Continental slope limestones of Lower and Middle Triassic, South China

Continental slope limestones of Lower and Middle Triassic, South China

ELSEVIER Sedimentary Geology 118 (1998) 77–93 Continental slope limestones of Lower and Middle Triassic, South China Bao Zhidong * Department of Geo...

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ELSEVIER

Sedimentary Geology 118 (1998) 77–93

Continental slope limestones of Lower and Middle Triassic, South China Bao Zhidong * Department of Geosciences, University of Petroleum, Beijing 100083, PR China Received 9 December 1996; accepted 27 June 1997

Abstract Continental slope limestones of the Lower and Middle Triassic in South China are composed of hemipelagic limestones, gravity-flow limestones, and contour-current limestones. The hemipelagic limestones are dark coloured and thin bedded, and contain substantial amounts of fine-grained terrigenous materials, volcanic materials, and sponge spicules. Gravityflow limestones are developed in all stages of the Middle and Lower Triassic in South China. Five fundamental types of gravity-flow limestones are recognized: slide limestones, debris-flow limestones, grain-flow limestones, turbidity-current limestones and rockfall limestones. They formed five types of amalgamated beds: slide–debris-flow limestones, slide–debris-flow–turbidity-current limestones, slide–debris-flow–grain-flow–turbidity-current limestones, debris-flow– turbidity-current limestones, and debris-flow–grain-flow–turbidity-current limestones. The first two types were formed mainly in the Early Triassic slopes. The Middle Triassic slopes were characterized by the widespread rockfall limestones. Growth faults, storms, earthquakes, and oversteepened slopes are probable triggers of gravity flows. Contour-current limestones are isolated lenses or thin, ripple-laminated beds of grainstones intercalated in hemipelagic argillaceous limestones and lime mudstones. They were formed at the base of the slopes. Palaeocurrent data indicated that the contour currents flowed perpendicularly to the slope. The contour-current limestones are not as common as the gravity-flow limestones, but they are important in the reconstruction of the palaeogeography and palaeotectonic setting in South China.  1998 Elsevier Science B.V. All rights reserved. Keywords: slope limestone; gravity flow; contour current; Lower and Middle Triassic; South China

1. Introduction South China in this paper refers to the area extending from the Qinling Mountains–Dabie Mountains in the north to the South China Sea in the south, from the Jinsha River–Yuan River in the west to the Yellow Sea and East China Sea in the east ranging from about 100º to 122ºE and from 21º400 to 33ºN. It is geologically bounded by the Qinling faults and Ł Fax:

C86 10 62341089; E-mail: [email protected]

Tancheng–Lujinag faults in the north and northeast, the Longmenshan and Honghe faults in the west, and the current coastline in the east and south. In South China the Lower and Middle Triassic strata are varied, commonly 400 to 2000 m thick, and can be divided into the Induan Stage and Olenekian Stage of the Lower Triassic, and the Anisian Stage and Ladinian Stage of the Middle Triassic. The Lower and Middle Triassic contain carbonates, siliciclastic rocks, cherty rocks, evaporites, volcanic rocks and other lithologies, carbonates dominating in the

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Fig. 1. Map of palaeogeography and gravity-flow and contour-current limestone distribution of Induan Age of the Early Triassic, South China (modified from Feng et al., 1997): 1 D land; 2 D littoral, 3 D carbonate platform; 4 D basin; 5 D slope; 6 D gravity-flow limestones, and contour-current limestones. Encircled numbers: 1 D Qingyan, Guizhou; 2 D Zhenfeng, Guizhou.

