Journal of the Royal Society of Western Australia, 76: 13-23,1993 Stratigraphy of the Lefroy and Cowan palaeodrainages. Western Australia J D A Clarke Western Mining Corporation, P.O. Box 157 Preston, Victoria 3072, Australia [communicated by J R Gozzard] Manuscript received July 1992; accepted November 1992 Abstract This paper revises the stratigraphy of the Lefroy and Cowan palaeodrainages, and compares it with that of the Eucla and Bremer Basins. The definition of the Eundynie Group is modified, and the terms Redmine Group, Revenge Formation, Gamma Island Formation, Polar Bear Formation, Roysalt Formation, and Beta Island Member are proposed. The Eocene palaeodrainage succession is placed in the Eundynie Group, and the post-Eocene succession in the Redmine Group. The Eundynie Group in the Lefroy Palaeodrainage consists of the non-marine to marginal marine Hampton Sandstone and Pidinga Formation, and Princess Royal Spongolite. In the Cowan Palaeodrainage, the Eundynie Group consists of the Werillup Formation, Norseman Limestone, and Princess Royal Spongolite. The Redmine Group consists principally of the clastic Revenge Formation. Carbonate units within the Redmine Group comprise the Cowan Dolomite in Lake Cowan (formerly placed in the Eundynie Group) and the Gamma Island Formation in Lake Lefroy. These formations are of probable Miocene age. Pliocene and younger evaporites are named the Roysalt and Polar Bear Formations in Lakes Lefroy and Cowan respectively. Gypsum dunes on Lake Lefroy comprise the Beta Island Member of the Roysalt Formation. Introduction Regional Setting Chains of linear playa lakes are a prominent physiogra¬ Lakes Lefroy, Cowan, and Dundas occur in the south¬ phic feature of the Kambalda and Norseman area (Fig 1). eastern part of the Archaean Yilgarn Craton, and are within The subject of investigation since Woodward (1897), these the palaeodrainage network defined by Bunting et al. (1974) lake chains are interpreted as the remnants of palaeodrain¬ and Van de Graaff et al. (1977). Lake Lefroy occurs within age systems as first proposed by Gibson (1909). Little the Lefroy palaeodrainage, and Lakes Cowan and Dundas published data exists on the sediments in the palaeodrain¬ occur in the Cowan palaeodrainage. The bedrock surface ages. beneath the lake surface defines a V-shaped palaeovalley (Figure 2). Recent drilling by Western Mining Corporation (WMC) during exploration for Archaean gold and nickel deposits has provided data on the stratigraphy of the Lefroy and Stratigraphy Cowan palaeodrainages. These data allow the resolution of Eundynie Group the relationship between the marginal stratigraphy of the Eucla and Bremer Basins. The presence of Tertiary marine sediments in the Norse¬ man and Lake Dundas region has been recognised since The aim of this paper is to describe the stratigraphy of early this century (Campbell 1906). They have not previ¬ Lakes Lefroy, Cowan, and Dundas and their adjacent areas; ously been reported from beneath Lake" Lefroy, although revise the definition of the Eundynie Group (Cockbain they are known from further east (Griffin 1989). The only 1968a); introduce the new terms Redmine Group, Revenge, outcrops of marine sediments in Lake Lefroy occur near Gamma Island, Polar Bear and Roysalt Formations, and Loves Find (Locality 7, Figure 1). Beta Island Member; and clarify stratigraphic relationships along the boundary of the Eucla and Bremer Basins. The Eocene carbonates and spongolites in Lake Cowan were sedimentary facies and evolution of the palaeodrainage fill termed the Eundynie Group (Figure 3) by Cockbain are the subject of a separate paper (Clarke in press). (1968a). The usage is here expanded to include all Eocene sediments in palaeodrainages marginal to the Eucla and The paper is based of geomorphological mapping the Bremer Basins. The Eundynie Group is thus laterally Lake Lefroy area, logging of percussion diamond and equivalent to the Eucla Group (redefined by Benbow et al., air-core drill holes, and petrography. Palynological analy¬ in press) of the Eucla Basin, and the Plantagenet Group sis of organic rich samples provided biostratigraphic con¬ (Cockbain 1968b) of the Bremer Basin. Some of the forma¬ trol. tions extend laterally from one group to another. Thus, the Princess Royal Spongolite, Pidinga Formation and Hamp¬ ton Sandstone occur in the Eucla and Eundynie Groups, and the Werillup Formation occurs in both the Plantagenet ® Royal Society of Western Australia 1993 and Eundynie Groups. 