Inferred bivalve response to rapid burial in a Pleistocene shallow-marine deposit from New Zealand

PALAEO ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 87-100

Inferred bivalve response to rapid burial in a Pleistocene shallow-marine deposit from New Zealand Yasuo Kondo *

Department of Geology, Faculty of Science, Kochi University, Kochi 780, Japan Received 12 May 1995; accepted 26 April 1996

Abstract

Orientations of 16 species of conjoined bivalves were analysed in relation to the sedimentary history of the middle Pleistocene Upper Castlecliff Shellbed and the overlying Karaka Siltstone in Wanganui Basin, New Zealand. The shellbed represents mid-cycle condensation, and the Karaka Siltstone (highstand systems tract) was deposited by coastal progradation. Most of the shell orientations represent either life orientation or reworked reclining orientation. However, orientations inverted compared to the normal feeding position are not uncommon for infaunal shallowburrowing bivalves, such as Notocallista multistriata, but only in the lower unit of the Karaka Siltstone. This orientation is interpreted as produced by upward migration of bivalves when they were buried rapidly. Such bivalve orientations are evidence for a rapid change in sedimentation rate from the starved condition represented by the midcycle condensed shellbed (Upper Castlecliff Shellbed), across a downlap surface, to the highstand systems tract (Karaka Siltstone) where episodic rapid sedimentation began.

Keywords: rapid burial; obrution; taphonomy; bivalve; pleistocene; New Zealand

I. Introduction

Biostratinomic analysis of infaunal bivalves is a potential sedimentological tool, from which erosional and rapid depositional events can be identified based on the preserved orientations of bivalves having different depths of burial and life orientations ( K o n d o , 1987, 1989; Kitamura, 1992), as well as differing abilities to escape burial by burrowing upwards ( K o n d o , 1990a; K o n d o and Iyoda, 1993). The highly diverse life positions of different infaunal bivalves, from near the sea floor surface down to 0.5 0.7 m below the surface, * Corresponding author. 0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved P I I S0031-0182 (96) 00039-9

along with their ontogenetic change in burrowing depth, enable us to assess quantitatively the substrate disturbance in post-Palaeozoic strata in which bivalves are dominant. Soft-bottom infaunal bivalves have been known to cope with rapid burial by migrating upward to maintain their position relative to the new sea floor (Reineck, 1958; McKnight, 1969; Shulenberger, 1970; Sch~ifer, 1972; Kranz, 1974; Nichols et al., 1978; K o n d o and Iyoda, 1993). In the case where such upward escape was attempted but unsuccessful, the bivalve would remain in the covering sediment in its escape orientation. Some bivalves, such as Solen, are known to escape upward by pushing, keeping roughly the same orientation with the life orientation in their burrow

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K Kondo/Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 8~100

(Kranz, 1974). The majority of bivalves, however, burrow by pulling with their foot, and their escape orientation is usually inverted with respect to their usual erect probing orientation (Kranz, 1974). Erect probing orientation refers to the bivalve's position during the initial stage of burrowing from a particular surface (Stanley, 1970). Normal life orientation may be the same as the erect probing orientation, or slightly different (Stanley, 1970). The reason why bivalves show inverted orientations when buried rapidly can be understood, by considering their mode of locomotion. Locomotion of bivalves is generally achieved by contraction of the foot to act as an anchor, with the result that the animal can only move in an anterior direction. However, inverted orientations during unsuccessful escapes have only rarely been documented in ancient sediments (Kondo, 1989, 1992). Pliocene-Pleistocene shallow marine cyclothems are ideal for documenting such bivalve orientations, because the repetitive nature of the sedimentary facies gives us valuable constraints on environmental reconstruction, and because much information is available on the modern ecology of the fossilized species. One such sedimentary cycle, the upper Castlecliff Shellbed and Karaka Siltstone, and the environmental reconstruction of the entire middle Pleistocene Castlecliff section (Fig. 1) were described in detail by Fleming (1953). Beu and Edwards (1984) suggested a glacio-eustatic origin for the cycle. Recently Carter et al. (1991) and Abbott and Carter (1994) provided revised descriptions and interpretations of the section in the light of sequence stratigraphy. Abbott and Carter (1994) studied the sedimentology and palaeoecology of the sequence in more detail. According to their studies, each cyclothem consists of a basal sandstone (transgressive systems tract), a mid-cycle shellbed and an overlying highstand systems tract mudstone. The depositional environment ranged from intertidal zone to midshelf, and unconformities between the cyclothems represent subaerial erosion during glacial lowstands. It is expected that many bivalve fossils suffered substrate disturbance by both current and wave activities and episodic storm agitation.

