Journal of the Geological Society, London, Vol. 162, 2005, pp. 789–799. Printed in Great Britain.
Neoarchaean (c. 2.58 Ga) halite casts: implications for palaeoceanic chemistry
K . A . E R I K S S O N 1 , E . L . S I M P S O N 2, S . M A S T E R 3 & G . H E N RY 4
Department of Geosciences, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, USA
(e-mail: kaeson@vt.edu)
2
Department of Physical Sciences, Kutztown University of Pennsylvania, Kutztown, PA 19530, USA
3
EGRI-HAL, School of Geosciences, University of the Witwatersrand, P. Bag 3, WITS 2050, Johannesburg, South Africa
4
Council for Geoscience, P. Bag X112, Pretoria, 0001, South Africa
1
Abstract: Possibly the most extensive and best-preserved Archaean halite casts yet discovered occur in the c.
2.58 Ga upper Black Reef and basal Oaktree formations, Transvaal Supergroup, in Mpumalanga Province,
South Africa. Halite casts are isolated on bedding planes, range in size from c. 1 mm to 20 mm, and have
cubic, dumbbell and triangular shapes, as well as hopper-like pyramidal hollows on cube faces. Some of the
casts display distinct hopper shapes characteristic of halite crystals. The halite cast-bearing pavements are
developed within silicified mudstone interbedded with siltstone or stromatolitic dolomite. Associated
sedimentary structures pointing to subaerial exposure include adhesion ripples and warts, desiccation and
prism cracks, rill marks and tepee structures. Halite cast-bearing beds are interpreted as supratidal flat or
sabkha deposits. The presence of isolated casts and hopper-shaped crystals suggests that halite resulted from
displacive growth within the sediment from supersaturated residual brines after mudstone deposition. Absence
of any indication of the former presence of gypsum or anhydrite supports previous contentions that the
Neoarchaean ocean was deficient in sulphate or contained a high bicarbonate to calcium ratio such that with
progressive evaporation, most calcium was consumed before the gypsum stability field was reached. The
association of halite and carbonate in the upper Black Reef and basal Oaktree formations constrains the
palaeolatitude of the Transvaal Basin at 2.58 Ga to subequatorial (10–308).
Keywords: Neoarchaean, seawater, composition, halite, palaeolatitudes.
those containing extensive aragonite crystal pseudomorphs.
Critical to the interpretation of palaeoceanic chemistry is the
unambiguous characterization of former evaporites on the basis
of pseudomorph morphologies.
In modern marginal-marine settings, halite accumulates in
subaerial and subaqueous hypersaline environments including
peritidal flats, and environments not affected by tides or storms
but flooded by marine waters that seep through a physical barrier
separating the evaporitic basin from the ocean (Handford 1991).
In each of these settings, progressive evaporation of seawater
leads to precipitation of calcite and gypsum followed by halite
(Handford 1991). Halite precipitation takes the form of subaqueous cumulates, or subaqueous bottom or intrasediment precipitates (Lowenstein & Hardie 1985). Later influx of fresh or storm
waters results in dissolution of halite such that, in the geological
record, halite is rarely preserved as a mineral but rather as casts
or moulds on bedding planes (Llewellyn 1968; Southgate 1982;
Demicco & Hardie 1994).
In modern settings and in the geological record extending back
to 1.8 Ga, gypsum is a common evaporite mineral often developed in association with halite. The pre-1.8 Ga record, in
contrast, is almost devoid of gypsum. The lack of gypsum is
considered to have important implications for the composition of
the early ocean (Grotzinger & Kasting 1993).
