A review of Permian–Carboniferous glacial deposits in Western Australia

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Chapter 2: “A review of Permian–Carboniferous glacial deposits in Western Australia” (Mory et al.), in Fielding, C.R., Frank, T.D., and Isbell, J.L., eds., Resolving the Late Paleozoic Ice Age in Time and Space: Geological Society of America Special Paper 441. This PDF file is subject to the following conditions and restrictions: Copyright © 2008, The Geological Society of America, Inc. (GSA). All rights reserved. Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in other subsequent works and to make unlimited copies for noncommercial use in classrooms to further education and science. For any other use, contact Copyright Permissions, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, fax 303-357-1073, [email protected]. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. This file may not be posted on the Internet.

The Geological Society of America Special Paper 441 2008

A review of Permian–Carboniferous glacial deposits in Western Australia A.J. Mory† Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia J. Redfern J.R. Martin Basin Studies and Petroleum Geosciences, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK ABSTRACT Widely distributed glacially derived material indicates that an extensive ice sheet covered Western Australia from at least the Gzhelian to mid-Sakmarian times. The earliest glacial sequences may be Bashkirian in the subsurface of the Southern Carnarvon and Canning Basins, although definitive glacial characteristics are less well defined. The younger glacially influenced successions are present in nearly all Phanerozoic basins in Western Australia, and typically comprise a lowermost glacial facies, middle marine mudstone facies, and uppermost fluvial–deltaic strata. Current palynological correlation show that the tripartite successions may not be coeval among all basins, which appears to contradict models of Gondwana-wide glaciation in which the end of glacial conditions is an inter-regional coeval event. However, detailed analysis is hampered by the existing low-resolution biostratigraphic scheme. There is some evidence that subsidence or penecontemporaneous faulting may have locally dominated relative sea-level change and modified regional glacial influences. A dramatic improvement in biostratigraphic resolution is required to resolve the controls on facies distribution, especially to differentiate between deglaciation patterns and periodic ice-sheet advance and retreat, and regional climatic changes and latitudinal differences within Gondwana. Keywords: biostratigraphy, correlation, stratigraphy, Western Australia. INTRODUCTION

Scotese et al., 1999). Facies associations of diamictites, massive sandstones and conglomerates, mudstones with exotic dropstones, and possible varved units, generally considered to be indicative of glacial conditions or rapid deposition during deglaciation (e.g., Eyles and Miall, 1984; Benn and Evans, 1998), are present in all Western Australian Paleozoic basins. In some of these basins, the diamictites have also been interpreted as facies redeposited under local tectonic influences, albeit with a glacial signature (Eyles et al., 2002).

All Paleozoic basins in Western Australia contain extensive Permian–Carboniferous glacial deposits (Figs. 1 and 2). Plate-tectonic reconstructions for this time place Western Australia between 75°S and 35°S, at similar latitudes to other glaciated Gondwanan basins (e.g., Zeigler et al., 1998; †

E-mail: [email protected].

Mory, A.J., Redfern, J., and Martin, J.R., 2008, A review of Permian–Carboniferous glacial deposits in Western Australia, in Fielding, C.R., Frank, T.D., and Isbell, J.L., eds., Resolving the Late Paleozoic Ice Age in Time and Space: Geological Society of America Special Paper 441, p. 29–40, doi: 10.1130/2008.2441(02). For permission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved.

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Figure 1. Distribution of Carboniferous–Permian glacial deposits in Western Australia (after Eyles et al., 2002), glacial striae directions (after Playford, 2002), 1:100,000 scale geological mapping by Geological Survey of Western Australia, and earliest Permian paleolatitudes (after Playford, 2002).

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Series

Guadalupian

Cisuralian

Pennsylvanian

Mississippian

Visean

Serpukhovian

Bashkirian

Moscovian

Kasimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadina

Wordian

Stage

Paterson Formation

inferred glacial influence in subsurface

glacial influence

Cullens Diamictite

Mosswood Formation

marine mudstone dominant

fluvial-deltaic influence dominant

Shotts Formation

Moorehead Formation

Woodynook Ss.

Rosabrook Coal Measures

Ashbrook Ss.

Redgate Coal Measures

Williespe Formation

Nangetty Formation

Holmwood Sh

High Cliff Sandstone

Irwin River Coal Measures

Carynginia Formation

Wagina–Dongara– Beekeeper

Northern Perth Basin

Harris Ss.

