Sedimenfology (1992) 39,983-1002

Halite saltern in the Canning Basin, Western Australia: a sedimentological analysis of drill core from the Ordovician-Silurian Mallowa Salt

D O N N A L . C A T H R O * ’ , J O H N K . W A R R E N ? ’ and G E O R G E E . WILLIAMS:

*Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia tSchool of Applied Geology, Curtin University of Technology, GPO Box U 1987, Perth, W.A. , 6001 Australia

$Department of Geology and Geophysics, University of Adelaide, GPO Box 498, Adelaide, South Australia 5001, Australia and Geosedimentary Services, PO Box 394, Marden, South Australia 5070, Australia

ABSTRACT

The Late Ordovician-Early Silurian Mallowa Salt of the Carribuddy Group, Canning Basin, north-west Australia, is the largest halite deposit known in Australia, attaining thicknesses of 800 m or more within an area of approximately 200 000 km2. Study of 675 m of drill core from BHP-Utah Minerals’ Brooke No. 1 well in the Willara Sub-basin indicates that the Mallowa Salt accumulated within a saltern (dominantly subaqueous evaporite water body) that was subject to recurrent freshening, desiccation and exposure. Textures and bromine signatures imply a shallow water to ephemeral hypersaline environment typified by increasing salinity and shallowing into evaporitic mudflat conditions toward the top of halite- mudstone cycles (Type 2) and the less common dolomite/anhydrite-halite-mudstone cycles (Type 1). The borate mineral priceite occurs in the capping mudstones of some cycles, reinforcing the idea of an increasing continental influence toward the top of mudstone-capped halite cycles.

The rock salt in both Type 1 and Type 2 cycles typically comprises a mosaic of large, randomly orientated, interlocking halite crystals that formed during early diagenesis. It only partially preserves a primary sedimentary fabric of vertically elongate crystals, some with remnant aligned chevrons. Intraformational biati, halite karst tubes and solution pits attest to episodic dissolution. Stacked Type 2 cycles dominate; occasional major recharges of less saline, perhaps marine, waters in the same area produced Type 1 cycles.

The envisaged saltern conditions were comparable in many ways to those prevailing during the deposition of halite cycles of the Permian Salado Formation in New Mexico and the Permian San Andres Formation of the Palo Duro Basin area in Texas. However, in the Canning Basin the cycles are characterized by a much lower proportion of anhydrite, implying perhaps a greater degree of continental restriction to the basin. The moderately high level of bromine in the Mallowa Salt (1 56.5 43.5 ppm Br for primary halite, 146.1 k54.7 ppm Br for secondary halite) accords with evolved continental brines, although highly evaporative minerals such as polyhalite and magnesite are absent. The bromine levels suggest little or no dissolution/reprecipitation of primary halite and yet, paradoxically, there is little preservation of the primary depositional fabric. The preservation of early halite cements and replacement textures supports the idea of an early shutdown of brine flow paths, probably at burial depths of no more than a few metres, and the resultant preservation of primary bromine values in the secondary halite.

INTRODUCTION

The sedimentology of ancient halite deposits remains a research frontier as almost all well preserved bedded halite is confined to the subsurface and drill core usually is unavailable for study. The recect acquisition

‘Formerly at the National Centre for Petroleum Geology and Geophysics, Adelaide, South Australia.

of continuous drill core from the early Palaeozoic Mallowa Salt in the Canning Basin of Western Australia, the largest halite deposit known in Australia (Wells, 1980), therefore has provided a rare opportu- nity to study the sedimentology and Petrography of a major halite deposit.

983

984 D . L. Cathro, J . K . Warren and G . E. Williams

The present study is based on 675 m of continuous drill core of mainly bedded rock salt from BHP-Utah Minerals' potash exploration well Brooke-1 (1 9"45'S, 122'55'E; see Fig. l), drilled in the Willara Sub-basin in 1988. Brooke-1 is located in the Canning Basin some 300 km south-west of the geologically renowned outcrop of Devonian barrier reefs. This study of halite sedimentology of the Mallowa Salt from Brooke-I complements an analysis of cyclicity as seen in the geochemistry of drill core of the Mallowa Salt from BHP-Utah Minerals' Gingerah Hill-1 (19"28'S, 122"22'E; Fig. I), drilled in the Willara Sub-basin in 1986 (Williams, 1991).

Onshore, the Canning Basin covers an area of more than 400 000 km2 (Fig. 1) and contains a 10-18 km thick sequence of shallow water marine, paralic and terrestrial rocks deposited from the Early Ordovician to Early Cretaceous (Purcell, 1984). Offshore, the basin covers approximately 165 000 km', and Palaeo- zoic rocks deepen considerably beneath a seaward- thickening wedge of Mesozoic and Tertiary deposits (Horstman & Purcell, 1988).

Six main tectonic elements divide the onshore Canning Basin into four depocentres, aligned as two parallel N W-SE trending major depressions separated by the Broome and Crossland platforms (Fig. 1). The Kidson and Willara sub-basins together form the southern depression, and the Fitzroy Trough and Gregory Sub-basin make up the northern depression (Forman & Wales, 1981 ; Purcell, 1984). The thickest sections of the evaporitic Carribuddy Group, of which the Mallowa Salt is the principal rock-salt formation, occur in the Kidson and Willara sub-basins. The stratigraphy of the southern Broome Platform/Willara Sub-basin is given in Fig. 2. The Mallowa Salt thins northward across the Broome Platform, and on the Jurgurra Terrace it forms patchy subcrop that is probably the remnant of once widespread deposits (Begg, 1987). Indeed, the limits of the Mallowa Salt in the Canning Basin are erosional and do not reflect the limits of the original brine seaway. Probable time- equivalent halite, now deformed into salt structures, occurs in the offshore Southern Bonaparte Basin some 1000 km north-east of Brooke-I (Durrant et al., 1990).

16's -

Fig. 1. Locality map showing Brooke-I and Gingerah Hill-1 within the Willara Sub-basin of the Canning Basin, Western Australia. Six main tectonic elements divide the onshore Canning Basin into four depocentres, aligned as two major parallel N W-SE trending depressions separated by the Broome and Crossland platforms. The Kidson and Willara sub-basins form the southern depression, and the Fitzroy Trough and Gregory Sub-basin make up the northern depression.

Halite saltern in the Canning Basin, NW Australia 985

,GE PERIOD LITHOLOGY FORMATION/ ENVIRONMENT

Manne

Wa

Fig. 3. Extent and generalized isopachs (in metres) of the Carribuddy Group within the Canning Basin (modified from Forman & Wales, 1981).

S N GROUP

Ordovician to Early Silurian age (Foster & Williams, 1991).

1.1 Sandstone Dolomite

Shale Halite (Mallowa Salt)

Glacials Basement

Limestone

_ _ _ _

Fig. 2. Stratigraphy of the Broome Platform/Willara Sub- basin (modified from Bentley, 1984).

Salt structures of probable equivalent age also have been intersected in the offshore Paqualin-1 well in the Vulcan Grabensome 700 kmnorthofBrooke-1 (Smith & Sutherland, 1991). The Mallowa Salt now forms a broad salt arch in the north-western Willara Sub- basin and similar structures occur elsewhere in the Canning Basin (Fig. 3 ; Wells, 1980).

