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Middle Triassic (Anisian) diversified bivalves: depositional environments and bivalve assemblages in the Leidapo Member of the Qingyan Formation, southern China
Palaeogeography, Palaeoclimatology, Palaeoecology 208 (2004) 207 – 223 www.elsevier.com/locate/palaeo
Middle Triassic (Anisian) diversified bivalves: depositional environments and bivalve assemblages in the Leidapo Member of the Qingyan Formation, southern China Toshifumi Komatsu a,b,*, Jin-hua Chen b, Mei-zhen Cao b, Frank Stiller c, Hajime Naruse d a Department of Earth Sciences, Kumamoto University, Kumamoto 860-8555, Japan Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China c Institute of Geology and Palaeontology, University of Mu¨nster, D-48149 Mu¨nster, Germany d Department of Earth and Planetary Science, Graduate School of Science, Kyoto University, Kyoto 606, Japan b
Received 17 July 2002; received in revised form 28 October 2003; accepted 23 March 2004
Abstract The Middle Triassic (Anisian) Leidapo Member of the Qingyan Formation, which crops out in southwestern China, consists of basin-floor and slope deposits with abundant benthic fossils and records two transgressions and one regression. The wellpreserved bivalves are grouped into four assemblages. The allochthonous Cassianella – Elegantinia and Protostrea – Cassianella assemblages as well as the autochthonous Palaeonucula assemblage are found in slope deposits. The allochthonous assemblages exhibit several modes of occurrence in sediment gravity-flow deposits. The parautochthonous Posidonia assemblage is preserved in slope and basin deposits. The genus-level compositions of these bivalve assemblages are similar to those from the Early Carnian of Tethyan Europe. It is probable that a significant diversification of bivalves took place in the Anisian. D 2004 Elsevier B.V. All rights reserved. Keywords: Bivalve; China; Depositional environment; Diversification; Gravity-flow deposits; Triassic
1. Introduction The Middle Triassic was an important time for the radiation of bivalve molluscs (Hallam and
* Corresponding author. Department of Earth Sciences, Faculty of Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan. E-mail addresses: [email protected] (T. Komatsu), [email protected] (J. Chen), [email protected] (M. Cao), [email protected] (F. Stiller), [email protected] (H. Naruse). 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.03.005
Wignall, 1997; McRoberts, 2001). Infaunal bivalves, in particular, substantially increased in generic diversity and acquired new niches partly as a result of the development of siphons and mucus feeding (Stanley, 1968). However, the precise time of the beginning of this bivalve diversification is unknown, although Hallam and Wignall (1997) inferred that it probably was delayed until the Ladinian or even later. Furthermore, Middle Triassic bivalve assemblages are somewhat enigmatic because the record of shallow-marine bivalves is relatively poor except
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for the Cassian Formation of the southern Alps (which mainly belongs to the Upper Triassic Carnian) and southwestern China (Bittner, 1895; Fu¨rsich and Wendt, 1977; Yang et al., 1983; Chen et al., 1992). The Anisian Leidapo Member of the Qingyan Formation is exposed in the Qingyan area, about 30 km south of Guiyang (provincial capital), Guizhou Province, China (Fig. 1). It is representative of Middle Triassic invertebrate fossils and sediments in southwestern China (Wei et al., 1996; Enos et al., 1997, 1998; Stiller, 1997; Chen et al., 1998). The Leidapo Member contains abundant, well-preserved macrofossils including bivalves, gastropods, brachiopods, sponges, corals, crinoids, echinoids, scaphopods, bryozoans, cephalopods and microfossils such as ostracods, foraminifers and calcareous algae (Hsu and Chen, 1943; Yang and Xu, 1966; Yin, 1974; Gu et al., 1976; Gan and Yin, 1978; Liao, 1978; Yin and Yochelson, 1983a,b; KristanTollmann, 1983a,b; He, 1984; Deng and Kong, 1984; Stiller, 1997, 1998, 1999, 2001a,b; Chen et al., 2001). The abundant and diverse bivalves are grouped into several autochthonous and allochthonous assemblages according to their composition, preservation and mode of occurrence. Palaeoenvironments of the Leidapo Member, especially its middle part, have been interpreted as shallow-marine, based on the abundance of fossils. However, based on a new, detailed analyses and taphonomic observations, some doubt has been cast on the
previous palaeoenvironmental interpretations of these facies. In the present study, the nature of the Anisian bivalve assemblages, and their habitats and taphonomy, are inferred for each depositional environment. Furthermore, relative sea-level fluctuations and the palaeoecology of the Middle Triassic bivalves are discussed.
2. Geologic setting The Middle Triassic rocks exposed in the vicinity of Qingyan, consisting of interbedded limestones, mudstones and siltstones (Fig. 2), attain a total thickness of about 1250 m. Usually, the rocks are divided into two formations, the Longtou Formation (Ladinian) and the Qingyan Formation (Anisian). Qingyan is the type locality of the latter formation. The Qingyan Formation is composed of limestones and mudstones and is about 800 m thick. In ascending order, it is subdivided into the Xiaoshan, Mafengpo, Yingshangpo, Leidapo and Yuqing members. According to palaeontological data, the latter two members are assigned to the upper Anisian, whereas the other three are regarded as lower Anisian in age (Stiller, 1997, 2001a; Chen et al., 1998). The Leidapo Member discussed in this paper, occupying the lower part of the upper Anisian, is about 200 m thick and consists of mudstones, shales and marls. This member is characterized by fossiliferous mudstones and marls (Fig. 3). It is grouped
Fig. 1. Location map of the study area.
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Fig. 2. The Middle Triassic Anisian facies distribution in central Guizhou province (modified from Enos et al., 1997). Study area is situated in the northern end of the Nanpanjiang Basin.
into three parts. The lower and upper parts are mainly composed of massive, laminated mudstone with abundant ‘‘paper shelled bivalves,’’ Daonella and Posidonia. The middle part consists of alternating beds of mudstone and marl, rich in fossils including abundant molluscs of Tethyan affinity. Brachiopods, gastropods and bivalves are especially conspicuous, and despite their Anisian age, their faunal aspects are remarkably similar to those described from the Carnian of the Alpine region of Europe (Hsu and Chen, 1943; Yang and Xu, 1966; Yin, 1974; Gu et al., 1976; Gan and Yin, 1978; Yin and Yochelson, 1983a,b). The associated ammonoids contain important taxa referred to the genera Hollandites, Judicarites, Phillippites, Discoptychites, Beyrichites, Bulogites, Paraceratites, Ptychites, Gymnites, Sageceras and Danubites. Especially, Paraceratites binodosus (Hauer) collected from the middle part of the member (ML-20) indicate a Late Anisian age (Liao, 1978; Stiller, 2001a,b). Anisian microfossils, ostracods and foraminifers also have been reported from this member (Kristan-Tollmann, 1983a,b; He, 1984). Anisian conodonts, e.g., Neo-
hindeodella triassica triassica (Mu¨ller), have also been found in the middle part of the Leidapo Member (Stiller, 2001a).
3. Methods We conducted a sedimentological study based on facies analysis in the field and on sectioning some blocks of rocks for detailed observation of sedimentary structures, grain size and fabric in the laboratory. We referred to the facies classifications proposed by Bouma (1962) and Lowe (1982), which mainly deal with gravity-flow deposits. The terminology for depositional environments and sedimentary structures follows Tucker and Wright (1990), Wright and Burchette (1996) and Stow et al. (1996). The Leidapo bivalves were mainly collected from siliciclastic to marly rocks. They are grouped into assemblages based on faunal composition, modes of fossil occurrence and shell preservation. Bulk rock samples were broken up in the laboratory, and all bivalves larger than 1 – 2 mm were counted. Modes of
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occurrence were mainly observed in the field. The life habits of the bivalves were reconstructed by analogy with closely related living taxa and also by reference to previous studies (Fu¨rsich and Wendt, 1977; Aberhan, 1994).
