3.a.1. FA1 – description

The majority of the exposure of FA1 occurs stratigraphically below FA2, as steeply dipping strata within the Hardabut Anticline (Hocking, Reference Hocking1991). The tectonic dip decreases downstream along the Murchison River and is approximately flat-lying at Ross Graham and Hawks Head (Fig. 1, locations 1–3). Above the first occurrences of FA2, a thick band of trough-cross-bedded sandstone assigned to FA1 is traceable from these locations north along the river as far as Fourways (Fig. 1, location 6). Cumulatively, FA1 accounts for c.460 m (38 % of total stratigraphic thickness) of the Tumblagooda Sandstone.

Facies association 1 is comprised of medium- to coarse-grained trough-cross-bedded sandstones, with occasional sandstone intraclasts and quartz pebbles (Fig. 5a). Mudrock is absent. Cross-beds occur in sets typically ranging from 15 to 60 cm thick, with rare instances up to 2.5 m thick. Palaeoflow measurements taken from foresets record a consistent flow direction to the northwest (Fig. 5e). FA1 only crops out as vertical cliffs, with negligible bedding plane exposures, so any sedimentary structures can only be viewed in cross-section. Convolute bedding (up to 50 cm thick) and scoured tops of cross-bedded sets are common, along with infrequent ripple cross-lamination. No trace fossils have been identified within FA1, either in this or previous studies.

Fig. 5.

A summary of the stratigraphy and palaeocurrent directions for FA1–FA4. (a) FA1 – the 12 m sedimentary log was taken at Four Ways (location 6, Fig. 1), showing the characteristic large-scale cross-bedded coarse sandstone of FA1, with some convolute laminated beds. (b) FA2 – the 20 m sedimentary log was taken at the Z-Bend (location 5, Fig. 1) showing the three characteristic facies of FA2: cross-bedded sandstone, densely bioturbated sandstone and planar-bedded sandstone. (c) FA3 – the 20 m sedimentary log was taken at Stone Wall (location 16, Fig. 1) showing the dominant trough-cross-bedded facies of FA3, with subordinate bioturbated horizons. (d) FA4 – the 20 m sedimentary log was taken at Rainbow Valley (location 22, Fig. 1) showing the characteristic facies of FA2: trough-crossbedded sandstone, densely bioturbated trough-cross-bedded sandstone, and planar-bedded sandstone. (e) The palaeocurrent rose for FA1 shows a strongly dominant flow to the north and northwest. (f) The palaeocurrent rose for FA2 shows variable flow directions, dominantly to the south/southeast, with a minor northerly flow. (g) The palaeocurrent rose for FA3 shows a very strongly unimodal flow to the northwest. (h) The palaeocurrent rose for FA4 shows a consistent flow to the northwest, with very minor evidence for an opposing southerly flow.

3.a.2. Facies association 1 – interpretation

The strongly unidirectional palaeoflow and ubiquitous trough cross-bedding support previous interpretations (Hocking, Reference Hocking1991) that FA1 was deposited in a braided fluvial setting. The fluvial sediments were apparently not colonized by burrowing organisms.

3.b.1. FA2 – description

Rocks belonging to FA2 crop out from just upstream of Ross Graham Lookout (Fig. 1, location 1) to Mount Yooadda Darlinoo (Fig. 1, location 13). They are frequently interbedded with FA1 in the lower reaches of the succession, with scattered thin bioturbated beds of FA2 increasing in frequency upwards through 20 m of strata until they make up most of the exposure at Ross Graham Lookout. The upper boundary between FA2 and FA3 rocks was not observed in this study, but Hocking (Reference Hocking1991) reports that it is an abrupt transition from the dominantly planar-laminated and bioturbated sandstones into the thick trough-cross-bedded packages of FA3, with no intercalation. The total thickness of FA2 is c. 410 m (34 % of the total stratigraphic thickness of the formation).

Facies association 2 is the most sedimentologically and ichnologically diverse part of the succession (Fig. 5b). The stepped style of outcrop exposure that is prevalent throughout FA2 reveals many bedding surfaces as well as vertical sections through beds. Sedimentary surface textures and surficial trace fossils are abundant on bedding planes, with ripple marks and adhesion marks co-occurring along with diverse burrows and trackways of FA2 (Fig. 6). Five lithofacies are recognized: 1. planar-bedded sandstones (c. 45 % of FA2); 2. Medium-scale cross-bedded sandstones (c. 45 % of FA2); 3. densely bioturbated sandstones (c. 10 % of FA2); 4. herringbone cross-stratified sandstones (<1 % of FA2); and 5. pinstripe-laminated sandstone (<1 % of FA2). These lithofacies are interbedded with variable frequency.

1. 1. FA2a: Planar-bedded medium-coarse sandstones. These typically occur as packages of 2–5 cm thick beds, with some beds up to 20 cm thick. Ripple and climbing ripple lamination are observed in vertical sections, and abundant surface textures occur on bedding planes. Ripple marks occur in symmetrical and asymmetrical forms which locally show evidence of having been washed out or deflected, or occur as ladder ripples (Fig. 7a–c). Some ripple marks contain water-level markings on their flanks (Fig. 7h), and one instance of weakly apparent V-shaped markings on a bedding plane was noted at Fourways, resembling the patterns that develop when water drains across a flat surface (Daerr et al. Reference Daerr, Lee, Lanuza and Clément2003) (Fig. 7g). Adhesion marks occur in high concentrations in FA2, both in association with ripple marks and in isolation, including adhesion warts, setulfs (inverted ‘flutes’ that record wind erosion of damp substrates (Friedman & Sanders, Reference Friedman and Sanders1974)) and adhesion ripples (Fig. 7d–f). Together these bedding plane markings reveal the existence of ‘true substrates’ (sensu Davies & Shillito, Reference Davies and Shillito2018) that indicate that the bedding planes are preserved sediment–fluid interfaces from the time of deposition.

Fig. 6.

The stratigraphic distribution and relative abundance of identified ichnospecies within the Tumblagooda Sandstone. Infaunal traces are illustrated to the left, and epifaunal traces to the right. Pale green highlights the parts of the stratigraphy that have been studied in detail (see Fig. 1). This shows by far the greatest abundance and diversity of trace fossils occurs within FA2, with a moderate abundance of traces in both FA3 and FA4, but a low diversity in both facies associations.

Fig. 7.