The Mallowa Salt, formerly termed the ‘Carribuddy Formation, Unit B’, was redefined by Lehmann (1984) and Middleton (1990). The presence of obligate, tetrahedral tetrads, assigned to Tetrahedraletes medi- nensis Strother & Traverse 1979, in drill core of the Mallowa Salt from Gingerah Hill-1 indicates a Late

C A R R I B U D D Y G R O U P

The Carribuddy Group consists of bedded evaporites, mudstones and dolomite that together have a maxi- mum thickness exceeding 3000 m (Fig. 3). It subcrops over an area of about 200 000 km2 within the onshore Canning Basin.

The Carribuddy Group comprises five formations in the Kidson Sub-basin (Middleton, 1990):

Sahara Formation

Mallowa Salt Halite, red brown

Red brown dolomite and (top) siltstone

mudstone, minor anhydrite and dolomite

siltstone

and dolomite

Nibil Formation Mudstone, dolomite and

Minjoo Salt

Bongabinni Red brown mudstone and

Halite, red brown mudstone

Formation (base) dolomite

In Brooke-1 the Mallowa Salt is unconformably overlain by an extensive unit of mottled mudstones

986 D . L. Cathro, J . K . WarrenandG. E. Williams

and anhydrite of probable Devonian age (Foster & Williams, 1991). The top of the rock salt occurs at 977 m depth and the formation continues to 1712 m where it passes into an argillaceous unit assignable to the Nibil Formation. Continuous drill core of the Mallowa Salt in Brooke-1 was obtained from the top of the formation at 977m to 1652m depth. As the sequence is flat lying, depths provide stratigraphic thicknesses. Core quality was excellent because an oil- based drilling mud was used and 100% core recovery was achieved. The 675 m of rock salt drill core obtained therefore spans most of the Mallowa Salt, which, based on wireline logs and seismic data, is up to 800 m thick in the Willara Sub-basin. The Brooke- 1 core is available for study and is now held in the core library facilities of the South Australian Depart- ment of Mines and Energy, Adelaide.

MALLOWA SALT

Halite-rich intervals of the Mallowa Salt were defined in Brooke-I using gamma ray and density logs. Core between 948 and 1475 m depth was then logged in detail to cover the top two-thirds of the Mallowa Salt and its contact with the overlying mottled mudstone and anhydrite. Particular attention was given to textural variations; core was back-lit on a specially constructed light table to highlight textural features and sedimentary cycle boundaries. Areas of textural interest were thin sectioned and bromine X-ray fluorescence analyses run to define any changes in brine chemistry during deposition.

The Mallowa Salt consists of several hundred stacked evaporite cycles of halite and terrigenous mudstone (red, green and less commonly grey and mottled), with lesser interbedded anhydrite and dolomite mudstone. Cycles are of two types, Type 1 and Type 2, with the bulk of the Mallowa Salt composed of repeatedly stacked Type 2 cycles. Meas- ured sections of stacked Type 1 and Type 2 cycles are shown in Fig. 4 and idealized Type 1 and Type 2 cycles are given in Fig. 5. Commonly, an incomplete cycle is terminated by a dissolution or karstic surface on the halite, indicating that less saline water came into contact with the deposited halite.

*Type 1 cycles consist of: terrigenous mudstone (top) banded halite with a variable, upwardly increasing clay content clear, coarsely crystalline halite massive to laminated anhydrite dolomite mudstone (base).

Type 1 cycles (Fig. 4) are generally 2-15 m thick and make up <5% of the cored sequence. The vertical sequence in a Type 1 cycle approximates the mineral sequence predicted from the theoretical evaporation of seawater, except that aragonite and gypsum, not dolomite and anhydrite, are the typical evaporite minerals produced by evaporating modern seawater (Usiglio, 1849). Type I cycles are seen best in the Mallowa Salt in Brooke-] at 1181-1196.5 m and 1330.25-1332 m depth. The dolomite and anhydrite at the base of these intervals may be correlated with the main horizons of anhydrite and minor dolomite at 1136-1 137 and 1306-1308 m depth in Gingerah Hill- 1 some 70 km to the west-northwest (Williams, 1991). This implies that Type 1 cycles are of regional extent. Dolomite and nodular anhydrite at 971.75-975.5 m depth in Brooke-1 immediately above the Mallowa Salt apparently are of different origin from the beds of anhydrite and dolomite at the base of Type 1 cycles, the former being related to the unconformity surface that developed on the Mallowa Salt during the Devonian.

.Type 2 cycles consist of: terrigenous mudstone (top) banded halite with a variable, upwardly increasing clay content clear, coarsely crystalline halite (base).

Type 2 cycles (Fig. 4) are generally 1-4 m thick. They are essentially incomplete Type 1 cycles in that they lack basal dolomite and anhydrite beds. These mineralogically simpler Type 2 cycles, composed mostly of halite and muddy halite, are dominant (95% of the Mallowa Salt in Brooke-1) and form multiple stacked cycles between the Type 1 cycles. The 675 m of cored Mallowa Salt in Brooke-1 contains several hundred Type 2 cycles, although many are incomplete with basal clear halite the most commonly absent facies.

Dolomite in Type 1 cycles

Two different types of fine grained, cream, tan, to dark grey-green dolomite mudstone beds (0.5-2.5 m thick) occur at the base of Type 1 cycles.

(1) Massive to poorly banded, soft dolomitic mud- stone (Fig. 6a) with local veins of halite; vein halite also occurs in the terrigenous mudstone beds.

Halite saltern in the Cunning Basin, NW Australia

LITHOLOGY TEXTURE LITHOLOGY TEXTURE

987

Mudstone

Anhydrite

Dolomite

p s e , c l e a r

J-~- Halite with minor

0 Mud clasts

Banded

g Vein halite

8 Upward aligned growth

Coarse, cleai O0 mosaic haiie

P Primary halite

D Displacive halite in mudstone Coarse, mosaic halite with minor impurities - KarsUdissolution surface

Incomplete Type 2 2 sequences

Fig. 4. Measured sections from Brooke-1 showing Type 1 and Type 2 cycles in core. The left column is from 1172 to 1198 m depth, the right column from 1242 to 1266 m depth.

(2) Massive hard dolomite mudstone with a conchoi- dal fracture; this typically grades down into soft dolomitic mudstone.

Both types of dolomite are micritic and typically associated with dark amorphous organic material (Fig. 6b) probably derived from cyanobacterial bind- ers. Some dolomite (including the dolomite at the base of the mottled sediments overlying the Mallowa Salt) retains a pelletoidal texture. The pellets are com- pressed and deformed but show no evidence of fracturing. Organic and clay mineral inclusions con-

centrate via microstylolitization to form wispy and ‘horse-tail’ laminae in the dolomite.

The dolomite probably formed early by brine reflux through pelletoidal limestones, while still somewhat unconsolidated, in a restricted hypersaline environ- ment. It was later subjected to compaction and microstylolitization. Dolomite is a syndepositional replacement mineral in many evaporitic environ- ments, both lacustrine and restricted marine (Smoot & Lowenstein, 1991; Warren, 1991). The lack of recognizable marine fossils does not necessarily imply a non-marine origin for the solutions that formed this

988 D . L. Cathro, J . K . Warren and G. E. Williams

Colou; banded micrite in (sometimes massive) dolomitic matrix

Fig.5. Idealized Type 1 and Type 2 cycles based on sequences in the Mallowa Salt from Brooke-I.

unit. Rather, it may reflect the high degree of continentality* of this type of evaporite deposition (see later discussion of overall depositional setting). It is also possible that some of the dolomite layers are residues left behind after meteoric and marine dissolution of substantial thicknesses of halite.

