Cold-seep carbonate hardgrounds as the initial substrata of coral reef development in a siliciclastic paleoenvironment of southwestern Taiwan

1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC 2 Division of Geology, National Museum of Natural Science, Taichung, Taiwan, ROC 3 Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan, ROC * Corresponding author address: Prof. Chang-Feng Dai, Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC; E-mail: corallab@ntu.edu.tw Abrupt facies changes from the underlying terrigenous mudstone of deep-water facies upward into reefal limestones were observed on Pleistocene scleractinian reefs in southwestern Taiwan. To reveal the initial mechanisms of reef development, we examined the lithologies and vertical facies changes of 7 outcrops and 37 borehole cores from the Takangshan Reef and performed petrographic and isotopic studies. Various occurrences of dolomitic mudstone were observed from 6 outcrops and in 11 borehole cores, containing massive dolomitic mudstones, carbonate pipes, dolomitic cobbles, and dolomitic pebbles. The δC values of 27 samples ranged from -53.7 0 00 to -10.4 0 00 , indicating that the carbonate cements of these mudstones were all cold-seep carbonates in origin. The majority of the coldseep carbonates and a funnel-shaped structure packed with dolomitic cobbles were precipitated and formed within fine-grained siliciclastic mudstones. The wide occurrence of seep carbonates in the study area suggests hydrocarbon seepage having occurred extensively. The compact nature and associated large lucinid bivalves in massive cold-seep carbonates further indicate a pronounced, long-lasting seepage of methane occurring antecedently to the development of Takangshan Reef. A schematic model was proposed to illustrate the occurrence of various associations of lithologies and lithofacies. The erosional surfaces on siliciclastic mudstones and the funnel-shaped structure, as well as the exhuming of massive cold-seep carbonates may have occurred concurrently during a tectonically unstable time in SW Taiwan. The deposition of fossiliferous mudstone interfingered TAO, Vol. 17, No. 2, June 2006 406 with the conglomerate lithofacies represents a rapid facies transition from a siliciclastic (non-carbonate) to a carbonate environment. The root of this rapid facies change is presumed to be tectonic movement, probably related to the westward thrust migration in the Pleistocene foreland basin. The exposed massive seep carbonates provided a substrate for the encrustations of corals as well as coralline algae and might have played a crucial role in the initial development of coral reefs in a siliciclastic paleoenvironment. To our best knowledge, this is the first case in the world of cold-seep carbonates acting as an initial colonization hardground for hermatypic corals and corallines. (

with the conglomerate lithofacies represents a rapid facies transition from a siliciclastic (non-carbonate) to a carbonate environment. The root of this rapid facies change is presumed to be tectonic movement, probably related to the westward thrust migration in the Pleistocene foreland basin. The exposed massive seep carbonates provided a substrate for the encrustations of corals as well as coralline algae and might have played a crucial role in the initial development of coral reefs in a siliciclastic paleoenvironment. To our best knowledge, this is the first case in the world of cold-seep carbonates acting as an initial colonization hardground for hermatypic corals and corallines.

