Carbonate Platform Evolution: from a Bioconstructed Platform Margin to a Sand-Shoal System (Devonian, Guilin, South China)

Home , Carbonate platform, Pebble, Progradation, Retrogradation

Sedimentology (2002) 49, 737–764

Carbonate platform evolution: from a bioconstructed platform margin to a sand-shoal system (Devonian, Guilin, South China)

DAIZHAO CHEN*, MAURICE E. TUCKER, JINGQUAN ZHU* and MAOSHENG JIANG* *Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing 100029, China (E-mail: [email protected]) Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK (E-mail: [email protected])

ABSTRACT The depositional organization and architecture of the middle–late Devonian Yangdi rimmed carbonate platform margin in the Guilin area of South China were related to oblique, extensional faulting in a strike-slip setting. The platform margin shows two main stages of construction in the late Givetian to Frasnian, with a bioconstructed margin evolving into a sand-shoal system. In the late Givetian, the platform margin was rimmed with microbial buildups composed mainly of cyanobacterial colonies (mostly Renalcis and Epiphyton). These grew upwards and produced an aggradational (locally slightly retrogradational) architecture with steep foreslope clinoforms. Three depositional sequences (S3–S5) are recognized in the upper Givetian strata, which are dominated by extensive microbialites. Metre-scale depositional cyclicity occurs in most facies associations, except in the platform-margin buildups and upper foreslope facies. In the latest Givetian (at the top of sequence S5), relative platform uplift (± subaerial exposure) and associated rapid basin subsidence (probably a block-tilting effect) caused large-scale platform collapse and slope erosion to give local scalloped embayments along the platform margin and the synchronous demise of microbial buildups. Subsequently, sand shoals and banks composed of ooids and peloids and, a little later, stromatoporoid buildups on the palaeohighs, developed along the platform margin, from which abundant loose sediment was transported downslope to form gravity-flow deposits. Another strong tectonic episode caused further platform collapse in the early Frasnian (at the top of sequence S6), leading to large-scale breccia release and the death of the stromatoporoid buildups. Siliceous facies (banded cherts and siliceous shales) were then deposited extensively in the basin centre as a result of the influx of hydrothermal fluids. The platform-margin sand-shoal/bank system, possibly with gullies on the slope, persisted into the latest Frasnian until the restoration of microbial buildups. Four sequences (S6–S9), characterized by abundant sand-shoal deposits on the margin and gravity-flow and hemipelagic deposits on the slope, are distinguished in the Frasnian strata. Smaller-scale depositional cyclicity is evident in all facies associations across the platform–slope–basin transect. The distinctive depositional architecture and evolution of this Yangdi Platform are interpreted as having been controlled mainly by regional tectonics with contributions from eustasy, environmental factors, oceanographic setting, biotic and sedimentary fabrics. Keywords Carbonate platform, microbially rimmed platform margin, sand- shoal system, South China.

Ó 2002 International Association of Sedimentologists 737 738 D. Chen et al.

INTRODUCTION northern Spain (Garcıá-Monde´jar, 1989, 1990; Agirrezabala & Garcıá-Monde´jar, 1992; Rosales Numerous studies on the formation and evolution et al., 1994; Garcıá-Monde´jar et al., 1996; Go´mez- of carbonate platforms within extensional tectonic Pe´rez et al., 1999; Rosales, 1999) and the Devo- settings have been documented from the geologi- nian of southern Hunan, South China (e.g. Jiang, cal record (e.g. Burchette, 1988; Cocozza & Gandi, 1989, 1990). 1990; Santantonio, 1993, 1994; Picard et al., 1994; In this paper, an example is presented of a Rosales et al., 1994; Bosence et al., 1998; Wilson, platform margin related to oblique, extensional 1999; Wilson et al., 2000). However, only a few faulting within a strike-slip fault zone in the studies have involved cases within a strike-slip Guilin region of South China (Fig. 1A; Chen et al., setting. Examples include the Lower Cretaceous of 2001a). The anatomy of the transition from

Fig. 1. (A) Sketch map showing the conﬁguration of basins (shaded areas) and carbonate platforms (non-ornamented areas + ornamented Yangdi Platform) in the Devonian of the Guilin region. The shaded spindle-shaped area is the Yangshuo Basin. (B) Geological map of the study area, where deposits from platform margin (nearly horizontally stratiﬁed) to slope (SW-dipping) and basin crop out along the highway from Zhongnan to Fuhe and farther west. The platform margin extends roughly in a NNE–SSW direction in this segment. Giv., Givetian; Fr., Frasnian; Fa., Famennian.

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 Carbonate platform evolution, Guilin, South China 739 platform margin to basin has been documented Yangshuo Basin to the south (Fig. 1A). The through detailed field observations and analysis southern margin was controlled by oblique of depositional facies and facies associations. A extensional faulting, which was induced by the well-exposed across-strike transect from the activity of a major strike-slip fault (cf. Chen et al., platform (the Yangdi Platform) to the basin (the 2001a). A transect of about 2 km across the pull-apart Yangshuo Basin, Fig. 1) allows an Yangdi Platform and Yangshuo Basin is well examination of the original architecture and exposed along the highway from Fuhe to Zhong- geometry of the platform margin. The mechanism nan, south-east of Guilin (Fig. 1B), where nearly of platform-margin development and evolution in horizontal strata at Zhongnan (back-margin such a tectonic setting is discussed, and special facies) gradually pass basinwards into inclined emphasis is placed on the syntectonic sedimen- slope strata (SW-dipping) around Wulibei (plat- tation during the late Givetian to Frasnian. The form margin to slope facies). This exhumed stratal stratigraphic context for this case study is largely pattern probably represents the original deposi- based on the work of Zhong et al. (1992), modified tional profile across the platform margin and by Chen et al. (2001a); a new stratigraphic unit is slope, as no post-depositional faulting has also proposed in this study for convenience. occurred at the platform edge (Fig. 1B). The strata of the basinal facies, although dipping very gently (2–10°), may not represent the original deposi- GEOLOGICAL SETTING tional declivity, in view of thrusting along strike AND STRATIGRAPHY of the basin margin to the east of Fuhe (Fig. 1B), where basinal strata were uplifted and eroded. The Yangdi Platform extended for about 25 km in Three outcrop sections at Zhongnan, Wulibei and a NW–SE direction and abutted the pull-apart Fuhe, representing the strata of back-margin,

Fig. 2. Nomenclature and distribution of lithostratigraphic units, depositional sequences (S1–S9) and platform phases (three phases) in the Guilin area and their correlation with the conodont stratigraphy. This study focuses on the last two platform phases (S3–S9). Two conodont zonation schemes are listed for convenience, based on Zhong et al. (1992).

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 740 D. Chen et al. platform margin-slope and basinal facies, respect- margin was locally scalloped along the platform ively, were measured and logged in detail. A margin (e.g. at Shihedong and south of Wulibei, further section at Shihedong was also recorded Fig. 1B), with gullies developed downslope (for location, see Fig. 1B). through which sediment was transported by The stratigraphic relationship between the plat- gravity flows. The dramatic change in platform form and basin is illustrated in Fig. 2. The basal morphology and architecture occurred close to Tangjiawan Formation, the first carbonate succes- the Givetian–Frasnian boundary. Seven sequen- sion overlying the clastic Xindu Formation, mainly ces have been recognized in the whole studied made of stromatoporoid biostromal facies repre- succession, three (S3–S5) in the upper Givetian senting the initial stage of platform–basin evolu- (platform development phase 2) and four (S6–S9) tion, is not included in this study. Thus, in the Frasnian (platform development phase 3) the stratigraphy mainly involved in this study (Fig. 2). Details of the sequence development and includes the Rongxian, Mintang, Gubi and Wuzhi- stratal patterns have been documented by Chen shan Formations and Zhongnan Limestone et al. (2001a,b). (Figs 1B and 2). The Rongxian Formation represents platform margin-to-slope and back-margin facies, ranging from the upper Givetian to Famen- BIOCONSTRUCTED PLATFORM nian; however, most back-margin deposits above SYSTEM: LATE GIVETIAN the middle Frasnian have been truncated as a result of Triassic uplift (Yanshanian orogenic stage). The From the late Givetian, the Yangdi Platform was mostly Upper Givetian Mintang Formation is char- rimmed with microbial buildups (reefs) with steep acterized by alternating laminated microbialites marginal slopes to basins, physiographically sim- and nodular pelagic limestones. The Zhongnan ilar to the rimmed-platform category of Read (1982, Limestone, a new stratigraphic unit synchronous 1985). The well-exposed southern margin permits with the Mintang Formation, is characterized by a detailed description of the depositional architec- microbial buildups with minor breccias. The Liuji- ture and facies changes across the margin. Based on ang Formation is characterized by starved basin depositional features and facies associations, plat- deposits of siliceous rocks with minor tentaculitid form-interior, platform-margin (back-reef/reef-flat, cherty limestones. The Gubi Formation (mainly fore-reef), slope (foreslope, lower slope) and basin Frasnian) is chiefly composed of gravity-flow environments have been distinguished within the deposits at the platform margin and pelagic nod- bioconstructed rimmed-platform system (Fig. 3A ular limestones with minor gravity-flow beds near and B). Description of the platform interior facies is the basin centre. The Wuzhishan Formation repre- given elsewhere (Chen et al., 2001b) and will not sents distal slope and basin deposits dominated by be considered in this paper. Only the anatomy of pelagic nodular limestones with minor calciturbi- the platform–basin transect from Zhongnan via dites, mainly of Famennian age (Fig. 2). Wulibei to Fuhe is presented here (see Fig. 1 for From middle Givetian to Frasnian times, three location and Figs 4 and 5). Metre-scale cyclicity main phases of platform–basin evolution have can be discerned in most of the facies except the been distinguished, each characterized by a dis- massive microbial buildups. tinctive stratigraphic succession and depositional package (Chen et al., 2001a). The emphasis of this Platform-margin environment paper is placed on the second and third phases (late Givetian and Frasnian respectively). In the Back-reef/reef-flat facies upper Givetian successions, depositional pack- Facies from the back-reef/reef-flat setting consist ages are characterized mainly by microbialites, of stromatoporoid rudstone/floatstone (PM1), with microbial buildups along the platform mar- microbial rudstone/grainstone (PM2), Amphipora gin (Zhongnan Limestone) grading into deep- grainstone/wackestone (PM3), skeletal wacke- water microbial laminites in the basin (Mintang stone/packstone (PM4) and minor microbial lam- Formation). In contrast, the Frasnian depositional inite (PM5) and fossil-poor mudstone (PM6) (see packages are characterized by shoal deposits the lithological logs of Zhongnan in Fig. 5; (oolitic–peloidal grainstones–packstones) on the Table 1). They are mostly present in the Rongxian margin (Rongxian Formation), extensive gravity- Formation. These deposits are generally light flow and hemipelagic deposits on the slope (Gubi coloured and thick to massive bedded (2–3 m Formation) and siliceous deposits in the basin common). Stromatoporoids and Amphipora are (Liujiang Formation). The antecedent platform