Anhydrite in Type I cycles

Anhydrite is the only sulphate mineral observed within the Mallowa Salt in Brooke-I, constitutingonly about 1% of the formation. The anhydrite lacks non- evaporite impurities other than very minor amounts of clay, indicating that the edges of the brine pool were distant at the time the sulphate beds were deposited. Typically, the anhydrite occurs as beds less

*Continentality is an informal term used to describe evaporite settings that may be fed by marine waters but are almost completely surrounded by a continental landmass. Much of the surface and pore waters in such systems are derived from a marine source, but as the sedimentary system is almost completely land-locked there is also an influence of continental waters especially in those parts of the system far removed from the open ocean.

than 4 cm thick. At some levels in the core the beds are laminated or interbedded with clean halite or form layers encasing halite pseudomorphs after aligned gypsum. Each layer of pseudomorphs originates from a common horizontal substrate (Fig. 7a,c).

Although now composed of halite, the pseudo- morphs are recognizable as bottom-nucleated, growth- aligned gypsum prisms. The shape indicates former gypsum prisms that were precipitated with near vertical c-axes and with terminating { I 1 1 } and {Ol 1) faces. Each pseudomorph is now centrally filled with halite, and rimmed with anhydrite laths that are arranged perpendicular and inward to the original gypsum crystal face (Fig. 7d). The anhydrite matrix surrounding the pseudomorphs shows both rosette- felted and aligned-felted textures (terminology of Maiklem et al., 1969). Some matrix anhydrite, away from the halite pseudomorphs, locally displays ‘flow- aligned’ textures in thin section; this is the result perhaps of transformation of matrix gypsum to anhydrite during the first 500 m of burial. A similar burial dewatering mechanism for the formation of ‘flow’ anhydrite in the Devonian of Canada was proposed by Shearman & Fuller (1969).

Analogous Holocene upward-aligned gypsum crys- tals typically grow in shallow subaqueous environ- ments with water depths of less than 10 m (e.g. Lake MacDonnell and Marion Lake, South Australia; Warren, 1982). The development of upward-aligned subaqueous crystals requires that for at least part of any year the gypsum-supersaturated conditions must dominate from the brine surface down to the brine pool floor, a condition that is most likely to occur in shallow brine pools. No shoaling sabkha sequence was identified in the anhydrite beds of the Mallowa Salt. The presence of relic gypsum ‘ghosts’ in the beds, the purity of the beds, and the typically laminated to layered nature of the anhydrite, as opposed to the enterolithically folded nature of sabkha anhydrite, imply that the bulk of the anhydrite formed during burial diagenesis as a replacement of subaqueous gypsum (see Warren & Kendall, 1985).

Halite cloning of subaqueous gypsum suggests also that replacement took place during early diagenesis when the brine flow paths in the gypsum bed were still relatively open, probably at depths of less than a few tens of metres (Shearman, 1985; Hovorka, 1988; Warren, 1989, 1991). Pseudomorphing by halite happened prior to, or simultaneously with, anhydriti- zation of the gypsum. At these shallow depths the gypsum sediments retained sufficient porosity to allow substantial reflux and replacement by halite-saturated

Halite saltern in the Canning Basin, N W Australia 989

Fig. 6 Dolomite in the Mallowa Salt. (a) Dolomitic mudstone above halite, 11814-1181.12 m depth. Up is toward top of photograph. (b) Photomicrograph of the dolomitic mudstone in (a), showing pelletoidal micrite and finely disaggregated dark organic material.

brines. Textures indicate that replacement of the larger gypsum crystals occurred before the remaining matrix gypsum was converted to nodular anhydrite. Halite pseudomorphs after gypsum are common in ancient sulphate beds intercalated with halite (Schaller & Henderson, 1932; Stewart, 1949; Jones, 1965; Holdoway, 1978 ; Hovorka, 1987). Hovorka (1988) has shown how halite-saturated brines flushing through a gypsum bed can dissolve the larger gypsum crystals and replace them with halite during shallow burial. One for one halite replacement of medium to coarse grained gypsum crystals was observed in the modern saline mudflat sediments of Salt Flat Playa, West Texas, at depths of less than 1 m in those areas subject to brine reflux and capillary evaporation (Hussain & Warren, 1989).

A tepee structured pressure ridge in a layer with halite pseudomorphs after subaqueous gypsum sur- vives within a stratiform anhydrite bed at 1195 m depth (Fig. 4). The ridge has an annealed crest (Fig. 7a). The pressure ridge probably formed by syndepositional crystal expansion as the upward growth of aligned gypsum pavement took place. Gypsum crystals do not start growing with an aligned texture. Rather, alignment occurs when crystals with their c-axes aligned subvertically (fastest growth

direction) begin to dominate the growth fabric. These fast growing crystals block off the growth of crystals whose c-axes are not subvertical (Warren, 1982). It is this blocking of sideways growing crystals that creates much of the lateral stress responsible for the pressure ridges. Modern gypsum tepees and pressure ridges can be found in both subaerial and subaqueous environments, but as the ridge crest in the core was at least in part healed, a subaqueous environment of formation is favoured. Gypsum pressure ridges, growing today around the subaerial edges of the gypsum ponds in the ICI salt works, St Kilda, South Australia, tend to show broken crests. Ridges forming in the permanently flooded portions of the same brine pans have reannealed crests or have ridges which have never been broken and overthrust into tepees. Similar reannealed tepees and gypsum pressure ridges are preserved within the Holocene gypsum sequences in Salt Lake on Kangaroo Island, South Australia (Warren, 1982).

Halite

The rock salt in both Type 1 and Type 2 cycles typically comprises a mosaic of large, randomly orientated, interlocking halite crystals that formed

990 D . L. Cathro, J . K . Warren and G . E. Williams

Fig. 7. (a) Clear, coarsely crystalline halite overlying a tepee structure, 1195.13-1195.26 m depth. The tepee occurs in an anhydrite unit where some original growth-aligned gypsum crystals are preserved as halite pseudomorphs. (b) Subvertical karst feature (arrows) filled with mud and halite (right), 1439.35-1439.54 m depth. (c) Laminated anhydrite interbedded with halite-filled pseudomorphs after gypsum, 1195.72-1 195.98 m depth. Top of sample shows sharp boundary with clear, coarsely crystalline halite. (d) Photomicrograph of halite-filled pseudomorph after growth-aligned gypsum, 1195.75-1 195.84 m depth. The pseudomorph is rimmed by anhydrite laths. Crossed polars.

during diagenesis and only partially preserve primary sedimentary features. The term ‘primary halite’ is used here fer any relict growth-aligned textures; much of the material termed primary halite has undergone some degree of early dissolution, cementation and recrystallization (see Lowenstein & Hardie, 1985). The complex spectrum of textures in the cored halite reflects the high solubility and reactivity of halite that occurs from the time a halite crystal first precipitates.

Halite in the Mallowa Salt occurs as four varieties, each with a preferred stratigraphic position within the cycles.

Clear, coarsely crystalline halite

This usually occurs as beds, 2-20 cm in thickness, at or near the base of the vertically elongate halite in a cycle. Beds display coarse, mosaic textures. The boundary between this coarsely crystalline halite and underlying mudstone or anhydrite is always very sharp whereas the passage up into the overlying colour-

banded halite is transitional. The beds of clear, coarsely crystalline halite show no obvious primary sedimentary features (Figs 7a & 8c) but they are cross- cut by syndepositional dissolution features implying the mosaic fabric was early (see later discussion). This lack of primary texture is puzzling. The coarsely crystalline halite may be either a diagenetic overprint (replacement) on finer grained halite cumulates* or a cement that infilled large voids created by karstifica- tion.