INTRODUCTION
Modern coral reefs generally develop in clear, warm, well-lit tropical shallow marine environments. Excessive terrigenous sediments and accompanying nutrients are usually considered a threat to coral reef health and growth (Rogers 1990). However, several studies have shown that modern coral reefs can sometimes survive or develop in turbid waters (Johnson and Risk 1987;Tudhope and Scoffin 1994;Perry 2005). Modern coral reefs have developed in muddy environments on the Queensland Shelf of northeastern Australia (Hopley et al. 1983;Johnson and Risk 1987), southeast of Phuket in Thailand (Tudhope and Scoffin 1994;Scoffin and Le Tissier 1998), and off the western coast of Hammond Island in the Torres Strait, Australia (Woodroffe et al. 2000). However, these reefs did not initially grow on muddy substrates, but on rocky bottoms, shoreline gravel and boulders, muddy coral rubble banks, and coarsegrained sediments. The fossil record also implies that the development of coral reefs are compatible with terrigenous deposits (e.g., Weiss et al. 1978;Sanders and Baron-Szabo 2005). It seems that coral reefs develop in a variety of shallow marine environments if hardground or coarse-grained siliciclastic substrate is available.
A special mode of modern ahermatypic coral buildup development in deep-water, noncarbonate environments closely associated with hydrocarbon seeps has previously been proposed (Hovland 1990;Hovland et al. 1998). Hovland (1990) suggests that such buildups formed at locations containing high concentrations of bacteria and other microorganisms suspended in water columns resulting from seeping fluids, which would have provided energy and carbon sources. Precipitation of early diagenetic minerals may have occurred with sediment grains and skeletal remains being cemented together to produce a firm substratum. Eventually, such carbonate buildups would have developed extensively, depending on other environmental factors and the intensity of the fluid seepage. This model may explain paradoxical buildups, both modern and fossil ones, found in various sedimentary environments (Hovland 1990). However, this seepage hypothesis is still being debated (De Mol et al. 2002;Hovland and Risk 2003;Hovland 2005).
In this study, we examine the initial mechanisms of reef development at the Gutingkeng Formation of southwestern Taiwan. These Pleistocene coral reefs occur as lenticular bodies in a siliciclastic mudstone unit and were constructed mainly by shallow-water colonial corals, including Acropora, Acanthastrea, Cyphastrea, Favia, Favites, Goniopora, Goniastrea, Galaxea, Pachyseris, Porites, and Turbinaria, with locally abundant free-living corals such as Cycloseris, Fungia, Heliofungia, and Herpolitha (Yabe and Sugiyama 1935a, b;Hoeksema and Dai 1991). These scleractinian reefs developed on several local topographic highs that are closely associated with anticlines and faults in a Plio-Pleistocene foreland basin (Gong et al. 1996(Gong et al. , 1998Lacombe et al. 1997Lacombe et al. , 1999Fig. 1). Several mud volcanoes in the vicinity are also closely related to the thrusts (Shih 1967; Fig. 1). Geochemical studies have revealed that the mud volcano fluids are composed of marine sedimentary pore water and the dehydrated water of clay minerals (Gieskes et al. 1992;You et al. 2004, Yeh et al. 2005. The exhaling gases of most mud volcanoes are methane-dominant (Yang et al. 2004). Active mud volcano fluid and  Teng (1992) and Lee (1992). Bold lines indicate major thrust faults; triangles are on the upthrown side. The large open arrow in the lower right corner shows the direction of convergence of the Philippine Sea Plate relative to the Eurasian Plate; moving rate is after Yu et al. (1997). (b) Map showing the locations of Pleistocene reef limestones and major structural features in southwestern Taiwan. Compiled from Shih (1967), Chinese Petroleum Corporation (1989, and Lacombe et al. (1999).
gas expulsions probably occurred concurrently during the evolving of folds and thrusts in southwestern Taiwan. It is believed that the thrust migration in the foreland basin caused rapid shallowing and formed local topographic highs along the anticlinal ridges that facilitated the development of coral reefs (Gong et al. 1998;Chow et al. 2001). Abrupt facies changes from the underlying terrigenous mudstone strata of deep-water facies upward into the reefal limestones were observed at the bottom of these reefs (Gong et al. 1996(Gong et al. , 1998. However, it is still uncertain how these reefs initiated on the siliciclastic muddy substrates. Herein we demonstrate the possible mechanism that initiated the development of these coral reefs in a siliciclastic paleoenvironment from a detailed study on the lithologies and facies changes.