Yangtze River area and siliciclastic rocks in the southeast (Figs. 1 and 2). Sedimentary environments include a shallow-water platform, a relatively deepwater slope, and a basin. The slope limestones consist of hemipelagic, gravity-flow, and contour-current deposits. During the Early and Middle Triassic in South China, tectonism was strong and the patterns of sea-level fluctuation were complicated (Wang et al., 1995; Bao and Feng, 1996), which produced varied sedimentary environments in each age and complex amalgamated beds of the slope limestones. A major influx of siliciclastic deposits marks the termination of carbonate deposition and ultimately marine deposition in South China during the Late Triassic. According to Feng (1988) and Feng et al. (1994a), there

are many indications of oil and gas in the Lower and Middle Triassic slope limestones in South China, and the study of slope limestones is therefore of both theoretical and practical significance. 2. Methods 2.1. Study of the temporal and spatial distribution of slope limestones The study of the petrology and sedimentology of the slope limestones followed an extensive investigation of the temporal and spatial distribution of the slope limestones while mapping the lithofacies distribution in the Lower and Middle Triassic rocks of South China (Feng, 1987, 1988; Feng et

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Fig. 2. Map of palaeogeography and gravity-flow and contour-current limestone distribution of Ladinian age, Middle Triassic, South China (modified from Feng et al., 1997; same legend as in Fig. 1).

al., 1994a,b; Feng and Bao, 1995; Bao and Feng, 1996). This project extended from 1985 to 1996. During the study, we measured 35 outcrop and well sections of Lower and Middle Triassic rocks and reconnoitred 22 more, collected 6853 rock and fossil specimens, and examined 5236 thin sections with polarizing microscope and cathodoluminescence. A substantial number of petrologic, palaeontologic, and geochemical analyses were carried out to provide data on sedimentary environments and facies. Sixty-seven single-factor maps of thickness of each stage, content of shallow-water carbonates, grainstones, penecontemporaneous dolostones, gypsum, shallow-water, coarse-clastic rocks, marine rocks, deep-water sedimentary rocks, and distribution of gravity-flow limestones, contour-current limestones, etc. were compiled to show palaeogeographic maps of 14 different ages of the Early and Middle Triassic at scales of 1 : 1,000,000 or 1 : 2,500,000. Two examples illustrate the sedimentary setting of the slope limestones (Figs. 1 and 2).

2.2. Study of petrology and sedimentology of slope limestones The petrologic and sedimentological study of the slope limestones is based on eight measured outcrop sections that contain gravity-flow limestones and contour-current limestones, selected from the 35 measured sections of the Lower and Middle Triassic in South China. Interpretations of sedimentary environments were based on stratal contact styles, rock structures and textures, fossil assemblages, and other sedimentary characteristics (Liu and Zeng, 1985; Reading, 1986; Feng, 1994; Feng et al., 1994b). The inference of fundamental types of gravity-flow limestones is based on rock constituents, grain-size analysis, and other sedimentary fabric, and on comparison with widely accepted models (Dott, 1963; Hampton, 1972; Middleton and Hampton, 1973, 1976; Cook, 1979; Lowe, 1982; Cook and Mullins, 1983; Reading, 1986). To determine the direction of gravity flows and contour currents, orientations of 87 elongate clasts and 24 synsedimentary fold axes

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in the gravity-flow limestones, and 58 cross-bedding dips in contour-current limestones were measured and beds were restored to the original position to determine the orientation for the rose diagrams. Twenty-six thin sections of probable contour-current limestones were analyzed for grain-size distribution to investigate transport mechanisms in the flow. 3. Spatial and temporal distribution of slope deposits Palaeogeography of South China was similar in both the Induan and Olenekian Ages of the Early Triassic (Feng et al., 1997). A shallow-water carbonate platform was embayed by two deep-water depositional areas, the north and south basins, and their marginal slopes (Fig. 1). The palaeogeography of South China was similar in both the Anisian and the Ladinian of the Middle Triassic (Feng et al., 1997). Only one deepwater depositional area remained, the south basin and its marginal slope (Fig. 2). The north basin and its marginal slope of the Early Triassic were evolved into a shallow-water platform in the Anisian. By the Late Triassic most of South China was a nonmarine facies as a result of major regression (Enos et al., 1998).

Fig. 3. Hemipelagic limestones. Trace fossils on surface. The grey circle is 25 mm wide. Lower Triassic, Ziyun, Guizhou.