4. Depositional facies and interpretation The Qingyan Formation is present in the northern part of the Middle Triassic Nanpanjiang Basin (or the Dian – Qian – Gui Basin) (Fig. 2), which was filled mainly with carbonate platform, bank and siliciclastic basinal deposits (Wei et al., 1996; Enos et al., 1997; Lehrmann, et al., 1998; Bao, 1998). The shallowwater carbonate platform is called the Yangtze Platform and was a stable palaeogeographic element from the Late Proterozoic to the Triassic (Enos et al., 1998). In the Middle Triassic, the Qingyan area was situated on the northeastern margin of the Nanpanjiang Basin (Enos et al., 1997; Chen et al., 1998). Enos et al. (1997) presented a detailed study of the depositional environments in the lower part of the Qingyan Formation, which is composed of retreating platformmargin deposits characterized by breccia and basinfloor deposits consisting predominantly of marls and mudstones. 4.1. Facies 1 4.1.1. Description Facies 1 consists of shell-supported muddy calcarenite (Figs. 3 –5). The calcarenite contains abundant ripped-up mud clasts (1– 20 cm in diameter), brachiopods, bivalves, gastropods, sponges and corals (Figs. 5.4 and 7). The bivalves are mainly preserved as disarticulated valves and fragments and form the framework of the shell-supported texture (Figs. 6.5 and 6). The sand-size particles are well-rounded bioclasts, not siliciclasts, in origin. These bioclastic calcarenite beds are massive or exhibit normal grading and have well-defined, flat or scoured bases (Figs. 5.2 and 6.6). Thinner beds (5– 10 cm thick) commonly have a sheet-like geometry, whereas thicker beds (average 10– 40 cm thick) fill incisions over 10 m in wide and 5– 30 cm in depth. The top parts rarely exhibit parallel lamination and cross-stratification suggesting south-directed
palaeocurrents. Facies 1 is commonly capped by thin, massive mudstone and parallel-laminated mudstone of Facies 4. 4.1.2. Interpretation Graded and massive beds containing abundant coarse bioclasts are a characteristic product of waning turbidity currents (Bouma, 1962; Walker, 1967; Lowe, 1982) and are similar to the S3 division of Lowe (1982) that is equivalent to Bouma’s (1962) Ta division. Overlying parallel-laminated beds correspond to Bouma’s (1962) Tb division formed under the bed load. Cross-stratified beds reflect deposition by traction currents. The shallowly scoured bases probably formed beneath a powerful turbidity current. 4.2. Facies 2 4.2.1. Description Facies 2 is composed of mudstones characterized by matrix-supported shell concentrations and rip-up clasts and capped by laminated mudstone (Fig. 6.2). The shelly mudstone beds (5 – 30 cm thick) are typically massive and have sharp, flat basal surfaces. They contain abundant shallow-marine brachiopods, bivalves, gastropods, sponges and corals. Ripped-up mud clasts and rounded limestone granules and pebbles are also abundant in these shelly mudstones (Figs. 6.1 – 3). The ripped-up clasts (1 – 20 cm in diameter) are discoidal in shape and are concentrated along with flat shells in the upper and top parts of these mudstone beds (Fig. 6.2). Occasionally, the top of the underlying bed is strongly deformed (Fig. 6.4) and shows minor overturned folds and flame structures. Facies 2 is commonly overlain by Facies 4, which is composed of thin, parallel-laminated and massive mudstone. 4.2.2. Interpretation This facies probably represents cohesive debrisflow deposits, although the beds described herein are very thin compared with typical debris-flow deposits. In the latter, large blocks, mainly composed of rip-up clasts and shell remains, appear to be floating in a mud matrix, which is characteristic of a typical matrix-supported framework (Stow et al., 1996). Outsized ripped-up clasts are commonly
T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 208 (2004) 207–223 Fig. 3. Columnar section of the Leidapo Member, showing the horizons of the samples examined and the stratigraphic occurrences of the bivalve species. LL: localities of the lower part of the Leidapo Member, ML: localities of the middle part of the Leidapo Member. 211
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Fig. 4. Facies codes and depositional environments.
present in the upper parts of debris flows and viscous sediment-flow deposits (Lowe, 1982; Postma et al., 1988). The underlying deformed beds were probably caused by shearing by the debris flow (Fig. 6.4).
4.3. Facies 3 4.3.1. Description Facies 3 consists of heterolithic units that show folds, overturned folds and ball and pillow structures
Fig. 5. (1) Type locality of the Leidapo Member at the Leidapo hill (arrow). (2) Minor channel-fill shell lens of facies 1 at locality ML-25. The basal part shows a convex-down erosional surface (arrows). (3) Vertical section of mudstone containing in situ preserved Palaeonucula qingyanensis at locality ML-12. Scale is 1 cm. (4) Horizontal section of basal part of shell-supported mudstone containing abundant brachiopods at locality ML-31. Scale is 1 cm.
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Fig. 6. Vertical sections of characteristic beds. Scale is 1 cm. (1 and 3) Debris-flow deposits (facies 2) containing rip-up clasts (black arrows), calcareous gravel (white arrow) and bioclasts (white and grey particles); they are characterized by matrix support and mud matrix (locality ML31). (2) Debris-flow deposits (facies 2) overlain by parallel laminations at locality ML-10. (4) Basal part of debris-flow deposits (facies 2) at locality ML-08. Underlying laminated mudstone is deformed by shearing by the debris flow. (5) Shell-supported massive shell bed (facies 1) at locality ML-34. White square materials are disarticulated crinoid remains. (6) Shell-supported shell bed (facies 1) showing grading at locality ML-24. Small burrows are found in underlying mudstone. (7) Slump mudstone (facies 3) at locality ML-17.
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(Fig. 6.7). The latter are found in a portion of the folded structures. The basal portions are usually chaotic. Facies 3 is generally overlain by mudstone of facies 4. These deformed beds are up to 70 cm thick, and their components originally were from facies 1, 2 and 4. 4.3.2. Interpretation Folds and overturned folds imply slope-induced slumping or deformation due to the shear stress of overriding currents (Stow et al., 1996). In the present case, facies 3 probably was not formed by the latter process but by slumping because this facies is generally covered by undeformed mudstone deposited from suspension and weakly bioturbated mudstone deposited under calm conditions. 4.4. Facies 4 4.4.1. Description Facies 4 is characterized by massive and laminated mudstone that rarely exhibits weak bioturbation and that yields a few bivalve taxa, such as Posidonia, Daonella and Palaeonucula. This weakly bioturbated mudstone lacks large burrows, but contains tiny Planolites-like burrows. Palaeonucula is sometimes preserved in life position. Posidonia is frequently preserved as articulated valves. In general, thin mudstone beds, 5 –60 cm thick, cover beds of facies 1 and 2 in the middle part of the Leidapo Member. The lower and upper parts of the Leidapo Member are composed of thick, massive mudstone (exceeding 20 m). This thick mudstone contains thin marl beds, thin calcarenite layers (about 0.5 cm thick), parallel-laminated siltstones and mudstones. Occasionally, parallel laminations are intercalated with abundant shell pavements composed of the small paper shelled bivalves, Posidonia and Daonella. 4.4.2. Interpretation Facies 4 was probably formed during the latest stage of turbidity currents and partly during the calm conditions after the passage of the turbidity currents. The parallel-laminated siltstones and mudstones are probably comparable to the Td interval of Bouma’s (1962) turbidite sequence. Generally, Posidonia oc-
curred in deep-sea deposits Triassic to Jurassic time (Kobayashi and Tokuyama, 1959; Fu¨rsich and Wendt, 1977; Chen et al., 1992; Aberhan, 1994; Etter and Tang, 2002). Palaeonucula, Posidonia and Daonella are typical members of oxygen-controlled bivalve assemblages in deep-sea settings (Aberhan, 1994). Planolites-like tiny burrows are also common in poorly oxygenated deep-sea areas (Bromley, 1996). The weakly bioturbated, thick, massive mudstone of facies 4 probably accumulated from suspension on the basin floor.
5. Depositional environments The different depositional facies occurring in the Leidapo Member record a development from basin floor (lower part of the Leidapo Member) to slope (middle part) and finally back to basin floor (upper part). Thick and massive mudstones suggesting a marginal basin-floor environment (facies 4) dominate the lower and upper parts of the Leidapo Member. Typical gravity-flow deposits develop in the middle part of the Leidapo Member (ML-01-30, ML-4344). They consist of turbidites (facies 1), debris flows (facies 2) and slump deposits (facies 3). These deposits are generally overlain by thin mudstones deposited under calm conditions (facies 4), although these mudstones (facies 4) are again overlain by sediment gravity-flow deposits. The dominance of these deposits suggests accumulation at the foot of a basin slope (Walker, 1992; Mullins et al., 1984; Wright and Burchette, 1996). In particular, outside of carbonate platforms, the lower slope facies is chiefly composed of distal turbidites and thin matrix-supported debris-flow deposits (Mullins et al., 1984). It is probable that gravity flows containing shallow-marine biota originated at the carbonate platform, bypassed the upper slope and finally were deposited on the lower slope. The upper part of the slope succession (ML-24-31) is dominated by turbidites (facies 1) characterized by grading and channeling. The channels are floored with coarse calcarenite, shell remains and gravels. These deposits probably filled minor channels on the upper and lower slopes. In addition, the gradient of the slope generally may have been relatively gentle because large-scale channel-fill sequences, thick slump depos-
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its and breccias with large components are absent from the Leidapo Member. The top of the middle part of the Leidapo Member suggests transgression because the member is capped by basin-floor deposits of the upper part of the Leidapo Member (ML-46). The lower part of the Leidapo Member, which is composed of basin-floor deposits, covers slope deposits of the Yingshang Member. These are composed mainly of limestones and breccias with sharp and flat basal surfaces (LL-01). This transgressive succession also suggests a relative sea-level change. Therefore, the Leidapo Member records two transgressions and one regression.
