Distinguishing borings and burrows in intraclasts: Evidence from the Cambrian (Furongian) of North China

Hardgrounds represent synsedimentary cemented stratigraphic beds that form at or near the sea ﬂ oor. Borings represent a key line of evidence for investigations of hardground development and record the evolution of bio-erosion and boring organisms. The unequivocal identi ﬁ cation of borings is done through identi ﬁ cation of the crosscutting relationship between the proposed boring and a hard substrate, such as lithoclasts and/or shells, with morphological criteria able to be used when dealing with a homogeneous substrate, such as micritic hardgrounds. Bioeroded hardgrounds and burrows with a micrite halo/lining are subject to fracturing and reworking, resulting in accumulations of intraclasts in ﬂ at-pebble conglomerates (FPC). The recognition of bor-ings and broken burrows with a halo can be challenging in FPC. Using trace fossils preserved in situ and in FPC from late Cambrian carbonates of North China, we establish a set of criteria for distinguishing borings from bur-rows with a halo in FPC. Features such as the relative volume of burrows and borings versus the host pebble and the number of traces per pebble, the cross-cutting relationship with different colored laminae, and the presence of pyrite or glauconite encrustations can all be invoked to aid recognition of borings. Examination of the cross-cutting relationship and encrustation of trace fossils are not suf ﬁ cient on their own. Our results suggest caution isnecessaryinde ﬁ ningboringsinFPC,particularlyassynsedimentarydeformationofburrowswithahaloinlate Cambrian FPC can create structures that resemble borings. © 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).


Introduction
Hardgrounds represent synsedimentary cemented stratigraphic beds that form at or near the seafloor (Christ et al., 2015).They are concentrated in particular periods of the geological record, and their presence has been closely linked to key climatic and evolutionary events (Christ et al., 2015).For example, early lithification of carbonate sediments in the Ordovician has been proposed to have facilitated the substantial increase in biodiversity associated with the Great Ordovician Biodiversification Event (Wright and Cherns, 2016).Identification of hardgrounds thus forms an integral component of documenting both geological and evolutionary events during the early Paleozoic.Recognition of hardgrounds, including early Paleozoic examples, is often not straightforward, usually because they lack encrustations and/or bioeroded grains and clasts.Due to their homogeneous, micritic composition, such features can not typically used for hardground identification (Christ et al., 2015).
Hiatal surfaces due to omission are often associated with the development of hard substrate (Wilson and Palmer, 1992;Lee et al., 2015).The occurrence of hiatal surfaces and hardgrounds is frequently accompanied by the presence of attached organisms, encrustations, truncated surfaces and signs of bioerosion (Wilson and Palmer, 1992;Dickson et al., 2008;Lee et al., 2015;Vinn and Toom, 2015;Peel, 2017).Borings created by organisms that once inhabited these environments have great potential for aiding in the recognition of hardgrounds, because borings are an important indicator of synsedimentary cementation.However, the identification of borings is often challenging.One lithology where identification of borings is a particular issue is flat-pebble conglomerates (FPC).Putative borings in FPC can be hard to define as true borings, because specimens are often reworked.For example, Lee et al. (2018Lee et al. ( , 2021) ) reported abundant borings from late Cambrian micritic intraclasts in FPC, without any borings preserved in situ.Reworked borings generally lack the features necessary for accurate identification, possibly leading to erroneous ichnotaxonomic assignment and issues with determining whether or not the source facies, in fact, was a hardground.FPC are globally distributed and particularly common in late Cambrian carbonates of the Sino-Korean Block (Saltzman, 1999;Pratt and Bordonaro, 2007;Chen et al., 2009Chen et al., , 2010;;Chen, 2014Chen, , 2015)).A reliable procedure to characterize trace fossils in FPC would aid the identification of Cambrian hardgrounds in both the Sino-Korean and broader region, and also increase understanding of the early evolution of macroborers during the Cambrian.
In this study, we describe trace fossils with a diagenetic halo that occur in situ, as well as fragmented portions of these trace fossils reworked as ichnoclasts and accumulated in FPC in late Cambrian carbonates of North China.By undertaking a comparison of specimens occurring in situ and borings preserved in FPC, we establish criteria for the recognition of borings in FPC and how they can be distinguished from similar structures.

