Platform margins, reef facies, and microbial carbonates; a comparison of Devonian reef complexes in the Canning Basin, Western Australia, and the Guilin region, South China

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Earth-Science Reviews 88 (2008) 33 – 59 www.elsevier.com/locate/earscirev

Platform margins, reef facies, and microbial carbonates; a comparison of Devonian reef complexes in the Canning Basin, Western Australia, and the Guilin region, South China Jian-Wei Shen a,⁎, Gregory E. Webb b , John S. Jell c a

Department of Marine Geology, South China Sea Institute of Oceanology; and CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China b School of Natural Resource Sciences, Queensland University of Technology, Brisbane, G. P.O. Box 2434 QLD 4001, Australia c Department of Earth Sciences, University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia Received 22 August 2007; accepted 10 January 2008 Available online 4 February 2008

Abstract Devonian reef complexes were well developed in Western Australia and South China, but no detailed direct comparison has been made between reef building in the two regions. The regions differ in several respects, including tectonic, stratigraphic and palaeoceanographic–palaeogeographic settings, and the reef building styles reflect minor differences in reef builders and reef facies. Similarities and differences between the two reef complexes provide insights into the characteristics of platform margins, reef facies and microbial carbonates of both regions. Here we present a comparison of platform margin types from different stratigraphic positions in the Late Devonian reef complex of the Canning Basin, Western Australia and Middle and Late Devonian margin to marginal slope successions in Guilin, South China. Comparisons are integrated into a review of the reefal stratigraphy of both regions. Reef facies, reef complex architecture, temporal reef builder associations, 2nd order stratigraphy and platform cyclicity in the two regions were generally similar where the successions overlap temporally. However, carbonate deposition began earlier in South China. Carbonate complexes were also more widespread in South China and represent a thicker succession overall. Platforms in the Canning Basin grew directly on Precambrian crystalline basement or early Palaeozoic sedimentary rocks, but in South China, carbonate complexes developed conformably on older Devonian siliciclastic strata. Pre-Frasnian reef facies in South China had more abundant skeletal frameworks than in Canning Basin reefs of equivalent age, and Famennian shoaling margins containing various microbial reefs may have been more common and probably more diverse in South China. However, Late Devonian platform margin types have been documented more completely in the Canning Basin. Deep intra-platform troughs (deep depressions containing non-carbonate pelagic sediments — Nandan-type successions) that developed along syndepositional faults characterize Devonian carbonate platforms in South China, but have no equivalent on the Lennard Shelf, Canning Basin where inter-reef areas were more shallow. The South China platform-to-depression pattern was generally continuous from the Lower to Upper Devonian, indicating that many pre-Devonian tectonic features continued to exercise considerable effect through deposition. Localized, fault-controlled subsidence was an important factor in both regions, but similarities in 2nd order aggradation– progradation cycles suggest that eustasy was also an important control on the larger scale stratigraphic development of both regions. © 2008 Elsevier B.V. All rights reserved. Keywords: reef complexes; Devonian; Canning Basin; Western Australia; Guilin; South China

1. Introduction Well preserved Devonian (Givetian to Famennian) reef complexes of the Canning Basin, Western Australia (Figs. 1, 2)

⁎ Corresponding author. Fax: +86 20 84458964. E-mail address: [email protected] (J.-W. Shen). 0012-8252/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2008.01.002

have been studied for decades using multidisciplinary techniques both for academic and economic purposes and have provided the basis for a ‘typical’ ancient reef model (Playford, 1980, 1984). That model has been useful for understanding ancient reef complexes that have not yet been studied in detail and/or are not as well preserved or exposed. Detailed information regarding the types of reefs and reef complexes in the Canning Basin include a variety of general and stratigraphic studies (Playford and Lowry,

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Fig. 1. Study areas and location of the Devonian reef complexes in Australia and South China.

1966; Playford, 1980, 1984; Playford and Cockbain, 1989; Kennard et al., 1992; Southgate et al., 1993; Whittam et al., 1994; Copp, 2000; Stephens and Sumner, 2003b), major palaeobiological studies (e.g., Veevers, 1959; Wray, 1967; Hill and Jell, 1970; Miles, 1971; Cockbain, 1984; Rigby, 1986; Becker et al., 1993; Long, 1995; Brownlaw and Jell, 1997; George, 1999; Brownlaw, 2000; Wood, 2000, 2004; Stephens and Sumner, 2003a; Chow and George, 2004), sedimentologic studies (Kerans, 1985; Kerans et al., 1986; Wallace, 1987; George et al., 1994, 1995; Holmes and Christie-Blick, 1993; George et al., 1997; Wood, 1998; Wood and Oppenheimer, 2000; Webb, 2001), and tectonic investigations (Begg, 1987; Kennard et al., 1994; Dörling et al., 1996; Ward, 1999; Chow et al., 2004). Devonian strata are also well preserved and widespread in South China (Fig. 1), with successions spanning Early Devonian (Siegenian) to Late Devonian (Famennian) time (Hou, 1978). Reefs and reef complexes were well developed from the Emsian continuing through to the Famennian (Wu, 1988; Yu and Shen, 1998), although no single reef complex spans the entire interval. Most previous work on Chinese Devonian reefs consists of local or sporadic studies typically concentrated on individual reefs with little discussion of related facies or regional context (Zhou et al., 1985; Tsien et al., 1988; Wu, 1988; Wu et al., 1988; Yu and Wu, 1988; Zhou et al., 1988; Shen et al., 1994; Zhou, 1996; Chen et al., 2001). However, Yu and Shen (1998) provided a comprehensive compilation on Devonian reef complexes in Guilin, South China, providing details on reef stratigraphy (e.g., biostratigraphy, lithostratigraphy and sequence stratigraphy), reef builders, and diagenesis. Playford (1969) compared Canning Basin reef complexes to Devonian reef complexes in western Canada. However, to date there have been no detailed comparisons of Devonian reef complexes in Western Australia and South China, despite both regions having extensively exposed carbonate platform successions in similar subtropical settings. The similar timing of extinction of major stromatoporoid reef builders prior to the end of the Frasnian has been recognized in the Canning Basin (George and Chow, 2002) and Guilin (Yu and Shen, 1998; Chen et al., 2002), and

Tsien et al. (1988) and Copper (2002a) drew attention to similarities between Devonian reef complexes in the two regions, but specific details were not provided. During Middle and Late Devonian time South China and Western Australia were positioned near each other in the southern subtropics east of the Palaeotethys (Fig. 3). Australia was on the northern margin of Gondwana, whereas South China occupied on an isolated microcontinent, although reconstructions are somewhat poorly constrained. Regardless, the two regions clearly were separated by a seaway by Late Devonian time (e.g., Golonka et al., 1994). The focus of this paper is to assess the similarities and differences between the reef complexes of these two relatively well-known areas in the southeast Palaeoththys, in order to place them in the larger context of Palaeotethyan palaeogeography. We compare the increasingly well-known Chinese Devonian reef complexes to coeval Australian complexes based on much previous work and some new petrologic and facies descriptions of particular reef margin examples in the Canning Basin. The increasing level of knowledge of the reef complexes in these two important regions allows a detailed comparison between them, and thereby an increased understanding of both. Aspects of platform margins, reef facies and microbial carbonates are described, compared and contrasted in particular. 2. Geological setting 2.1. Devonian stratigraphy and reef complexes in the Canning Basin Devonian rocks are well developed in the onshore and offshore extension of the Lennard Shelf in the northern Canning Basin (Forman and Wales, 1981, Towner and Gibson, 1983) (Fig. 2). The Canning Basin was an active intracratonic sag basin that opened into the eastern end of the Palaeotethys during Devonian time (Fig. 3) (Metcalfe, 1996; Li and Powell, 2001; Golonka, 2002). Lennard Shelf limestones were first described by Hardman (1884), and much of the subsequent work was summarised by Playford (1980, 1984). Devonian reef complex

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Fig. 2. Devonian outcrops in the Canning Basin, Western Australia (modified from Playford, 1980). Localities of primary platform margin examples discussed in text are indicated as follows: MG Menyous Gap; WP Wagon Pass; GG Galeru Gorge; WG Windjana Gorge; HR Horseshoe Range.

development in the Canning Basin occurred on the shallow northeastern and southwestern sides of fault-bounded blocks flanking the northwest-southeast trending Fitzroy Trough, which formed through significant crustal extension (Yeates et al., 1984; Drummond et al., 1991). The Lennard shelf bordered the Kimberley block landmass to the northeast and adjoined open ocean to the northwest with a large continental embayment to the south and southwest. Reefs developed on the opposing side of the trough are not exposed and are poorly known. Reefs of similar age are also known from the Carnarvon to the south and from the Bonaparte Basin to the northeast (Cockbain and Playford, 1988; Mory and Beere, 1988). Reef complexes on the Lennard Shelf are spectacularly exposed as a series of limestone ranges that crop out for 350 km along the northern margin of the Canning Basin (Playford, 1980) and extend into the subsurface to the northwest of the exposed section (Southgate et al., 1993; Copp, 2000). The width of total Devonian carbonate exposure on the Lennard Shelf varies from ∼ 3 km at Napier Range to ∼ 50 km at Pillara Range. Individual

complexes are more than 1000 m thick, and the maximum thickness of the entire sequence is up to 2000 m (Playford, 1984). Devonian rocks on the Lennard Shelf may be conveniently subdivided into (a) reef complexes, (b) conglomerates associated with the reef complexes, (c) pre-reef-complex units, and (d) post-reef-complex units (Playford, 1980, 1984). Several schemes have been devised for the stratigraphic nomenclature of the reef complexes (Guppy et al., 1958; Playford, 1980, 1984), but the most commonly used nomenclature is that of Playford (1984), wherein broadly transgressive upper Givetian and Frasnian strata are grouped into the Pillara Cycle, and the regressive Famennian strata are grouped into the Nullara Cycle (Fig. 4). To avoid confusion with current usage of the term ‘cycle’, the terms ‘Pillara Cycle’ and ‘Nullara Cycle’ are from here on termed Pillara phase and Nullara phase except where specific reference is made to the original usage in works by, for example, Copp (2000). Through the 1990s, many researchers focused their studies on types of carbonate platforms, sequence stratigraphy and eustasy in the Devonian reef complexes (Becker

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Fig. 3. Plate tectonic map of Middle–Late Devonian, 380–359 Ma (after Fig. 13 of Golonka, 2002). 1. Oceanic spreading center and transform faults; 2. Subduction zone; 3. Thrust fault; 4. Normal fault; 5. Transform fault; 6. South China Block; 7. Australia, 8. Lennard Shelf, Western Australian and Guilin, South China.