6. Bivalve assemblages Over 100 species of Bivalvia have been collected from the Leidapo Member. They are grouped into four assemblages: the Palaeonucula, Posidonia, Cassianella – Elegantinia and Protostrea – Cassianella assemblages (Figs. 3, 7, 8 and 9). 6.1. Palaeonucula assemblage 6.1.1. Composition and life habits This assemblage is composed of Palaeonucula strigillata (Goldfuss), Palaeonucula qingyanensis Chen and rare Cassianella ecki sulcata Chen. Palaeonucula was an infaunal, deposit-feeding bivalve that is a characteristic element in Mesozoic muddy-shelf and oxygen-controlled assemblages (Aberhan, 1994). 6.1.2. Modes of occurrence These bivalves are found very sporadically in the lower slope mudstones (facies 2 and 4) in the middle part of the Leidapo Member. Almost all Palaeonucula strigillata (Goldfuss) and Palaeonucula qingyanensis Chen are preserved as closed and articulated valves. Some are oriented with their sagittal plane perpendicular to the bedding planes with their posteriordorsal margin aligned upwards. They occur preferentially in the top parts of the beds composed of gravity-flow deposits (Figs. 5.3 and 9). These modes of occurrence probably reflect their living position. These bivalves probably inhabited the lower slope environments. However, Cassianella ecki sulcata Chen was probably trans-
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ported from shallow marine environments. This species is usually preserved in the form of disarticulated valves which indicates an allochthonous occurrence. 6.2. Posidonia assemblage 6.2.1. Composition and life habits This assemblage is dominated by Posidonia pannonica (Mojsisovics) and small Posidonia sp. and further contains rare Posidonia wengensis Wissmann, Daonella boeckhi Mojsisovics and Daonella sp. D. boeckhi Mojsisovics and Daonella sp. are common in the lower part of the Leidapo Member. These paper shelled bivalves lived epifaunally, though their exact modes of life are enigmatic. In general, Posidonia is abundant in hemipelagic and pelagic facies and often forms monospecific shell concentrations (Kobayashi and Tokuyama, 1959; Fu¨rsich and Wendt, 1977; Chen et al., 1992; Aberhan, 1994; Etter and Tang, 2002). However, P. wengensis Wissmann exceptionally has been reported from inner shelf to basin-floor deposits in the Yuqing and Leidapo members of the Qingyan Formation (Komatsu et al., submitted for publication). 6.2.2. Modes of occurrence In the basin-floor mudstone (facies 4), this assemblage occurs in two different modes: the first type is represented by shell-supported, lenticular (a few centimeters wide) concentrations or bedding, whereas in the second type, shells occur sporadically in a sediment layer (Figs. 8 and 9). In both modes of occurrence, articulated valves are more abundant than disarticulated ones, and they commonly preserved in an open position (butterfly position). There are no signs of abrasion or breakage, even though the valves are extremely thin. These occurrences suggest that the Posidonia assemblage is parautochthonous. Shell concentrations are abundant in laminated mudstones (facies 4) within the basal part of the Leidapo Member (LL-01, 02) and in a few horizons of the middle part of the Leidapo Member (ML-36, 38). In particular, within the basal part (about 5 m) of the Leidapo Member, there are many layers with shell concentrations composed of disarticulated Posidonia valves. Isolated shells are common in the thin mudstones (facies 4) overlying the gravity-flow deposits (facies 1 and 2) of the slope environment.
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Fig. 7. Bivalves from the middle part of the Leidapo Member. (1) Mysidioptera aff. vixcostata (Stoppani), x1, ML-31. (2) Mysidioptera fornicata Bittner, x1, ML-17. (3) Cassianella qingyanensis Chen, Ma and Zhang, x1, ML-17. (4) Gervillaria subelegans (Chen), x1, ML-18. (5) Pteria cf. sturi (Bittner), x1, ML-17. (6) Modiolus paronai Bittner, x1, ML-17. (7) Schafhaeutlia laubei (Bittner), x1, ML-29. (8) Palaeonucula qingyanensis Chen, x1, ML-12. (9) Parallelodon beyrichi (Strombeck), x1, ML-14. (10) Quadratia quadrata Yin, x1, ML-19. (11) Costatoria proharpa multiformis Chen, x3, ML-17. (12 and 13) Elegantinia elegans (Dunker), x1, x3, ML-17. (14) Protostrea sinensis (Hsu), x1.5, ML-17. (15) Enantiostreon difformis (Goldfuss), x2, ML-17. (16) Elegantinia cf. venusta Chen, x5, ML-17. (17) Praechlamys schroeteri (Giebel), x1.5, ML-17. (18) Plagiostoma striatum (Schlotheim), x2, ML-14. (19 and 20) Posidonia sp., x3, ML-38.
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Fig. 8. Detailed columnar sections showing stratigraphic occurrences of bivalves and compositions of assemblages. (A) Basin deposits composed of massive and laminated mudstone containing the Posidonia assemblages (facies 4) and thin debris-flow deposits (facies 2). (B) Debris-flow deposits (facies 2) of slope environments containing the Cassianella – Elegantinia assemblage in the middle part of the Leidapo Member.
6.3. Protostrea– Cassianella assemblage 6.3.1. Composition and life habits This assemblage is mainly composed of Protostrea sinensis and several species of Cassianella, i.e. Cassianella ecki sulcata Chen, Cassianella qingyanensis Chen, Ma and Zhang, Cassianella subcislonensis Hseu and Chen, Cassianella gryphaeatoides Hseu and Chen and Cassianella simplex Chen. These bivalves account for about 50– 75% of the assemblage. The following epifaunal species are subordinate elements of this assemblage: Pteria guizhouensis Chen, Pteria rugosa Chen, Ma and Zhang, Leptochondria paradoxica Chen, Leptochondria subillyrica (Hsui), Entolium sp., Praechlamys schroeteri (Gie-
bel), Enantiostreon difformis (Goldfuss), Mytilus eduliformis praecursor (Frech), Mysidioptera aff. vixcostata (Stoppani), Plagiostoma striatum (Schlotheim) and Lopha sp. Infaunal bivalves are not abundant (Figs. 3, 8 and 9). Protostrea. sinensis (Hseu), Enantiostreon difformis (Goldfuss) and Lopha sp. exhibit distinct attachment areas, which are typical for cemented epifaunal bivalves. Pteria was epibyssate, whereas Leptochondria paradoxica Chen, Praechlamys schroeteri (Giebel), Mysidioptera aff. vixcostata (Stoppani), M. eduliformis praecursor (Frech) and Plagiostoma striatum (Schlotheim) were epibyssate or free-living and usually attached themselves to stationary objects and hard objects with a byssus. Cassianella was also
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Fig. 9. Detailed columnar sections showing stratigraphic occurrences of bivalves and compositions of assemblages. (A) Debris-flow deposits of slope environments containing the Cassianella – Protostrea assemblage. (B) Slope deposits composed of mudstone yielding in situ preserved Palaeonucula qingyanensis and parautochthonous Posidonia sp., and debris-flow deposits containing the allochthonous Cassianella – Protostrea and Cassianella – Elegantinia assemblages.
an epifaunal, free-living suspension feeder (Fu¨rsich and Wendt, 1977). 6.3.2. Modes of occurrence This assemblage is found in turbidites (facies 1), debris flows (facies 2) and slump deposits (facies 3). It forms shell- and matrix-supported shell concentrations that, in addition, contain abundant brachiopods, gastro-
pods, ammonoids and corals. In particular, flat bivalve shells, including Protostrea, Pteria and Praechlamys, are concentrated in the upper parts of these shell beds, lying parallel to one another. The shells are preserved chiefly as disarticulated valves and fragments. They are commonly abraded, and their margins are chipped. This bivalve assemblage is allochthonous. The cemented bivalves Protostrea and Lopha are attached to
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shells and tiny corals. However, these small colonies could have also been transported by gravity flows since some of them were preserved upside down. 6.4. Cassianella – Elegantinia assemblage 6.4.1. Composition and life habits This assemblage is characterized by species of Cassianella and Elegantinia. The genus Cassianella is represented by Cassianella ecki sulcata Chen, Cassianella qingyanensis Chen, Ma and Zhang, Cassianella subcislonensis Hseu and Chen, Cassianella gryphaeatoides Hseu and Chen, and Cassianella simplex Chen, the genus Elegantinia by Elegantinia elegans (Dunker), Elegantinia cf. venusta Chen and Elegantinia kunlunensis Lu and Chen. These two genera account for over 50% of the specimens in this assemblage. The following species are common: Quadratia quadrata Yin, Costatoria proharpa multiformis Chen, Costatoria inaequicostata (Klipstein), Schafhaeutlia astartiformis (Muenster), Schafhaeutlia laubei (Bittner), Protostrea sinensis (Hseu), Pteria guizhouensis Chen, Praechlamys schroeteri (Giebel), Elegantarca subareata (Chen, Ma and Zhang), Lopha sp., Modiolus paronai Bittner, Modiolus dimidiatus Muenster and Gervillaria subelegans (Chen) (Figs. 3, 8, and 9). Infaunal elements make up 25 –50% of this assemblage. Elegantinia, Costatoria, Quadratia and Astarte are shallow-infaunal burrowers. In these species, siphons were probably short or lacking because their pallial lines are not sinuate. The deep burrower Pachymya is rarely encountered, Modiolus paronai Bittner, Modiolus dimidiatus Muenster and Elegantarca subareata (Chen, Ma and Zhang) were semi-infaunal (endobyssate) suspension feeders and Gervillaria subelegans (Chen) was epibyssate or a recliner. 6.4.2. Modes of occurrence The shells are commonly preserved randomly as disarticulated valves and fragments in shell concentrations within turbidites (facies 1), debris-flow deposits (facies 2) and slump deposits (facies 3). Hence, this assemblage is typically allochthonous. Occurrences and preservation of most flat epifaunal bivalves are similar to those in the Protostrea – Cassianella assemblage, and they are preserved as
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disarticulated valves and fragments in the upper parts of the shell beds. In addition, articulated infaunal bivalves are found mainly in the lower part of the shell beds in gravityflow deposits, although they do not retain their life orientations. In turbidites, articulated valves are not abundant, and shell ornamentation and growth lines are not occasionally preserved, owing to abrasion; moreover, the shell margins of disarticulated valves commonly are chipped. In contrast, tightly conjoined valves are relatively common in debris-flow deposits. Geopetal fabrics are abundantly present inside of them, though they are not in a position of growth. Shell preservation is moderately good. The commarginal ribs and faint growth lines of Elegantinia elegans (Dunker) are preserved (Fig. 7), and even the fragile anterior and posterior wings of Cassianella are not chipped.
