Trace fossils and paleoenvironmental control of ichnofacies in a late Quaternary gravel and loess fan delta complex, New Zealand

Palaeogeography, Palaeoclimatology, Palaeoecology, 81 (1991): 253-279

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Elsevier Science Publishers B.V., Amsterdam

Trace fossils and paleoenvironmental control of ichnofacies in a late Quaternary gravel and loess fan delta complex, New Zealand A. A. Ekdale a and D. W. Lewis b aDepartment of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA bDepartment of Geology, University of Canterbury, Christchurch 1, New Zealand (Received J u n e 8, 1989; revised a n d accepted July 12, 1990)

ABSTRACT Ekdale, A. A. and Lewis, D. W., 1991. Trace fossils and paleoenvironmental control of ichnofacies in a late Quaternary gravel and loess fan delta complex, New Zealand. Palaeogeogr., Palaeoclimatol., Palaeoecol., 81: 253-279. Remarkably well-preserved trace fossil assemblages of a small, Late Quaternary ( > 3000 yr B.P.) fan delta complex are exposed in sea cliffs south of Conway Flat, North Canterbury, New Zealand. Marine trace fossils are common and wellpreserved in the bar, embayment and prodelta facies, but deltaic distributary facies contain no trace fossils at all. The preceding paper in this journal (Lewis and Ekdale, 1991) describes the lithofacies and their paleoenvironmental setting in this fan delta complex. Common trace fossils include the ichnogenera Anconichnus, Arenicolites, Asterosoma, Cylindrichnus, Diplocraterion, Gordia, Helminthoida, Ophiomorpha, Piscichnus, Planolites and Skolithos. One new ichnogenus (Arborichnus), two new ichnospecies (Arborichnus sparsus and Diplocraterion asymmetrium), and three informal varieties of Diplocraterion parallelum (var. lingum, arcum and quadrum) are introduced. Local ichnofacies include (a) low-diversity Anconichnus horizontalis Ichnofacies of densely packed burrows of depositfeeding animals in shallow-marine, fine-grained sediment of bar and distal embayment facies, (b) high-diversity Diplocraterion parallelum var. lingum Ichnofacies of burrows of suspension-feeding animals in interbedded mud, clean sand and gravelly sand of the bar facies, (c) moderate-diversity Diplocraterion parallelum var. quadrum Ichnofacies in interbedded mud and sand of distal embayment facies, and (d) low-diversity Planolites montanus Ichnofacies of deformed burrows in bioturbated mud of the prodelta facies. In addition, sparse animal escape structures (fugichnia) occur locally in interbedded sand and gravel of the bar facies. The trace fossil associations of the Conway Flat fan delta complex are unusual in their excellent state of preservation, and many trace fossils are exposed in full three-dimensional relief. Because of their very young age, they must reflect similar associations of biogenic sedimentary structures in modern deltaic settings that are as yet undescribed. This report appears to be the first documentation of a fully marine trace fossil assemblage in loessial sediments.

Introduction Fan deltas are unusual but widespread in the geologic record, and their component facies exhibit a broad spectrum of primary sedimentary structures and bed forms (Nemec and Steel, 1988). However, biogenic sedimentary structures have been reported in only a very few fan delta complexes, and their treatment generally has been superficial. A major reason for the scarcity (if not total absence) of trace fossils in fan deltas is that 0031-0182/91/$03.50

such systems consist mainly of rapidly deposited gravel that provides an unsuitable substrate for most burrowers and other trace-makers. The preceding paper in this issue describes a fan delta complex in the late Quaternary Hawera Series on the South Island of New Zealand (Lewis and Ekdale, 1991, fig.l). This is a marine fan delta system containing large amounts of very finegrained sediment, and it contains remarkably diverse and well-displayed trace fossil assemblages representing several different ichnofacies. Trace

© 1991 - - Elsevier Science Publishers B.V.

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fossiliferous strata are well-exposed in sea cliffs south of Conway Flat Station, south of the Conway River mouth in north Canterbury. This paper describes the well-preserved trace fossils and documents the relationships between ichnofacies and depositional environments in the Conway Flat fan delta complex.

Depositional setting In the preceding paper, Lewis and Ekdale (1991) described marine sedimentary facies in the Conway Flat fan delta sequence representing distributary, bar, proximal embayment, distal embayment and prodelta depositional environments. Although definitive biostratigraphic information is lacking, a late Quaternary age is inferred for the bulk of the Conway Flat sequence on the basis of stratigraphic position, paleogeographic setting and radiocarbon dating. In situ tree stumps in proximal embayment deposits in the upper part of the section yield radiocarbon dates of 8400-7670 yr B.P. (Ota et al., 1984). These beds underlie, overlie and are laterally equivalent to distal embayment and bar facies containing marine trace fossils. The top of the fan delta sequence is overlain by fluvial gravel, which contain detrital logs in debris flow deposits that have been dated as 3550-3050 yr B.P. (Ota et al., 1984). The base of the Conway Flat cliff section (in prodelta deposits) has not been dated radiometrically. Sediment composition is siliciclastic, and sediment texture ranges from clay-size particles to cobbles. Gravel and sand were derived from the nearby unglaciated Hawkeswood Range, which is composed of Mesozoic metasedimentary rocks of the Torlesse Supergroup. Clay-size sediment in this sequence is composed of very fine sedimentary rock debris of quartz and feldspar; the clay mineral content is minimal. The sediment is loess, which was derived from glaciated mountains farther to the west and carried by strong winds and possibly also major rivers into the coastal environment, where it was burrowed by marine organisms. We recognize our somewhat unconventional usage here of the term "loess", which generally is applied to very fine, light-colored, wind-deposited

A.A. EKDALE AND D. W. LEWIS

sediment that is cohesive enough to form vertical cliffs. The very fine-grained, buff-colored, bioturbated sediment forming the cliffs at Conway Flat is indeed loess, which has been derived from bedrock by glacial erosion and transported into nearshore marine environments largely by wind. This sediment has not been re-transported appreciably in the marine environment, nor has it been diluted substantially with other detrital sediment. The original quartz-feldspar composition and extremely fine texture of the loessial sediment has been maintained. It is a quartz- and feldspar-rich sediment that is much too fine-grained to be called "sand". However, to call it "clay" or "mud" based solely on grain size would be highly misleading, because sedimentologically, it has behaved more like loose sand or silt than like sticky clay or mud. Thus, we refer to the loessial sediment that was blown and washed into the nearshore marine environment in the Conway Flat sequence as "marine loess" in order to differentiate it from clayey mud, which may be texturally the same, but which is mineralogically and hydrodynamically very different. The subaerial delta plain portion of the Conway Flat deltaic complex is not exposed in the cliff section. Distributary facies within the fan deltas comprise coarse gravel in horizontal topset and steeply dipping foreset beds, as well as in channels that were incised into prodelta deposits. No trace fossils were observed in any of the distributary gravel beds. Lateral to the distributary facies of the fan deltas lie the distal embayment facies, composed of reworked loess, and the bar facies, consisting of well sorted sand and fine gravel. Lateral to and overlying the bar facies is the proximal embayment facies, consisting of rhizolith-bearing loess, fluvial debris flows and crevasse splay deposits. No animal trace fossils or body fossils are visible in this facies. Tree stumps (podocarp conifers) occur in situ in shoreline loessial deposits, and detrital logs occur in subaqueous debris flow deposits. None of this wood contains Teredolites (traces made by woodboring mollusks), which would be expected if it had been submerged in the sea. Identifiable marine trace fossils are restricted to distal embayment, bar and prodelta facies, and they are abundant in

TRACEFOR~IL~ANDICFINOFACIEgINLATEQUATERNARYFANDELTACOMPLEX,NEWZEALAND those units. Selective weathering and erosion expose an exceptionally well-preserved assemblage of trace fossils in three-dimensional relief within many beds of the bar facies (Fig.l). The only invertebrate macrofossils in the sequence are gastropods (Zeacolpus symmetricus and Cominella nassoides) and bivalves (Chlamys delicatula and unidentified venerids), all of which are restricted to prodelta mud (Lewis and Ekdale, 1991, fig.2). It is doubtful that any of the trace fossils in this facies were produced by these gastropods or bivalves.

~ff

Trace fossils

Trace fossils are abundant and diverse in the Conway Flat sequence. The most common ichnogenera are Anconichnus, Arborichnus, Arenicolites,

Asterosoma, Cylindrichnus, Diplocraterion, Gordia, Helminthoida, Ophiomorpha, Piscichnus, Planolites and Skolithos (see Appendix). Preservation All the trace fossils are burrows; no surface tracks or trails and no borings in rock or wood

Fig. 1. Three-dimensional preservation of Diplocraterion and other trace fossils in full relief in deposits of alternating marine loessial mud and gravelly sand. A. Cliff face; one-meter stick for scale. B. Closer view of same; 7 cm arrow points to Cylindrichnus

concentricus.