Examples of the variable forms of ripple marks and adhesion marks known from FA2 of the Tumblagooda Sandstone. (a) Washed-out ripples – suggesting a high-energy flow across parts of the substrate removed the existing bedforms. (b) Deflected ripples – variable ripple crest directions across a small area reveal a variable flow direction, or flow outwards away from a source. (c) Ladder ripples – revealing a later secondary flow perpendicular to the major ripple crests, recording drainage of water from the substrate. (d) Adhesion warts – small adhesion marks revealing that no preferential wind direction was involved in their formation. (e) Setulfs – elongate adhesion marks, with the direction of elongation parallel to the prevailing wind under which they were formed. (f) Adhesion ripples – ripple marks covered in adhesion structures, with the formational wind direction perpendicular to ripple crests. (g) V-ing drainage marks on a surface on the Fourways trail. Faint Vs are highlighted at the top right and centre bottom of the image, and point towards the bottom. (h) Drainage marks on ripple crests, showing where water has sat at different levels within the troughs of the ripples. (i) Tadpole nest ripples – ripples in multiple different directions coalesced, showing drainage has occurred in differing directions to the main flow direction. Black scale bar – 10 cm.

The true substrates host a range of different trace fossils, both solitary and in infrequent dense patches, dominated by trackways but with short vertical burrows, resting traces and horizontal burrows also present (Beaconites, Crescentichnus, Cruziana, Didymaulichnus, Didymauliponomos, Diplichnites, Fascifodina, Heimdallia, Helicodromites, Paleohelcura, Paradictyodora, Planolites, Polarichnus, Protichnites, Psammichnites, Rusophycus, Siskemia, Skolithos, Taenidium, Trusheimichnus, Tumblagoodichnus). These are accompanied on some surfaces by bubble marks (Fig. 8).

1. 2. FA2b: Planar- and trough-cross-bedded sandstones. These occur as 20 to 60 cm thick packages of fine- to coarse-grained sandstone. Cross-bedded packages can be traced along the dip direction of the foresets on the scale of tens of metres, with a particularly prominent example of such observed at The Loop (Fig. 9). These are distinct from the cross-bedded sandstones of FA1 because they host a number of trace fossils, both in vertical section cross-cutting bedding, and on the foresets in association with sedimentary surface textures such as ripple marks and adhesion marks (Fig. 9c, d).

2. 3. FA2c: Densely bioturbated sandstones. These are fine- to coarse-grained sandstone beds, ranging from 10 to 40 cm in thickness, that are densely bioturbated with 6 to 13 mm J-shaped burrows, predominantly belonging to the ichnogenus Heimdallia (with subordinate Paradictyodora) and often overprinted with Tumblagoodichnus from overlying beds. The beds are typically individual and separated by 0.1 m and 2 m of less bioturbated strata, but less frequently can be seen stacked directly on top of one another. In many instances all original sedimentary bedding surface features are absent due to total overprinting with burrows but, in less densely bioturbated examples, ripple marks can be observed on bedding surfaces. In vertical section, sedimentary structures are totally overprinted by the ichnofabric. This lithofacies typically weathers proud of the walls of the river gorges, forming a large proportion of stepped platforms throughout the exposure of FA2.

3. 4. FA2d: Herringbone cross-stratified sandstone. Apparent bi-directional cross-strata are not uncommon within FA2, and one 50 cm thick instance at The Loop can be conclusively diagnosed as herringbone cross-strata due to the three-dimensional nature of the outcrop (Fig. 10a). Asymmetric ripples occur over the top of cross-bedded medium sandstone dipping towards the NNE, and are succeeded by cross-beds dipping to the SSW. At 12.5 m stratigraphically below this, ripples climbing to the south top medium cross-bedded sandstone dipping to the north (Fig. 10). This same section contains a solitary occurrence of mud-drapes overlying asymmetric sand ripples, where the mud-drapes have been worked into a symmetric ripple form that is discordant with the underlying sediment (Fig. 10b).

4. 5. FA2e: Pinstripe-laminated sandstone. This is observed in two 20-cm thick packages c. 70 cm apart near the base of the section at Z-Bend. It comprises very finely laminated fine–medium sandstone, lacking any bioturbation or physical sedimentary structures. Sand grains are very well rounded and have a frosted appearance. In one example, the fine laminations are truncated by an irregular surface of coarser sediment lag, potentially recording a deflation surface (Fig. 11).

Fig. 8.

A surface with bubble marks (black arrows), patchy ripples and Rusophycus (white arrows). Interpreted as potentially representative of a microbial film covering the original substrate. (a) Overview of the surface. (b) Close-up showing Rusophycus, truncated ripples marks, and bubble marks. (c) Close-up showing bubble marks on an otherwise unmarked part of the bedding surface. Black scale bar – 10 cm.

Fig. 9.

An example of a barform preserved as cross-bedded sandstone within FA2 at The Loop. (a) A view of part of the barform, with the topset and bottomset marked with thick black lines, and reactivation surfaces marked in blue. (b) A depositional surface from within the barform covered in adhesion marks, showing that this surface was exposed subaerially during a period of non-deposition. (c) A depositional surface with several examples of Diplichnites, revealing that animals traversed the surface during sedimentary stasis. (d) A vertical burrow, cutting obliquely through a sub-horizontal depositional surface. This burrow appears to originate from higher up within the barform, likely the horizontal top surface. Geologist is 1.8 m tall; black scale bar – 10 cm.

Fig. 10.

Sedimentological evidence for tidal deposition in parts of FA2. All features were observed at The Loop. (a) Herringbone cross-laminated sandstone. The beds in the base of the photo are dipping to the right of the image, and the overlying beds (above the ripple-marked surface) are dipping to the left. (b) Mud-draped ripple marks, recording low-energy deposition associated with the slack-water phase of a tidal cycle. (c, d) Reverse climbing ripples – ripple marks climbing in an opposing direction to the underlying cross-beds, suggesting reversing flow direction. Black scale bar – 10 cm.

Fig. 11.

Sedimentological evidence for aeolian deposition in parts of FA2. (a) Pinstripe-laminated fine sandstone, with a band of coarse sandstone at the base and cutting through the middle, recording the original topography of the surface. This is overlain by further pinstripe-laminated sandstone and a top surface with adhesion marks. Observed in cliffs opposite the base of the trail, Z-Bend. (b) A line drawing highlighting the key features in (a). The coarse surface carving through the middle of the pinstripe-laminated sandstone is interpreted as an aeolian deflation surface. The area shown in the photo (a) is highlighted by the blue box. (c) A close-up of pinstripe-laminated fine sandstone. (d) A close-up of a layer of highly rounded coarse sandstone, interpreted as ‘millet seed’ texture, from Fourways. Black scale bar – 10 cm; red scale bar – 1 cm.