MATERIALS AND METHODS
The two quarries in Takangshan were chosen for field studies (Fig. 2). We focused on vertical facies changes from the underlying siliciclastic mudstones upward into the basal parts of the Takangshan Reef limestone. The field study sites were designated as Outcrops 1 to 7 (Fig. 2). Thirty-seven borehole cores stored in the National Museum of Natural Science (NMNS) which show facies change strata were also examined. These include 19 boreholes from the quarry at northwestern (NW) Takangshan and 18 boreholes from eastern (E) Takangshan (Fig. 2).
Rock specimens were examined using hand samples, polished slabs, and thin sections, under a light microscope. In addition, typical samples of siliciclastic mudstone and fossiliferous mudstone were picked from the longest (68 m) borehole core (Core 9 of E Takangshan) for grain size analysis. Owing to weak consolidation, samples were loosened in distilled water within 1 to 2 weeks, and only gentle shaking was required before sieving operations. Grains of each grade were identified using a binocular microscope, and mineralogies were determined by X-ray diffraction (XRD) analysis. In addition, the very fine to coarse sand (grain size = 0.062 -1.0 mm) of the fossiliferous mudstone was well mixed. About half of the sample (dry wt = 5.59 g) was soaked with a 10% HCl solution to remove skeletal carbonates, then reweighed to obtain the relative percentage of carbonate and terrigenous components.
Carbonate-cemented mudstones for mineralogical composition analyses were sampled (n = 105) using a hand-held microdrill or steel vise, and veined samples were avoided. Then 27 samples with single mineralogical composition were selected for carbon and oxygen isotopic analyses. Bulk mineralogy was determined by XRD on powdered samples using a Rigaku diffractometer with Cu K α radiation (1° min −1 ) at the NMNS. Carbon and oxygen isotope compositions of samples were analyzed using a Micromass IsoPrime mass spectrometer equipped with a Multicarb automatic system at the National Taiwan Normal University. Rock powders were reacted with 100% phosphoric acid at 90°C. The carbonate standard NBS-19 (National Bureau of Standards; δ 13 C = 1.95 0 00 , δ O 18 = -2.20 0 00 ) was used to calibrate to the Vienna Peedee belemnite (V-PDB) standard. The average precision based on the NBS-19 carbonate standard was 0.03 0 00 for δ 13 C and 0.06 0 00 for δ O 18 (n = 177). No correction was made for the difference in the phosphoric acid fractionation factors between dolomite and calcite.

Siliciclastic Mudstone
The typical siliciclastic mudstone was composed of fine-grained siliciclastics (wt% of clay and silt fractions approximately= 98%) (Fig. 3a, Table 1). Most of the siliciclastic mud- stones are monotonous and with no discernible bedding features (Figs. 4a, b), but some subangular to subrounded dolomitic mudstone cobbles and pebbles were observed at 4 outcrops and in 10 borehole cores (Figs. 4c, d).

Fossiliferous Mudstone
Fossiliferous mudstone is siliciclastic mudstone containing abundant bioclasts (Fig. 5b, Table 2), and this varied in thickness from 15 cm to more than 2 m. This weakly consolidated lithology was observed at Outcrops 1, 4, and 7 (Figs. 5a, b) and in all borehole cores except one (Core N-10) at NW Takangshan. Faint bedding and mud chips (Fig. 5c) could be observed locally at the outcrops. Some subangular to subrounded dolomitic mudstone cobbles and pebbles occurred in Core 3 and H-1 at the E Takangshan quarry. The majority of coarse bioclasts within this lithology were abraded and/or stained a brown color, and only a few intact colonial coral, free-living fungiids, and clypeiform urchins were found, suggesting that most of the bioclasts were not deposited in situ. The results of acid digestion showed that only 12.5% was acid insoluble residues, revealing that the majority of the sand grade fraction of this lithology consisted of bioclasts rather than terrigenous siliciclastics.