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were recognized. Preservation is generally excellent for viewing but not for collecting, because all the biogenic structures are composed of friable sediment. Our attempts to stabilize specimens for collection by spraying on various fixatives were only marginally successful, and impregnation techniques proved unsuccessful. Many delicate forms, both large and tiny, are preserved in exquisite detail. Full relief exposure occurs in the bar facies, where loess-filled burrows extend down from loessial sediment into underlying layers of sand and sandy gravel. As the coarser sediment is eroded from the cliff face via wind and rain action, the slightly more cohesive, loess-filled burrows stand out in spectacular weathered relief. However, none of the sediment in the Conway Flat sequence is well-cemented, and full-relief trace fossils crumble readily when touched. Cliff erosion proceeds rapidly, and during the six months of our field observations at Conway Flat, substantial destruction of some of the best trace fossil exposures occurred by natural collapse. In the distal embayment and prodelta facies, endichnial trace fossils are displayed by means of color variation, textural contrast and slightly weathered relief. Three-dimensional geometry of burrows can be determined, because the general abundance of specimens allows numerous views of each taxon from various orientations, and also because the sediment can be scraped easily to reveal the complete structure via serial sectioning. However, this mode of preservation likewise does not allow easy collection of specimens.

Biologic affinities Biologic affinities Of most of the trace fossils in the Conway Flat sequence are clear. Because the sequence is late Quaternary in age, it is likely that most if not all of the burrowing organisms represented in the geological record here still survive along the New Zealand coast. "U"-shaped burrows, such as Diplocraterion and Arenicolites, can be constructed by a wide variety of creatures. Morton and Miller (1968) list several macroinvertebrate taxa that create "U"-shaped structures in unconsolidated sediments along New Zealand's modern coast. Most are sand-dwelling

A. A. EKDALE AND D. W. LEWIS

worms, including some polychaete annelids (Axio-

thella quadrimaculata, Abarenicola assimilis affinis and Scolecolepis sp.), sipunculans (Sipunculus mundanus), enteropneusts (Balanoglossus australiensis) and echiurans ( Urechis novaezealandiae). U. novaezealandiae, which exceeds 20 cm in length, and which burrows as deeply as 60 cm below the sediment surface, is a likely candidate for the creator of the large Diplocraterion parallelum var. quadrum. A similar modern echiuran (Echiurus echiurus) inhabits muddy marine environments along the North Sea coast, where it constructs vertical, 30cm-deep, "U"-shaped burrows that commonly exhibit retrusive and/or protrusive spreiten structures (Hertweck, 1970). Simple vertical burrows, such as Skolithos and Cylindrichnus, also can be constructed by various organisms. Many polychaete annelids, such as Notomastus zeylandieus and Timarete anchylochaeta, create vertical shafts in shallow-marine sediments around New Zealand today (Morton and Miller, 1968). Similarly, horizontal pascichnial trails, such as Helminthoida and Planolites, can be created by a number of burrowing taxa, Deposit-feeding worms with an anterior mouth and posterior anus, such as many annelids, echiurans, enteropneusts and holothuroids, are capable of producing feces-filled grazing trails within the sediment. The knobbly-walled burrow, Ophiomorpha nodosa, typically is constructed by certain thalassinidean crustaceans, such as the ghost shrimp, Callianassa major, along the Atlantic coast of North America (Frey et al., 1978), and Upogebia wuhsienweni, along the Yellow Sea coast of Korea (Frey et al., 1987). Callianassafilholi and its close relatives Upogebia hirtifrons and U. danai are common burrowers in nearshore sand environments of New Zealand today (Morton and Miller, 1968), but there are no published records that their burrows possess knobbly walls. Piscichnus ichnosp, almost certainly was produced by the feeding activities of elasmobranch fish (rays), because modern rays have been observed in the process of excavating very similar biogenic structures along the sea coasts of New Zealand and North America today (Howard et al., 1977; Gregory et al., 1979).

TRACE FOSSILS AND ICHNOFACIES IN LATE QUATERNARY FAN DELTA COMPLEX, NEW ZEALAND

The tiniest burrows on the Conway Flat fan delta sequence are Anconichnus, Gordia and the smallest form of Skolithos, each of which has a tunnel diameter less than 2 ram. Some of these structures could have been made by tiny worms that stuffed their burrows with fecal material. Burrowing amphipod crustaceans also are potential candidates, because modern haustoriid amphipods in North America have been observed making burrows that resemble some of the three tiny ichnogenera in the Conway Flat sediments (Howard and Elders, 1970). Haustoriid amphipods are abundant in sediments along New Zealand coastlines today (Morton and Miller, 1968), but details of their burrowing patterns have not been described in the literature. No modern organisms have been observed in the process of making burrows identical to Arborichnus and Asterosoma, and no body fossils are associated with these trace fossils in the Conway Flat sequence. It is possible that the creators of these trace fossils were polychaete annelids (such as Glycera and Nereis) or tellinid bivalves, which sometimes produce sparsely branched burrows in modern sediments (R. G. Bromley, pers. commun., 1990). Several types of common burrowers in modern intertidal sediments of New Zealand are not represented in the Conway Flat sequence. For example, a variety of actinian anemones, brachyuran crabs and bivalves create distinctive burrows in mudflats and sandy beaches along South Island shores today (Morton and Miller, 1968; Jones, 1983). However, nothing resembling their burrows has been recognized in the late Quaternary sequence at Conway Flat. Local ichnofacies

Distinctive trace fossil associations characterize the marine sedimentary facies of the fan delta complex at Conway Flat. Proximal embayment facies contain abundant plant root penetration structures (rhizoliths). Distributary channel facies contain no biogenic structures at all. All other facies of the fan delta complex contain burrows of marine animals. Recurrent associations of trace fossils and ichno-

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fabrics are grouped into four ichnofacies within the Conway Flat depositional system (Table l). These local ichnofacies probably are not unique to this deltaic example. The fact that they occur in a Holocene setting suggests that similar ichnofacies may occur in modern deltaic environments in New Zealand and elsewhere, but modern examples have not been documented.

Anconichnus horizontalis Ichnofacies Anconichnus horizontalis is nearly ubiquitous in marine loess beds. Although it can be seen in direct association with many other ichnogenera present in the Conway Flat sequence, monospecific occurrences of A. horizontalis are distinctive and common enough to warrant designation of a local ichnofacies based on A. horizontalis alone. Sediments in this monospecific (or low-diversity) ichnofacies were thoroughly bioturbated by a dense population of small deposit-feeding organisms, possibly worms or amphipod crustaceans. Diplocraterion parallelum var. quadrum and some other trace fossils occur sparsely in this ichnofacies, but they are much more prominent in other ichnofacies. Diplocraterion parallelum var. lingum Ichnofacies The interbedded marine loess, sand and gravel of the bar facies contain the greatest trace fossil diversity and show the best trace fossil preservation in the Conway Flat delta system. Owing to the unique consistency of the compacted loess, many trace fossils weather out in full relief as exichnia attached to the soles of overlying beds. Diplocraterion parallelum is the most prominent and striking trace fossil in this association. D. parallelum var. lingum dominates, but several other ichnotaxa are common as well, including especially D. parallelum var. arcum. Diplocraterion asymmetrium, Arenicolites, Cylindrichnus concentricus and Skolithos linearis are locally abundant. Arborichnus sparsus, Asterosoma, Gordia and Ophiomorpha nodosa occur sparsely. Anconichnus horizontalis permeates some beds, and often it is seen preferentially occupying the spreite of D. parallelum var. arcum.

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A. A. EKDALEAND D. W. LEWIS

TABLE 1 Trace fossil composition of ichnofacies in the Conway Flat fan delta complex Ichnofacies

A. horizontalis

D. p. lingum

D. p. quadrurn

Anconichnus horizontalis Arborichnus sparsus Arenicolites sp. Asterosoma ichnosp. Cylindrichnus concentricus Diplocraterion asymmetrium Diplocraterion parallelum var. lingum Diplocraterion parallelum var. arcum Diplocraterion parallelum var. quadrum Gordia ichnosp. Helminthoida ichnosp. Ophiomorpha nodosa Piscichnus ichnosp. Planolites montanus Skolithos linearis

A

C S

C

Fugichnia

A S

P. montanus

C

C S A A A

S

C

S

S

S C

S

A

A S

A = abundant; C = c o m m o n ; S = sparse.