7. Preservation potential of shells in debris-flow deposits Traction currents and repeated reworking generally result in shell abrasion and fragmentation (Driscoll, 1967, 1970; Brett, 1990). Therefore, it is unexpected that shell beds in debris flow deposits often contain fairly well-preserved shell remains. No marks of collisions and abrasions are visible on such shell surfaces, although shells in turbidite and traction deposits are commonly fragmented. Probably this good preservation potential results from the rapid burial and the rheological flow behavior in cohesive debris flows. The rheology of a debris flow is represented by plastic (bingham) behavior (Stow et al., 1996). In this rheological situation, rigid plugs are formed by the interaction of shear stress and strain with decelerating flow velocity, and clasts maintain their relative positions in the flow. As a consequence, sediments containing shell remains are packed together, and the shells do not crush and abrade each other. In addition, it is significant that the grains of the mud matrix hardly damage the shell surfaces. The shells within the allochthonous assemblages of the Leidapo Member were probably transported within cohesive debris flows; hence, they did not undergo current transportation and reworking by current; finally, these unabraded remains were rapidly buried.
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Palaeoenvironments of the middle part of the Leidapo Member have been interpreted to represent shallow-marine conditions around a carbonate platform because abundant shallow-marine fossils are found in this part of the member, and some occasionally are preserved in articulated form. However, these fossils are preserved in gravity-flow deposits accumulated in slope environments; they are typically allochthonous. Probably, the Leidapo Member was formed in deeper water environments.
8. Diversification and palaeoecology of Middle Triassic bivalves The bivalves from the Anisian Leidapo Member are rather similar to those of the Early Carnian Cassian Formation in the southern Alps (Bittner, 1895), at both the genus and species levels. Chen (2003) reported the following Tethyan species from the Leidapo Member: Palaeonucula strigillata (Goldfuss), Palaeoneilo distincta (Bittner), P. oviformis (Eck), Parallelodon beyrichi (Strombeck), Modiolus dimidiatus Muenster, Modiolus paronai Bittner, Modiolus salzstettensis (Hohenstein), Mytilus eduliformis (Schlotheim), Modiolus pygmaeus (Muenster), Protopis joannae (Wagen), Pteria cassiana (Bittner), Pteria caudate (Stoppani), Pteria sturi (Bittner), Pteria bittneri (Woehrmann), Cultriopsis angusta (Goldfuss), Cultriopsis angulata (Muenster), Leptochondria albertii (Goldfuss), Daonella boeckhi Mojsisovics, Entolium subdemissum (Muenster), Praechlamys badiotica (Bittner), Plicatula sessilis Koken, Plicatula filifera Bittner, Enantiostreon difformis (Goldfuss), Enantiostreon spondyloides (Schlotheim), Mysidioptera gremblichii Bittner, Mysidioptera ornata laevigata Bittner, Mysidioptera tenuicostata Bittner, Mysidioptera fornicata Bittner, Palaeolima pichleri (Bittner), Plagiostoma tingi (Fan), Costatoria inaequicostata (Klipstein), Pseudomyoconcha goldfussi Dunker, Pseudomyoconcha maximilianileuchtenbergensis (Klipstein), Schafhaeutlia laubei (Bittner), Schafhaeutlia astartiformis (Muenster), Schafhaeutlia mellingi (Hauer), Myophoricardium lineatum (Woehrmann) and Myophoriopis carinata Bittner. All of these species are also known from the Cassian Formation. The predominant and characteristic genera of the Cassian Formation
(Fu¨rsich and Wendt, 1977) are also abundant in the Leidapo Member, including Cassianella, Palaeonucula, Gervillia, Parallelodon, Pteria, Mysidioptera, Costatoria and Schafhaeutlia. Furthermore, about 100 species of Tethyan bivalves were reported from the Anisian Junzihe Formation, western China (Yang et al., 1983; Chen, 2003) and are also similar to those of the Cassian Formation. Hallam and Wignall (1997) inferred that Claraia, Eumorphotis and Promyalina explosively diversified in the Early Triassic and went extinct by Anisian times and suggested that the bivalve radiation was delayed until the Ladinian or even later. However, McRoberts (2001) showed that bivalves began to diversify slowly during the Anisian, although the time of the highest diversity was the Carnian. Yin (1985) observed that the diversity of pectinaceans at the generic and subgeneric levels clearly increased during the Anisian. Chen (2003) reported a rich benthic bivalve fauna, amounting to more than 100 species (in over 50 genera), from the Anisian Qingyan Formation; up to 25 genera previously were inferred to range only from the Griesbachian to Spathian in China. It seems likely that Tethyan bivalves began to diversify during the Anisian, and that the characteristic and dominant species of the Ladinian and Carnian bivalve faunas originated at that time. From a palaeoecological perspective, Fu¨rsich and Wendt (1977) distinguished several autochthonous fossil associations, as well as allochthonous fossil assemblages, in the Cassian fauna in the southern Alps. They reported that allochthonous elements are relatively dominant in slope and proximal basin-floor environments. Transported allochthonous assemblages also predominate in the slope environments of the Leidapo Member. Since most bivalves in these assemblages of the Leidapo Member belong to the same inner reef genera and species as those in the St. Cassian Basin, the bivalves of the Leidapo Member were probably transported mainly from inner reef regions by turbidity currents and debris flows. Although Palaeonucula in the St. Cassian Basin predominates in shallow, marginal basin and reef environments, an autochthonous assemblage composed of Palaeonucula qingyanensis Chen and Palaeonucula strigillata (Goldfuss) also formed under calm conditions between gravity-flow events on the lower slope environments of Qingyan.
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Posidonia pannonica (Mojsisovics), Posidonia wengensis Wissmann, Posidonia sp., Daonella boeckhi Mojsisovics and Daonella sp. inhabited basin-floor environments, though the habitat of P. wengensis Wissmann is exceptionally wide and covers inner shelf to basin-floor environments (Komatsu et al., submitted for publication). Generally, paper-shell assemblages composed of Posidonia and Daonella are typical for Triassic basin-floor conditions worldwide (Kobayashi and Tokuyama, 1959; Fu¨rsich and Wendt, 1977; Chen et al., 1992; Aberhan, 1994; Etter and Tang, 2002). The mode of life of these bivalves was predominantly epifaunal. Many of these bivalves were pterioids. In particular, Cassianella simplex Chen, Cassianella ecki sulcata Chen and Protostrea sinensis (Hseu) (Family Dimyidae) were quite abundant. Fu¨rsich and Wendt (1977) observed that several species of autochthonous Cassianella are common in some faunal assemblages within basin environments of the Cassian Formation, and that they were characteristic species of muddy-shelf environments in the Middle and Upper Triassic. The infaunal bivalves of the Leidapo Member mainly comprise suspension-feeding shallow burrowers, such as Elegantinia, Costatoria, Quadratia, Neoschizodus (Myophoriidae) and Astarte. The family Myophoriidae commonly occurs in Triassic shallow-marine deposits worldwide. The characteristic Jurassic to Cretaceous genus Astarte is present in Anisian deposits, although it is extremely rare.
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typically allochthonous Cassianella – Elegantinia and Protostrea – Cassianella assemblages. The latter two assemblages are preserved in gravityflow deposits. (3) The Anisian Leidapo Member contains characteristic Early Carnian Tethyan bivalves, which are much alike at the generic level. It is probable that a notable diversification of Middle Triassic bivalves had already begun in the Anisian (early Middle Triassic).
Acknowledgements We would like to express our sincere thanks to the staff of the Nanjing Institute of Geology and Palaeontology, Academia Sinica, for various help and to Prof. Franz T. Fu¨rsich, Universita¨t Wu¨rzburg, for his useful suggestions. We are grateful to the two referees for their critical comments. This work was financially supported by the National Natural Science Foundation (NSFC projects no. 49872006, 40172003), the Ministry of Science and Technology of China (MSTCN project no. G2000077708), the German Research Foundation (DFG travelling funds 446 CHV 111/3/02) and the Postdoctoral Programs of the Nanjing Institute of Geology and Palaeontology, Academia Sinica, which is gratefully acknowledged.