3.b.2. FA2 – interpretation

The five lithofacies within FA2 reflect deposition in distinct littoral sub-environments, predominantly under subaqueous conditions but with evidence for recurrent subaerial reworking of substrates.

FA2a, the planar-bedded medium–coarse sandstones, exhibit diverse ripple marks (Fig. 7a–c, h, i) that were formed during subaqueous sediment motion, and adhesion warts (Fig. 7d–f) formed during intermittent periods of emergence, when dry sand was blown across, and adhered to, damp substrates. Evidence of draining water is seen in the ladder ripples and water-level marks of this lithofacies. Bubble marks are potentially circumstantial evidence for microbial films covering some substrates (Davies et al. Reference Davies, Liu, Gibling and Miller2016, Reference Davies, Shillito and McMahon2017): such would not be unexpected in an emergent littoral environment but there is no accessory evidence to definitively confirm such an interpretation.

FA2b, sets of fine- to coarse-grained cross-bedded sandstone, show evidence of periods of emergence during their deposition through adhesion marks on their foresets (Fig. 9). This lithofacies can be attributed to subaqueous dunes that occasionally became subaerially exposed. Some of the foresets host arthropod trackways, indicating that they were traversed during intervals of sedimentary stasis associated with emergence. Vertical burrows cutting through the cross-beds originate only from topsets (Fig. 9), indicating that they were emplaced after resubmergence.

FA2c, densely bioturbated sandstones, bear physical similarities with the planar-bedded sandstones, but there is no evidence to suggest that the burrows were generated subaerially. Occasional overprinting by adhesion marks proves that the burrows were excavated under subaqueous conditions and later emergent. Due to the penetrative nature of the burrows, they cross-cut through earlier sediment layers that were both subaqueously deposited and intermittently emergent.

Hocking (Reference Hocking1991) reports restricted occurrences of bioturbated medium-grained sandstones containing Heimdallia in thin (<30 cm) planar beds identical to these bioturbated horizons from the top of FA1, 35 m below the first reported occurrence of FA2. These few horizons do not conform to the description of FA1 as coarse, trough-cross-bedded sandstones, and do not occur in the lower 400 m of exposed stratigraphy. As there is strong evidence for the interbedding of FA1 and FA2 after this point at Ross Graham Lookout and downstream from there, we consider these bioturbated beds to instead mark the first occurrence of FA2 in the stratigraphy.

The above three lithofacies indicate only that the strata of FA2 were subaqueously deposited and recurrently emergent. However, FA2d, the instances of herringbone cross-stratified sandstone and mud-draped ripple marks, provide very strong evidence from the Tumblagooda Sandstone of a tidal influence to sedimentation, and are incompatible with the fluvial–aeolian depositional model proposed by Trewin (Reference Trewin1993a). Evidence for both repeated flow direction reversal and stillstand conditions (including evidence for a switch from directed currents to oscillatory currents during waning flow: Fig. 10.b) are common motifs in tidal settings. Whilst, in isolation, these could potentially have plural environmental explanations (Dalrymple, Reference Dalrymple, James and Dalrymple2010), the weight of evidence from the combined suite of sedimentary structures within this lithofacies implicates fully submerged and marine conditions. The close association of FA2d with the other lithofacies of FA2 suggests that it was marginally offshore from the other littoral lithofacies, the intermittent emergence of which can best be explained by their having been deposited within an intertidal prism between maximum high and low tide.

Fa2e, the pinstripe-laminated sandstone with highly rounded, frosted grains, provides a counterpoint to this, as it is direct evidence for aeolian deposition. The occurrence of this lithofacies within FA2 can be explained as a marginally supratidal or high-intertidal setting within a broad sandy tidal flat environment.

In summary, there is very strong direct evidence that FA2 was deposited within a tidally influenced environment. There is little evidence for fully subaerial deposition apart from the two instances of FA2e, but evidence for the subaerial reworking and emergence of subaqueously deposited sand is a common motif. In agreement with other interpretations (Hocking, Reference Hocking1991; Bradley et al. Reference Bradley, Redfern, Hodgetts, George and Wach2018), the recognition that FA2 can only represent littoral deposition on tidal sand flats means that the fluvial–aeolian depositional model for FA2 of the Tumblagooda Sandstone (Trewin, Reference Trewin1993a, b; Trewin & McNamara, Reference Trewin and McNamara1994; McNamara, Reference McNamara2014) must be rejected. This has crucial implications for the significance of the unit for understanding the terrestrialization process. Close association between arthropod trackways and emergent indicators shows that some organisms were traversing subaerial substrates, but all of these substrates were littoral and likely resubmerged within a diurnal or synodic tidal cycle: there is no evidence that any of these tracemakers were permanent non-marine denizens. Significantly, there is no evidence that any of the burrows in FA2 were created subaerially, and their partial reworking by adhesion structures implies that they were fully subaqueous constructions that were either abandoned or used as refuges during low tide (see Sections 4 and 5 for more detail on the environmental preferences of these traces).

3.c.1. FA3 – description

Exposures of FA3 occur in the Murchison River Gorge downstream of Mount Yooadda Darlinoo (Fig. 1, location 13) where it occurs as stepped outcrops, and vertical coastal cliffs near Kalbarri (Fig. 12). Wave-cut ledges revealing bedding planes occur along the entire length of the coastal exposures from Jakeʼs Point (Fig. 1, location 20) to Grandstand Rock (Fig. 1, location 24). The total thickness of FA3 is measured as c. 280 m (23 % of the total stratigraphic thickness of the Tumblagooda Sandstone).

Fig. 12.