13 Journal of the Royal Society of Western Australia, 76 (1), March 1993 Figure 1. Locality map, showing Lakes Lefroy, Cowan and Dundas. 14 Journal of the Royal Society of Western Australia, 76 (1), March 1993 The Eundynie Group has maximum thicknesses of about (1990) extended the Rollos Bore Formation to include all 100 metres in both the Cowan and Lefroy palaeodrainages, similar lithologies in palaeodrainages on the Yilgarn Cra¬ and rests unconformably on Archaean and Proterozoic ton. Kern and Commander (in press) reviewed problems basement. with the Rollos Bore locality as a type section, and proposed that the name Wollubar Sandstone, based on The Cowan Dolomite (Fairbridge 1953) was placed in the work in the Roe and Yindarlgooda palaeodrainages, be Eundynie Group by Cockbain (1968a). Drilling by WMC used in its place. Similar lithologies are present in the however indicates that the unit occurs within the later Rebecca and Raeside palaeodrainages to the northeast Tertiary lacustrine sequence, and is thus part of the (Smyth and Button 1989). overlying Redmine Group. The multiplicity of stratigraphic terms for Eocene palaeodrainage sediments along the margins of the western Pidinga Formation part of the Eucla Basin contrasts with that of the eastern part. In the eastern palaeodrainages, the lignitic marginal Numerous names have been proposed for the Eocene marine to non-marine sediments are all placed into the Sediments in the palaeodrainages of the Yilgarn Craton. Pidinga Formation, and the non-lignitic, predominantly Non marine sediments of Eocene age were named the coarse-grained marginal marine to non-marine sediments Rollos Bore Beds by Balme and Churchill (1959), after the Hampton Sandstone (Benbow et al. in press). This is type locality near Coolgardie. Hocking and Cockbain practice is continued in this paper. 15 L Journal of the Royal Society of Western Australia, 76 (1), March 1993 Figure 3. Stratigraphic correlations for the Eucla Platform, palaeodrainage, and Bremer Basin. In the Lefroy palaeodrainage, the Pidinga Formation The Hampton Sandstone consists of fine to gravelly consists of laminated red-brown to green silts, white, grey quartz sand. The unit is sparsely to moderately fossilifer- or black clays and silts, and lignite. Gravelly, lignitic sand ous; the most common fossils are siliceous sponge spicules. bodies are locally common. Authigenic pyrite is common Opalised gastropods, foraminifera, calcareous sponge spic¬ throughout the sequence. Organic content varies from very ules, together with brachiopod and echinoderm fragments, high in lignites to low in white clays. The organic matter are also present. The greatest thickness of Hampton Sand¬ occurs as dispersed and comminuted fragments, woody stone yet encountered in Lake Lefroy is 24 metres in KD material (most commonly roots, but also stems, seeds, and 3001 (Figure 4). leaves), and lignite. Rare siliceous spicules are sometimes present. Marine fossils are absent from marginal tributaries The Hampton Sandstone overlies and interfingers with such as the Mt Morgan palaeodrainage. Proportions of the Pidinga Formation and Princess Royal Spongolite. It is different lithologies are highly variable. Sedimentary struc¬ most readily differentiated from sands in the Pidinga tures, apart from faint laminations, are rare. Thin interbed- Formation by the abundance of carbonaceous matter in the ded gravels are locally present. Scattered quartz granules latter. Differentiation from the Princess Royal Spongolite is occur throughout the sequence. A palynomorph assem¬ based on the higher sand and lower spicule content of the blage containing 26 species belonging to 12 genera has been Hampton Sandstone. recovered from the Pidinga Formation in Lake Lefroy has been described by Parker (1988a, b) and Harris (1989). Jones (1990) cited palynological and micropalaeontologi- cal data which indicate a Middle to Late Eocene age for the Hampton Sandstone. This age indicates that the formation The Pidinga Formation rests unconformably on is pene-contemporaneous