A number of horizons in the transition beds between the Black
Reef Formation and Oaktree Formation of the Chuniespoort
Group, Transvaal Supergroup, South Africa (Figs 1 and 2)
contain a variety of casts that represent the oldest evidence for
halite precipitation described from the geological record. The
sedimentology of these and associated facies has been investigated along the eastern escarpment in Mpumulanga Province,
South Africa (Fig. 1) with a view to understanding: (1) the
depositional environment of halite precipitation; (2) the mode of
precipitation of halite; (3) the implications for Neoarchaean
ocean chemistry of halite precipitation in rock units that underlie
Geological setting
The Transvaal Supergroup is a late Archaean to early Palaeoproterozoic succession of siliciclastic and chemical sedimentary, and
subordinate volcanic rocks that are preserved within three
separate sub-basins: Transvaal, Kanye and Griqualand West
(Catuneanu & Eriksson 1999). Rocks of the Transvaal Supergroup unconformably overlie either the Witwatersrand Supergroup or the Ventersdorp Supergroup and make up the floor
rocks of the Bushveld Complex. In Mpumulanga Province, South
Africa, the Transvaal Supergroup overlies the protobasinal Wolkberg Group, and is subdivisible into the Black Reef Formation,
Chuniespoort Group and Pretoria Group (Figs 1 and 2). The
Black Reef Formation is gradational into the Oaktree Formation
at the base of the Chuniespoort Group. A latest Archaean age for
the Black Reef–Oaktree study interval is constrained by U–Pb
dating of tuffs intercalated within the Oaktree Formation NW of
Johannesburg (2550 3 Ma and 2588 7 Ma) and in Mpumulanga Province (2583 5Ma) (Walraven & Martini 1995; Martin
et al. 1998).
Along the Mpumulanga escarpment, the Black Reef Formation
varies in thickness from 10 to 20 m (Henry et al. 1990; Eriksson
& Reckzo 1995) and consists of conglomerates, sandstones
and mudstones. A lower upward-fining interval is interpreted
as a braided-alluvial to floodplain transition, whereas two overlying upward-coarsening intervals are considered to represent
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K. A. ERIKSSON ET AL.
Fig. 1. Geological map of the Transvaal
Supergroup along the eastern Mpumulanga
escarpment with inset of South Africa. Also
shown are the locations of the measured
sections (Fig. 4).
progradational braid-delta deposits (Fig. 3; Henry et al. 1990).
Braid-delta deposits (sensu McPherson et al. 1987) are confined
to the northern and southern parts of the outcrop belt and the
halite-bearing units are located in the northern portion of the belt
(Fig. 3). The study interval overlies cross-bedded sandstones of
braided-alluvial origin and is capped by large stromatolitic
domes of the Oaktree Formation that are interpreted as subtidal,
marine facies (Truswell & Eriksson 1975; Beukes 1987).
Facies descriptions and interpretations
Sections were measured through halite-bearing intervals in the
upper Black Reef Formation and Oaktree Formation at the base
of the Chuniespoort Group at three locations along the Mpumu-
langa escarpment: Dientje–Old Stone Bridge and Ses-I-se-Draai
(Fig. 4). In addition, a section was measured through the upper
Black Reef Formation and Oaktree Formation at Three Rondawels, where no halite casts were observed.
Pebbly sandstone facies
The lower Black Reef Formation along the escarpment is less
than 5 m thick (Fig. 3; Henry et al. 1990) and consists of pebbly
and coarse-grained sandstones containing medium- to large-scale
trough cross-beds, and small-scale planar cross-beds within
which the foresets are defined by grain flows (see Buck 1985).
Palaeocurrent data indicate strongly unidirectional flow towards
the west and SW (Fig. 5). Pebbly sandstones of the lower Black
N E OA R C H A E A N H A L I T E C A S T S A N D O C E A N C H E M I S T RY
791
Reef Formation are sharply overlain by siltstone–shale, stromatolitic dolomite or tuff (Fig. 4). Large-scale, symmetrical ripples
with wavelengths up to 20 cm (Fig. 6a) define the top of the
pebbly sandstone facies at Ses-I-se-Draai (Fig. 4b).
The lower Black Reef Formation has been interpreted as a
mainly braided-alluvial deposit by Henry et al. (1990) on the
basis of its coarse grain size and dominance of unidirectional
cross-beds. Comparable facies are developed in modern (e.g.
Coleman 1969; Cant & Walker 1978), and ancient braidedalluvial deposits including those of Archaean age (e.g. Eriksson
1978; Beukes & Cairncross 1991; Els 1998). The lack of
meandering-river deposits in the Black Reef Formation is
consistent with an absence of bank stabilization by vegetation in
the Neoarchaean landscape (see Schumm 1968). Large symmetrical ripples developed on the top of the lower Black Reef
Formation reflect wave reworking associated with initial transgression. Similar ripples have been described from relict Pleistocene sediments that were reworked during the Holocene sea-level
rise (Leckie 1988).