et al., 1993; Southgate et al., 1993; Kennard et al., 1994; George et al., 1997; Becker and House, 1997; Ward, 1999). Kennard et al. (1992), Southgate et al. (1993) and Whittam et al. (1994) considered the Pillara and Nullara phases to be parts of a 2nd order transgressive–regressive cycle based on subsurface seismic and well data. They recognized numerous 3rd order sequences within the two phases, and subsurface data from cores allowed Copp (2000) to define three sequential facies associations (i.e., subdivisions) within the Pillara phase. George et al. (2002) recognized seven distinct phases (four in Pillara time and three in Nullara time) of platform development in the Napier Range. Three basic settings, the inter-reef areas, marginal slopes and reefal limestone platforms, are recognized in both phases of reef building (Playford, 1984). The reef complexes range in age from middle Givetian to late Famennian and mostly grew on Precambrian basement (e.g., Playford and Lowry, 1966; Playford, 1980; Johnson and Webb, 2007), except in the Emanuel Range, where a veneer of Ordovician strata separate the reef complex from Precambrian basement. Earliest Givetian carbonates are more ramp-like. No complete successions from Givetian to Famennian crop out; most individual sections expose only part of the Givetian, Frasnian or Famennian owing to low regional dips, the relatively small area of many platforms, and the low degree of subsequent tectonic deformation and dissection. Rocks of Givetian age, based on the occurrence of Geminospora lemurata (Hocking et al., 1996), are known primarily from complexes in the Pillara–Home–Emanuel Ranges area. Reef limestones in the Emanuel Range overlie the Givetian Cadjebut Formation, which consists of dolomitic carbonates, siltstones and evaporites that were deposited in a restricted setting above the basement un-

conformity (Grey, 1992; Hocking et al., 1996). The Givetian– Frasnian boundary is difficult to trace in platform and platform margin facies owing to discontinuous outcrops and because most exposed complexes are Frasnian in age. Overlying Famennian reef complexes largely have been removed by erosion. However, Famennian complexes were originally more extensive than those of the Frasnian over much of the area (Playford, 1984; Playford et al., 1989). The Frasnian–Famennian (F–F) boundary has been documented in several sections (Becker et al., 1991; George and Chow, 2002; George et al., 2002). Regardless, biostratigraphic boundaries are very difficult to recognise in the shallow platform facies leading to difficulties in correlating between different shallow-water complexes (e.g., Copp, 2000; George et al., 2002). The carbonate sequence is overlain conformably by the Upper Devonian to Mississippian siliciclastic-dominated Fairfield Group. Playford (2002) attributed the loss of carbonate deposition and extinction of the reefs to an exposure event followed by inundation by siliciclastic facies with renewed transgression in the latest Famennian and Tournaisian. 2.2. Devonian stratigraphy and reef complexes in South China South China was located east of the Palaeotethys during Middle–Late Devonian time, near the margin of Gondwana northwest of Australia (Fig. 3) (Metcalfe, 1996; Golonka, 2002). Devonian rocks in South China extend over a region covering Hunan, Guizhou, Guangxi, the eastern part of Yunan and the northern part of Guangdong Provinces and include the southwestern part of the Yangtze Craton and the Huanan fold-belt (Fig. 5). Devonian reefs and reef complexes developed in shallow

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Fig. 4. Schematic diagram of Canning Basin Devonian reef complex development through time with nomenclature (after Playford, 1980; Copp, 2000). Approximate relative stratigraphic positions of platform margin examples discussed in text are indicated in boxes. Exact stratigraphic positions of examples cannot be constrained owing to the lack of detailed biostratigraphic data.

settings on the northern side of the Huanan Sea, which formed due to subsidence of the western part of the Huanan fold belt as South China rifted from Gondwana. The Huanan Sea covered the southern parts of the continent while the northern Huanan Landmass was emergent. Reefal platforms (Xiangzhou facies) extended 210–380 km from nearshore to offshore and were up to 650 km long. Middle and Late Devonian shallow-marine carbonate platforms in Guilin were nearly 40 km wide from east to west and ∼80 km north to south. The Devonian System was deposited on a subsiding shelf, with individual depositional centres and subsidence rates, bordering the Jiang Nan Shield on the north, the Yun Kai Island on the east and the Nieu Sheu Shan Island to the west. Thus, palaeogeography was quite heterogeneous compared to the more linear Lennard Shelf of the Canning Basin. South China Devonian rocks have been differentiated broadly into littoral facies (Qujing facies), shallow carbonate platform facies (Xiangzhou facies), and pelagic facies (Nandan facies) (Ruan and Mu, 1983; Liao and Ruan, 1988; Yu, 2000, Fig. 6). Lower Devonian strata are characterized mainly by siliciclastic rocks, typically resting with angular unconformity on pre-Devonian strata. Middle

and Upper Devonian strata are characterized principally by widespread shallow-water platform carbonates (Xiangzhou facies) surrounded by deeper pelagic deposits (Nandan facies) that occur in a system of relatively deep intra-platform troughs (Hou and Wang, 1988). Reefs and microbial mounds were extensively developed in shallow platform settings through the interval (Zhou, 1996; Wang, 1996; Yu and Shen, 1998). Eight local stages in the Chinese section (Yang et al., 1981; see Tsien et al., 1988, their Table 1) have been correlated with the European standard conodont succession (Bai et al., 1978; Wang and Ziegler, 1983). Reef-bearing formations (Xiangzhou Facies) were mainly developed on pre-Devonian and Early Devonian siliciclastic substrates whose topography was controlled by pre-Devonian or reactivated pre-Devonian tectonic features (e.g., Chen et al., 2001). In the Guilin area, the Eifelian (Middle Devonian) to Viséan (Mississippian) sequence is mostly conformable and almost complete, with thicknesses of carbonate deposits being as much as 1300 m in restricted platform and back-reef successions and 1800 m in the platform margin succession. Although, most biostratigraphic boundaries are difficult to define in the platform margin facies, the

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Fig. 5. Famennian palaeogeographic reconstruction of South China and position of Guilin platform.

Eifelian–Givetian, Givetian–Frasnian, Frasnian–Famennian, and Famennian–Tournaisian boundaries are discernible in shallow carbonate platform facies on the basis of changes in lithology and fossil content. The boundaries are traceable in the fore-reef slope facies using conodont biostratigraphy (Yu et al., 1988; Yu and Shen, 1998). 3. Comparison of Devonian platform margins and reef facies in the Canning Basin and South China Carbonate platform margins may have variable architecture and constituent facies ranging from almost continuous frame-constructed barrier reefs to oolitic or bioclastic sand shoals with or without isolated patch reefs or microbial mounds (e.g., James, 1983; Fagerstrom, 1987). A variety of different margin types occurred on the Devonian platforms of the Lennard Shelf and South China. Facies terminology used below is modified slightly from Playford (1984, his figure 19), with: 1) marginal slope divided into proximal reefalslope and more distal fore-reef facies or fore-bank facies, 2) platform margin divided into reef-margin or shoaling margin

facies, and 3) limestone platform divided into reef-flat immediately adjacent to the margin and more distal backreef facies or bank facies. 3.1. Devonian platform margins in the Canning Basin Devonian carbonate platform margin facies in the Canning Basin are generally narrow, commonly discontinuous, and may be missing completely due to margin collapse, exposing reef-flat or back-reef facies at the margin in central and northern reef complexes (e.g., Napier Range, northern Lawford Range–McIntyre Knolls area; Playford, 1981; Ward, 1999). Platform margin facies represent a variety of water depths, but were mostly close to sea level, commonly with high relief and differing topographic configurations ranging from steep to gentle (Playford, 1984). Kerans (1985) provided the most detailed summary of lithofacies in the reef complexes. Lithofacies at platform margins range from rudstones, grainstones or packstones containing bioclastic debris, ooids, oncoids, peloids, and intraclasts to boundstones dominated by stromatoporoids and/or calcimicrobes. Where present, reef margin boundstones are typically massive, commonly a few tens

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Fig. 6. Palaeozoic stratigraphic framework in major Devonian sedimentary basin of South China (from Guangxi, Guizhou and Hunan Bureau of Geology and Mineral Resources, 1985, 1987 and 1988).

of meters wide, or less, and grade into thickly bedded reef-flat facies on the platform and more thinly bedded, reefal-slope facies sloping towards the adjoining basin to the south or inter-reef areas more locally. Reef-flat facies may extend up to 1 km into the platform where they pass into shallow back-reef or bank facies with gradational and interfingering contacts (Playford, 1984). Kerans (1985) considered the contact between reef flat and backreef facies to be marked by the last occurrence of Renalcis behind the reef margin. Back-reef facies commonly exhibit paraseqencescale cyclicity (Read, 1973a,b; Playford et al., 1989; Brownlaw et al., 1996, 1998; Hocking and Playford, 2001), which in some

cases appears to reflect Milankovich periodicities (Brownlaw et al., 1998). Pervasive early cementation played an important role in lithification of marginal reef framework, grainstones, rudstones and even lime mud-rich sediments to form rigid wave-resistant margins on many of the Lennard Shelf platforms, even in shoaling settings (Playford, 1980; Kerans, 1985; Kerans et al., 1986; Wallace, 1987). The abundant cementation of the proximal reefalslope facies combined with the narrow reef-margin facies makes identification of the reef-margin facies difficult where the proximal slope stands in high relief and the backreef is recessive

Table 1 Comparison of platform margin characteristics on the Lennard Shelf, Canning Basin and in South China

Width (Reef Crest Zone) Age Lateral subfacies change Marginal facies Frasnian reef builder in margin Famennian reef builder in margin Shallow water shoals

Canning Basin

Guilin (South China)

Narrow, tens of meters wide Givetian to Famennian Backed by reef-flat subfacies Biogenic clasts and fragments, ooids, peloids, micritic intraclasts, and lime mud Stromatoporoids, algae, microbes and corals, commonly forming more or less conspicuous frameworks Calcimicrobes and algae, sponges, synsedimentary cements Common in Frasian and Famennian

Narrow Givetian to Famennian Not commonly backed by reef- flat subfacies Ooids, algae/microbes, microbial peloids, microbial aggregates, oncoids, and lime mud Givetian stromatoporoid framework, Frasnian massive and tabular stromatoporoids and microbes Microbe, algae, cemenstone, sponges are not common Well developed in Famennian