An overview of the three distinct types of outcrop of Tumblagooda Sandstone that occur in the river gorges and coastal cliffs of Kalbarri National Park. Block diagrams show the hypothetical distribution of observed trace fossils in a 3×3 unit cube, if units contain vertical (blue), horizontal (red) or no (yellow) traces. Grey shows areas which cannot be observed, and absent units show those lost to erosion. Pie charts show proportion of existing vertical traces (blue/grey) and horizontal traces (red/grey) which can be observed with a given outcrop style, and the relative bias towards observed vertical/horizontal traces (red/blue) given equal proportions existing within the volume. Type 1 – Stepped Exposure. (a) An example of stepped exposure from The Loop. (b) Block diagram and pie charts given vertical and horizontal traces in every unit cube. (c) Block diagram and pie charts given vertical and horizontal traces in every alternate unit cube. Type 2 – Coastal Exposure. (d) An example of coastal exposure from Eagle Gorge. (e) Block diagram and pie charts given vertical and horizontal traces in every unit cube. (f) Block diagram and pie charts given vertical and horizontal traces in every alternate unit cube. Type 3 – Cliff Exposure. (g) An example of cliff exposure from Hawks Head Lookout. (h) Block diagram and pie charts given vertical and horizontal traces in every unit cube. (i) Block diagram and pie charts given vertical and horizontal traces in every alternate unit cube.

Facies association 3 consists almost entirely of stacked, medium- to very coarse-grained, trough-cross-bedded sandstones with frequent pebbly horizons (Fig. 5c). Trough-cross-bedded co-sets are separated by second-order (sensu Allen, Reference Allen1983) bounding surfaces which are typically near-horizontal (Fig. 13f). Occasionally channelized scour pathways cut across these surfaces, resulting in topographic lows lined with coarse gravel lags (Davies & Shillito, Reference Davies and Shillito2018, their fig. 1B). In the upper part of FA3 these trough-cross-bedded sandstones are occasionally separated by thin planar-bedded packages of medium- to coarse-grained sandstone containing vertical burrows. Measurements from foresets indicate a strongly unimodal palaeoflow to the NW (Fig. 5g).

Fig. 13.

An overview of the depositional environment of the Tumblagooda Sandstone. Main schematic shows the broad depositional environment for all facies associations. Small schematics show a more detailed plan view highlighting the different sub-environments that are included within each facies association. ESB – emergent sand bar; SSB – submerged sand bar. Photos illustrate features from different areas of the facies associations as they are observed in the rock record. (a) Large-scale cross-bedded coarse sandstone containing rip-up clasts. (b) Pinstripe lamination. (c) Adhesion marks. (d) Densely bioturbated ‘Heimdallia Bed’. (e) Cross-beds from barform deposits. (f) A scoured horizontal surface showing truncated tops of trough cross-beds. (g) Psammichnites on the top of truncated cross-beds. (h) Skolithos in a thin planar coarse sandstone bed. (i) Dense piperock fabric cutting through large trough cross-beds.

With the exception of ubiquitous cross-bedding, few other physical sedimentary structures are seen in FA3, and no ripple marks or any other sedimentary surface features are seen on bedding planes. Trace fossils are infrequent and consist only of the horizontal trail Psammichnites, and, restricted to the top of the facies association, Planolites and Skolithos. The Psammichnites traces are notable because they can be seen to colonize the tops of truncated dunes, revealed by planform cross-sectional exposures of foresets. This implies that some of the erosional bounding surfaces are true substrates – in other words, not every erosional disruption of the mobile bedforms was immediately followed by further sedimentation, but rather by an interval of sedimentary stasis (Davies & Shillito, Reference Davies and Shillito2018).

The facies association contains the only mudstone in the whole Tumblagooda Sandstone (with the exception of the singular mud-draped ripple horizon in FA2d (Fig. 10b)), but this is not in situ, instead occurring as rare intraclasts up to 10 cm in diameter. The mud clasts are massive and barren of any microfossils (from our own micropalaeontological analyses).

3.c.2. FA3 – interpretation

The unimodal NW palaeoflow (perpendicular to the palaeo-coastline) and coarse sandstone lithology of FA3 resembles that of the fluvial strata within FA1, but is distinct because of the presence of trace fossils, infrequent planar-bedded horizons (Fig. 5c) and occasional mud clasts. It is unlikely that the strata record non-marine fluvial deposition because of (a) the absence of trace fossils in similar facies of FA1, and (b) the absence of burrows in all fluvial strata of similar age worldwide (Minter et al. Reference Minter, Buatois, Mángano, Davies, Gibling and Labandeira2016b). As FA2 consists of tidal flat sediments, and FA4 contains evidence for fully marine deposition (see Section 3.a.4), an explanation for the abundance of trace fossils within unidirectional cross-bedded strata may be found by considering this facies association to record the distal, marine-influenced reaches of a fluvial system, where physical fluid flow was dominantly river-dominated, but saline or brackish waters promoted the colonization of sediments by a restricted marine infauna. The lack of any physical evidence for emergence in FA3 suggests it was likely deposited in a fully submerged setting. These features fit with an estuarine depositional setting, although this is a cautious assertion as the scale of the outcrop does not permit identification of any incised valley.

The presence of scoured bounding surfaces that were later colonized by Psammichnites and Planolites traces can be explained by fluctuations in fluvial discharge or increased wave action (for example, as could be triggered by storm surges). The latter is considered most likely here, as the interval of stasis following erosion indicates that the dunes did not immediately readjust to a new flow regime (Davies & Shillito, Reference Davies and Shillito2018). Such a situation would arise due to a stochastic external disturbance to dune migration, as occurs when storm wave-levelling of dunes shifts the location of concentrated flow and deposition within an estuarine setting (e.g. Fruergaard et al. Reference Fruergaard, Andersen, Johannessen, Nielsen and Pejrup2013).

3.d.1. FA4 – description

Outcrop of FA 4 is largely restricted to coastal cliffs, and bedding-plane exposures are only seen at localities 0–40 m from the top of FA4 (e.g. Red Hill, Tumblagooda Hill; Fig. 1 locations 18 and 19 respectively). These exhibit a moderate diversity of trace fossils, with an assemblage of sub-vertical and horizontal burrows (Arenicolites, Diplocraterion, Heimdallia, Laevicyclus, Phycodes, Rusophycus, Tumblagoodichnus) in association with vertical burrows (Daedalus, Funalichnus, Skolithos). The latter three ichnogenera are the only trace fossils that can be recognized in the vertical cliff outcrops that dominate FA4.

FA4 is similar to FA3 in that it is composed mostly of trough-cross-bedded medium to very coarse sandstones with foreset palaeocurrent measurements indicating a unimodal flow direction to the NW (Fig. 5d, h). However, unlike in FA3, these trough-cross-bedded sandstones are densely bioturbated, with Skolithos and Daedalus creating a piperock fabric. Additionally, the cross-bedded sandstones typically present as scour-based sets that are thicker and narrower than those in FA3, and, near the top of FA4, planar-bedded horizons separating packages of cross-bedded sandstones are seen, similar to those visible in FA2.