Dolomitic Mudstone
This lithology is represented by the fine-grained mudstones that were cemented mainly by microcrystalline dolomites. They occurred as well-lithified massive rocks, cobbles, pebbles, and carbonate pipes in the siliciclastic mudstone, and/or as redeposited cobbles, pebbles, and carbonate pipes in the conglomerate and fossiliferous mudstones. Majorities of the dolomitic mudstones were precipitated and/or formed within the fine-grained siliciclastic mudstones. The results of XRD analyses (n = 115) showed that these mudstones were cemented mainly by dolomite. Therefore, this lithology was named the 'dolomitic mudstone'.
Massive dolomitic mudstones were observed at Outcrops 3, 5, and 6. The lateral extent of this lithology could reach 13 m, and the thickness varied from 0.5 to more than 3.2 m. Many fossil lucinids preserved in a living position were frequently found in this outcropped lithology.
For example, at Outcrop 5, more than 100 large fossil lucinids (Fig. 5d), several turrid, trochid, and turbinid gastropods, and a few mytilid bivalves were found in the blocks excavated from the outcrop. Dolomitic cobbles and pebbles composed of fine-grained dolomitic mudstone could be observed from (i) mudstones exposed at Outcrops 2, 4, 6, and 7; (ii) the conglomerate exposed at Outcrop 2 (Fig. 6a); (iii) mudstones in 6 borehole cores (Cores G, at the NW Takangshan quarry and 4 cores (Cores 2, 3, B-5, and D-3) at the E Takangshan quarry (Fig. 4d); and (iv) the fossiliferous mudstone in 2 borehole cores (Cores 3 and H-1) at the E Takangshan quarry. Most of the dolomitic cobbles and pebbles in the borehole cores were recognized from the siliciclastic mudstone; they were neither encrusted nor bored into by organisms except those from Outcrop 4 which had been densely bored into by bivalves (Fig. 4c). At Outcrop 7, a funneled structure was observed in the non-consolidated siliciclastic mudstone (Fig. 5b), and this structure was filled with dolomitic cobbles and pebbles.
Carbonate pipes were observed in the mudstones exposed at Outcrops 4 and 7 (Fig. 6b). Redeposited carbonate pipes were observed in the conglomerate exposed at Outcrop 2 (Fig. 4b), and in the fossiliferous mudstone of Core 3 from the E Takangshan quarry. These carbonate pipes ranged from 3 to 30 cm in diameter, with an elliptical or nearly circular cross-section. Each carbonate pipe possesses a hollow or filled conduit-like tubular structure at its center. Orientations of most of these carbonate pipes were vertical, while some were slightly tilted or nearly horizontal. No encrusting organisms were found on the outer walls of the carbonate pipes except for some scars produced by boring.

Conglomerate
The conglomerate consisted of angular to well-rounded carbonate-cemented cobbles and pebbles in a matrix of weakly consolidated mud. The lateral extent of this lithology could be traced to 5 m, and the thickness varied from 10 to more than 30 cm (Fig. 4b). Weakly consolidated cobbles with fractures and/or brecciated fabrics (Fig. 6a), carbonate pipes (Fig. 4b), redeposited large lucinids, and free-living fungiids were also observed.

VERTICAL FACIES CHANGE
Four types of vertical facies change from the underlying mudstone upward into the Takangshan Reef were discriminated based on close examination of the 7 outcrops and 37 borehole cores.

Siliciclastic Mudstone-Fossiliferous Mudstone-Bioclastic Floatstone
This type of facies change was observed at 3 outcrops (Outcrops 1, 4, and 7) and in 36 borehole cores (Figs. 4d, 5a, b). At Outcrop 1, the tubular burrows of the siliciclastic mudstone lacked outer linings and were infilled by bioclasts and muds (Fig. 4a), suggesting a firmground nature for the mudstones, i.e., a stiff but uncemented substrate. Both the top and bottom of the fossiliferous mudstone with a vertical thickness of about 60 cm were bounded by scoured contacts. Many vertically oriented carbonate pipes were observed in the siliciclastic mudstone exposed at Outcrops 4 (Fig. 6b) and 7 (Fig. 5b). Broken carbonate pipes and dolomitic cobbles with dense bivalve bores (Fig. 4c) were observed at Outcrop 4. At Outcrop 7, a funnel-shaped structure was developed within the mudstone and was truncated at the upper facies contact, forming a convex upper boundary (Fig. 5b). This is analogous to a mud volcano conduit, in which the semi-consolidated cobble-and pebble-sized mudstones are expelled onto the surrounding muddy substrata and eventually embedded in the unconsolidated matrix of siliciclastic mudstones. Fossils of the fossiliferous mudstone that exposed at this outcrop consisting of fragments of foliaceous corals, rhodoliths, pectens, oysters, turbinid gastropods, free-living corals Cycloseris, and an intact coral colony of Stylocoeniella guentheri. A few carbonatecemented cobbles were observed at the bottom of this lithology. These carbonate-cemented cobbles, pebbles, and carbonate pipes were also observed in siliciclastic mudstones and/or fossiliferous mudstones in 11 borehole cores (e.g., Fig. 4d).

Siliciclastic Mudstone-Bioclastic Floatstone
This type of facies change occurred only in Core N-10 of NW Takangshan. In this core, the vertical facies change occurred abruptly from the siliciclastic mudstone upward into the basal part of Takangshan Reef limestone, showing strong contrast to those that occurred in the 36 other borehole cores. The fossil burrows in the mudstone were truncated on the facies contact and infilled by granule-to pebble-sized limestones.