with the Pidinga Formation. The Archaean basement, and interfingers with the Hampton lower part of the Hampton Sandstone is correlated with the Sandstone. In Lake Lefroy the formation reaches a maxi¬ Tortachila transgression and the upper part with the mum known thickness of 60 metres in CD 1916. Aldinga transgression of McGowran (1989). The Pidinga Formation interfingers with the Hampton Sandstone in the Palynomorphs described by Parker (1988b) from the eastern and central parts of the Eucla Basin (Benbow et al. in Pidinga Formation in Lake Lefroy range in age from Early press). Sand and sandstones in the Werillup Formation are to Late Eocene. Similar ages were recorded by Balme and equivalent to the Hampton Sandstone in the Bremer Basin Churchill (1959) at Rollos Bore, and by Smyth and Button and Cowan Palaeodrainage. (1989) from the Rebecca and Raeside palaeorivers. Werillup Fortnation The Hampton Sandstone interfingers with the Pidinga The Werillup Formation (Cockbain 1968b) comprises the Formation. The equivalent formation in the Bremer Basin basal part of the Plantagenet Group in the Bremer Basin. and Cowan Palaeodrainage is the Werillup Formation. The formation is widespread along the south coast of Western Australia. The lignitic and carbonaceous sedi¬ ments of Lake Cowan are correlated with the Werillup Formation because of their similar stratigraphic position, Hampton Sandstone lithology, and depositional environment. The Hampton Sandstone is a widespread unit at the base The Werillup Formation consists of carbonaceous clays, of the Tertiary succession of the Eucla Basin (Lowry 1970). silts and sands, together with lignite lenses. A palynologi¬ Benbow (1990a) described Hampton Sandstone interfinger¬ cal assemblage of 84 species from 57 genera has been ing with and overlying Wilson Bluff Limestone and spon- recovered from the Werillup Formation at Norseman golites (equivalent to the Princes Royal Spongolite below). (Parker 1988b, Harris 1989). The presence of 12 dinoflagel- Its presence in the Lefroy Palaeoriver to the west of lates species belonging to 11 genera was reported by Harris Kambalda was recognised by Jones (1990). The formation is (1989). Siliceous sponge spicules and insect remains are absent from the Cowan palaeoriver. also locally present. 16 Journal of the Royal Society of Western Australia, 76 (1), March 1993 Figure 4. Sub-surface sequence for Lake Lefroy. 17 Journal of the Royal Society of Western Australia, 76 (1), March 1993 Limestones interfingering with the lower part of the for the Nanarup Limestone Member (McGowran 1989). The Werillup Formation are termed the Nanarup Limestone southern extension of the Cowan Palaeodrainage into the Member along the south coast and Norseman Limestone in Bremer Basin at Esperance contains calcareous marine and around Lake Cowan. sandstones in the lower Werillup Formation (Cockbain 1967; Morgan & Peers 1973). These are almost certainly The Werillup Formation rests unconformably on equivalent to the Nanarup and Norseman Limestones. This Archaean and Proterozoic basement. The formation is stratigraphic position is contrary to Hocking (1990a), who disconformably overlain in the palaeodrainages by the placed the Nanarup Limestone at the top of the Werillup Princess Royal Spongolite. Along the south coast it is Formation. The two limestones are thus likely to be the overlain by the Pallinup Siltstone. same unit, although more extensive drilling is necessary to confirm this correlation. It is recommended that Nanarup McGowran (1989) demonstrated that the Nanarup Lime¬ Limestone Member be abandoned as a stratigraphic term, stone was Middle Eocene. This suggests that elastics and the earlier name Norseman Limestone Formation be underlying the Nanarup Limestone may be Early Eocene, used for all marine carbonates that interfinger with the and those overlying late Eocene. Middle Eocene to Late Werillup Formation. Eocene palynomorphs were described by Parker (1988b) and Harris (1989) from the Werillup Formation from Lakes Princess Royal Spongolite Cowan and Dundas. The Werillup Formation is equivalent The Princess Royal Spongolite (Glauert 1926) occurs to the Pidinga Formation and Hampton Sandstone of the extensively in palaeodrainages along the eastern margin of