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 Carbonate platform evolution, Guilin, South China 741

Fig. 3. Sketch diagram showing the depositional environments and facies changes from the platform to the basin. (A) Depositional environments and facies across the platform–slope–basin transect during the early microbially rimmed platform stage (S3, middle Givetian). The platform margin-to-slope rimmed with microbial buildups aggraded vertically, and few sediments were exported downslope. (B) Depositional environments and facies across the platform–slope–basin transect during the latest stage of the microbially rimmed platform system (S5, latest Givetian). The rimmed margin and slope were undercut with gullies, through which sediments from the platform were transported downslope, leading to a less well-protected platform margin.

Fig. 4. A panoramic sketch from a photomosaic showing the transition from the horizontal platform strata (the hill right back in the distance) to the inclined foreslope strata of the upper Givetian. Note the aggradational architecture of the microbial buildup on the slope. Sequences (S2–S5) are separated by the thick lines. Box at the bottom of hill is 3Æ5 m high. Dashed arrows represent measured sections shown in Figs 5 and 10. The Zhongnan section is measured at the back of the hill (right back in the distance).

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Fig. 5. Lithological logs and correlations of platform growth phase 2 across the platform margin to the basin from Zhongnan via Wulibei to Fuhe. An aggradational microbial buildup on the platform margin- to-slope is well illustrated and granular sediments were funnelled through, inducing downslope gravity-flow deposits in the late stage of buildup construction. The microbial buildup was finally terminated after a significant collapse at the top of the Givetian. Thin lines on the right of the logs from Fuhe and Zhongnan mark the high-frequency cycles (parasequences); arrows mark the boundaries of cycle sets. Refer to Tables 1 and 2 for facies codes. See Figs 1B and 4 for section locations. LST, lowstand deposits; TST, transgressive deposits; HST, highstand deposits. M, mudstone; W, wackestone; P, packstone; G, grainstone; Co, conglomerate; B, breccia/ megabreccia; Bs, boundstone; Bi, bindstone; Rs, rudstone; Fs, floatstone. the most common fossils with subordinate ostra- around the world (e.g. Read, 1973; Elrick, 1995; cods and foraminifera. These biotic associations Chen et al., 2001b). Grains are usually abraded characterize semi-restricted, subtidal to peritidal and more rounded than those in platform interior back-reef environments of Devonian platforms lagoons (e.g. Chen et al., 2001b), suggesting

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 Ó 02ItrainlAscaino Sedimentologists, of Association International 2002

Table 1. Depositional facies of the bioconstructed platform system.

Depositional Facies association/ environment facies (codes) Description Biota Interpretation Platform Back-reef/reef-flat margin Stromatoporoid Thick to massively bedded (2–3 m common); Stromatoporoids, Stachyodes, Moderate- to high-energy Rs/Fs (PM1) abraded skeletal particles, microbial Amphipora, gastropods and reef-flat fragments and peloids common, minor ostracods; microbial colonies oncoids; bulbous and domal stromatoporoids constitute Fs Microbial Several m thick; microbial fragments Thrombolitic microbes Moderate- to high-energy Rs/Gs (PM2) (mm- to cm-sized) dominant, minor cyanobacterial colonies back-reef flat oncoids and peloids, rare skeletal grains (i.e. Epiphyton and Renalcis), gastropods and ostracods Amphipora 10’s cm to a few m thick; current-abraded, Amphipora dominant, minor Semi-restricted, moder- Gs/Ws (PM3) parallel-prone skeletal grains common, Stachyodes, gastropods, ate-energy, back-reef minor peloids, oncoids and microbial ostracods; cyanobacterial setting Sedimentology fragments; laminite intercalations colonies locally China South Guilin, evolution, platform Carbonate Skeletal Thin- to medium-bedded (10–50 cm); Gastropods, ostracods and Restricted, low-energy Ws/Ps (PM4) relatively dark in colour; restricted Amphipora behind margin skeletal grains, minor peloids, (i.e. lagoon) ,

49, micritic matrix Laminite (PM5) Cm-scale lamination, commonly indicated Microbial colonies; Amphipora Restricted, lower inter- 737–764 by peloidal Gs/Ws, microbialite/peloidal tidal to shallowest Gs couplets, rarely by intergrown subtidal environment microbialites; thin Amphipora Gs intercalations common; fenestral and dissolution cavities occur locally Fossil-poor Thin-bedded (<20 cm); rare argillaceous Rare ostracods and gastropods Restricted, quiet shallow Ms (PM6) seams; rare restricted fauna subtidal Fore-reef Massive microbial Locally thick-bedded (>50 cm); microbial Microbial colonies: Renalcis, Microbial buildups on the Bs (FR1) Bi/Bf dominant, rare Fs, mainly Epiphyton and thrombolitic fore-reef and upper- consisting of cyanobacterial and microbes common, minor red most foreslope thrombolitic microbes with minor skeletal algae; skeletal fauna: attached fauna; Renalcis dominate the upper brachiopods, Amphipora, fore-reef and decrease in abundance stromatoporoids; rare ostracods, downslope; stromatactis and cavities ﬁlled gastropods with early marine cement common; aggradational architecture 743 Table 1. Continued. 744

Depositional Facies association/ environment facies (codes) Description Biota Interpretation al. et Chen D. Slope Upper slope (foreslope) Bedded microbial Generally 1–5 m thick; stromatolites and Microbial colonies: Epiphyton Microbial bioherms Bs (US1) thrombolites common, especially the (or Angulocellularia), Renalcis, and/or biostromes former; LLH- and SH-type laminae (up to thrombolitic microbes, minor on the mid- to lower several 10’s cm high) common in the Wetheredella and Rothpletzella, foreslope former, indicated by intergrown (mono- red algae; rare gastropods, or heterospeciﬁc) cyanobacterial colonies, ostracods and Amphipora rare skeletal fauna; the latter, thrombolitic microbes dominant, minor aggregates of cyanobacterial colonies; stromatactis common Lower slope (toe-of-slope) Foreslope breccia/ 5–15 m thick and lens-shaped with basal Microbial colonies similar to the Breccia talus spalled erosion; composed mainly of microbial marginal buildups, minor from the buildup and Ó megabreccia (LS1)

02ItrainlAscaino Sedimentologists, of Association International 2002 Bs from fore-reef and foreslope, with skeletal fauna such as redeposited on the minor microbial laminites; disorganized stromatoporoids, brachiopods lower slope and clast-supported breccias with no and gastropods (toe-of-slope) obvious grading, locally calciturbidites intercalated, particularly in the upper part Pebble conglomerate Lime Ms tabular clasts with rounded ends, Rare Viscous debris ﬂow (LS2) usually mud-supported, parallel-prone to deposits derived from bedding and upslope imbricated locally the mid-upper slope Calciturbidite 0Æ2–3 m thick; ooids and peloids common, Brachiopods, gastropods, Turbidity ﬂow deposits (LS3) minor intraclasts and skeletal grains, calcispheres, rare ostracods on the lower slope locally ooids dominant, forming ooidal Gs; normal grading common; coquina intercalations locally Coquina Thin- to thick-bedded (10–100 cm); Brachiopods dominant, Shells resedimented to (LS4) articulated to disarticulated, abraded minor ostracods and gastropods; lower slope by currents shells displaying parallel-prone orientation microbialites as lumps or or storms and geopetal fabrics; other grains include aggregates microbial aggregates, intraclasts Basin Basin Microbial laminites Cm- up to 50 cm thick; wavy to planar Epiphyton (or Angulocellularia) Deep, quiet, eutrophic