Colour-banded halite with minor clay

This variety typically occurs in the middle of the halite portion of a cycle and is common throughout most of

*Cumulate is a term borrowed by Lowenstein & Hardie (1985) from igneous petrology. In evaporite studies it is used to define halite crystals that precipitated from a brine, most commonly at the air-brine interface, which only later settle to the bottom of the brine pool under the effects of gravity (Handford, 1991).

Halite saltern in the Canning Basin, N W Australia 99 1

Fig. 8. Halite in drill core of the Mallowa Salt. (a) Transition upward from displacive halite to a fractured, red and green mottled, terrigenous mudstone that may be infilling a vase-shaped microkarst pit; 1012.33- 1012.71 m depth. (b) Displacive halite in a highly disrupted terrigenous mud matrix, 1017.10-1017.41 m depth. (c) Clear, coarsely crystalline mosaic halite above a terrigenous mudstone containing red fibrous vein halite, 1197.20-1 197.54 m depth. (d) Colour-banded halite showing inclined bedding that may reflect proximity to the edge of a salt polygon, 1417.00-1417.21 m depth. (e) Same core as in (d), backlit to show improved definition of banding. Upward-aligned crystal growth (1) and randomly interlocking crystals (2) can be seen. Pipes of clear halite cross- cut the depositional banding (3). Clear, coarsely crystalline halite (4) caps the specimen. (f) Thin section of colour-banded halite core shown in (d) and (e). showing zones of vertically elongate halite, upward-aligned chevrons in traceable horizons (boxes) and random mosaic halite (2) infilling solution pits; 1417.02-1417.10 m depth.

992 D . L. Cathro, J . K . Warren and G . E. Williams

the halite in the Mallowa Salt in Brooke-1. Beds composed of alternating bands of colourless, red and brown halite generally range in thickness from 0.5 to 50 cm. The colours result from different amounts of clay and other impurities, including iron oxides. The banding commonly is cross-cut by clear, coarsely crystalline halite. Colour banding defines depositional bedding in this facies (Fig. 8d,e) and ranges from horizontal to inclined. The inclined bands represent either low angle collapse into syndepositional karst features, or the inclined margins of halite pressure ridges and tepees.

Crystal aggregates within the bands are either vertically elongate crystals with locally preserved primary chevrons (Fig. 8d-f) or coarse, randomly interlocking crystalline mosaics. Vertically elongate halite crystals indicate original precipitation on the subaqueous floor of a shallow brine pool that was less than 1 or 2 m deep (Lowenstein & Hardie, 1985). Colour-banded halite that lacks vertically elongate crystals in Permian evaporites of West Texas has been interpreted as reflecting grain enlargement and recrys- tallization of original cumulate halite crystals (Ho- vorka, 1987).

Interlocking coarse crystal mosaics (Figs 8e,f & 9a,b) make up the bulk of the colour-banded halite and truncate many of the vertically elongate textures. An unequivocal interpretation of the precursor to the mosaic texture requires recognition of relict primary textures within the larger recrystallized and cemented crystal mosaics. Hovorka (1987) recognized relic halite rafts in coarse crystalline mosaics in the Permian evaporites of West Texas. No such relic feature was observed in the crystal mosaics in colour-banded salt from Brooke-]. The 'foam' textures in Brook-1 halite, and the triple enfacial junctions with angles approxi- mating 120", show that the crystals are diagenetic (see Spencer & Lowenstein, 1989), but the precursor morphology before diagenesis is uncertain.

The timing of the recrystallization and grain enlargement that formed the mosaics is also uncertain, but based on Holocene analogues it is probable that much of the coarsely crystalline halite formed soon after deposition. Arakel (1988, his fig. lob) shows matching textures in diagenetically altered clastic halite in the uppermost Holocene sediments of Hutt Lagoon, Western Australia. Casas & Lowenstein (1989) also illustrate identical mosaic textures in various Pleistocene/Holocene halites (their figs 4 ,9 & 14).

The colour-banded halite unit contains some layers with recognizable vertically elongate halite. Such

layers are uncommon and only faintly visible in hand specimens of core. Elongate crystals were more obvious when 1 cm thick slabs were backlit. Vertically orientated crystals are most obvious in highly coloured and distinctly banded halite (Fig. 8e,f), especially where the vertical crystals are terminated by an overlying muddy layer. Most backlit halite showing vertically elongate growth does not contain inclusion- rich chevrons or coronets in thin section (Fig. 8f); the thin sections reveal mostly inclusion-free elongate halite. Relic primary chevrons can be found in these zones of elongate halite but make up no more than 10% of any thin section.

Aligned inclusion-rich chevrons occur typically as distinct traceable subhorizontal layers (Fig. 8f). In most chevrons, the shape of individual crystals is emphasized by the internal alternation of cloudy fluid inclusion-rich zones and clear inclusion-poor halite zones (Fig. 9c,d). Most vertically elongate halite crystals lack fluid inclusions and so are not true chevrons* yet still exhibit a near vertical growth alignment. Relic crystal growth faces are bordered by grain residues of dust and other detrital material that probably originally formed fine layers on the crystal faces.

Three types of halite with a depositional texture persist in the core.

(1) The halite that is most obviously primary and least affected by diagenesis (replacement and cemen- tation) consists of internally zoned crystals with distinct vertical growth alignment (height to width ratio of approximately 3 : 1 ; Fig. 8f). Similar chevron halite is found in modern shallow brine pools where it forms as a bottom-nucleated precipitate (Arthurton, 1973). The rapid fluctuation in growth rate needed to produce internal zonation implies growth took place in extremely shallow but subaqueous conditions (Roedder, 1982; Lowenstein & Hardie, 1985). Some of the chevron layers are cross-cut by sharp truncation surfaces (Fig. 8f) that plane off individual halite chevrons (Fig. 9c).

(2) The second variety is halite displaying distinct subvertical growth in core, but with few or no internal fluid inclusions to define chevrons in thin section (Fig. 8d-f). In this type of halite the crystals were originally outlined by fine detrital material. Fluid inclusions may have been present but migrated to the crystal margins during diagenesis and dewatering. Migration of inclusions has not been observed in

*The AGI glossary defines chevrons as halite with trapped solid or fluid inclusions.

Halite saltern in the Canning Basin, NW Australia 993

Fig. 9. Halite in thin section. (a) Mosaic of large, randomly interlocking halite crystals, 1374.02-1374.10 m depth. (b) Mosaic halite, 1417.02-1417.10 m depth. (c. d) Chevron halite within vertically aligned halite, 1417.02-1417.10 m depth. Note truncation of chevrons at top of photomicrograph in (c). Up is toward top of photomicrographs. Scale bar applies to all photomicrographs.

bedded halite but is common in halokinetic halite. Alternatively, and perhaps more probably, much of the subvertically aligned primary halite may have precipitated without abundant inclusions. Inclusion- free vertically elongate halite constitutes much of the primary halite now forming in Hutt Lagoon (Arakel, 1988; his fig. 7). The mantling of the crystals may reflect the settling of aeolian dust through the water

column or a mantling of the crystals by mucilage from cyanobacterial or halobacterial plexi.

(3) The third variety has a subvertical alignment still faintly distinguishable in some portions of a layer when back-lit. Crystals have ragged irregular shapes so that the original crystal orientation is not readily conspicuous (Fig. 9a). Chevrons defined by inclusions are not evident in thin sections of this type of halite.