4. Hemipelagic limestones

5.1. Five fundamental types

The hemipelagic limestones make up volumetrically 50 to 80% of all slope limestones in the Lower and Middle Triassic of South China. They are composed of lime mudstones, argillaceous limestones, and nodular limestones. The limestones are dark grey to black, thin-bedded and millimetre-laminated. They contain substantial amounts of fine-grained terrigenous materials, volcanic materials, sponge spicules, and calcispheres. Trace fossils (Fig. 3) and body fossils are abundant, containing ammonites and thinshelled bivalves (Entolium sp., Leptochondria sp., Posidonia wengensis, Daonella bulongensis, Halobia subcomata, Cassianella sp., etc.) and others. The fossils are usually well preserved, but some are broken by compaction. Bed contacts range from planar, nearly parallel and continuous for long distances to more wavy and discontinuous. The hemipelagic limestones are often interbedded with thin-bedded cherty rocks. They were locally weakly folded or truncated dur-

On the basis of their structures and textures, reflecting internal mechanical behaviour and dominant sediment support mechanism, five fundamental types of gravity-flow limestones can be distinguished (Fig. 4), i.e. slide, debris-flow, grain-flow, turbiditycurrent and rockfall deposits (Dott, 1963; Middleton and Hampton, 1973; Cook and Enos, 1977; Cook and Egbert, 1981; Gao and Liu, 1983; Cook and Mullins, 1983; Wang, 1986; Chen, 1991; Loucks et al., 1991; Miller and Heller, 1994; Duncan, 1995; Bao and Feng, 1996). Although rockfall limestones are strictly gravity deposits rather than gravity-flow deposits (Feng et al., 1994; Wang et al., 1995), they are included in Fig. 4 because in the study area they typically grade upward into gravity-flow limestones.

ing deposition of gravity-flow limestones in the upper slopes. Locally they are overlain by the contour-current limestones with sharp contacts. 5. Gravity-flow limestones Gravity-flow limestones were developed on the slope around the north basin during the Induan of the Early Triassic, and on the slope around the south basin in the Early and Middle Triassic. Five fundamental types of gravity-flow limestones formed five types of amalgamated beds.

5.1.1. Type I: slide deposits The lower part of each deposit consists of darkcoloured, thin limestones which are highly folded,

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Fig. 4. Fundamental types of gravity-flow limestones of the Lower and Middle Triassic, South China. (I) Slide deposit. The rock shows different degrees of deformation, from small overfolds at the base to entirely broken clasts in the top. Note the truncation surface at the base. (II) Debris-flow limestone. Note obvious inverse to normal graded bedding upward. (III) Grain-flow limestone. Note weak reverse grading in base and water-escape structures in lower and middle parts. (IV) Turbidity-current limestone. Obvious normal graded bedded and Bouma units A, C, D, and E. (V) Rockfall limestone. Note the large angular clasts of neritic limestone. Redrawn from field sketches.

sometimes overfolded, and slightly broken. They were developed on the slope, close to the sedimentary environments from which they were derived (Moussa, 1977; Bao and Feng, 1996). Upward in

the middle sequence the limestones generally are broken and the intraclasts are mainly thin platy chips. The top part of the sequence is composed of calcirudite in which over 80% of the intraclasts

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are slope-derived subparallel-oriented tabular clasts, lithologically similar to the underlying beds. The slide limestones are intraclast-supported (Fig. 4-I), and some clasts are in sutured contact. These limestones were deposited in the preliminary stage of the development of the slope (Wang et al., 1995). If the slide flow continued to move downslope with more and more intraclasts entrained, then it would form a debris flow. Fig. 5 illustrates slide development and transition to debris flow (Cook, 1979). The slide limestones locally grade upward into debrisflow limestones and typically overlie the underlying beds with an erosional contact (Fig. 5B). 5.1.2. Type II: debris-flow limestones Debris-flow limestones are the most widespread of the gravity-flow limestones in South China. Many appear gradational on the top of the slide or rockfall deposits. Beds are typically several metres thick with clasts from granule- to boulder-sized in a matrix of lime mud. The clasts were derived from grainstones of the platform and from dark, thin-bedded carbonate mudstone of the slope, and are generally poorly sorted and rounded (Fig. 6). A few cobble- to boulder-sized clasts are supported by relatively fine clasts (Fig. 6A). Intraclasts are typically randomly oriented; they are subparallel only near the base (Fig. 6B). Clasts are matrix-supported with normal or inverse graded bedding (Fig. 4-II). The deposits generally grade upward into the other type of gravity-flow limestones and truncate the underlying beds by as much as several metres (Fig. 6A). The debrisflow limestones are commonly lenticularly shaped or channel-like, and their thickness varies laterally, suggesting deposition in channels on the slope. Debris flows may evolve into turbidity currents as ambient fluid is mixed with more slurry (Hampton, 1972) or give rise to grain flows as they rapidly flow down a steep slope (Bagnold, 1954; Leeder, 1982). 5.1.3. Type III: grain-flow limestones These are mainly sparry grainstones with a small amount of lime-mud matrix (Fig. 4-III). Single beds are generally 5–50 cm thick. The intraclasts are sand- to gravel-sized, mainly derived from limestones deposited on shallow-water platforms. The rocks show moderate sorting, moderate to good roundness and are mainly grainstones (Fig. 7). Both