Sedimentology (B1) laminae indicated by intergrown (mono- and Ursoscopulus common, conditions in the basin or heterospeciﬁc) cyanobacterial colonies, minor Rothpletzella close to the toe-of-slope benthic fauna absent; microstromatactis common; micromounds (5–15 cm high) locally Platy- to thin-bedded Lime Ms dominant, rich in organic matter, with Tentaculitids, foraminifera and Suspension fall-out in

, lime Ms (B2) argillaceous partings; coquina lenses gastropods, rare Stringocephalus deeper waters well 49, intercalated locally; ﬁne lamination common; below storm wave base

737–764 bioturbation weak, pelagic to hemipelagic fauna dominant, rare benthic fauna

Gs, grainstone; Ps, packstone; Ws, wackestone; Ms, mudstone; Bs, boundstone; Bi, bindstone; Bf, bafflestone; Fs, framestone. Carbonate platform evolution, Guilin, South China 745 relatively vigorous current activity. Overturned the platform interior (Elrick, 1995; Chen et al., and abraded stromatoporoids with peloidal grain- 2001b). The microbial laminites commonly form stone/packstone matrix characterize relatively the caps to the subtidal facies and are thus high-energy conditions, most probably in the considered as having been deposited under peri- reef-flat area, and microbial rudstones/grain- tidal conditions. stones formed under similar conditions, but closer to the buildups. The extensively distri- Fore-reef facies buted and abraded Amphipora suggest relatively Organic buildups of the fore-reef (facies FR1) are strong current activity close to the reef-flat in a composed of massive microbial boundstones in platformward direction. The skeletal wacke- which cyanobacterial microbes, such as Renalcis, stones/packstones reflect a decrease in current Epiphyton and thrombolites (Table 1), acted as the activity and were deposited in shallow to inter- framebuilders with some attached brachiopods mediate subtidal environments towards the plat- (mostly rhynchonellids). Generally, the upper form interior. The fossil-poor mudstones are very fore-reef of the buildups is dominated by Renalcis rare in the measured section and reflect restricted, (Fig. 6A and B), and those that are close to the quiet and shallow subtidal environments closer to

Fig. 6. (A) Polished slab of microbial framestone/rudstone composed overwhelmingly of monospecific microbial Renalcis (?), which is slightly fragmented. Cavity-filling calcite is extensively recrystallized. Scale bar is 3 cm long. (B) Polished slab of microbial bindstone/framestone composed mainly of Renalcis, with minor Epiphyton (1), red algae (Solenopora) (2) and unidentified microproblematica. Upward-growing structures are preserved (apparently in the mid–lower part). Early marine cements commonly rim the stromatactis (in the middle) and cavities. Scale bar is 3 cm long. (C) Photomicrograph of deep-water microbial laminite, indicated by intergrown Epiphyton (or Angulo- cellularia) laminae. Scale bar is 1 mm long. Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 746 D. Chen et al. reef-flat updip are commonly fragmented on the fore-reef, but with a higher abundance of (Fig. 6A), indicating relatively active currents. Epiphyton (Figs 5 and 6A and B). The lower However, their abundance decreases downslope. foreslope mainly consists of thick-bedded stro- In contrast, microbial Epiphyton increase volu- matolite boundstone (US1), locally with thick- metrically basinwards. Cavities and stromatactis, bedded microbial boundstone (Wulibei section in extensively filled with early marine cements, are Fig. 5; Table 1), forming metre-scale, domal bio- common (Fig. 6A and B), suggesting a constructive herms and/or biostromes (Fig. 7B). Their features role of early marine cementation in stabilizing the are similar to those reported in the Canning Basin microbial buildups. Such buildup margins are (Playford et al., 1976; George, 1999). Similarly, generally about 500 m across. The cyanobacterial cyanobacterial colonies played an important role microbes initiated on the steep platform margin in the maintenance of the steep slope through and grew upwards to form an aggradational (sta- framebuilding, calcification and early marine tionary) margin, with minor pulses of progradation cementation, as discussed above. (Figs 3–5) and local retrogradation in the final Facies associations indicate a low- to moderate- stage of platform growth (e.g. at Shihedong, Chen energy slope environment from the local photic et al., 2001a). The fore-reef clinoforms have (upper foreslope) to aphotic zone (lower fore- original dips up to 45° but, in the early stage of slope). The planar cross-strike distance from the platform construction, the dips were generally fore-reef to the lower foreslope is in the range lower in the range 25–30° (Fig. 4). 500–800 m (cf. Fig. 1B). The lowest value of 25° This type of architecture suggests that sedimen- (a tangent of 0Æ47) is taken as the average clino- tation at the platform margin may have approxi- form dip in view of burial compaction and the mately kept pace with the subsidence there, concave-up slope profile. In this case, the depo- induced by deep-seated faulting. The vertical sitional depth to the lower foreslope would have accretion and stability of the margin were largely been in the range 230–370 m. the result of the microbial buildups, which bound and trapped most of the grains and lime mud Lower slope (toe-of-slope) facies exported from the platform, the early induration The depositional dip on the lower slope decrea- (calcification) of the microbialites themselves and ses from about 15° to 1–5°. Deposits in this early marine cementation. These processes environment consist of foreslope breccias (LS1), helped to prevent the steep slopes from collapse. pebble conglomerate (LS2), calciturbidites (LS3) Similar situations to this have been documented and coquinas (LS4) (Fig. 3B; Table 1). At the base by Mountjoy & Riding (1981), Burchette (1988), of the Zhongnan Limestone (upper Givetian; S3), Drachert & Dullo (1989), Kenter (1990), Purser & coquinas lie above microbial buildups (Figs 5 and Plaziat (1998) and Bahamonde et al. (2000). The 7C). They colonized these areas or were transpor- microbial binding may also have led to sediment ted from the platform margin and accumulated as starvation in the basinal area. shell banks in the front of the buildups and towards the base of the clinoforms. During this Marginal slope environment phase, calciturbidites were not developed owing to the relatively gentle slope during the initial Upper slope (foreslope) facies acceleration of basin subsidence, but they do The upper slope is morphologically composed of occur in the upper Zhongnan Limestone. Grain- clinoforms with original dips of 25–45° (e.g. stones/packstones with ooids, peloids, minor Fig. 4). The values would be a little lower if intraclasts and skeletal grains, and pebble con- compaction is taken into account. The values are glomerates, mostly occur in the upper part of the confirmed by the measurement of geopetal fabrics Zhongnan Limestone (i.e. S5, Figs 3B and 5). in the deposits. A decrease in the depositional These represent more granular sediments shed off dip of the clinoforms occurs downslope, from the platform under relatively high-energy condi- about 45° to around 15° in the lower slope, and tions, which may have induced turbidity currents the profile is concave-up especially in the lower or debris flows on the now relatively steep slope. segment. Compositional changes in facies occur Two breccia wedges occur in the upper part of downslope as well. The uppermost foreslope is the microbial succession and are mainly com- composed of microbial bindstone/bafflestone posed of clasts derived from fore-reef and fore- (FR1) with extensive early marine cement slope microbial buildups; they taper and pinch (Fig. 7A), similar to that in the microbial buildups out both basinwards and platformwards over a

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 Carbonate platform evolution, Guilin, South China 747

Fig. 7. (A) Irregular to networked cavities in the microbial boundstone, which are ﬁlled with radiaxial ﬁbrous cements on the cavity walls and equant calcite crystals in the centre (big cavities common). Key for scale (4 cm). (B) Microbial boundstone showing domal topography (bioherm) in the lower foreslope. Hammer for scale (37 cm). (C) Abundant small brachiopods (Leiorhynchus sp. rhynchonellids mostly) concentrate to form coquinas. Articulated shells are common. Key for scale (4 cm).