Lyons Group

Carrandibby Formation

Callytharra Formation

Wooramel Group

Byro Group

Kennedy Group

Southern Carnarvon Basin

Grant Group

Betty/Reeves Formation

? Winifred Formation ? ?

Clianthus Fm Carolyn Calytrix Fm ? Formation Hoya Fm

Nura Nura Mbr

Poole Sandstone

Noonkanbah Formation

Liveringa Group

Northern and Central Canning Basin

Figure 2. Interbasinal correlation of Carboniferous–Permian glacial units, Western Australia (modified after Eyles et al., 2002).

G. maculosa

S. ybertii

D. birkheadensis

D. tenuistriatus

Stage 2

P. confluens

P. pseudoreticulata

S. fusus Westralia Sandstone

Ewington Coal Measures

M. trisina

D. byroensis

Allandale Ss.

Premier Coal Measures

Muja C.M.

Southern Perth Basin Gunbarrel and Southern Canning Collie Coal Field Vasse Coal Field Basins

P. sinuosus

D. ericianus D. granulata M. villosa

P. rugatus

D. parvithola

Palynology

Stockton Group

Age (Ma)

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Weaber Group

Wadeye Group

?

Kurriyippi Formation

Treachery Shale

Keyling Formation

Fossil Head Formation

Cape Hay Fm Pearce Fm

Kulshill Group

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In addition, striated surfaces on basement highs and within the sedimentary successions provide direct evidence of glacial action. Permian–Carboniferous sections deposited under glacial influence reach thicknesses of over 2000 m in the major depocenters (Fig. 1), such as the northern Perth Basin (~4000 m, Coolcalalaya subbasin); the Southern Carnarvon Basin (~3500 m, Merlinleigh subbasin); the Canning Basin (~2500 m, Fitzroy Trough); and the Southern Bonaparte Basin (?>4000 m, Petrel subbasin). The Permian–Carboniferous section also extends onto basement areas where the direct record of glaciation is meager, and the limited outcrops are extremely difficult to date. For example, thin boulder deposits on the Yilgarn craton, which are generally less than 10 m thick and have previously been mapped as Cenozoic, are possibly the weathered remnants of Permian glacial accumulations (Playford, 2001). Isolated pockets of diamictite and glacial striae on Proterozoic rocks just west of the Gunbarrel Basin (Jones, 2004) support this hypothesis. Evidence for Glacial Affinity Evidence for extensive Permian–Carboniferous glaciation across Western Australian basins is compelling. Glacially influenced deposits are widespread, and most basins show a succession with a lower glacial package, a middle mud-dominated package, and an upper sand-rich package (Fig. 2). Neither the middle nor upper package contains direct indicators of glacial affinities. Eyles and Eyles (2000) and Eyles et al. (2001, 2006) questioned the primary glacial character of many of these successions, noting the high degree of modification by resedimentation processes such as mass flow. They interpreted a tectonic control from the nonsynchronous shift between basins in depositional style from underfilled with a glacial influence to overfilled with little or no glacial influence. By comparison, O’Brien et al. (1998) interpreted a succession of highstand and lowstand glacial deposits on the edge of the Fitzroy Trough of the Canning Basin as recording the interplay between sea level, subsidence, and sediment supply rates. There is little direct evidence for significant fault activity during deposition because of the discontinuous nature of Western Australian outcrops and variable seismic quality. The dramatic thickening of the Carboniferous section into the central grabens of the Bonaparte and Canning Basins and half grabens in the Southern Carnarvon and northern Perth Basins does, however, imply significant local penecontemporaneous fault movements. The available seismic data in the Canning Basin suggest that significant fault movement had mostly ceased by the Early Permian, when thermal sag became the dominant subsidence mechanism. The widespread distribution of facies with a glacial affinity, demonstrated by exotic outsize clasts and dropstones, and their stratigraphic relation to striated pavements (Figs. 1 and 3A) along the basin margins support a primary depositional mechanism associated with glaciation. Resedimentation, while possibly related to fault movements, is just as likely to be due to rapid