normal and reverse grading is developed. Dishshaped water-escape structures are developed in the lower and middle sequence. Sutured grain contacts are relatively common in the grainstones, which suggests slow cementation and diagenetic compaction in marine and early burial environments (Wang, 1991; Feng et al., 1994b). Grain-flow limestones are rarely developed independently. They usually grade into another type of gravity-flow limestones both upward and downward, or have small-scale scour at the base. These grainflow limestones were developed on relatively steep slopes from cohesionless sediment supported by dispersive pressure (Middleton and Hampton, 1976; Lowe, 1982; Leeder, 1982). 5.1.4. Type IV: turbidity-current limestones Turbidity-current limestones are widespread, second only to debris-flow limestones amongst the gravity-flow limestones in the Lower and Middle Triassic in South China. They consist of relatively fine packstones and wackestones, forming complete or incomplete Bouma sequences (Fig. 4-V; Middleton and Hampton, 1976). Thickness varies little laterally. Thick-bedded turbidity-current limestones are generally coarse-grained, moderately to poorly graded and laminated. Thin-bedded turbidity-current limestones are relatively fine-grained, well graded, and laminated. Basal scour marks and casts are well developed on both the thick- and the thin-bedded limestones. The turbidity-current limestones are typically interbedded with or overlain by dark-coloured, thin-bedded hemipelagic limestones. Some are gradational from other types of gravity-flow limestones. 5.1.5. Type V: rockfall limestones The rockfall limestones are mainly distributed in the slope deposits around the south basin. The limestones are massive, usually from several to over ten metres thick with single intraclasts up to 15 m wide, exhibiting a rather magnificent view. The intraclasts are mostly derived from bioclastic packstones, oncolitic packstones and lime mudstones, showing that they came from the shallow-water platforms. The intraclasts are very poorly sorted and angular. There is no size grading; the texture is clast-supported (Fig. 4-V).

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Fig. 5. Stages in slide development and possible transition to debris flow. (A), (C) and (D) show progressive deformation of a semiconsolidated slide moving downslope. (A) Small overfold developed and shear zone deformed into thin beds of breccias in middle and base of photograph. The grey circle is 55 mm wide. (B) Erosional contact (dashed line) overlain by a breccia of remolded clasts. Note small overfold above the remoulded clasts (lower left). The hammer is 28 cm long. (C) Clasts are parallel to subparallel, and in sutured contact with a little matrix. Note platform-derived bioclastic packstone (lower elongated clast). White circle is 25 mm wide. (D) Higher content of platform-derived clasts (middle and upper right). The slide that produced (D) could grade into debris flow with more and more mud entrained. The black circle is 55 mm wide. The Olenekian Stage of the Early Triassic, Dongmen, Guangxi.

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Fig. 6. Debris-flow deposits. (A) Debris flow truncated the underlying beds. Large clasts (dashed lines circled) are supported by fine clasts and matrix. (B) Clasts subparallel in debris-flow limestone base. Note subparallel shallow-water fenestral limestone clast ( f ) and grainstone clast (g). The Induan Stage of the Early Triassic, Qingyan.