This association includes domichnia (dwelling structures of suspension-feeding animals), represented by Arenicolites, Cylindrichnus concentricus,

Diplocraterion asyrnmetrium, Diplocraterion parallelum var. arcum and Skolithos linearis. Also, fodinichnia (feeding burrows of deposit-feeding animals) are represented by Diplocraterion parallelum var. lingum, Anconichnus horizontalis and Asterosoma ichnosp. Relatively high-energy depositional conditions prevailed in the bar environments during deposition of the sand and gravel, but low- to moderate-energy conditions occurred during deposition and bioturbation of the marine loess.

Diplocraterion parallelum var. quadrum Ichnofacies In layered sand and loess beds of the distal embayment facies, there is a moderately diverse trace fossil association dominated by Diplocraterion parallelum var. quadrum. Several other trace fossils, including Asterosoma, Helminthoida and Piscichnus, are common in this ichnofacies as well. Anconichnus occurs throughout, but D. parallelum var. quadrum dominates.

This is a very different, lower diversity ichnofacies than that of the D. parallelum var. lingum association. Primary bedding is generally continuous, but many beds are thoroughly burrowed, and internal stratification is obscured by bioturbation. Diplocraterion parallelum var. quadrum here is the same as that which occurs in the Anconichnus horizontalis and Planolites montanus Ichnofacies, except that in this association it is the most abundant, dominant form. Asterosoma ichnosp, is the same as that in the D.parallelum var. lingum association, except that here it is much more common. The Diplocraterion parallelum var. quadrum association represents burrowing by deposit-feeding animals in a more open or energetic sedimentary environment than the Anconichnus horizontalis association, but less so than the D. parallelum var. lingum association. Asterosoma ichnosp., with its irregularly concentric infill and ragged upper lobes, and D. parallelum var. quadrum with its upward migration of a vertical "U"-tube, both represent burrowing by organisms that spent considerable time in one place exploiting the sediment for food.

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TRACE FOSSILS AND ICHNOFACIES IN LATE QUATERNARY FAN DELTA COMPLEX. NEW ZEALAND

Planolites montanus Ichnofacies Prodelta mud commonly exhibits a totally bioturbated, color-mottled ichnofabric, dominated by Planolites montanus and including other indistinct burrows that were produced in soft or soupy substrates. Burrowing was intense, and numerous generations of burrows are superimposed upon one another. Burrow outlines commonly are indistinct, and burrow margins indicate that compaction and synsedimentary shearing have deformed many of the structures. Individually identifiable burrows cannot be discerned easily, but smeared Planolites is predominant. Helminthoida and Diplocraterion parallelum var. quadrum are locally common. Because the majority of the burrows appear to be simple, unlined, horizontal and subhorizontal tunnels without meniscate fill, they are referred to as Planolites in this paper. It is possible that some of the trace fossils designated as Planolites here actually may be portions of Helminthoida or other pascichnial trails, but soft-sediment deformation and lack of adequate outcrop exposures prevents certain identification. Paleoenvironmental controls of ichnofacies

Trace fossil associations in the Conway Flat fan delta complex demonstrate the response of infaunal benthic communities to numerous aspects of the physical environment (Table 2). The most important of these appear to be salinity, interstitial oxygen, sediment composition and texture, and hydrodynamic energy of the depositional environ-

ment (in terms of rate of sedimentation, frequency of erosional events, and orientation of waves and currents). Substrate stability, water temperature and bathymetry were of lesser importance.

Salinity The problem with interpreting salinity facies from trace fossil assemblages is that so many common ichnotaxa can be found in both marine and non-marine settings (Ekdale, 1989). Several common trace fossils in the Conway Flat sequence, such as Arenicolites, Ophiomorpha and Skolithos, have been reported in a variety of marine and non-marine environments, so their use in recognizing paleosalinity regimes is not straightforward. Some, such as Asterosoma, Diplocraterion and Helminthoida, usually are regarded as clear indicators of a marine environment. No diagnostic freshwater trace fossils were observed. Salinity-related controls on trace fossil distribution in the Conway Flat sequence are not immediately obvious. Gravel units in the distributary facies contain no trace fossils at all, but it is not clear if that is due to a very low salinity, a very high sedimentation rate or very coarse-grained sediment texture. Perhaps all three factors operated to inhibit colonization of the sediment by burrowers. Beds of structureless loess in the proximal embayment facies contain rhizoliths of salt marsh or terrestrial plants, but the root structures are not sufficiently well-preserved to determine which. The fact that these rhizolith-bearing beds lack obvious animal burrows, which typify the rest of

TABLE 2 Paleoenvironmental factors that influencemarine ichnofaciesin the Conway Flat fan delta complex Ichnofacies

A. horizontalis

D. p. lingum

D. p. quadrum

P. montanus

Sediment type Salinity Interstitial oxygen Temperature Hydrodynamic energy Bathymetry Paleoenvironment

loess variable(?) low unknown low shallow bars and distal embayment

loess, sand, gravel marine moderate to high unknown high shallow bars

loess, sand marine low to moderate unknown moderate shallow distal embayment

loess, clay marine low cold water low offshore prodelta

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the loessial sediments in the Conway Flat succession, suggests that brackish or even fresh water has excluded marine burrowers from this facies. Ota et al. (1984) noted that diatoms from the main tree trunk horizon in the proximal embayment facies indicate brackish water. The very low diversity of the Anconichnus horizontalis Ichnofacies, and its occurrence in the distal embayment facies just seaward of the brackishwater proximal embayment facies, possibly indicates that salinity variations have excluded all but the most euryhaline benthos from colonizing the sediment. Because the other three ichnofacies contain a high diversity of trace fossils, many of which have been reported only in marine settings, those ichnofacies appear to represent fully marine conditions. Estcourt (1967) studied the ecology of 19 species of burrowing polychaete annelids in a modern estuary located only 120 km southwest of the Conway Flat Quaternary section. He observed a distinct salinity zonation of these worms, with various species tolerating fresh to brackish to fully marine water. Although the depositional and hydrographic setting of the modern estuary and the Quaternary Conway Flat fan delta complex are quite different from one another, Estcourt's example demonstrates that salinity can be an important influence on trace fossil distribution along the New Zealand coast.

Interstitial oxygen Oxygen content of the interstitial waters is an important influence on the behavior and distribution of infaunal, deposit-feeding organisms. In the Conway Flat sequence, only the monospecific Anconichnus horizontalis association appears to be related to oxygen control, as well as perhaps to salinity. The Conway Flat strata provide no evidence of bottom water anoxia above the sea floor, nor of strongly reducing conditions, because sulfides are generally absent. However, plant debris is common in the distal embayment sediment that contains Anconichnus, indicating insufficient interstitial circulation of pore waters to oxidize all organic material. The abundance of very fine-grained car-

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bonaceous matter and the low oxygen conditions in the substrate are compatible with our interpretation that the distal embayment facies was protected from open marine circulation by bars and spits (Lewis and Ekdale, 1991). Anconichnus also occurs in loessial beds of the bar facies, but in this setting it is concentrated mainly inside the sediment filling Diplocraterion parallelum var. lingum and D.parallelum var. arcum burrows. The Diplocraterion burrow fill may have included feces that contained sufficient unoxidized organic matter to attract the makers of Anconichnus, who not only tolerated low interstitial oxygen levels but moreover thrived on the unoxidized organic matter in the sediment. Domichnial vertical burrows (Arenicolites, Cylindrichnus and Skolithos) indicate that bottom water circulation was sufficient to fully oxygenate the sea floor. Endostratal pascichnia (Helminthoida and Planolites) and fodinichnia (Diplocraterion parallelum var. lingum and Asterosoma) may indicate dysaerobic interstitial conditions, but oxygen still had to be sufficiently high to allow an abundance of diverse deposit feeders to flourish in the sediment at these sites.

Sediment composition and texture Composition and texture of the sediment certainly influenced the composition of trace fossil associations in the loessial sediment of bar, embayment and prodelta facies. The marine loess is a non-cohesive mudstone, which allows organisms that normally are restricted to non-cohesive sand to inhabit a muddy substrate. Sediment texture influenced the distribution of Anconichnus, which occurs only in loessial sediment at Conway Flat (and claystone or micrite in other geologic settings). The tiny animals that created Anconichnus were very sensitive to sediment grain size. They were no larger than sand grains themselves (i.e., generally a millimeter or less in diameter). Although they may have been able to displace sand grains, they preferred to burrow through the finer-grained loessial sediment as they foraged for food in the interstitial environment. The unique texture of marine loess permitted Anconichnus tunnels to remain open without col-

TRACE FOSSILS AND ICHNOFACIES IN LATE QUATERNARY FAN DELTA COMPLEX, NEW ZEALAND

lapsing, which in turn allowed the burrowers to circulate water through their tunnel systems for respiration. The meiofaunal deposit feeders had plentiful organic matter to eat while they occupied their burrows, which were aerated with oxygenladen water brought down from the watersediment interface. Diplocraterion and several other trace fossils, which are abundant in the marine loess deposits of the Conway Flat section, generally are known only in sandy substrates. It is the noncohesive character of clay-poor loessial mud at Conway Flat that has allowed the burrowers to flourish in such a very fine-grained substrate.

