other arboreal pollen (AP) which usually comprises various subtropical bushveld taxa, are indicative of relatively warm palaeoclimates, while the presence of temperate fynbos elements may indicate cooler conditions. The contrast between tropical woodland forms and upland temperate forms is also reflected in the first principal component weights of the pollen data set (Fig. 9). The first principal components curve (Fig. 10) is therefore indicative of palaeotemperature trends. These results together with the substantial intervals over which pollen is absent suggest that the coolest phase during the Last Glacial Maximum and the warmest peak of the Last Interglacial are not reflected in this curve. On the basis of the above criteria, the palaeoenvironmental record from the Saltpan is provi-. sionally summarized below, pending the results of the more detailed analyses presently in progress:

ca. 88-81 m: Deposition in relatively deep, fresh water under cool and wet conditions. ca. 79-72 m: Swampy conditions in relatively shallow, moderately fresh water with warmer temperatures. ca. 71-70 m: Moderately cool, wet conditions. ca. 70-34 m: No pollen except at the ca. 47 m level which reflects relatively cool conditions, possibly with dry summers accounting for strongly evaporative conditions and " C h e n o / A m " pollen. The high Podocarpus pollen percentage is somewhat contradictory, but may be interpreted as the result of relative over-representation caused by long distance transport to the site where local pollen production was low. ca. 34 32 m: Warm, wet conditions with local fresh water swamp in crater. ca. 32-23 m: Deposition in deep water under a moderately cool, wet climate. ca. 20 19 m (ca. 40,000 yr B.P.): Swampy environment in crater under intermediate temperature and moisture conditions. ca. 17 12 m (ca. 39,000--30 000 yr B.P.): Cool, moderately dry conditions. ca. 12 6.5 m: No pollen.

328

T.C. PARTRIDGEET AL. ZOUTPAN PC WEIGHTS PC1 va PC2 PC2 0.4 Tribulua •

Typha

Olea

Acanthaceae

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ca. 6.4 m (ca. 8000 yr B.P.): Dry, moderately warm. ca. 6-2 m (ca. 7000-2000 yr B.P.): Relatively warm, wet. Surface pollen sample: Modern environment

with bush encroachment due to agricultural disturbance. The lack of pollen in the two intervals 6-12 and 34-72 m is difficult to explain on palynological grounds, but the distribution of fern spores in the

PRETORIA SALTPAN: 200,000 YEAR SOUTHERN AFRICAN LACUSTRINE SEQUENCE

core sequence may provide some clues. These spores, which are mainly of the smooth, trilete variety, usually referred to as the Pellaea type (Scott, 1982), seem to be relatively prominent in samples with the lowest pollen concentrations, i.e. those immediately adjacent to the intervals without pollen (Fig. 11). Since these spores are generally more robust than most pollen types it is likely that their relative over-representation may be the result of a process of selective preservation in sediments where conditions were conducive to the destruction of the more vulnerable palynomorphs. The palaeoenvironmental significance of the fern spores should therefore be assessed with caution. More importantly, it can be inferred that, when destructive conditions were at their most extreme, these spores would also have disappeared leading to a complete absence of palynomorphs. Destruction of palynomorphs may have occurred in shallow sediments soon after deposition as a result of seasonal drying, but also where subsequent episodes of severe desiccation affected more deeply buried deposits whose pollen had previously been preserved under reducing conditions. This might have been the case during a dry phase extending from well before the Last Glacial Maximum into the early Holocene.

~29

The diatom record

The 90 m of lacustrine sediments recovered from the Pretoria Saltpan are currently being analysed with respect to their diatom content. Freshwater diatoms are known to be sensitive to changes in the physical, chemical and nutrient properties of their environment. The changing species composition of samples taken down core may, therefore, allow the reconstruction of changes within a basin due to factors such as climatic fluctuations, or human impact. The value of diatoms in palaeoenvironmental reconstruction has been clearly established elsewhere in Africa, particularly in relation to the lakes of the Rift Valley (Gasse, 1975; Gasse et al., 1980, Habeyran and Hecky, 1987) and in the regions bordering the Sahara (e.g. Gasse et al., 1987). In spite of extensive work on diatom taxonomy in South Africa, most recently by F.R. Schoeman and R.E.M. Archibald, little has been done to exploit the potential of diatoms as palaeoecological indicators in this part of Africa. The present study represents the first long diatom sequence to come from South Africa. Diatom samples have been prepared at 50 cm intervals from 0 to 20 m and at 1 m intervals from 20 to 83 m in the Saltpan core. The bottom 7 m

'°°/ 60

% 40

2(Y

0~

0

100

2O Ferns

Depth (m) ---0--- Cyperaceae

Fig. I I. Fern spore percentages compared with those of Cyperaceae pollen in the Pretoria Saltpan lacustrine sequence.