Fine- to medium-grained sandstone facies
Fig. 2. Stratigraphic column for Transvaal Supergroup (modified from
Catuneanu & Eriksson 1999).
This facies dominates the Three Rondawels section, where it is
over 5 m thick (Fig. 4c). Sandstone is fine- to medium-grained
and has horizontal stratification and small-scale trough crossbeds. Bedding planes exhibit a range of sedimentary structures
including adhesion warts and ripples (Fig. 6b), desiccation cracks
that display evidence for multiple generations of shrinkage and
infill (Fig. 6c), raindrop impressions preserved as casts with
crater rims (Fig. 6d), aligned, lenticular sandstone dykelets
developed on desiccated mudstone polygons (Fig. 6e), and
various ripples including symmetrical, asymmetrical, ladderback
and interference forms; some ripples contain desiccated mudstone drapes.
The association of sedimentary structures in this facies
indicates very shallow water and periodically emergent conditions. Adhesion structures are produced by wind-blown sand
adhering to a wet or damp surface (Kocurek & Fielder 1982).
Such conditions would also promote the formation and preservation of raindrop impressions. Alternating submergence and
emergence of the depositional surface is indicated by the
desiccation cracks. The symmetrical and interference ripple
Fig. 3. Cross-section illustrating the
geometry and facies of the Black Reef
Formation along the Mpumulanga
escarpment (modified from Henry et al.
1990). c.u., coarsening upwards; f.u., fining
upwards.
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K. A. ERIKSSON ET AL.
Fig. 4. Detailed measured sections through
the Black Reef Formation and basal
Oaktree Formation. (a) Old Stone Bridge–
Dientjie; (b) Ses-I-se-Draai; (c) Three
Rondawels. (See Figure 1 for locations of
cross-sections.)
forms indicate an environment influenced by waves whereas the
ladderback forms support an intertidal setting (see Klein 1985).
Suspension settling of mud on ripples during slack-water periods
was followed by exposure and desiccation. The origin of the
lenticular sandstone dykelets is more problematic. Geometrically
they resemble syneraesis cracks (e.g. van Straaten 1954) but such
an origin would not explain their preferred alignment. A
plausible explanation is that a mud layer cracked subaqueously
in response to downslope creep of a semi-consolidated mud layer
with the cracks subsequently infilled with sand. A similar origin
has been inferred for linear shrinkage cracks in the Green River
Formation (Picard 1966).
Siltstone–mudstone facies
Massive and laminated siltstone and mudstone form the upper
Black Reef Formation at Ses-I-se-Draai (Fig. 4b). Tractionproduced sedimentary structures are notably lacking. This facies
is also interbedded with stromatolitic dolomite of the lower
Oaktree Formation at the same location. Sedimentary structures
are dominated by horizontal laminations, but symmetrical ripples
as well as starved ripples of siltstone within mudstone are present
locally. Symmetrical ripple crests vary in orientation from
north–south to east–west.
Lack of traction-produced structures in this facies in the Black
Reef Formation indicates slow suspension sedimentation below
wave base. The presence of ripples reflects gradual shoaling into
the basal Oaktree Formation.
Silicified mudstone–siltstone facies
This facies is developed in the lower Oaktree Formation at Ses-Ise-Draai and Old Stone Bridge (Fig. 4a and b) and as eight
horizons interbedded with sandstone in the upper Black Reef
Formation at God’s Window (Fig. 1). Individual horizons range
in thickness from 2 to 30 cm. Facies are dominated by silicified
N E OA R C H A E A N H A L I T E C A S T S A N D O C E A N C H E M I S T RY
N
000°
A
Dientje
090°
270°
N = 60
180°
N
000°
B
Ses-I-Se-Draai
270°
090°
N = 34
180°
N
000°
C
Three Rondawels
270°
090°
N = 47
180°
Fig. 5. Palaeocurrent data for braided-alluvial facies of the lower Black
Reef Formation.