deglaciation and high sedimentation rates. These processes can effectively rework glacial deposits and hinder identification of the primary depositional mechanism. Age Control The majority of Western Australia’s glacial deposits are dated from palynological and macrofaunal assemblages. The most widespread extend from the Gzhelian (Late Carboniferous) to Tastubian (early Sakmarian; Fig. 2). In addition, thick sections of possible glacial origin span the Late Carboniferous in the Canning and Southern Carnarvon Basins and extend down into the Early Carboniferous in the Perth Basin. Glacial deposits are also known from the Kungurian (late Early Permian) in the Perth and Southern Carnarvon Basins (Figs. 2, 3D, and 3E). Macrofaunal biostratigraphy is limited to brachiopod and ammonite faunas from Permian outcrops, but macrofossils are rare in the Asselian–Tastubian and tend to show low diversity due to cold-water temperatures (~8 °C; Lowenstam, 1964; Archbold and Shi, 1995). Records of such faunas from the subsurface are even rarer, which further limits their biostratigraphic usefulness. By comparison, there is a widely applicable palynozonation (Kemp et al., 1977; Foster, 1982; Backhouse, 1993, 1998a; Mory and Backhouse, 1997; Apak and Backhouse, 1999) that has been established mainly using subsurface data. Correlation of the brachiopod and palynozones to international stages has largely depended on the ages deduced from coexisting ammonite faunas (Archbold, 1993, 1998, 2002; Foster and Archbold, 2001), but it has been supplemented by zircon dating of volcanic horizons in eastern Australia (Roberts et al., 1995, 1996). While there is a growing amount of palynological information from shallow boreholes and petroleum exploration wells, much of it is not published, and the earlier work, in particular, needs updating. Many outcrops are still poorly dated, and their position within the glacial succession relies heavily on lithostratigraphic correlations. The Australian mid- to Late Carboniferous palynological zonation is crude, and individual zones have greater duration than international stages (Fig. 2). Helby (2006) indicated a somewhat different zonation based on four wells from the Arafura Basin in the Northern Territory and commented that the mid-Carboniferous to Asselian palynological zonation in Australia is in need of revision. The ages of the Diatomozonotriletes birkheadensis and Deusilites tenuistriatus zones, in particular, are poorly constrained, especially since their duration as depicted in Figure 2 follows Roberts et al.’s (1995) correlation, in which there are no data for the upper Pennsylvanian. Mid- to Late Carboniferous macrofaunas have yet to be reported from Western Australia, although palynology from shallow drilling (Backhouse, 2002) indicates that at least some supposedly Permian faunas in the Southern Carnarvon Basin described by Dickins and Thomas (1959) may be late Carboniferous. In Western Australian basins, the relationship between outcrop of glacial sections, which usually are along basin margins, and deeper sections, toward basin centers, is often unclear,

A

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D

E

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Figure 3. Outcrops showing glacial characters: (A) Glacially striated basement, Oakover River, northern Pilbara; (B) intraformational glacial striae in sandstone, Carolyn Formation, Grant Group, Canning Basin; (C) striated and faceted clast from diamictite, Nangetty Formation, Perth Basin; (D) boulder beds, Nangetty Formation, Perth Basin; (E) possible glacial varves, Lyons Group, Southern Carnarvon Basin; and (F) massive diamictite and siltstone overlain by fluvial-deltaic sandstone, Carolyn Formation, Grant Group, Canning Basin, with outcrop gamma superimposed.

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Mory et al.

largely because the former are so poorly dated due to the rarity of nonoxidized samples. In addition, the present Australian zonation is of low resolution in comparison with some international biostratigraphic schemes. Therefore, even if outcrop sections can be dated palynologically from shallow drilling, a considerable degree of uncertainty will remain in intra- and interbasinal correlations. Refinements of Carboniferous–mid-Sakmarian palynological or macrofaunal zonations in Western Australia do not appear to be forthcoming unless significant economic incentives, such as major mineral deposits or petroleum accumulations, are found within strata of this age. Regional Stratigraphic Framework Lithostratigraphic nomenclature for Western Australia’s glacial deposits varies considerably among basins, from a single formation in the Gunbarrel Basin to a group with three or more subdivisions in the Canning Basin (Fig. 2). These differences reflect not only varying lithological heterogeneity, but also the variable state of knowledge between basins for these successions. In places, historical factors have produced markedly different stratigraphic hierarchies for overall similar successions in adjacent basins; for example, in the northern Perth Basin, the basal sandstone unit within the otherwise undifferentiated succession has member status, whereas the comparable unit in the Southern Carnarvon Basin has formation status in a largely undivided group (Fig. 2). Another significant difference in nomenclature is that parts of the overlying postglacial successions in the Canning and Bonaparte Basins are grouped with the underlying glacial successions, but this is not the case in the more southerly basins (Fig. 2). The rationalization of lithostratigraphic schemes between basins would facilitate interbasinal correlations. PERTH BASIN Glacial facies have been recorded from the ?latest Visean– Sakmarian Nangetty Formation, the Sakmarian High Cliff Sandstone, and the Artinskian–Kungurian Carynginia Formation (Fig. 2). In the northern Perth Basin, the Nangetty Formation is a predominantly fossiliferous marine mudstone that shows clear glacial features, including diamictites with striated and faceted clasts (Figs. 3C, D) and distinctive clasts from the adjacent Yilgarn craton and marginal Proterozoic rocks (Playford et al., 1976). A basal sandstone member includes redeposited diamictites (Eyles et al., 2006) and rare glacial striae trending to 335° in sandstone of uncertain age (Fig. 1; Mory et al., 2005). A few shallow bores in the member yielded palynomorphs from the D. birkheadensis and ?Grandispora maculosa palynozones (Backhouse, 1996, 1998b). Until further samples become available, it is not possible to say if this material is reworked, or if lack of the Spelaeotriletes ybertii and D. tenuistriatus zones is due to incomplete sampling or breaks in the succession. The Nangetty Formation is thickest against the eastern margin of the basin along the Darling and Urella faults,