References 9. Conclusions (1) The upper Anisian Leidapo Member consists of marginal basin-floor and slope deposits that accumulated outside of a carbonate platform. It records two transgressions and one regression. (2) Abundant bivalves are mainly found in the middle part of the Leidapo Member. They are divided into four assemblages. The basin mudstone contains a parautochthonous low diversity Posidonia assemblage occasionally containing the articulated paper shells of the genera, Posidonia and Daonella. The slope deposits commonly yield the autochthonous Palaeonucula assemblage, a parautochthonous Posidonia assemblage and the
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(Eds.), Faunal Recovery After the Palaeozoic Three Mass Extinctions. Science Press, Beijing. In press, in Chinese, with English summary. Chen, J., Wang, Y., Wu, Q., Li, L., Zhou, R., Chen, H., 1992. A study of bivalve zonal succession from upper part of Middle Triassic in northwest Guangxi, south China. Acta Palaeontol. Sin. 31, 403 – 422 (Plates 1 – 6, in Chinese, with English abstract). Chen, J., Xu, K., Xu, R., 1998. On some problems of Triassic and Jurassic biogeography in South China. Acta Palaeontol. Sin. 37, 97 – 107 (in Chinese with English abstract). Chen, J., Cao, M., Stiller, F., 2001. Preliminary palaeosynecological analyses on the Upper Anisian (Middle Triassic) Qingyan fauna. Acta Palaeontol. Sin. 40, 262 – 268 (in Chinese with English abstract). Deng, Z., Kong, L., 1984. Middle Triassic corals and sponges from southern Guizhou and eastern Yunnan. Acta Palaeontol. Sin. 23, 489 – 503 (Plates 1 – 3, in Chinese, with English abstract). Driscoll, E.G., 1967. Experimental field study of shell abrasion. J. Sediment. Petrol. 37, 1117 – 1123. Driscoll, E.G., 1970. Selective bivalve shell destruction in marine environments, a field study. J. Sediment. Petrol. 40, 898 – 905. Enos, P., Wei, J., Yan, Y., 1997. Facies distribution and retreat of Middle Triassic platform margin, Guizhou province, south China. Sedimentology 44, 563 – 584. Enos, P., Wei, J., Lehrmann, D.J., 1998. Death in Guizhou – Late Triassic drowning of the Yangtze carbonate platform. Sediment. Geol. 118, 55 – 76. Etter, W., Tang, C., 2002. Posidonia shale: Germany’s Jurassic Marine Park. In: Bottjer, D.J., Etter, W., Hagadorn, J.W., Tang, C. (Eds.), Exceptional Fossil Preservation. Columbia Univ. Press, New York, pp. 265 – 291. Fu¨rsich, F.T., Wendt, J., 1977. Biostratinomy and palaeoecology of the Cassian Formation (Triassic) of the Southern Alps. Palaeogeogr. Palaeoclimatol. Palaeoecol. 22, 257 – 323. Gan, X., Yin, H., 1978. Lamellibranchiata. Palaeontological Atlas of Southwest China. Carboniferous – Quaternary. Geological Publishing House, Beijing, pp. 305 – 393. Guizhou volume, No. 2, in Chinese. Gu, Z., Huang, B., Chen, C., 1976. Fossil Lamellibranchiata of China. Science Press, Beijing. 522 pp., 120 pls., in Chinese. Hallam, A., Wignall, P., 1997. Mass Extinctions and Their Aftermath. Oxford Univ. Press, Oxford. 320 pp. He, Y., 1984. Middle Triassic Foraminifera from central and southern Guizhou, China. Acta Palaeontol. Sin. 23, 420 – 431 (in Chinese with English abstract). Hsu, T., Chen, K., 1943. Revision of the Chingyen Triassic fauna from Kueichou. Bull. Geol. Soc. China 23, 129 – 138. Kobayashi, T., Tokuyama, A., 1959. Daonella in Japan. J. Fac. Sci., Univ. Tokyo, Sect. 2 12, 1 – 26. Komatsu, T., Akasaki, M., Chen, J., Cao, M., Stiller, F., 2003. Benthic fossil assemblages and depositional facies of the Middle Triassic (Anisian) Yuqing Member of the Qingyan Formation, South China. Paleontol. Res. (submitting for publication). Kristan-Tollmann, E., 1983a. Ostracoden aus dem Oberanis von Leidapo bei Guiyang in Su¨dchina. Schr.reihe Erdwiss. Komm. 5, 121 – 176. Kristan-Tollmann, E., 1983b. Foraminiferen aus dem Oberanis von
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Middle Triassic (Anisian) diversified bivalves: depositional environments and bivalve assemblages in the Leidapo Member of the Qingyan Formation, southern China Toshifumi Komatsu a,b,*, Jin-hua Chen b, Mei-zhen Cao b, Frank Stiller c, Hajime Naruse d a Department of Earth Sciences, Kumamoto University, Kumamoto 860-8555, Japan Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China c Institute of Geology and Palaeontology, University of Mu¨nster, D-48149 Mu¨nster, Germany d Department of Earth and Planetary Science, Graduate School of Science, Kyoto University, Kyoto 606, Japan b
Received 17 July 2002; received in revised form 28 October 2003; accepted 23 March 2004
Abstract The Middle Triassic (Anisian) Leidapo Member of the Qingyan Formation, which crops out in southwestern China, consists of basin-floor and slope deposits with abundant benthic fossils and records two transgressions and one regression. The wellpreserved bivalves are grouped into four assemblages. The allochthonous Cassianella – Elegantinia and Protostrea – Cassianella assemblages as well as the autochthonous Palaeonucula assemblage are found in slope deposits. The allochthonous assemblages exhibit several modes of occurrence in sediment gravity-flow deposits. The parautochthonous Posidonia assemblage is preserved in slope and basin deposits. The genus-level compositions of these bivalve assemblages are similar to those from the Early Carnian of Tethyan Europe. It is probable that a significant diversification of bivalves took place in the Anisian. D 2004 Elsevier B.V. All rights reserved. Keywords: Bivalve; China; Depositional environment; Diversification; Gravity-flow deposits; Triassic
1. Introduction The Middle Triassic was an important time for the radiation of bivalve molluscs (Hallam and
* Corresponding author. Department of Earth Sciences, Faculty of Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan. E-mail addresses: [email protected] (T. Komatsu), [email protected] (J. Chen), [email protected] (M. Cao), [email protected] (F. Stiller), [email protected] (H. Naruse). 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.03.005
Wignall, 1997; McRoberts, 2001). Infaunal bivalves, in particular, substantially increased in generic diversity and acquired new niches partly as a result of the development of siphons and mucus feeding (Stanley, 1968). However, the precise time of the beginning of this bivalve diversification is unknown, although Hallam and Wignall (1997) inferred that it probably was delayed until the Ladinian or even later. Furthermore, Middle Triassic bivalve assemblages are somewhat enigmatic because the record of shallow-marine bivalves is relatively poor except
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for the Cassian Formation of the southern Alps (which mainly belongs to the Upper Triassic Carnian) and southwestern China (Bittner, 1895; Fu¨rsich and Wendt, 1977; Yang et al., 1983; Chen et al., 1992). The Anisian Leidapo Member of the Qingyan Formation is exposed in the Qingyan area, about 30 km south of Guiyang (provincial capital), Guizhou Province, China (Fig. 1). It is representative of Middle Triassic invertebrate fossils and sediments in southwestern China (Wei et al., 1996; Enos et al., 1997, 1998; Stiller, 1997; Chen et al., 1998). The Leidapo Member contains abundant, well-preserved macrofossils including bivalves, gastropods, brachiopods, sponges, corals, crinoids, echinoids, scaphopods, bryozoans, cephalopods and microfossils such as ostracods, foraminifers and calcareous algae (Hsu and Chen, 1943; Yang and Xu, 1966; Yin, 1974; Gu et al., 1976; Gan and Yin, 1978; Liao, 1978; Yin and Yochelson, 1983a,b; KristanTollmann, 1983a,b; He, 1984; Deng and Kong, 1984; Stiller, 1997, 1998, 1999, 2001a,b; Chen et al., 2001). The abundant and diverse bivalves are grouped into several autochthonous and allochthonous assemblages according to their composition, preservation and mode of occurrence. Palaeoenvironments of the Leidapo Member, especially its middle part, have been interpreted as shallow-marine, based on the abundance of fossils. However, based on a new, detailed analyses and taphonomic observations, some doubt has been cast on the
previous palaeoenvironmental interpretations of these facies. In the present study, the nature of the Anisian bivalve assemblages, and their habitats and taphonomy, are inferred for each depositional environment. Furthermore, relative sea-level fluctuations and the palaeoecology of the Middle Triassic bivalves are discussed.
2. Geologic setting The Middle Triassic rocks exposed in the vicinity of Qingyan, consisting of interbedded limestones, mudstones and siltstones (Fig. 2), attain a total thickness of about 1250 m. Usually, the rocks are divided into two formations, the Longtou Formation (Ladinian) and the Qingyan Formation (Anisian). Qingyan is the type locality of the latter formation. The Qingyan Formation is composed of limestones and mudstones and is about 800 m thick. In ascending order, it is subdivided into the Xiaoshan, Mafengpo, Yingshangpo, Leidapo and Yuqing members. According to palaeontological data, the latter two members are assigned to the upper Anisian, whereas the other three are regarded as lower Anisian in age (Stiller, 1997, 2001a; Chen et al., 1998). The Leidapo Member discussed in this paper, occupying the lower part of the upper Anisian, is about 200 m thick and consists of mudstones, shales and marls. This member is characterized by fossiliferous mudstones and marls (Fig. 3). It is grouped
Fig. 1. Location map of the study area.
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Fig. 2. The Middle Triassic Anisian facies distribution in central Guizhou province (modified from Enos et al., 1997). Study area is situated in the northern end of the Nanpanjiang Basin.
into three parts. The lower and upper parts are mainly composed of massive, laminated mudstone with abundant ‘‘paper shelled bivalves,’’ Daonella and Posidonia. The middle part consists of alternating beds of mudstone and marl, rich in fossils including abundant molluscs of Tethyan affinity. Brachiopods, gastropods and bivalves are especially conspicuous, and despite their Anisian age, their faunal aspects are remarkably similar to those described from the Carnian of the Alpine region of Europe (Hsu and Chen, 1943; Yang and Xu, 1966; Yin, 1974; Gu et al., 1976; Gan and Yin, 1978; Yin and Yochelson, 1983a,b). The associated ammonoids contain important taxa referred to the genera Hollandites, Judicarites, Phillippites, Discoptychites, Beyrichites, Bulogites, Paraceratites, Ptychites, Gymnites, Sageceras and Danubites. Especially, Paraceratites binodosus (Hauer) collected from the middle part of the member (ML-20) indicate a Late Anisian age (Liao, 1978; Stiller, 2001a,b). Anisian microfossils, ostracods and foraminifers also have been reported from this member (Kristan-Tollmann, 1983a,b; He, 1984). Anisian conodonts, e.g., Neo-
hindeodella triassica triassica (Mu¨ller), have also been found in the middle part of the Leidapo Member (Stiller, 2001a).
3. Methods We conducted a sedimentological study based on facies analysis in the field and on sectioning some blocks of rocks for detailed observation of sedimentary structures, grain size and fabric in the laboratory. We referred to the facies classifications proposed by Bouma (1962) and Lowe (1982), which mainly deal with gravity-flow deposits. The terminology for depositional environments and sedimentary structures follows Tucker and Wright (1990), Wright and Burchette (1996) and Stow et al. (1996). The Leidapo bivalves were mainly collected from siliciclastic to marly rocks. They are grouped into assemblages based on faunal composition, modes of fossil occurrence and shell preservation. Bulk rock samples were broken up in the laboratory, and all bivalves larger than 1 – 2 mm were counted. Modes of
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occurrence were mainly observed in the field. The life habits of the bivalves were reconstructed by analogy with closely related living taxa and also by reference to previous studies (Fu¨rsich and Wendt, 1977; Aberhan, 1994).
