Tilting marks: Observations on tool marks resembling trace fossils and their morphological varieties

Sedimentary Geology 288 (2013) 60–65

Contents lists available at SciVerse ScienceDirect

Sedimentary Geology

j ourna l homepage: www.e lsev ie r .com/ locate /sedgeo

Tilting marks: Observations on tool marks resembling trace fossils and theirmorphological varieties

Andreas Wetzel ⁎Geologisch-Paläontologisches Institut, Universität Basel, Switzerland

⁎ Geologisch-Paläontologisches Institut, Universität BasBasel, Switzerland. Tel.: +41 61 2673585; fax: +41 61 2

E-mail address: [email protected].

0037-0738/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.sedgeo.2013.01.003

a b s t r a c t

Article history:Received 6 November 2012Received in revised form 24 January 2013Accepted 25 January 2013Available online 4 February 2013

Editor: J. Knight

Keywords:Tool markCrawling traceTrace fossilOscillatory wavesForeshore

Tilting marks, defined here as linear tool marks having transverse ornamentation, are produced in shallowwater when the oscillatory action of waves of short wavelength tilt grounded objects rhythmically in sucha way that they move and push sediment aside. These tool marks can resemble trace fossils, particularly ifthey are bilaterally symmetrical. Even asymmetrical objects can produce symmetrical tilting marks becausethe shape of the mark only depends on the geometry of the ground-touching part of the object, which maybe partially floating. Objects of either soft or hard consistency, such as jellyfish or wood, respectively, can pro-duce tilting marks. Tilting marks are normally produced linearly parallel or at an angle to the direction ofwave propagation and do not show sharp bends or curves. Tilting marks can be formed on plane beds aswell as rippled surfaces. Tilting marks can be distinguished from trace fossils by taking into account the ge-ometry (symmetry), the direction of movement, and the mainly linear course and the internal pattern.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The term “tilting marks” was introduced to describe those sedi-mentary structures that are produced in quite shallow water depthwhen waves tilt grounded or partially floating objects repeatedly insuch a way that they move and push sediment aside while tilted,leaving a structure behind (Wetzel, 1999). Tiltingmarks can thereforeresemble trace fossils. An object may move in the direction of wavepropagation or at an angle to it. Tilting marks are always producedon the surface and have no subsurface expression, which is whatdistinguishes them from many trace fossils. Tilting marks are oftenelongated, nearly straight or slightly curved, and oriented parallel oroblique to the direction of wave propagation. In addition, tiltingmarks are quite similar to tool marks that can be produced by objectswhich are moved by wind on land (Jones, 2006). Recently, ob-servations of Precambrian sedimentary structures at Mistaken Point,Newfoundland (Canada) were interpreted as metazoan traces (Liuet al., 2010a, 2010b) or tilting marks (Retallack, 2010). This discus-sion shows that there is still some confusion regarding the originand significance of tilting marks.

In order to discuss these issues, this study describes (1) the typesand geometry of objects that may produce tilting marks, (2) the fac-tors that affect the morphological variability of tilting marks, and(3) the influence of the substrate on the characteristics of the tilting

el, Bernoullistrasse 32, CH-405667 3613.

rights reserved.

marks. These new findings may be of use to recognize tilting marksand to distinguish them from other structures in the rock record orpresent day environment.

2. Observations

The morphology of tilting marks and how they form has beenmainly studied on tidal flats because abundant tilting marks havebeen observed in small ponds and runnels formed during ebb tide(e.g., Davis and Fitzgerald, 2004). Tilting marks can be found verywell developed on surfaces of fine-grained, unconsolidated, soft orcohesionless sediment because it is displaced easily. Tilting marksare found on rippled as well as smooth surfaces (Fig. 1A and D).Smooth surfaces form in the swash zone at high tide/water level or,alternatively, during ebb tide when in a shallow runnel where flowvelocity is very high for some time. At low tide with some waterremaining in a pond or runnel, small-scale waves may induce the for-mation of tilting marks even on such smooth surfaces (Fig. 1A). Onrippled surfaces, tilting marks often show a differentiated morpholo-gy being more deeply incised and structurally more detailed on thestoss side than on the lee side of a ripple (Fig. 1D).