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causing a disjunction at the slope break where the margin should occur. On the outcrop it is common to progress up an exhumed reefal-slope facies only to find it terminate at a small valley with recessive back-reef facies sloping away in the opposite direction due to differential compaction caused by increased stylolitization of the platform interior (Playford, 1980); facies representing the exact slope break are commonly difficult to identify, being a narrow, irregular belt of typically massive facies or being absent due to collapse. However, well exposed examples of a variety of platform margins have been studied, and facies at platform margins varied both spatially and temporally (e.g., see Playford, 1984; Playford et al., 1989; Webb, 2001). Although the lack of detailed biostratigraphic data for many margin outcrops precludes detailed investigation of temporal changes in margin facies, and thus disentanglement of geographic and temporal variations, five temporally successive platform margin examples are discussed briefly below to illustrate different margin facies associations. They include: 1) a cyclic Givetian to Frasnian bank margin at Menyous Gap, Pillara Range (Webb and Brownlaw, 2000); 2) a high-energy, early Frasnian (possibly ‘Pillara Cycle association 2’ of Copp, 2000) Stachyodes-dominated margin at Wagon Pass, Lawford Range; 3) a later Frasnian high-energy stromatoporoidRenalcis margin with collapse-prone reefal-slope at Galeru Gorge, Lawford Range; 4) a late Frasnian (latest ‘Windjana Gorge Sequence’ of Whittam et al., 1994) Renalcis-dominated framework margin at Windjana Gorge in the Napier Range (e.g., Kerans, 1985; Wood, 1998); and 5) a prograding Famennian margin at Horseshoe Range (Webb, 2001). The geographic and stratigraphic positions of the margins are shown in Figs. 2 and 4. Although the exact ages of the specific margin examples are unknown owing to the lack of biostratigraphic data, their general temporal succession is constrained on the basis of mapping by Playford and Hocking (1999) and stratigraphic reconstructions of Playford (1980), Hall (1984), Whittam et al. (1994) and Copp (2000). Low-relief, cyclic, commonly retreating Givetian to early Frasnian bank and platform margins are well exposed on the northeastern flank of the Pillara Range (Fig. 7) (lower Pillara Cycle of Copp, 2000). Cyclic, low-relief bank deposits, such as those exposed in Menyous Gap, consist mostly of subtidal cycles that represent parasequences wherein sub-fair weather wave base facies alternate with shallower, but still subtidal facies near or above wave base. Exposure surfaces are absent at the tops of most cycles (Brownlaw et al., 1996), but Read (1973a,b) described rare examples with exposure. Complete individual cycles in the platform interior consist of 1) rare recessive carbonate mudstone-wackestone at the base; followed by 2) recessive coral facies dominated by Disphyllum, Argutastrea, Alveolites, and/or Thamnopora in growth position, but crushed, with minor thin stromatoporoids in a muddy matrix; 3) more resistant tabular stromatoporoid facies characterized by tabular to low domal stromatoporoids in growth position in muddy matrix; 4) much more resistant domal stromatoporoid facies with more abundant, large (60 cm) stromatoporoids in growth position in variable wackestone–packstone matrix; 5) resistant Stachyodes facies consisting of abundant Stachyodes (stromatoporoid) floatstone with peloidal packstone–grainstone

matrix; 6) Amphipora (stromatoporoid) facies consisting of Amphipora floatstone in peloidal wackestone–packstone matrix; 7) resistant peloidal facies consisting of well sorted peloidal packstone–grainstone; and 8) resistant fenestral limestone facies consisting of peloidal packstone–grainstone containing abundant fenestrae (Read, 1973a,b; Brownlaw et al., 1996). Cyclicity is also evident at the margin (Webb and Brownlaw, 2000). A generalized complete cycle from just behind the margin includes (1) recessive, poorly cemented coral facies containing ramose thamnoporid and alveolitid tabulate corals with less abundant thin stromatoporoids in fine-grained matrix, overlain by (2) mounded, resistant tabular stromatoporoid reef-mound facies wherein isolated, constratal tabular-low domal stromatoporoids occur in growth position in packstone–wackestone matrix, (3) resistant stromatoporoid reef facies with tabular to domal stromatoporoids in growth position, rarely encrusting each other, and forming lowrelief, cavity-bearing framework, and (4) coarse (N2 cm) debrisbearing, large oncoid floatstones. At Menyous Gap the uppermost (shallowest) two facies were not developed, and the margin succession alternates between the recessive coral facies and the more resistant tabular stromatoporoid reef-mound facies. At the slope break the tabular stromatoporoid reef-mound facies passes laterally into a massive tabular tabulate coral facies that consists of carbonate mudstone containing isolated tabulate corals in growth position, brachiopods, debris and cement-filled cavities. That facies passes down slope into well-bedded proximal forebank slope facies. The recessive coral-dominated facies was continuous from the bank interior over the bank margin during intervals of higher sea level (Webb and Brownlaw, 2000), except where they pinched out, or were subsequently eroded, over mounded facies in somewhat higher energy (i.e., shallower) cycles, such as at Menyous Gap (Fig. 7). The retreating bank margins gave way to more abrupt, steeper Renalcis-dominated reef margins higher in the Frasnian succession in the Pillara area (Hall, 1984; Benn, 1984; Cooper et al., 1984; Kerans, 1985). In other areas, such as the Emanuel Range, ooid shoals occurred at the Givetian platform margin (Warren and Kempton, 1997). Frasnian reef-margin facies in the Canning Basin were constructed primarily by stromatoporoids and microbes (e.g., Renalcis and abundant micritic microbialites, Playford, 1984) that formed more or less conspicuous frameworks, with abundant to uncommon in situ frame-building skeletal organisms. However, despite similar constructors, reef facies differed substantially from place to place. The early to middle Frasnian margin at Wagon Pass (Fig. 8A) occurs on both sides of a narrow ridge (a ‘spine’ using the term of Playford, 1980) with reefal-slope facies dipping away both to the east and west. Margin facies consist of a very narrow (3–4 m wide) zone characterized by scarce, in situ columnar and domal stromatoporoids (Fig. 8B) typically isolated within Stachyodes rudstone with varying, but generally minor, degrees of binding by Renalcis. The margin facies on both sides of the spine pass into proximal reefal-slope facies consisting of well-winnowed, bedded Stachyodes rudstone (Fig. 8C). Towards the interior of the spine the margin passes into a very limited reef flat facies consisting of more massively bedded Stachyodes rudstone with laminar stromatoporoids on bedding planes proximally and

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Fig. 7. Platform margin at Menyous gap, Lennard Shelf. Platform has 14° structural dip to east. (A) Cyclic platform (bank) facies consist of alternating recessive coral (RC) and tabular stromatoporoid (TS) facies. At the margin, tabular stromatoporoid facies passes into mounded stromatoporoid reef-mound facies (SRM) and then into mud-mound-like tabular tabulate coral facies (TT) before passing into proximal forebank-slope (MS). The recessive coral (RC) facies passes over the margin, but pinches severely over the mounded tabular tabulate facies. Scale bar equals 5 m. (B) Section through cyclic bank facies behind margin showing alternation of resistant tabular stromatoporoid facies and recessive coral facies. (C) Section through stromatoporoid reef-mound facies and massive tabular tabulate facies at margin.

more overturned stromatoporoids distally. The entire reef-flat was only tens of meters wide between the oppositely directed margins at Wagon Pass. Farther north in the Lawford Range the apparently younger Frasnian margin at Galeru Gorge to the west of McIntyre Knolls consisted of abundant individual relief-bearing patch or pinnacles reefs (Fig. 9A) that consist of massive Renalcis-bound laminar stromatoporoid facies (Fig. 9B). The very thin (b 1 cm) stromatoporoids are inconspicuous on the outcrop, and Stachyodes

debris is much less abundant than to the south at Wagon Pass. Reefal-slope facies is massive proximally and contains abundant intraclastic rudstones and debris beds distally. The reef flat was dominated by Stachyodes proximally, in some cases well bound by Renalcis, and passed into Amphipora facies distally to the north. The margin was affected by collapse in many places, but relatively smooth transitions from reefal-slope into more massive margin facies occur in places. Farther to the east, margin collapse gave rise to the McIntyre Knolls debris blocks (Playford, 1981).

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Fig. 8. (A) Schematic diagram of Frasnian platform margins on reef spine at Wagon Pass, Lawford Range, view towards south. (B) Domal stromatoporoids in reef margin facies. (C) Proximal fore-reef slope composed of bedded Stachyodes rudstone. Lens cap is 6 cm in diameter.

At Windjana Gorge the reef margin has a near vertical scarp termed an ‘upright-scarp’ by Playford (1984) (Fig. 9C). In some places it was clearly controlled by collapse, and large blocks occur in the proximal fore-reef slope facies. In some cases, well bedded reef-flat facies containing abundant, commonly overturned domal stromatoporoids (Fig. 9D), Stachyodes, Renalcis and commonly dolomitised siliciclastic quartz-rich beds abut bedded reefal-slope facies with a very sharp vertical contact due to margin collapse (e.g., Playford et al., 1989). The margin facies, where it is preserved, is dominated by stromatoporoid and Renalcis boundstone and passes into thickly bedded columnar stromatoporoid-Renalcis boundstone interbedded with stromatolitic beds towards the reef flat (Playford and Cockbain, 1992). The proximal reefal-slope abuts the steep margin abruptly and also contains abundant stromatoporoids and Renalcis, with framework components becoming increasingly isolated within finer grained facies distally in the fore-reef facies (e.g., Wood, 1998). Apart from reef margins, many Frasnian platform margins consisted of well-cemented oolitic shoals (Playford, 1984). Playford (1980) emphasized the importance of shallow-water shoals in the platform margins of the Canning Basin, and Warren and Kempton (1997) presented more detailed sedimentological information on the shoals. Frasnian oolitic shoal margins (e.g., near Nadji Cave in Bugle Gap in the Lawford Range) consist of massive oolitic and coated grain grainstones, but the structure of

individual shoals remains to be documented. Scattered stromatoporoid patch reefs occurred behind the margin amid shoaling sands, and abundant tabular stromatoporoids stabilized some horizons in the most proximal reefal-slope facies, but the margin was stabilized largely by pervasive early cementation of the carbonate sand (Playford, 1980). Although small-scale collapse events modified steep margins in many places, large-scale platform margin collapse occurred in the northern Lawford and southern Laidlaw Ranges where platform margin facies and even reef-flat facies are missing from the edges of platforms, and back-reef facies abut collapse scarps. Large collapse blocks (e.g., McIntyre Knolls and the nearby McPhee Knoll) east of Galeru Gorge south of the northern Lawford Range contain thickly bedded facies of reef flat affinity (with minor back-reef) containing abundant Renalcis and Stachyodes boundstones. Reef-margin facies from the same region contain laminar stromatoporoids, Renalcis and abundant micritic microbialite. Neptunian dykes are common in the reef-margin, reef flat facies and some parts of the marginal-slope facies in many parts of the reef complex, but they are rare in the back-reef facies (Playford, 1980, 1984). They are a consequence of abundant early marine cementation combined with synsedimentary seismicity and over-steepening due to compaction of basinal muds (Playford, 1984). Where the reef margin fringed the continent (e.g., at Windjana Gorge, Napier Range), neptunian dykes commonly contain siliciclastic deposits rich in coarse quartz sand.

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Fig. 9. (A) Massive mounded Frasnian reef margin facies passing into slope on eastern margin of Galeru Gorge, Lawford Range, Lennard Shelf, view towards south. (B) Laminar stromatoporoid-Renalcis boundstone in relief-bearing margin facies at Galeru Gorge. (C) Massively bedded Frasnian reef flat facies abutting fore-reef slope at collapse margin on top of range, northern side of Windjana Gorge, Lennard Shelf, across from the ‘classic face.’ (D) Domal stromatoporoids in Frasnian reef flat facies at Windjana Gorge.