and it thins dramatically to the west toward the mid-basinal Beagle–Dongara Ridge, where it is absent (Eyles et al., 2006). The presence or absence of the unit further west (offshore) is as yet unproven. The unit probably also thins into the Dandaragan Trough to the south, based on the overall thinning of the Permian in that direction on seismic sections (Mory and Iasky, 1996). The lithostratigraphically equivalent Stockton Group in the Collie subbasin and Vasse Shelf in the south of the basin is less than 50 m thick and contains a basal Asselian diamictite unit in both areas (Le Blanc Smith, 1993; Le Blanc Smith and Kristensen, 1998). The group has yet to be recognized from the deeper parts of the basin in that region. The glacial affinities of the mid-Sterlitamakian (late Sakmarian) Holmwood Shale above the Nangetty Formation are uncertain. Intervals with rare glacial erratics reported by Playford et al. (1976) from the base of the unit were mapped as uppermost Nangetty Formation by Le Blanc Smith and Mory (1995)—they found none in situ within the Holmwood Shale. Small granite clasts in the overlying High Cliff Sandstone, and locally abundant dropstones up to 1.65 m across in the latest Artinskian– Kungurian Carynginia Formation, indicate that glacial influence (floating ice) continued into the Artinskian following intervening nonmarine conditions in which no glacial facies are recorded (Eyles et al., 2006). SOUTHERN CARNARVON BASIN The Lyons Group (Condon, 1967) is undifferentiated apart from the basal Harris Sandstone and uppermost Carrandibby Formation (Hocking et al., 1987). Deposition of the group commenced in the northwest of the basin during the early Pennsylvanian, based on the recognition of microfossils representing the S. ybertii zone in cores from Waroora-1 (Backhouse, 2002) at a level associated with small dropstones. To the south, outcrops of glacial facies range in age from about Kasimovian to mid-Sakmarian, based on data from shallow-water bores (Backhouse, 2002), and include silty mudstone facies with centimeter-thick plane laminations in the southern part of the basin (Fig. 3E). These facies have been interpreted as possible varves (Hocking et al., 1987), whereas similar laminated facies in the northern Perth Basin have recently been interpreted as possible distal turbidites (Eyles et al., 2006). A glacial origin is clearly indicated by abundant diamictites and large boulders up to 2 m across in fossiliferous marine mudstones, and distinctive glacial striae. The latter lie within the Harris Sandstone of likely late Carboniferous age (?Kasimovian; based on the D. tenuistriatus zone described from shallow drill holes on Mt. Lyons Station by Backhouse, 2002), and they indicate ice movement to the NNW and to the west (Fig. 1). Rare glacial striae to the NNW on basement are probably also late Carboniferous in age, and, together with the striae within the Harris Sandstone, they represent the only clearly pre-Permian ice transport directions in Western Australia. Some erratic boulders in the group appear to be derived from the north (Grey et al., 1977) or east (R.M. Hocking, 2006,