4. Depositional facies and interpretation The Qingyan Formation is present in the northern part of the Middle Triassic Nanpanjiang Basin (or the Dian – Qian – Gui Basin) (Fig. 2), which was filled mainly with carbonate platform, bank and siliciclastic basinal deposits (Wei et al., 1996; Enos et al., 1997; Lehrmann, et al., 1998; Bao, 1998). The shallowwater carbonate platform is called the Yangtze Platform and was a stable palaeogeographic element from the Late Proterozoic to the Triassic (Enos et al., 1998). In the Middle Triassic, the Qingyan area was situated on the northeastern margin of the Nanpanjiang Basin (Enos et al., 1997; Chen et al., 1998). Enos et al. (1997) presented a detailed study of the depositional environments in the lower part of the Qingyan Formation, which is composed of retreating platformmargin deposits characterized by breccia and basinfloor deposits consisting predominantly of marls and mudstones. 4.1. Facies 1 4.1.1. Description Facies 1 consists of shell-supported muddy calcarenite (Figs. 3 –5). The calcarenite contains abundant ripped-up mud clasts (1– 20 cm in diameter), brachiopods, bivalves, gastropods, sponges and corals (Figs. 5.4 and 7). The bivalves are mainly preserved as disarticulated valves and fragments and form the framework of the shell-supported texture (Figs. 6.5 and 6). The sand-size particles are well-rounded bioclasts, not siliciclasts, in origin. These bioclastic calcarenite beds are massive or exhibit normal grading and have well-defined, flat or scoured bases (Figs. 5.2 and 6.6). Thinner beds (5– 10 cm thick) commonly have a sheet-like geometry, whereas thicker beds (average 10– 40 cm thick) fill incisions over 10 m in wide and 5– 30 cm in depth. The top parts rarely exhibit parallel lamination and cross-stratification suggesting south-directed
palaeocurrents. Facies 1 is commonly capped by thin, massive mudstone and parallel-laminated mudstone of Facies 4. 4.1.2. Interpretation Graded and massive beds containing abundant coarse bioclasts are a characteristic product of waning turbidity currents (Bouma, 1962; Walker, 1967; Lowe, 1982) and are similar to the S3 division of Lowe (1982) that is equivalent to Bouma’s (1962) Ta division. Overlying parallel-laminated beds correspond to Bouma’s (1962) Tb division formed under the bed load. Cross-stratified beds reflect deposition by traction currents. The shallowly scoured bases probably formed beneath a powerful turbidity current. 4.2. Facies 2 4.2.1. Description Facies 2 is composed of mudstones characterized by matrix-supported shell concentrations and rip-up clasts and capped by laminated mudstone (Fig. 6.2). The shelly mudstone beds (5 – 30 cm thick) are typically massive and have sharp, flat basal surfaces. They contain abundant shallow-marine brachiopods, bivalves, gastropods, sponges and corals. Ripped-up mud clasts and rounded limestone granules and pebbles are also abundant in these shelly mudstones (Figs. 6.1 – 3). The ripped-up clasts (1 – 20 cm in diameter) are discoidal in shape and are concentrated along with flat shells in the upper and top parts of these mudstone beds (Fig. 6.2). Occasionally, the top of the underlying bed is strongly deformed (Fig. 6.4) and shows minor overturned folds and flame structures. Facies 2 is commonly overlain by Facies 4, which is composed of thin, parallel-laminated and massive mudstone. 4.2.2. Interpretation This facies probably represents cohesive debrisflow deposits, although the beds described herein are very thin compared with typical debris-flow deposits. In the latter, large blocks, mainly composed of rip-up clasts and shell remains, appear to be floating in a mud matrix, which is characteristic of a typical matrix-supported framework (Stow et al., 1996). Outsized ripped-up clasts are commonly
T. Komatsu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 208 (2004) 207–223 Fig. 3. Columnar section of the Leidapo Member, showing the horizons of the samples examined and the stratigraphic occurrences of the bivalve species. LL: localities of the lower part of the Leidapo Member, ML: localities of the middle part of the Leidapo Member. 211
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Fig. 4. Facies codes and depositional environments.
present in the upper parts of debris flows and viscous sediment-flow deposits (Lowe, 1982; Postma et al., 1988). The underlying deformed beds were probably caused by shearing by the debris flow (Fig. 6.4).
4.3. Facies 3 4.3.1. Description Facies 3 consists of heterolithic units that show folds, overturned folds and ball and pillow structures
Fig. 5. (1) Type locality of the Leidapo Member at the Leidapo hill (arrow). (2) Minor channel-fill shell lens of facies 1 at locality ML-25. The basal part shows a convex-down erosional surface (arrows). (3) Vertical section of mudstone containing in situ preserved Palaeonucula qingyanensis at locality ML-12. Scale is 1 cm. (4) Horizontal section of basal part of shell-supported mudstone containing abundant brachiopods at locality ML-31. Scale is 1 cm.
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Fig. 6. Vertical sections of characteristic beds. Scale is 1 cm. (1 and 3) Debris-flow deposits (facies 2) containing rip-up clasts (black arrows), calcareous gravel (white arrow) and bioclasts (white and grey particles); they are characterized by matrix support and mud matrix (locality ML31). (2) Debris-flow deposits (facies 2) overlain by parallel laminations at locality ML-10. (4) Basal part of debris-flow deposits (facies 2) at locality ML-08. Underlying laminated mudstone is deformed by shearing by the debris flow. (5) Shell-supported massive shell bed (facies 1) at locality ML-34. White square materials are disarticulated crinoid remains. (6) Shell-supported shell bed (facies 1) showing grading at locality ML-24. Small burrows are found in underlying mudstone. (7) Slump mudstone (facies 3) at locality ML-17.
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(Fig. 6.7). The latter are found in a portion of the folded structures. The basal portions are usually chaotic. Facies 3 is generally overlain by mudstone of facies 4. These deformed beds are up to 70 cm thick, and their components originally were from facies 1, 2 and 4. 4.3.2. Interpretation Folds and overturned folds imply slope-induced slumping or deformation due to the shear stress of overriding currents (Stow et al., 1996). In the present case, facies 3 probably was not formed by the latter process but by slumping because this facies is generally covered by undeformed mudstone deposited from suspension and weakly bioturbated mudstone deposited under calm conditions. 4.4. Facies 4 4.4.1. Description Facies 4 is characterized by massive and laminated mudstone that rarely exhibits weak bioturbation and that yields a few bivalve taxa, such as Posidonia, Daonella and Palaeonucula. This weakly bioturbated mudstone lacks large burrows, but contains tiny Planolites-like burrows. Palaeonucula is sometimes preserved in life position. Posidonia is frequently preserved as articulated valves. In general, thin mudstone beds, 5 –60 cm thick, cover beds of facies 1 and 2 in the middle part of the Leidapo Member. The lower and upper parts of the Leidapo Member are composed of thick, massive mudstone (exceeding 20 m). This thick mudstone contains thin marl beds, thin calcarenite layers (about 0.5 cm thick), parallel-laminated siltstones and mudstones. Occasionally, parallel laminations are intercalated with abundant shell pavements composed of the small paper shelled bivalves, Posidonia and Daonella. 4.4.2. Interpretation Facies 4 was probably formed during the latest stage of turbidity currents and partly during the calm conditions after the passage of the turbidity currents. The parallel-laminated siltstones and mudstones are probably comparable to the Td interval of Bouma’s (1962) turbidite sequence. Generally, Posidonia oc-
curred in deep-sea deposits Triassic to Jurassic time (Kobayashi and Tokuyama, 1959; Fu¨rsich and Wendt, 1977; Chen et al., 1992; Aberhan, 1994; Etter and Tang, 2002). Palaeonucula, Posidonia and Daonella are typical members of oxygen-controlled bivalve assemblages in deep-sea settings (Aberhan, 1994). Planolites-like tiny burrows are also common in poorly oxygenated deep-sea areas (Bromley, 1996). The weakly bioturbated, thick, massive mudstone of facies 4 probably accumulated from suspension on the basin floor.
5. Depositional environments The different depositional facies occurring in the Leidapo Member record a development from basin floor (lower part of the Leidapo Member) to slope (middle part) and finally back to basin floor (upper part). Thick and massive mudstones suggesting a marginal basin-floor environment (facies 4) dominate the lower and upper parts of the Leidapo Member. Typical gravity-flow deposits develop in the middle part of the Leidapo Member (ML-01-30, ML-4344). They consist of turbidites (facies 1), debris flows (facies 2) and slump deposits (facies 3). These deposits are generally overlain by thin mudstones deposited under calm conditions (facies 4), although these mudstones (facies 4) are again overlain by sediment gravity-flow deposits. The dominance of these deposits suggests accumulation at the foot of a basin slope (Walker, 1992; Mullins et al., 1984; Wright and Burchette, 1996). In particular, outside of carbonate platforms, the lower slope facies is chiefly composed of distal turbidites and thin matrix-supported debris-flow deposits (Mullins et al., 1984). It is probable that gravity flows containing shallow-marine biota originated at the carbonate platform, bypassed the upper slope and finally were deposited on the lower slope. The upper part of the slope succession (ML-24-31) is dominated by turbidites (facies 1) characterized by grading and channeling. The channels are floored with coarse calcarenite, shell remains and gravels. These deposits probably filled minor channels on the upper and lower slopes. In addition, the gradient of the slope generally may have been relatively gentle because large-scale channel-fill sequences, thick slump depos-
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its and breccias with large components are absent from the Leidapo Member. The top of the middle part of the Leidapo Member suggests transgression because the member is capped by basin-floor deposits of the upper part of the Leidapo Member (ML-46). The lower part of the Leidapo Member, which is composed of basin-floor deposits, covers slope deposits of the Yingshang Member. These are composed mainly of limestones and breccias with sharp and flat basal surfaces (LL-01). This transgressive succession also suggests a relative sea-level change. Therefore, the Leidapo Member records two transgressions and one regression.