Frasnian reefal-slope and fore-reef facies in the Canning Basin are variable but consist mainly of well-bedded bioclastic or intraclastic rudstone–floatstone, bioclastic packstone, wackestone, and mudstone containing debris flow breccias in some areas (Playford, 1984; Kerans, 1985). Microbial boundstones are also common in some areas (George et al., 1997; George, 1999), but shale, cherty layers and concretions are rare. Neptunian sills are abundant in fore-reef slope deposits in some places (Playford, 1984). Proximal reefal-slope facies at Windjana Gorge include stromatoporoids and Renalcis in relatively massive boundstone interbedded with wackestone (e.g., Kerans, 1985; Wood, 1998). In the somewhat deeper setting at Menyous Gap, Pillara Range, proximal fore-bank facies consist of thinly bedded floatstone, packstone and wackestone. Poorly winnowed packstones with abundant cement-filled shelter porosity suggest that some beds are tempestites. Stachyodes rudstone dominates the proximal reefal-slope at Wagon Pass and McWhae Ridge in the Lawford Range (Fig. 8C), whereas collapse breccias are more abundant farther north in the Lawford Range at Galeru Gorge and to the east. Although the steepness of platform margins and degree of back-stepping have broad temporal controls imposed by sea level (e.g., transgression, Playford, 1980, 1984; Kerans, 1985; Playford et al., 1989; Copp, 2000) and possibly facies changes (e.g., development of steep, well-cemented calcimicrobe frameworks), syn-depositional tectonic controls clearly played a

major role in the backstepping geometry of the Frasnian reef complexes in many places (Ward, 1999; George et al., 2002; Playford, 2002). Frasnian reef flat facies in the Canning Basin consist of medium- to thick-bedded limestones (Playford et al., 1989). Early Frasnian banks contained proximal Thamnopora, and massive rugose corals such as Argutastrea (e.g., Menyous Gap, Pillara Range) occurring with cemented peloids, bioclasts, and mudstone. Late Frasnian reef flats contained tabular and massive stromatoporoids proximally passing into Stachyodes- and then Amphipora-dominated facies distally. Renalcis is common locally where it may bind in situ Stachyodes into rigid, low-relief frameworks (e.g., at Galeru Gorge, northern Lawford Range). Stachyodes rudstones containing scattered massive and tabular stromatoporoids occur in the reef flat facies of Wagon Pass and McWhae Ridge, Lawford Range. Large oncoids (N 3 cm) occur in proximal shallow backreef shoals in the southern Oscar Range (Johnson and Webb, 2007) and in the shallowest cycles in the Pillara Range southeast of Menyous Gap. Famennian platform margins in the Canning Basin were built primarily by microbes, calcareous algae, and synsedimentary cement (Playford, 1980; Kerans, 1985; Webb, 2001; Stephens and Sumner, 2003a; Chow and George, 2004) with minor, locally abundant sponges. Microbial frameworks are best known from Windjana Gorge and nearby areas along the Napier Range. Margins were stabilized dominantly by microbes, primarily

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Renalcis, but Rothpletzella and Girvanella occupied minor roles as reef builders (Playford, 1980, 1984; Kerans, 1985; Stephens and Sumner, 2003a). Famennian reefal fabrics are also well preserved in allochthonous blocks as described by Playford (1984), Kerans (1985), and George et al. (1997). Large skeletal organisms are rare, although sponges occur in the Napier Range northwest of Windjana Gorge, and the solenoporoid calcareous alga Parachaetetes occurs there and at northeastern Horse Spring Range (Webb, 2001). Thin stromatoporoids that are inconspicuous on outcrops occur in small mounds in the proximal reefalslope facies (Webb, 2001). They may also occur in the margin facies but have not been identified there for certain. Large (N 40 cm diameter) dome-shaped stromatoporoids occur in Famennian margin facies in fault slivers south of Horseshoe Range, but they appear to have been isolated in occurrence. In general margin facies contain abundant massive cementstone with cement-rich crusts, radiaxial spar cements, and minor amounts of bound sediment. Margin facies in some places (e.g., parts of the southern Horseshoe Range) are completely dolomitised. In the southwestern Horseshoe Range area, typical massive microbialcementstone margin facies pass sharply into well-bedded reefalslope facies (Fig. 10A) (Webb, 2001). The width of the margin facies is difficult to determine owing to the progradational geometry, but massive facies abut and overlie steeply dipping strata over lateral distances approaching 100 m in some cases, suggest-

Fig. 10. (A) Massive, prograding Famennian reef margin facies abutting well bedded fore-reef slope facies at southwestern edge of Horseshoe Range, Lennard Shelf, view towards north. (B) Famennian columnar stromatolites contained within pebble-rich siliciclastic facies in platform interior, Horseshoe Range, Lennard Shelf.

ing progradation at near still stand conditions. Playford (2002) suggested that later Famennian reef margins, below the termination of the Nullara phase, were steeper, but the facies have not been described in detail. In general, margin facies are poorly known though the Famennian owing to their common removal by erosion in many areas and the lack of biostratigraphic control. Famennian reef-flat to proximal back-reef facies consist of thick-bedded oolitic and oncoid shoaling facies containing stromatolites and thrombolites passing into cyclic fenestral mud-lump and pelloidal grainstones and less abundant interbedded siliciclastic units containing columnar stromatolites away from the margin (Webb, 2001; Stephens and Sumner, 2003a). Conglomerate beds grade to sandy limestone with well-developed cross bedding, and rounded pebbles are common in the sandy limestone. Columnar stromatolites are most abundant within siliciclastic intervals (Fig. 10B). Chow and George (2004) described tepee-shaped agglutinated microbial mounds from the Famennian carbonate platform in the Chedda Cliffs area, Napier Range, and deeper buildups composed of sponges and stromatolites occur in the forereef facies (Playford et al., 1976; Paul, 1996; George, 1999). 3.2. Devonian platform margins in South China Devonian platform margin facies in South China are also relatively narrow (i.e., laterally restricted) and are composed of massive and thick-bedded limestones (Fig. 11). However, successive outcrops from platform margin to fore-reef slope are lacking in South China, so it is difficult to describe the types of platform margin geometries. Regardless, known platform margin and fore-reef slope outcrops in Guilin suggest similar facies transitions as in the Canning Basin, and lithofacies of the platform margin are similar in composition, being characterised mainly by ooids, algae, microbes, peloids, oncoids, and micrite. In general, shoaling margins are more common and diverse in Guilin. Abundant shallow-water shoals along platform margins include oolitic shoals, oolitic and algal/microbial grain shoals, oncoid shoals, and bioclastic shoals dominated by nautiloids or brachiopods. The transition from reef-flat to reef margin facies is also gradational and indistinct in Guilin (Fig. 12). Large-scale wedge-shaped grainstone deposits (Fig. 13) occur in the reefalslope in Guilin carbonate platform systems, and they are considered to have originated as subaqueous shoals resting on truncated bedding structures. Comparison of platform margin characteristics in the Canning Basin and South China are summarised in Table 1. Early Givetian platforms in Guilin were generally low-relief stromatoporoid-coral-microbe/algal banks containing some biostromes. Little or no reef framework occurred around the margins of the early Givetian platforms in Guilin. However, unlike in the Canning Basin, true reef platforms occurred during the late Givetian in Guilin and continued growing until the Mississippian. Late Givetian platform margins in Guilin are characterized by stromatoporoid framestones with rare algae and microbes and are exemplified by the platform margins at Yangshuo Bridge (Yu and Wu, 1988) and Yanshan (Shen and Yu, 1996). Associated fore-reef slope facies occur at Liangshuijing and Wuguishan (Fig. 14) (Shen, 2002a). Fore-reef

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Fig. 11. Frasnian (Upper Devonian) retreating carbonate platform margin and marginal slope at Qifeng Zhen, Guilin. Outcrops of platform margin facies are mounded with ∼ 90 m of relief, composed of light colored, massive microbial limestone that contains Renalcis, red algae, and corals, and thick-bedded grainstone and packstone that contains ooids, bioclasts, and intraclasts. Outcrops of marginal slope facies consist of grey to dark grey, thin-bedded nodular limestones, turbiditic and graded limestones, and oncoid limestone.

slope facies at Liangshuijing near Yangshuo and Wuguishan near Yanshan were dated as late Givetian on the basis of conodonts (Shen et al., 1994) and indicate that true platform margins developed by that time. Earlier Givetian bank facies had restricted interiors with cyclic deposition (Yu and Shen, 1998). Dolomites occur commonly in the stromatoporoid limestones of Givetian restricted platform interior facies (Yu and Shen, 1998; Chen et al., 2004).

The Frasnian carbonate platform in Guilin was differentiated into an open platform facies, restricted platform or back-reef facies, reef flat facies, platform margin facies, and fore-reef slope facies (Shen and Zhang, 1994). The extent of reef flat facies is not clear, and it is characterized by bivalve-bearing cortoid floatstone, microbial-Stachyodes nodular floatstone, and rudstone/grainstone in which debris consists mostly of stromatoporoids, solenoporoid algae, corals, and micritic intraclasts

Fig. 12. Schematic reconstruction of Frasnian reef flat, marginal reef and proximal slope at Houshan, Guilin, South China.

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Fig. 13. Debris-flow related large-scale sediment wedge in the Frasnian (Upper Devonian) marginal slope facies (Qifengzhen Formation), Guilin.

Fig. 14. Types of Devonian platform margin facies successions and reef subfacies in Guilin, Guangxi, South China 1. wackestone and turbiditic limestone 2. large-scale crossbeddings 3. debris-flow breccias 4. bioturbated wackestone 5. slump structure 6. microbial nodule 7.bioclastic packstone 8. oncoid floatstone 9.gastropod wackestone 10. tentaculitid wackestone 11. Amphipora floatstone 12. stromatolite 13. Laminane 14. micrite 15. rudstone 16. brachiopods 17. chert layers 18. argilliceous limestone 19. dolomitic microbal-laminae 20. flat-pebble floatstone 21. lenticular limestone 22. shale 23. dolostone 24. peloidal micrite 25. brachiopod shelly limestone 26. marls 27. chert 28. graded grainstone 29. Siliciclastic rock.

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Fig. 15. Frasnian platform marginal reefal limestones, Guilin. (A) Rothpletzella-receptaculitid boundstone. Coin diameter = 24 mm. (B) Rothpletzella boundstone. Hummer length = 28 cm. (C) Stromatolite boundstone. Coin diameter = 24 mm. (D) Thrombolite boundstone. Coin diameter = 24 mm. (E) Renalcis framestone. Pen length = 14.5 cm. (F) Thin-sention microphotography showing Renalcis framestone.