Tilting marks form also on the surface of incipient microbial mats.Because the microbes bind sediment particles together, only thestronger impacts of the tilted object resultant from larger than aver-age waves may displace sediment. Therefore, on such mats the pat-tern constituted by pushed sediment is wider spaced and lessdetailed the more intense the sediment is bound by microbes, whilethe objects need to displace sediment-binding microbes and adhering

Fig. 1. (A) Tilting mark produced by the medusae Aurelia aurita on a plane bed; the tilting mark resembles crawling traces produced by large gastropods (see text). (B) Tilting markproduced by medusae Aurelia aurita on a plane bed covered with incipient microbial mat; note the slightly curved course of the tilting mark. (C) Small tilting mark produced bylarge seaweed while partly floating; the resultant tilting mark resembles crawling traces of gastropods. (D) Seaweed produced a tilting mark resembling the trace fossilPolykampton; note ornamentation resembling bundles of probes or spreiten on both sides of a central furrow. At the stoss side the tilting mark is more deeply incised andshows more detail than on the lee side. (E) Tilting marks may form over a considerable distance; same object than shown in (D).

61A. Wetzel / Sedimentary Geology 288 (2013) 60–65

material (Fig. 2). Structures typically of (incipient) microbial matssuch as irregular lamination and wrinkle structures may be present(e.g., Noffke, 2010; Carmona et al., 2012).

Objects of very different consistency may produce tilting marks.Tilting marks can be produced by hard objects, such as empty bivalveshells (Wetzel, 1999). However, also soft, flexible objects like sea-weed or jellyfish can produce tilting marks (Fig. 1). Wave tilting ofa soft object can induce a wave-like deformation that propagatesthrough the object, such as the jellyfish Aurelia aurita (Fig. 3). Anapproaching oscillatory wave lifts up the proximal tip of such a medu-sae lying on the sediment, starts to fold this proximal part, and then

the fold migrates through the soft body concomitantly with the prop-agating wave (Fig. 3). When the medusae's tip that was lifted anddeformed first touches the ground again at an acute angle, a little sedi-ment is pushed. In this way the medusae moves in the direction ofwave propagation or at an acute angle to it and leaves a mark behind(Fig. 3). The size of the produced tilting mark depends on the size ofthe sediment-pushing part of the object, in case of a soft medusae likeA. aurita, the margin of the umbrella. The resultant tilting mark resem-bles crawling traces produced by gastropods; for instance, the speciesBullia rhodostoma can produce traces as wide as 6 cm (Abel, 1935, p.207–2014; figs. 176–195). In addition, resting seagulls can fluidize a

Fig. 2. Tilting mark produced by medusae Aurelia aurita on microbial mat. (A) On incipient microbial mat the tilting mark is still wide, but the detailed meniscate pattern is onlyslightly developed. (B) On surface covering, thin mats, only ephemeral imprints are left, that are more widely spaced and smaller than those left in loose sand.

62 A. Wetzel / Sedimentary Geology 288 (2013) 60–65

sandy substrate by repeatedly moving their legs up and down (e.g.,Hertweck, 1978; Wehrmann and Hertweck, 1998); the resultant, notyet named, traces on the surface are very similar to tilting marks (Fig. 4).