330

of the core have yet to be analysed. The sediment is dried at 40°C and 0.5 g weighed out. A mixture of concentrated H / S O 4 and HNO3 is then added to each sample to remove non-siliceous material. The samples are then washed several times in distilled water before slides are prepared. The diatoms are mounted in a high resolution mountant, Naphrax resin. When necessary, the samples may receive additional treatment, such as dilution or the use of ultrasound to disperse clumps. Counting was carried out using a Zeiss photomicroscope at x 1250 magnification, under oil immersion. Where possible, at least 400 valves were counted. The results have been expressed as percentages of each taxa occurring within a sample. The diatom diagram for the top 50 m of the core is shown in Fig. 12. The diatom flora from parts of the present-day Saltpan and its feeder artesian spring has been described by Schoeman and Ashton (1982), although they did not study the open water of the pan. This work has proven to be a valuable point of reference. Other information concerning the distribution and ecology of diatom taxa has been taken primarily from Archibald (1983), Gasse (1980, 1986, 1987), Gasse et al. (1983) and Kilham et al. (1986). The diatom record from the Saltpan is not a continuous one; within the top 50 m there are four major breaks: (1) from the surface to about 650 cm; (2) from 1750 to about 2100 cm; (3) from 2700 to 2900 cm and (4) from 3250 to 3800 cm. In some samples diatoms were too scarce or too poorly preserved to allow a full count, but percentages are shown on the diagram for illustrative purposes using a hatched symbol. As a basis for discussion, four diatom zones have been recognised: zone 1 - 5000-3800 cm; zone I I - - 3 2 5 0 - 2 9 0 0 c m ; zone III--2700-2100 cm and zone I V - - 1 7 5 0 - 6 5 0 cm. In the samples between 50 and 83 m, diatoms are generally present, quite well preserved and often very abundant. The dominant taxa in this portion of the core are Pseudostaurosira brevistriata, Cyclotella meneghiniana, C. stelligera and Aulacoseira granulata var. angustissima. Overall, the assemblages indicate water of moderate alkalinity and conductivity (EC about 300-1200 gS cm -1 ). There are signs of shallowing and

T.C. PARTRIDGE ET AL.

increased alkalinity between about 57 and 54 m. This flora largely continues in to the lower part of diatom zone I in Fig. 12. The most abundant taxa in zone I are Aulacoseira granulata, Pseudostauro-

sira brevistriata, Rhopalodia gibberula, R. gibba, Epithemia argus, Cyclotella meneghiniana, Achnanthes lanceolata var. dubia, Fragilaria atomus (?) and Mastagloia elliptica. The bottom sample in this zone (4960-4962 cm) is distinguished by a high proportion of A. granulata (including its variety angustissima) (53%). According to Kilham et al. (1986), this is "the most widely distributed species of the genus in Africa". It is described by Kilham (1990) as being meroplanktonic, entering the plankton opportunistically when lakes are well mixed. It has a relatively low light requirement, indicates eutrophic conditions and needs high amounts of available silica. Although it does occur in other samples in this zone, the percentages in these are much lower. Moving up through the zone, P. brevistriata becomes dominant (74% at 4768-4771 cm) and in the top sample of the zone (3890-3894 cm) Navicula elkab (ca. 52%). Both P. brevistriata and N. elkab may be littoral or facultatively planktonic, although the latter may only enter the plankton under certain conditions (Hecky and Kilham, 1973). The water chemistries indicated by these two taxa are, however, quite different. The lower part of the zone (below about 4500 cm) seems to record moderately alkaline conditions (pH 7.8-8.5), electrical conductivity (EC) of about 1000 g S c m -1, low salinity (probably < 0.5%o), with water of CO3-HCO3 type (probably Na rich). The water may have been deepest at the time of deposition of the lowermost sample and then shallowed; the lake was well mixed. Aquatic macrophytes or fringing vegetation may be indicated by the presence of a number of epiphytic taxa (Epithemia spp., Rhopalodia spp. ). The sample from 4111 to 4113 cm indicates a change to more alkaline and saline conditions with C. meneghiniana, R. gibberula and N. elkab abundant. Salinity was probably in the range 5-30% (Gasse, 1987). This trend appears to have continued to the top of the zone, where N. elkab dominates. This is a diatom of strongly alkaline water ( > 26 meq l - i ) , pH > 9 and EC >2500 g S c m -1 (Gasse, 1986). It may also indicate that C1 was a relatively