short distance (typically 200–500 m) (Figs 5 and of the Devonian reef-flank in the Canning Basin, 8). The lower one is intercalated with ooidal Western Australia, where the slope angle was calciturbidites and exhibits an upward-thicken- about 25–30°, and slope relief was estimated to ing stratal pattern, suggesting an overproductive have been in the range 300–400 m (Playford, carbonate factory on the platform, relative to 1981). accommodation creation, and an unstable slope. This scenario reflects a less protected and stabil- Basinal environment ized microbial buildup system, on which gullies and grooves may have been developed locally. Basinal deposits consist mainly of deep-water The upper wedge of breccia contains rare calci- microbial laminites (B1) and thin- to platy-bed- turbidite beds and exhibits an undulatory basal ded lime mudstones (B2) (Table 1). These are surface, suggesting strong slope erosion during typical of a deep, quiet and sediment-starved the onset of breccia formation. environment with hemipelagic to pelagic fauna, Although most of the lower slope is not weak bioturbation, fine lamination and high exposed, the planar cross-strike distance of the organic content. The intergrowth of cyanobacte- present lower slope is estimated to have been in rial Epiphyton (or Angulocellularia; Fig. 6C) (cf. the range 500–800 m in view of the location of Riding, 1991) and Ursoscopulus (cf. Weller, 1995; exhumed basinal deposits (cf. Fig. 1B). Accord- Chen et al., 2001a) in the laminites indicates ingly, the relief of the lower slope could have deep-water conditions with no current agitation been in the range 45–75 m if the low dip value and sparse sediment influx. In general, an (5°, a tangent of 0Æ09) is taken as the average upward-deepening trend occurs in the upper estimate for the lower slope. Cumulatively, the Givetian, which is indicated by an upward relief of the overall slope from the platform edge increase in the abundance of clay and planktonic to the basin would have been in the range 275– fauna and a decrease in storm intercalations. The 450 m. This estimate for the relief of the steep, depositional depth could have been deeper than bioconstructed slope is broadly similar to the case that of the lower slope (275–450 m), as discussed

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 748 D. Chen et al. above, particularly in the late stage of this micro- fenestral types, such as irregular to laminoid. bially rimmed platform, based on the preserved Such deposits are widely distributed in plat- platform-to-basin profile. This would have been form-interior settings elsewhere in South China well below storm wave base. from the shallowest subtidal to intertidal environments (Chen et al., 2001b). Amphipora grainstone/wackestone, similar to that described SAND SHOAL-DOMINATED PLATFORM earlier, is the common back-margin deposit. SYSTEM: FRASNIAN Only one subaerial exposure horizon with microkarstic features such as reddening and vadose From the earliest Frasnian, a pronounced change (meniscus and pendant) cements has been occurred in platform architecture as a result of recognized (Fig. 10). large-scale collapse of the platform margin near the end of the Givetian. Under these circumstances, Stromatoporoid buildup facies the bioconstructed platform margin was locally Stromatoporoid buildups (PM8) are not exposed scalloped in a platformward direction (i.e. at but are inferred to have been present at least in the Shihedong and to the south of Wulibei; Fig. 1B) upper part of S6 in the early Frasnian from the and gullied on the slope. Bank-derived sediments occurrence of clasts in the slope megabreccias at were funnelled through the gullies and bypassed the base of S7 (Fig. 11A). The buildups were the mid–upper slope to feed the lower slope dominated by stromatoporoid bindstone/baffle- aprons, where many coarse gravity-flow deposits stone and would have been similar to those were deposited. Stacked breccia/megabreccia described elsewhere (e.g. Chen et al., 2001a). units occur in the lower Frasnian (S6 and S7) at However, branching stromatoporoids and corals, Dongcun (see Fig. 1A for location; Chen et al., such as Stachyodes and Thamnopora, are abun- 2001a; fig. 9). At this time, sand shoals dominated dant, implying they were low wave-resistant the platform margin in the absence of organic structures (Fig. 11B). Moreover, the coeval slope barriers reducing wave action (Figs 3B and 9). This deposits lack buildup clasts and breccias but are situation persisted throughout the Frasnian until dominated by gravity-flow deposits (mainly calci- the restoration of microbial buildups on the plat- turbidites) (Fig. 10). These points suggest that the form margin in the latest Frasnian (e.g. Wulibei buildups were limited in size and did not prevent logs in Fig. 10). In the platform–basin transect granular sediments from being shed off the plat- from Zhongnan via Wulibei to Fuhe, platform- form by binding and baffling. It is quite likely that margin (back-margin, stromatoporoid buildup and the buildups were only developed on local pal- sand shoal/bank), slope (upper slope, lower slope) aeohighs along the platform margin that survived and basin environments are recognized (Figs 9 the intense platform collapse in the latest Givetian. and 10), based on the depositional features, geom- This buildup episode is probably the youngest etries and facies associations. Measurements indi- horizon of this type occurring in Frasnian strata. cate an increase in slope inclination (35–50°) during this platform growth phase, especially in Sand-shoal and bank facies the early part (Fig. 8), probably in response to more rapid basin subsidence and some progradation of Depositional facies of this environment com- the platform margin. monly consist of peloidal–ooidal grainstones/ packstones (PM10) and peloidal–oncoidal rudstones/packstones (PM9) (Table 2); they are the Platform-margin environment most common facies in the marginal shoal envi- Back-margin facies ronment, although the majority of the upper Frasnian strata have been eroded at outcrop. The deposits of this environment are only partly These deposits are generally light coloured and exposed, and only the basal part of the Frasnian thin to medium bedded. Compositionally, peloids is seen. Facies are dominated by fenestral lime- are abundant, commonly mixed with ooids, onc- stone (PM7) with minor Amphipora grainstone/ oids and other grains (Fig. 10). These deposits wackestone (PM3) (Table 2). The fenestral characterize a relatively shallow, moderate- to limestones are thick to massive bedded, but moderately high-energy shoal/bank environment, small-scale internal cycles ranging from several with either peloidal–ooidal or peloidal–oncoidal centimetres to tens of centimetres in thickness sands (Fig. 9). are common and defined by the changes in

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 Carbonate platform evolution, Guilin, South China 749

Fig. 8. Sketch from photomosaics showing the general depositional succession and geometry changing from a microbially rimmed platform to sand-shoal platform system from the uppermost Givetian to the Frasnian. The two lower megabreccia/breccia units are composed of microbialites derived from the updip microbial buildups. The upper breccia unit at the base of S7 is mainly composed of clasts of stromatoporoid buildup facies. See Fig. 10 for detailed stratigraphic logs (conducted along the dashed arrow). This diagram is the westward-extending part of Fig. 4 approximately in the dip direction of the slope. Bush-like patterns mean heavy vegetation. Box at the bottom is 3Æ5 m high.

basinwards. Four main breccia horizons occur in Marginal slope environment the Frasnian strata, and the lower two horizons, at Upper slope facies the bases of S6 and S7, respectively, are relatively thick fore-reef breccias and megabreccias com- Autochthonous deposits of this environment are posed of clasts of microbial and stromatoporoid sparse, but gravity-driven contortion of previ- boundstones (Figs 5 and 10). These breccias are ously deposited, semi-consolidated sediments most likely related to the erosion and/or collapse (slumps, US2) is extensive because of the steep of the organic buildups. The rest of the breccias slope. The sediments shed off the platform-mar- are mostly slope-derived and, compositionally, gin banks may have bypassed the upper slope and are dominated by clasts of peloidal mudstone/ been transported farther downslope. The slumps wackestone, originally deposited in deep water commonly comprise thin-bedded peloidal mud- through suspension fall-out or muddy turbidity stones/wackestones, which locally exhibit boud- flow settling (cf. Chen et al., 2001a), locally with inage bedding. Different degrees and scales of grainstones of peloids and skeletal grains (e.g. the soft-sediment deformation, such as translational base of S8). Similar Devonian examples have been and rotational slides, are present in the contorted documented in western Canada (e.g. Hopkins, horizons (Fig. 11C and D), and local break-up of 1977) and southern Hunan (Jiang, 1989). These the overfolded part into clasts is observed. These breccias/megabreccias are usually interpreted as features are typical of small-scale creeps and debris-flow deposits formed through relatively glides down the slope. large-scale slope failure and erosion, triggered by various mechanisms including tectonic-induced Lower slope facies seismic activity, tsunamis, storms and rapid The deposits of this environment are mainly relative sea-level falls (cf. Cook & Taylor, 1977; composed of breccias/megabreccias (LS1), Cook & Mullins, 1983; Spence & Tucker, 1997). In pebble conglomerates (LS2), calciturbidites (LS3) such circumstances, semi-consolidated masses on and muddy calciturbidites (hemipelagic peloidal the slope were forced to move and accelerate mudstones/wackestones, LS5) (Fig. 11C; Table 2). downslope as soon as the shear strength of the Breccias/megabreccias are usually wedge- masses was exceeded. These slides and slumps shaped with basal erosion, ranging from several were transformed into debris flows through the metres up to 20 m thick and less than 800 m increasing entrainment of water and mud in the across; they pinch out both platformwards and course of downward movement. The remobilized

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Table 2. Depositional facies of sand-shoal platform system. al. et Chen D.