6. Bivalve assemblages Over 100 species of Bivalvia have been collected from the Leidapo Member. They are grouped into four assemblages: the Palaeonucula, Posidonia, Cassianella – Elegantinia and Protostrea – Cassianella assemblages (Figs. 3, 7, 8 and 9). 6.1. Palaeonucula assemblage 6.1.1. Composition and life habits This assemblage is composed of Palaeonucula strigillata (Goldfuss), Palaeonucula qingyanensis Chen and rare Cassianella ecki sulcata Chen. Palaeonucula was an infaunal, deposit-feeding bivalve that is a characteristic element in Mesozoic muddy-shelf and oxygen-controlled assemblages (Aberhan, 1994). 6.1.2. Modes of occurrence These bivalves are found very sporadically in the lower slope mudstones (facies 2 and 4) in the middle part of the Leidapo Member. Almost all Palaeonucula strigillata (Goldfuss) and Palaeonucula qingyanensis Chen are preserved as closed and articulated valves. Some are oriented with their sagittal plane perpendicular to the bedding planes with their posteriordorsal margin aligned upwards. They occur preferentially in the top parts of the beds composed of gravity-flow deposits (Figs. 5.3 and 9). These modes of occurrence probably reflect their living position. These bivalves probably inhabited the lower slope environments. However, Cassianella ecki sulcata Chen was probably trans-
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ported from shallow marine environments. This species is usually preserved in the form of disarticulated valves which indicates an allochthonous occurrence. 6.2. Posidonia assemblage 6.2.1. Composition and life habits This assemblage is dominated by Posidonia pannonica (Mojsisovics) and small Posidonia sp. and further contains rare Posidonia wengensis Wissmann, Daonella boeckhi Mojsisovics and Daonella sp. D. boeckhi Mojsisovics and Daonella sp. are common in the lower part of the Leidapo Member. These paper shelled bivalves lived epifaunally, though their exact modes of life are enigmatic. In general, Posidonia is abundant in hemipelagic and pelagic facies and often forms monospecific shell concentrations (Kobayashi and Tokuyama, 1959; Fu¨rsich and Wendt, 1977; Chen et al., 1992; Aberhan, 1994; Etter and Tang, 2002). However, P. wengensis Wissmann exceptionally has been reported from inner shelf to basin-floor deposits in the Yuqing and Leidapo members of the Qingyan Formation (Komatsu et al., submitted for publication). 6.2.2. Modes of occurrence In the basin-floor mudstone (facies 4), this assemblage occurs in two different modes: the first type is represented by shell-supported, lenticular (a few centimeters wide) concentrations or bedding, whereas in the second type, shells occur sporadically in a sediment layer (Figs. 8 and 9). In both modes of occurrence, articulated valves are more abundant than disarticulated ones, and they commonly preserved in an open position (butterfly position). There are no signs of abrasion or breakage, even though the valves are extremely thin. These occurrences suggest that the Posidonia assemblage is parautochthonous. Shell concentrations are abundant in laminated mudstones (facies 4) within the basal part of the Leidapo Member (LL-01, 02) and in a few horizons of the middle part of the Leidapo Member (ML-36, 38). In particular, within the basal part (about 5 m) of the Leidapo Member, there are many layers with shell concentrations composed of disarticulated Posidonia valves. Isolated shells are common in the thin mudstones (facies 4) overlying the gravity-flow deposits (facies 1 and 2) of the slope environment.
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Fig. 7. Bivalves from the middle part of the Leidapo Member. (1) Mysidioptera aff. vixcostata (Stoppani), x1, ML-31. (2) Mysidioptera fornicata Bittner, x1, ML-17. (3) Cassianella qingyanensis Chen, Ma and Zhang, x1, ML-17. (4) Gervillaria subelegans (Chen), x1, ML-18. (5) Pteria cf. sturi (Bittner), x1, ML-17. (6) Modiolus paronai Bittner, x1, ML-17. (7) Schafhaeutlia laubei (Bittner), x1, ML-29. (8) Palaeonucula qingyanensis Chen, x1, ML-12. (9) Parallelodon beyrichi (Strombeck), x1, ML-14. (10) Quadratia quadrata Yin, x1, ML-19. (11) Costatoria proharpa multiformis Chen, x3, ML-17. (12 and 13) Elegantinia elegans (Dunker), x1, x3, ML-17. (14) Protostrea sinensis (Hsu), x1.5, ML-17. (15) Enantiostreon difformis (Goldfuss), x2, ML-17. (16) Elegantinia cf. venusta Chen, x5, ML-17. (17) Praechlamys schroeteri (Giebel), x1.5, ML-17. (18) Plagiostoma striatum (Schlotheim), x2, ML-14. (19 and 20) Posidonia sp., x3, ML-38.
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Fig. 8. Detailed columnar sections showing stratigraphic occurrences of bivalves and compositions of assemblages. (A) Basin deposits composed of massive and laminated mudstone containing the Posidonia assemblages (facies 4) and thin debris-flow deposits (facies 2). (B) Debris-flow deposits (facies 2) of slope environments containing the Cassianella – Elegantinia assemblage in the middle part of the Leidapo Member.
6.3. Protostrea– Cassianella assemblage 6.3.1. Composition and life habits This assemblage is mainly composed of Protostrea sinensis and several species of Cassianella, i.e. Cassianella ecki sulcata Chen, Cassianella qingyanensis Chen, Ma and Zhang, Cassianella subcislonensis Hseu and Chen, Cassianella gryphaeatoides Hseu and Chen and Cassianella simplex Chen. These bivalves account for about 50– 75% of the assemblage. The following epifaunal species are subordinate elements of this assemblage: Pteria guizhouensis Chen, Pteria rugosa Chen, Ma and Zhang, Leptochondria paradoxica Chen, Leptochondria subillyrica (Hsui), Entolium sp., Praechlamys schroeteri (Gie-
bel), Enantiostreon difformis (Goldfuss), Mytilus eduliformis praecursor (Frech), Mysidioptera aff. vixcostata (Stoppani), Plagiostoma striatum (Schlotheim) and Lopha sp. Infaunal bivalves are not abundant (Figs. 3, 8 and 9). Protostrea. sinensis (Hseu), Enantiostreon difformis (Goldfuss) and Lopha sp. exhibit distinct attachment areas, which are typical for cemented epifaunal bivalves. Pteria was epibyssate, whereas Leptochondria paradoxica Chen, Praechlamys schroeteri (Giebel), Mysidioptera aff. vixcostata (Stoppani), M. eduliformis praecursor (Frech) and Plagiostoma striatum (Schlotheim) were epibyssate or free-living and usually attached themselves to stationary objects and hard objects with a byssus. Cassianella was also
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Fig. 9. Detailed columnar sections showing stratigraphic occurrences of bivalves and compositions of assemblages. (A) Debris-flow deposits of slope environments containing the Cassianella – Protostrea assemblage. (B) Slope deposits composed of mudstone yielding in situ preserved Palaeonucula qingyanensis and parautochthonous Posidonia sp., and debris-flow deposits containing the allochthonous Cassianella – Protostrea and Cassianella – Elegantinia assemblages.
an epifaunal, free-living suspension feeder (Fu¨rsich and Wendt, 1977). 6.3.2. Modes of occurrence This assemblage is found in turbidites (facies 1), debris flows (facies 2) and slump deposits (facies 3). It forms shell- and matrix-supported shell concentrations that, in addition, contain abundant brachiopods, gastro-
pods, ammonoids and corals. In particular, flat bivalve shells, including Protostrea, Pteria and Praechlamys, are concentrated in the upper parts of these shell beds, lying parallel to one another. The shells are preserved chiefly as disarticulated valves and fragments. They are commonly abraded, and their margins are chipped. This bivalve assemblage is allochthonous. The cemented bivalves Protostrea and Lopha are attached to
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shells and tiny corals. However, these small colonies could have also been transported by gravity flows since some of them were preserved upside down. 6.4. Cassianella – Elegantinia assemblage 6.4.1. Composition and life habits This assemblage is characterized by species of Cassianella and Elegantinia. The genus Cassianella is represented by Cassianella ecki sulcata Chen, Cassianella qingyanensis Chen, Ma and Zhang, Cassianella subcislonensis Hseu and Chen, Cassianella gryphaeatoides Hseu and Chen, and Cassianella simplex Chen, the genus Elegantinia by Elegantinia elegans (Dunker), Elegantinia cf. venusta Chen and Elegantinia kunlunensis Lu and Chen. These two genera account for over 50% of the specimens in this assemblage. The following species are common: Quadratia quadrata Yin, Costatoria proharpa multiformis Chen, Costatoria inaequicostata (Klipstein), Schafhaeutlia astartiformis (Muenster), Schafhaeutlia laubei (Bittner), Protostrea sinensis (Hseu), Pteria guizhouensis Chen, Praechlamys schroeteri (Giebel), Elegantarca subareata (Chen, Ma and Zhang), Lopha sp., Modiolus paronai Bittner, Modiolus dimidiatus Muenster and Gervillaria subelegans (Chen) (Figs. 3, 8, and 9). Infaunal elements make up 25 –50% of this assemblage. Elegantinia, Costatoria, Quadratia and Astarte are shallow-infaunal burrowers. In these species, siphons were probably short or lacking because their pallial lines are not sinuate. The deep burrower Pachymya is rarely encountered, Modiolus paronai Bittner, Modiolus dimidiatus Muenster and Elegantarca subareata (Chen, Ma and Zhang) were semi-infaunal (endobyssate) suspension feeders and Gervillaria subelegans (Chen) was epibyssate or a recliner. 6.4.2. Modes of occurrence The shells are commonly preserved randomly as disarticulated valves and fragments in shell concentrations within turbidites (facies 1), debris-flow deposits (facies 2) and slump deposits (facies 3). Hence, this assemblage is typically allochthonous. Occurrences and preservation of most flat epifaunal bivalves are similar to those in the Protostrea – Cassianella assemblage, and they are preserved as
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disarticulated valves and fragments in the upper parts of the shell beds. In addition, articulated infaunal bivalves are found mainly in the lower part of the shell beds in gravityflow deposits, although they do not retain their life orientations. In turbidites, articulated valves are not abundant, and shell ornamentation and growth lines are not occasionally preserved, owing to abrasion; moreover, the shell margins of disarticulated valves commonly are chipped. In contrast, tightly conjoined valves are relatively common in debris-flow deposits. Geopetal fabrics are abundantly present inside of them, though they are not in a position of growth. Shell preservation is moderately good. The commarginal ribs and faint growth lines of Elegantinia elegans (Dunker) are preserved (Fig. 7), and even the fragile anterior and posterior wings of Cassianella are not chipped.