994 D . L. Cathro. J . K . Warren and G . E. Williams

This is the most enigmatic type of colour-banded halite that still contains recognizable remnants of its depositional texture. Where transitional into clear, coarsely crystalline halite the boundary between the two units cannot be defined, as both are dominated by coarse, randomly interlocking crystal mosaics. Similar inclusion-poor ragged halite is a common facies in much of the halite from the Middle Permian San Andres Formation of West Texas (Fracasso & Ho- vorka, 1986).

All three types of colour-banded halite can be truncated by sharp subhorizontal erosion surfaces (Figs 8f & 9c) and cross-cut by subvertical clay- or halite-filled fissures (Fig. 8a,e). The depositional sig- nificance of these cross-cutting structures is discussed further in a following section on dissolution features.

Chaotic muddy halite

This occurs near the top of the halite portion of a cycle. The halite is composed of large (1.0-2.5 cm diameter) anhedral to euhedral aggregates in a structureless red muddy matrix. Terrigenous mud- stones in the Mallowa Salt are rarely halite-free and commonly contain such large crystals (Fig. 8b) or bladed-fibrous red vein halite (described below; Fig. 8c). The large intrasediment halite crystals range from isolated to randomly orientated interlocking crystals. Many of the halite crystals in the mudstone- rich horizons entrained concentric zones of included detrital material to form a skeletal or ‘pagoda’ halite texture (Fig. 8b). Such a texture implies that the halite grew rapidly and displacively just below the surface of an evaporitic mudflat composed of brine-soaked terrigenous sediments (Gornitz & Schreiber, 1981 ; Handford, 1991 ; Smoot & Lowenstein, 1991).

Repeated episodes of wetting and halite dissolution, followed by halite cementation and displacive halite growth, combined to destroy any primary bedding in the terrigenous mudstone matrix. This process of churning and destroying of the original sedimentary structures in the mudstone by halite crystal growth has been described as ‘haloturbation’. Modern ana- logues are the displacive halites in the haloturbated mudflats of Bristol Dry Lake, California (Handford, 1982; Rosen & Warren, 1990).

Vein halite

Vein halite usually occurs within the terrigenous mudstone and dolomitic mudstone interbeds. It is most common in the red terrigenous mudstones

(Fig. 8c) and is constructed of halite fibres a few millimetres across that grew perpendicular to the fracture wall. The red colour is due to clays and iron oxide impurities between the fibres. The fibres probably grew during burial diagenesis as fractures appeared in the mud matrix. The fracturing was probably a response to overpressuring and hydraulic fracturing of the muds during burial dewatering. Entrained pore and structural waters in the clays were unable to escape easily via halite units, because halite beds are tight and impermeable after only a few tens of metres of burial (see Casas & Lowenstein, 1989). Hovorka (1987) noted similar vein halite in the terrigenous mudstones of the San Andres Formation of West Texas.

Terrigenous mudstones

Red and green terrigenous mudstones with little or no entrained halite characterize the upper portions of both Type 1 and Type 2 cycles. The mudstones are composed of kaolinite, montmorillonite and illite with minor amounts of disseminated dolomite and anhy- drite. Terrigenous mudstones are not important volumetrically within the total Mallowa Salt but are locally important at the top of many cycles. The mudstones usually are massive beds without internal sedimentary stratification. Primary sedimentary struc- tures were probably destroyed by syndepositional cycles and precipitation and dissolution of intrasedi- ment evaporites (haloturbation) perhaps associated with freshening episodes related to the beginning of the next cycle.

A hydrated borate mineral, priceite (4Ca0.5B203.7H,0), was precipitated displacively within some mudstone beds. Its presence was deter- mined by X-ray diffraction analysis and confirmed by boron analyses carried out by Classic Comlabs, Adelaide, using colorimetry (mean of eight analy- ses=75 ppm B, range=30-140ppm B; Table 1). Boron is usually attached to clay minerals and is liberated during diagenesis within a concentrating brine. Hydrated borates typically occur in mudstones deposited in intracontinental lakes fed either by thermal springs or groundwaters in volcanic terrains (Kyle, 1991). All economic accumulations of boron occur in lacustrine sediments. However, hydrated borates can also occur in minor amounts in what are interpreted as marine-associated evaporites (Sonnen- feld, 1984). A firm environmental interpretation of the priceite cannot be made from its identification in the terrigenous mudstones of only one borehole,

Halite saltern in the Canning Basin, NW Australia 995

Table 1. The boron content of mudstones in the Mallowa Salt from Brooke-1.

Depth (m)

1181.00 1194.47* 1196.50 1 196.87 1330.55 133 1.43 1131.90 1 132.00

Boron (ppm)

30 80

140 50 70

110 50 70

*Priceite occurrence.

Brooke-1. Its presence does, however, support the idea of increasing continentality toward the top of each cycle.

Dissolution of halite: pits, karst and erosional surfaces

Microkarst pits (> 1 cm wide and up to I m deep) are common in the clear, coarsely crystalline halite and also cross-cut muddy halite and colour-banded halite units. Some karst pits show a brown, muddy, geopetal lining. The pits are near-vertical dissolution features that cross-cut the halite layers. The exact morphology of the microkarst pits, other than being subvertical and irregular, cannot be determined from drill core. Usually, neither the palaeosurface nor the base of the pit can be seen in the core.

Microkarst pits and erosional surfaces can form in three ways in a shallow to ephemeral brine body.

(1) Subaerial exposure of the halite crust after the water that formed the crust has completely dried up. The water table is then located several centimetres to metres below the crust surface. Dissolution ensues as shallow groundwater and occasional rainwater pene- trate and leach along intercrystalline boundaries that subsequently widen to form pits and pipes (Powers & Hassinger, 1985). In the modern salars of Chile the dissolution features form in the lower parts of small depressions in the halite crust and lead down to the water table, which may be just a few centimetres or several metres below the crust (Stoertz & Ericksen, 1974). This is the likely mechanism that formed most of the pits and all of the larger subvertical karst features observed in the Mallowa Salt (Fig. 7b). Such large dissolution features can only form when the crust is vadose. A subsequent flood of brine into the area covers the pits with a new halite crust and fills them with a clear halite cement.

(2) Alternatively, some of the smaller pits (< 10 cm deep) may reflect repeated dissolution episodes from

early morning dew that widens thermally induced fractures in freshly exposed subaerial salt crust. The fractures are subsequently lined by silt blown in by dust storms. Evaporation of the next flood of storm water produces a new halite crust and fills the pits with a halite cement. This mechanism of dew-induced pit formation was observed in the highly humid brine- soaked coastal flats of A1 Zibaya sabkha east of Abu Dhabi city (J.K.W., personal observation).

(3) Dissolution and partial fractionation of halite about the edges of a halite brine pool due to a flushing by diluted water may also create pits. Unsaturated brines, formed about the edge of a brine pool, can flood and mix with halite-saturated brines and groundwaters all the way out to the centre of an extremely shallow pool (Lowenstein & Hardie, 1985). Their path across and through a bedded halite is marked by dissolution surfaces. However, due to the density-stratified nature of the hydrology of an evaporite basin, this mechanism lacks the gravity drive to form deep pits in subaqueous halite crusts that remain subaqueous (see below).

Narrower vertical dissolution features 0.5-1 cm wide, called pipes, also cross-cut the horizontal bedding surfaces of much of the colour-banded halite and have been filled with the clear, coarse halite (Fig. 8e). Such pipes are thought to result from dissolution and cementation influenced by fractures (see Hovorka, 1987).