Also seaweed can form tilting marks. Once again, the size of the re-sultant tilting marks depends on the size of the ground-touching part.In fact, large seaweed can produce small tilting marks, as several sea-weed have a rigid thallus and localized buoyancy may be induced bysporangia or gas-filled chambers (Fig. 5). Then, only a small part of

Fig. 3. Scheme illustrating the production of tilting mark by a soft, flexible object overtime (1–7) by wave-induced oscillatory deformation of the soft object migratingthrough it. The tilting mark is produced while the proximal end of the object, thatwas lifted first, hits the bottom at an acute angle.

even large seaweed may touch the ground while partly floating inwater (Fig. 1C–E). Seaweed can produce symmetrical structures similarto crawling traces of gastropods, or asymmetrical marks that resemblebundles of probes extending to either side of a central furrow or ridgethat very vaguely resembles the trace fossil Polykampton (Fig. 1D and E).

The morphological variability of tilting marks also depends on theground-touching part of the tilted object. The geometry and densityas well as the weight distribution within the object are of importance(e.g., Branagan, 1976). Therefore, different objects can produce similartiltingmarks, as illustrated by the examples given in Fig. 6. For instance,seaweed can produce tilting marks very similar to that produced bymedusae (Fig. 6). Also hard objects consisting of styrofoam or plasticproduce tilting marks resembling that of medusae and correspondingtraces (Fig. 6). Such anthropogenic material has only existed a shortwhile, but materials of similar density like peat clasts or pebblesconsisting of unconsolidated mud could have produced similar tiltingmarks in the geologic record. Furthermore, furrow-like tilting marksresembling the trace fossil Lophoctenium may be produced by shells,wood or plant parts (Fig. 7). Tilting marks formed by hard elongatedobjects may maintain their orientation and course because the longground-touching edge of the object in fact acts as keel in the soft sub-strate (Fig. 8). Where the tilting mark exceeds a curve of about 30° theinternal pattern of the tilting mark changes normally (Fig. 9).

These examples show that asymmetrical objects can producesymmetrical tilting marks (Fig. 1C–E). Similar structures that result

Fig. 4. Trace produced by a seagull on a sandy tidal flat by repeated movement of itslegs that fluidized the sediment (for details see text and Hertweck, 1978).

Fig. 5. Formation of a tilting mark by a partially floating object; waves move the objectup (1) and down (2); repeated impact of the object produces the tilting mark (3).

63A. Wetzel / Sedimentary Geology 288 (2013) 60–65

from dewatering, liquefaction, seismic waves, hydrofracturing or softsediment deformation have a subsurface expression and, thereforecan be clearly distinguished from tilting marks, which are producedon the surface only.

3. Discussion

Observations on tilting marks can be made best on tidal flats be-cause a major requirement for their production is met. The effectivewave base – where surface sediments start to be moved by waves –

needs to be just above the sediment surface to provide kinetic energyto tilt and move objects on the sediment surface. Such conditions arequite unlikely in the open sea while waves show a continuous sizespectrum (e.g., Peters and Loss, 2012). If the wave spectrum is toowide, large ground-touching waves may occur too often and over-print any pre-existing structures on the sediment surface (includingtilting marks). If the wave base is too far above the bottom, objectson the sediment surface cannot be tilted and moved. Conditions fa-vorable for the production of tilting marks are met along coastlineswhere shallow ponds and runnels may form after storms or duringebb tide. Therefore, the sediment surface on which tilting marks are

Fig. 6. Different objects can produced very similar tilting marks: (A) Seaweed. (B) Plastic diswave propagation.

produced might have been affected by other wave/current regimesthan that leading to the formation of tilting marks. Therefore, thealignment of tilting marks does not need to show a relationship tothe wave/current indicators preserved on/below the sediment sur-face. In addition, on land rain or floods in rivers may lead to the for-mation of shallow ponds. These shallow ponds are short-lived andmay be filled by washed-in sediment, may dry out when the waterlevel falls, or are reworked by waves or currents when the waterlevel rises. Consequently, the time window to produce tilting marksis small.