On the basis of these recent advances in the interpretation of these well-known marine cyclothems, this paper provides biostratinomic observations of bivalve shell orientations, including descriptions of inverted, probable escape orientations, and discusses the use of bivalve orientation in the interpretation of sedimentary dynamics and depositional environment.

2. Field setting and observati~

The sedimentary cyclothems of the Castlecliff section record fairly complete transgressiveregressive environmental changes originating from middle Pleistocene glacio-eustatic sea-level changes. Only the uppermost units of many highstand systems tracts are missing due to subaerial erosion at the superjacent sequence boundary. Observations in this study were made in the Upper Castlecliff Shellbed and the Karaka Siltstone near the eastern end of the sea coast exposure known as Tainui cliff, at Castlecliff in Wanganui (Fig. 1). These beds, along with the underlying Shakespeare Cliff Sand represent the Cyclothem 10 of Abbott and Carter (1994), which is correlated with Oxygen Isotope Stage 11. The Upper Castlecliff Shellbeds and the Karaka Siltstone have been subdivided into 10 units for the detailed analysis for the present study (Fig. 2). Composition of fossil assemblage in each unit was analysed in terms of life habit group; infauna, semi-infauna, epifauna and deposit-feeder were distinguished for bivalves, together with gastropods. Stratigraphic change in the life habit composition is generally considered to reflect sedimentation rate change. For disarticulated bivalves, concavo-convex shell orientation was counted for each unit, to assess the stratigraphic change in current activity. Shell orientation of a total of 78 individuals of conjoined bivalves was described and interpreted in terms of either life orientation, upward escape orientation or reworked reclining orientation, based on the available ecological information for similar modern species. Also, a more general observation was made for the lower cycles, particularly for the HST beds, from the Shakespeare Cliff Siltstone (Cyclothem

Y. KondolPalaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 87-100

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9) down to the Omapu Shellbed (Cyclothem 5), to confirm whether the observation in Cyclothem 10 equally applies here (see Fig. 3).

3. Depositional environment of the cyclothem As for the other Wanganui cyclothems, the lower unit of fine to medium sandstone, the Shakespeare Cliff Sandstone, is interpreted as a transgressive systems tract (TST). The middle densely fossiliferous mudstone, the Upper Castlecliff Shellbed, rep-

resents a mid-cycle shellbed (MCS), and the overlying mudstone, the Karaka Siltstone, represents a highstand systems tract ( H S T ) (Abbott and Carter, 1994)• The TST contains intertidal and shallow subtidal bivalves, such as Paphies donacina and Dosinia subrosea, which live today on highly to moderately exposed ocean beaches, within a well-sorted fine sandstone showing parallel and low-angle lamination. This sandstone is therefore interpreted as deposited in a very shallow environment off an ocean beach.

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Y. Kondo/Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 87-100

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The TST is overlain by 1.3 m of densely fossiliferous mudstone of the MCS, with a burrowed basal surface. Compositional, and hence environmental changes are recognized within this midcycle shell bed (Fig. 2, right and Fig. 4). Within the shellbed, the epifaunal ratio increases and then decreases upwards with a maximum ratio in Unit 4. This probably represents a stratigraphic change in the degree of sediment-starvation. In accordance

with this, the ratio of concave-up shells increases and then decreases upwards. This indicates that current activity became weak in the middle, and strong again towards the top. Both the fossil composition and shell orientation suggest the sea became deepest in the middle and shallower again towards the top of the MCS. The lowermost unit (Unit 1) contains pristine, conjoined specimens of Gari lineolata, Tucetona

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The upper contact of the MCS with the HST appears transitional. In the uppermost unit (lower part of Unit 6), disarticulated and parallel-embedded shells of Tiostrea are common. The density of molluscan shells gradually decreases upwards, across the MCS-HST contact, passing upward into a much less fossiliferous mudstone. In the HST beds (Units 6-10) showing parallel lamination or bedding, fossils are extremely rare or absent, and fossils occur preferentially in unstratified intervals. Schaubcylindrichnus burrows are common in this fossiliferous interval. The compositional changes suggest an upward-shallowing trend within the HST, from middle to inner shelf to a nearshore environment. In the uppermost unit of the HST (about 5 m above Unit 10), a molluscan fossil assemblage dominated by Tawera spissa was found in wellsorted sandy siltstone (Kondo, unpublished data). The fossil assemblage includes Myadora striata, Gari lineolata, Resania lanceolata and