Geological background
This study centers on Cambrian strata near Xuzhou City, located at the southeastern margin of the North China Platform, in central China (Fig. 1A).Our focus is the Chaomidian Formation, which forms the uppermost unit of the Cambrian succession in the study area (Chough et al., 2010;Zhu et al., 2021).Samples of trace fossils from the Chaomidian Formation that form the basis of our study come from three sections; the Dananzhuang and the Zhaishan sections from Xuzhou City, and the Mangshan section from Yongcheng County (Fig. 1).In the Dananzhuang section, the Chaomidian Formation is about 250 m thick and represented by thinly bedded micrites, mudstone, shales, oolitic limestones, bioclastic limestones and FPC with a thickness of 3-20 cm (Fig. 2).In general, FPC are composed of micritic intraclasts with a grainstone matrix.Identification of the Chuangia, Changshania-Irvingella, Kalishania, Ptychaspis-Tsinania, Quadraticephalus, and Mictosaukia trilobite biozones within the Chaomidian Formation indicates a late Cambrian age (Xu et al., 1984).The Mangshan section (Fig. 1B) is similar to the Dananzhuang and Zhaishan sections (Fig. 2B, C) but with the lower and middle Cambrian strata not exposed in the Mangshan section, and only some parts of the upper Cambrian carbonates interbedded with mudstone and dolostone (Fig. 1C).

Material and methods
The herein studied material is from the micritic limestone and clasts within the FPC of the Chaomidian Formation, as exposed in the Zhaishan, Dananzhuang and Manshan sections (Fig. 2).Burrows with a halo were collected from rhythmic beds of micrite limestone and mudstone (Fig. 2B, C).Rare borings occurring in situ are found in micrite layers.
The morphological characters of the in situ and reworked trace fossils, such as shape, size and branching were assessed visually in outcrop and in polished slabs.Polished slabs were also used for Micro-X-ray fluorescence (XRF) tests.Micro-XRF mapping results were obtained using a Broker M4 Tornado, located at the Northwest University, with point size set to 50 kV and 20 μm.Scanning time per  pixel was set to 15 ms and the distance of capture was 20 μm.Trace fossil details were photographed with a ZEISS Smart Zoom 5 Stereo micrographic camera.The percentage of burrows or borings versus the host rock was measured using Image J based on photographs from the field, and reference material (see Supplementary materials).All samples used in this study (except those that were not collected and which are included based on field observations) are deposited in the collection of the Early Life Institute at Northwest University, Xi'an, China.

In situ burrows with a halo
Burrows preserved in situ are comprised of a passive fill surrounded by a micritic diagenetic halo (Figs. 3, 4).In most cases, their cross section varies between circular, oval and irregular I-shaped (Figs. 3A, B, 4A, B).The surrounding matrix is usually mudstone (Fig. 3C).The halo surrounding the burrows consists of micrite and no micro-structure is visible.The burrow margin provides a clear contrast between the matrix and the burrow (Fig. 3D).The burrow fill consists of lime mud (Fig. 3D), dolomite (Fig. 3B) or sparry calcite (Fig. 6A).On bedding planes, burrows occur as both unbranched, slightly curved tunnels (?Planolites, Fig. 3C) and horizontal burrows that branch multiple times (i.e.Thalassinoides horizontalis, Fig. 3D).The diameter of the burrows ranges from 3 to 10 mm and the thickness of the diagenetic halo is about 1 to 5 mm.Some of the burrows are slightly deformed (Fig. 3B).Micro-XRF mapping results show that the highest concentration of Ca is in the burrow fill, and an elevated concentration of Ca in the burrow halo (Fig. 7).The highest values for K, Al and Si are in the matrix, with values for all three elements elevated in the halo, and there is little of these elements in the burrow fill.Fe is also highly concentrated in the matrix.The distribution of P appears to be homogeneous for all parts of the burrow (fill and matrix, Fig. 7).Some in situ burrows with a halo appear diffuse in polished slabs (Fig. 8A) but are much clearer in micro-XRF mapping (Fig. 8C-G).In cross section these burrows display Y-shaped branching.The burrow systems are truncated and stained with pyrite, with a high concentration of Fe and S on the truncated surface (Fig. 8A).Micro-XRF mapping results indicate similar composition for the unbranched burrows (Fig. 7), with higher concentrations of Fe, Si and Al in the halo and a higher concentration of Ca in the fill (Fig. 8C-G).