Facies association 4 is restricted to the uppermost 50 m (< 5 % total stratigraphic thickness) of the Tumblagooda Sandstone, and its base is marked by the first appearance of a Skolithos–Daedalus piperock ichnofabric.

3.d.2. FA4 – interpretation

The medium to very coarse sandstone lithology and unidirectional palaeoflow recorded in the lower parts of FA4 suggest it was deposited under the influence of the same estuarine system that deposited FA3. However, following the first instance of piperock fabric in the Tumblagooda Sandstone, the ichnodiversity of the succession increases (though not to levels seen in FA2). Ichnodiversity thus distinguishes FA4 from FA3 where the sedimentary facies are similar, and implies that the succession was deposited in the more distal reaches of the interpreted estuarine system.

The diversity of the richest ichnofauna in the stratigraphically higher exposures of FA4 is boosted by surficial trace fossils, which could be explained by the simple fact that more bedding planes are exposed here. However, some bedding surfaces are seen in large fallen blocks of the lower parts of FA4, along the coastal exposures, where, other than Psammichnites and Planolites, no surficial trace fossils are observed. Additionally, such ichnological diversity is not witnessed in any FA3 exposures that are rich in bedding planes. As such, the higher ichnodiversity towards the top of FA4 appears to relate to a genuine syn-depositional change in ecological conditions; albeit one that is recorded by only minimal change in the physical sedimentary character (i.e. occasional planar-bedded packages). As such, the terminal part of the Tumblagooda Sandstone could record the culmination of a transgression of saline waters, away from freshwater input from any river system (e.g. either due to being further offshore, or due to a shift in position or discharge from the adjacent fluvial–estuarine system).

4.a.1. Bilobate trails and paired grooves

Four distinct ichnogenera from within this category have been previously identified; Cruziana, Didymaulichnus, Tumblagoodichnus and a ‘Meander-loop Trail’ (Trewin & McNamara, Reference Trewin and McNamara1994; Fig. 14). Of these, Cruziana, Didymaulichnus and the ‘Meander-loop Trail’ are observed uniquely on bedding surfaces, whereas Tumblagoodichnus, the most frequently observed trace of this style, is seen both on bedding planes and in vertical sections. This architectural style of trace fossil is constrained to FA2a, FA2b and FA2c, aside from a single example of Tumblagoodichnus observed in the stratigraphically highest levels of FA4.

4.a.2. Trackways and scratch imprints

There is a high abundance of trackways on bedding surfaces of the Tumblagooda Sandstone, from five ichnospecies of four ichnogenera. Of these, Diplichnites gouldi, ?Paleohelcura, Protichnites isp. and Siskemia isp. A have been previously identified by Trewin & McNamara (Reference Trewin and McNamara1994) (Fig. 14). A second, newly recognized ichnospecies of Siskemia (Siskemia isp. B) has been observed on a single bedding surface on the south side of The Loop. These trace fossils are only observed within FA2a and FA2b.

Siskemia isp. B

Five well-preserved examples of this ichnospecies occur on the single bedding surface from which it is known (Fig. 15a). As in Siskemia isp. A, two grooves spaced 1–2 mm apart occur between the track rows; however, in S. isp. B these paired grooves are typically discontinuous and repeated, 30–50 mm in length with gaps of 50–70 mm between the repetitions. The footprints forming the external part of the trackways are approximately circular, and up to 5 mm in diameter. These occur as symmetrical series of six footfall impressions, arranged with very little overlap and with repeat distance 90–110 mm. This is identified as a distinct ichnospecies due to the defined series length, the shape of the footprints, and typically discontinuous medial grooves.

Fig. 15.

Trace fossils in the Tumblagooda Sandstone identified for the first time in this study. (a) Siskemia isp. B in negative epirelief, from FA2 at The Loop. Note the paired medial grooves and six discernible footfall impressions per cycle. (b) Beaconites capronus on a bedding surface from FA2 at The Loop. Note the sharply angular meniscate infill and clear burrow lining. (c) Taenidium serpentinum in positive epirelief from FA2 at Fourways. Note the large, homogeneous meniscate infill and lack of a burrow lining. (d) Phycodes isp. in positive epirelief from FA4 at Red Hill. (e) Helicodromites isp. in positive epirelief from FA2 at The Loop. The shaft of the burrow forms a tight, translated spiral. (f) Paradictyodora isp. in positive epirelief from FA2 at the Z-Bend. In association with many examples of Heimdallia chatwini, this trace differs from the shape of the translated J-tube forming the main structure of the burrow, as it tapers downwards. (g) Trusheimichnus isp. in both positive and negative epirelief from FA2 at The Loop. The sole example of this trace observed in the Tumblagooda Sandstone, and a new ichnospecies; note the circular depression to the right of the image, and the pyramidal mound to the left of this hollow. (h) Funalichnus strangulatus, as a loose trace from ?FA4 at Eagle Gorge. This trace is distinguished most clearly from the frequently observed Skolithos by the pronounced ribbing down one side. (i) Laevicyclus isp. observed in cross-section from FA4 at Red Hill. Note the broad conical burrow top. (j) Paired burrows observed in FA4 at Red Hill, assigned to Arenicolites isp. The lack of any evidence of internal spreite distinguishes these from the previously reported Diplocraterion parallelum (Trewin & McNamara, Reference Trewin and McNamara1994). (k) Polarichnus isp. observed in positive epirelief from FA2 at The Loop. Note the characteristic reversing menisci marking the top margin of the burrow. (l) Fascifodina isp. observed in oblique section from FA2 at The Loop. Note the wide sub-vertical branches from the main shaft. Black scale bar – 10 cm; red scale bar – 5 cm.

4.a.3. Bilaterally symmetrical short, scratched impressions and burrows

Two ichnospecies from this category have previously been identified: Rusophycus trefolia and ‘Selenichnites langridgei’ (Trewin & McNamara, Reference Trewin and McNamara1994; Fig. 14), the latter now reassigned to Crescentichnus langridgei (after Romano & Whyte, Reference Romano and Whyte2015). These ichnospecies are rare throughout the Tumblagooda Sandstone, with a cumulative 12 examples known. Both are observed on bedding plane exposures, and in most examples they are observed in association with one another. Aside from one example of Rusophycus known from the top of FA4, the traces are restricted to FA2a and FA2b.