Rockfall limestones are more widespread in the Middle Triassic than in the Lower Triassic. Late Permian foraminifers (e.g. Palaeofusulina, Nankinella) can be found in a few clasts. Probably it was the ac-

tivities of basement growth faults (Hou and Huang, 1984) that triggered the Upper Permian to Triassic deposits of the platform margin to fail and form the rockfall deposit.

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Fig. 7. Grain-flow limestone. (A) Grainstone deposited by grain flow. Note the wavy top (arrow) overlain by packstones derived from turbidity currents. The rule is 28 cm long. (B) Photomicrograph of grain-flow limestone (layer b in A) composed of sand-sized clasts mainly derived from algal limestones of the platform. The clasts are moderately sorted, moderately to well rounded, and cemented by spar. The Olenekian Stage of the Early Triassic, Ziyun.

5.2. Amalgamated beds Five types of amalgamated beds are recognized in South China: slide–debris-flow limestones, slide–debris-flow–turbidity-current limestones, slide–debris-flow–grain-flow–turbidity-current limestones, debris-flow–turbidity-current limestones, and debris-flow–grain-flow–turbidity-current limestones (Fig. 8). Deposits result from the sequential operation of several processes and a change of the dominant internal mechanical behaviour (Middleton and Hampton, 1976; Scholle et al., 1983; Ross et al., 1994). 5.2.1. Amalgamated bed A: slide–debris-flow limestones The lower part of a bed is a slide deposit, and the clasts are mostly derived from the limestones of the slope, deposited parallel to subparallel. The upper part of a bed consists of debris-flow limestones, amongst which clasts derived from the shallow-water platform dominate. The thickness ratio of the two parts is nearly 1 : 1 (Fig. 8A). This type of amalgamated bed was the outcome of the preliminary stage of the development of the grav-

ity flow. This amalgamated bed probably resulted from a slide of the water-saturated hemipelagic sediments which quickly passed downslope and began to break and liquefy, and formed a matrix-supported structure in the top of the flow owing to a relatively high velocity. Before complete liquefaction the gravity flow came to the lower gentle slope and was quickly deposited, forming this amalgamated bed. This amalgamated bed shows a sharp erosional contact with the underlying slope deposit. A rapid emplacement of a debris-flow deposit may trigger sliding of the water-saturated substrate, and form a slide–debris-flow limestone amalgamated bed. However, this should produce a sharp or even erosional contact between top and base sequences, which was not found in the amalgamated bed. 5.2.2. Amalgamated bed B: slide–debris-flow–turbidity-current limestones The vertical petrologic variation of the slide– debris-flow–turbidity-current limestones is obvious in outcrop sections (Fig. 8B). The lower part of the beds consists of floatstone slide deposits, grading up into matrix-supported debris-flow limestones upward in the middle part, and the upper part consists of tur-

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bidity-current limestones containing Bouma units A, C, D and E. The thickness ratio of the three parts is about 1 : 2 : 1. About 70% of the clasts are derived from lime-mud sediments of the slopes, suggesting that the gravity-flow might have travelled a relatively long way in a deep-water slope environment. This amalgamated bed could have resulted from a gravityflow which passed through the upper slope and was deposited on the gently sloping zone of the lower slope. Amalgamated bed types A and B are well developed on the slope around the north and south basins, especially the south basin, in the Early Triassic. 5.2.3. Amalgamated bed C: slide–debris flow–grain flow–turbidity-current limestones Seen in outcrop in Ziyun County, Guizhou Province, this amalgamated bed is a rather spectacular deposit. The lower part of a bed consists of slide deposits with clast-support and parallel clast orientation, typically 5 m thick. The middle part consists of debris-flow limestones, matrix-support with random clast orientation, about 20 m thick. The overlying unit consists of granule- and pebble-sized rudstones with water-escape structures. These are grain-flow deposits, nearly 3 m thick and mainly spar-cemented with a little matrix. The top part consists of about 5 m of relatively fine-grained turbidity-current limestones with Bouma units. Typically grain-flow deposits are not thick since theoretically steep slopes are required to initiate and maintain grain-to-grain interactions (Lowe, 1976; Leeder, 1982; Cook and Mullins, 1983; Feng et al., 1994b), while steep and relatively long slope are rarely developed. But relatively thick grainflow deposits may occur when growth faults produce a steep slope during a special period just as in Ziyun, Guizhou, South China (Hou and Huang, 1984; Huang and Chen, 1987). The process was