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332 important anion, possibly due to evaporative concentration of the pan's water. In the upper part of the zone mineral debris was quite abundant and some valves appeared thinly silicified. The presence of "blank" valves of N. elkab was recorded. This has been found previously in a core from a hyperalkaline crater lake in central Mexico, where SEM and microprobe examination revealed that the valves were actually completely encrusted with silica (Metcalfe and Hales, in press). It may be that this is a response to the high availability of dissolved silica under highly alkaline conditions (Richardson et al., 1978). The sediments above 3800 cm did not preserve a diatom record. Diatom zone II (3250-2900cm) is dominated by Nitzsehia pusilla (taken here to include some forms of N. perspicua) (Krammer and LangeBertalot, 1988). A diatom recorded as Nitzschia sp. (affin. pusilla) was present and abundant in most of the Saltpan samples analysed by Schoeman and Ashton (1982). N. pusilla may be littoral or faculatively planktonic and it is apparently indifferent in terms of its anionic preference. It is, however, very halotolerant, often occurring in eusaline waters (30-40%) (Gasse, 1987). Valve preservation was generally quite poor in this zone, with intact frustules rare (e.g. of Rhopalodia gibberula). A shallow, alkaline and highly saline environment may be indicated. It should be noted that the occurrence of halite begins above about 34 m. An assemblage similar to that in zone II was described as group IVD of Gasse et al. (1983), present in strongly alkaline (> 50 meq 1-1) closed lakes or hot springs. Following a short break in the record, zone III includes the samples between 2700 and 2711 cm. A wide range of species are present. Amongst the most abundant are R. gibberula, N. elkab, Nitzschia liebetruthii, Achnanthes exiqua and Nitzschia palea. The bottom sample in the zone (2650-2652 cm) is dominated by R. gibberula (65%), often present as frustules. This taxon is usually a littoral epiphyte, but may occur in the plankton in well mixed, shallow lakes. It probably reflects a pH > 8, high salinity (30-40%o according to Gasse, 1987) and alkalinity. The accompanying species (e.g. Mastagloia elliptica, Cymbella pusilla) would support such an interpretation. Coeconeis engelbrechtii, a brack-

T.C. P A R T R I D G E E T AL.

ish water species apparently only found in South Africa (Archibald, 1983), is also quite abundant at this level. A shallow, highly alkaline and saline pond, possibly with vegetation, may be indicated. In the remainder of zone III, N. palea and A. exiqua are most common, being replaced by N. liebetruthii and N. elkab up core. The assemblages appear rather mixed in terms of their apparent pH and salinity preferences. Taxa such as A. exiqua, N. palea and Gomphonema spp. usually prefer low salinity ( < 0.5%0). There are a number of possible explanations for such a mixture: (1) the sampling interval has "captured" a range of different environments, (2) vertical stratification in the water body allowed the development of two different environmental niches (see Gasse, 1988), (3) the assemblages reflect a mix of distinct seasonal growths, with low salinity conditions in the rainy season and high salinity developing in the dry season (Gasse, 1987). Schoeman and Ashton (1982) point out that localised storms, occurring mainly between October and April, today bring 450-700 mm of rain to the catchment. Ashton and Schoeman (1983) describe the impact of this rain on the chemistry of the Saltpan waters, although its effect on the flora has not been studied. Towards the top of the zone, there is some indication that high salinity/alkalinity conditions were more prevalent with taxa such as Navicula halophila and Anomoeoneis sphaerophora f. seulpta becoming more abundant. There is a further break in the diatom sequence between 2100 and 1750 cm, which is overlain by diatom zone IV (1975-650cm). The two most abundant taxa in this zone are N. pusilla and Chaetoceros muelleri (valves and cysts). The presence of C. muelleri is believed to reflect a significant change in the water chemistry of the pan to a situation in which CI was the dominant anion. C. muelleri is a planktonic species of C1 rich, saline water (5-30%0) (Gasse, 1987; Bradbury, 1989; Metcalfe and Hales, in press). Valves of C. muelleri are abundant in the lower half of the zone, but above 12 m, the taxon is represented mainly by cysts, possibly indicating environmental stress. Although highly alkaline and highly saline throughout the deposition these upper sediments, it seems likely that the water shallowed above