Reworked burrows
In cross section, the outline of reworked burrows (or ichnoclasts) varies from circular or oval to irregular shape (Fig. 5).Some ichnoclasts are relatively complete, still preserving their original burrow shape and branching pattern (Fig. 6A), with a circular or oval cross section (Fig. 5B).Other burrows are fragmentary, and their associated halo is broken and are C-or I-shaped in cross-section (Figs. 5, 6).The halo of the burrows is comprised of homogeneous micrite and the burrows do not cross-cut or co-occur with carbonate grains or laminae (Fig. 6B, C).The color of the burrow halo is grayish, and the burrow fill is similar to the matrix of the FPC (Fig. 5G, H).The diameter of the burrows (without halo) ranges from 2 to 6 mm and the thickness of the haloes varies considerably, in the range of 1.5-10 mm.Some of the fill material found in reworked burrows is the same as in the matrix, but others are filled with sparry calcite (Fig. 6).No pyrite or glauconite encrustation is found in the reworked burrows.Micro-XRF mapping returns similar values as recorded for the in situ burrows and the results for the fill and matrix are lower for Ca, K, Al, Si, and Fe than for in situ material (Fig. 8).The distribution of P appears to be homogeneous for reworked burrows (Fig. 8).

In situ borings in the carbonate hardground
Borings occurring in situ in the carbonate hardgrounds are observed in the Mangshan section (Fig. 8).These borings are ~5 mm deep and 2-5 mm wide (Fig. 8A, B).The borings are encrusted by pyrite (Fig. 8A).Pyrite layers overlay the borings and the fill of the borings.The boundary between the borings and the matrix is sharp (Fig. 8C-G).These borings do not occur with an associated diagenetic halo.Results of micro-XRF mapping show high concentrations of Fe, S, Si and Al in the borings (Fig. 8C-G).In contrast, the fills of borings have a high concentration of Ca, and there is a high concentration of Fe, Si and Al in the surrounding matrix.The bioeroded hardground overlaps with the substrate that contains burrows with a thick halo (Fig. 8A, C-G).

Borings occurring in pebbles of the FPC
Borings commonly occur in pebbles.The outline of these borings is I-, C-, J-or clavate shaped (Figs.6C, 9) and their diameter ranges from 1 to 4 mm.The borings appear red in outline and the thickness of the ironstained layer is about 0.5-0.8mm.At high magnification, glauconite can be seen encrusting the surface of the red outline.The thickness of the glauconitic encrustation is about 0.3 mm.Pebbles containing borings are composed of micrite.Some white and brown colored laminae are usually seen in pebbles found in FPC where the intraclasts have no halo.The fills inside the borings is similar in composition to the surrounding matrix (e.g., dolomite).

