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nary reefs in the southernmost Belize lagoon, British Honduras. Geol Soc Am Bull 93:116—126. Choi DR, Holmes CW (1982) Foundations of Quarternary reefs ...
( Springer-Verlag 1998

Coral Reefs (1998) 17 : 215—222

REP OR T

S.-Y. Gong · S.-W. Wang · T.-Y. Lee

Pleistocene coral reefs associated with claystones, Southwestern Taiwan

Accepted: 21 April 1998

Abstract Scleractinian coral reefs, when coexistent with siliciclastic sediments, usually occur in association with deltaic or coastal sands. Nevertheless, Pleistocene reef limestones in southwestern Taiwan are developed in association with thick claystones that were deposited in a deeper-water environment. These reef limestones are characterized by: (1) rapid transition from underlying claystones upward to reefal limestones, (2) lateral interfingering with open-shelf claystones, (3) being overlain by terrestrial deposits or exposed with no covering strata, and (4) being located in close association with anticlines. The authors propose that these reef limestones developed on anticlinal ridges raised above the adjacent sea floor by thrust-front migration in a foreland setting.

coincident with deltaic or coastal sands (Choi and Ginsburg 1982; Choi and Holmes 1982; Purser et al. 1987; Santisteban and Taberner 1988; Friedman 1988; Acker and Stearn 1990; Braga et al. 1990; Fay et al. 1992, among others). In the northern Great Barrier Reef, however, Holocene reefs are developing on a foundation of lagoonal mud (Johnson and Risk 1987). In this study, we present an example in which Pleistocene scleractinian reefs of southwestern Taiwan occurred in association with open-shelf claystones in a foreland setting. The objectives are: (1) to describe the unusual occurrence of the Pleistocene reef limestones in southwestern Taiwan and (2) to propose an explanation of how these coral reefs developed in association with deeper-water claystones.

Key words Coral reefs ) Tectonic control ) Siliciclastics ) Foreland wasin ) Pleistocene ) Taiwan Geological setting and stratigraphy Introduction Transition and mixing of carbonate and siliciclastic sediments have received much attention since the 1980s. Mount (1984) proposed four types of mixed siliciclastic and carbonate sedimentation: (1) punctuated mixing; (2) facies mixing; (3) in situ mixing; and (4) source mixing. Many other cases of transition and mixing of carbonate and siliciclastic sediments were collected in Doyle and Roberts (1988), Budd and Harris (1990) and Lomando and Harris (1991). Among the cases of carbonate and siliciclastic transition, scleractinian coral reefs typically develop in shallow waters S.-Y. Gong ( ) ) S.-W. Wang National Museum of Natural Science, Taichung, Taiwan, R.O.C. e-mail: [email protected] T.-Y. Lee Department of Earth Science, The National Taiwan Normal University, Taipei, Taiwan, R.O.C.

The structural style of southwestern Taiwan is dominated by thrusts and folds (Suppe 1980; Ho 1986). Southwestern Taiwan is part of a foreland basin formed by tectonic loading of the Central Range Orogen during Plio-Pleistocene arc-continent collision (Covey 1986; Teng 1990). Westward migration of the thrust front has deformed and exposed what was once the eastern and deeper part of the basin. More than 5 km of siliciclastic strata was deposited in the basin with a sedimentation rate (before decompaction) estimated to be about 0.4 m/ky during the Early Pliocene, and 2—3 m/ky during the Late Pliocene and Pleistocene (Chen et al. 1977; Chang and Chi 1983). In these thick siliciclastic sequences several scleractinian coral reefs occur that are worthy of our attention (Gong et al. 1996). The Pleistocene reef limestones in southwestern Taiwan occur as lenticular bodies in the Gutingkeng Formation in the vicinity of Kaohsiung (Fig. 1). The stratigraphy of the limestones is summarized in Fig. 2. As Figs. 1 and 2 show, the ages of the studied reef