(Fig. 12). Receptaculitid-microbial bioherms and Smithiphyllum bioherms (Shen and Zhang, 1997) occur in the reef flat facies, and stromatactis textures are well developed (e.g., Frasnian reef-flat facies at Houshan; Fig. 12). Back-reef facies in Guilin typically contain a variety of wackestones, commonly dominated by gastropods, Amphipora, other stromatoporoids, or brachiopods. Oolitic facies are less common in Frasnian platforms. Cyclic deposition was well developed in dolomitic peloidal limestones interbedded with fine, crystalline dolomite containing desiccation cracks, fenestral limestones, laminated limestones, and intraclas-

tic facies (Shen, 1993; Shen et al., 1994). Gong et al. (2001) suggested that cyclicity observed in the slope and basin facies reflects Milankovitch forcing. Frasnian platform margins contain massive and tabular stromatoporoids and microbes (Renalcis), although debris breccias occur in some fore-reef successions (Chen et al., 2001). The greater depth of the intra-platform troughs in South China may have isolated any large collapse blocks well away from their original margins, thus disguising their origin. Regardless, Frasnian reef facies commonly developed on platform margins and include examples such as the reef

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and mounds at Houshan (Shen and Zhang, 1997; Yu and Shen, 1998) and algal-microbial mounds at Qifengzhen (Yu, 1988; Yu and Shen, 1998). At Hantang, about 7 km northwest of Guilin, Frasnian platform margin reefs are characterized by Rothpletzellareceptaculitid boundstone, Rothpletzella boundstone, stromatolite boundstone, thrombolite boundstone, and Renalcis framestone (Fig. 15). Frasnian fore-reef slope facies in Guilin are characterized by debris flow breccias, bioturbated wackestones, shale, micrite, oncoid floatstone, chert layers, and concretions. Slump structures occur in places. Although Devonian carbonate platform evolution in Guilin previously was considered to reflect largely evolution from Givetian and Frasnian bioconstructed platform margins to Famennian sand-shoal systems (Regional Geological Survey Institute of Guangxi, 1994; Chen et al., 2002), recent studies have documented Famennian platform margins in Guilin that were dominated by microbial fabrics (Shen et al., 1997; Yu and Shen, 1998; Shen and Webb, 2004a,b). Most Famennian reefs in South China are barrier types that developed on platform margins with proximal reefal-slopes. The Famennian Yunghsien Formation (Fig. 6) in the Guilin area represents marginal reef facies that typically consist of microbial and oolitic limestones. Dominant margin lithologies are light grey, thick to massive microbial framestones and microbial bindstones intercalated with micritic limestone, peloidal grainstone, oolitic grainstone, and stromatolitic boundstone. Oncoid floatstone and rudstone occur but are not common. Calcimicrobes were the most abundant fossil group, and stromatactis fabrics are common in the reef facies (Shen and Zhang, 1997). Shoals formed by ooids, microbial peloids, nautiloids, and brachiopod shells also were well developed within margin facies at Guilin. Most of the known individual Famennian reefs in South China were described from the Guilin area. They were well developed along margins and proximal fore-reef slopes of three Bahamaslike carbonate subplatforms — the Guilin, Yangshuo and Yanshan subplatforms (Shen, 2002a). Individual reefs occur on the subplatform margins and proximal reefal-slopes of Zhaijiang, Miaomen, Shatang, and Yantang (Zhou, 1996; Shen et al., 1997; Yu and Shen, 1998; Shen and Webb, 2004a,b). Microbes, especially Renalcis, Epiphyton, and a laminated microbe (probably Rothpletzella), constructed these reefs without skeletal metazoans. Individual buildups were dominated either by Epiphyton, Renalcis and cement, or stromatolites. The microbe-cement frameworks represent high-energy shallow-water conditions. Stromatoporoid limestones are rare in the Famennian reefs and mounds, but occur and coexist with syringoporoid tabulate corals in latest Famennian biostromes of the open-platform facies. Neptunian dykes occur in the reef margin or reefal facies at Miaomen (Yu and Shen, 1998). The microbial reefs and mounds described from the Guilin area are typical of Famennian reef types in South China. However, relatively little work has been done on other coeval outcrops in South China, and more buildup types may be discovered with further detailed work. Microbes were not confined to reefs on Famennian platform margins and fore-reef slopes in South China; they also occupied low-relief facies over entire carbonate platforms. Such facies are characterized by common and well-developed microbial lami-

nites with fenestral fabrics (e.g. Dushan, South Guizhou, Issacson et al., 1999). Finely crystalline dolomites with desiccation cracks and fenestral fabrics, edgewise breccia, and laminated limestones have been documented in Famennian back-reef facies in Guilin (Yu and Shen, 1998), and strata are characterized by typical carbonate platform cyclicity (Shen et al., 1994). Famennian microbial buildups were poorly developed in nearshore settings of Guizhou, Hunan and Yunnan due to a rapid and extensive sealevel fall in the early Famennian, leading to deposition of siliciclastic sediments and low-relief microbial carbonates bordering the Jiang Nan Shield (Fig. 5) (e.g., Yu and Shen, 1998). Pervasive early submarine cementation occurred on Guilin Frasnian and Famennian margins, and radiaxial-fibrous calcite, multiple generations of cement in shelter-voids and stromatactis fabric are common in the reef margin and reef-flat facies (Shen and Zhang, 1997; Shen et al., 1997; Shen and Webb, 2004a,b). In summary, Devonian platform margins in South China are similar to those in the Canning Basin, although successive outcrops from platform margin to fore-reef slope as occur in the Canning Basin (e.g., Playford, 1984) are lacking in South China. Lithofacies and biota are similar in many cases, but some differences have been recognised (Fig. 16). Givetian and Frasnian back-reef and bank facies in the Canning Basin and South China are similar. Givetian platforms in the Pillara Range on the Lennard Shelf and the Guilin region were generally low-relief stromatoporoid-coral-microbe/algae banks containing some biostromes. This Givetian bank facies represents restricted interiors with cyclic deposition in both regions. Many Frasnian platform margins in the Canning Basin and Guilin have similar fabrics containing massive and tabular stromatoporoids and microbes (Renalcis). Wedge-shaped downlapping and onlapping grainstone deposits have been observed in the Frasnian fore-reef slope

Fig. 16. Lithologic characteristics of the marginal reef facies in the Canning Basin and Guilin area.

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facies both in the Canning Basin (e.g., the Napier Range) (George et al., 1997; George and Chow, 2002) and Guilin (this paper). Famennian margins are dominantly microbial in both regions. Comparison of platform margin characteristics in the Canning Basin and South China are summarized in Table 1. Differences in platform architecture and stratigraphy resulted from local-regional differences in palaeogeography and tectonic history. Several platform margin types documented in the Canning Basin reef complexes (Playford and Cockbain, 1989) are poorly represented in the Devonian complexes in South China (e.g., collapsed, pinnacle and back-stepping margins). Also, large-scale collapsed platform margins such as those in the northern Lawford Range, Lennard Shelf have not been observed in South China. However, isolated large collapse blocks, such as the Macintyre Knolls in the Canning Basin, may occur at greater depth in the intra-platform troughs in South China. The extent of reef flat facies is less clear in South China than in the Canning Basin. Receptaculitid-microbial bioherms and Smithiphyllum bioherms so far have not been reported from reef-flat facies in the Canning Basin. Oolitic facies are more common on the Lennard Shelf as compared to platforms in Guilin, whereas dolomitic facies are more abundant in the Chinese back-reef facies. Siliciclastic sediments influenced the development of reefs/carbonate platforms in both the nearshore settings of the Lennard Shelf and South China (Guizhou, Hunan and Yunnan). However, siliciclastic sediments are more common in the Famennian platforms on the Lennard Shelf than in those of the more distal Guilin region, South China, possibly resulting in more common and more diverse microbial reefs in South China. 4. Discussion The distribution, geometry and morphology of carbonate platforms and reef growth are heavily constrained by palaeogeography, pre-existing topographic features, and current circulation patterns. Changes in basin subsidence rates, tectonic deformation episodes, sea level history, climate change, and biological evolution impose additional constraints and controls on the temporal evolution of reefal platforms (e.g., Fagerstrom, 1987; Bosscher and Schlager, 1993; Harris et al., 1999; Hallam and Wignall, 1999; Stanley, 2001). One major characteristic shared by Devonian reefs and carbonate platforms in South China and the Canning Basin is their close relationship to antecedent topography. Devonian reef complexes in the Canning Basin mostly developed directly on structural, fault-controlled highs that involved Precambrian and locally Ordovician basement. Some platforms on the Lennard Shelf, such as the Oscar Range, clearly evolved from fringing reefs around long-term offshore islands of faulted Precambrian basement through barrier reefs and finally isolated platforms as the islands were covered (e.g., Playford and Lowry, 1966; Hurley, 1986; Johnson and Webb, 2007). Coeval reefs and reef complexes in South China also developed on structural highs but were more or less conformable on Lower and lower Middle Devonian marine siliciclastic deposits, thus evolving from siliciclastic shelves to reefal platforms. However, differences in the distribution of

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basement highs and their relative topography as well as differences in synsedimentary structural style and subsidence rates led to differences in carbonate development between the two regions, and these differences are discussed below. Differences in postDevonian structural evolution also led to very different styles of preservation of carbonate complexes in the two regions. Devonian reef complexes of the Canning Basin are exhumed with exceptional exposure over a large area and have suffered relatively little subsequent tectonic deformation and only limited dolomization (Playford, 1980, 1981; Playford et al., 1989), although post-Devonian deformation of the limestones was more intense along the Precambrian margins (Playford and Hocking, 1999). In contrast, coeval reef complexes in South China were extensively disturbed by subsequent tectonism and are more commonly dolomitized (Table 2). Those factors, combined with the more complicated initial palaeogeography and wider area of exposure, have rendered the Chinese reef platforms more difficult to study. The 15–20 million years (mid-Givetian to Famennian) of reef complex development in the Canning Basin consisted primarily of reef- or shoal-rimmed carbonate platforms flanked by steep fore-reef slopes that trended into adjoining inter-platform areas (Playford, 1980). By comparison, South China recorded a continuous ∼34 million-year (early Emsian to late Famennian) history of carbonate platform development (Yu and Shen, 1998), but true reef margins did not occur throughout the region at all times. Givetian reefs are abundant in the Xiangzhou carbonate facies, but Frasnian reefs are less common, generally occurring only along platform margins (e.g., reefs in Hunan; Wang, 1996). In the Guilin area, reef complexes grew for ∼20 million years from the early Givetian to late Famennian, (Yu and Shen, 1998), much as in the Canning Basin. Famennian reefs in South China are known only from Guilin, but their rarity may reflect the lack of recognition in other areas. 4.1. Tectonic and eustatic controls on Devonian stratigraphy in the Canning Basin and South China Palaeogeographically, Western Australia was on the continental margin of eastern Gondwana, and South China was a separate microcontinent across a seaway from eastern Gondwana during Middle–Late Devonian time. Both areas had relatively passive tectonic styles, but syndepositional rifting on the Lennard Shelf was associated with an intracratonic sag rather than a continental margin, as was the case for South China. Hence, Western Australia was relatively stable, being part of Gondwana, while South China rifted away from Gondwana to the northwest. Hence, the two regions had different phases of extension, rifting and subsidence. Devonian extension in the Canning Basin started in the Emsian, and a rift succession was initiated by major extension in the Late Devonian (Kennard et al., 1994). Subsidence took place throughout Emsian (Early Devonian) to Viséan (Mississippian) time with reef development largely confined to the Frasnian and Famennian. Devonian extension began earlier in South China, in the Lochkovian. Subsequent rifting led to the formation of NE–SW and NW–SE trending grabens (Zhao et al., 1996), and reefs/carbonate platforms were developed in Emsian,