mudstone in laminae between 2 and 30 mm thick. Mudstone is
mainly massive with rare faint parallel laminations. Intercalated
within the mudstone are 1 to 4 mm thick, massive, gradedbedded and rippled, lenticular-bedded siltstone laminae (Figs 6f
and 7a) that locally infill desiccation cracks and angular depressions in the underlying mudstone (Fig. 7b). Locally, scouring is
present at the base of siltstone beds. Chaotic intraclast breccias
are common within this facies typically in association with
small-scale syndepositional faults and dismembered folds overlain by intact laminations. Other structures developed in this
facies include halite casts in extensive pavements up to 5 m by
20 m, rare tepee structures (Fig. 7c), rill marks, and desiccation
and prism cracks (Fig. 7a). Halite casts range from ,1 cm to
793
2 cm in size, vary from square- to triangular- to dumbbell- to
hopper-shaped, and typically are isolated from one another (Figs
7d and 8a–c). Casts commonly display internal zoning (Fig. 8d)
and hopper-like pyramidal hollows on cube faces, and are
commonly associated with desiccation cracks.
Evidence for the former presence of sulphates is lacking.
Specifically, the following criteria are not evident on outcrop or
in thin section: (1) nodules of former anhydrite replaced by
quartz, calcite or dolomite; (2) pseudomorphs after swallow-tail
gypsum or after anhydrite laths; (3) displacive crystallization
associated with non-cubic pseudomorphs (see Demicco & Hardie
1994). Radiating crystal fans in overlying limestones that were
previously interpreted as gypsum pseudomorphs (Bertrand-Sarfati 1976) are now considered to be the product of neomorphism
of botryoids of aragonite, on the basis of textures and elevated
strontium contents (Sumner & Grotzinger 2000).
Associations of lithologies in this facies indicate overall lowenergy conditions, favouring mud accumulation, interrupted by
frequent higher-energy pulses that introduced silt to the depositional setting. Massive siltstone laminae probably reflect storm
processes whereas ripple cross-laminae record weak traction
reworking. Desiccation structures indicate periodic exposure of
the depositional surface. The rare tepee structures may have
formed as a result of the expansive growth of halite and/or early
carbonate cement in the zone of evaporative pumping (see
Warren 1983; Lowenstein & Hardie 1985). Chaotic breccias are
mainly products of desiccation and/or incipient tepee formation.
Isolated halite casts and the presence of hoppers with concave
margins support displacive growth of halite probably as a result
of evaporation of capillary brines (see Shearman 1978; Gornitz
& Schreiber 1981; Handford 1988). Less common casts with
internal zoning support incorporative growth within the sediment
(see Handford 1988). Halite formation by upward rather than
downward diffusion is favoured by the shallow-water setting
implied by the desiccation cracks (Handford 1988). Angular
depressions at the base of siltstone laminae represent casts of
halite that was dissolved by lower-salinity waters that introduced
silt. Casts indicate that halite precipitation occurred at very
shallow depths within the sediment or on the sediment surface.
Stromatolitic dolomite facies
Stromatolitic dolomite is the predominant facies in the Oaktree
Formation and, in general, the size of stromatolitic domes
increases upwards (Fig. 4). At Ses-I-se-Draai, silicified stromatolite bearing horizons between 5 and 25 cm thick are interbedded
with siltstone–mudstone, tuff and carbonate mudstone–grainstone (Fig. 4b). Stromatolites in these horizons consist of linked,
low-relief domes, or vertically stacked domes that increase in
width and complexity upwards (Fig. 8d). Laminations on a
millimetre scale are well preserved by secondary silicification.
Stromatolitic domes higher in the Oaktree Formation are up to
5 m wide, display relief of up to 50 cm, and also are characterized by well-defined millimetre-scale laminations. Secondary
silicification is lacking. Stromatolites are elongated east–west to
SE–NW.
All structures documented in this facies are similar to
isopachously laminated stromatolites that consist of encrusting
layers of former (high-magnesium?) calcite, and have been
described from stratigraphic units higher up in the Chuniespoort
Group and from other Archaean and Palaeoproterozoic successions (Grotzinger & James 2000; Pope & Grotzinger 2000). The
encrusting layers are interpreted as products of abiotic precipitation of carbonate mud related to progressive oversaturation of
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K. A. ERIKSSON ET AL.