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laticostata and Oxyperas elongata. The presence of these species indicates inner-shelf sandy mud deposition. The fossil composition changes gradually upward to become dominated by Tiostrea chilensis lutaria (Unit 5). This is a common oyster species in New Zealand living in the intertidal zone down to about 60 m (Beu and Maxwell, 1990). The middle to upper part of the MCS is represented by little-disturbed dense colonies of Tiostrea.

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The sequence boundary with the overlying Mosstown Sand is locally bored by an intertidal bivalve, Barnea similis. This is evidence for subaerial erosion between the deposition of the HST and the Mosstown Sand, and probably the near-beach sediments containing Paphies or Dosinia were eroded from the top of the HST.

RAPIDLY BURIED BIVALVES SHOWING INVERTED ORIENTATION

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Wanganui coast at Castlecliffsection (after Abbott and Carter, 1994). Arrows indicate that a rapid burial event is inferred.

4. Shell orientations of conjoined bivalves Fig. 5 summarises the stratigraphic distribution of preserved life orientations, reworked reclining orientations and escape orientations for each species. Most importantly, the inverted orienta-

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E Kondo/Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 87 100

Fossil Composition

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tions are found only in the K a r a k a Siltstone, especially in the lower part (Units 6, 7, and 8). Fig. 6 illustrates bivalve shell orientations in various situations, taking the shallow-burrowing siphonate suspension-feeding bivalve Ruditapes philippinarum as an example. The normal life orientation of this species is with the posterior up and with the commissure plane upright. When it is buried suddenly, it shows an anterior-upward upright orientation ( K o n d o and Iyoda, 1993), called an inverted erect probing orientation (Kranz, 1974). When it is washed out, and is reclined alive on the sea floor, its horizontal orientation is physically most stable (a reworked reclining orientation). These positions apply to most infaunal siphonate bivalves, including Notocallista multistriata, Nemocardium pulchellum and Dosinia greyi. However, other forms which differ in life orientation and mode of locomotion respond

differently, so the observed orientations need interpretation for each species. As demonstrated by Kranz (1974), the response and the ability to escape from rapid burial differ greatly depending on the life habit of the species. As stated earlier, bivalves burrowing upwards to escape from rapid burial are in inverted orientation, but individuals found in inverted orientation in the strata have not necessarily been buried alive. There is no direct evidence demonstrating this. But, there are some observations supporting this interpretation. (1) Intermediate orientations between upright life orientation and reworked reclining orientation, e.g., a tilted orientation, are rare. This suggests that bioturbation was not an important agent in reorienting bivalve life orientation. (2) Some of the individuals, about one out of five specimens, are not filled with sediment, and

Y. Kondo/Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 87-100

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the valves are tightly closed. This is consistent with the interpretation that they were buried alive. Therefore, in this paper, I interpret inverted bivalve orientation as representing escape orientation during rapid burial. 4.1. Epifauna and serni-infauna Tiostrea chilensis lutaria is the dominant epifaunal bivalve in the MCS. Some of the individuals are found lying parallel to the bedding plane, with their right valves facing upwards in the middle to upper units (Units 4 and 5) of the MCS. This is interpreted as the original life attitude. The disarticulation ratio is generally high, ranging from 71 to 100%. We do not need to interpret this as a result of disturbance, because epifaunal bivalves

are easily disarticulated without disturbance. Rather, this species was probably seldom disturbed, due both to the relatively deep environment and to the relatively large size of the shell. The gregarious mode of life may also have enhanced preservation in life position. Modiolarca impacta is a small endobyssate (probably semi-infaunal) mytilid bivalve, mostly found in probable life position in the lowermost unit (Unit 6) of the HST, with inverted specimens of Notocallista multistriata and Serratina eugonia. Disarticulated valves of this species were found rarely. In general, the epifaunal benthos avoids environments of rapid sedimentation, particularly those characterised by muddy sediments, because they need a constant current to provide suspended

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E Kondo/Palaeogeography, Palaeoclimatolog.v, Palaeoecology 128 (1997) 87 100

Fig. 6. Orientation of the shallow-burrowing suspension-feeding bivalve Ruditapes philippblarum in various burrowing situations. Internal features are shown. Note the inverted orientation during rapid burial.