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Table 2 Comparison of Devonian reefal platforms in the Canning Basin and South China Canning Basin

South China

Size

Limestone zone ranging about 350 km long and 3–50 km wide

Subseequent tectonic disturbance Dolomization Exposure Preservation Palaeogeographic position

Negligible Limited Spectacular, exhumed Good Shallow-water fault-bounded flanks of the Fitzroy Tough, relatively near-shore Middle and Late Devonian crustal extension Frasnian and Famennian

Xiangzhou platform facies is 650 km long and 210–380 wide; Guilin crabonate platform 40 km wide and 80 km long Extensive Common Well exposed Good Shallow water northern side of Huanan trough, relatively farther off-shore Devonian rifting from Gondwand Mostly Givetian, Frasnian reefs are not common, Famennian reefs are known only from Guilin Fringing reefs and biostromes in on-shore setting, marginal and patch reefs in off-shore setting, and reef mounds in off-shore platform-depression setting Well-differenitiated Narrow, reef platform margins are not well described Narrow Stromatoporoid-coral biostromes, calcarenite interbedded with laminated dolomitic limestones, dolomite, and microbial laminated limestone. Amphipora abundant, Thamnopora uncommon. Dolomitic fenestral limestone, fine-crystal dolomite, edgewise conglomerate and laminated limestone 34 million years in South China and 20 millon years in Guilin Off-shore fault-controlled reef-rimmed platforms Successive section from back-reef, reef-flat, platform margin to marginal slope is lacking Givetian to Famennian, Early Devonian to Eifelian strata are well developed Early Devonian to Eifelian marine siliciclastic rocks Nearly continuous sea-level rise. Four major episodes of platform emergence (Shen et al., 1994)

Tectonic stage Reef forming time Reef types

Barrier to fringing reef belt along landmass of the Kimberley block, marginal reefs developed on off-shore platform margins

Reef subfacies Reef platform margin Reef flat Givetian to Frasnian back-reef

Well-differenitiated Narrow, many reef platform types occur Extend 1 km into the interior of platform Stromatoporoid-coral biostromes,abundant Amphipora and Thamnopora interbeded with fenestral limestone and peloid calcarenite, ooids, terrigenous sediment common

Famennian back-reef

Fenestral limestone, ooids, pelloid, mudlump grainstone with terrigenous siliciclastic sand 15–20 million years

Reef history Major controlling factor Outcrops Reef strata Platform basement Sea-level

Major cycles Platform development Stratigraphic boundary in platform Stratigraphic succession Cyclic deposition

Fault-controlled reef-rimmed platforms Entire section from back-reef, reef-flat, platform margin to marginal slope can be traced Late Givetian to Famennian, Early Devonian to early Givetian strata are absent Mostly Precambrian crystalline rocks Continuous relative rise and major subsidence, three episodes of platform emergence were described (Playford and Cockbain, 1989; George and Powell, 1997; George and Chow, 1999) Givetian–Frasnian Pillara transgression and Famennian Nullura regressivion Bank stage to platform stage Locally not distinct No single outcrops show successive section from Givetian, through Frasnian, to Famennian Well developed in Givetian–Famennian platform interiors

Eifelian, Givetian, Frasnian and Famennian times with relatively continuous subsidence. However, a more localized starved pullapart basin occurred in the Guilin area (Regional Geological Survey Institute of Guangxi, 1994; Chen et al., 2001). Syndepositional faulting was very important in controlling carbonate platform development in South China (e.g., Yu and Shen, 1998; Chen et al., 2001) and on the Lennard Shelf (Hall, 1984; Begg, 1987; Ward, 1999; Copp, 2000; George et al., 2002). Carbonate breccias and debris flow breccias are common in fore-reef slope facies in both regions, and neptunian dykes in the Canning Basin and Guilin are similar in distribution. The dominant faults controlling Devonian sedimentation in South China were NE–SW trending. However, NW–SE trending rifts also developed and formed a series of interconnected troughs (intra-platform depressions) that were adequately deep and

Eifelian–Givetian cycle and Frasnian–Famennian cycle Bank stage to platform stage Distinct with lithological and fossil characteristics Successive sections from Eifelian to Famennian Well developed in Givetian to Famennian back-reef subfacies

restricted to accumulate hemipelagic to pelagic deposits. Devonian sedimentation along the Huanan Sea was largely controlled by the position of these troughs (Zhao et al., 1996). Intra-platform troughs containing non-carbonate Nandan-type pelagic successions bounded by syndepositional faults are thus typical features of Devonian carbonate platforms in South China, and the platform-to-depression pattern generally continued from the Lower to Upper Devonian, suggesting that many of the pre-Devonian tectonic features continued to exercise considerable control on sedimentation through the period (Shen, 2002b). The position of, and facies transitions within, reef complexes in the Canning Basin also were controlled by basement-involved faults, with platforms largely confined to fault-controlled palaeo-highs, but local topography on the emergent Precambrian basement controlled complex coastal

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morphology (e.g., Johnson and Webb, 2007) to a greater extent than has been demonstrated in China. Structurally-controlled carbonate deposition on the Lennard Shelf was largely governed by NW–SE trending syn-depositional faults and associated half grabens along the northern margin of the Fitzroy Trough, and those principal structural features indicate north-northeast syn-depositional extension (Shaw et al., 1994). Local rifting occurred from the late Givetian creating zones of weakness, as pre-existing normal faults were reactivated associated with extension and block tilting events. These NW–SE trending faults focused synsedimentary deformation along the edge of the platform margins (Shaw et al., 1994). Faults trending NE–SW superposed the NW–SE trending faults and led to the formation of the somewhat deeper inter-reef areas (e.g., in the Pillara Range, Dörling et al., 1996). However, these inter-reef areas were not as deep as the intra-platform troughs in South China, and they did not accumulate deep-water pelagic or siliceous successions. Tectonic structure was more complex in South China. Rifting during the Lochkovian–Pragian led to the formation of NE–SW and NW–SE trending grabens (Zhao et al., 1996), and the NE–SWand NW–SE trending faults controlled Emsian to Frasnian carbonate shelf sedimentation. The pattern of faulting also became more complex with a serious of new faults, trending N–S, E–W, NW–SE, and NE–SW, that cut pre-existing troughs. Subsidence associated with the new faults resulted in interconnections between pre-existing shelf troughs (Tsien et al., 1988; Zhao et al., 1996). Nearshore palaeogeographic features and terrigenous sedimentation (e.g., as documented on the Lennard Shelf by Holmes and Christie-Blick, 1993; George et al., 1994; Johnson and Webb, 2007) were less important in controlling Chinese carbonate platforms because faults that were active during the Frasnian and Famennian dissected the distal Givetian carbonate platforms to form the three offshore subplatforms in the Guilin area (Shen, 2002a). These platforms were relatively isolated from the Huanan landmass. Hence, Famennian reefs generally developed along the offshore subplatform margin and fore-reef slope facies far from exposed terrigenous sources (Fig. 5). Playford et al. (1989, see their figure 18) suggested a largely continuous growth history for reef complexes in the Canning Basin with only one small hiatus at the F–F boundary. That history was attributed primarily to extensive tectonic subsidence, but eustasy, at least during the Frasnian Pillara phase, was also seen to have a role. The globally regressive Famennian was recorded by the broadly prograding Nullara phase due to continued subsidence, but the major Famennian regressions known from Euramerica (Johnson et al., 1985) were not recognized on the Lennard Shelf (Playford et al., 1989). Southgate et al. (1993) and Kennard et al. (1994) considered the Pillara and Nullara phases to be parts of a 2nd order transgressive–regressive cycle, but they interpreted the cycle to reflect primarily tectonic controls with an early active extension phase characterized by high subsidence rates (=Pillara time) followed by a slower thermal sag phase leading to much slower subsidence during Nullara time. The transgressive Pillara phase was characterized by back-stepping to aggradational platform growth with well-developed, essentially vertical reef

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margins adjoining progressively deeper basins up to the midFrasnian. However, Playford et al. (1989) documented two major and several minor episodes of widespread platform drowning, margin backstepping, and development of isolated platform atolls and pinnacle reefs in the late Frasnian (Fig. 4). They attributed the major back-stepping and platform drowning phases of the late Frasnian to global transgressive pulses, but more recently, Playford (2002) attributed the major 3rd order backstepping events to “strong pulses of faulting and subsidence.” Ward (1999) considered some back-stepping in the Napier Range to reflect fault movement, and Chow et al. (2004) suggested that synsedimentary uplift and tilting occurred in the Napier Range farther to the northwest, although Playford and Hocking (2006) disputed the findings. Recent work (Ward, 1999; George et al., 2002) also suggests that platform progradation began in the latest Frasnian. Minor platform emergence occurred in the lower Frasnian (George and Chow, 1999), at the Frasnian–Famennian boundary, with a small part of platforms above sea level (e.g., Chedda Cliffs and Hawkstone Creek area of the Napier Range, Playford et al., 1989; Playford, 2002), in the lower Famennian (George and Powell, 1997), and prior to deposition of the Fairfield Group, which represents a transgressive sequence after termination of Nullara sedimentation (Copp, 2000; Playford, 2002). On the whole, the history of reef and carbonate complex development in South China also illustrates a largely transgressive interval from Early Devonian to Late Devonian time as a result of major subsidence throughout deposition, and the succession clearly shows sequential onlap with transgressive pulses onto eroded Lower Palaeozoic basement offshore and onto the Jiang Nan Shield onshore (Tsien et al., 1988). However, four episodes of platform emergence were recognized in Guilin on the basis of exposure-erosion surfaces in platform successions (Yu and Shen, 1998), and the carbonate succession can be broken into two broad cycles of platform development: the Eifelian–Givetian cycle and Frasnian–Famennian cycle. The primary platforms in South China were developed during Emsian time, and were characterized by carbonate sedimentation in widespread shelf settings overlying siliciclastic deposits. However, the first carbonate deposition recorded in Guilin was early Givetian in age, about equivalent to the hemiansatus conodont Zone overlying siliciclastic deposits characterized by littoral gravel–sand flat and sand–mud flat sediments (Yu and Shen, 1998). The Eifelian–Givetian cycle consists of Eifelian siliciclastic rocks and Givetian carbonate bank to platform deposits. The thick to massive lower and middle Givetian bank deposits underwent extensive dolomitization, but upper Givetian limestones are well bedded and show parasequence scale cyclicity in restricted platform settings (Yu and Shen, 1998). The Frasnian– Famennian cycle of Guilin reef platform development was characterized by Frasnian transgression and Famennian regression similar to that of the Pillara and Nullara phases of the Lennard Shelf. Thus platforms in Guilin were characterized by essentially vertical growth with widespread drowning and backstepping in the latest Frasnian and extensive regression and shrinking platform size in the early Famennian. Relative sealevel in Guilin rose steadily in the Frasnian and dropped quickly