Hydrodynamic energy In the bar facies, sedimentation rates and frequency of erosional events profoundly affected the benthos. Trace fossils occur mainly in the marine loess intervals, which in nearly all cases are totally bioturbated and are characterized by very abundant Diplocraterion parallelum var. lingum and D. parallelum var. arcum. These burrows not only permeate the loess beds but also extend down into the tops of intervening sand and gravel layers. Although the Diplocraterion burrows are mostly loess-filled, the distal portions of the burrows obviously had to be emplaced in the sand. Even so, Diplocraterion creators apparently preferred to inhabit the marine loess, and upon reaching the sand at depth in the substrate, did not extend down much further. Other vertical burrows in this association (especially Skolithos) were constructed through the entire thickness of some sand layers but are not observed within the loess beds. This observation suggests that Skolithos progenitors preferred life in a sandy substrate to life in a finer-grained loessial substrate. A possible explanation of these trace fossil occurrences is that the bar facies attest to episodic high-energy sedimentation followed by post-storm settling of loessial sediment, which subsequently was bioturbated thoroughly (see Fig.20). According to this scenario, the major portion of each sand bed was deposited rapidly under high-energy conditions, and the intervening fair-weather phase

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allowed burrowers to colonize the substrate from the top down. The next high-energy depositional event ensued before the burrowing infauna was able to completely bioturbate the previous unit of sediment. An alternative explanation, which seems more likely in an accreting bar environment, is that the marine loess units and the sand units represent different depositional episodes and different endobenthic communities. According to this interpretation, the burrowers inhabiting loessial substrates produced Diplocraterion parallelum var. lingum in great profusion. Burrowers inhabiting the uppermost layers of sand were abundant and diverse, including both suspension-feeding and depositfeeding organisms. Their small open burrows (especially Arborichnus, Arenicolites, Gordia and Diplocraterion asymmetrium) would be filled passively with marine loess deposited under lowenergy conditions. Organisms that created deep, vertical burrows (such as Skolithos) in the sand beds were much less abundant and diverse, and they included mainly suspension feeders. The frequent alternation between deposition of loessial sediment and sandy sediment allowed the three different suites of trace fossils to overlap in the preserved sedimentary record and reflect a complex ichnofabric. No matter which of two depositional interpretations is correct, the ichnogenera in bar facies units at Conway Flat are those that characterize the Diplocraterion parallelum var. lingum association. All of these trace fossils represent vertical burrowing by suspension-feeding and deposit-feeding worms (probably polychaetes, enteropneusts and/or echiurans). Diplocraterion, because of its protrusiveretrusive (up-and-down) nature, is well-known as a trace fossil indicator of episodic sedimentation and erosion. Goldring (1964) demonstrated in Devonian sandstones from England that "Diplocraterion yoyo'" (junior synonym of D. parallelum) contained a retrusive sprete in situations of fairly continuous sediment accumulation and a protrusive spreite in response to a stripping away of sediment ~cover by submarine erosion. In cases where repeated episodes of minor deposition and erosion occurred, Goldring noticed that individual

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Diplocraterion specimens actually contained both retrusive and protrusive spreite segments. The situation in the Conway Flat fan delta system is more complicated than Goldring's (1964) Devonian example, because three different varieties of Diplocraterion parallelum, including both retrusive and protrusive spreiten, occur in the sequence. Enigmatically, Diplocraterion parallelum var. quadrum are mostly retrusive, D. parallelum var. arcum may be retrusive and/or protrusive, and D. parallelum var. lingum are nearly all protrusive. It is likely that the three varieties of Diplocraterion parallelum were produced by different kinds of organisms. The sedimentologic setting and the types of animal behavior represented were not the same for the three varieties of Diplocraterion parallelum, even though the burrows often (although not invariably) occur side-by-side in the same beds. The large, retrusive Diplocraterion parallelum var. quadrum probably was the dwelling of an echiuran worm. This trace fossil is abundant in the distal embayment facies and also occurs sparsely in the prodelta facies. Both depositional environments were situated in low-energy, subtidal settings that apparently received continuous deposition (relative to the distributary and bar facies). The animal maintained an open "U"-shaped burrow, which it shifted gradually upward in order to remain a constant distance beneath the watersediment interface as mud gradually accumulated on the sea floor. Based on the height of the very regular, retrusive spreite ofD. parallelum var. quadrum, the burrower must have occupied its burrow for a long time (i.e., during deposition of 25 cm or more of muddy sediment). The medium-sized Diplocraterion parallelum var. arcum, which occurs mainly in the bar facies, is the most variable of the D. parallelum forms. Its spreite may be retrusive, protrusive or both, and the internal structure of the spreite may be symmetrical and even or asymmetrical and ragged. Usually the spreite is filled with loess, but in some cases it is filled with coarser-grained material piped down from overlying gravel layers, which presumably were deposited immediately following an episode of erosional scour by traction currents that were transporting the gravel. Perhaps D. parallelum var. arcum was the burrow of a suspension feeder that

A. A. EKDALE AND D. W. LEWIS

moved its burrow up and down in response to alternating episodes of sedimentation and erosion in the bar facies, in the manner described by Goldring (1964) for the "Diplocraterion yoyo" in the Devonian sandstones of England. An alternative hypothesis of deposit feeding and/or microbial gardening on the burrow walls is possible but seems less likely. The small, protrusive Diplocraterion parallelum var. lingum is restricted to loessial sediment of the bar facies. Generally they are filled with clean loess that is no different from the surrounding matrix sediment, but the bottom part of the burrow commonly extends down into laminated sand layers. These structures apparently are fodinichnial trace fossils produced by the downward extension of burrows during feeding in the sediment. Diplocraterion usually is thought of as a domichnial trace of suspension-feeding organism, but we believe that D. parallelum var. lingum in particular must be the fodinichnial burrow of a subsurface deposit feeder for several reasons. First, the spreiten are always protrusive in an accreting (rather than eroding) sedimentary regime, which suggests that the organism was progressively lengthening its burrow as it worked the sediment for food. Second, they are very densely packed, and the sediment has been totally overturned by D.parallelum var. lingum bioturbation, as might be expected for a contemporaneous population of active deposit feeders. Third, intersection of adjacent burrows is common, which is unlikely for a population of sedentary suspension feeders. Fourth, the internal structure of the spreite usually is irregular and asymmetrical, which is not suggestive of a burrow that has been gradually shifted to keep up with sedimentation and/or erosion. Wave and/or tidal current action influenced the orientation of both D. parallelum var, lingum and 19.parallelum var. arcum, which are preferentially oriented perpendicular to the depositional dip of the delta (see Fig.21). The fact that these oriented burrows occur in bar facies suggests that the direction of waves and/or tidal currents was important to the burrowers, either in terms of irrigating the burrows or transporting a constant food supply to the animals. Similar preferred orientations of densely populated 19iplocraterion were reported in

TRACE FOSSILS AND ICHNOFACIES IN LATE QUATERNARY FAN DELTA COMPLEX, NEW ZEALAND

intertidal and shallow subtidal siliciclastic deposits of late Jurassic age in England by Fiirsich (1975) and of Permian age in South Africa by Mason and Christie (1986).

Substrate stability Fugichnia (animal escape structures) reflect very unstable substrates. They occur in gravelly sand bar deposits but are not widespread. The burrowers apparently could not maintain an open burrow with a load of non-cohesive sand and gravel piled suddenly on top, so they were forced to crawl up and out, allowing the sediment to cave into the site vacated by the escaping organism. A wide variety of invertebrates and vertebrates are known to create escape structures in modern marine settings (Sch~fer, 1972). In the Conway Flat sequence, no body fossils of the escaping animals are preserved, so their identity is unknown. The typical size of the fugichnia is within the range of Diploeraterion parallelum var. areum or D. parallelum var. quadrum, suggesting that they may be escape structures of D.parallelum var. arcum or D. parallelum var. quadrum progenitors.

Temperature Little, if any, published data is available concerning the influence of water temperature on trace fossil distribution. The trace fossil assemblages at Conway Flat give no direct paleotemperature information. However, the molluscan body fossils in the prodelta facies, characterized by the Planolites montanus Ichnofacies, indicate colder water than is present at this locality today, because they include species that typify offshore environments off the southern coast of New Zealand's South Island today. Although it is unwise to generalize on such sparse data, it is worth noting that Planolites today is especially typical of deep-sea pelagic and hemipelagic sediments, which are deposited in very cold water.