detritus and they cannot cope with rapid burial (Kranz, 1974). Preservation of epifaunal species in mudstone in life orientation can, therefore, be interpreted in two ways: (1) normal death in their preferred habitat, or (2) an accidental death due to rapid burial. Fully-grown shells of Tiostrea and their gregarious occurrence suggest that the former case applies, except in the uppermost unit of the MCS where substrate disturbances begin. In the case of Modiolarca impacta, rapid burial without a recognizable response is more likely. Specimens of this species are found almost invariably articulated. There are no butterflied specimens (i.e., no bivalves that are still articulated but that have sprung open upon death). This supports the interpretation of rapid burial. However, the fact that Modiolarca impacta occurs only in the lower unit of the K a r a k a Siltstone and not in the shellbed suggests that this species is adapted to frequently disturbed environments. All the brachiopods and most of the epifaunal bivalves, such as Pecten benedictus marwicki and Limatula sp. occur only in the Upper Castlecliff Shellbed. But, Chlamys gernmulata is abundant also in the lower unit of the K a r a k a Siltstone. All the individuals observed are disarticulated, both in the shellbed and in the mudstone.

4.2. Infaunal shallow-burrowing suspension feeders The stratigraphic change in mode of occurrence of Notocallista multistriata is the best example demonstrating the relation of sedimentary history and the response of bivalves to rapid sedimentation. Notocallista is preserved in a reworked, physically stable orientation in the lower unit of the MCS ( Unit 1 ), and in a normal life orientation or more commonly in an inverted orientation in the lower unit of the H S T (Units 6 and 7) (Figs. 5 and 7). It is notable that intermediate orientations, e.g., a tilted orientation, are rare. The preserved life orientation for Notocallista is assumed to be with the long axis almost vertical. No direct information on the life orientation of this species is available. It is, however, safe to assume that this is the normal life orientation of this species, because this is well-known to be the life orientation of other members of the same family, Veneridae. Large adult individuals of Macrocallista nimbosa are known to assume a life orientation with the long axis tilted (Stanley, 1970), but juveniles are oriented more nearly vertically. The preserved life orientation of Notocallista multistriata is therefore similar to juveniles of Macrocallista. Also, a member of the same family, Ruditapes philippinarum, having a similar shell

Y. Kondo/Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 87-100

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morphology, has a life orientation with the long axis almost vertical (Kondo, 1987). The inverted orientation shown by this species was with the long axis almost vertical, but sometimes with the shell venter upwards. The upward changes in the mode of occurrence of Notocallista multistriata suggest an increase in frequency and/or depth of reworking; the ratio of the reworked specimens increases upward, although the number of observations is small. An upward decrease in number of specimens preserved in life orientation and in inverted orientation may result from increased frequency and/or depth of reworking. Average shell length of N. multistriata found in the Karaka Siltstone is about 2 cm, being smaller than the ordinary adult size of the species. This

may be interpreted as the result of frequent disturbance occurred in this environment. Nemocardium pulchellum also occurs conjoined, and inverted, in the lowermost unit of the HST, along with inverted shells of Notocallista. Observation of the same genus (Nemocardium samarangae) in Japan (Kondo, 1987) confirms that individuals of this genus show a similar life orientation to other cardiid bivalves, that is, a posterior-up, upright orientation. Nemocardium pulchellum is a small cardiid bivalve, commonly found on a muddy substrate on the inner shelf (Luckens, 1972). An inverted specimen of Dosina zelandica is found in the lowermost unit of the Karaka Siltstone. Specimens of this species are found in life position or in reclining orientation in the upper

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i4 Kondo/Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 8~100

unit of the MCS. Such different preservational conditions of this species confirms the change in sedimentary dynamics from the sediment-starved MCS to the HST where intermittent rapid sedimentation began. A single specimen of Venericardia purpurata in the upper unit of the MCS was preserved in a vertical posterior-up life orientation. This species is a non-siphonate burrower and must have been buried very shallowly in the substrate. The preservation in life position of this species appears to be fortuitous.