P R E T O R I A S A L T P A N : 200,000 Y E A R S O U T H E R N

AFRICAN

333

LACUSTRINE SEQUENCE

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334

T.C. PARTRIDGE ET AL.

about 12 m. Valve preservation deteriorated moving up through this zone and only a partial count was possible from 690 to 692 cm at the top of the zone. Previous work on central Mexican

lakes indicates that a flora such as this may be associated with highly enriched brines, from which Ca has been lost and CO3 replaced by C1 (Metcalfe, 1988; Metcalfe and Hales, in press). It should be

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Fig. 14. Synthetic pH curve for the uppermost 50 m of the Pretoria Saltpan lacustrine sequence (based on the data from Gasse and Tekaia, 1983).

PRETORIA SALTPAN: 200.000 YEAR SOUTHERN AFRICAN LACUSTRINE SEQUENC[-

noted that calcite deposition stops above about 15 m and the precipitation of gaylussite begins. In the top part of the Saltpan core few samples contained any diatoms and no counts were possible. It appears that either alkalinities were such that dissolution of frustules upon death prevented their preservation in the fossil record, or that salinity became high enough to limit the development of diatoms (cf. Hecky and Kilham, 1973 and Gouleau and Noel, 1984). Although much work remains to be done on the Saltpan sequence, some trends may be identified. Further information may be derived from the study of the changing abundance of silica microfossils (diatoms, phytoliths, cysts) through the core (Fig. 13). In this figure the decline in diatom numbers towards the top of zone IV is apparent. Phytolith abundance generally follows diatom abundance and may be compared with percentages of epiphytic taxa. The significance of the changes in numbers of cysts is not clear, especially as things identified as chrysophyte cyst-type forms under the LM may prove to be something else. Using information of apparent ecological preferences published elsewhere, an attempt has been made to construct a synthetic palaeo-pH curve for the Saltpan (Fig. 14). Although lacking the precision of PH curves based on transfer function data from particular lakes and their surrounding areas, it is still an interesting exercise. The main feature of the curve is the lower pH (8-8.6) recorded in zone I relative to the upper parts of the core (pH >8.6). The reconstructed pH for zone III is regarded as highly dubious due to the low percentage of species included. A similar reconstruction is being attempted for salinity. Overall, the diatoms from the top 50 m of the Pretoria Saltpan appear to indicate a eutrophic, alkaline lake, probably at its deepest at the base of the studied sequence, which subsequently became generally shallower and more chemically concentrated, with CI superseding CO3 as the dominant anion. The parts of the core where diatoms are not preserved do not necessarily reflect periods of drier climate, but according to the pollen evidence they may coincide with warm periods. It is possible that the development of C. muelleri valves in the lower part of zone IV reflects

335

the final, temporary, deepening of the lake (presumably prior to the Last Glacial Maximum), before its gradual desiccation over the terminal Pleistocene and Holocene.

Conclusions From the data which have been assembled and the interpretations that have been possible to date there is no doubt that the Pretoria Saltpan core represents one of the most important long sequences from the southern African subcontinent. Its mid-latitude situation, in a zone which is likely to have been subject to high-amplitude changes associated with shifting climatic belts and patterns of atmospheric circulation during the Pleistocene, gives it special significance as a source of palaeoenvironmental data, particularly of changes in moisture availability. Good chronological control is thusfar available only for the uppermost 20 m of the succession, yet preliminary interpretation of this zone, spanning the Last Glacial Maximum and the Holocene, indicates that mid-latitude desiccation, which accompanied the lowest temperatures of the Last Glacial, was probably more severe and prolonged than has hitherto seemed likely. Similar fluctuations are evident within samples from the lower part of the core, and the results of attempts to date these older deposits are awaited with interest. There seems to be every prospect that ongoing analyses will permit the reconstruction of palaeoclimatic indices for the mid-latitudes of southern Africa parallel with those derived from the Vostok ice-core for the high latitudes of Antarctica.