Bathymetry There is no clear trace fossil evidence for bathymetric zonation of the various fan delta facies

263

exposed at Conway Flat, because none of the trace fossils in this sequence are particularly reliable as indicators of absolute water depth. Whereas it is possible to infer relative bathymetry within the fan delta system from proximal-distal (i.e., distributary bar-embayment-prodelta) facies relations, the primary controls of trace fossil distribution in the Conway Flat sequence are not related to absolute water depth. The interbedded loessial mud, clean sand and gravelly sand of the delta front facies contain a predominance of vertical domichnial burrows (e.g., Arenieolites, Cylindrichnus, Diplocraterion and Skolithos). Such burrows are especially characteristic of Seilacher's (1964, 1967) universal "Skolithos Ichnofacies", which often is interpreted to represent intertidal conditions. However, the Diplocraterion parallelum var. lingum trace fossil association in the Conway Flat sequence probably does not represent an intertidal setting, because other typical tidal flat features were not observed. The D. parallelum var. lingum Ichnofacies at Conway Flat more likely represents shallow subtidal marine conditions in shoal bars, as described by Lewis and Ekdale (1991). The Diplocraterion parallelum var. quadrum Ichnofacies contains abundant fodinichnia (D. parallelum var. quadrum, Asterosoma and Piseiehnus) and pascichnia (Helminthoida). It apparently represents somewhat quieter conditions in the distal embayment facies, but there is no evidence that water depth was any greater (or shallower) than that represented by the D. parallelum var. lingum Ichnofacies. Bioturbated, fossiliferous mud of the prodelta facies is organic-rich, and macroscopic carbonaceous material occurs throughout. Pascichnial grazing trails (Planolites and Helminthoida) and fodinichnial burrows (Diploeraterion parallelum var. quadrum) are common, and most layers are totally bioturbated. Unabraded shells of coldwater, offshore mollusks occur in localized patches. Thus, trace fossils and body fossils, as well as sediment texture and bedding character, suggest that these muddy units were deposited in a relatively distal marine environment below normal wave base, most likely in an offshore setting. Stratigraphic position distal to the delta front

264

facies suggests that the prodelta sediments were deposited in deeper water than any of the other ichnofacies exposed at Conway Flat. However, no typically deep-water trace fossils, such as complex fodinichnia (e.g., Zoophycos and Chondrites) or agrichnia (e.g., Paleodictyon and Cosmorhaphe), which are known from the Quaternary only in deep-sea deposits, were observed. The shallowest-water facies appear to be the distributary and proximal embayment facies. Seaward and perhaps slightly deeper are the bar and distal embayment facies, but these still represent very shallow subtidal environments. Offshore, the prodelta facies is the deepest of the deposits in the Conway Flat fan delta sequence, but determination of absolute water depths is impossible. Conclusions

Ichnofacies within this fan deltaic system are intimately linked with paleoenvironmental conditions, because salinity, interstitial oxygen, character of the sea floor and energy of the depositional environment directly influenced the types of benthic organisms and their burrowing habits. Paleoenvironmental interpretations based on ichnofacies support and complement the sedimentologic interpretations discussed by Lewis and Ekdale (1991). In the deltaic distributary facies, where current energy was strongest, depositional rates were highest and sediment grain size was greatest, no trace fossils were produced. Marine loess deposits of similar texture and composition occur in four different sedimentary facies, which can be differentiated readily on the basis of their trace fossil associations. The proximal embayment facies is characterized by abundant rhizoliths and no animal burrows. The bar, distal embayment and prodelta facies are characterized by four distinctive ichnofacies. The Anconichnus horizontalis Ichnofacies was influenced primarily by very fine sediment texture, low interstitial oxygen concentrations and variable salinity regimes. This ichnofacies occurs patchily in most of the marine sedimentary environments represented at Conway Flat. The Diplocraterionparallelum var. lingum Ichnofacies, which characterizes the bar facies, was influenced mainly by sediment texture and hydro-

A.A. EKDALE AND D. W. LEWIS

dynamic energy. Some members of the association (especially D. parallelum var. lingum) preferentially occur in marine loess units, which accumulated during quiet-water phases of deposition, and other members (especially Skolithos) preferentially occur in sandy intervals, which were deposited under higher-energy conditions. Wave and/or current action was high enough to provide abundant food for suspension-feeding organisms, but not too high to allow many burrowers to withstand frequent sedimentation and erosional events. The Diplocraterion parallelum var. quadrum Ichnofacies, which exemplifies the distal embayment facies, was influenced chiefly by very fine sediment texture, abundant carbonaceous matter and lowerenergy hydrodynamic conditions than in the bar facies. The Planolites montanus Ichnofacies, which typifies the prodelta, apparently was influenced by sediment texture and possibly by deeper (and perhaps colder) water than the other ichnofacies. The fan delta complex at Conway Flat is unusual, if not unique, in that very fine-grained, loessial sediment has been reworked in various marine environments by both biologic and physical processes. The trace fossil associations in this fan delta complex are remarkable in their high diversity for such a sedimentologic setting and their excellent state of preservation. Acknowledgements

This project was partially supported by research grants from National Science Foundation (to A. A. E.) and the University of Canterbury Geology Department (to D. W. L.). We thank William Bull, Malcolm Laird and Gerrit van der Lingen for their helpful discussions in the field, Albert Downing for photography, Lee Leonard for drafting, Carol Wiltshire for sediment analyses, and Richard Bromley and Susan Ekdale for suggesting improvements to the manuscript. Appendix - - Systematic descriptions of trace fossils

Ichnologic terminology used here conforms to that in Ekdale et al. (1984, pp. 301-316), and trace fossil taxonomy is in accord with that in

TRACE FOSSILS AND ICHNOFACIES IN LATE QUATERNARY FAN DELTA COMPLEX, NEW ZEALAND

H/intzschel (1975). One new ichnogenus and two new ichnospecies are described. In addition, we distinguish three different varieties of the ichnospecies, Diplocraterion parallelum, by informal names. All trace fossil specimens occur in the Haweran (late Quaternary) sea cliff located directly south of Conway Flat Station, North Canterbury, South Island, New Zealand. Anconichnus Kern 1978

Anconichnus hor&ontalis Kern 1978 (Fig.2) Description: Highly arcuate, elbow-shaped, fodinichnial burrows, always are densely spaced. Each burrow consists of a tiny (about 0.5 mm in diameter), twisted, U-shaped tunnel, which many be oriented at virtually any angle. Kern (1978) wrote that the " U " typically is oriented sideways so that one of the two limbs of the " U " lies directly above the other, and the connecting tunnel at the distal end of the "U" is situated vertically or subvertically. However, we observed no consistent orientation of Anconichnus in the Conway Flat sequence. Anconichnus is preserved within the distal embayment facies as an endichnial trace (Fig.2A,B). It appears by sharp color contrast (and sometimes also textural contrast) on scraped outcrop surfaces, where dark burrows are surrounded by a light-colored patch of sediment, suggesting that oxidation of organic substances occurred in the sediment immediately adjacent to the burrows. Within the bar facies, especially in the fill of some Diplocraterion parallelum var. arcum specimens, Anconichnus stands out in full relief (Fig.2C). In vertical cross-section, Anconichnus may be confused with Chondrites, but no Chondrites-style branching is discernible. Chondrites is a common trace fossil throughout the geologic record, including the Tertiary of New Zealand, but it could not be recognized anywhere in the Conway Flat sequence. Anconichnus, on the other hand, is the most widespread and abundant trace fossil in the fan delta complex. Arborichnus new ichnogen.

Diagnosis: Vertical trunk-like shaft that splits at acute angles and irregular intervals into multiple, upward-directed branches.