4.3. Moderately to very deep burrowers Four specimens of Diplodonta globus occur in an inverted orientation, that is with the commissure plane vertical and the umbo downward, in the upper unit of the MCS and the lower unit of the HST. The only described life orientation for a related living species is the western Atlantic Diplodonta notata (Stanley, 1970) which lives buried deeply with the umbo downward. The rather unusual inverted orientations of Diplodonta globus may, therefore, be interpreted as normal life orientation. Exactly the same shell orientation is found commonly in the closely related lucinacean species, Cycladicama cummingii in the Pliocene-Pleistocene of Japan (Kondo, unpublished data), supporting this interpretation. There is no direct evidence to determine whether these specimens died normally or were killed by rapid burial without recognizable response. Considering that Diplodonta notata is the slowest burrower among those species studied by Stanley (1970), the latter possibility is the more likely. Four out of five conjoined specimens of Dosinia (Kereia) greyi were found in life orientation in the MCS and the HST. The deep pallial sinus, more than half of the shell length, suggests a deep burrowing habit for this species. On the basis of the established relation between pallial sinus depth and burrowing depth (Kondo, 1987), this bivalve is inferred to burrow to 2 4 times its shell length, that is 5 15 cm for specimens of the common size. The orientation is with the commissure plane vertical, and the umbo pointing laterally or slightly downward. This orientation is similar to those for

most other bivalves belonging to the Dosiniinae, as described by Stanley (1970) and Kondo (1987, 1990b). Four out of four conjoined specimens of Zenatia acinaces were found in life orientation in the lower units of the HST (Units 6, 7 and 8), with the long axis inclined about 40 ° from the horizontal. All the specimens are about 4-5 cm long, being much smaller than the ordinary adult size of the species. Zenatia acinaces is a mactrid bivalve endemic to New Zealand, considered to be adapted to a deep burrowing habit, as inferred from the anteriorly located umbo and the large siphonal and pedal gapes (Beu, 1966). There is no information available on life orientation and depth of burial of this species. The preserved life orientation of Zenatia is thus a first record suggesting the ecology of this animal. In life position, the pallial sinus is aligned roughly vertical, and siphons are, therefore, extended vertically upward. This is reasonable and is a reconstruction consistent with those for other deep burrowing bivalves such as Mya arenaria, Tresus keenae and Panopea japonica recorded from the Pleistocene of Japan (Kondo, 1990b). Three specimens of Panopea zelandica occur in the lower unit of the MCS and one individual was found in the upper unit of the HST. All of them retain their life position. Panopea zelandica is an extremely deep-burrowing suspension-feeder. It is generally described as burrowing at 46 cm below the surface (Morton and Miller, 1968), or down to 0.7 m (Beu and Maxwell, 1990). Preservation in life orientation is fairly common in this genus, apparently because of the extremely deep burrowing habit, and suggests that the depth of sediment reworking during formation of the MCS did not exceed the depth of burial, i.e. several tens of centimetres. No traces of upward migration were recognized.

4.4. Deposit feeders Deposit-feeding bivalves observed include a siphonate deposit feeder, Serratina eugonia, and the labial palp deposit feeders, Neilo australis and Nucula sp. In the lower unit of the HST (Units 6 and 7), Serratina is fairly common together with Notocallista, but only two individual show an

Y. Kondo/Palaeogeography, Palaeoclimatology,Palaeoecology128 (1997) 87-100 inverted orientation among 11 conjoined specimens (18%), a much smaller ratio than for Notocallista (9 out of 19; 47%). This may suggest a higher escape ability of this species than Notocallista. This is reasonable, because deposit feeders generally are better adapted to coping with muddy sediments; they must move about in search of nutritious muddy sediments. The distinction of life orientation from reworked orientation is very difficult for S. eugonia, because the life orientation is inferred to be horizontal with the bent right valve upward; this life orientation cannot be distinguished from the physically stable, reworked orientation. The orientations shown in Fig. 4 are thus purely descriptive; only vertical and horizontal orientations are distinguished, besides the inverted orientation. Most of the specimens showing reclining orientations are probably those preserved in-situ. Only one conjoined specimen of Neilo australis was observed; it was in the probable life orientation, an upright position with its posterior end upward.