N E OA R C H A E A N H A L I T E C A S T S A N D O C E A N C H E M I S T RY
795
Fig. 7. Photographs of facies. (a) Photomicrograph showing graded-bedded and rippled lenticular-bedded siltstone laminae, and prism cracks within
silicified mudstone at Ses-I-se-Draai. Photograph is 2.5 cm wide. (b) Photomicrograph showing a halite mould infilled with siltstone. Photograph is 1.5 cm
wide. (c) Tepee structure in silicified siltstone–sandstone facies at Ses-I-se-Draai (scale is 15 cm long). (d) Casts of halite hopper crystals from silicified
siltstone–mudstone facies at Ses-I-se-Draai. Some casts are internally zoned and indicate incorporation of sediment during growth. Casts with concave
margins provide evidence for displacive growth of halite (scale is in centimetres).
seawater as a result of increasing temperature and salinity (Pope
& Grotzinger 2000). Radiating crystal pseudomorphs indicative
of aragonite precipitation (Sumner & Grotzinger 2000) were not
identified but these are commonly destroyed by dolomitization
and silicification (C. Schreiber, pers. comm.). Thus, the former
presence of aragonite cannot be excluded. The upward increase
in size of domical stromatolites (Fig. 4) is considered to reflect
progressive deepening.
Fig. 6. Photographs of facies. (a) Large-scale symmetrical ripples defining a ravinement zone above the lower Black Reef Formation at Ses-I-se-Draai. (b)
Adhesion ripples or warts in fine- to medium-grained sandstone facies of the upper Black Reef Formation at Three Rondawels. (c) Desiccation cracks
displaying multiple generations of shrinkage and infill in fine- to medium-grained sandstone facies at Three Rondawels (scale is 6.5 cm long). (d)
Raindrop casts in fine-grained sandstone at Three Rondawels. (e) Oriented, lenticular sandstone dykelets in fine- to medium-grained sandstone facies at
Three Rondawels. (f) Photomicrograph showing graded siltstone laminae within silicified mudstone at Ses-I-se-Draai. Angular depressions at the base of
some siltstone laminae represent infilled halite casts. Photograph is 2.5 cm wide.
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K. A. ERIKSSON ET AL.
Fig. 8. Photographs of halite casts. (a) Casts of halite hopper crystals in silicified shale, Lower Oaktree Formation, near Bourke’s Luck potholes. The cast
in the centre shows internal zoning as a result of sediment incorporation during growth (scale in centimetres). (b) Halite casts in silicified mudstone,
Lower Oaktree Formation, near Burke’s Luck potholes. The large halite cast, 15 mm across, and triangular or tricuspate shapes of casts of the corners of
halite crystals, should be noted. (c) Casts of hopper-shaped halite cubes with depressed cubic faces, as evidence for displacive growth, from silicified
siltstone–mudstone facies at Ses-I-se-Draai (coin is 1 cm in diameter). (d) Small stromatolite domes displaying an upward increase in width in
stromatolitic dolomite facies at Ses-I-se-Draai (coin is 2 cm in diameter).
Mafic tuff
Mafic tuffs are developed in each of the measured sections and
range in thickness from 50 cm to 5 m (Fig. 4). Locally, the tuffs
contain glass shards up to 1 cm in length. The tuffs are mostly
massive but locally display evidence of reworking in the form of
horizontal stratification and symmetrical ripples. Halite casts
similar in size and structure to those discussed above are developed on the top of the tuff bed at Ses-I-se-Draai.
Tuffaceous horizons are widespread in the lower part of the
Chuniespoort Group (Walraven & Martini 1995; Martin et al.
1998) but the location(s) of the explosive volcanic centres is not
known. Also unclear is whether the tuffaceous horizons in
different parts of the basin reflect a single or multiple explosive
events. In the study area, deposition of the tuffs occurred in
shallow water, as implied by the traction-produced structures. In
addition, the local presence of halite casts supports a shallowwater, evaporitic setting.
Palaeolatitudinal constraints
Because certain rock types such as evaporites (halite, gypsum
and anhydrite), carbonates, coals and tillites are climatically
sensitive sediments, and tend to be deposited under restricted
conditions, they are useful in palaeoclimatic studies. Evaporites
occur in the subtropics, where it is dry, and where evaporation
exceeds the total of precipitation plus inflow of surface water
(Gordon 1975). Carbonates, in particular those of the Bahamian
type, occur in equatorial, subtropical, and warm temperate
regions, where it is warm and where there is adequate sunlight
penetration (Scotese & Barrett 1990).