265

Etymology: arbor (Latin, "tree"); iknos (Greek, "trace"). Arborichnus sparsus new ichnosp. (Figs. 3 and 4) Diagnosis: Arborichnus with a small number of gently curved branches; diameter of trunk generally larger than that of branches; diameter of trunk and branches may vary slightly throughout their length. Etymology: sparsus (Latin, "spread out"). Average dimensions: Trunk diameter 2-6mm; branch diameter 2 to 4 mm; entire structure less than 5 cm high. Type section: Hawera Series, late Quaternary, sea cliffs south of Conway Flat Station, North Canterbury, South Island, New Zealand (42°40'S, 173°26'E). Type material: Holotype (fragmented specimen) no. UUIC-263P, University of Utah, Department of Geology and Geophysics. The poorly consolidated nature of the sediment prevented collection of undamaged type specimens. Description: The general aspect of this trace fossil is one of a deciduous tree in winter (devoid of leaves). It can be recognized only when preserved in full relief. A. sparsus is sparsely branched, and the limbs are smoothly arcuate. The attachment points of the limbs to the trunk commonly are constricted. Arenicolites Salter 1857

Arenicolites ichnosp. (Fig.5) Description: Simple "U"-shaped burrow without a spreite. There are two general size classes of Arenicolites: Tunnel diameter of the larger Arenicolites is about 8 mm. The size and shape of the " U " are quite variable, although most specimens possess symmetrical " U " shapes. The smaller, most common form of Arenicolites at Conway Flat has a tunnel about 2mm in diameter. The " U " is about 4 cm wide and 2 cm deep. This form also exhibits a wide variation in the shape of the "U", which is always asymmetrical. Another trace fossil (Diplocraterion asymmetrium) is very similar to the smaller form of Arenicolites, but it possesses an imperfect, incomplete, spreite-like structure attached to one or both of

266

A . A . E K D A L E A N D D. W . L E W I S

.... i. . . .

i%~.~¸¸¸

Fig.2. Anconichnus horizontalis in bar facies. A, B. Endichnial specimens with light-colored halo in distal embayment facies; scale bars equal 1 cm. C. Burrows in full relief in loess-rich mudstone within bar facies; scale bar equals I cm.

the side limbs o f the " U " , indicating that the b u r r o w e r shifted its b u r r o w slightly to the side during occupation. The creators of D. asymmetrium and the smaller f o r m o f Arenicolites m a y have been the same.

Asterosoma yon O t t o 1854 Asterosoma ichnosp. (Fig.6) Description: Straight, vertical b u r r o w with the upper (proximal) end o f the shaft flaring out and dividing into several subhorizontal tunnels that

TRACE FOSSILS AND ICHNOFACIES IN LATE QUATERNARY FAN DELTA COMPLEX, NEW ZEALAND

Arborichnus sparsus

I

lcm

Fig.3. Sketch of idealized morphology of Arborichnus sparsus (new ichnogenus and ichnospecies).

267

radiate from the center. The shaft contains an irregular concentric fill, and the flared end appears crudely lobate. Horizontal sectioning does not reveal a well-developed, radially symmetrical, rosette pattern of regular lobes that would be typical of idiomorphic Asterosoma in the geologic record, according to H/intzschel (1975). It is possible that a few specimens of Skolithos and/or Cylindrichnus recognized at Conway Flat actually are incomplete specimens of Asterosoma, although

Fig.4. Arborichnus sparsus in bar facies. A. Loess-filled specimens on sole of a marine loess bed; 7 cm arrow for scale. B. Loessfilled specimens in full relief; scale bars equal 1 cm.

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A.A. EKDALE AND D. W. LEWIS

and contains a cone-in-cone fill of sediment brought down from above. While Cylindrichnus is not as widespread as Skolithos in the Conway Flat sequence, the two often occur side by side.

Diplocraterion Torell 1870 Diplocraterion parallelum Torell 1870 Description: Vertical "U"-shaped burrows with parallel burrow shafts and a spreite that has developed vertically but not laterally (Fiirsich, 1974).

Diplocraterion parallelum, variety lingum (Figs.7A and 8)

Fig.5. Arenicofites ichnosp, in full relief in bar facies sand bed; scale bar equals 1 cm.

Fig.6. Asterosomaichnosp, in full relief in thin sand bed in distal embaymentfacies; scale bar equals 2 cm. that cannot be asserted definitely where tops of burrows cannot be seen.

Cylindrichnus Howard 1966 Cylindrichnus concentricus Howard 1966 (Fig. 1B) Description: Long, conical, concentrically lined shaft. Cylindrichnus closely resembles Skolithos, because both ichnogenera are unbranched vertical burrows. However, Cylindrichnus always tapers downward, possesses multiple concentric linings,

Etymology: lingua (Latin, "tongue"). Description: Long, rounded, tongue-shaped D. parallelum; nearly always protrusive; oriented subvertically rather than strictly vertically; spreite depth usually greater than spreite width; spreite planar, slightly bent or even gently twisted. Diameter of causative burrow 7 mm; width of protrusive spreite (including causative burrow) 4 cm; depth of protrusive spreite (including causative burrow) greater than 4 cm. D. parallelum var. lingum is the smallest of the three forms at Conway Flat. It has a rounded, tongue-shaped spreite with parallel, subvertical limbs of the causative burrow. In at least half of the observed specimens, the spreiten structure between the limbs is highly irregular and asymmetrical. Almost all of the D. parallelum var. lingum specimens at Conway Flat are entirely protrusive; a very few specimens exhibit a small retrusive portion of a mainly protrusive spreite. D.parallelum var. lingum usually occurs in denser populations (up to 750 individuals per m 2) than the larger Diplocraterion parallelum var. quadrum. Spacing is crowded, and intersection of burrows is common. Type material: Fragmented specimens no. UUIC264P, 265P, 266P, and 267P, University of Utah, Department of Geology and Geophysics. The poorly consolidated nature of the sediment prevented collection of undamaged specimens.

Diplocraterion parallelum, variety arcum (Figs.7B, 9 and 10)

Etymology: arcus (Latin, "bow"). Description: Broad, rounded, bow-shaped D. parallelum; may be protrusive, retrusive or both;

269

TRACE FOSSILS AND ICHNOFACIES IN LATE QUATERNARY FAN DELTA COMPLEX, NEW ZEALAND

D. para/Io/um var. lingum

O. parallelum vat. arcum

~6cm i?.porallelum vat.

quadrum

Fig.7. Sketch of idealized morphology of three distinct varieties of Diplocraterionparallelum. A. D.parallelum var. lingum. B. D.parallelum var. arcum. C. D.parallelum var. quadrum, c=causative burrow; p= protrusive spreite; r= retrusive spreite. Note differences in shape, size and style of spreite development. oriented strictly vertical; spreite width usually greater than spreite depth; spreite planar and straight. Diameter of causative burrow 16mm; width of retrusive spreite 15 cm; depth of retrusive spreit (excluding causative burrow) less than 10 cm. D. parallelum var. arcum is the intermediatesized form at Conway Flat. Although usually retrusive, many specimens have a protrusive spreite, and a few contain both retrusive and protrusive elements in the same specimen. The broadly arcuate, bow-shaped spreite locally appears ragged in vertical cross-section, but the internal structure of the spreite is generally symmetrical. Commonly, but not invariably, the fill of the causative burrow is permeated with Anconichnus horizontalis. Type material: Fragmented specimens no. UUIC268P and 269P, University of Utah, Department of Geology and Geophysics. The poorly consolidated nature of the sediment prevented collection of undamaged specimens. Diplocraterion parallelum, variety quadrum (Figs.7C and 11) Etymology: quadrum (Latin, "square"). Description: Squared-off D. parallelum with a nearly straight horizontal tunnel connecting with the two straight vertical shafts of the causative burrow at right angles; nearly always retrusive; oriented strictly vertical; spreite width and depth approximately equal, yielding a slightly rounded,

quadrate shape; spreite planar and straight. Diameter of causative burrow 12 mm; length of causative burrow 20 cm or more; width of retrusive spreite 20 cm; depth of retrusive spreite (excluding causative burrow) typically 25 cm or more. D. parallelum var. quadrum is the largest form of the ichnospecies at Conway Flat. It is nearly always retrusive with a rounded quadrate spreite that is oriented strictly vertically. Commonly, although not invariably, the spreite and causative burrow contain small (0.5 mm in diameter and 1.5 mm long), chaotically arranged rod-shaped, fecal pellets (Fig. 11B). D. parallelum var. quadrum is very easily confused with Teichichnus rectus (not found at Conway Flat), which has a wall-like, vertical spreite, produced by the upward migration of a nearly straight, horizontal causative burrow. In D. parallelum var. quadrum, however, the causative burrow is a vertical "U" rather than a straight, horizontal tunnel. If the vertical limbs of the Diplocraterion "U" are not seen, it is likely that the trace fossil will be identified as Teichichnus. Type material: The endichnial preservation of this large trace fossil and the poorly consolidated nature of the sediment prevented collection of representative specimens (see Figs.7C and l lA). Discussion: of all three Diplocraterion varieties: Diplocraterion parallelum, as defined by Ffirsich (1974), is the appropriate ichnospecies name for vertical spreiten burrows with parallel limbs. The

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A.A. EKDALE AND D. W. LEWIS

Fig.8. Diplocraterion parallelum vat. lingum in bar facies. A. Vertical, flat spreite; 3 cm arrow for scale. B. Subvertical, slightly twisted spreite; 3.5 cm arrow for scale. C. Subvertical, slightly twisted spreite, which is intersected by a Skolithos shaft; 3 cm arrow for scale. D. Densely packed specimens in full relief on sole of marine loess bed; scale bar equals 4 cm.