On tidal flats where the most tilting marks form, the preservationpotential is extremely low because subsequent tides restructure thesediment surface and hence overprint pre-existing structures. Tiltingmarks in such settings may be preserved most likely where ponds orrunnels are filled by sand or wind blown dust. Aeolian material cancover and cast existing structures in some detail. This process hasbeen suggested as a possible taphonomic process (Müller, 1985;Bruton, 1991;Wetzel, 1999). When produced on the surface of micro-bial mats, tilting marks have a higher preservation potential if theyare overgrown later by microbes forming “overmarks”. The term“overmark” is coined in analogy to that in use for tracks that are over-grown by microbial mats constituting the so-called “overtracks”(Marty et al., 2009). In addition, a seldom but instantaneous sedimen-tation event such as an ash fall may preserve tilting marks (e.g.,Heikoop et al., 1997).

If preserved under such conditions (others are not yet known),tilting marks in the geological record are expected to be associatedwith shallow-water indicators, such as swash zone plane beds, planebeds formed in very shallow water associated with rhomboid ripplesor similar structures, parallel-crested ripples, short wavelength ripples,bird foot prints, signs of subaerial exposure and microbial inducedsedimentary structures (Noffke, 2010; Carmona et al., 2012).

The observations described in this study were made by relativelylarge objects like medusae or seaweed that produce large tilting

k. (C) Styrofoam. (D) Crushed stiff plastic object. Note movement of object is oblique to

Fig. 7. Hard objects producing tilting marks resembling bundles of probes or spreiten-like structure like the trace fossil Lophoctenium. (A) Piece of wood. (B) Partly floating plank.

Fig. 8. Hard elongated object producing a tilting mark. The course of the mark is givenby the long edge(s) of the object that may act a keel in the soft substrate.

Fig. 9. Empty gastropod shell (Buccinum sp.) producing a tilting mark; while the shellturned during displacement, the resultant tilting mark is bent, but concomitantly theinternal pattern of the tilting mark changed.Photograph courtesy of R.G. Bromley.

64 A. Wetzel / Sedimentary Geology 288 (2013) 60–65

marks that have a low potential to be mistaken as trace fossils. Thelarge tilting marks exhibit the structure and patterns in more detailthan small ones and, hence, allow for clear observation.

4. Conclusions

• In most instances, tilting marks are produced in very shallow water,although theoretically they can form in the open ocean at waterdepths close to wave base. As such, tilting marks are normally asso-ciated with shallow water indicators and very likely with signs ofemergence. Most tilting marks are produced on tidal flats and inshallow ponds or runnels formed after storms, which are very fa-vorable for production of tilting marks because during fallingwater level the critical water depth and water velocity to push ob-jects are achieved. However, the fossilization potential of thesestructures produced there is low.

• Objects of various consistencies, soft to hard, can produce tiltingmarks. A soft object is moved roughly in direction of wave propaga-tion (±15–20°), while a hard object may move obliquely (>30°) towave propagation if an elongated edge of it acts as keel into thesubstrate.

• Geometrically different object can produced similar marks, whilegeometry, size and internal pattern of the tilting marks only dependon the size and the shape of the ground-touching part of the object.Similarly, asymmetrical objects can produced symmetrical tiltmarks (and vice versa).

• On stiff substrates or surfaces covered by microbial mats only thepart of an object having the strongest impact leaves tilting marks,so the marks tend to be smaller and less detailed on such substratesthan on soft sediment.

Acknowledgments

While discussing the origin of Precambrian sedimentary struc-tures at Mistaken Point (Newfoundland, Canada) during anICHNIA-3 conference field trip, A. Liu (Cambridge, UK) provided en-couragement to write up this short note. This paper benefittedfrom lively discussions with field trip participants, in particular,G. Mángano and L. Buatois (both Saskatoon, Canada), N. Carmona(Roca, Argentina), and J. Scott (Edmonton, Canada). R.G. Bromley(Copenhagen, Denmark) provided the photograph shown in Fig. 9.A. Wehrmann (Wilhelmshaven, Germany) contributed backgroundinformation about trampling traces of seagulls. A. Reisdorf (Basel)drew the figures. The Swiss National Science Foundation providedfinancial support (SNF grant 200020-140217). J. Knight (Wits, SouthAfrica) critically read the manuscript, made helpful comments

65A. Wetzel / Sedimentary Geology 288 (2013) 60–65

and streamlined the English. All these contributions are gratefullyacknowledged.