Lefroy Palaeodrainage and Eucla Basin. the Eucla Basin (Jones 1990). The Formation is common only along the margins of Lefroy and Cowan palaoedrain- ages, having been removed by erosion from much of the Norseman Limestone central portions, but is more continuous further east. The The Norseman Limestone (Gregory 1916) is a fossilifer- Princess Royal Spongolite is absent from tributaries such as ous marine carbonate that crops out along the south eastern the Mt Morgan Palaeodrainage. margins of Lake Cowan, and also occurs to the east in Dog Lithologies consisting of greater than 50% siliceous Lake (Hooper 1959). Stratigraphic relationships are not sponge spicules with lesser amounts of silt and clay evident at the type locality (Clarke et al. 1948), but drilling by WMC has revealed that the formation is widespread overlying or interfingering the Pidinga and Werillup beneath Lake Cowan (see two drill holes ET120R, CW200R Formations are recognised as Princess Royal Spongolite. Minor quartz sand and glauconitic peloids are also present. and SN74R in Figure 5). Other fossils, apart from centric diatoms, are absent. The only description of the spicule assemblage is that of Hinde The Norseman Limestone varies from skeletal wack- (1910), who recognised 15 genera at the type locality. estone and grainstone to calcareous sandstone. Fossils include bryozoans, gastropods, bivalves, brachiopods, and The Princess Royal Spongolite reaches a thickness of 12 foraminifera. Cementation varies from complete to nonex¬ metres beneath Lake Lefroy in drill hole CD 1916 (Figure 4). istent. Rocks at the type locality are silicified, and patchy A thickness of 21 metres on the northern shores of Lake dolomitisation has occurred Ln drill hole BLD 13. The Cowan near Bingeringie was reported by Hooper (1959). formation beneath Lake Cowan, even though largely uncemented, is extensively dolomitised. Trough cross A disconformable contact marked by a gravel bed bedding is well developed in outcropping calcarenites near separates the Princess Royal Spongolite from the Pidinga the site of BLD 13 (Figure 6a). Glauconitic material is Formation in drill hole CD 1916. The Hampton Sandstone present in minor amounts, most commonly near the base interfingers with the Formation at Loves Find, and overlies and top of the formation. The Norseman Limestone reaches it in CD 1916. The Princess Royal Spongolite onlaps directly a maximum known thickness of 3/ metres beneath Lake onto Archaean basement along the shores of Lake Lefroy near Loves Find (Figure 6b). Cowan in drill hole ET 120R. Palynomorphs described by Hos (1977) from Harris Lake Previously unclear stratigraphic relationships have been on the margins of the Eucla Basin gave a Late Eocene age resolved by drilling in and around Lake Cowan. In BLD 13, for the Princess Royal Spongolite. The laterally equivalent the Norseman Limestone gradationally overlies lignitic Pallinup Siltstone in the Bremer Basin contains a Late siltstone of the Werillup Formation and is separated from Eocene marine fauna (Cockbain 1968b). The Princess Royal silicified Princess Royal Spongolite by a thin bed of sandy Spongolite was probably deposited during the Aldinga Werillup Formation. The Werillup Formation overlying the Transgression of McGowran (1989). Norseman Limestone towards the middle of Lake Cowan (Figure 5) is, however, up to 30 metres thick. The formation passes laterally into the Wilson Bluff Limestone to the east (Jones 1990), while the Pallinup Foraminiferal data reported by Cockbain (1968a) indi¬ Siltstone of the Bremer Basin passes into the Toolina cated a Late Eocene age. Palynological studies by Parker Limestone of the Eucla Basin, east of Israelite Bay (Lowry (1988b) indicated that the Werillup Formation immediately 1970). The Bring Member of the Pidinga Formation (Ben- underlying the Norseman Limestone in BLD 13 is of Early bow 1986) is the time-equivalent lithology in the Tallaringa to Middle Eocene age. This indicates that the formation is Palaeodrainage of the eastern Eucla Basin. Middle Eocene or younger. Late Eocene lignitic sediments (Harris 1989) are found overlying the Norseman Lime¬ Redmine Group stone. Successions of non-fossiliferous, yellow, brown, and red Cockbain (1968a) correlated the Norseman Limestone alluvial and lacustrine sediments were mentioned by with the lithologically similar Toolina Limestone in the Smyth and Button (1989) and Jones (1990). This paper Eucla Basin. The Norseman Limestone is however more formalises the stratigraphy of the post Eocene succession in likely a correlative of the Nanarup Limestone Member of Lakes Lefroy and Cowan. The names and their coordinates the Werillup Formation in the Bremer Basin. A Middle are taken from localities on the Lefroy 1:100 000 geological Eocene age (Tortachila transgression) has been determined map (sheet 3235), and are shown in Figure 1. 