Differences between burrows with a halo and borings in FPC
As shown by our results, the herein described fragmentary burrows with a halo (Figs. 5,6,7) and in situ preserved burrows with a halo (Figs. 3, 4, 7) raise the question of how to distinguish burrows with a halo from borings in FPC.Burrows with a diagenetic halo are fractured at different angles, due to both brittle deformation and soft deformation, producing "intraclasts" with various shapes (for instance, oval, C, I and J-shaped; Fig. 5B, G, H).The morphology of these ichnoclasts can be very similar to borings (Fig. 5G, H) and makes it difficult to distinguish both, particularly if overall outline is the only diagnostic feature utilized.In addition, if burrows with a halo occur in a micritic substrate, as they do in our study area, indicators of a hiatal surface, such as bioeroded grains or bioclasts, are often absent and further hinder a correct identification.
However, some important differences between burrows with a halo and borings in FPC can be recognized.The most important difference is the relative size of burrows and borings versus their hosting pebble.The mean volume of burrows occupying the clast is much larger than for borings (Fig. 10A-D; Table S1) and the number of burrows per clast is much less than borings (Fig. 10E, F; Table S1).
In our polished slabs, some trace fossils in intraclasts that penetrate differently colored laminae are likely borings (Figs.6C, F, 9A, I).Burrows with a halo do not crosscut, or even co-occur, with differently colored laminae.Both pyrite and glauconite are commonly present in hardgrounds, however neither in situ nor reworked burrows with a halo are seen encrusted with pyrite or glauconite, whereas some possible borings are (Fig. 9).

Criteria to distinguish burrows with a halo and borings in FPC
Criteria that may be useful for distinguishing between fragmentary burrows with a halo or lining and borings in FPC at the microfacies scale include the relative volume of burrows and borings versus hosting pebbles, the number of trace fossils per clast, whether or not traces penetrate differently colored laminae, and the presence of pyrite or glauconite encrustations.

Relative volume of burrows and borings versus host pebbles and number of trace fossils per clast
In some cases, burrow linings (Keighley and Pickerill, 1995;Wang et al., 2019) or synsedimentary diagenetic haloes around burrows (Bertling, 1999;Knaust, 2021;Matysik et al., 2022;Wilson and Palmer, 1992;Bromley, 1975) are thought to have been firm or even hard at the time of construction.For instance, in early Cambrian carbonates, burrows with a thick diagenetic halo can be reworked synsedimentary, causing multiple sets of fractures in the resulting ichnoclasts, which then become filled with soft sediment (e.g.mud).However, the substrate around the burrow can be locally lithified around the burrow.With the unlithified hosting sediment remaining soft, the bond between the matrix and the diagenetic halo of the burrow would not have been sufficient to allow for synsedimentary fracturing of the surrounding matrix, meaning that most of the intraclasts only contain one burrow (Fig. 11) and leading to burrows occupying a larger area versus the surrounding intraclast (Fig. 10A-D).While the unlithified matrix could still easily be eroded, the lithified burrows with a halo were more resistant and subject to fragmentation and redeposition in breccia and intraformational conglomerates.Burrows without a halo in soft to firm substrate can also be reworked by storms and reworked, but it would be difficult to identify burrows without a lining or diagenetic halo in FPC, due to the lack of information about the burrow margin.
Borings preserved in pebbles have been reported from the early Cambrian onwards (Cherns, 1980;Siggerud et al., 2000;Vinn and Wilson, 2010;Schwarz and Buatois, 2012;Vinn et al., 2015, Vinn andToom, 2016;Bassi et al., 2017;Brlek et al., 2018;Figs. 6C, 9).The fracturing of the hard substrate results in intraclasts containing borings, however the fully lithified substrate is more stable and homogeneous.Thus, fracturing in a fully lithified substrate does not occur around or near borings, leading to a smaller volume of borings versus the volume of the surrounding pebble and more borings per clast (Fig. 10; Table S1).In rare cases, the ratio of the volume of borings versus the volume of the surrounding pebble can be similar to the ratio of the volume of burrows versus the volume of the surrounding pebble, depending on the degree of hardground fracturing.Consequently, based on comparison with burrows and borings from this study and other late Cambrian and Mesozoic deposits (Fig. 10; Table S1), most of the 'borings' described in Lee et al. (2018) should be reconsidered, because those 'borings' occupy a larger relative volume versus the hosting intraclast than known examples of borings and most intraclasts contain only one trace (Fig. 10).