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in the early Famennian, then slowly rose again in the middle and late Famennian (Shen, 1995). Although sea level falls are associated with palaeokarst features (e.g., latest Famennian palaeokarst in Eryaochang and Etoucun sections, Yu and Shen, 1998) in the Siphonodella praesulcata Zone of the latest Famennian, there was a transgression during the Mississippian, which is evidenced by Tournaisian shales (∼ equivalent to the Fairfield Group on the Lennard Shelf) and thin-bedded micrite with rare macrofossils overlying the thick and massive Famennian limestones. Hence, the overall Late Devonian succession in Guilin is stratigraphically similar to that of the Lennard Shelf. The increased subsidence between platforms in South China led to the formation of deeper intra-platform troughs than occurred on the Lennard Shelf. While that may be the result of the faulting being associated with continental rifting rather than an intracratonic sag, the platforms in Guilin themselves paradoxically have similar thicknesses to those on the Lennard Shelf. The similarity in timing of 2nd order sequences in South China and Western Australia is intriguing. Second order cycles could reflect eustasy or similar tectonic evolution. While Playford et al. (1989) noted a role for eustatic control, at least for the Pillara phase of the Lennard Shelf, subsequent authors (e.g., Southgate et al., 1993; Kennard et al., 1994) emphasized tectonic controls on stratigraphy, rather than eustasy. Southgate et al. (1993) and George et al. (2002) even suggested that exposure near the F–F boundary was associated with tectonic uplift in some areas, not necessarily global regression. As the Lennard Shelf and South China were deposited during active rifting, it is possible that the similar stratigraphy is a coincidence of similar tectonic activity. However, the Frasnian and Famennian stratigraphy of western Canada is also characterized by a highly transgressive Frasnian section with backstepping platforms and pinnacle reefs that gave way to a broadly regressive, prograding sequence in the latest Frasnian following an interval of shelf exposure near the F–F boundary, and in that case eustasy was interpreted as the likely driving mechanism (e.g., Whalen et al., 2000; Schwab et al., 2004). Whalen et al. (2000) noted that the 2nd order sea level cycle was consistent with the sea level curve of Johnson et al. (1985). While differential subsidence affected the stratigraphy of different carbonate platforms differently (e.g., Miette and Ancient Wall platforms), the dominant control was found to be eustasy (Whalen et al., 2000). Hence, the major 2nd order cycle in Western Australia and South China may also reflect a largely eustatic signal with a variable tectonic overprint. As pointed out by Playford et al. (1989) the Famennian succession of the Lennard Shelf does not correlate well to the 3rd order regressions of the Johnson et al. (1985) sea level curve. However, Southgate et al. (1993) recognized six 3rd order cycles that they interpreted as reflecting eustatic changes within the Nullara phase, and George et al. (2002) recognized three 3rd order sequences within the Famennian of the Napier Range, although they did not attribute them to eustasy. In summary, tectonism played a major role in controlling the Late Devonian sequence stratigraphy of both regions, but the coincidence of major Frasnian platform aggradation and backstepping followed by Famennian progradation in Western Australia, South China and the Canadian Rocky

Mountains suggests a eustatic control of the 2nd order sequence in any case. However, differentiation of tectonic from eustatic controls on 3rd order cycles will require more detailed biostratigraphic control in shallow platform facies than currently exists. 4.2. Geographic differentiation of carbonate platform evolution in the Canning Basin and South China Devonian reefs and reef complexes in South China grew as a series of fringing reefs and biostromes in onshore siliciclasticcarbonate mixed settings near the adjoining land mass and as reefrimed platforms with marginal and patch reefs in offshore settings, such as Guangxi. Reef-rimed platforms are not common in nearshore settings (e.g., Hunan, Guizhou, and Yunnan), and most of the reefs proximal to the land mass were small fringing reefs, patch reefs, or reef mounds. Small reef mounds (several meters diameter) also developed in intra-platform troughs. Regardless, South China carbonate platforms were best developed in offshore settings, and from Givetian to Famennian time outer platform deposits contain only rare terrigenous sediments. Although isolated carbonate platforms developed on the Lennard Shelf by the Frasnian, located mostly on basement-involved fault blocks, some platforms fringed and then overgrew continental islands of Precambrian basement (e.g., Oscar Range; Playford and Lowry 1966; Playford, 1980; Playford et al., 1989; Johnson and Webb, 2007). By the Famennian, all exposed reefs were located along the margins of very broad fringing platforms, although the exact outline of platforms is unclear in most cases due to subsequent erosion. Known Famennian reefs and deeper-water mounds occur mostly along the Napier Range (Playford, 1980; Wood, 2004), Horse Spring area and southern Horseshoe Range (Webb, 2001), although Playford (1984) suggested that the northern Horseshoe Range margin was more a low-relief bank than a reef margin. Napier, Horseshoe and Horse Spring Ranges abut the Kimberley Landmass to the northeast, so known Famennian reefs and mounds in the Canning Basin are interpreted to have developed in relatively nearshore settings (Playford, 1980; Webb, 2001; Stephens and Sumner, 2003a; Chow and George, 2004) that fringed the old landmass (Fig. 2), and deposits of several ages interfinger with coastal conglomerates (e.g., Holmes and Christie-Blick, 1993; Johnson and Webb, 2007). Very small stromatolite buildups that occur in Famennian strata west of McWhae Ridge may be an exception — as they may have occurred relatively far offshore, but on relief held up by the underlying Frasnian reef spine. Regardless, the dominant geometries of platforms were controlled by nearshore palaeogeographic patterns overprinted on basement involved fault blocks that roughly paralleled the palaeo-coastline. The occurrence of the Lennard Shelf along the margin of an intracratonic basin may have provided some protection from open ocean swells, although the basin was large for an epicontinental sea, and the limestones preserve rare earth element geochemistry suggestive of open marine conditions (Nothdurft et al., 2004). Regardless, little evidence of windward versus leeward orientation has been documented on most carbonate platforms. The asymmetry of Lloyd Hill Reef in Paddys Valley north of the

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Emanuel Range may be an exception (Playford, 1981), and Hall (1984) documented evidence of windward and leeward reef margins in the western Pillara Range, although one or both are commonly destroyed. Regardless, most of the fringing platforms provide relatively little information. Long-shore drift patterns on the southern side of the Oscar Range suggest that on-shore wind patterns may have dominated in any case (Johnson and Webb, 2007). Many isolated platforms, such as the Pillara Range, have only one margin well exposed, in that case the landward-facing leeward margin. The narrow, ridgelike platforms of the Lawford Range (e.g., at Wagon Pass) have equally well winnowed proximal Stachyodes rudstone-dominated reefal-slopes on both east and west sides. The South China carbonate platforms occurred in a more exposed position bordering the Palaeotethys (Fig. 3), and windward and leeward margins are more easily differentiated (Chen et al., 2001). Reefs on the Guilin subplatform grew at several locations around the fault-dissected margins, with the three well documented reefs and mounds developed on the leeward sides of platforms, with other less well known reefs occurring along the fault-controlled windward margins. 4.3. Climatic controls on carbonate platform evolution in the Canning Basin and South China Although temperature clearly controlled the distribution of middle Palaeozoic reefs (Copper, 2002b), both the Canning Basin and South China were located within 30° of the palaeoequator during the Middle–Late Devonian (Fig. 3) (Hurley and Van der Voo, 1987; Michael et al., 1990; Golonka, 2002). The Lennard Shelf (the Canning Basin) and Guilin (South China) were at very similar latitudes, between 10° to 20°, in Givetian to Famennian time (Fig. 3) (Golonka, 2002). The similar latitude and general proximity may explain many of the similarities between the two reef systems. The low latitude also may have allowed reef growth even during times of global cooling (Copper, 2002a), and may help explain why carbonate reef growth was so extensive through the Famennian on the Lennard Shelf relative to other regions. Although South China was closer to the equator, both regions experienced climates generally favorable for warm-water chlorozoan carbonate production. Parasequence scale carbonate cyclicity in both regions has been interpreted as the result of Milankovitch cycle-related climate fluctuations (Brownlaw et al., 1998; Gong et al., 2001), possibly including eustatic sea level fluctuations of tens of meters during the Frasnian (Webb and Brownlaw, 2000). Although Hocking and Playford (2001) still considered repeated tectonic episodes to be a possible mechanism on the Lennard Shelf, Copper (2002b) noted that similar carbonate cycles in the Givetian appear to be globally distributed and pointed out that such eustatic cycles were caused by minor ice volume changes even during warm intervals such as the Late Cretaceous (Miller et al., 1999). The larger relative sea level falls documented in the early Famennian in Western Australia, South China and North America (Yu and Shen, 1998; Issacson et al., 1999; George et al., 2002; Playford, 2002) are associated with exposure of platforms in many regions and widespread

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evaporites in Montana (Sandberg et al., 1983), the Pripyat Depression, and the Dnieper–Donetz Basin (Manukalova-Grebenjuk, 1974; Avkhimovich and Demidenko, 1985), thus suggesting that shallow, restricted settings were globally distributed during the early Famennian. Additionally, shale sequences that overlie the carbonate successions in the Canning Basin and in South China have been attributed to latest Famennian to Tournaisian transgression (Playford et al., 1989; Yu and Shen, 1998). The early Famennian sea-level fall and the sudden global appearance of shallow-marine conditions followed by transgression probably reflect latest Devonian (Famennian) and Mississippian glacioeustasy in both regions. Copper (2002b) suggested that global cooling that peaked in the Famennian (Frakes et al., 1992) was a major factor in the decline of reefs in most regions, and the cooling may also help explain the faunal changes across the F–F boundary in Western Australia and South China. Globally, stromatoporoid reefs in particular were distributed in higher latitudes during the very warm Emsian and retracted during cooler intervals (Copper, 2002b). Hence, the continuation of stromatoporoids in Famennian reef facies on the Lennard Shelf, although in much reduced capacity, may reflect the low latitude setting. The reason for the more diminished reef growth and lower progradation of carbonate platforms in the Famennian of South China at similar latitudes is more difficult to explain. 4.4. Role of microbes in reef evolution and succession of reef building communities in the Canning Basin and South China Towards the Late Devonian, reefs and reef complexes worldwide were increasingly dominated by calcimicrobes, micritic microbialite, and biocementstone (Webb, 1998, 2002, Shen and Webb, 2004b), and microbial carbonates are widespread in the Upper Devonian of South China (Yin et al., 1990; Shen et al., 1997; Shen and Webb, 2004a,b), Western Australia (Wray, 1967; Playford, 1980; George, 1999; Webb, 2001; Stephens and Sumner, 2003a; Chow and George, 2004), Canada (Machielse, 1972; Maurin, 1972; Nishida et al., 1985, Bourque and Boulvain, 1993; Whalen et al., 2002) and Europe (Bathurst, 1982; Dreesen et al., 1985; Tsien, 1994; Weller, 1995). Although extinction of many reef-building stromatoporoids near the F–F boundary has been considered the dominant reason for the increase in microbial reef frameworks in the Late Devonian, some important changes affecting microbial boundstone abundance in Devonian framework communities occurred before the extinction event (Webb, 1996), and some reefal communities crossed the F–F boundary more or less intact (Wood, 1995, 2000). Hence, the evolution of Devonian reef framework communities was complex. In the Canning Basin, tabular, massive and dome-shaped stromatoporoids were largely replaced as dominant skeletal organisms in platform margins by bulbous, branching (stachyodiform), and irregular stromatoporoids after the Givetian (e.g., Copp, 2000). Calcimicrobe frameworks, and true rigid frameworks of any type, were very scarce in Givetian platform margins (Pillara Cycle associations 2 and 3, Copp, 2001), but they became increasingly abundant from some point in the Frasnian. For example, calcimicrobes are rare or absent in the low-relief,