18 Journal of the Royal Society of Western Australia, 76 (1), March 1993 T L SI S U O E C A N O B R A C Figure 5. Sub-surface sequence for Lakes Cowan and Dundas. 19 Journal of the Royal Society of Western Australia, 76 (1), March 1993 The Eocene sediments in the Lefroy and Cowan palaeodrainages lack iron oxide bearing sediments; iron oxides and hydroxides are common in the post-Eocene sediments succession. Eocene sediments are fluvial to shallow marine in origin, contrasting with the predomi¬ nantly lacustrine sediments of the post-Eocene succession. The Eocene palaeodrainage succession can also be corre¬ lated with equivalent units on the Eucla Basin, but this cannot be done with the post-Eocene sediments. These differences are sufficient to justify placing the post-Eocene succession into a separate lithostratigraphic unit, the Redmine Group. The name Redmine Group is taken from Redmine Siding, the railway siding at the Kambalda Nickel Opera¬ tions concentrator mill (GR 740488). The Redmine Group rests disconformably on the Eocene sediments of Lake Lefroy, up to 20 metres of relief has been incised into the top of the Eundynie Group (Figure 4). The Redmine Group comprises the post-Eocene portion of palaeodrainage sediments. Red clastic facies occur in Lake Lefroy, further east in the Lefroy Palaeoriver (Jones 1990), and in Lake Cowan. Dark green-grey sediments of presumed post-Eocene age are also present in Lake Cowan. Red-brown sediments in the Raeside and Rebecca palaeodrainages (Smyth & Button 1989) may be equivalent to the Redmine Group. Similar post-Eocene sediments, apart from possible Cowan Dolomite (see below), have not been demonstrated in the Lake Dundas area, but it is highly likely that they are present. The Redmine Group in the sub-surface consists of predominantly red-brown silts, clays, sand, and gravel. Some sands and gravels are cemented by iron oxides. Dark green elastics are locally present within the group in Lake Cowan. Outcrops are similar, but cemented by iron oxides. Interbedded lenses of oolitic to fenestral dolomite are present. Outcrops of dolomites and iron cemented elastics common along the northern and western margins of Lake Lefroy are also included in the Redmine Group. The upper part of the group contains bedded gypsum deposits and gypsum aeolianites. Up to 26 metres of predominantly red-brown sediments overlying the Eocene succession are present beneath Lake Lefroy. Similar thicknesses occur beneath Lake Cowan. The base of the Redmine Group rests disconformably on palaeotopography eroded into Eocene sediments or uncon- formably on Archaean basement. The variable thickness of the Group is interpreted to be the result of infill of a variably eroded surface. The upper limit of the Redmine Group is the present depositional surface. The Redmine Group post-dates the late-Late Eocene marine sediments on which it rests, with deposition contin¬ uing through to the present. Earliest Pliocene palynomor- phs are present in the upper part of the group in Lake Lefroy (Parker 1988a). The Redmine Group comprises, the Cowan Dolomite together with Revenge, Gamma Island, and Roysalt Forma¬ tions. Figure 6. (A) Outcropping Norseman Limestone on shore of Lake Cowan, near drill hole BLD 13. (B) Outcropping Princess Royal Spongolite and upper Hampton Sandstone Revenge Formation on shore of Lake Lefroy, near Loves Find. (C) Outcropping The formation is named after Revenge open pit, shown fenestral dolomite of Gamma Island Formation at type on the Lefroy map sheet at GR 765412, but in reality locality on Lake Lefroy. (D) Cross-bedded gypsarenite of occurring 500 metres further east at GR 770412. Beta Island Member, Beta Island. 