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limestones become younger toward the north (Lee 1990). In terms of structural geology, the study area is predominated by thrusts that trend NNE-SSW or NESW. Before the arc-continent collision, the area was part of the Tainan rift basin and was located below the paleo- shelf break (Fig. 1A, also see Yang et al. 1991). The Gutingkeng is traditionally divided into the Upper Gutingkeng Formation and the Lower Gutingkeng Formation (Fig. 2). The Upper Gutingkeng measures about 500 m in thickness and consists of gray, bedded silty claystones, occasionally with thin sandstone layers displaying climbing-ripples and parallel to low-angle cross-laminae (Covey 1986; Lin 1991). Benthic foraminifera present in the Upper Gutingkeng Formation include Bolivina robusta pacifica, ºvigerina proboscidea, Melonis nicobaranse, Bulimina marginata, Bolivina semicostata, Bolivina formosana, Bolivina subangularis, Bigenerina nodosaria, and Brizalina aenariensis. Presence of these taxa indicates a continental shelf break to shelf environment of deposition (Wang 1992). The Lower Gutingkeng measures 1590 m in thickness, and consists of thick gray, massive claystones, occasionally with thin claystone/siltstone laminae. It commonly forms badlands in the field. Its benthic foraminifera fauna includes Cassidulina carinata, Bulimina rostrata, Bolivina robusta, ºvigerina peregrina, Bulimina aculeata, and Bulimina striata mexicana, indicating a low oxygen to anaerobic middle abyssal environment (Wang 1992). Sedimentological studies interpreted the Upper Gutingkeng Formation to have been deposited in outer shelf environments, whereas the

Fig. 1A Major geotectonic elements of Taiwan. Compiled from Teng (1992) and Lee (1992). Bold lines represent major thrusts, triangles on upthrown sides. Note the —200 m and !100 m contour line and their bending in offshore southwestern Taiwan. The open arrow in the lower right corner shows the current moving direction of the Philippine Plate with respect to the Eurasia Plate; the moving rate is also shown. B Map showing the locations of the reef limestones studied and major structural elements in southwestern Taiwan. Limestones are indicated by letters. ¹, Takangshan; H, Hsiaokangshan; P, Panpingshan; K, Kaohsiung. Location marked in A

Lower Gutingkeng was deposited in upper continental slope environments (Covey 1986; Lin 1991). Faunal analyses of benthic foraminifera of the Gutingkeng also agree with such an interpretation (Wang 1992). The limestones discussed herein crop out in hills of the Kaohsiung area. These limestones have been quarried by the cement industry. Fieldwork was carried out at well-exposed outcrops at quarries. Core materials studied by the authors were obtained from the cement companies and are now stored in the National Museum of Natural Science, Taiwan.

Facies of the reef limestones Algal-coral boundstone facies

This facies appears cream white to light tan in color, and is mainly composed of algal-coral boundstones

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valves, gastropods, fungiid corals, and fragments of foliaceous corals. This facies usually occurs below the bioclastic wackestone facies and above the calcareous silty claystones of the Gutingkeng Formation (Fig. 3D). In borehole cores, this facies marks the beginning of carbonate sedimentation.

Stratigraphic characteristics

Fig. 2 Stratigraphy of the Pleistocene reef limestones in southwestern Taiwan based on coccolith biostratigraphy (Chi 1979; Chang and Chi 1983; Lee 1990). According to Chang and Chi (1983), the G. oceanica Zone is equivalent to Martini’s (1971) NN20; the P. lacunosa Zone is equivalent to NN19; the C. macintyrei Zone is equivalent to NN16—18; the R. pseudoumbilica Zone is equivalent to NN13—15. The limestone units are represented by letters. as in Fig. 1

built by scleractinian corals such as Porites, Acropora, Favia and Favites, and also by encrusting calcareous red algae, encrusting bryozoans and mollusks (Fig. 3A). The relative predominance of coral over encrusting red algae varies from place to place. Filling in the reef framework are bioclastics, principally consisting of larger foraminifers, such as Amphistegina, Baculogypsinoides, Homotrema, and Calcarina, as well as fragments of mollusks, corals, bryozoans and articulated coralline algae. This facies occurs in all the limestones on top of, or laterally in contact with the bioclastic wackestone facies. Bioclastic wackestone facies