A review of Permian–Carboniferous glacial deposits in Western Australia personal commun.), suggesting major changes in ice-transport directions in this region. Striated boulders are present but rare: most are well rounded or weakly faceted, so transportation could have been largely by fluvial processes. Seismic data show the Lyons Group thickening to the west against the Kennedy Range fault system (Eyles et al., 2003), indicating that movements on the system continued from at least the late Carboniferous into the Early Permian. Erratic boulders are also present in the Billidee and Coyrie Formations (uppermost Wooramel and lowermost Byro Groups; Hocking et al., 1987), indicating that floating ice persisted into the late Artinskian–early Kungurian. NORTHERN AND CENTRAL CANNING BASIN The lower part of the Sakmarian Grant Group extends across most of the basin and includes diamictite and mudstone with dropstones typical of glacial deposition in a predominantly submarine environment (Fig. 4; Crowe and Towner, 1976a). Although the underlying Middle to Upper Carboniferous Reeves Formation (formerly the Betty Formation of the Grant Group) was excluded from the group by Apak and Backhouse (1999), it is here included on genetic grounds: at least some conglomerate in the uppermost Carboniferous part of the unit in the Fitzroy Trough may be diamictite with a glacial affinity. The paucity of cores from this unit hinders the identification of glacial characters, but distinct glacial features in coeval sections from the adjacent Bonaparte and Southern Carnarvon Basins imply that such an affinity is likely. Crowe and Towner’s (1976b) division of the Grant Group (Betty, Winifred, and Carolyn Formations) based on outcrops and limited drilling in the Fitzroy Trough has a significant difficulty in that the type section of the Winifred Formation is an isolated, deeply weathered exposure some 400 km to the south of the Fitzroy Trough. Until the type section is dated, definitive correlation to the succession within the Fitzroy Trough is impossible. Other difficulties with Crowe and Towner’s (1976b) formations include the poorly defined base of the Carolyn Formation, the small number of wells to which their division has been applied, and the necessity of revising the existing palynology from those wells. Kemp et al. (1977) and Powis (1984), for example, reported stage 2 (presently considered Asselian) palynomorphs from intervals largely assigned to the Winifred Formation, but subsequent work by Backhouse (in Apak and Backhouse, 1999) found Sakmarian palynofloras. The absence of Asselian strata in the basin has yet to be proved conclusively: a revision of the palynozonation of a significant number of wells is required, including the two wells that Kemp et al. (1977) used as their principal reference sections for the basin (Blackstone-1 and Meda-1). Given that Blackstone-1 is one of the few wells in which the Winifred Formation has been identified, the Asselian age is accepted tentatively until the palynology for this well (amongst others) is revised. Similar correlation difficulties exist with some members of the Carolyn Formation, especially in the

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southeast of the Lennard Shelf, where silty diamictites and overlying sandstones were recently defined as the Deeadeea Claystone and Ngumban Sandstone members (Fig. 3D; Playford, 2002). Lithostratigraphically, these units most probably correlate with part of the Wye Worry Member of Crowe and Towner (1976a). The difficulty in correlating Crowe and Towner’s (1976b) stratigraphic scheme to the extensive subsurface data available in the Barbwire Terrace, where petroleum exploration companies have drilled a number of fully cored sections through the Grant Group, led to a new stratigraphic division being proposed: the Hoya, Calytrix, and Clianthus Formations (Redfern, 1991; Redfern and Millward, 1994; Apak and Backhouse, 1999). All three formations lie within the early Sakmarian Pseudoreticulatutispora confluens zone, making them approximately equivalent to the Carolyn Formation. Archbold (1995) correlated a brachiopod fauna from the base of the Calytrix Formation with another near the top of the Wye Worry Member in Carolyn Valley, but it is unclear whether the top of the member is eroded in this outcrop section. A striated pavement in the Lennard Shelf, north of the Fitzroy Trough, indicates ice movement to the NNW (Playford, 2002). Strongly karstified Devonian limestone draped by Grant Group diamictites on this margin of the basin was interpreted by Playford (2002) as the result of extensive planation and carbonate dissolution below an ice sheet. The similar marked unconformity across the Barbwire Terrace at the base of the Hoya Formation, which is clearly evident on seismic and in core sections, is interpreted as a glacially eroded surface overlain by an extensive basal diamictite that could be a lodgement tillite (Fig. 4; Redfern, 1990; Redfern and Williams, 2002). The basal Hoya Formation contains thick diamictites (Figs. 4A–4E), mudstones with dropstones (Figs. 4C and 4F), and massive sandstone that have been interpreted as proximal sediments following deglaciation from a large ice sheet (Redfern and Williams, 2002), or as redeposited deeper-water facies (Eyles and Eyles, 2000). One of the most striking glacial features in outcrop are striae within a 10-m-thick interval of the lower part of the Wye Worry Member in central Grant Range (O’Brien and Christie-Blick, 1992) and 40 km to the SSE next to Mount Wynne. These indicate grounded ice moving toward the northwest (330° and 290°, respectively; Fig. 3B). Significant glacial activity probably ceased within the Tastubian, based on brachiopod faunas near the top of the Wye Worry Member of the Carolyn Formation above distinctly glacial facies (Dickins, 1996; Archbold, 1995). The presence of glacial indicators, such as striated dropstones in marine claystone and siltstone, at the top of this member in Saint Georges Range is in direct contrast with the lack of such features in the thick, predominantly coarse-grained facies at the same stratigraphic level in the Grant Range to the northwest. These latter facies are interpreted as rapid fluvial-deltaic outwash, which at times overwhelmed marine sedimentation. However, the equivalence is still uncertain given the lack of biostratigraphic data from the Grant Range outcrops.