Traces penetrating different colored laminae or not
Trace fossils occurring in pebbles are often observed penetrating differently colored laminae (e.g., fig. 4 in Vinn and Wilson, 2010;fig. 8i in Matysik et al., 2022).This implies that these traces are most likely borings, as the trace maker clearly colonized the carbonate pebbles with laminae after their lithification (Figs.6C, 9I).The studied material shows that burrows occurring in hardground always have a diagenetic halo (Fig. 8), which is not the case for intraclasts found within carbonate laminae (Fig. 7; Bromley, 1975;Matysik et al., 2022).In contrast to these probable borings, the burrows never crosscut differently colored laminae or shells (Fig. 6C-F).This indicates that the formation of burrows and their associated halos occurred almost at the same time.Given that it is relatively easy to tell if a trace fossil is cross-cutting differently colored laminae or not, this cross-cutting relationship between differently colored laminae and trace fossils (or lack thereof) is one of the useful criteria for distinguishing burrows with a halo and borings occurring in FPC.

Pyrite and glauconite encrustation
Due to the hiatus in sedimentation during the subaquatic hardground stage, pyrite or manganese oxide and phosphate encrustations are usually developed (Eren and Tasli, 2002;Immenhauser et al., 2000;Mutti andBernoulli, 2003 Tucker andWright, 2009).Thus, pyrite and glauconite encrustations usually indicate the presence of a hardground.Element distributions established using micro-XRF mapping allow comparison of the chemical composition of burrows with a halo and borings.As our results show, substrates with in situ preserved borings are highly concentrated in Fe and S, particularly on the surface of the borings (Fig. 8), implying the presence of pyrite rather than hematite or limonite.The exposure of the hardground to an oxygen-rich environment results in the transformation of this pyrite into iron oxide (Fig. 9A, F, I) (Kafri and Sass, 1996;Immenhauser et al., 2000) but the reason for the Fe staining could also be bacterial iron reduction (Chen et al., 2020 and references therein).Additionally, glauconite encrustation is often associated with borings (Figs.8B, 9I, J).Where borings lack encrustation, this is likely because the borings were reworked when the boring organism was still alive and occupied the boring, or shortly after the boring was abandoned, allowing little or no time for pyrite or glauconite encrustation.However, using chemical signatures for identification should be done with caution, as some borings without pyrite encrustation do occur in FPC from previously described deposits (Cherns, 1980;Siggerud et al., 2000;Vinn and Wilson, 2010;Schwarz and Buatois, 2012;Vinn et al., 2015, Vinn andToom, 2016;Bassi et al., 2017;Brlek et al., 2018).Our micro-XRF results also indicate that burrows with a halo lack pyrite or glauconite encrustation but, it is worth noting that burrows, in rare cases, can be encrusted by or cooccur with pyrite (e.g.fig.4-6e, f in Li, 2019;Wang et al., 2019) and encrusted traces maybe not necessarily be true borings.

Suggestions
This overview represents a potential set of diagnostic criteria for defining fragmentary burrows with a halo/lining and borings from intraclasts.In practice, this is not a simple process, and identifying fragmentary burrows with a halo/lining and borings requires several essential steps.First, it requires identifying possible borings from FPC based on morphology.Secondly, the characteristics of in situ trace fossils near the relevant FPC must be determined, because the degree of fracturing of the hardground could lead to borings in pebbles that resemble fragmentary burrows with a halo.Thirdly, the relative volume of traces versus the volume of the hosting pebble and the number of traces per pebble should be calculated.Finally, the cross-cutting relationship between traces and their hosting clast should be determined.This includes examination of pyrite, phosphate and glauconite encrustations on the pebble and the cross-cutting relationship with the traces (Fig. 12).Considering the lack of data for both a wide range of localities and time periods, our criteria represent an important initial step in distinguishing borings and burrows with a halo in FPC.S1.