Fig. 9. Contourites. (A) Rippled and lens-shaped grainy limestones probably derived from contour currents, Middle and Lower Triassic in South China. I D grainy limestones; II D lime mudstones. (B) The grains are mainly well-sorted and well-rounded, platform-derived ooids and intraclasts in the contour-current limestone. The Anisian Stage of the Middle Triassic, Zhenfeng.

helped by the presence of some lime-mud matrix, both the matrix and some turbulence probably aiding dispersive pressure in supporting the grains above the base of the bed (cf. Cook and Mullins, 1983). This amalgamated bed may result from a welldeveloped gravity-flow which has passed a relative

Fig. 8. Gravity-flow limestone amalgamated beds of the Lower and Middle Triassic, South China. (A) Slide–debris-flow limestones. The lower one metre is slide limestone composed of syn-sedimentary-folded beds and tabular chips. The upper is matrix-supported debris-flow limestone, sharp lower contact. (B) Slide–debris-flow–turbidity-current limestones. The lower one metre is slide limestone, the middle two metres is debris-flow limestone, and the upper one metre is turbidity-current limestone. Note that the three types of limestones were continuously deposited upward. (C) Slide–debris-flow–grain-flow–turbidity-current limestones. The grain-flow limestone, upper middle part, is mainly grain-supported and shows a water-escape structure. (D) Debris-flow–turbidity-current limestones. (E) Debris-flow– grain-flow–turbidity-current limestones. Note that the Bouma unit C is well developed in the turbidity-current limestone. Redrawn from field sketches.

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Fig. 10. Rose diagrams demonstrating the directions of the contour currents parallel with the strike of the slopes. The black petals indicate the direction of the contour current and the white petals indicate the dips of the slopes. (A) Rose plot based on data of 48 tabular clasts and 13 overfold axes (white petals) in limestones, and 31 cross-lamination in the contour-current limestones (black petals) in Qingyan, Guizhou. (B) Rose plot based on data of 39 tabular clasts and 11 overfold axes in limestones, and 27 cross-lamination in the contour current limestones in Zhenfeng, Guizhou.

steeper slope. The amalgamated bed has an erosional base, and usually grades upward into dark-coloured, thin, cherty mudstones or marls (Fig. 8C). These features are consistent with deposition at the base of the slope or the margin of the basin. 5.2.4. Amalgamated bed D: debris flow–turbidity-current limestones The lower part of the amalgamated bed consists of poorly sorted and weakly graded debris-flow limestones. The clast size becomes finer and the clast content increases upward. The top part contains Bouma units deposited by turbidity currents. The thickness ratio of the lower and upper parts is about 2 : 1. This amalgamated bed typically has an erosional base and grades upward into dark-coloured, thin-bedded lime mudstones or argillaceous limestones (Fig. 8D). The large proportion of deep-water intraclasts in the amalgamated bed indicates that the gravity-flow was deposited on the lower slope after passing over a relatively long middle and lower slope. This amalgamated bed is well developed in the slope around the north basin.

5.2.5. Amalgamated bed E: debris-flow–grain-flow–turbidity-current limestones The lower part consists of debris-flow deposits with reverse to normal graded bedding. The middle part consists of grain-flow limestones with dish structures and rare tubular water-escaped structures. The turbidity-current cap is relatively thin, with normal graded bedding and a well developed Bouma unit C. The overlying and underlying beds are typically dark-coloured, thin-bedded hemipelagic limestones (Fig. 8E). The thickness ratio of the parts is about 3 : 1 : 3. The sedimentary characteristics suggest this amalgamated bed was deposited on the lower slope. 6. Contour-current limestones A few grainy limestones were intercalated with carbonate mudstones or marls, in the form of isolated lenses or thin beds with ripple bedding. The wave length of ripples is 5 to 10 cm and ripple height is 0.5 to 2 cm. The grainstones are generally thin, about 0.5 to 5 cm (Fig. 9). Constituents are silt-sized