This facies appears buff to light tan in color, and consists of coral fragments, rhodoliths and fragments of mollusks, bryozoans and articulated red algae in a micritic matrix (Fig. 3B). Two types of bioclastic wackestones can be distinguished: rhodolith-dominated wackestones and coral fragment-dominated wackestones, but there is a whole spectrum between these two end members. In the field, this facies tends to occur below the algal-coral boundstone facies and above the fossiliferous claystone/argillaceous limestone facies. Fossiliferous claystone/argillaceous limestone facies

This facies appears gray in color, and consists of thin layers of encrusting coralline algae and some bioclastics in an argillaceous matrix (Fig. 3C). Bioclastics in this facies include larger foraminifera, thin-shell bi-

Stratigraphically, the limestones are characterized by: (1) rapid transition from underlying terrigenous mudstones of deeper-water facies to reef limestones, (2) lateral interfingering with fine-grained siliciclastics, (3) being overlain by terrestrial deposits or subaerially exposed with no covering strata, and (4) being located in close association with anticlines. Rapid transition from the underlying siliciclastic mudstones to reef limestones

The outcrops and borehole cores of the limestones reveal rapid facies transition from the underlying claystones to reef limestone within a few meters (Figs. 3, 4, 5 and 6). Such a vertical facies transition is best represented by the Takangshan Limestone. At Takangshan, the gray to dark gray claystones of the Gutingkeng Formation change upward into gray clayey siltstones/sandstones, then into argillaceous rhodolith-bioclasts wackestones, and finally into coral boundstones in a few to about 15 m (Figs. 3 and 6). Sharp facies contacts are also observed in places where bioclastic limestone was deposited directly above a scoured surface into claystones (Fig. 4). The Gutingkeng claystones are massive rich in planktonic foraminifers and small benthic foraminifers. Macrofossils are rare. In places, vertical pipe-like trace fossils were found at the very top of the claystones. The clayey siltstones/sandstones below the limestones may contain larger foraminifers, mollusks, and fungiid corals. They represent a transition from siliciclastic to carbonate deposition. Acropora fragments were found locally in the clayey siltstones/sandstones (Fig. 5). Commonly, a vertical facies transition from claystone to coral boundstone occurs within a few meters (Figs. 3 and 6). The Hsiaokangshan Limestone also exhibits the same vertical facies change from dark, massive claystones to reef limestone within a few meters (Fig. 6C). Similar facies transitions from the Gutingkeng Formation upward into reef limestones were observed in the Panpingshan Limestone where the gray claystones below the Panpingshan Limestone change upward into siltstones, argillaceous limestones, bioclastic limestones, and then finally into coral boundstones in less than 10 m. At the bottom of the Kaohsiung Limestone, Chi (1989) reported a similar vertical facies transition within about 15 m.

218 Fig. 3A—D Photograph of polished rock slab showing the lithofacies and evidence for a rapid upward shallowing of the Takangshan Limestone. The sample was taken from borehole 9 drilled at the eastern flank of Takangshan. A centimeter scale is placed on the right side of the samples. A Algal-coral boundstone, !50 m. Note Porites (Po) in the middle. B Bioclastic wackestone, !55 m. Note Acropora fragment (Af ) and shell fragment (Sf ) in the lower middle. C Fossiliferous claystone/argillaceous limestone, !60 m. The white fragments are bioclastics and the dark area is argillaceous matrix. D Calcareous silty claystone of the Gutingkeng Formation, !68 m

Lateral interfingering with fine-grained siliciclastics

As mentioned earlier, the reef limestones in southwestern Taiwan occur as lenticular bodies. Laterally interfingering with these reef limestones are gray to dark gray, massive claystones and silty claystones of the Gutingkeng Formation. From the geological profiles, the Takangshan, Hsiaokangshan, Panpingshan, and Kaohsiung Limestones can be seen to grade east-

ward or southeastward into claystones of the Gutingkeng Formation (Fig. 7). Such stratigraphic relation can be further verified at quarries on the eastern or southern margins of the Takangshan, Panpingshan, and Kaohsiung Limestones where the limestones are found to be interbedded with gray, massive to thickbedded silty claystones of the Gutingkeng Formation. Applying Walther’s Law, this vertical succession represents lateral facies transitions.