Acknowledgements Recovery of the 1988/89 Saltpan core was funded by the Geological Survey of South Africa through the good offices of its Chief Director, Dr. C. Frick, and through grants to T.C. Partridge and L. Scott from the South African Foundation for Research Development; the research of J.C. Vogel was also supported by the latter body. The ongoing support of these organizations, and of the many researchers who have contributed their time

336

and expertise to aspects of this multidisciplinary study, are acknowledged with gratitude. References Acocks, J.P.H., 1953. Veld types of South Africa. Mem. Bot. Surv. S. Afr., 28: 1-192. Archibald, R.E.M., 1983. The diatoms of the Sundays and Great Fish Rivers in the eastern Cape province of South Africa. In: Bibliotecha Diatomologica, 1. Cramer, Vaduz. Ashton, P.J. and Schoeman, F.R., 1983. Limnological studies on the Pretoria Salt Pan, a hypersaline maar lake. I: Morphometric, physical and chemical features. Hydrobiologia, 99: 61-73. Bond, G.W., 1946. A geochemical survey of the underground water supplies of the Union of South Africa. Dep. Mines, Geol. Surv. Mem., 41,208 pp. Bradbury, J.P., 1989. Late Quaternary lacustrine palaeoenvironments in the Cuenca de Mexico. Quat. Sci. Rev., 8: 75-100. Feuchtwanger, T., 1973. Zoutpan: carbonatite-alkaline volcano. Dep. Geol., Univ. Witwatersrand, 41 pp. (unpublished). Fudali, R.F., Gold, D.P. and Gurney, J.J., 1973. The Pretoria Salt Pan: astrobleme or cryptovolcano? J. Geol., 81: 495-507. Gasse, F., 1975. L'rvolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du plio-plristocrne ~. l'actuel: Reconstitution des palromilieux lacustres h partir de l'rtude des diatomres. Thesis. Univ. Paris VI. Gasse, F., 1980. Les diatomres lacustres Plio-P16istocrnes du Gadeb (Ethiopie). Systrmatique, palrorcologie, biostratigraphie. Rrv. Algol. Mem. H. S., 3. Gasse, F., Rognon, P. and Street, F.A., 1980. Quaternary history of the Afar and Ethiopian Rift Lakes. In: M.A. Williams and H. Faure (Editors) The Sahara and the Nile, Quaternary Environments and Prehistoric Occupation in Northern Africa. Balkema, Rotterdam, 361-400 pp. Gasse, F., Tailing, J.F. and Kilham, P., 1983. Diatom assemblages in East Africa: Classification, distribution and ecology. Rev. Hydrobiol. Tropicale, 16: 3-34. Gasse, F., 1986. East African diatoms. Taxonomy, ecological distribution. In: Bibliotecha Diatomologica, 11. Cramer, Vaduz, 201 pp. Gasse, F., 1987. Diatoms for reconstructing palaeoenvironments and palaeohydrology in tropical semi-arid zones. Examples of some lakes from Niger since 12,000 BP. Hydrobiologia, 154: 127-163. Gasse, F., 1988. Diatoms, palaeoenvironments and palaeohydrology in the western Sahara and Sahel. Wurzburg Geogr. Arb., 69: 233-254. Gasse, F., Fontes, J.-C., Plaziat, J.C., Carbonel, P., Kaczmarska, I., De Deckker, P., Soulir-Marsche, I., Callot, Y. and Dupeuble, P.A., 1987. Biological remains, geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the Holocene lakes from North Sahara. Palaeogeogr., Palaeoclimatol., Palaeoecol., 60: 1-46. Gouleau, D. and Noel, D., 1984. L'importance des diatomres dans le cycle de la silice dissoute des saumures libres et des