References

Abel, O., 1935. Vorzeitliche Lebensspuren. Fischer, Jena 644 p.Branagan, D.F., 1976. Jelly fish trails. Journal of Sedimentary Petrology 46, 240–242.Bruton, D.L., 1991. Beach and laboratory experiments with the jellyfish Aurelia and re-

marks on some fossil “medusoid” traces. In: Simonetta, A.M., Conway Morris, S.(Eds.), The Early Evolution of Metazoa and the Significance of Problematic Taxa.Cambridge University Press, pp. 125–129.

Carmona,N.B., Ponce, J.J.,Wetzel, A., Bournod, C.N., Cuadrado, D.G., 2012.Microbially inducedsedimentary structures in Neogene tidal flats from Argentina: Paleoenvironmental,stratigraphic and taphonomic implications. Palaeogeography, Palaeoclimatology,Palaeoecology 353–355, 1–9.

Davis Jr., R.A., Fitzgerald, D.M., 2004. Beaches and Coasts. Blackwell, Oxford 419 pp.Heikoop, J.M., Tsujita, C.J., Heikoop, C.E., Risk, M.J., Dickin, A.P., 1997. Effects of volcanic

ashfall recorded in ancient marine benthic communities: comparison of a near-shore and an offshore environment. Lethaia 29, 125–139.

Hertweck, G., 1978. Die Bewohner des Wattenmeeres in ihren Auswirkungen auf dasSediment. In: Reineck, H.-E. (Ed.), Das Watt. Senckenberg-Buch, 50. W. Kramer,Frankfurt, pp. 145–172.

Jones, A.T., 2006. Wind-generated tool marks resembling trace fossils. Australian Journalof Earth Sciences 53, 631–635.

Liu, A.-G., McIlroy, D., Brasier, M.D., 2010a. First evidence for locomotion in the Ediacarabiota from the 565 Ma Mistaken Point Formation, Newfoundland. Geology 38,123–126.

Liu, A.-G., McIlroy, D., Brasier, M.D., 2010b. First evidence for locomotion in the Ediacarabiota from the 565 Ma Mistaken Point Formation, Newfoundland. Geology 38, e224.

Marty, D., Strasser, A., Meyer, C.A., 2009. Formation and taphonomy of human foot-prints in microbial mats of present-day tidal-flat environments: implications forthe study of trace fossil footprints. Ichnos 16, 127–142.

Müller, A.H., 1985. Aktualistische Beiträge zur Taphonomie fossiler Quallen. Teil II.Freiberger Forschungshefte, C400, pp. 80–89.

Noffke, N., 2010. Geobiology. Microbial Mats in Sandy Deposits from the Archean Era tothe Today.Elsevier, Amsterdam (194 pp.).

Peters, S.E., Loss, D.P., 2012. Storm and fair-weather wave base: a relevant distinction.Geology 40, 511–514.

Retallack, G.J., 2010. First evidence for locomotion in the Ediacara biota from the565 Ma Mistaken Point Formation, Newfoundland — comment. Geology 38, e223.

Wehrmann, A., Hertweck, G., 1998. Lebensspuren. In: Türkay, M. (Ed.), Wattenmeer.Kleine Senckenberg-Reihe, 29. W. Kramer, Frankfurt, pp. 53–57.

Wetzel, A., 1999. Tilting marks: a wave-produced tool mark resembling a trace fossil.Palaeogeography, Palaeoclimatology, Palaeoecology 145, 251–254.