4.a.4. Passively filled horizontal burrows

A single ichnospecies of passively filled horizontal burrow, Didymauliponomos rowei, was identified in previous studies (Trewin & McNamara, Reference Trewin and McNamara1994; Fig. 14). We observed two examples, both occurring unassociated with any other trace fossils within FA2a, and both occurring on bedding planes as with all previous examples known from the formation.

4.a.5. Simple actively filled (massive) horizontal to oblique structures

This architectural style is observed in a single ichnospecies, Planolites isp., which is known to occur in FA2a, FA3 and FA4, only recorded on bedding surfaces. This trace is most abundant in FA2a, with infrequent dense clusters occurring on true substrates within FA3, in association with Psammichnites (Davies & Shillito, Reference Davies and Shillito2018). A single example is known from the stratigraphically highest exposures of FA4.

4.a.6. Simple actively filled (meniscate) horizontal to oblique structures

A single meniscate burrow ichnospecies was identified from the Tumblagooda Sandstone by Trewin & McNamara (Reference Trewin and McNamara1994), ‘Beaconites antarcticus’. However, the subsequent revision of the ichnotaxonomy of meniscate burrows (Keighley & Pickerill, Reference Keighley and Pickerill1994) means that examples of these traces are more accurately assigned to two different ichnotaxa: B. capronus and Taenidium serpentinum. A cumulative five examples are known from bedding plane surfaces within FA2a.

Beaconites capronus Howard & Frey Reference Howard and Frey1984

B. capronus is known from two long horizontal burrows exhibiting thin linings and a heterogeneous tight chevronate pattern to their meniscate infill (Fig. 15b). The grain size of the infill matches that of the surrounding medium–coarse sediment, although in places it appears to be ordered into alternating packages of slightly finer and coarser sand.

Taenidium serpentinum Heer Reference Heer1877

Three specimens are referred to T. serpentinum, occurring on the same bedding surface. These burrows are unlined, and have meniscate infill divided into well-spaced, sub-rectangular packages (Fig. 15c). The infilling sediment is homogeneous medium sand, the same as the surrounding lithology. They are distinguished from alternative ichnospecies of Taenidium by shallow curvature of the menisci and the homogeneous nature of the infill.

4.a.7. Complex actively filled horizontal structures

‘Aulichnites’ was described from FA3 by Hocking (Reference Hocking1991) and Trewin & McNamara (Reference Trewin and McNamara1994) (Fig. 14). However, this has since been reassigned to Psammichnites (Minter et al. Reference Minter, Buatois, Mángano, MacNaughton, Davies and Gibling2016 a; Davies & Shillito, Reference Davies and Shillito2018), following the redescription of Aulichnites as a junior synonym (Mángano et al. Reference Mángano, Buatois and Rindsberg2003). Psammichnites is dominantly observed in patches of high abundance on bedding surfaces within FA3, although three examples were observed on bedding surfaces of FA2a and FA4.

4.a.8. Horizontal burrows with horizontal to vertical branches

One example of a branching horizontal burrow was observed upon the surface of a loose block within FA4 at Red Hill. This trace is previously undescribed from the Tumblagooda Sandstone, and has been assigned to an unknown ichnospecies of Phycodes.

Phycodes isp. Richter Reference Richter1850

The single burrow exhibiting multiple curved sub-horizontal branches was observed in medium sandstone on the surface of a loose block from the stratigraphically highest parts of FA4. The burrow shaft is 50 mm in diameter before it branches, and the individual branches are a maximum of 25 mm in diameter, narrowing towards their distal ends (Fig. 15d). The observed branches overlap one another, curving a full 180° along their length.

4.a.9. Horizontal helicoidal burrows

A new discovery of a single example of a horizontal helicoidal burrow was observed within FA2a, in a loose block found on the north side of The Loop. This trace has been assigned to Helicodromites mobilis.

Helicodromites mobilis Berger Reference Berger1957

H. mobilis occurs as a specimen 95 mm long with a shaft diameter of 13 mm. Two full whorls are visible in the specimen, and the total diameter of the spiral is 25 mm (Fig. 15E). The burrow is preserved in positive epirelief in medium sandstone. As only one specimen is known from a loose block, detailed palaeoenvironmental context of this trace remains unknown beyond the fact that it was formed within FA2.

4.a.10. Burrows with complex vertically oriented spreiten

Burrows with complex vertical spreite are the most abundant trace fossils known from the Tumblagooda Sandstone. Two ichnospecies have previously been reported, Daedalus isp. and Heimdallia chatwini (Trewin & McNamara, Reference Trewin and McNamara1994; Fig. 14) occurring within FA2a, FA2c and FA4, and are supplemented by Paradictyodora isp., identified here for the first time. Daedalus is commonly observed in vertical sections within FA4, although it is also observed on cross-section on some bedding surfaces. Heimdallia is observed both upon bedding surfaces and in vertical section within FA2, with two known examples from FA4 at Tumblagooda Hill. Beds densely packed with Heimdallia are a distinctive motif within FA2, with 42 distinct Heimdallia beds observed in a 68-m section at Z-Bend – 10.2 % of the total stratigraphic thickness.

Paradictyodora isp. Olivero et al. Reference Olivero, Buatois and Scasso2004

A single example of Paradictyodora has been positively identified in association with Heimdallia within the densely bioturbated facies of FA2 (Fig. 15f). Like H. chatwini, Paradictyodora is formed of horizontally stacked, vertically oriented spreiten which curve downwards. However, unlike Heimdallia the plane containing the J-tube is approximately orthogonal to the direction of displacement, whereas in Heimdallia the plane is parallel to displacement direction.

4.a.11. Mound-shaped structures

Trusheimichnus Schlirf Reference Schlirf2006 n. isp.

A short, unlined, unbranched, vertical burrow with a circular cross-section associated with a triangular pyramidal mound next to the opening (Fig. 15g). The shaft of the burrow remains unfilled and is 25 mm wide by 23 mm deep. The mound is steep-sided, 17 mm tall and the same width as the burrow along the edge which the two components of the trace share. The length of the mound (i.e. the maximum distance that it reaches from the shaft of the burrow) is 36 mm. This specimen differs from other known examples of Trusheimichnus due to the shortness and straightness of the burrow shaft and the lack of a terminal chamber, and as such it is concluded to belong to an unidentified ichnospecies.