Siliciclastic Mudstone-Conglomerate-Bioclastic Floatstone
This type of facies change was observed only at Outcrop 2 (Fig. 4b). The vertical facies changes from the underlying siliciclastic mudstone upward into conglomerate, and then into Takangshan Reef limestone which occurred abruptly within 30 cm. An encrusting coral preserved in situ was found 40 cm above the bottom of the bioclastic floatstone, suggesting a rapid facies change from bioclastic floatstone to coral boundstone. Both the top and bottom of the muddy conglomerate were bounded by scoured contacts. Many well-cemented dolomitic cobbles with negative δ 13 C values were collected from the siliciclastic mudstone (Table 3).

Siliciclastic Mudstone-Massive Dolomitic Mudstone-Bioclastic Floatstone
This type of facies change occurred at Outcrops 3, 5, and 6, although not all of the massive dolomitic mudstones were well exposed. This type of facies change is best represented by Outcrop 6 that mainly exposed in a cave. Some carbonate-cemented cobbles were observed from the underlying siliciclastic mudstone. The facies change from the underlying siliciclastic mudstone upward into massive dolomitic mudstone was transitional. However, the vertical facies change from the underlying massive dolomitic mudstones to bioclastic floatstone was abrupt, suggesting the results of bottom scouring (Fig. 6c). Localized coralline algae and scleractinian corals encrusted on the top of massive dolomitic mudstones was observed at Outcrop 6 with a lateral extent of 24 cm and a maximum thickness of 11 cm. Abrupt facies changes were also observed at Outcrop 3 where the partly exposed massive dolomitic mudstone was directly overlain by the bioclastic limestone with an irregular contact (Fig. 6d). At Outcrop 5, the massive dolomitic mudstone was encrusted by scleractinian corals and coralline algae, then overlain by the bioclastic floatstone (Figs. 4a, 5a, c). Vertical thickness of this encrustation varied from 16 to 20 cm and was constructed firstly by encrusting Porites and Favia, then by encrusting coralline algae and faviids, and finally by Porites. Many bivalves that bored into this encrustation were observed, including Parapholas quadrizonata, Jouannetia sp., and Lithophaga sp.. Thin sections of this encrustation (Fig. 8c) revealed sessile organisms other than corallines, such as encrusting bryozoans, foraminifera Acervulina, and barnacles.

RESULTS OF STABLE ISOTOPE ANALYSES
The carbon and oxygen stable isotope compositions of 27 samples of carbonate-cemented mudstone were analyzed. The δ 13 C values of 27 rock samples ranged from -53.7 0 00 to -10.4 0 00 (Fig. 7, Table 3). The massive dolomitic mudstone of Outcrop 5 showed the most-negative δ 13 C value. The dolomitic cobbles and pebbles yielded values that ranged from -50.8 0 00 to  Table 3 for the sample locations and occurrences.