5. Observation in the other cycles

Fossil assemblages containing conjoined bivalves displaying shell orientation inverted from the normal life orientation are not unusual examples found only in the Karaka siltstone; they are found to be common in other offshore muddy sediments of lower cycles exposed at the Castlecliff section. For example, in the base of the Shakespeare Cliff Siltstone in Cyclothem 9 (Abbott and Carter, 1994), specimens of Serratina eugonia showing anterior-upward orientation were found. Inside of the conjoined valve remains unfilled with sediment. Both in life orientation and inverted orientation are found for Notoeallista multistriata. Other fossils found in this bed include in-situ Amygdalum striatum and in-situ or reclining individuals of Modiolarca impacta. Inverted orientations of Neilo australis are found near the base of the Lower Westmere Siltstone in Cyclothem 5 (Abbott and Carter, 1994). Here the fossil assemblage consists of Macoma sp., NeiIo australis and Chlamys gemmulata. In Cyclothems 6, 7 and 8 (Abbott and Carter,

97

1994), however, no similar inverted bivalves were found. The lower unit of the HST in these cycles are generally sandier than those in Cyclothems 5, 9 and 10. This may have caused the difference. These observations confirm that unsuccessful escape recognized as inverted orientation of bivalves are not uncommon in the Castlecliff section, and that such rapid burial occurred repeatedly in offshore muddy shelfs during the cyclic sea-level changes in the middle Pleistocene.

6. Discussion

Storm process may be erosional or depositional, depending on the environment and severity of the storm. In beaches or very shallow seas, a large amount of sediment may be removed during storms. This is demonstrated by our common experience of finding coastal erosion after heavy storms. Alternatively, on the middle to lower shelf well below the normal wave base, there may be rapid deposition from suspended muddy sediments after a storm, without significant erosion. Across most of the level bottom between these two extreme environments, substrate erosion during a storm is compensated by subsequent rapid deposition. The sedimentological record of a storm event thus differs greatly depending on the water depth and basin characteristics. Hummocky cross-stratification, which is often emphasized as a characteristic sedimentary structure of storm sedimentation, is in fact only one of the sedimentary structures originating from storm sedimentation. The proximality gradient in storm sedimentation and its taphonomic consequences, has been discussed by Aigner (1982), Seilacher (1982), Seilacher et al. (1985), Brett (1990), and Brett and Seilacher (1991). All these studies are based on observations on Mesozoic and Palaeozoic strata, and information about the effect of storms on burrowing benthos like bivalves is much scarcer than epibenthos like trilobites, brachiopods and crinoids. Kondo (1990a, 1992) reported inverted orientations for the shallow-burrowing siphonate suspension feeding bivalves Callista chinensis, Dosinorbis troscheli and Fulvia mutica, from the shallow marine transgressive sequence of the middle

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Pleistocene Kiyokawa Formation of Japan. The inverted orientation probably represents escaping responses to rapid sedimentation during storms. The observation that only shallow-burrowing siphonate suspension feeding bivalves assume this orientation implies that they have the highest escape potential from rapid burials, as has been confirmed by experimental rapid burial of living bivalves of different life habit groups (Kranz, 1974). Substrate erosion was estimated to be 20 30 cm below the surface, based on the different preservation of life orientation between deep and shallow burrowing bivalves. Extremely deep burrowing bivalves such as Panopea japonica and Tresus keenae are invariably preserved in life position, while the shallow burrowers mostly occur embedded parallel to the bedding, that is, in reworked reclining orientations. The reworked bivalves in the Kiyokawa Formation are commonly conjoined and the shells are pristine. There is no indication that these bivalve remains were repeatedly washed out and transported. Thus I concluded that (1) the shellbeds in the Kiyokawa Formation were formed by alternating erosion and

subsequent deposition of roughly the same thickness; (2) the shellbeds in the Kiyokawa formation were successively deposited, without large-scale erosion exceeding 20-30cm, and even the reworked shells were not washed out repetitively; and (3) the storm reworking did not involve largescale lateral transport (Kondo, 1989, 1990a). The inferred escape orientation described in this paper occurs in deeper water facies (Fig. 8), where storm sedimentation was dominantly depositional, judging from the sediment texture. Preservation of these escaping orientations only in the lowest unit of the HST appears reasonable, because fine sediment blanketing after storms is common in offshore environments, which also suffer less common and less deep subsequent storm erosion. In shallower environments, on the other hand, given that rapid sedimentation occurred and bivalves escaped from the rapid burial, it is unlikely that the escape orientation will be preserved, because of subsequent erosion. Thus, rapid sedimentation and a relatively deeper water depositional environment are the two main factors which act to preserve upward-escaping orientations of bivalves. sea-level

depositional surface after storm X

.ro''o:

normal wavebase storm wavebase

little or no response

unsuccessful common

escape

successful escape common ? ~ ~

Tio. Mod.