A

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nn ar

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os

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nd

5 cm elf

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ug h

ir bw Bar

rR ove Oak

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e 3 Pl Ter at rac for e m

sin

CANNING BASIN Basement

24°S

120°E

AJM706

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128°

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A review of Permian–Carboniferous glacial deposits in Western Australia Figure 4. Core from the basal Hoya Formation, Grant Group, on the Barbwire Terrace: (A) sandy diamictite with angular clasts of Devonian carbonate in a sandy matrix, and less common granitic and metamorphic clasts, Calytrix-1, 398.72 m; (B) basal unconformable contact of Grant Group, with fractured Devonian siltstone overlain by a diamictite containing angular carbonate clasts and exotic clasts in a sandy matrix, Hoya-1, 433.80 m; (C) deformed balls of siltstone to very fine-grained sandstone in a dark gray mudstone, indicating significant remobilization/loading, Clianthus-1, 366.66 m; (D) softsediment deformation within a sandy diamictite with large subangular clasts, Eremophilia-3, 3221.1 m; (E) large granitic dropstone (>20 cm across) in a sandy diamictite, Halgania-1, 254.7 m; and (F) rhythmically bedded siltstone to mudstone with upward-fining laminae, containing small random outsized clasts of rock fragments and rare muddy diamictite clasts interpreted as till pellet dropstones, Eremophilia-1, 175.85 m.

At present, the correlation between the stratigraphic successions developed for outcrop and the subsurface is hindered by the lack or imprecision of available biostratigraphic control. It is further complicated by the apparent interfingering of glacial and fluvial members or facies in outcrop of the Carolyn Formations. Ongoing work aims to better date and delineate glacial facies in the Canning Basin outcrops to resolve this problem. GUNBARREL BASIN AND SOUTHERN CANNING BASIN The Gzhelian–Sakmarian Paterson Formation is a mixed succession of diamictite, sandstone, and siltstone that extends from the Gunbarrel Basin into the southern Canning Basin (Fig. 1). A basal diamictite, with varied exotic clasts, is commonly overlain by cross-bedded, texturally and compositionally immature sandstone, in a succession tens of meters thick at most (R.M. Hocking, 2006, personal commun.). The age indicates a partial equivalence with the Reeves or Betty Formation up to the Carolyn Formation in the northern and central Canning Basin; it is uncertain whether the tripartite division, evident elsewhere in the state, is present or not. Glacial tunnel valleys on basement in the Pilbara region along the southern margin of Canning Basin indicate ice movement between 330° and 030°, as do the associated striated basement surfaces (Figs. 1 and 3A; Playford, 2001). The tunnel valleys probably represent the last significant ice movement across basement in this area, possibly in the Sakmarian or Asselian based on sparse Sakmarian palynofaunas from the region described by Backhouse (1974, 1976). Significant manganese deposits are associated with the tunnel valleys at Woodie Woodie (Ferguson et al., 2006). Glacial tunnel valleys on the east side of the Yilgarn craton imply ice movement to the east (Eyles and de Broekert, 2001). Thin diamictite deposits of the Paterson Formation extend at least 100 km west of the Gunbarrel Basin and locally overlie east-west–oriented glacial striae on Paleoproterozoic basement