The Dolomitic Mudstones Are Cold-Seep Carbonates
The fossil macrofauna of the massive dolomitic mudstones was dominated by large lucinids, and most of the fossil lucinids on the outcrop were preserved in living positions. These are features typical of ancient seep assemblages (Bottjer et al. 1995). The co-occurrence of fossil Table 3. Isotopic compositions of carbonate-cemented mudstones from the Takangshan (TKS).
lucinids and carbonate-cemented mudstones in the study area is likely analogous to the Jurassic 'pseudobioherm' in SE France (Gaillard et al. 1992), the Cretaceous 'seep-related limestone mounds' of NE Greenland (Kelly et al. 2000), the 'Tepee Buttes' in the Cretaceous Pierre Shale Formation of Colorado (USA) (Kauffman et al. 1996), the Miocene 'calcari a Lucina' (limestones with Lucina sp.) in the Italian north Apennines (Conti and Fontana 1999), and the Cenozoic seep-related Type III chemosynthetic assemblages in Japan (Majima et al. 2005).
The existing species of the Lucinidae are burrow-dwelling bivalves that occur globally over a wide range of marine habitats from intertidal to deep sea (Fisher 1990). Many lucinids burrow deeply and live near the interface of oxic and anoxic zones, or within the latter (Taylor and Glover 2000). All known species of the Lucinidae possess sulfide-oxidizing, chemosymbiotic bacteria housed in bacteriocytes of their gill filaments. Chemosymbiosis was suggested to be an inherent character of this family, which can be traced to the Silurian lucinid Ilionia (Liljedahl 1992;Taylor and Glover 2000).
The features of dolomitic mudstones of the study area, including clotted and stromatactoid petrographic fabrics (Fig. 8a), carbonate pipes (Fig. 6b), co-occurrence of fossil lucinids (Fig. 5d), and active tectonic settings (McDonnell et al. 2000), suggest that they are probably the coldseep carbonates. It has been shown that those carbonates formed at methane-seeps are typified by low δ 13 C values (reviewed in Peckmann and Thiel 2004). The δ 13 C values obtained from 27 rock samples of carbonate-cemented mudstones ranged from -53.7 0 00 to -10.4 0 00 (Table 3), which further verify this speculation.
Those massive cold-seep carbonates were characterized by abundant chemosynthetic lucinids suggesting that they were possibly formed beneath the oxic zone of sediments, perhaps no deeper than several centimeters to a few meters below the muddy seafloor (Ritger et al. 1987;Gaillard et al. 1992;Campbell and Bottjer 1993). However, a few epibenthic mollusks (turrid, trochid, and turbinid gastropods, and mytilid bivalves) associated with cold-seep carbonates were also observed at Outcrop 5, suggesting that parts of the cold-seep carbonates might have been exposed to the muddy seafloor.

Carbon and Oxygen Stable Isotope Compositions of Cold-Seep Carbonates
Biogenic methane is extremely depleted in 13 C and exhibits δ 13 C values in the range of -50 0 00 to -110 0 00 (Whiticar 1999), whereas thermogenic methane typically yields values ranging from -30 0 00 to -50 0 00 (Sackett 1978). In addition, the δ 13 C values of the cold-seep carbonates are generally higher than those of seeping hydrocarbons at the same sites. The very low δ 13 C values of ancient carbonates would indicate a biogenic methane source (Peckmann and Thiel 2004;Pierre and Rouchy 2004). Accordingly, the δ 13 C values of 27 rock samples of cold-seep carbonates are possibly the result of a mixture of deep thermogenic hydrocarbons and shallow biogenic methane (Díaz-del-Río et al. 2003).
The δ O 18 values of 26 dolomitic samples are positive and range from 1.9 0 00 to 5.0 0 00 . Although the high δ O 18 values of seep carbonates have been ascribed to the dissociation of gas hydrates at modern seeps, this approach has not been successfully applied to fossil coldseep carbonates (reviewed in Peckmann and Thiel 2004). Alternatively, the higher δ O 18 val-  (Allen and Matthews 1982). The calcitic carbonate pipe that yielded negative δ O 18 values (-5.4 0 00 ) was collected from the massive cold-seep carbonate that exhibited positive δ O 18 values (2.9 0 00 ). This was probably the result of carbonate formed with different seeping fluid sources or partly altered during diagenesis.