20 Journal of the Royal Society of Western Australia, 76 (1), March 1993 The Revenge Formation occurs widely beneath Lakes Lenses of oolitic (L. Killigrew, quoted by Loftus-Hills Lefroy and Cowan, and also beneath some of the smaller 1981) and fenestral dolomite are present in drill hole KD lakes marginal to them. Similar lithologies occur along the 3006 (Figure 4). White oncolitic, fenestral, peloidal, and length of the Lefroy palaeodrainage. Lateral continuity massive dolomites to dolomitic carbonate mudstones are between the Lefroy and Cowan palaeodrainages is proba¬ present on the margins and estuaries of Newtown, Mer- ble, hence the use of the same stratigraphic name for these ougil and Muldolia Creeks (Figure 1). lithologies in the two palaeodrainages. The type locality is exposed in the walls of the Revenge open pit, where a The thickness varies from at least 2 metres in the type thickness of 5 metres is attained. locality, to 2.5 metres in KD 3006. Boundary relationships are not exposed at the type The Revenge Formation beneath Lakes Cowan and locality. The Gamma Island Formation occurs within the Lefroy consists largely of massive to faintly laminated Revenge Formation in KD 3006, and overlies it along the red-brown silts. Small lenses of fine sand and ferruginous western shores of Lake Lefroy. sandstone occur within the formation beneath Lake Lefroy. Red sands and ferruginous sandstones predominate along A Miocene age is favoured, given the widespread the margins of the lake. Pebble beds are present at the base preservation of lacustrine sediments, especially carbonates, of the sequence, and also at intervals higher in the section; from this time elsewhere in Australia (De Deckker 1988). these represent internal disconformities. Dark grey-green coloured sediments are locally present. Outcrops of iron The Gamma Island Formation is placed in a separate oxide cemented conglomerate and sandstone. These well- formation from other units in the Redmine Group because indurated sediments are marginal facies of the Revenge its marked lithological contrast to them, and for consis¬ Formation. The Revenge Formation in Lake Cowan is tency of usage with equivalent units, the Cowan Dolomite composed of similar lithologies to those found in Lake in Lake Cowan, and the Garford Formation is the Lefroy (Figure 5). palaeodrainages of the eastern Eucla Basin (Benbow et al. in press). The lack of continuity between the otherwise similar The formation reaches a known thickness of 17 metres in lacustrine carbonates of Lakes Lefroy and Cowan justifies Lake Lefroy. Marginal facies of the Revenge Formation the use of different formation names. exposed on the shores of Lake Lefroy occur at elevations 8 metres above the present lake surface, indicating original Cowan Dolomite Formation thicknesses at least 8 metres greater than that presently The relationship of the Cowan Dolomite (Fairbridge preserved. The formation may reach thicknesses of 25 1953) to the Norseman Limestone and Princess Royal metres in Lake Cowan (Figure 5). Spongolite is not evident at the type locality, although the three units are spatially closely associated. Aircore drilling The base of the formation rests unconformably on at Lake Kirk south of Norseman has revealed similar Archaean bedrock, as displayed in the Revenge open pit, or lithologies occurring at different levels with lacustrine disconformably on Eocene sediments. The upper limit of sediments overlying the Princess Royal Spongolite. The the unit is the disconformable base of the overlying Roysalt formation is thus placed within the Redmine Group. Formation in Lake Lefroy and the Polar Bear Formation in Lake Cowan. The Cowan Dolomite crops out along the shores of Lakes Cowan and Brazier. The formation consists of white to The oxidised nature of the Revenge Formation is not buff-coloured dolomite to dolomitic carbonate mudstone. favourable for the preservation of palynomorphs, and none The maximum thickness of the Cowan Dolomite is un¬ have been recovered. A lower limit for the formation is set known, in SN 78R it is 2 metres thick. The Cowan Dolomite by the underlying Late Eocene sediments, and the overly¬ is equivalent to the Gamma Island Formation in Lake ing Pliocene pollen assemblage in the Roysalt Formation