The sole example of Trusheimichnus was found within a loose block in proximity to FA2 at the south side of The Loop, so its precise environmental context cannot be determined. However, as the trace-bearing surface preserves the original topography of the biogenic structure it records a true substrate, with the burrow formed during a period of sedimentary stasis. The sharp, steep sides of the mound beside the burrow show that it was formed in damp, subaerially exposed sediment, as dry subaerial sand would collapse, and wet submerged sand would form a shallower slope (Tegzes et al. Reference Tegzes, Albert, Paskvan, Barabási, Vicsek and Schiffer1999).

4.a.12. Vertical unbranched burrows

Vertical burrows occur in high abundance in the Tumblagooda Sandstone, dominantly within the piperock of FA4 (Hocking, Reference Hocking1991; Trewin & McNamara, Reference Trewin and McNamara1994). Two ichnospecies have been previously described, ‘Lunatubichnus wittecarrensis’ and Skolithos linearis (Fig. 14). ‘Lunatubichnus wittecarrensis’ (Trewin & McNamara, Reference Trewin and McNamara1994) is here considered to be an invalid ichnotaxon because further investigation of the traces reported from the coastal cliffs instead reveals them to be weathered cross-sections through Skolithos and Daedalus (Fig. 14). Two other ichnospecies of vertical unbranched burrows have been recognized for the first time: Funalichnus strangulatus, known from a single loose example at Eagle Gorge, and Laevicyclus, observed in two examples from the stratigraphically highest parts of FA4 at Tumblagooda Hill.

Skolithos is by far the most common trace fossil from within this grouping, forming a dense piperock in the coastal assemblages of FA4. It is most frequently observed in vertical section, although burrow cross-sections are also observed on bedding planes. Skolithos is also seen on rare marker horizons towards the top of FA3, again in vertical section, and sparsely distributed throughout the rocks of FA2a, FA2b and FA2c as short burrows on bedding surfaces and occasionally in vertical section. The variable presentation of this trace is a direct result of the different outcrop exposure between the inland river gorges and the coastal cliffs.

Funalichnus strangulatus Fritsch Reference Fritsch1883

A straight, unbranched burrow, 30 mm long by 10 mm wide with a slightly off-round cross-section. The burrow is ornamented with closely spaced transverse ridges along one side, whereas the opposite side of the shaft is smooth and unmarked (Fig. 15h). The infill is structureless and compositionally the same as the medium- to coarse-grained sandstones of bioturbated beds from close to where the specimen was discovered.

As only one example was identified, occurring within a loose block, the broader sedimentological and palaeoenvironmental context of the burrow remains unknown. However, its location amidst a scree slope towards the top of the coastal exposure at Eagle Gorge (Fig. 1, location 22) restricts it to the upper reaches of FA3 or FA4.

Laevicyclus Quenstedt Reference Quenstedt1879

Two examples of vertical, cylindrical burrows from Tumblagooda Hill are assigned to Laevicyclus isp. (Fig. 15i). These burrows are unbranched and open out into a cone towards the top of the shaft, differentiating them from the multitude of other vertical burrows known from FA4. Both examples are at least 75 mm long, although they disappear into the bed below so their total lengths are unknown. At the widest point of the cone the burrows reach 20 mm in diameter, narrowing to 10 mm for the majority of the shaft. The two examples occur in medium–coarse planar-bedded sandstone, and are in association with Skolithos.

4.a.13. Vertical single U- and Y-shaped burrows

All U-shaped burrows previously described from the Tumblagooda Sandstone were assigned to Diplocraterion parallelum, with ‘paired burrows’ also considered to belong within Diplocraterion due to the presence of spreite (Trewin & McNamara, Reference Trewin and McNamara1994; Fig. 14). Here two other ichnospecies are reported, Arenicolites isp. and Polarichnus isp. Arenicolites occurs as paired burrows with no evidence for internal spreite, on bedding surfaces near the top of FA4 at Tumblagooda Hill. A single specimen from a loose block of FA2 at The Loop is assigned to Polarichnus.

Arenicolites isp. Salter Reference Salter1857

Paired burrows observed on bedding surfaces near the top of Tumblagooda Hill are assigned to Arenicolites isp. (Fig. 15j). These burrows are distinguished from the tops of the simple vertical burrows that dominate much of FA4 as the burrow tops are consistently closely associated with one other, with a separation of c. 30–50 mm. The diameter of the burrow shafts ranges from 5 to 12 mm, and there is no evidence of spreite between the two shafts, distinguishing these from Diplocraterion.

Polarichnus isp. Narbonne et al. Reference Narbonne, Gibling and Jones1979

Polarichnus is considered a U-shaped burrow based upon the type specimen (Narbonne et al. Reference Narbonne, Gibling and Jones1979), but the burrow shaft is not seen in the Tumblagooda example; instead being recognized by the up-thrusted medial surficial epirelief ridge of sediment, exhibiting opposing directions of meniscate ridges (Fig. 15k). The same presentation is seen in trace fossils also assigned to Polarichnus in similar facies of the Ringerike Group of Norway (Davies et al. Reference Davies, Sansom and Turner2006). A single specimen of Polarichnus was observed in the Tumblagooda Sandstone, in a loose block of medium sandstone from within FA2 at The Loop. The trace is 125 mm long and 58 mm wide.

4.a.14. Burrows with shaft or bunch with downwards-radiating probes

A single example of a previously undescribed trace fossil with a shaft and downwards-radiating branches was observed in vertical section from within FA2a at The Loop. This trace has been assigned to an unknown ichnospecies of Fascifodina.

Fascifodina isp. Osgood Reference Osgood1970

Fascifodina isp. is interpreted based on one trace cutting obliquely through planar-bedded medium sandstone at The Loop. The main shaft of the trace is cylindrical and 150 mm long with a diameter of 18 mm. This shaft then branches into four probes which all originate from the same part of the main shaft, radiate downwards and outwards and bulge along their length (Fig. 15l). The trace is believed to record the deposit-feeding behaviour of a vermiform organism (Osgood, Reference Osgood1970).