The Paleoenvironment before the Formation of Coral Reefs
The occurrence of cold-seep carbonates observed at 6 outcrops (Outcrops 2 to 7) and in 11 borehole cores, suggests that hydrocarbon seepage occurred extensively in the Takangshan area. The compactness and association of large lucinid bivalves in massive cold-seep carbonates further indicate pronounced, long-lasting seepage of methane (Kelly et al. 2000) occurring antecedently to the development of Takangshan Reef. The carbonate pipes might have been formed as a result of sporadic expulsion of methane-related fluid seepages (Kulm and Suess 1990;Sakai et al. 1992;Campbell and Bottjer 1993;Díaz-del-Río et al. 2003). The stromatactoid cavities (Fig. 8a) occurring in a rock sample collected from Outcrop 5 were probably formed by the erosion of less indurated sediments between the well-cemented intercrusts (Peckman et al. 2002). We propose a schematic model to illustrate the occurrence of various lithologies and lithofacies associations (Fig. 9). The erosional surfaces on siliciclastic mudstones (Figs. 4a, b, 5a) and the funnel-shaped structure (Fig. 5b), as well as the exhuming of massive cold-seep carbonates (Figs. 6c, d) might have occurred concurrently during a tectonically unstable time in SW Taiwan. The deposition of fossiliferous mudstone (Figs. 5a, b) interfingered with conglomerate lithofacies (Fig. 4b) represents a rapid facies transition from a siliciclastic (non-carbonate) to a carbonate environment. The results of grain size analyses show a significant increase in the sand grade fractions of fossiliferous mudstone being due to an increase in bioclasts rather than an increased supply of coarser terrigenous siliciclastics, suggesting an environment more suitable for the initial colonization of reef-building organisms. The root of this rapid facies change is presumed to be tectonic movement, probably related to the westward thrust migration in the Pleistocene foreland basin. Although the precise geological time on these abrupt facies changes is not available, the successive occurrence of these events is probable judging by the active tectonic setting in SW Taiwan (Lacombe et al. 1997(Lacombe et al. , 1999. Although the massive cold-seep carbonates, carbonate pipes, and the conglomerate strata exposed in the E Takangshan quarry occurred with a limited lateral extent, much-wider occurrences of dolomitic cobbles and pebbles were observed from the borehole cores (Fig. 2). Many studies have proposed that violent gas escaping through the sediment surface may lead to the formation of pockmarks, craters, and brecciated carbonates (Bottjer et al. 1995;Kauffman et al. 1996;Díaz-del-Río et al. 2003). The occurrences of dolomitic lithoclasts in massive mudstones and muddy conglomerates in the study area suggest that similar events might have occurred. The funnel-like structure observed at Outcrop 7 (Fig. 5b) was likely a mud volcano conduit analogous to that of the Miocene mud volcano at Monferrato, NW Italy (Clari et al. 2004). This structure possibly served as a channel for expelling cobble-and pebble-sized mudstones onto the surrounding muddy substrata, and resulted in the occurrences of cobbles and pebbles floating in weakly consolidated matrices of siliciclastic mudstones. The conglomerate consisting of mud breccias and methane-derived carbonate was possibly a deposition resulting from mud volcano activities. It is likely that various hydrocarbon seepages occurred extensively and vigorously on the muddy seafloor of the Takangshan area before the development of coral reefs.

The Initial Development of Coral Reefs
Reef formation is often dependent on the topography of the seabed, and topographic highs favor the colonization and growth of reef builders, especially where the substrate is a hardground or composed of coarse bioclasts (Fagerstrom 1987). At Outcrops 5 and 6, the massive coldseep carbonates were encrusted by scleractinian corals Porites, Favia, and coralline algae . Associations of such encrustations and massive cold-seep carbonates are unusual in the geological record. The exposed massive seep carbonates might have served as 'carbonate factories' for the surrounding muddy environments, and probably played a crucial role in the initial development of tropical coral reefs in a siliciclastic paleoenvironment of active tectonic setting. To our best knowledge, this is probably the first report that documents cold-seep carbonates acting as initial colonization hardgrounds for hermatypic corals and corallines.

CONCLUSIONS
1. Various occurrences of cold-seep carbonates were recognized at 6 outcrops and in 11 cores at the Takangshan, including massive cold-seep carbonates, carbonate pipes, and dolomitic cobbles/pebbles. The δ 13 C values of 27 rock samples of seep carbonates range from -53.7 0 00 to -10.4 0 00 , suggesting the mixing of deep thermogenic hydrocarbons and shallow biogenic methane. The wide occurrences of seep carbonates in the study area also suggest that the hydrocarbon seeping occurred extensively. 2. A schematic model was proposed to illustrate the occurrence of various lithologies and lithofacies associations observed in the study area. The erosional surfaces on the siliciclastic mudstones and the funnel-shaped structure, as well as the exhuming of massive cold-seep carbonates might have occurred concurrently during a tectonically unstable time in SW Taiwan. The deposition of fossiliferous mudstone lithofacies indicate a rapid facies transition from a siliciclastic (non-carbonate) to a carbonate environment presumed to be a result of tectonic movement. 3. The encrustations of hermatypic corals and coralline algae on the massive cold-seep carbonates are unusual in the geological record, suggesting that the exposed seep carbonates played a crucial role in the initial development of coral reefs in a siliciclastic paleoenvironment.