Lefroy. Small outcrops of dolomite in the Cowan (Parker 1988a). A Miocene age is favoured, given the palaeodrainage east of Salmon Gums (to the south of Lake widespread preservation of lacustrine sediments from this Dundas) were correlated with the Cowan Dolomite by time elsewhere in Australia (De Deckker 1988). This also Doepel (1973). coincides with the last major transgression on the Eucla Basin that deposited the Nullarbor Limestone (Lowry The age of the formation is unknown, but is probably 1970), the elevated base level resulting in lacustrine deposi¬ Miocene. Carbonate facies are common in Miocene lacus¬ tion within marginal palaeodrainages. trine sediments elsewhere in Australia (De Deckker 1988), the Formation may be partly equivalent to the Miocene- Pliocene Garford Formation in palaeodrainages of the The Perkolilli Shale (Kern & Commander, in press) in the eastern Eucla Basin (Benbow et al. in press). Roe Palaeodrainage is lithologically very similar, and is probably correlative with the Revenge Formation. The Miocene fluvial to marginal marine Plumbridge Formation Roysalt Formation (Hocking 1990b) and the marine Yarle Sandstone (Benbow 1990b) along the margins of the Eucla Basin are likely to be The name Roysalt Formation is derived after Roysalt distal equivalents. Some of the numerous Cainozoic forma¬ Siding, GR 688271, near the old Lake lefroy salt works. The tions defined by Glassford (1987) from Yeelirie may be unit occurs right across the floor of Lake Lefroy and on its partly equivalent to the Revenge Formation. margins. Type exposures of the formation occur in the walls of the Revenge open pit, where thicknesses of 1.5-2m are attained. Gamma Island Formation The Roysalt Formation consists of sandy silts and clays on the margins of Lake Lefroy, passing into bedded The Gamma Island formation is named after Gamma gypsum crystals in a silty, carbonaceous, and pyritic matrix Island, the local name for the Lake Lefroy island shown at towards the centre of the lake. Gypsum crystals in the GR 775539 on the Lefroy map sheet. The formation occurs lower part of the formation have grown together to form a patchily beneath Lake Lefroy, and along its northern and boxwork structure. The sediments in the gypsum dunes western margins. The shore of Lake Lefroy at the Newtown have been separated into the Beta Island Member (see Creek estuary (Figure 1, 6c). below). 21 Journal of the Royal Society of Western Australia, 76 (1), March 1993 The greatest known thickness is 9 metres (KD 3010, The Beta Island Member is closely associated with Figure 4). Horizontally bedded gypsum occurs up to 1 siliciclastic dune sediments, both on the lake islands and metres above the present lake floor in gvpsum dune shores. The gypsum dunes underlie siliciclastic dunes in islands, where overlying gypsum aeolianite has protected many localities, the stratigraphic relationship between the the sediments from erosion. Beta Island Member and older, degraded dune systems is unclear. The member is equivalent to part of the Miranda The Roysalt Formation rests disconformably on the Member of the Darlot Formation at Yeelirie (Glassford erosional top of the Revenge Formation. Preservation of 1987). similar sediments above the current lake floor in gypsum dune islands indicates a minimum thickness of 1 metres for eroded material, which demonstrates that the current lake floor is an erosional surface. Polar Bear Formation The Polar Bear Formation is named after Polar Bear Palynomorphs from the base of the formation at Revenge Peninsula, the large peninsula extending out into Lake give a Pliocene age (Parker 1988a). Deposition of the Cowan north of Norseman. The Polar Bear Peninsula Roysalt Formation may have continued into the Holocene. occurs on the Cowan topographic sheet at GR 870680. The Polar Bear Formation occurs across the floor of Lake The Roysalt Formation is a time equivalent of Pliocene Cowan and on its margins. The type section is in CNG air sediments at Lake Tay, west of Lake Dundas (Bint 1981), core hole ET 120R, where the formation is 3 metres thick. the Darlot Formation (in particular the Miranda Member) at Yeelirie (Glassford 1987), and of the Pliocene Narlaby Formations (Benbow et al. in press) in South Australia. The The Polar Bear