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(Jones, 2004). These directions match the transport direction deduced from erratics in the western part of the Gunbarrel Basin (Jackson and van de Graaff, 1981). In the east and northwest of the basin, Jackson and van de Graaff (1981) found erratics derived from the northeast and northwest, respectively, implying that basement topography surrounding this basin strongly influenced ice-transport directions. However, these directions may require revision in the light of more recent mapping of the adjacent Precambrian terrains. BONAPARTE AND ARAFURA BASINS Glacial strata in the Bonaparte Basin are restricted to the Pennsylvanian–Artinskian Kulshill Group, which reaches a considerable thickness in the Petrel subbasin based on deep seismic data acquired by the Australian Geological Survey Organisation (now Geoscience Australia; Bradshaw, 1990). Much of the 10,000-m-thick succession in the center of the Petrel subbasin previously assigned to this unit (e.g., Eyles et al., 2002) may be from older units. The group disconformably overlies Serpukhovian–early Bashkirian strata of the Wadeye Group (Gorter et al., 2005), which is coeval with the basal part of likely glacial successions in more southerly basins. Subsurface sections of the group have been divided into the Kurriyippi Formation, Treachery Shale, and Keyling Formation, in which the lower Kurriyippi Formation contains a “preglacial” succession dominated by thick fluvial-deltaic channel facies (Mory, 1991). Although the sedimentology of the group has not been studied systematically, diamictites are described in a number of well completion reports from levels corresponding to the upper part of the Kurriyippi Formation, part of the Treachery Shale, and in the lower Keyling Formation. Thinning of the basal Kurriyippi Formation toward the edge of the basin resembles thinning of the Reeves Formation toward the edge of the Fitzroy Trough in Canning Basin. There is also some evidence that the Treachery Shale is locally disconformable on the Kurriyippi Formation, possibly due to remobilization of Silurian salt at depth in the Sakmarian. The succeeding Keyling Formation represents a return to dominantly fluvial-deltaic facies with relatively little glacial signature (Mory, 1991), similar to the uppermost part of the better studied Grant Group in the Canning Basin. Recent unpublished work by ENI Australia has delineated a number of glacial tunnel valleys with lower Sakmarian fill in the offshore southeastern part of the Petrel subbasin. The valleys are mostly directed to the northeast and northwest (J.D. Gorter, 2006, personal commun.), implying that ice was perched on nearby Precambrian basement. This work also indicates an apparent lack of late Moscovian–Asselian strata (corresponding to the D. tenuistriatus zone and stage 2), which was not appreciated by Mory (1991), as the P. confluens zone and stage 2 were not always differentiated at the time. The implication is that none of the underlying Carboniferous succession is glacial in origin, and that strata assigned to much of the Kurriyippi Formation may need to be excluded from the Kulshill Group.

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This suggests that a palynological review of the succession is required, and existing cores need to be reexamined to confirm whether the break is present. Onshore sections (Keep Inlet Formation) show a series of thick fining-up cycles, presumably due to episodic movement on faults along the basin margin, and no glacial indicators, but they have been placed within the Kulshill Group because they cut across Lower to mid-Carboniferous units (Mory and Beere, 1988). The lack of glacial indicators and the presence of palynomorphs of the P. confluens zone in a coal exploration bore near the coast (reported as Sakmarian in Mory and Beere, 1988) imply a correlation with the upper postglacial part of the Keyling Formation and the uppermost Grant Group in Canning Basin. Strata designated as “Kulshill Group equivalents” in the Goulburn graben of the Arafura Basin, over 400 km northwest of the Bonaparte Basin, extend in age at least from the Asselian into the lower Sakmarian (Helby, 2006; Struckmeyer, 2006), but the lack of core makes it uncertain as to whether there is any glacial signature. Similarly, it is unclear whether mid-Carboniferous palynomorphs from the base of this 1700-m-thick succession are reworked, as claimed by Bradshaw et al. (1990). SUMMARY Widespread glacially derived material strongly supports the interpretation that an extensive ice sheet covered large parts of Western Australia. All basins, with the exception of Gunbarrel Basin, contain a glacially influenced tripartite stratigraphic succession consisting of a lowermost glacial diamictite-mudstonesandstone facies association, middle mudstone-dominated package, and uppermost sandstone-rich, commonly coal-bearing, deltaic strata. The present Australia biostratigraphic zonation for the Middle Carboniferous to Lower Permian is of low resolution in comparison with some international schemes. Uncertainty in correlation remains at both intra- and interbasinal scales. Refinement of the biostratigraphy and a rationalization of lithostratigraphic schemes between and within basins are required to facilitate interbasinal correlations. Eyles et al. (2002) speculated that the tripartite facies were diachronous between basins, suggesting tectonics dominated subsidence and relative sea level, and that the middle shale units record high relative sea levels during times of maximum synrift subsidence. Although tectonics clearly influenced accommodation space in some basins, especially in the major depocenters during the Late Carboniferous, and major penecontemporaneous faults possibly locally remobilized glacial sediments, the main deglaciation would have been the most significant control on relative sea level. Accordingly, glacial processes are interpreted to be the dominant influence on deposition, where facies suites were influenced by periodic ice advance and retreat, and the selective preservation of successions. It is uncertain whether the end of glacial conditions can be used as an interregional coeval event, as implied by the models for Gondwana-wide glaciation proposed, for example, by Wopfner (1999). The differentiation