N E OA R C H A E A N H A L I T E C A S T S A N D O C E A N C H E M I S T RY
The distribution of climatically sensitive sedimentary rocks
has been used to independently test and verify the palaeolatitudes
calculated from palaeomagnetic studies assuming a geocentric
axial dipole model for the Earth’s magnetic field in the past
(Irving & Briden 1962; Opdyke 1962; Briden 1968, 1970). For
the Mesozoic and Cenozoic, palaeolatitudinal positions of the
major continents are known with great precision through the use
of seafloor magnetic anomalies (Ziegler et al. 1983). Detailed
palaeomagnetic studies have also allowed for accurate reconstructions of Palaeozoic palaeogeography (Cocks & Torsvik
2002; Torsvik & Cocks 2004), but palaeomagnetic reconstructions for the Precambrian are much less certain, because of the
paucity of well-defined ages. For the Mesozoic and Cenozoic,
Scotese & Barrett (1990) plotted the latitudinal distribution of
known climatically sensitive sedimentary rocks (evaporites,
carbonates, coals and tillites) in the form of pole-to-pole
histograms. They successfully used these histograms and their
associated probability functions to calculate a Palaeozoic Apparent Polar Wander (APW) path for the Gondwana Supercontinent,
which is in fairly good agreement with the palaeomagnetically
determined APW of Bachtadze & Briden (1990). Scotese &
Barrett (1990) showed that climatically sensitive sedimentary
rocks can be used to successfully predict the location of
palaeolatitudes, and hence of palaeopoles, assuming that the
zonal distribution of climate patterns was the same throughout
the Phanerozoic eon as it is today. This assumption can be used
for most of Precambrian Earth history, except for those periods
when the equator-to-pole temperature gradient was very different
from now, such as during the global periods of glaciation during
the Palaeoproterozoic and Neoproterozoic eras.
The analytical results of Scotese & Barrett (1990), based
partly on the data of Parrish et al. (1982), show that evaporites
are restricted mainly to latitudes (N or S) of between 58 and 358,
with an occurrence probability of 0.72, and most modern
carbonates are restricted to latitudes (N or S) of between 108 and
308, with an occurrence probability of 0.55. Thus, for the
Transvaal Supergroup halite–carbonate association, there is a
maximum probability that this pair of climatically sensitive
lithologies was formed at between 108 and 308 palaeolatitude at
c. 2.58 Ga. This subequatorial palaeolatitude result fills an
important gap in our palaeogeographical knowledge of the
Transvaal Supergroup, because the only well-constrained palaeomagnetic palaeopole from this sequence is from the much
younger 2222 13 Ma Ongeluk lavas (Cornell et al. 1996).
Evans et al. (1997) inferred an equatorial palaeolatitude
(118 58) for the Ongeluk lavas; these occur in the Postmasburg
Group that overlies the Ghaap Group, a correlative of the
Chuniespoort Group. The preserved rocks of the Transvaal
Supergroup thus appear to have been deposited while the
Kaapvaal Craton was situated in low palaeolatitudes, ,308 N or
S. Because of the absence of palaeomagnetic data, little can be
said about the movement of the Kaapvaal Craton between the
period of deposition of the Black Reef Quartzite Formation and
outpouring of the Ongeluk lavas. However, a 64.5 17.58
palaeolatitude for the 2782 5 Ma Derdepoort basalt (Wingate
1998) suggests a northward migration of the Kaapvaal Craton
between 2.8 and 2.2 Ga.
Discussion
Vertical successions of facies (Fig. 4) suggest progressive
deepening from alluvial settings in the lower Black Reef
Formation to a subtidal environment at the time of deposition of
the Oaktree Formation. The presence of stromatolites produced
797
by precipitation from oversaturated seawater supports a marine
rather than a lacustrine depositional environment. In the Ses-I-seDraai location, wave ripples developed above alluvial facies of
the lower Black Reef Formation define a ravinement surface and,
together with the overlying facies, indicate rapid deepening. The
upper Black Reef Formation at this locality consists of a
progradational parasequence that was followed by deepening into
the basal Oaktree Formation. A similar vertical transition (except
for the later deepening phase) from continental sandstone to thin
transgressive-phase deposits followed by thicker subtidal to
supratidal, regressive-phase deposits is well documented from
sabkhas of Abu Dhabi (Kinsman & Park 1976; Wright 1984).