Formation consists of sandy silts and formation passes laterally into, and interfingers with fluvial clays along the edges of Lake Cowan, and bedded gypsum and aeolian clastic sediments along the margins of Lake crystals in a silty matrix towards the centre of the lake. Low Lefroy. The Polar Bear Formation is the equivalent unit in relief gypsum dunes occur in some localities along the floor Lake Cowan (see below). Different formation names are of Lake Cowan and its margins. These dunes are generally used owing to the lack of continuity between the different less than 2 metres high. evaporitic lithologies of Lakes Cowan and Lefroy. The Formation is 3 metres thick in ET 120R and CW 200R (Figure 5). Scattered low relief gypsum dunes along the Beta Island Member margins of the lake and across its floor add another 2 m. The name Beta Island Member is taken from Beta Island, the local name for the gypsum dune island at GR 765412, The Polar Bear Formation rests disconformably on the site of the Lake Lefroy land sailing club. erosional top of the Revenge Formation. The top of the formation is defined by the present land surface. The Beta Island Member comprises the gypsum dunes that occur across the lake floor and on its western margins. The Formation is earliest Pliocene to Holocene in age, by Most islands on Lake Lefroy are at least in part gypsum analogy with the lithologically and stratigraphically equiv¬ dunes. Some, such as Delta Island, are composite, with a alent Roysalt Formation in Lake Lefroy. Early Pliocene bedrock core and accreted gypsum and silica dunes. The pollen in sediments in Lake Tay 125 kilometres to the member also occurs along the lake shore, particularly at south-west (Bint 1981) indicate the presence of non-arid Sandalwood. The 8 metres high cliffs that surround most of lake shore vegetation. This indicates that gypsum precipi¬ the margin of Beta Island (Figure 6d) comprise the type tation in Lake Tay began after the Early Pliocene. Thus the locality for the Member. base of the Polar Bear Formation in Lake Cowan (about half way between Lakes Lefroy and Tay), may be slightly The Beta Island Member is composed of gypsum ce¬ younger than earliest Pliocene. mented, steeply cross-bedded, gypsum sand. Quartz sand, silt and clay are minor components. Individual cross bed sets range in thickness from 4-8 metres. Two gypcrete horizons are present within the member on Beta Island. The uppermost horizon corresponds with the present surface in each case. Conclusions The member reaches a maximum thickness of 11 metres This proposed stratigraphic framework is intended to on Oyster Island. simplify and clarify the stratigraphy of the Cowan and Lefroy palaeodrainages, and relate them to the stratigraphy The Beta Island Member overlies the Roysalt Formation. of the Bremer and Eucla Basins. It is hoped that the same The contact is exposed approximately 1 metre above the stratigraphic nomenclature, with appropriate modifica¬ lake floor on Beta Island where cross bed toes transgress tions for local units (particularly in the Redmine Group) across flat bedded gypsum. The present land surface can be extended to other palaeodrainages along the mar¬ corresponds with the top of the member. gins of the Bremer and Eucla Basins. The age of the Beta Island Member is unknown. The gypsum dunes must post date the Pliocene and younger Roysalt Formation from which it is derived. The member is currently being eroded, with cliff faces developed on Acknowledgments: The author thanks P Nguyen and P Day for discussion nearly all exposures. Mixed quartz-clay dunes have also on Lake Lefroy sediments. D Miller, B Hearder, S Peters, N Archer, and L partly buried gypsum dunes on Delta Island and at Offe assisted in data collection at Norseman. M Benbow, P Commander and Sandalwood; the member is thus unlikely to be Holocene in A Cockbain advised on stratigraphic and regional issues. N Alley gave much useful advice on palynology. Various drafts were read by A van age. Bowler (1976) dated many gypsum dunes in arid Bentum, J Reeve, D Miller, and Mike Rosen. The figures were drafted by I Australia at 13-18 000 years BP, a possible age for the Beta Chew, J Savage, and T Richards. Permission to publish was granted by Island Member. WMC through D Haynes. 22