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Fig. 11. Schematic drawings of carbonate tempestites in the carbonate platform margin adjacent to the slope. (A) Base is erosional with scours and tool marks. Lower part is matrix-supported with tabular clasts oriented randomly or vertically. Hummocky cross-bedding developed in upper part. (B) Clasts are derived from various rocks with various shapes, suggesting deposition after some transport. (C) Carbonate wackestones with well-developed hummocky cross-bedding, and shell lags. The tempestites in type C are interbedded with thin-bedded lime mudstones. The sedimentary characteristics of the clasts and the sequences suggest that the depositional environments from tempestites A to C are progressively closer to the slope break. Redrawn from field sketches.

to medium sand-sized bioclasts, ooids and intraclasts (Fig. 9B), quartz sands, and a few heavy minerals with a median size 0.088 to 0.144 mm (3.5 to 2.8 phi). Sorting is typically 0.5 to 0.7 phi, and roundness is medium to good. The convex faces of the bioclasts are upwards. Cross-lamination is well developed in the beds and lenses. The limestones are overlain by hemipelagic lime mudstones, and usually underlain by the top unit of gravity-flow limestones or hemipelagic lime mudstones with sharp contact. The grain-size distribution analyses of the grainy limestones illustrate that the grainstones originated by deposition from traction currents. Rose diagrams illustrate the dip of laminae in the cross-bedding, indicating that the directions of ripple migration are generally perpendicular to the dips of the long axes of the clasts in the gravity-flow limestones (Fig. 10). This demonstrates that directions of the currents were parallel with the strike of the slopes, consistent with an interpretation as contour currents (Cook and Egbert, 1981). The lithofacies maps and the depositional sequences enclosing the contour current deposits show that they were deposited at the base of the slope (Heezen et al., 1966; Stow and Lovell, 1979; Shor et al., 1984; Liu et al., 1990; Hollister, 1993; Gao et al., 1995, 1996).

7. Discussion The distribution of the rockfall limestones and the amalgamated bed of debris-flow–grain-flow– turbidity-current limestones suggests that there were steep growth faults under the sedimentary slope during the Early and Middle Triassic (Hou and Huang, 1984; Huang and Chen, 1987; Jin, 1989; Mu et al., 1989; Feng et al., 1994a). Several factors may have helped initiate gravity flows. One probable factor was instability of soft or semi-cemented sediments on the slopes. As platform margins prograded into the basins, the sediments became unstable, especially if the slope angle approached the angle of repose of sediments. A second probable factor was that growth faults steepened the slope and fragmented platformmargin and slope limestones. These fragments slid downslope and formed gravity flows. A third probable factor concerned storms, which may have been responsible for the formation of some gravity flows on the slope around the north basin. Carbonate tempestites have been identified in the platform margin adjacent to the upper slope (Fig. 11; cf. Ager, 1973; Aigner, 1982, 1985). Earthquakes also could trigger gravity flows (Hampton, 1972; Reading, 1986). The thin, ripple-laminated beds of grainstones,

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Table 1 Comparison between the contour-current limestones and the gravity-flow limestones, Lower–Middle Triassic, South China Contour-current limestone

Turbidity-current limestone

Grain-flow limestone Debris-flow limestone

no obvious vertical sequence, interbedded with hemipelagic sedimentary sequences Bed thickness typical 0.5–2 cm

analogue to Bouma sequences, grades downward into other gravity-flow limestone

no obvious vertical sequence, often grades upward and downward into other gravity-flow limestone generally 5–50 cm

Sedimentary fabric

flutes, tool mark cast structure well developed; massive bedding, graded bedding, horizontal bedding; clasts random or subparallel orientation; sharp basal contact, grades into overlying beds gravel to clay; poorly to moderately sorted; cumulative frequency curve consists of one grain population

Sedimentary sequence(s)

ripples, small-scale cross-bedding, weak normal grading; sharp contacts with both overlying and underlying beds