Q Not. epifaunal & infaunal s u s p e n s i o n feeder

.]

infaunal si )honate suspension feeder

infaunal si )nonate deposit-feeder

Fig. 8. Diagrammaticrepresentationof inferredsituationwhereinvertedbivalveorientationwas formed.

E Kondo/Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 8~100

Speyer and Brett (1985) and Speyer (1987) have described clustered occurrences of trilobites from the muddy offshore sediments from the Middle Devonian Hamilton Group in western New York. Common enrolled trilobites in laterally continuous beds were interpreted as representing a response to rapid burial, probably below the storm wave base. This is an example of a biotic response to storm disturbance in a similar shelf environment to that at Casltlecliff. Kranz (1974) and Kondo and Iyoda (1993) concluded that rapid burial with muddy sediment was more harmful to suspension-feeding bivalves, and that the ratio of successfully recovered individuals was much smaller when individuals are buried in mud rather than sand. The soupy nature of muddy sediment deposited from suspension may be harmful in two ways; (1) it smothers the gill of buried bivalves and (2) soupy mud is physically difficult to move through for bivalves. In the experiment by Kondo and Iyoda (1993), most individuals of Ruditapesphilippinarum buried with muddy sediment remained near the original sediment water interface, and only some of the individuals showed inverted orientations. This suggests that even specimens preserved in life orientation may have been killed by storm sedimentation. The inverted shell orientations of siphonate shallow-burrowing bivalves in the Karaka Siltstone are thus remarkable evidences of a sedimentation rate change from the almost starved condition shown by the underlying MSC to the HST, where rapid sedimentation results from episodic storm sedimentation. These processes and environmental conditions can be inferred from sedimentary structures and generally sparse macrofossils, but bivalve orientation analysis may be an important tool for confirming that storm sedimentation processes have occurred. In addition to the conventional paleoecologieal information about composition of fossil assemblages within the cycle, this study shows that taphonomic features indicating responses to rapid burial can be useful for sedimentologic and environmental interpretation.

Acknowledgements This study was conducted during my sabbatical leave funded by the Ministry of Education, Science

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and Culture of Japan. I thank Robert Carter of James Cook University of North Queensland, who encouraged me to write this paper and revised an early version of this manuscript. My special thanks are due to Steve Abbott of the University of Tasmania, who allowed me to access unpublished information on the stratigraphy and palaeontology of the Castlecliff section. I am also indebted to Peter Kamp of the University of Waikato for the hospitality during my field work and for giving me general information including stratigraphy of the section. This manuscript was greatly improved by the review by Alan Beu of the Institute of Geological and Nuclear Sciences Ltd. and Carlton Brett of University of Rochester.

References Abbott, S.T. and Carter, R.M., 1994. The sequence architecture of mid-Pleistocene (c. 1.1-0.4 Ma) cyclothems from New Zealand: facies development during a period of orbital control on sea-level cyclicity. Spec. Publ. IAS, 19: 367-394. Aigner, T., 1982. Calcareous tempestite: storm-dominated stratification in Upper Muschelkalk Limestone (Middle Trias, SW-Germany). In: G. Einsele and A. Seilacher (Editors), Cyclic and Event Stratification. Springer, Berlin, pp. 180 198. Beu, A.G., 1966. The molluscan genera Lutraria, Resania and Zenatia in New Zealand. Trans. R. Soc. N. Z. Zool., 8: 63 91. Beu, A.G. and Edwards, A.R., 1984. New Zealand Pleistocene and late Pliocene glacio-eustatic cycles. Palaeogeogr. Palaeoclimatol. Palaeoecol., 46:119 142. Beu, A.G. and Maxwell, P.A., 1990. Cenozoic Mollusca of New Zealand. N. Z. Geol. Surv. Palaeontol. Bull., 58, 518 pp. Brett, C.E., 1990, Obrution deposits. In: D.E.G. Briggs and P.R. Crowther (Editors), Palaeobiology, a Synthesis. Blackwell, Oxford, pp. 239-243. Brett, C.E. and Seilacher, A., 1991. Fossil Lagerstfitten: a taphonomic consequence of event sedimentation. In: G. Einsele et al. (Editors), Cycles and Events in Stratigraphy. Springer, Berlin, pp. 283-297. Carter, R.M., Abbott, S.T., Fulthorpe, C.S,, Hayweck, D.W. and Henderson, R.A., 1991. Application and global sea-level and sequence stratigraphic models in southern hemisphere Neogene strata from New Zealand. In: D.I.M. MacDonald (Editor), Sedimentation, Tectonics and Eustacy at Active Margins. Spec. Publ. IAS, 12: 41-65. Fleming, C.A., 1953. The geology of Wanganui Subdivision, Waverley and Wanganui sheet districts (N 137-138). N. Z. Geol. Surv. Bull., 52: 1-362. Kitamura, A., 1992. Time resolution in paleoenvironmental