"U"-shaped

causative burrow of all three D. parallelum varieties described here was shifted up and/or down in the sediment by the burrower to produce the distinctive Diplocraterion spreite, which can be either protrusive or retrusive or both. The three forms of Diplocraterion parallelum comprise the most prominent and striking trace fossils in the Conway Flat fan delta sequence. All fit within the ichnospecific diagnosis of D. parallelum, as presented by Fiirsich (1974), but they differ significantly and consistently in size, shape and

occurrence. In fact, it is likely that the three forms of D. parallelum represent different taxa of burrowers and possibly even different behavior patterns. The causative burrow of all three forms usually has remained hollow. D. parallelum var. lingum and D. parallelum var. arcum weather out in full relief from the less indurated matrix sediment. Because most are filled with sediment from above, they hang down from the sole of the overlying bed and are exposed in three dimensions within the more friable sand and gravelly sand. D. parallelum

TRACE FOSSILSAND ICHNOFACIESIN LATE QUATERNARYFAN DELTACOMPLEX, NEW ZEALAND

271

Fig.9. Diplocraterion parallelum var. arcum specimens extending down from bioturbated loess bed into laminated sand bed in bar facies; 7 cm arrow for scale.

Fig. 10. Diplocraterion parallelum var. arcum in bar facies. A. Longitudinal view of retrusive spreite in full relief; 3 cm arrow for scale. B. Cross-section of retrusive spreite in full relief; 7 cm arrow for scale.

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A.A. EKDALEAND D, W. LEWIS

Fig. 11. Diplocraterion parallelum var. quadrum in distal embayment facies. A, Longitudinal view of retrusive spreite; scale bar equals 10 cm. B. Closeup of "Teichichnus-like" spreite of D. parallelum var. quadrum, containing tiny fecal pellets; scale bar equals 1 cm.

var. lingum and D. parallelum var. quadrum typically are filled with loessial mud, but D. parallelum var. arcum may be filled with either loessial or gravelly sediment.

Diplocraterion

asymmetrium

new

ichnosp.

(Figs. 12, 13)

Diagnosis: Diplocraterion with a very asymmetrical "U"-tube and an incomplete, asymmetrical spreite that may be protrusive, retrusive or both. Spreite typically is developed laterally as well as vertically.

Etymology: ab symmetria (Latin, "departure from symmetry").

Average dimensions: Diameter of causative burrow 2 mm; width of "U"-tube 4 cm; depth of "U"tube (excluding spreite) 2 cm; width and depth of incomplete spreite quite variable, but never more than a few millimeters. Type section: Hawera Series, Late Quaternary, sea cliff south of Conway Flat Station, North Canterbury, South Island, New Zealand (42°40'S, 173°26'E).

TRACE FOSSILS AND ICHNOFACIES IN LATE QUATERNARY FAN DELTA COMPLEX. NEW ZEALAND

form of Diplocraterion reported here does not fit any currently recognized ichnospecies.

D/plocroferion osymme trium

C

Fig.12. Sketch of idealized morphology of Diplocraterion asymmetrium (new ichnospecies), c= causative burrow; p= protrusive spreite; r= retrusive spreite; l =laterally protrusive spreite.

I

..........

273

!7-

Gordia Emmons 1844 Gordia ichnosp. (Fig.15A) Description: Tiny knotted structure that consists of single sand-filled tunnel (less than 2 mm in diameter), which is tightly and asymmetrically intertwined. Although most Gordia specimens figured in previous literature are more loosely intertwined (H~intzschel, 1975), the form preserved at Conway Flat fits the ichnogeneric diagnosis. The irregular, three-dimensional morphology resembles that of serpulid worm tubes, but this unlined trace fossil is not a calcareous tube, nor is there any evidence that it is the fill of a calcareous tube that has dissolved away. The knotted morphology resembles Lapispecus, which is a boring, but this trace fossil is a burrow emplaced in uncemented sediment. Gordia is an uncommon trace fossil at Conway Flat, where it occurs only in association with Diplocraterion parallelum var. lingum. Helminthoida Schafhiiutl 1851 Helminthoida ichnosp. (Fig. 14) Description: Strictly horizontal, irregularly meandering trail produced by an active deposit feeder grazing within the sediment. Trail geometry is a tight, irregular scribble with abundant crossovers rather than a regular, phobotaxic meander pattern,

Fig.13. Diplocraterionasymmetrium in bar facies. A, B. Specimens in full relief, illustrating variability of the incomplete, asymmetrical spreite; scale bars equal 1 cm.

Type material: No holotype or syntypes designated (see Figs. 13A and B). The extremely delicate fullrelief preservation of this trace fossil and the poorly consolidated nature of the sediment prevented collection of representative type specimens. Discussion: Although some ichnologists may argue that it is unwise and confusing to designate a spreite-forming variant on the basic Arenicolites pattern by a different ichnogenus, the diagnosis of Arenicolites precludes the presence of any type of spreite. A vertical or subvertical "U"-shaped burrow with a spreite clearly fits the diagnosis of Diplocraterion. However, the highly asymmetrical

Fig.14. Helminthoida ichnosp, on horizontal plane in distal embayment facies; scale bar equals 10 cm.

274

as in H. labyrinthica (not present in the Conway Flat sequence). Tunnel diameter is 4-6 mm. Helminthoida ichnosp, occurs as an endichnial trace in mudstone in the distal embayment and prodelta facies. It is visible where horizontal surfaces have been exposed by erosion at the base of the cliff. The burrow is always filled with mud that differs from the surrounding sediment, presumably because it is fecal matter from the sediment-ingesting burrower.

Ophiomorpha Lundgren 1891 Ophiomorpha nodosa Lundgren 1891 (Fig. 15B) Description: Knobbly-walled burrow. Short, straight, unbranched, subvertical Ophiomorpha

Fig.15A. Gordia ichnosp, in full relief in bar facies; scale bar equals 1 cm. B. Ophiomorpha nodosa (incomplete specimen) in full relief in bar facies; 7 cm arrow for scale.

A.A. EKDALE AND D. W. LEWIS

(generally about 2 cm in diameter) occurs rarely in some bar sandstones in association with Diplocraterion parallelum var. lingum. This trace fossil generally is interpreted as the permanent dwelling of infaunal crustaceans, and it is widespread throughout Mesozoic and Cenozoic sedimentary rocks around the world. It is uncommon in the Conway Flat sequence.

Piscichnus Feibel 1987 Piscichnus ichnosp. (Fig. 16) Description: Broad, asymmetrical, conical or cylindrical, round-bottom depression of variable size, ranging from 20 to 40 cm in diameter at the top and up to half a meter deep. In most cases, there is a cone-in-cone fill that has been extensively reburrowed, mainly by animals producing Diplocraterion parallelum var. quadrum. These rather large trace fossils occur in many, but not all, parts of the distal embayment facies. Piscichnus apparently is the feeding trace of rays (elasmobranch fish). Biogenic structures of this type have been described by Gregory et al. (1979) for modern eagle rays along the North Island coast of New Zealand and by Howard et al. (1977) for modern stingrays along the United States Atlantic coast. Howard et al. (1977) and Kamola (1984) also described fossil ray holes in Pleistocene and Cretaceous shallow-marine sequences, but they did not assign in ichnogeneric name to the structures. M. R. Gregory (pers. comm., 1989) has discovered similar ray structures in Miocene deposits of northern New Zealand. Because of their shape and size, we believe that fossil ray holes in the Conway Flat sequence should be designated Piscichnus ichnosp. They do not belong in the only valid ichnospecies of this ichnogenus, P. brownii, which was interpreted by Feibel (1987) as a shallow feeding depression made by non-marine cichlid fish. Because of the lack of adequate three-dimensional views of the Conway Flat ray holes, morphologic details of the structures are not known sufficiently to designate a new ichnospecific name for them in this paper. Planolites Nichoison 1873 Planolites montanus Richter 1937 (Fig. 17) Description: Simple, unbranched, unwalled, sinuous, subhorizontal tunnel, which is filled with

TRACE FOSSILSAND ICHNOFACIESIN LATEQUATERNARYFAN DELTACOMPLEX,NEW ZEALAND

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Fig. 16. Piscichnus ichnosp, in cross-section in distal embayment facies. A. Several specimens in unscraped cliff face; scale bar equals 7 cm. B. Specimen scraped to show subsequent burrowing by Diplocraterion parallelum var. quadrum; scale bar equals 6 cm.

Fig.17. Planolites montanus in vertical section in prodelta mud; scale bar equals 2 cm.