Depositional Facies association/ environment facies (code) Description Biota Interpretation Platform Back-margin margin Amphipora See Table 1 See Table 1 See Table 1 Gs/Ws (PM3) Fenestral limestone Thick-bedded (>0Æ5 m), creamy-white or pink in Calcispheres common; Semi-restricted, shallowest (PM7) colour; compositionally peloida l Ms/Ps with rare ostracods and subtidal to intertidal minor ooids; fenestral cavities variable from gastropods environments irregular to laminoid in shape, internal sediments in cavities common Ó Stromatoporoid buildup 02ItrainlAscaino Sedimentologists, of Association International 2002 Stromatoporoid Mainly made up of Bi/Bf of massive, spherical, Massive, spherical and bulbous Stromatoporoid buildups Bs (PM8) bulbous (binders) and branching (bafflers) stromatoporoids and branching developed on stromatoporoids, minor corals and brachiopods, ones (i.e. Stachyodes and palaeohighs which based on the succeeding breccia horizon on Amphipora); corals, survived the the toe-of-slope and back-reef deposits (PM3) brachiopods and others platform collapse Sand shoal/bank Peloidal–oncoidal Mainly composed of oval and oblate oncoids Ortonella? problematic microbes; Moderate-energy, marginal Rs/Ps (PM9) and peloids with minor ooids and shells; ostracod shells shoals and banks oncoids, mm to 10’s cm in size, with influenced by waves concentric and conical laminae and nucleus; and storms sharp internal erosion surfaces locally Peloidal–ooidal Ooids and peloids common, oncoids Rare Moderately high-energy Gs/Ps (PM10) subordinate; minor erosion surfaces common, sandy shoals and banks but current-related structures absent, fenestral fabrics locally Slope Upper slope Slump (US2) Contorted horizons of semi-lithified, hemipelagic Originally planktonic fauna, Small-scale slides/glides deposits, locally grading into breccias i.e. tentaculitids, of previous on upper slope before

Sedimentology basinwards; variable degrees and scales of unconsolidated masses slope failure deformation structures Lower slope Breccia/megabreccia Foreslope breccias: stromatoporoid Bs clasts Stromatoporoids as buildup Breccias or talus spalled (LS1) up to a few m in size (olistoliths); constructor; Stachyodes, from the upslope build-

, clast-supported in general, poorly sorted Amphipora, corals, brachiopods ups and redeposited at 49, with no obvious grading, and wedge-shaped and others toe-of-slope 737–764 Ó 02ItrainlAscaino Sedimentologists, of Association International 2002 Table 2. Continued.

Depositional Facies association/ environment facies (code) Description Biota Interpretation Slope breccias: cm- to m-sized clasts; clasts Rare corals, brachiopods and Transported by debris of lime Ms/Ws dominant, minor peloidal crinoids; hemipelagic to pelagic flows and redeposited Gs/Ps, rare microbial Bs and skeletal Gs; fauna dominant in the clasts at the toe-of-slope disorganized, mud- to clast-supported, crude grading of clasts locally Pebble conglomerate Tabular clasts with rounded ends, up to several Rare Similar to the above, (LS2) 10’s cm long, mainly made up of lime Ms; but smaller scale usually mud-supported and parallel-prone oriented, locally upslope imbricated; locally they grade upwards into calciturbidites Calciturbidite Thin to thick bedded (5–100 cm), commonly Rare Transported by turbidity (LS3) normal graded with incomplete Bouma flows and redeposited at sequences; compositionally peloidal or toe-of-slope ooidal-peloidal Gs/Ps dominant, with minor Sedimentology intraclasts, oncoids and rare skeletal grains; local slumps; interfingering with basinal China South Guilin, evolution, platform Carbonate deposits basinwards Muddy Dark coloured; platy to thin-bedded (1–20 cm), Planktonic tentaculitids, Hemipelagic suspension calciturbidite local boudinage; peloidal Ws/Ms in ostracods, rare benthic fall-out and/or muddy ,

49, (LS5) composition; locally cherty nodules/bands fauna turbidity flows abundant, calciturbidites intercalated; 737–764 hemipelagic fauna; different degrees and scales of soft-deformation structures Basin Basin Cherty lime Dark grey to black; thin-bedded (5–20 cm), Abundant planktonic Deep, oxygen-deficient, mudstone black lime Ms abundant in organic matter tentaculitids, minor quiet environment (B3) and chert nodules, locally banded cherts ostracods and calcisponges; intercalated, upward-increasing amounts rare radiolarians in cherts; fine lamination Banded Black to brownish; platy to thin bedded Tentaculitids and minor Oxygen-depleted, stagnant, chert/siliceous (1–20 cm), fine laminated; basal banded radiolarians deeper-waters with rare shale (B4) cherts grade upwards into siliceous shales; carbonate influx manganese nodules enclosed Nodular Lime Ms nodules fitted with greenish and Planktonic tentaculitids; Pelagic suspension fall-out, limestone red argillaceous seams, locally exhibiting intensive bioturbation locally weakly oxygenated (B5) brecciated appearance; commonly bottom waters calciturbidites interbedded

Gs, grainstone; Ps, packstone; Ws, wackestone; Ms, mudstone; Bs, boundstone; Bi, bindstone; Bf, bafﬂestone. 751 752 D. Chen et al.

Fig. 9. Sketch showing depositional environments and facies across the platform–slope–basin proﬁle during the stage of the sand- shoal platform system in the Fras- nian. Abundant sediments were shed offbank and transported downslope as gravity ﬂows, because of a lack of any protection at the platform margin.

parts would settle down in places where the slope to the increasing energy level and slope instabil- became gentle enough (2–5°), although the possi- ity (see above). bility of coeval deposition on the mid–upper slope also existed. Basinal environment Pebble conglomerates are commonly present in the successions below horizons of breccia and In the lower Frasnian, basinal deposits are mainly megabreccia and are relatively thin (commonly made up of cherty lime mudstones (B3) and <3 m). Locally, they grade upwards into calcitur- banded cherts/siliceous shales (B4) (Fig. 10; bidites. They are also interpreted as debris-flow Table 2). Salient features are the extensive occur- deposits (e.g. Cook & Taylor, 1977; Cook & rence of the siliceous facies, abundant planktonic Mullins, 1983). Their occurrences suggest an fauna and organic matter and weak bioturbation, increasing energy level, probably as a result of all characteristics of a deep, oxygen-depleted, the acceleration of accommodation loss on the sediment-starved environment with poor circula- platform (relative sea-level fall) or slope instabil- tion. It is probable that biogenic silica only made ity (Chen et al., 2001a). a minor contribution to the silica source, and that Calciturbidites are widely distributed in the most of the silica was supplied by syndepositional lower slope successions, and generally increase in volume and coarsen upwards towards the breccia/megabreccia horizons. Their composition, dominated by peloids and ooids, suggests Fig. 10. Lithological logs and correlations of platform derivation mostly from the platform. Deposits at phase 3 from the platform margin to the basin from Zhongnan via Wulibei to Fuhe. See Fig. 1B for loca- the distal toe-of-slope fringing the basin are tions. The section of the platform margin at Zhongnan mainly muddy calciturbidites composed of fine- was mostly eroded and is exaggerated on the right. grained peloidal mudstones/wackestones (cherty Thin lines on the right of the logs for Zhongnan mark nodules intercalated locally), and they grade the high-frequency cycles (parasequences). Refer to basinwards into cherty lime mudstones, banded Tables 1 and 2 for facies codes. Conodonts: 1, Meso- cherts/siliceous shales and nodular limestones taxis asymmetricus;2,Pa. punctata; 3, Pa. jamieae;4, (Fig. 10). They onlap the upper slope facies as Pa. gigas gigas;5,Pa. triangularis;6,Polygnathus sp.; 7, Pa. punctata;8,Pa. triangularis. LST, lowstand relative sea-level rose rapidly. In the succeeding deposits; TST, transgressive deposits; HST, highstand sea-level fall, these unconsolidated, fine-grained deposits. M, mudstone; W, wackestone; P, packstone; hemipelagic deposits were readily subjected to G, grainstone; Co, conglomerate; B, breccia/megabrec- soft-sediment deformation and slumping owing cia; Bs, boundstone; Rs, rudstone.

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Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 754 D. Chen et al.

Fig. 11. (A) Megabreccia at the base of S7, which overlies platy calciturbidites and hemipelagic carbonates (at the lower right corner). Hammer for scale (37 cm long). Gubi Formation, Wulibei. (B) Details of the megabreccia in (A). The clast is mainly composed of stromatoporoids (Stachyodes common, see arrows), Amphipora, ostracods and brachiopods. The matrix is mainly lime mudstone and wackestone. Scale bar is 3 cm long. (C) Platy to boudinage- bedded, hemipelagic cherty limestones. Cherty bands with relatively high relief above the weathered surface. Thin- bedded calciturbidites are intercalated (see the lower right and top left corners), which constitute the upper part of an upward-thickening cycle. Slight slump folds occur here. Arrows point to the top of cycles. Hammer for scale (37 cm long). Gubi Formation, Wulibei. (D) Strongly deformed syndepositional tight slump folds in the hemipelagic limestones, which are intercalated with thin-bedded calciturbidites (see the top of the hammer). Hammer for scale (37 cm long). Gubi Formation, Shihedong (see Fig. 1B for location). hydrothermal fluids rising up through deep-sea- dites close to the toe-of-slope. Locally, breccias ted fault zones (Zhou, 1990; Wang & Chen, 1996; occur in the basinal succession (see Fuhe logs in Chen et al., 2001a). The bathymetric level of the Fig. 10). In the upper Frasnian strata, siliceous basin floor could have been deeper than that of material disappears rapidly along with the plank- the earlier phase of platform growth in view of tonic fauna and organic matter. In contrast, accelerated basin subsidence supported by facies bioturbation intensifies locally, and clay content evidence and increased slope declivity (docu- increases a little. These features suggest that a mented earlier), but the magnitude is difficult to striking environmental change occurred on the determine. basin floor, such as the attenuation of volcano- In the upper Frasnian, basinal deposits are genic hydrothermal activity and an increase in dominated by nodular limestones (B5, Table 2) the oxygen content of the water column, from and are commonly interfingered with calciturbi- increased water circulation.