T.C. PARTRIDGEET AL. eaux interstitielles dans le marais salant de Salin-de-Giraud (S.E. de la France). Rev. Grol. Dyn. Grogr. Phys., 25: 177-186. Grunow, J.O., 1967. Objective classification of plant communities: a synecological study in the Sourish Mixed Bushveld of Transvaal. J. Ecol., 55: 691-710. Haberyan, K. and Hecky, R.E., 1987. The Late Pleistocene and Holocene stratigraphy and palaeolimnology of Lakes Kivu and Tanganyika. Palaeogeogr., Palaeoclimatol., Palaeoecol., 61: 169-197. Hecky, R.E. and Kilham, P., 1973. Diatoms in alkaline, saline lakes, ecology and geochemical implications. LimnoL Oceanogr., 18: 53-71. Jeppe, F., 1868. Die Transvaalsche oder S.A. Republik nebst einem Anhang: Dr. Wangemann's Reise in Suidafrika. Petermann's Geogr. Mitt. Ergebn., 24, 24 pp. Kilham, P., 1990. Ecology of Melosira species in the Great Lakes of Africa. In: M. Tilzer and C. Serruya (Editors), Large Lakes Ecological Structure and Function. Springer, New York, pp. 414-427. Kilham, P., Kilham, S. and Hecky, R.E., 1986. Hypothesized resource relationships among African planktonic diatoms. Limnol. Oceanogr., 31: 1169-1181. Krammer, K. and Lange-Bertalot, H., 1988. Bacillariophyceae. 2. Bacillariaceae, Epithemiaceae, Surirellaceae. In: SiJsswasserflora von Mitteleuropa. Fischer, Stuttgart, 585 pp. Leonard, F.C., 1946. Authenticated meteorite craters of the world: a catalog of provisional co-ordinate numbers for the meteoritic falls of the world. Univ. New Mexico. Publ. Meteoritics, 1, p. 54 Levin, G., 1991. The Pretoria Salt Pan: the historical aspects. Geobulletin, 34 (2): 13-16. Metcalfe, S.E., 1988. Modern diatom assemblages in Central Mexico: The role of water chemistry and other environmental factors as indicated by Twinspan and Decorana. Freshwater Biol., 19: 217-233. Metcalfe, S.E. and Hales, P.E., in press. Holocene diatoms from a Mexican crater lake La Piscina de Yuriria. Proc. 1 lth Symp. Living and Fossil Diatoms, San Francisco, 1990. Milton, D.J. and Naeser, C.W., 1971. Evidence for an impact origin of the Pretoria Salt Pan, South Africa. Nature, 299: 211-212. Partridge, T.C., Reimold, W.U. and Walraven, F., 1990. The Pretoria Zoutpan crater: first results from the 1988 drilling project. Meteoritics, 25, p. 396. Partridge, T.C., Reimold, W.U., Kerr, S.J. and Stanistreet, I.G., 1991. The Pretoria Saltpan--the scientific aspect. Geobulletin, 34: 16-22. Reimold, W.U., Koeberl, C., Kerr, S.J. and Partridge, T.C., 1991. The Pretoria Saltpan--the first firm evidence for an origin by impact. Lunar. Planet. Sci. Lett., 22: 11171118. Richardson, J.L., Harvey, T. and Holdship, S.A., 1978. Diatoms in the history of shallow East African lakes. Pol. Arch. Hydrobiol., 25: 341-353. Rohleder, H.P.T., 1933. The Steinheim Basin and the Pretoria Salt Pan: volcanic or meteoritic origin? Geol. Mag., 70: 489-498. Scott, L., 1982. Late Quaternary fossil pollen grains from the

PRETORIA SALTPAN:200,000YEAR SOUTHERN AFRICAN LACUSTRINESEQUENCE Transvaal, South Africa. Rev. Palaeobot. Palynol., 36: 241 278. Scott, L., 1988. The Pretoria Salt Pan: a unique source o1" Quaternary palaeoenvironmental information. S. Afr. J. Sci., 84: 560-562. Schoeman, F.R. and Ashton, P.J., 1982. The diatom flora of

337

the Pretoria Salt Pan, Transvaal, Republic of South Africa. Bacillaria, 5: 63-99. Trollope, A., 1878. South Africa. Balkema, Cape Town, 504 pp. (reprinted edition edited by J.H. Davidson, 1973). Wagner, P.A., 1922. The Pretoria Saltpan. Geol. Surv. S. Afr. Mem., 20, 136 pp.