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structureless sediment (possibly fecal matter) that differs from the immediately surrounding sediment. The Conway Flat specimens are preserved as endichnial traces that appear to be quite irregular in size, shape and orientation. They are neither straight nor strictly horizontal, so they are referred to P. montanus, as described by Pemberton and Frey (1982). Skolithos Haldeman 1840 Skolithos linearis Haldeman 1840 (Fig. 18) Description: Simple, unbranched, vertical or subvertical shaft. In the Conway Flat sequence there

A.A. EKDALEAND D. W. LEWIS

is a large size variation among Skolithos specimens, suggesting that several different kinds of animals produced simple vertical burrows. The largest S. linearis (up to 2 cm in diameter and 15 cm long) are straight and nearly vertical. Some specimens possess a slightly flaring upper end, superficially resembling that of Cylindrichnus concentricus. However, the infill of the Skolithos shaft is structureless rather than cone-in-cone, as in Cylindrichnus. The largest Skolithos often possess a sandy wall, which appears to be of diagenetic rather than biogenic origin. The smaller S. linearis exhibit much greater variation in size and orientation than

Fig. 18. Skolithos linearis in laminated sand o f bar facies. A. Several specimens in full relief; 4.5 cm white ruler for scale. B. Specimen dissected to show concentric (probably diagenetic) wall structure; scale bar equals 2 cm.

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Fig.19. Fugichnia (animal escape structures) in laminated sand and gravel of bar facies; scale bar equals 6 cm.

Fig.20. Burrowed units in marine bar facies. Diplocraterion parallelum var. lingum and other vertical burrows extend down from totally bioturbated loess into laminated sand, but they do not penetrate gravel; scale bar equals 10 cm.

the large forms. M o s t are a p p r o x i m a t e l y 1 m m in diameter, and they m a y be vertical, subvertical or irregularly curved.

Fugichnia (escape trace fossils) Interbedded sandstone and c o n g l o m e r a t e in the bar facies contain a small n u m b e r of patchily distributed animal escape structures (fugichnia).

Such structures vary considerably in size and shape. Usually they consist of a roughly cylindrical or conical collapse feature with an infill that m a y be chaotically organized or structureless (Fig.19). In several cases, the structure contains an irregular vertical shaft, positioned at the edge of the collapse feature itself. A l t h o u g h the implication of such a separate shaft is unclear, it is possible that it

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A.A. EKDALEAND D. W. LEWIS N

A D. para/le/ura

var.

arcum

B D. paralle/um

vat.

lingum

Fig.21. Rose diagrams illustrating orientation of Diplocraterion parallelum specimens on a single bedding plane in bar deposits. A. D. parallelum var. arcum (n=27). B. D.parallelum var. lingum (n = 30). s = approximate strike of the modern cliff face; d= approximate depositional dip of the Pleistocene fan delta; N = north. Note that both varieties exhibit a preferred orientation roughly parallel to the inferred paleocoastline.

represents part of a Diplocraterionparallelum var. quadrum whose inhabitant escaped upward from the causative burrow, allowing non-cohesive gravelly sand to collapse. Because of the typically high morphologic variability of fugichnia, they cannot be assigned to any particular ichnotaxon. References Bromley, R. G. and Ekdale, A. A., 1984. Chondrites: a trace fossil indicator of anoxia in sediments. Science, 224: 872-874. Ekdale, A. A., 1985. Paleoecology of the marine endobenthos. Palaeoclimatol., Palaeogeogr., Palaeoecol., 50: 63-81. Ekdale, A. A., 1989. Pitfalls of paleobathymetric interpretation based on trace fossil assemblages. Palaios, 3: 464-472. Ekdale, A. A., Bromley, R. G. and Pemberton, S. G., 1984. Ichnology: trace fossils in sedimentology and stratigraphy. Soc. Econ. Paleontol. Mineral., Tulsa, 317 pp. Ekdale, A. A. and Mason, T. R., 1988. Characteristic trace fossil associations in oxygen-poor sedimentary environments. Geology, 16: 720-723. Estcourt, I. N., 1967. Ecology of benthic polychaetes in the Heathcote Estuary, New Zealand. N.Z.J. Mar. Freshwater Res., 1: 371-394. Feibel, C. S., 1987. Fossil fish nests from the Koobi Fora Formation (Plio-Pleistocene) of northern Kenya. J. Paleontol., 61: 130-134. Frey, R. W., Howard, J. D. and Hong, J. S., 1987. Prevalent lebensspuren on a modern macrotidal fiat, Inchon, Korea: ethological and environmental significance. Palaios, 2: 517-593. Frey, R. W., Howard, J. D. and Pryor, W. A., 1978. Ophiomorpha: its morphological, taxonomic, and environmental signi-

ficance. Palaeogeogr., Palaeoclimatol., Palaeoecol., 23: 199-229. Ffirsich, F. T., 1974. On Diplocraterion Torrell 1870 and the significance of morphological features in vertical spreitenbearing, U-shaped trace fossils. J. Paleontol., 48: 952-962. Ffirsich, F. T., 1975. Trace fossils as environmental indicators in the Corallian of England and Normandy. Lethaia, 8: 151-172. Goldring, R., 1964. Trace fossils and the sedimentary surface in shallow-water marine sediments. In: L. M. J. U. van Straaten (Editor), Deltaic and Shallow Marine Deposits. Elsevier, Amsterdam, pp. 136-143. Gregory, M. R., Ballance, P. F., Gibson, G. W. and Ayling, A. M., 1970. On how some rays (Elasmobranchia) excavate feeding depressions by jetting water. J. Sediment. Petrol., 49: 1125-1130.

H/intzschel, W., 1975. Trace fossils and problematica. In: C. Teichert (Editor), Treatise on Invertebrate Paleontology, Part W, Miscellanea, Supplement 1. Geol. Soc. Am. and Univ. Kansas Press, Lawrence, 2nd ed., pp. 1-269. Hertweck, G., 1970. The animal community of a muddy environment and the development of biofacies as effected by the life cycle of the characteristic species. In: T. P. Crimes and J. C. Harper (Editors), Trace Fossils. Seel House Press, Liverpool, pp. 235-242. Howard, J. D. and Eiders, C. A., 1970. Burrowing patterns of haustoriid amphipods from Sapelo Island, Georgia. In: T. P. Crimes and J. C. Harper (Editors), Trace Fossils. Seel House Press, Liverpool, pp. 243-262. Howard, J. D., Mayou, T. V. and Heard, R. W., 1977. Biogenic sedimentary structures formed by rays. J. Sediment. Petrol., 47: 339-346. Jones, M. B., 1983. Animals of the Estuary Shore: Illustrated Guide and Ecology. Univ. Canterbury Publ., Christchurch, 162 pp. Kamola, D. L., 1984. Trace fossils from marginal-marine facies of the Spring Canyon Member, Blackhawk Formation (Upper Cretaceous), east-central Utah. J. Paleontol., 58: 529-541. Kern, J. P., 1978. Paleoenvironment of new trace fossils from the Eocene Mission Valley Formation, California. J. Paleontol., 52: 186-194. Lewis, D. W. and Ekdale, A. A., 1991. Lithofacies relationships in a Late Quaternary gravel and loess fan delta complex, New Zealand. Palaeogeogr., Palaeoclimatol., Palaeoecol., 81: 229-251. Mason, T. R. and Christie, A. D. M., 1986. Palaeoenvironmental significance of ichnogenus Diplocraterion Torell from the Permian Vryheid Formation of the Karoo Supergroup, South Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 51: 249-265. Morton, J. and Miller, M., 1968. The New Zealand Sea Shore. Collins, London, 638 pp. Nemec, W. and Steel, R. J., 1988. What is a fan delta and how do we recognize it? In: W. Nemec and R. J. Steel (Editors), Fan Deltas: Sedimentology and Tectonic Settings. Blackie, Glasgow, pp. 3-13. Ota, Y., Yoshikawa, T., Iso, N., Okada, A. and Yonekura, N., 1984. Marine terraces of the Conway coast, South Island, New Zealand. N.Z.J. Geol. Geophys., 27: 313-325.

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Pemberton, S. G. and Frey, R. W., 1982. Trace fossil nomenclature and the Planolites-Palaeophycusdilemma. J. Paleontol., 56: 843-881. Sch/ifer, W., 1972. Ecology and palaeoecology of marine environments. Univ. Chicago Press, Chicago, IL, 568 pp.

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Seilacher, A., 1964. Biogenic sedimentary structures. In: J. Imbrie and N. D. Newell (Editors), Approaches to Paleoecology. Wiley, New York, pp. 296-316. Seilacher, A., 1967. Bathymetry of trace fossils. Mar. Geol., 5: 413 -428.