Low salinity waters typically penetrate the vadose portions of a salt crust first along crystal grain boundaries or thermal fractures that then widen to form the karstic pits and pipes. This is not a one-stage process; recurring episodes of dissolution and cement- fill cross-cut and overprint earlier episodes in the salt bed, so destroying much of the original primary aligned texture of the crusts (e.g. uppermost part of Fig. 9c). It leaves behind a complex intermixture of halite cements, recrystallized halite and local areas that preserve the primary subaqueous texture (Fig. 9d).

Warren (1989, p. 63) discusses the schizohaline hydrology of such regions and how it creates a salt bed that, although originally deposited as subaqueous halite, ends up containing only minor volumes of salt with preserved primary textures. Lowenstein & Hardie (1985) stressed that the unequivocal signature of both marine and non-marine ephemeral salt pan deposition is inscribed in halite layers by dissolution events that occur during subaerial exposure and periodic flooding of halite environments by less saline waters. This complex record of dissolution events is repeated

996 D . L. Cathro, J . K . Warren and G . E. Williams

throughout the entire thickness of the salt pan succession in the Mallowa Salt.

Major cross-cutting dissolution processes cannot occur within a deeper perennial brine body. Any fresher water entering the brine pool, i.e. water undersaturated with respect to halite, is less dense than the halite-depositing brines residing in the pore fluids and overlying a recently formed subaqueous halite crust. Being less dense, the fresher water will flow or float out over the top of the halite-saturated brine body. The two water bodies will maintain a density stratification unless the brine pool is extremely shallow and complete mixing can occur. Hence, no halite-dissolving waters can penetrate a subaqueous salt crust. Penetration by freshwater can occur only when the brine pool dries up and the salt crust moves into the vadose zone. For a full discussion of density stratification in marine and non-marine halite basins the reader is referred to Warren (1989) and Handford (1 991).

Table 2. A statistical analysis of the Br content (ppm) for primary and secondary halite in the Mallowa Salt from Brooke-1.

Primary halite Secondary halite

Mean 156.5 146.1 SD 43.51 54.68 SE 4.87 9.24 Variance 1893-3 2989.8 Coefficient of variance 27.80 31.42 n 80 35 Minimum 60 60 Maximum 270 270 Range 210 210 No. missing 0 0 No. <loth% 8 3 10th % 102.5 70 25th % 125 103.7 50th % 150 140 75th % 187.5 170 90th % 220 220 No. >90th % 5 3 Kurtosis -0.274 -0.177 Skewness 0.40 0.42

Bromine geochemistry

Profiles of the halite Br content for six Type 2 cycles are shown in Fig. 10 and the statistics summarized in Table 2. Up to 29 samples, spaced no more than 40 cm apart, were obtained for each cycle. Whenever

possible, halite crystals displaying remnant aligned textures (interpreted as primary) were sampled, along with adjacent clear mosaic halite crystals (secondary) for comparison. Our initial working assumption was that Br levels of the primary and secondary halites would differ, as secondary halite is either a cement in

60 ' 70 . 80 ' 90 U -1185 I , , , , I , I

90 110 130 150

-1 254 Profile 3

'r -1 258

-1260 E

d0 150 160 260 2iO 280

-1 396 E - -1 400 5 0" -1404

I

Q

v -1408 , , , , , I

160 200 240 280 Br (PPW

-1 31 4 -1 31 6 -1 31 8 -1 320 -1 322

60 100 140 180 220

-1 466

-1468

-1 470

-1 472

I I I

' . a

4 t Fig. 10. Bromine profiles of six Type 2 cycles in the Mallowa Salt from Brooke-1. Solid circles=primary halite, open circles = secondary halite.

Halite saltern in the Canning Basin, N W Australia 997

dissolution voids or a replacement of the primary halite. All halite was hand picked to include a minimum of muddy material, which a previous analysis of the Mallowa Salt from Gingerah Hill-1 had shown to be anomalously high in Br, probably due to the presence of tachyhydrite and carnallite (X- ray diffraction analysis; Williams, 1991). Fluid inclu- sions were not removed from the halite crystals prior to Br analysis. Analyses were carried out by Classic Comlabs Ltd, Adelaide, using X-ray fluorescence.

The irregularity of Br profiles for the halite and the textural evidence of syndepositional karstification of the halite suggest that the depositional environment for the halite ranged from shallow and highly restricted to ephemeral brine pools. Bromine contents of selected primary and secondary halite samples from various cycles range from 60 to 270 ppm, implying a range from marine to highly restricted, enclosed basin conditions. Bromine values cannot be used to deter- mine if an evaporite was deposited under marine or non-marine conditions (Hardie, 1984). Modern non- marine Br values completely overlap with those of seawater. The rapidity of up-section changes in Br does, however, give an indication of water depth and stability of the brine pond (see Dean, 1978; Raup & Hite, 1978).

The Br profiles within individual halite cycles in the Mallowa Salt are irregular, with variation of as much as 150 ppm between adjacent samples < 0.5 m apart (profile 3 at - 1257 m). Bromine values for most of the profiles range typically between 100 and 180 ppm with occasional peaks at up to 270 ppm (Fig. 10). The cycle at - 1016 to - 1021 m has Br values of 60-85 ppm (profile I), and another at - 1255 to - 1261 m has Br values of 100-270 ppm (profile 3). No obvious overall vertical trend is displayed by individual halite cycles, although the lower portions of profiles 3 and 4 appear to show negative trends, and variability increases toward the tops of profiles 3, 4 and 6. The large degree of fluctuation in Br values from halite layers a few tens of centimetres apart suggests halite deposition in highly saline waters that were shallow to ephemeral. If the water body depositing the halite was deeper, the water volume would have buffered the bromine levels and so a more consistent profile and trend would be apparent within the cycles.

Analysis of halite with vertically elongate textures compared to adjacent samples of mosaic halite from the same stratigraphic level gave an initially surprising result: the two types of halite were statistically indistinguishable in terms of their Br content

(Table 2). That is, Br values show no statistically significant difference (r-test) between halites with remnant growth-aligned textures and adjacent halites composed of mosaic crystals.

The statistical similarity of Br signatures for the two types of halite implies that the dissolution/ replacement/cementation of primary halite by mosaic halite was very early and occurred in the shallow subsurface at depths of a few centimetres. This process would generate homogeneous bromine levels in any individual depositional layer. That is, the depositional fluid and the diagenetic fluid was one and the same. However, adjacent layers of crusts less than 0.5m apart can show differences in Br levels of more than 150 ppm, suggesting an environment of syndeposi- tional diagenesis that was also characterized by rapidly fluctuating salinities. The Br chemistry implies that most of the Br re-equilibration in each crust must have occurred very early, soon after each halite layer was deposited and before the deposition of the next major halite bed. Brine flow pathways were thus shut down by pervasive halite cementation and diagenesis only tens of centimetres below the sediment surface. Diagenesis in the halite beds of the Mallowa Salt was essentially complete at depths of a few to tens of centimetres below the crust surface.

Mean Br values of several halite cycles that are separated by tens of metres of strata are significantly different (Fig. 11); such differences probably reflect temporal variation in salinity during evaporite depo- sition within the Willara Sub-basin. Average Br values suggest environments ranging from less saline, to highly restricted very saline brine pools. The Br values of 60-85 ppm for the uppermost cycle at - 1016 to - 1021 m (profile 1) that caps the studied sequence suggest either meteoric water recycling or marine incursion at this stratigraphic level. The high average Br content of 208 ppm for the cycle at - 1396 to - 1406 m (profile 5 ) implies that a laterally equivalent unit may contain potash salts (see Raup & Hite, 1978),

Mean and Standard deviation for each Br

"5 -1200 -2

% -1300 - 4

Br (ppm)

Fig. 11. Mean and standard deviation of Br values for halite from the six profiles sampled.