The parasequence at Ses-I-se-Draai provides constraints on the
depositional setting of the halite-cast hosting facies and by
implication the environment of halite precipitation. Lack of
traction-produced structures in the siltstone and mudstone facies
at the base of the parasequence implies a sub-wave base
environment, whereas evidence for exposure throughout the
upper half of the parasequence supports a peritidal setting in
which small-scale stromatolitic domes and carbonate muds and
sands together with evaporites were deposited. The preponderance of evidence for intrasediment growth and dissolution of
evaporites supports a sabkha and, in particular, a saline mudflat
setting (see Handford 1991; Demicco & Hardie 1994). In such
an environment, evaporite crystals close to the surface commonly
dissolve during floods, resulting in crystal moulds, although a
decrease in salinity could also be related to influx of marine
waters during storms.
The geomorphology of the sabkha setting can be evaluated
with reference to the cross-section of the Black Reef Formation
(Fig. 3). This section utilized the top of the last coarse-grained
facies as the datum. If the cross-section is hung from the base of
the first occurrence of large-scale, stromatolitic domes, it would
demonstrate that the halite-bearing and associated facies occupy
the northern margin of the fan-delta deposits. In a palaeogeographical sense, this depression may have taken the form of an
embayment between a fan delta to the south and an undocumented fan delta further north. If so, it is likely that halite-bearing
facies may also be present between the two fan deltas shown on
the cross-section (Fig. 3), but poor outcrop precludes testing of
this model.
The lack of evidence for sulphate minerals in the studied
sections has important implications for Neoarchaean palaeoceanic and/or palaeoatmospheric chemistry. Rare gypsum pseudomorphs reported from the Neoarchaean Carawine Dolomite in
the Hamersley Basin, Australia (Simonson et al. 1993) represent
the oldest evidence of gypsum precipitation. Mesoarchaean barite
from the Warrawoona Group in the Pilbara Block of Australia,
previously interpreted as a replacement of gypsum (e.g. Buick &
Dunlop 1990), is now considered to represent a primary hydrothermal precipitate (Runnegar et al. 2001). Evaporite pseudomorphs from the Onverwacht Group in the Barberton Greenstone
Belt, South Africa, represent silicified nahcolite, a sodium
bicarbonate (Lowe & Worrell 1999). The virtual absence of
gypsum from the early Earth record is attributed to low sulphate
concentrations in early Precambrian oceans related to the anoxic
state of the atmosphere or to a high bicarbonate-to-carbonate
ratio in early Precambrian oceans such that during progressive
evaporation calcium would have been exhausted before the
gypsum field was reached (Grotzinger & Kasting 1993). The
results of this study support the model of Grotzinger & Kasting
(1993) but do not resolve the alternative interpretations for the
absence of gypsum. The presence of aragonite crystal pseudomorphs that make up as much as 50% of Neoarchaean carbonate
798
K. A. ERIKSSON ET AL.
successions including the Malmani Dolomite indicates oversaturation of the sea-water with respect to calcium carbonate
(Grotzinger & James 2000; Sumner & Grotzinger 2000). However, the lack of any ferric iron pigmentation in the Black Reef
alluvial facies (Twist & Cheney 1986) and in older Archaean
alluvial facies such as the Moodies Group and Pongola and
Witwatersrand Supergroups (Eriksson 1978; Beukes & Cairncross 1991; Els 1998) indicates overall anoxic atmospheric
conditions favouring a low sulphate content of the oceans.
Conclusions
(1) Halite cast-bearing beds in the 2.58 Ga upper Black Reef and
lower Oaktree formations accumulated in supratidal or sabkha
palaeoenvironments.
(2) Halite precipitation occurred by displacive growth within
the host sediment.
(3) Evidence for the former presence of sulphates is lacking,
thereby supporting previous hypotheses that the Neoarchaean
ocean was deficient in sulphate or contained anomalously high
bicarbonate contents.
(4) A subequatorial palaeolatitude for the Transvaal Basin at
2.58 Ga is implied by the association of carbonate and halite,
and provides a new data point for palaeogeographical reconstructions.
Field work on which this paper is based was funded by National
Geographic Society grant 6003-97. We thank C. Schreiber and P. Turner
for their insightful reviews, and M. Fowler for his constructive comments.
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Received 15 September 2004; revised typescript accepted 13 January 2005.
Scientific editing by Mike Fowler