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cyclic stromatoporoid mound platform margins of the Pillara Range around Menyous Gap, but somewhat higher in section, still within the Frasnian, Renalcis frameworks formed steep platform margins (e.g., Hall, 1984). It is possible that the introduction of Renalcis-bound framework may have allowed the change from low-relief retreating banks to the near-vertical aggrading reef margins documented by Hall (1984) and Benn (1984). Although many stromatoporoid taxa became extinct near the Frasnian–Famennian boundary, calcimicrobes and algae survived largely unscathed and thrived in the Famennian platform environments. The diversity and guild structure of the Famennian reefs differed from earlier Frasnian reefs (Fagerstrom, 1994; Webb, 1996), but, as pointed out by Wood (2000), the early Famennian microbial margin facies had precursors in the Frasnian, thus representing more or less continuations of particular community types across the extinction boundary. Although, the potential for Famennian calcimicrobe reef frameworks to support vertical reef scarps is difficult to test due to the relatively low rate of accommodation generation during most of the Nullara phase, they presumably would have been equivalent to late Frasnian cement-rich microbial facies. Framework facies associated with steep latest Famennian reef margins (Playford et al., 1989) have not been documented, but presumably represent typical Famennian calcimicrobe frameworks. In South China, Givetian platform margins also were dominated by tabular and domal stromatoporoids in biostromal or biohermal facies (Chen et al., 2001), but stromatoporoid framestone was also common on margins. Calcimicrobes generally were not abundant in Givetian platform margin facies (Shen, 2002b), but some steep late Givetian margins contained Renalcis (Chen et al., 2001). As on the Lennard Shelf, microbes became much more important in reef construction on Frasnian platform margins (Shen, 2002b) and dominated Famennian reef margins where they helped bind rare stromatoporoids and calcareous algae. Stromatoporoids did not form frameworks by themselves; they were generally encrusted by microbes, algae, and cement to form rigid framework. However, Renalcis was not as abundant in Chinese Frasnian reef margins as in the Canning Basin. The major loss of stromatoporoid volume and diversity at the F–F boundary was a major event for reef building in South China, but, as in Western Australia, reef building was more or less continuous through the boundary, except for a small hiatus due to eustatic sea level fall. Famennian platform margin facies in both areas are characterized by microbial reefs and mounds, skeletal shoals, and oolitic shoals. Thus, major shifts in platform margin buildup assemblages in both regions occurred from the Givetian to the Famennian and reflect: 1) replacement of low-relief stromatoporoid biostrome facies with increasingly rigid calcimicrobestromatoporoid communities during the Frasnian; and 2) loss of most of the large, conspicuous, reef-building stromatoporoids near the Frasnian–Famennian boundary. Although calcimicrobes were the major Famennian reef builders in both regions, the specific communities and abundance of particular microbes differed (Fig. 17). In the Canning Basin, Renalcis (senso latto) was the most conspicuous Frasnian and Famennian frame-building microbe, and other microbes (e.g.,

Fig. 17. Occurrence of calcimicrobes and algae in the Canning Basin and Guilin area. (data from Wray, 1967; Playford, 1980, 1984; Shen et al., 1997; Yu and Shen, 1998; Webb, 2001; Stephens and Summer, 2003a; Shen and Webb, 2004a,b; Chow and George, 2004).

Rothpletzella, Girvanella, Ortonella and Palaeomicrocodium) were less important as reef builders, although Rothpletzella was very abundant in some places, especially on drowned Frasnian platform slopes. Agglutinated and micritic stromatolites and thrombolites occurred in shallow subtidal, transitional back-reef to reef-flat settings (Webb, 2001; Chow and George, 2004), and deep-water stromatolitic bioherms and biostromes occurred on fore-reef slopes (Playford et al., 1976; George, 1999) and on subvertical platform margins (Ward, 1999). The occurrence and distribution of stromatolites were discussed in more detail by Shen and Webb (2004b). Skeletal algae were less important, but include Parachaetetes (Webb, 2001). In the Guilin area, calcimicrobes were more diverse, with Epiphyton and Renalcis being abundant reef builders, and other microbes also being common (e.g., Garwoodia, Izhella, Rothpletzella, and a “Keega”-like microbe, Fig. 17). Chen et al. (2001) suggested that microbes dominated reefs only in restricted settings due to reduced current circulation associated with subsidence of the basin center in Guilin, but microbial communities on platform margins occur with normal marine flora and fauna suggesting that adverse conditions in water quality cannot explain the abundance of microbial facies. Rothpletzella stromatolites have been documented in South China only in the Givetian fore-reef slope facies. Hence, its setting is similar to that in the Canning Basin, but it occurred earlier in China. Less common microbes in the Guilin reefs include Wetheredella, Girvanella, Ortonella, Rivularia, and a Tharama-like microbe. Famennian deep-water agglutinated

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stromatolites have not been observed in the Guilin area, but shallow stromatolite-rich reefs occur (Shen and Webb 2004b). Calcareous algae are represented mainly by the solenoporoids, Parachaetetes and Solenopora, which are common in stromatolitic mounds but are not important in Epiphyton- and Renalcisdominated frameworks. Dasycladacean green algae also occur (Shen, 2002b). 5. Conclusions (1) Devonian carbonate platforms covered a larger area in South China than on the Lennard Shelf, Canning Basin, Western Australia. Devonian carbonate successions in South China are generally thicker than those of the Lennard Shelf, but the thickness of carbonate deposits in the Guilin area is similar to that of the Lennard Shelf. (2) Fault-controlled antecedent topography localized carbonate platforms and reefs in both regions, but underlying structure differed. Reef complexes on the Lennard Shelf grew directly on faulted Precambrian basement and Ordovician strata along the mainland and around and over continental islands of the Precambrian Kimberley Block (Playford, 1980). Locally, coastal carbonates were separated from basement by, and interfingered with, a veneer of siliciclastic coastal conglomerate (e.g., Holmes and Christie-Blick, 1993; George and Chow, 1999; Johnson and Webb, 2007). In South China most Devonian sediments overlie Precambrian and Cambrian rocks unconformably in nearshore settings and Cambrian and Ordovician sediments farther offshore. Initial Devonian deposits were transgressive siliciclastic sediments, and the large carbonate complexes developed mostly on those units in more offshore settings (Regional Geological Survey Institute of Guangxi, 1994). Thus latest Devonian carbonate platforms were more isolated from terrigenous sources than in the Canning Basin. (3) Devonian reef complexes are well preserved on the Lennard Shelf with well exposed, well differentiated back-reef, reef flat, reef margin, and reefal-slope facies. Strata commonly are preserved relatively close to their original depositional dips with palaeogeographic relationships well preserved in outcrop, but as a result, no complete Givetian to Famennian stratigraphic succession is available for study in a single outcrop section. Chinese Devonian reef complexes have similar facies as those on the Lennard Shelf, but individual facies transitions are more poorly preserved owing to tectonic deformation and partial dolomitization. Few extensive exposures occur where lateral facies transitions and palaeogeographic relationships can be documented. However, the more extensive tectonic deformation has allowed documentation of more or less continuous vertical sections through strata from the Eifelian to Tournaisian in several places. (4) Platform margin facies are similar in both regions, possibly owing to their very similar latitudinal positions, and they generally represent very narrow belts of poorly bedded to massive limestone. On the Lennard Shelf, the

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Givetian was dominated by low-relief banks with stromatoporoid biostromes and reef-mounds at the margins. Reef-mounds gave way to higher-relief frameworks at margins during the Frasnian, possibly with the increased role of calcimicrobes such as Renalcis. Stromatoporoids, although volumetrically abundant, were generally secondary to microbes and early marine cement and rarely formed rigid frameworks by themselves. Famennian framework constructors were dominantly calcimicrobes and solenoporoid algae. In Guilin, South China, encrusting domal and tabular stromatoporoids and massive corals were the principal framework constructors in Givetian patch reefs and reef margins. Dominant Frasnian constructors in South China were calcimicrobes and stromatoporoids, but as in Western Australia, stromatoporoids were less important than microbes in constructing rigid reef margin facies. Famennian reef constructors in South China were also similar to those of the Lennard Shelf (i.e., mostly microbes and algae with early cement), but microbes were more diverse. (5) Second order stratigraphic patterns of Devonian reef growth in the Canning Basin and South China are basically similar, possibly reflecting Late Devonian eustasy, but local tectonic factors made for differences in individual backstepping and drowning events. The formation of deep intraplatform troughs in South China may reflect greater subsidence associated with rifting on a continental margin rather than in an intracratonic sag. However, associated carbonate platforms do not appear to have experienced greater subsidence. Progradation of platforms began in both regions during the latest Frasnian and continued through the Famennian during an interval of low accommodation generation. (6) Termination of Devonian carbonate complexes on the Lennard Shelf at the end of the Famennian resulted from platform exposure during a lowstand followed by increased flux of siliciclastic sediments from the adjacent Kimberly block during renewed transgression. Devonian offshore reef complexes in Guilin were largely drowned by rapid subsidence and eustatic sea level rise, but reefs in Hunan, a nearshore setting, were terminated as their basins were filled by terrigenous deposits as in the Canning Basin. Acknowledgements This research was supported by CAS (The Chinese Academy of Sciences) Hundred Talents Program and the University of Queensland (IPRS and UQIPRS). We thank A. D. George and anonymous reviewers for critical comments on an earlier version of this manuscript. Special thanks to Steve Kershaw and Rachel Wood for reviewing this paper and providing many constructive suggestions and comments. Sampling and photography at Windjana Gorge were carried out by GEW under license no. NE002227 of the Western Australian Department of Conservation and Land Management. G. E. Webb was supported by Australian Research Council grant A39701501.

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