of possible local controls on facies distribution (such as tectonics or sediment input) from deglaciation patterns (periodic ice-sheet advance and retreat), and regional climatic changes or latitudinal differences within Gondwana, awaits a dramatic improvement in biostratigraphic resolution. ACKNOWLEDGMENTS We thank Roger Hocking, Annette George, and Chris Fielding for their helpful reviews of the manuscript. Redfern and Martin acknowledge financial support from Natural Environment Research Council (UK) Ph.D. grant NER/S/A/2004/13012 and Shell International Exploration and Production. Mory publishes with the permission of the Director, Geological Survey of Western Australia. REFERENCES CITED Apak, S.N., and Backhouse, J., 1999, Stratigraphy and Petroleum Exploration Objectives of the Permian-Carboniferous of the Barbwire Terrace and Adjacent Areas, Northeast Canning Basin: Geological Survey of Western Australia Report 68, 30 p. Archbold, N.W., 1993, A zonation of the Permian brachiopod faunas of Western Australia, in Findlay, R.H., Unrug, R., Banks, M.R., and Veevers, J.J., eds., Gondwana Eight: Assembly evolution and dispersal: Proceedings of the 8th Gondwana Symposium, Hobart, Tasmania, Australia, 21–24 June 1991: Rotterdam, A.A. Balkema, p. 313–321. Archbold, N.W., 1995, Studies on Western Australian brachiopods: 12. Additions to the late Asselian–Tastubian faunas: Proceedings of the Royal Society of Victoria, v. 107, p. 95–112. Archbold, N.W., 1998, Marine biostratigraphy and correlation of the west Australian Permian basins, in Purcell, P.G., and R.R., eds., The Sedimentary Basins of Western Australia, Volume 2: Perth, Western Australia, Petroleum Exploration Society of Australia, p. 553–568. Archbold, N.W., 2002, Peri-Gondwanan Permian correlations: The mesoTethyan Margins, in Keep, M., and Moss, S.J., eds., The Sedimentary Basins of Western Australia, Volume 3: Perth, Western Australia, Petroleum Exploration Society of Australia, p. 223–240. Archbold, N.W., and Shi, G.R., 1995, Permian brachiopod faunas of Western Australia: Gondwanan–Asian relationships and Permian climate: Journal of Southeast Asian Earth Sciences, v. 11, p. 207–215, doi: 10.1016/0743-9547(94)E0015-6. Backhouse, J., 1974, Palynology of a Borehole in the Southern Part of the Yarrie Sheet: Geological Survey of Western Australia Palaeontology Report 1974/28, 1 p. Backhouse, J., 1976, Palynology of the Oakover River Boreholes: Geological Survey of Western Australia Palaeontology Report 1976/6, 3 p. Backhouse, J., 1993, Palynology and correlation of Permian sediments in the Perth, Collie and Officer Basins, Western Australia: Geological Survey of Western Australia Report 34, p. 111–128. Backhouse, J., 1996, Preliminary Palynology of CRAE YCH 1 and YCH 2 in the Coolcalalaya Sub-Basin: Geological Survey of Western Australia Palaeontology Report 1996/7, 4 p. Backhouse, J., 1998a, Palynological correlation of the Western Australian Permian: Proceedings of the Royal Society of Victoria, v. 110, p. 107–114. Backhouse, J., 1998b, Palynology of Samples from the Coolcalalaya Sub-Basin Collected in 1998: Geological Survey of Western Australia Palaeontology Report 1998/7, 3 p. Backhouse, J., 2002, Palynology Report on Samples from the Onshore Carnarvon Basin: Geological Survey of Western Australia, Statutory Petroleum Exploration Report G31560A1, 6 p. Benn, D.I., and Evans, D.J.A., 1998, Glaciers and Glaciation: London, Arnold Publishers, 734 p. Bradshaw, J., 1990, Geological cross-section across the Petrel sub-basin, Bonaparte Basin: Bureau of Mineral Resources, Geology and Geophysics Report, v. 1990, no. 72, 16 p.

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