998 D . L. Cathro, J . K . Warren and G . E. Williams

but no potash seam has yet been reported in the Canning Basin.

Overall, the Br values in the Brooke-1 core are higher than those observed by Glover (1973) for halite in the Mallowa Salt from other wells in the Canning Basin (mean of 28 analyses= 114.3 ppm Br, range = 56-1 86 ppm Br). This supports the hypothesis that Brooke-1 is located in what was one of the more restricted parts of the Canning Basin during the deposition of the Mallowa Salt.

In summary, the statistically distinct average Br values for some cycles and the highly variable Br values within each cycle suggest that there was no pervasive late-stage flushing and bromine re-equilibra- tion of the Mallowa Salt. Rather, most of the Br re- equilibration was apparently early, and differences in Br values were locked in by pervasive halite cemen- tation and replacement at burial depths of no more than 1-2 m. A syndepositional closure of brine flow paths early in the burial history is supported by work in modern halite deposits that shows that much massive Quaternary halite is tight, having lost its porosity by dissolution/cementation episodes after burial to only a few tens of metres (Casas & Lowenstein, 1989). In the Qarhan salt pan in Qaidam Province, China, porosity in the halite is less than 5% at the surface and in all the Quaternary salt lakes studied the porosity in the halite units was zero by 40 m depth.

DISCUSSION AND CONCLUSIONS

The Mallowa Salt was deposited within a saltern, a dominantly subaqueous evaporite water body that at times was marine influenced and at other times almost completley continental (terminology from Warren, 1989, p. 24). Individual depositional episodes shoaled upward into evaporitic mudflat conditions that ter- minated each halite cycle (Fig. 5) . Even at its deepest, the brine pool of the saltern was probably no more than 1-2m deep. Throughout each halite cycle the brine pool was subject to recurrent freshening, desiccation and exposure.

Sedimentary textures and Br signatures imply a very shallow to ephemeral water depositional environ- ment typified by variable salinities and increasing restriction (continentality) toward the top of both types of cycle. Type 2 cycles were formed and stacked within a highly restricted portion of the Canning Basin where occasional recharges of less saline brine (probably marine-derived) freshened the basin brines

sufficiently to produce Type 1 cycles. Somewhat similar depositional conditions prevailed during de- position of the Permian Salado Formation in New Mexico and the Permian San Andres Formation in the Palo Duro Basin area of Texas (see below).

Type 1 and Type 2 cycles both record the progressive evolution of the basin-fill from subaqueous brine pool to ephemeral mudflat (Fig. 5). Similarly, geochemical data from Gingerah Hill-1 show cyclical trends toward more continental terrigenous facies (decreasing con- tent of total salts) following deposition of major beds of anhydrite (Williams, 1991). Dolomites and anhy- drites of Type 1 cycles define periods of greater brine pool freshening, perhaps due to episodic marine incursions in the Willara Sub-basin. Well preserved pseudomorphs of vertically aligned gypsum in the anhydrite indicate deposition was subaqueous. This conclusion is consistent with the regional extent of the thickest anhydrite beds. Possible passageways for seawater into the sub-basin are not well defined due to a paucity of subsurface data. Likely connections to the open sea lay to the east or the north-west of the Willara Sub-basin.

Freshening events producing the basal dolomite and anhydrite of the Type 1 cycles were followed by gradual basin restriction typified by deposition of first saltern, and then evaporitic mudflat halite in both Type 1 and Type 2 cycles. The first halite to be deposited in both types of cycle was a clear, coarsely crystalline unit, possibly originally deposited as clear, porous, mud-free cumulates. Cumulates grow first as hoppers at the air-brine interface to form rafts that continue to float and expand in area until they reach several square centimetres in size. The rafts finally become so large and heavy that they take on water, founder, and sink to the bottom of the brine pool. As the process of pyramidal hopper growth, formation of rafts and sinking continues during evaporation of the brine pool, the bottom builds up layer upon layer of rafts to form a porous salt unit that Lowenstein & Hardie (1985) called ‘immature halite’. This porous cumulate unit was quickly altered to mosaic halite during the subsequent drying/dissolution period of each cycle. Although the environment must have been restricted at the time the cumulates were building up on the subaqueous brine pan floor, the terrigenous edges of the brine body were still distant enough to allow the deposition of halite without a significant detrital influx.

The next unit to be deposited comprised an interlayering of vertically elongate halite formed as growth-aligned subaqueous halite crystals and colour-

Halite saltern in the Canning Basin, NW Australia 999

banded halite that lacked obvious growth-aligned crystals but probably was also deposited subaqueously. Most of the primary aligned halite texture was dissolved, cemented or recrystallized within this unit, probably by syndepositional subaerial exposure and brine flux. Relic layers of inclusion-rich chevrons that survived suggest, however, that the brine pool con- taining the original halite was very shallow, perhaps <<2 m deep. Logan (1987) documented fine grained cumulate halite forming in Lake McLeod, Western Australia, in brine deeper than 2 m while halite crusts precipitated from shallower brines. At times sheet floods or aeolian transport spread thin layers of detrital clay grade material across the Mallowa brine pool.

The presence of intraformational hiati and halite karst tubes cross-cutting the coarsely crystalline halite, vertically elongate halite and colour-banded halite layers, as well as obvious subhorizontal erosional truncation surfaces within all three halite types, show that throughout this time of predominantly sub- aqueous deposition the halite-covered floor of the Willara Sub-basin was also subjected to freshening and dissolution episodes. Many of these episodes were associated with subaerial exposure. Large subvertical karst pits cannot form without a substantial lowering of the saline water table; only salt crust within the vadose zone can undergo such dissolution. If the crust remained phreatic but covered by freshwater then there would be no gravitational head to drive the less dense halite-dissolving waters through the crust. The top of the crusts would thus be planed off by subhorizontal dissolution surfaces down to the level of the halite-saturated porewaters residing in the crust. This bevelling mechanism can be observed today not only in halite crusts but also in most marine and non- marine evaporites subject to a periodic cover by unsaturated waters (Warren, 1982; Smoot & Lowen- stein, 1991).

With progressive drying of the basin during each brine pan cycle, deposition of terrigenous mud became more widespread as increasing amounts of terrigenous material reached the central portion of the sub-basin where Brooke-1 is located. This led to the deposition of red siliciclastic mudstone (Fig. 5). In contrast to the green colour of the thin clay layers in the underlying halite units that were deposited under saline reducing conditions, the red colours of the terrigenous muds imply that oxidizing conditions dominated during deposition of the terrigenous mudstone units that cap both types of cycle. Displacive halite crystals grew in this muddy matrix within brine-soaked sediments just below the mudflat surface to form haloturbated

mudstones. The environment was a subaerial-domi- nated evaporitic mudflat, that at times was covered by thin ephemeral sheets of brine. These brine sheets were probably no more than a few centimetres deep and blown about the mudflat by wind action. Such wind-blown brine sheets can be seen today on the saline mudflats of many modern playas such as Bristol Dry Lake in California and Lake Frome and the Holocene coastal salinas of South Australia.

The much greater total volume of Type 2 compared to Type 1 cycles in the Mallowa Salt indicates that a highly restricted shallow to ephemeral salt pan environment was dominant at the locality of Brooke- 1. For most of its depositional history the area was far removed from any influx areas of less saline CaS0,- depositing brines. This is supported by the generally high Br values in most of the sedimentary cycles studied in Brooke-1.