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pulses of small-scale progradation, but metre- DEPOSITIONAL SEQUENCES scale cycles are generally not recognized in the AND PLATFORM EVOLUTION microbial buildups, especially in the fore-reef zone (Figs 4 and 5). Each episode of microbial Bioconstructed platform system (sequences upbuilding was interrupted by a rapid facies S3–S5) change, commonly involving a deeper water facies During this late Givetian platform development (e.g. coquinas, platy hemipelagic peloidal wacke- phase (actually phase 2 in the development of the stones/mudstones) or non-deposition (common platform as a whole, see above), microbialites on the upper foreslope and bankwards) (Fig. 5). were deposited from platform margin to slope to These surfaces are interpreted as the disconform- basin floor, resulting in a distinctive architecture. able boundaries between the constructional epi- Three sequences (S3–S5) are identified in the sodes of microbial buildup growth, and they are succession. In the back-reef/reef-flat deposits, physically linked updip to the surfaces of exten- upward-shallowing, metre-scale cycles (com- sive platform shallowing (± exposure); thus, they monly 1–5 m thick) are extensive (see Zhongnan correspond to depositional sequence boundaries. log in Fig. 5), but the cyclicity is commonly During the later growth phase of the microbial obscured by the light-coloured appearance and buildups, increases in gravity-flow deposits and weak stratification. Typically, these cycles slope declivity of the lower foreslope deposits, consist of upward-shallowing units from especially in the upper part of S4 and S5 (Figs 5 fossil-poor lime mudstone (PM6) to skeletal and 10), indicate an enhancement of energy level, wackestone/packstone (PM4) to stromatoporoid slope instability and tectonic activity on deep- rudstone/floatstone (PM1) to Amphipora grain- seated faults or a relative sea-level fall (cf. Mullins stone/wackestone (PM3) and laminites (PM5) at et al., 1986, 1991; Bosellini, 1989; Bosellini et al., the top. The cycles reflect aggradation of subtidal 1993). facies up to near sea level. More commonly, Small-scale erosive grooves and/or gullies may however, not all of these facies are present in each have been formed on the slope, down which cycle; common cycles are PM1–PM3, PM1–PM5, eroded sediments were transported gravitational- PM3–PM5, PM6–PM4 (or PM3); minor ones are ly to the toe-of-slope. During phases of non- PM4–PM1–PM3 (or PM5). Some are dominantly deposition, carbonate productivity on the platform subtidal cycles (Fig. 5). Similar Devonian cycles and early cementation of the microbial buildups have been extensively described by many authors were depressed. These processes would have led (Read, 1973; Elrick, 1995, 1996; Lamaskin & to the instability of oversteepened clinoforms and Elrick, 1997; Chen et al., 2001b). Locally, cycles given rise to local scalloped embayments of the of microbial rudstone/grainstone (PM2) or micro- platform margin or collapse of the platform mar- bial boundstone (FR1) to laminites (PM5) are gin itself (Fig. 1B), as seen in the late stage of recognized. bioconstruction. This scenario would result in the The metre-scale cycles can be grouped into shedding of large olistoliths (or megabreccias) to larger scale cycle sets, and depositional sequences the lower slope, as relative sea-level fell, possibly (S3–S5) are identified within the back-reef/reef-flat enhanced by contemporaneous tectonic activity. succession based on the vertical stacking patterns The coquinas near the base of the Zhongnan of the metre-scale cycles (five cycle sets in S3–S4 Limestone represent shell banks in front of the and four cycle sets in S5). These can be correlated foreslope (toe-of-slope), and they are considered well with cycles in the basinal succession at Fuhe as transgressive deposits. There are two horizons (Fig. 5). In general, more subtidal cycles are of buildup-derived breccia/megabreccia wedges present in the lower parts of the sequences and on foreslope (Figs 5 and 8). The lower one is more peritidal cycles in the upper parts, where intercalated with ooidal grainstone/packstone they display increasing-upward vadose cementa- (turbidites), particularly in the upper part tion. Nevertheless, persistent subaerial exposure is (Fig. 5), and the breccia beds themselves display only found at the top of S5 in the form of an upward-thickening pattern, indicating a grad- microkarstic features (i.e. reddening and vadose ual increase in energy level, slope instability and cementation) (Fig. 5). This subaerial exposure off-platform shedding of sediment. This is reinter- surface is widely recognizable in platform succes- preted here as having been formed through sions across South China (Chen et al., 2001b). highstand shedding (cf. Chen et al., 2001a). The The stratal patterns from fore-reef to foreslope upper breccia wedge on the other hand is inter- display an aggradational architecture, with minor preted as a lowstand deposit in view of its strong

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 756 D. Chen et al. basal erosion surface, rare tapering updip, calci- cycles [PM9–PM3 (PM5), PM3–PM5, minor turbidite intercalations and normal grading in the PM10–PM5] are extensive and can be further upper part of the breccia. This wedge was prob- grouped into larger scale cycle sets (Fig. 10). ably a response to a rapid loss of accommodation Upwards, the sequence is dominated by sand- space on the platform margin. The undulatory shoal deposits comprising peloidal–ooidal pack- basal surface of this wedge-shaped breccia unit is stone/grainstone. Metre-scale, thickening- and thus interpreted as the sequence boundary, and it coarsening-upward cycles, indicating the upward- can be traced updip to join the subaerial exposure increasing energy, are widely developed, and they surface (Fig. 5). The two breccia wedges are are also grouped into larger-scale cycle sets. overlain by thin-bedded peloidal mudstone/ Four breccia units, including the basal one wackestone (LS5), representing the consequence documented earlier, are present in the lower of a rapid sea-level rise (transgression). slope area of the platform. These breccias/megab- In the basinal succession, the thin-bedded lime reccias are considered here as lowstand deposits, mudstones (B2) generally grade upwards into and their basal surfaces are thus the sequence deep-water microbial laminites (B1), constituting boundaries in view of their erosive contact with upward-shallowing cycles (Fig. 5; Chen et al., underlying horizons, rare calciturbidite beds and 2001b), although they are not a real reflection of upward-thinning stratal patterns in response to changing water depth. Vertical stacking of these rapid decrease in accommodation space on the cycles gives lower order cycle sets, in which the platform (cf. Sarg, 1988; Garcıá-Monde´jar & lower components of the cycles thin upwards, Fernańdez-Mendiola, 1993; Spence & Tucker, whereas the upper parts thicken upwards. The 1997; Go´mez-Pe´rez et al., 1999; Chen et al., stacking patterns of these cycle sets provide the 2001a). Four sequences are recognized in the basis for sequence identification and for their Frasnian strata and, ideally, each starts with a correlation with the back-reef succession. The top breccia horizon with a basal erosion surface in the of these cycle sets, which possess the largest slope setting and locally grades upwards into amount of microbial laminite (B1), is considered calciturbidites; overlying hemipelagic deposits as the sequence boundary, and this is overlain by are interpreted as transgressive deposits (Figs 5 a cycle set commencing with a thick unit of thin- and 10). bedded lime mudstone (B2) (Fig. 5). Three sequ- Within the lower slope facies association, ences are identified in the Mintang Formation, upward-coarsening cycles commencing with and they can be correlated well with the back-reef hemipelagic peloidal mudstone/wackestone sequences on the basis of cycle sets. passing up into calciturbidites and/or pebble conglomerates are common (Figs 10 and 11C). These cycles may further group into larger-scale Sand-shoal/bank system (sequences S6–S9) cycle sets, in which the turbidite/debris beds After the large-scale platform collapse and funda- commonly increase in volume and frequency mental environmental change at the end of the upwards, reflecting increasing energy and slope Givetian, the bioconstructed platform system instability. Thus, the basal cycles usually pos- came to an end, and a new sand-shoal system sess the thickest fine-grained, hemipelagic depos- was established in the Frasnian (Fig. 8); this was its and are interpreted as transgressive deposits. phase 3 in the history of the platform as a whole. The overlying upward-coarsening cycle sets are Stromatoporoid buildups and microbial bioherms considered as the highstand deposits in view of colonized the platform margin and upper slope in their occurrence and composition, as proposed the early and latest Frasnian respectively. Four by some authors (e.g. Schlager et al., 1994). depositional sequences are recognized in this During times of relative sea-level highstand, platform phase (Fig. 10), representing four stages more sediment is exported off the platform and of platform construction. transported downslope, forming the turbidity Only a small part of the platform-margin/back- flows or debris flows, as the speed of accommo- margin deposits is preserved from this platform dation creation is slower than the rate of car- phase at outcrop. In S6, the lower part is domin- bonate production. ated by shoal deposits mainly comprising peloi- In the basinal deposits, the sequences are dal–oncoidal rudstone/packstone, and the upper dominated by hemipelagic to pelagic deposits part is dominated by back-margin deposits, mainly with minor calciturbidites and breccias Amphipora packstone/wackestone. Within this (Fig. 10). It is noted that the deepest basinal succession, metre-scale upward-shallowing facies associations in S6 and S7 correspond to