4.b.1. Association Group A – Heimdallia-dominated (T5, T6, T8, T9, T10, T11, T12)

This group of shallow infaunal trace fossil associations is dominated by Heimdallia, is the most common and is consistently present for the entire thickness of FA2 (Fig. 5b). The extent of bioturbation varies from occasional overlapping traces to total destruction of any primary bedding. The degree of bioturbation likely reflects a spatial patchiness to burrowing intensity (Marenco & Hagadorn, Reference Marenco and Hagadorn2019). On areas of the bedding surfaces where the burrows are sparse, surface features remain intact and the traces are often seen in association with subaqueous and emergent sedimentary surface features including wave ripple marks and adhesion marks. Skolithos is a common minor element of this assemblage, appearing in low abundance on some less densely bioturbated bedding surfaces. The other traces which occur in this group are: Beaconites, Didymaulichnus, Pardictyodora, Planolites and Tumblagoodichnus.

Whilst Tumblagoodichnus is recognized in association with this group (T5, T6), in these instances it always originates from beds stratigraphically above those containing Heimdallia, demonstrating that it is a palimpsest upon the original ichnofabric. The association between Heimdallia and Tumblagoodichnus has previously been noted, and interpreted as both coincidental (with Tumblagoodichnus as the record of temporary subsurface refuges made by arthropods) (Trewin & McNamara, Reference Trewin and McNamara1994) or as ‘hunting’ behaviour (with the animal responsible for Tumblagoodichnus scooping down through the sediment to eat the animal creating Heimdallia) (McNamara, Reference McNamara2014; Bradley et al. Reference Bradley, Redfern, Hodgetts, George and Wach2018). The latter contention is improbable given that the Tumblagoodichnus, originating from overlying beds, would have been formed on timescales of days to years after the Heimdallia burrowing activity.

4.b.2. Association Group B – Tumblagoodichnus-dominated (T1, T7)

Tumblagoodichnus is one of the most abundant trace fossils in the Tumblagooda Sandstone (Table 2), and occurs in direct association with sedimentary surface textures indicative of subaerial emergence, including adhesion marks, setulfs and wave ripples. Whilst there is some overlap with associations in groups A and C, where Tumblagoodichnus also plays an accessory role, it is the major trace in two associations which also contain Rusophycus and Skolithos. In these associations, burrow tops occurring on adhesion-marked surfaces suggest that some of the burrows may have been formed subaerially, although it is difficult to ascertain the relative timing of burrowing and emergence of the substrate. This group of associations is known to occur throughout the full stratigraphic thickness of FA2, specifically within FA2a and FA2b.

4.b.3. Association Group C – Trackways-dominated (T4, T13, T14, T15, T16, T17, T18)

This group of trace fossil associations is predominantly epifaunal, comprised of arthropod trackways (Diplichnites, Protichnites, Siskemia) which are occasionally associated with burrows (Skolithos, Tumblagoodichnus and Psammichnites). This group of associations is observed from the full thickness of FA2, occurring within FA2a and FA2b within similar facies to those where group B is found. The traces are associated with a range of both subaqueous and emergent sedimentary surface textures; wave ripple marks, and frequently adhesion marks, indicating that some of the traces in this group were formed subaerially.

4.b.4. Association Group D – Skolithos-dominated (T21, T22)

This group of trace fossil associations is dominantly comprised of vertical infaunal traces which together produce a piperock fabric. The main constituent traces are Skolithos and Daedalus, with Arenicolites and Laevicyclus in the upper exposures of FA4. The burrows occur only within subaqueous sedimentary strata, within trough-cross-bedded medium to very coarse sandstones. The longest burrows penetrate to depths of over 1 m: lengths that are not witnessed when Skolithos appears as an accessory trace in the other associations (Fig. 17). Burrows originate only from topsets in the trough-cross-bedded sandstones and are not seen terminating against foreset surfaces, indicating burrowing activity commenced during intervals of sedimentary stasis post-dating dune migration (Fig. 18).

Fig. 17.

Comparison of the typical length of Skolithos in the piperock fabric of FA4 vs those found as accessory traces in other associations, particularly in FA2. (a) Representative Skolithos from FA4, beginning at the top surface beside the shrub and cutting down through all of the cross-bedding. (b) A representative Skolithos from FA2 occurring in planar bedding sandstone, showing the vastly shorter length (approximately 8× shorter). Black scale bar – 10 cm.

Fig. 18.

A schematic view of coastal exposures of FA4, illustrating the consistency of palaeoflow and the locations within trough-cross-bedded units where burrows originate. (a) A photo of an exposure at Rainbow Valley. (b) A schematic drawing of the outcrop, highlighting the different trough-cross-bedded sediment packages. Flow directions are reported on bars below the line drawing following Davies et al. (Reference Davies, McMahon and Shillito2018), with colours corresponding to packages. The bar is aligned with the outcrop, drawn looking east. The red line illustrates the only surface where it is clear burrows originate – it appears that the system was only bioturbated during periods of stasis, following the deposition of all of the observed trough-cross-bedded sandstones. (c) A photo of an exposure between Red Bluff and Mushroom Rock. (d) The corresponding line drawing. Colours of sediment packages correspond to palaeocurrent measurements on the bars to the right of (c). Bars are aligned with the outcrop, drawn looking NE, and palaeoflow measurements are very similar to those reported in (b). Again, the red line marks the only surface where it is clear burrows originated, at the top of the outcrop. Black scale bar – 1 m.

4.b.5. Association Group E – Psammichnites-dominated (T19, T20)

This group is mostly comprised of shallow infaunal traces and represents the major ichnological signature in FA3, containing Psammichnites, Planolites and Skolithos. Psammichnites occurs in high-density patches on erosional bounding surfaces between subaqueously deposited trough-cross-bedded co-sets (Davies & Shillito, Reference Davies and Shillito2018), alongside occasional patches of Planolites. Skolithos rarely occurs in association with Psammichnites, and where it does it is as a result of a later overprinting of the original fabric from a higher bed.

4.b.6. Association Group F – Rusophycus-dominated (T2, T3)

This group of trace fossil associations is the least common, restricted to disparate locations throughout FA2. The associations contain Crescentichnus, Rusophycus and Skolithos, and dominantly occur in mixed subaqueous-emergent strata of FA2a. A solitary occurrence in FA2b is on an erosional bedding surface, with both Rusophycus and Crescentichnus in very high concentrations (Trewin & McNamara, Reference Trewin and McNamara1994). All other examples contain a much lower density of trace fossils on planar-bedded surfaces with no sedimentary surface textures. Whilst it is likely that Rusophycus and Crescentichnus were produced at the same time (they are thought to be produced by the same euthycarcinoid animals; Trewin & McNamara, Reference Trewin and McNamara1994), in the association between Rusophycus and Skolithos, Skolithos post-dates and overprints the fabric from above horizons.