100

K Kondo/Palaeogeography, Palaeoclimatology, Palaeoecology128 (1997) 87-100

analysis based on the mode of occurrence of bivalve fossils a case study for the middle part of the early Pleistocene Omma Formation, Kanazawa. J. Geol. Soc. Jap., 98: 1031-1039 (in Japanese with English abstract). Kondo, Y., 1987. Burrowing depth of infaunal bivalves--observation of living species and its relation to shell morphology. Trans. Proc. Palaeontol. Soc. Jap. N.S., 148:306 323. Kondo, Y., 1989, A method of analysing mode of occurrence of bivalve fossils-comparison of position and orientation of living and fossil bivalves. Benthos Res., 37:73-82 (in Japanese with English abstract). Kondo, Y., 1990a. Storm sedimentation, bivalve response and taphonomy: an observation from the Pleistocene Shimosa Group, Japan. In: Abstr. 139 Reg. Meet. Palaeontol. Soc. Jap., Mizunami, p. 25 (in Japanese). Kondo, Y., 1990b. Preserved life orientations of soft-bottom infaunal bivalves: documentation of some Quaternary forms from Chiba, Japan. Nat. Hist. Res. (Nat. Hist. Mus. Inst., Chiba), 1:31 42. Kondo, Y., 1992. Taphonomy of bivalves: analysis of the mode of occurrence in some shallow marine deposits in the Pleistocene of Japan. In: 29th Int. Geol. Congr. Abstr., 2, p. 355. Kondo, Y. and lyoda, R., 1993. Response and taphonomy of bivalves to rapid burial: an experimental approach using Ruditapes philippinarum (Veneridae). In: Abstr. 1993 Annu. Meet. Palaeontol. Soc. Jap., Tsukuba, p. 102 (in Japanese). Kranz, P.M., 1974. The anastrophic burial of bivalves and its paleoecological significance. J. Geol., 82: 237-265. Luckens, P.A., 1972. Distribution of Nemocardium (Pratulum) pulchellum (Gray, 1843) (Mollusca: Bivalvia: Cardiidae) in

the New Zealand region. N. Z. Oceanogr. Inst. Rec., 1: 47-63. McKnight, D.G., 1969. A recent, possible catastrophic burial in a marine molluscan community. N. Z. J. Mar. Freshwater Res., 3: 177-179. Morton, J. and Miller, M., 1968. The New Zealand Seashore. Collins, London, 2nd., 638 pp. Nichols, J.A., Rowe, G.T., Cliford, C.H. and Young, R.A., 1978. In situ experiments on the burial of marine invertebrates. J. Sediment. Petrol., 48:419 425. Reineck, H.-E., 1958. Walbau-Gefgge in Abhangigkeit yon Sediment-Umlagerungen. Senkenbergiana Lethaea, 39: 1 24. Sch~fer, W., 1972. Ecology and Paleoecology of Marine Environments. Univ. Chicago Press, 568 pp. Seilacher, A., 1982. General remarks about event deposits. In: G. Einsele and A. Seilacher (Editor), Cyclic and Event Stratification. Springer, Berlin, pp. 16l 174. Seilacher, A., Reif, W.E. and Westphal, F., 1985. Sedimentological, ecological and temporal patterns of fossil Lagerstfitten. Philos. Trans. R. Soc. London B, 311:5 23. Shulenberger, E., 1970. Responses of Gemmagemma (Mollusca: Pelecypoda) to a catastrophic burial. Veliger, 13: 163-170. Speyer, S.E., 1987. Comparative taphonomy and palaeoecology of trilobite Lagerstfitten. Alcheringa, 11:205 232. Speyer, S.E. and Brett, C.E., 1985. Clustered trilobite assemblages in the Middle Devonian Hamilton Group. Lethaia, 18: 85-103. Stanley, S.M., 1970. Relation of shell form to life habit in Bivalvia. Geol. Soc. Am. Mem., 125, 296 pp.