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 Carbonate platform evolution, Guilin, South China 757 the thickest slope facies (including breccia acceleration of basinal subsidence resulted in units), particularly in the latter, suggesting a differential carbonate production across the plat- rapid subsidence of the basin floor at the onset form–slope–basin transect; this led to steepening of their formation. In contrast, an obvious of the slope and sediment starvation there owing shallowing and/or subaerial exposure trend in to low carbonate production below the photic the platform succession was recorded before zone. This situation would also have allowed the and at the beginning of the formation of these bioconstructors to aggrade upwards rather than two sequences (Fig. 10). Similar trends during prograde at the platform margin. Cyanobacterial this time interval have been reported in other microbes (mainly Renalcis and Epiphyton) on the platforms elsewhere in south China (Chen et al., margin-to-slope acted as the framebuilders 2001a,b). through binding, trapping and baffling sediment exported from the platform, and prevented the sediment being shed to the basin floor; thus, CONTROLS ON THE ARCHITECTURE calciturbidites were rarely deposited on the slope AND EVOLUTION OF THE at this time (Fig. 3A). PLATFORM–SLOPE–BASIN SYSTEM The aggradational architecture of the microbial buildups reflects a crude balance between Tectonism the rate of accommodation space increase controlled by relative sea-level rise and the rate of During Devonian times in South China, tecton- sediment supply controlled by environmental ism exerted a fundamental control on the factors (e.g. Go´mez-Pe´rez et al., 1999), partic- development and evolution of carbonate plat- ularly in the early stages of platform growth. forms and interplatform basins (e.g. Wu et al., With time, however, this balance became vul- 1987; Zhong et al., 1992). A sinistral transten- nerable as the platform margin thickened, and sional tectonic setting was considered to be the relief between the platform and basin responsible for their origin (Shen et al., 1987; increased through progressive acceleration of Jiang, 1990; Liu et al., 1993; Liu, 1998; Chen basin subsidence and steepening of the slope. et al., 2001a). The sinistral strike-slip faulting During the latest Givetian (top of S5), large-scale was initially reactivated along the rigid intra- platform collapse occurred synchronous with continental antecedent basement fault zones, significant uplift (subaerial exposure) and scal- and this induced further secondary strike-slip loping of the platform margin (backstepping of faulting within the blocks. The Yangshuo Basin the platform margin locally, e.g. at Shihedong) was formed in such a secondary synthetic and demise of the microbialites (Fig. 5). After (sinistral) strike-slip fault zone; a spindle- to this, the platform evolved into the sand-shoal- rhomboidal-shaped basin was created through dominated platform system as a result of col- the pull-apart process between the strike-slip lapse and slope erosion; organic buildups made faults (cf. Fig, 12; Chen et al., 2001a). This of stromatoporoids and some other biota colon- confined basin geometry restricted and dam- ized the palaeohighs surviving the collapse of pened the current flows and favoured the the margin. growth of cyanobacterial microbes in the basinal A subsequent large-scale platform collapse in realm. The Yangdi Platform was tectonically the early Frasnian (top of S6) occurred in relation located on the northern side of the Yangshuo to the following significant uplift (and/or plat- pull-apart basin, along one of the oblique, form shallowing). Huge masses of megabreccia extensional faults (Chen et al., 2001a). This were released to the toe-of-slope in these cata- fault delineated the rectilinear alignment of strophic events. Simultaneously, the jerky subsi- the southern margin of the Yangdi Platform. dence of the basin led to rapid deepening and From the late Givetian to Frasnian, the Yangdi poor circulation, encouraging the deposition of Platform changed from a microbially rimmed chert in the basin (Fischer & Arthur, 1977) as seen platform system to a sand-shoal system. The at outcrop (Figs 5 and 10). This type of tectonic long-term evolution of carbonate platform–basin dynamics and associated depositional response architecture is attributed to local and regional between the platform and basin generally charac- tectonism. The intensification of extensional tec- terizes extensional tectonic settings (e.g. Barr, tonic activity, induced by the strike-slip faulting 1987; Leeder & Gawthorpe, 1987; Jackson et al., from the late Givetian, accelerated the differential 1988; Yielding, 1990; Ward, 1999), fitting in well subsidence between platform and basin. The

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Fig. 12. Palaeogeography and palaeotectonic setting of the Yangdi Platform and Yangshuo Basin in Devonian times. (A) Palaeogeography of Frasnian time in eastern Guangxi–Hunan, South China (modiﬁed from Chen et al., 2001a) with platforms alternating with interplatform basins. Inset box is the area of (C). (B) Interpreted syndepositional structural styles of the platform–slope–basin system in (A). A large-scale sinistral transtensional movement (F1, F2) was interpreted as the primary mechanism responsible for the formation of the platform–basin system in (A), which induced a series of secondary fault systems (f1–f5; Chen et al., 2001a). (C) Exaggeration of the area of the inset box in (A). The spindle- to rhomb-shaped Yangshuo Basin was developed in the north of Yangshuo, south-eastern Guilin, and abutted the Yangdi Platform to the north. Two dashed circles show the location of subdeeps in the basin. Anatomy of cross-strike transition from platform–slope–basin is carried out for this study (see the shaded bar). (D) Structural style of the Yangshuo Basin and Yangdi Platform, which was formed as a result of the secondary, synthetic sinistral strike-slip faulting (f3 and f4). The northern margin of the Yangshuo Basin was controlled by the extensional faulting induced by f3 and f4, and delineated the orientation of the platform margin. Compare (B) and (D) for the structural interpretation.

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 Carbonate platform evolution, Guilin, South China 759 with the proposed tectonic setting of the Yangdi deduced from the sequences is not consistent Platform. with the proposed eustatic curve of Johnson et al. Contemporaneous intensification of volcanic (1985). In addition, the pattern of accommodation activity, indicated by the occurrence of intrusive space changes recorded on platform successions sills in the Givetian platform successions and elsewhere in South China is different from the pillow lavas and tuffs in the Frasnian basinal eustatic curve (cf. Chen et al., 2001b), probably successions farther to the south in Guangxi (Wu reflecting a regional tectonic influence. Further- et al., 1987; Zhong et al., 1992), also suggests more, recent work on the Devonian stratigraphy enhanced tectonism from the latest Givetian to around the world indicates that the pattern of the early Frasnian. Under these circumstances, a relative seal-level change is variable (House & large quantity of volcanogenic hydrothermal Ziegler, 1997). fluids rich in siliceous and toxic matter was Taking into account all the information, introduced into the basins through the deep- regional tectonism probably played a major role seated fault zones, leading to the deposition of in relative changes in sea-level through the significant chert in the basin in the early Fras- Givetian–Frasnian, driving the development of nian (e.g. Fig. 10). Furthermore, hydrothermal the depositional sequences and probably over- fluids rising up through deep-seated faults, riding the eustatic signals. An intraplate stress which commonly delineated the orientation of tectonic model for vertical crustal movements platform margins, may also have affected the proposed by Cloetingh et al. (1985) may explain stability of margin-to-slope through corrosion the short- to long-term (105)107 years) relative and hydrofracturing. changes in sea level in the basin and on the The strong episode of tectonism and associated platform, as proposed for other carbonate se- environmental changes across the Givetian/Fras- quences developed in active tectonic settings nian transition finally led to the decline of the (Go´mez-Pe´rez et al., 1999; Rosales, 1999). The bioconstructed platform system and the onset of accumulation of the transtensional stress would the sand-shoal/bank system, which persisted into induce periods of relative sea-level rise, whereas the latest Frasnian. During this stage, large quan- the sudden stress release would result in normal tities of granular and muddy sediments were fault movement at depth for each tectonic pulse, exported off the platform to induce turbidity with uplift of the footwall and downthrow of the flows and suspension clouds (Figs 9 and 10). hangingwall. This process reached a climax in After this, the restoration of microbial buildups the early Frasnian. allowed a bioconstructed platform margin to Thus, it is considered that the depositional develop again in the latest Frasnian (see Wulibei sequences were formed largely through the effect logs in Fig. 10). of regional tectonic activity, which may have been superimposed on a long-term eustatic cycle. The higher frequency (metre-scale) depo- Relative sea-level changes sitional cyclicity on the platform is more likely Depositional sequences and their internal organ- to have been driven by Milankovitch orbital ization are produced by fluctuations in relative forcing, as postulated by numerous authors (e.g. sea level, with contributions from variations in Goodwin & Anderson, 1985; Goldhammer et al., carbonate production and sediment input, and 1990; Balog et al., 1997; Chen et al., 2001b). This environmental factors (e.g. Sarg, 1988). Seven cyclicity, however, is commonly modulated or third-order sequences have been recognized in interrupted by lower frequency eustatic and the Devonian strata discussed in this paper (Chen tectonic signals and, in such a tectonically active et al., 2001a,b), with three sequences (S3–S5) in basin, jerky subsidence could also be a driving the upper Givetian and four sequences (S6–S9) in mechanism. the Frasnian. The two sequence sets represent the bioconstructed platform and the sand-shoal-dom- Environmental factors and oceanographic inated platform systems respectively. Although it setting seems reasonable that regional tectonism played an important role in generating the sequences in Environmental factors, such as current circulation, the latest Givetian to early Frasnian (see above), nutrient levels, oxygen content and oceanographic the role of relative sea-level change in sequence setting (windward vs. leeward), are also very formation is still uncertain. Regardless of their important controls on the architecture and evolu- origin, the relative sea-level change history tion of carbonate platforms. On the Yangdi