Grain-size distribution

fine sand to clay; moderately to well sorted; cumulative frequency curve consists of two grain populations Orientation of parallel to slope flow

generally 20–50 cm

perpendicular to slope

Slide deposit

Rockfall limestone

no obvious vertical sequence, often grades upward into other gravity-flow limestone generally 1–3 m

no obvious vertical sequence, often grades upward into debris-flow limestone 0.5–5 m

no obvious vertical sequence

massive bedding, or reverse to normal grading; grades into underlying and overlying beds

massive bedding, or reverse to normal grading; grades into underlying and overlying beds

massive bedding; grades into overlying beds, and erosional contact with underlying beds

gravel to clay; moderately sorted

boulder to clay; poorly sorted; cumulative frequency curve consists of one grain population

gravel to clay with platy gravel dominated; no obvious sorting

cobble to sand; no sorting at all

perpendicular to slope

perpendicular to slope

perpendicular to slope

perpendicular to slope

several to over 10 m massive bedding; clasts angular, clast-supported

Table 2 Comparison of transport mechanism between the contour-current limestones and the gravity-flow limestones, Lower–Middle Triassic, South China Transport mechanism

Contour-current limestone

Gravity-flow limestone

Fluid properties Flow characteristics Driving factor(s)

Newtonian fluid, traction current stable, continuous, low to medium velocity (5–30 cm/s) thermohaline circulation, Coriolis force, submarine storm

non-Newtonian fluid, density current abrupt, relatively high velocity (10–2500 cm/s) slope failure, fault activities, storm

enclosed in hemipelagic limestones, look to be dilute, distal turbidites at first sight, but as pointed out by Cook and Taylor (1977) and Cook and Egbert (1981), they have a different origin which is indicated by: (1) sharp lower and upper contacts, (2) their near perfect hydraulic sorting, (3) the common lack of a lime matrix, (4) laterally isolated lenses to continuous evenly spaced current ripples, and (5) a transport direction parallel to the slope strike (Ta-

bles 1 and 2). Similar limestones deposited on a Cretaceous continental slope have been ascribed a contour-current origin (Bein and Weiler, 1976). Between gravity flows, contour currents parallel to the slope reworked the unconsolidated sediments and bioclasts of hemipelagic deposition or Bouma unit E of the gravity-flow sequences, and formed the contour-current limestones. Modern contour currents are mainly found on the continental rise (Heezen et

. Bao Zhidong / Sedimentary Geology 118 (1998) 77–93

al., 1966; Hollister and Elder, 1969; Liu and Zeng, 1985; Hollister, 1993). Contour-current limestones could reveal the trend of the slope. The cross-lamination in the limestones shows the direction of the current which flowed parallel to the slope. According to the rose diagrams (Fig. 10), the direction of the contour current, i.e. the strike of the northern and western slopes around the south basin, varied from 270º to 206º in the Early and Middle Triassic in South China. As pointed out by Cook and Mullins and others (Cook and Mullins, 1983; Feng et al., 1994b), it is increasingly important to understand the nature, origin, and facies associations of the slope limestones as petroleum exploration continues into deeper water. More and more source beds and reservoirs will probably be sought and found in carbonate slope and basin settings. A well known example of the petroleum production from slope carbonates is from the Cretaceous of Mexico (Enos, 1977); the reservoirs are mainly in carbonate rocks derived from gravity flows. In South China there are many indications of oil and gas in the slope limestones, and 111.6 m3 of crude oil have been produced from two wells in Jurong County, Jiangsu Province. The reservoirs are Lower Triassic slope limestones. Source beds of the oil are also the slope limestones (Feng, 1988; Feng et al., 1994b), owing to good conditions for formation and preservation of organic matter in slope environments. It may be herein predicted that the slope limestones will be potential targets for the exploration of petroleum. Acknowledgements This research was funded by the Chinese National Petroleum Corporation. The paper is mainly extracted from my doctoral dissertation. I am grateful to my advisor Professor Feng Zengzhao for his supervision throughout the research. I am indebted to Professor Ian J. Fairchild for his editorial advice. Sincerely I thank Dr. A.K. Satterley and Dr. Paul Enos for their constructive suggestions which have greatly improved earlier versions of the manuscript. Special thanks are due Professor Hou Fanghao and Professor Fang Shaoxian for their kind advice and generous support during the research. I also acknowledge senior engineer Yang Hong, Dr. Jin Zhenkui,

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