Comparison with other ancient halite cycles

Cyclicity in the Mallowa Salt is akin to the barred marginal marine to salt pan cyclicity in halites in the McNutt Zone of the Permian Salado Formation, New Mexico (Lowenstein, 1988), and the halites of the San Andres Formation in the Palo Duro Basin, Texas (Hovorka, 1987), where it records salinity variations in a highly restricted environment. However, there are also some important differences (Table 3).

Table 3. A comparison of cyclicity in the Late Ordovician to Early Silurian Mallowa Salt with that in the Permian halites of West Texas and New Mexico (see Lowenstein, 1988).

Similarities

0 Near identical halite textures in stacked cycles composed of saltern halite passing up into evaporitic mudflat deposits

marked by carbonate and sulphate beds

bodies subject to desiccation

diagenetic events

Rare cycles probably recording marine spillover events

0 Water depths rarely exceeded a few metres and brine

Primary halite textures grossly overprinted by early

Differences

Anhydrite common in West Texas, uncommon in Mallowa Salt

0 Polyhalite and magnesite common in West Texas, not observed in Mallowa Salt in Brooke-1 and Gingerah Hill-1

Mallowa Salt 0 Elevated boron levels and occurrence of priceite in

1000 D . L. Cathro, J . K . Warren and G . E. Williams

The McNutt Zone containsseveral economic potash horizons, but to date no potash seam has been reported in wells that intersect the Mallowa Salt (see below). Lowenstein (1 988) distinguished between two types of cycle, Type I and Type 11, in the McNutt Zone of the Salado Formation.

Type I cycles consist of: halite with mud (top) halite anhydrite-polyhalite mixed siliciclastic and carbonate (magnesite) mud- stone (base).

Type I1 cycles are essentially incomplete Type I cycles and consist of:

muddy halite (top) halite (base).

Type I1 cycles are typically thinner than the more complete Type I cycles, but Type I1 are volumetrically dominant. The succession of facies in Type I and Type I1 cycles in the Salado Formation compares well with that of Type 1 and Type 2 cycles in the Mallowa Salt at Brooke-1. However, in the McNutt Zone, the carbonate in the cycles is magnesite within the potash zone, where extremely arid conditions existed during deposition, and dolomite outside the potash zone. Polyhalite was a common diagenetic precipitate. The carbonate in Brooke-1 is dolomite and polyhalite is absent from the sampled sequence. Ignoring these mineral differences, the bulk of the Mallowa Salt in Brooke-1 has close resemblances to the Type I1 cycles. Type 2 cycles could be termed stacked Type I1 cycles with similar thicknesses, textures and stacking to those seen in the Salado Formation*. Similarly, the Type 1 cycles in Brooke-I are comparable to the Type I cycles of Lowenstein (1988).

The differences in mineralogy, the B levels, and the presence of Br-rich brines not associated with potash salts in Brooke-I do require explanation (Table 3). Of course the problem cannot be completely solved by studying the sedimentology and geochemistry of halite in one well. Moreover, little is known concerning the composition of contemporaneous evaporites in the adjoining, structurally deeper Kidson Sub-basin. Elevated B levels and a lack of potash-associated minerals in the Mallowa Salt in Brooke-1 probably indicate a brine chemistry that was not completely marine. It may well be that much of the brine

*The mineralogical differences between cycles in the Mallowa Salt and Salado Formation were the main reason for naming the Mallowa cycles Types 1 and 2, rather than Types I and 11.

depositing the halite was originally marine-derived but became mixed with potash-depleted and B- enriched continental waters in the restricted setting of Brooke-1. This is perhaps why the priceite occurs in the red terrigenous mud facies. At the time the terrigenous muds were deposited on top of each cycle, the Brooke-1 location was probably many hundreds of kilometers from the open ocean. If so, the question to be answered is where did such brines come from? Alternatively, was there a climatic, topographic or hydrological control to the evolution of the brines that could explain the lack of potassic salts in Brooke-I? The area does contain halites characterized by Br values that indicate evolved brines; according to the literature (e.g. Baar, 1966; Holser, 1966; Anderle et al., 1979) the brines should have been capable of depositing potash minerals. These questions cannot be answered until the character of the Mallowa Salt over the entire Canning Basin, including the Kidson Sub-basin, is known.

Most modern evaporite deposits in areas of low relief, even those in highly restricted continental basins such as the Rann of Kutch and the interdunal mudflats of Abu Dhabi, rarely preserve primary crusts containing minerals more saline than halite. The only modern area where carnallite is perhaps a primary precipitate is Lake Qarhan, a continental playa hundreds of kilometres from the coast in the high relief rain-shadow region of Qaidam Province, China. Even there, the potash-rich brines come from the dissolution of the uplifted deposits of an earlier Plio- Miocene potash lake that formed as a highly restricted depression in a transform belt (Kezoa & Bowler, 1985).

Lowenstein & Hardie (1985) have clearly shown that most Phanerozoic potassic salts are syndiagenetic or diagenetic in origin. Even in Lake Qarhan most of the potash is replacive. Precipitation of widespread potash minerals is intimately associated with the alteration and replacement of earlier bedded deposits of less saline evaporite minerals such as gypsum and halite. To form widespread potassic salts the K-rich brines must be able to flush through the precursor halite or gypsum. The Br signatures in the Mallowa Salt are high, and those of the vertically elongate primary halite and adjacent mosaic halite in the same layer are indistinguishable. However, Br values can vary by as much as 150 ppm over a depth of less than 50cm (profile 3, Fig. 10). Taken together, these characteristics imply a syndepositional closure of brine flow paths very early in the burial history (see earlier). It may well be that this cementation and

Halite saltern in the Canning Basin, N W Australia 1001

replacement completely shut down the hydrology in the area of Brooke-1 so early and so pervasively that if any brines capable of forming potash replacements were present they simply could not enter the halite unit.

The cyclicity in the Mallowa Salt, as indicated by spectral analysis of stratigraphic geochemical data from Gingerah Hill-1 (Fig. l), suggests climatic oscillations forced by orbital cycles (Williams, 1991). The spectral data imply a precession-eccentricity- dominated pattern, which accords with the low palaeolatitude (< 15") of north-western Australia in Late Ordovician to Early Silurian time (see Embleton, 1984; Scotese & McKerrow, 1990). As discussed by Williams (1991), deposition of the Mallowa Salt was coeval with the Late Ordovician to Early Siluran circum-polar glaciation of north Africa, Europe and North America; hence the cyclicity of halite deposi- tion, particularly the deposition of Type 1 cycles, may partly reflect glacio-eustatic changes in sea level. Anderson (1982a,b) also recognized orbitally forced influences on evaporite cycles in the Permian evapor- ites of the Castile Formation, West Texas.

ACKNOWLEDGMENTS

This paper is based on an NCPGG BSc (Hons) study by D.L.C. with the supervision and collaboration of J.K.W. and G.E.W. We thank BHP Minerals' Exploration Department for supporting this research and permitting publication of the results. Thanks go to Marc Davies, Dave Russell and Dave Brisbane for valuable geological and technical support during the drilling of Brooke-1 and to Vic Gostin for discussion. Thanks also go to Tina Lindemann for computer plotting and to Sherry Proferes for her drafting skills. Comprehensive reviews by Sue Hovorka and Tim Lowenstein greatly improved the paper.

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(Manuscript received 3 April 1991 ; revision received 29 June 1992)