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Platform, the extensive microbialites (microbial and sediment redistribution to the basin. The buildups on the margin-to-slope, microbial lami- cyanobacterial microbes colonized the platform nites in the basin) in the upper Givetian reflect a margin as framebuilders through binding and supersaturated environment with poor circula- baffling sediment being exported from the plat- tion and oxygen-deficient and eutrophic bottom form interior. In this case, little sediment was waters. Such an environment would have favoured shed off the platform to the basin, leading to the blooming and early calcification of cyanobacterial progressive steepening of the slope and increase colonies, but would have excluded normal-marine in relief between the platform and the basin. On benthic organisms through the serious environ- the other hand, the growth of cyanobacterial mental stress (e.g. Feldmann & McKenzie, 1997; colonies on the slope would have helped to Neuweiler et al., 1999; Riding, 2000). maintain the steep slope and protect it from The occurrence of siliceous facies in the collapse through encrusting and early lithifica- lower Frasnian suggests a more euxinic basinal tion (Mountjoy & Riding, 1981; Burchette, 1988; milieu and more serious environmental stress Drachert & Dullo, 1989; Kenter, 1990; Purser & still (Vogt, 1989). The cherts are unlikely to Plaziat, 1998; Bahamonde et al., 2000). In the late have been formed in depths of several thousand Givetian, the aggradational (locally slightly retro- metres like their modern analogues in view of gradational in the late stage of bioconstruction) the geometry of the platform-to-basin profile. architecture of the platform margin was therefore The long residence time of oxygen-depleted generated under circumstances in which the bottom waters with poor circulation and rare vertical growth of the microbial buildups was sediment influx may have exerted a profound approximately keeping pace with the creation of control on the precipitation of silica (cf. Fischer new accommodation space controlled by relative & Arthur, 1977). The upwelling of anoxic, sea-level changes. nutrient- and silica-rich volcanogenic fluids Sediment fabrics and grain size mainly affect from deep-seated fault zones would have accel- slope angles (Kenter, 1990; Drzewiecki & Simo, erated the expansion of oxygen-depleted waters 2000). Generally, mud-supported deposits build (Vogt, 1989), favouring substantial accumulation slope angles lower than those constructed by of silica for chert. Simultaneously, they would grain-supported sediments. More commonly, have increased the environmental stress, poi- fine-grained sediments are unstable on slopes soned the environment and led to the demise of greater than about 5° (e.g. Kenter, 1990). In the the microbialites in the basin realm, in spite of Frasnian strata, most of the slope facies are fine- their high tolerance to eutrophication. Similarly, grained, micrite-rich hemipelagic deposits (e.g. such an environmental setting could also have peloidal mudstone/wackestone). Thus, deforma- caused the termination of the stromatoporoid tion structures from creeps, rotational–transla- buildups in the early Frasnian, which were tional slides to slumps on all scales, are common replaced by the sand-shoal system. in the slope deposits. Palaeogeographically, the southern margin of the Yangdi Platform was situated in a windward location to the southern margin. The CONCLUSIONS prevailing winds from the west and south-west (cf. Wu et al., 1987; Zhong et al., 1992) were 1 From late Givetian to Frasnian, a carbonate also significant to platform construction and platform was developed on a palaeohigh located would have provided a continuous energy north of the Yangshuo pull-apart basin. Its flux, favouring prolific growth of microbial southern margin was delimited by one of the buildups and development of carbonate sand oblique, extensional faults trending NW–SE in a shoals on the platform margin. They may also sinistral strike-slip fault zone, and this exerted a have inhibited the progradation of the plat- fundamental control on platform development form, which is usually more pronounced on and evolution. leeward margins (e.g. McLean & Mountjoy, 2 In the late Givetian, the platform was rimmed 1993). with microbial buildups consisting mainly of cyanobacterial colonies (Renalcis and Epiphyton dominantly), which grew upwards, generating an Biotic constituents and sediment fabrics aggradational to slightly retrogradational archi- Biotic constituents can influence carbonate plat- tecture with a steep foreslope (up to 45° in dip). form morphology, stratal patterns, slope declivity This reflected the rough balance between the

Ó 2002 International Association of Sedimentologists, Sedimentology, 49, 737–764 Carbonate platform evolution, Guilin, South China 761 growth rate of the microbial frame constructors with rare sediment influx and abundant supply and the rate of accommodation creation con- of nutrient- and silica-rich hydrothermal fluids trolled by relative changes in sea level. Platform- upwelling from deep-seated faults, increased the margin, slope and basinal environments are environmental stresses and poisoned the eco- identified across the platform–slope–basin pro- logical conditions. This also caused the demise file, and each is characterized by distinctive of the benthic organisms (i.e. cyanobacterial depositional features and facies associations. colonies, stromatoporoids and corals) in the Three sequences (S3–S5) are distinguished in the periplatform area, and simultaneously promoted microbialite-dominated successions of the upper the extensive precipitation of silica in the basin. Givetian, representing three stages of biocon- The less protected platform margin of sand structed platform evolution. shoals and banks provided more sediment for 3 From the latest Givetian to early Frasnian, off-bank downslope export by turbidity currents two phases of extensional tectonism caused large- and debris flows. The fine-grained, micrite-rich scale collapse of the platform margin and slope deposits on the slope were easily subjected to erosion (at the top of both sequences S5 and S6) at synsedimentary deformation. times of significant platform uplift (± subaerial 5 Depositional sequences were formed by exposure) and basin subsidence. The two col- fluctuations in relative sea level. It is likely that lapse events brought about the demise of the synsedimentary tectonic activity made an organic buildups and created scalloped embay- important contribution to the changes, although a ments along the platform margin. This led to superimposed eustatic signal cannot be ruled out. substantial release of megabreccia downslope and The tectonic activity was related to the exten- the subsequent sedimentation of siliceous sional faulting induced by the strike-slip tectonic deposits within the basin in response to block activity. tilting and the introduction of significant hydrothermal fluids into the basin. Afterwards, a platform system dominated by sand shoals on the ACKNOWLEDGEMENTS platform margin was established, characterized by grainstone shoals on the margin and extensive This work was supported financially by the gravity-flow and hemipelagic deposits on the National Natural Science Foundation of China slope. This persisted into the latest Frasnian through grant 49872043 to C.D.Z. Partial support until the restoration of microbial buildups on the from the Foundation for Returned Overseas platform margin. In the Frasnian, platform-mar- Scholars, Chinese Academy of Sciences, is grate- gin, slope and basin environments, characterized fully acknowledged. We are much indebted to by different depositional features and facies senior geologists Zhengliang Li and Bao’an Yin, associations, are distinguished across the plat- who introduced several outcrops, from the form–basin transect, and four sequences (S6–S9) Regional Geological Survey and Research of are recognized. Guangxi. Thanks also go to Hongjin Lu for his 4 The onset of the transtensional extensional assistance in the field and identification of con- movements from the late Givetian accelerated odonts. Support from the K. C. Wong Fellowship the differential subsidence between the platform of the Royal Society of London for the senior and basin. A confined basin displaying a spin- author’s stay at Durham University is gratefully dle- to rhomboidal-shaped configuration was acknowledged, as is a grant from the Royal formed, which gave rise to slope steepening, Society for M.E.T.’s fieldwork in China. sediment starvation and poorer circulation of waters in the basinal realm. This favoured the colonization of the periplatform zone by cyano- REFERENCES bacterial microbes, and their existence also helped to maintain steep slopes by processes of Agirrezabala, L.M. and Garcıá-Monde´jar, J. (1992) Tectonic frame-building, encrusting and early lithifica- origin of carbonate depositional sequences in a strike- tion. The windward position of the platform slip setting (Aptian, northern Iberia). Sed. Geol., 81, margin was also a factor influencing platform 163–172. construction. Nevertheless, the energy of the Bahamonde, J.R., Vera, C. and Colmenero. J.R. (2000) A steep- fronted Carboniferous carbonate platform: clinoformal prevailing winds may have been dampened by geometry and lithofacies (Picos de Europa, NW Spain). the stagnant waters in the confined basin. Rapid Sedimentology, 47, 645–664. expansion of oxygen-depleted waters, coupled

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