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veloped at the top of the Takangshan Limestone and are now filled with red soil. No erosional remnant of marine strata is found on top of the Takangshan Limestone. The Hsiaokangshan, Panpingshan, and Kaohsiung Limestones are also exposed without covering strata.

Closely related to anticlines

As shown in Fig. 1, the reef limestones in southwestern Taiwan are located on thrusts/anticlines. This is especially clear in the Kaohsiung area where the Takangshan, Hsiaokangshan, Panpingshan, and Kaohsiung Limestones all occur very close to the anticlines or thrusts along the western front of the foothills. The majority of them are located on the eastern flank of the anticlines, and form west-vergent terraces (Fig. 7). Recent paleostress studies, using calcite twin analysis, revealed a local synsedimentary NW-SE extension, parallel to the regional NW-SE compression, developed within these reef limestones (Rocher et al. 1996). Fig. 4 Photograph of outcrop showing a scoured surface at the top of the claystone (Cl). Bioclastics of the Takangshan Limestone (Lm) deposited above the surface. The hammer measures 33 cm long and is placed upside down

Fig. 5 Photograph showing coral fragments (Cf ) at top of the claystone in the transition zone from the underlying claystone to reef limestones above at Panpingshan. The pencil is 15 cm in length

Overlain by terrestrial facies or exposure surface

The Pleistocene sequences in southwestern Taiwan are predominantly marine deposits. However, Pleistocene reef limestones of southwestern Taiwan are overlain by terrestrial deposits or by no strata at all. The Takangshan Limestones are overlain by well-developed red soil/conglomerates (Fig. 8). Dissolution cavities de-

Discussion Origin of reef development

Stratigraphic characteristics of the reef limestones described are certainly bound to raise questions. It is considered that the reef limestones in southwestern Taiwan were deposited on paleo-topographic highs that stood above the sea floor and were, therefore, not buried by siliciclastic sediments. In a basin with a high subsidence rate and high siliciclastic influx, two conditions are necessary for scleractinian reef development: (1) water depth shallow enough for coral growth, and (2) isolation from the surrounding siliciclastic influx. A topographic high could meet both of these requirements. We believe that these scleractinian reefs were developed on a topographic high while fine-grained siliciclastics were still being deposited in the adjacent area. As for the rapid shallowing of depositional sites, it can result from eustatic or tectonic origins. It is well known that the Pleistocene eustatic sea level fluctuated due to advances and retreats of glaciers (Prell et al. 1986, among others). Although the rapid shallowing could have been caused by glacioeustatic sea-level fall, no evidence of such basin-wide rapid upward-shallowing during the NN19 period along sections in southwestern Taiwan that are synchronous to the limestones (Covey 1986; Lin 1991; Wang 1992). Therefore, this shallowing was a local event that cannot be explained solely by a eustatic sea-level fall. In addition, reef limestones in southwestern Taiwan are covered by red soil or exposed at the surface. No

220 Fig. 6A –C Lithologic columns displaying facies transition from the underlying claystone to the reef limestone at Takangshan and Hsiaokangshan. A Borehole 8 of Chia-Hsin Cement Ltd. drilled at the eastern flank of Takangshan. B Borehole 3 of Chia-Hsin Cement Ltd. drilled at the eastern flank of Takangshan. C Composite lithocolumn of a quarry at southeastern Hsiaokangshan

Fig. 7 Geological profiles of the A Tagangshan Limestone and B Panpingshan Limestone

Fig. 8 Photograph showing that dissolution cavities developed at the top of the Takangshan Limestone (Lm) and are now filled with red soil (Rs). The note book is 19 cm long

erosional remnants of marine strata were observed on top of these limestones nor above the red soil. Dissolution cavities developed at the top of the Takangshan Limestone and subsequently were filled exclusively with red soil. This implies that the reef limestones were uplifted above sea level since their formation, and favors a tectonic over an eustatic origin for the upwardshallowing of the depositional sites. If the subaerial exposure was caused by a eustatic sea-level fall, the subsequent eustatic sea-level rise should have resulted in marine sediments on top of the exposure surface but that is not the case. The question now is: what mechanism was responsible for the formation of the paleotopographic high.