7. Preservation potential of shells in debris-flow deposits Traction currents and repeated reworking generally result in shell abrasion and fragmentation (Driscoll, 1967, 1970; Brett, 1990). Therefore, it is unexpected that shell beds in debris flow deposits often contain fairly well-preserved shell remains. No marks of collisions and abrasions are visible on such shell surfaces, although shells in turbidite and traction deposits are commonly fragmented. Probably this good preservation potential results from the rapid burial and the rheological flow behavior in cohesive debris flows. The rheology of a debris flow is represented by plastic (bingham) behavior (Stow et al., 1996). In this rheological situation, rigid plugs are formed by the interaction of shear stress and strain with decelerating flow velocity, and clasts maintain their relative positions in the flow. As a consequence, sediments containing shell remains are packed together, and the shells do not crush and abrade each other. In addition, it is significant that the grains of the mud matrix hardly damage the shell surfaces. The shells within the allochthonous assemblages of the Leidapo Member were probably transported within cohesive debris flows; hence, they did not undergo current transportation and reworking by current; finally, these unabraded remains were rapidly buried.
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Palaeoenvironments of the middle part of the Leidapo Member have been interpreted to represent shallow-marine conditions around a carbonate platform because abundant shallow-marine fossils are found in this part of the member, and some occasionally are preserved in articulated form. However, these fossils are preserved in gravity-flow deposits accumulated in slope environments; they are typically allochthonous. Probably, the Leidapo Member was formed in deeper water environments.
8. Diversification and palaeoecology of Middle Triassic bivalves The bivalves from the Anisian Leidapo Member are rather similar to those of the Early Carnian Cassian Formation in the southern Alps (Bittner, 1895), at both the genus and species levels. Chen (2003) reported the following Tethyan species from the Leidapo Member: Palaeonucula strigillata (Goldfuss), Palaeoneilo distincta (Bittner), P. oviformis (Eck), Parallelodon beyrichi (Strombeck), Modiolus dimidiatus Muenster, Modiolus paronai Bittner, Modiolus salzstettensis (Hohenstein), Mytilus eduliformis (Schlotheim), Modiolus pygmaeus (Muenster), Protopis joannae (Wagen), Pteria cassiana (Bittner), Pteria caudate (Stoppani), Pteria sturi (Bittner), Pteria bittneri (Woehrmann), Cultriopsis angusta (Goldfuss), Cultriopsis angulata (Muenster), Leptochondria albertii (Goldfuss), Daonella boeckhi Mojsisovics, Entolium subdemissum (Muenster), Praechlamys badiotica (Bittner), Plicatula sessilis Koken, Plicatula filifera Bittner, Enantiostreon difformis (Goldfuss), Enantiostreon spondyloides (Schlotheim), Mysidioptera gremblichii Bittner, Mysidioptera ornata laevigata Bittner, Mysidioptera tenuicostata Bittner, Mysidioptera fornicata Bittner, Palaeolima pichleri (Bittner), Plagiostoma tingi (Fan), Costatoria inaequicostata (Klipstein), Pseudomyoconcha goldfussi Dunker, Pseudomyoconcha maximilianileuchtenbergensis (Klipstein), Schafhaeutlia laubei (Bittner), Schafhaeutlia astartiformis (Muenster), Schafhaeutlia mellingi (Hauer), Myophoricardium lineatum (Woehrmann) and Myophoriopis carinata Bittner. All of these species are also known from the Cassian Formation. The predominant and characteristic genera of the Cassian Formation
(Fu¨rsich and Wendt, 1977) are also abundant in the Leidapo Member, including Cassianella, Palaeonucula, Gervillia, Parallelodon, Pteria, Mysidioptera, Costatoria and Schafhaeutlia. Furthermore, about 100 species of Tethyan bivalves were reported from the Anisian Junzihe Formation, western China (Yang et al., 1983; Chen, 2003) and are also similar to those of the Cassian Formation. Hallam and Wignall (1997) inferred that Claraia, Eumorphotis and Promyalina explosively diversified in the Early Triassic and went extinct by Anisian times and suggested that the bivalve radiation was delayed until the Ladinian or even later. However, McRoberts (2001) showed that bivalves began to diversify slowly during the Anisian, although the time of the highest diversity was the Carnian. Yin (1985) observed that the diversity of pectinaceans at the generic and subgeneric levels clearly increased during the Anisian. Chen (2003) reported a rich benthic bivalve fauna, amounting to more than 100 species (in over 50 genera), from the Anisian Qingyan Formation; up to 25 genera previously were inferred to range only from the Griesbachian to Spathian in China. It seems likely that Tethyan bivalves began to diversify during the Anisian, and that the characteristic and dominant species of the Ladinian and Carnian bivalve faunas originated at that time. From a palaeoecological perspective, Fu¨rsich and Wendt (1977) distinguished several autochthonous fossil associations, as well as allochthonous fossil assemblages, in the Cassian fauna in the southern Alps. They reported that allochthonous elements are relatively dominant in slope and proximal basin-floor environments. Transported allochthonous assemblages also predominate in the slope environments of the Leidapo Member. Since most bivalves in these assemblages of the Leidapo Member belong to the same inner reef genera and species as those in the St. Cassian Basin, the bivalves of the Leidapo Member were probably transported mainly from inner reef regions by turbidity currents and debris flows. Although Palaeonucula in the St. Cassian Basin predominates in shallow, marginal basin and reef environments, an autochthonous assemblage composed of Palaeonucula qingyanensis Chen and Palaeonucula strigillata (Goldfuss) also formed under calm conditions between gravity-flow events on the lower slope environments of Qingyan.
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Posidonia pannonica (Mojsisovics), Posidonia wengensis Wissmann, Posidonia sp., Daonella boeckhi Mojsisovics and Daonella sp. inhabited basin-floor environments, though the habitat of P. wengensis Wissmann is exceptionally wide and covers inner shelf to basin-floor environments (Komatsu et al., submitted for publication). Generally, paper-shell assemblages composed of Posidonia and Daonella are typical for Triassic basin-floor conditions worldwide (Kobayashi and Tokuyama, 1959; Fu¨rsich and Wendt, 1977; Chen et al., 1992; Aberhan, 1994; Etter and Tang, 2002). The mode of life of these bivalves was predominantly epifaunal. Many of these bivalves were pterioids. In particular, Cassianella simplex Chen, Cassianella ecki sulcata Chen and Protostrea sinensis (Hseu) (Family Dimyidae) were quite abundant. Fu¨rsich and Wendt (1977) observed that several species of autochthonous Cassianella are common in some faunal assemblages within basin environments of the Cassian Formation, and that they were characteristic species of muddy-shelf environments in the Middle and Upper Triassic. The infaunal bivalves of the Leidapo Member mainly comprise suspension-feeding shallow burrowers, such as Elegantinia, Costatoria, Quadratia, Neoschizodus (Myophoriidae) and Astarte. The family Myophoriidae commonly occurs in Triassic shallow-marine deposits worldwide. The characteristic Jurassic to Cretaceous genus Astarte is present in Anisian deposits, although it is extremely rare.
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typically allochthonous Cassianella – Elegantinia and Protostrea – Cassianella assemblages. The latter two assemblages are preserved in gravityflow deposits. (3) The Anisian Leidapo Member contains characteristic Early Carnian Tethyan bivalves, which are much alike at the generic level. It is probable that a notable diversification of Middle Triassic bivalves had already begun in the Anisian (early Middle Triassic).
Acknowledgements We would like to express our sincere thanks to the staff of the Nanjing Institute of Geology and Palaeontology, Academia Sinica, for various help and to Prof. Franz T. Fu¨rsich, Universita¨t Wu¨rzburg, for his useful suggestions. We are grateful to the two referees for their critical comments. This work was financially supported by the National Natural Science Foundation (NSFC projects no. 49872006, 40172003), the Ministry of Science and Technology of China (MSTCN project no. G2000077708), the German Research Foundation (DFG travelling funds 446 CHV 111/3/02) and the Postdoctoral Programs of the Nanjing Institute of Geology and Palaeontology, Academia Sinica, which is gratefully acknowledged.
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