Lee (1992) reported that the deformation front of the Taiwan orogenic belt follows a trend of approximately N55°E northwest of Kaohsiung City and extends NNE on land. This deformation front, passing through the northern flank of the Panpingshan anticline, can be linked to the Meilin thrust fault (Fig. 9) which cuts through the western flanks of the Takangshan anticline (Fig. 1). Lee (1992) also recognized incipient thrusts and anticlinal ridges located to the west of the thrusts offshore southwestern Taiwan (Fig. 8). The thrust sheets behind the deformation front may also extend onshore and connect to the thrust systems east of the Meilin fault. The authors herein propose that the anticlinal ridges associated with the thrusts offshore provide an

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the fault-propagation folding, and have actually been reported from onshore and offshore Taiwan (Suppe 1980; Ho 1986; Lee 1992). Such anticlinal ridges might have provided ideal sites for reef development because they became shallow enough for coral growth, and were not buried by the siliciclastic influx in the surrounding area. Although uncertainties remain at this time, we believe this anticline hypothesis explains most geological attributes discussed above and in Gong et al. (1996). Coral reefs developed on local anticlinal ridges that resulted from fault propagation while the Gutingkeng Formation was being deposited in the surrounding deeper water.

Geological significance

Fig. 9 Cross section and generalized map showing the possible relationship between structures on land and the offshore deformation front. Note the possible offshore extension of the Panpingshan Anticline. The interpreted profile is based on seismic stratigraphy (Lee 1992)

Fig. 10 Schematic model of reef development on anticlinal ridges induced by the thrust. Fine-grained siliciclastics were deposited in deeper waters around the reef

If the model presented here is valid, its geological implications are significant. First, it provides a model for scleractinian reef development associated with claystones. Reef development along structural highs is common, and at least has been reported in the Devonian carbonate sequences in the Timan-Pechora basin (Yentsov 1992), in Miocene sequences of Russia (Znamenskaya and Chebanenko 1986), in the Paleogene carbonate buildups of the south Pyrenean basin (Eichenseer et al. 1988), and in Miocene reefs of Cyprus (Robertson et al. 1991), among others. However, those analogues occurred in carbonate-dominated basins. Southwestern Taiwan thus represents an interesting example of tectonic control on reef development in a foredeep dominated by deep-water siliciclastics. In addition, the temporal and spatial distribution of the reef limestones may provide valuable information about the history of thrust-front migration. If the Pleistocene reefs were indeed initiated by fault propagation, the ages of these reefs should place a constraint on the timing of each related thrust and the history of thrustfront migration (Gong et al. 1996). As mentioned earlier, the reef limestones in the studied area generally become younger toward the north, and the thrusts trend NNE-SSW or NE-SW in the area (Figs. 1 and 2). This is consistent with the orogen moving toward the northwest as evidenced by NW-SE compressional stresses (Rocher et al. 1996). Therefore, the temporal and spatial relationships of the reef limestones may well represent the history of northwestward thrust-front migration in southwestern Taiwan (Gong et al. 1996).

Conclusions explanation for reef development in southwestern Taiwan (Fig. 10). In a foreland basin, thrust sheets migrate toward the basin and cause basin subsidence due to tectonic loading of the orogen (Flemings and Jordan 1990). Anticlinal ridges are often created along

1. Reef limestones in southwestern Taiwan developed on local structural highs that were isolated from siliciclastic influx. Synchronous siliciclastics deposition occurred in deeper water adjacent to the reef limestones.

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2. Structural highs on which the Pleistocene reef limestones in southwestern Taiwan were deposited are interpreted to be anticlinal ridges of fault-propagation folding. Thrusting caused rapid shallowing and formed local topographic highs that became the sites of reef development. Southwestern Taiwan offers an interesting example in which scleractinian coral reefs developed in a foreland basin dominated by deep-water siliciclastics. Not only does it offer a model to explain reef development associated with claystones, but it also provides valuable information toward a better understanding of the history of thrust-front migration in southwestern Taiwan. Acknowledgements The authors thank H.W. Chen, W.R. Chi, and O. Lacombe for helpful discussions. We also thank Dr. G.M. Friedman, Dr. J.L. Carew, and two anonymous reviewers for their comments on an earlier version of the manuscript.

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