Conclusions
Burrows with a thick diagenetic halo and associated borings are preserved both in situ and in FPC from late Cambrian carbonates of North China.Differences between burrows with a halo and borings occurring in FPC are described and discussed.Based on these differences, possible criteria to distinguish borings from burrows in a broken homogeneous substrate are established for the first time and represent an important first step in distinguishing borings and burrows with a halo in FPC.
Both the relative volume of traces versus their hosting pebble and the number of traces per pebble are important criteria.Borings in FPC occupy a relatively smaller volume of the surrounding pebble, ranging from 0.05 to 17.88 % (mean value = 3.06 %), when compared with burrows, which range from 5.68 to 70.70 % (mean value = 26.01%) of pebble volume.The number of burrows per pebble (in most cases n = 1) are significantly different from the number of borings (n = 2-10).In addition, pyrite and glauconite encrustation on the surface of the pebbles, and a cross-cutting relationship between traces and the hosting pebbles (including encrustations and differently colored laminae) are further potential indicators.Supplementary data to this article can be found online at https://doi.org/10.1016/j.sedgeo.2022.106302.

Fig. 1 .
Fig. 1.Satellite images showing the geographic locations of the studied sections.A. Overview of central China, showing the locations of Yongcheng county and Xuzhou City (shown by asterisks).B. Enlarged view, showing the distribution of the Mangshan, Dananzhuang and Zhaishan sections (asterisks).C. The Mangshan section near Mangshan Town (indicated by an asterisk).D. The Dananzhuang section and Zhaishan section in Xuzhou City (line and asterisk, respectively).

Fig. 2 .
Fig. 2. Lithological column for the study area.A. Summary of the Cambro-Ordovician succession around Xuzhou City.B. Lithological column of Gushan and Chaomidian formations at the Dananzhuang section.C. Lithological column of the Chaomidian Formation from the Mangshan section.

Fig. 3 .
Fig. 3. Burrows with halo preserved in situ, co-occurring with soft-sediment deformation, from late Cambrian deposits of the Mangshan Section, Yongcheng County, North China.A, B. Vertical section with oval-shaped burrows surrounded by yellowish carbonate.C. Bedding-plane view showing a slightly curved unbranched burrow (?Planolites), some of which exhibit an oval-shaped cross section.D. Bedding-plane view showing a branching burrow (Thalassinoides horizontalis) with a thin halo and filled with yellowish lime mud.

Fig. 4 .
Fig. 4. Interpretative line drawing of Fig. 3, blue represents micrite halo, yellow represents burrow fill, and orange and pink indicating matrix.

Fig. 5 .
Fig. 5. Broken burrows with a diagenetic halo (i.e., ichnoclasts) preserved in flat-pebble conglomerates. A. Reworked burrows with a thick halo from the Chaomidian Formation, Zhaishan section.B. Fragmentary burrows with a thin halo and oval cross section from the lower part of the Chaomidian Formation, Dananzhuang section.C. Fragmentary branched burrows with a thick halo from the Mangshan section.D-F.Interpretative line drawings of A-C, respectively, with yellow representing the burrow fills, blue representing the micritic diagenetic halo, and gray representing the matrix.G, H. Dense accumulation of reworked burrows with a micritic halo from the upper most of the Chaomidian Formation in the Dananzhuang section, in vertical view (G) and bedding-plane view (H).The red dashed lines indicate erosional surfaces, white arrows pointing to fragmentary burrows with a micritic diagenetic halo.The diameter of the coin is 2 cm.FPC B1 = flat-pebble conglomerates bedding 1, FPC B2 = flat-pebble conglomerates bedding 2.

Fig. 6 .
Fig. 6.Polished slab composed of in situ preserved and reworked burrows with a halo and possible borings in pebbles recovered from the upper part of the Chaomidian Formation, upper Cambrian, Zhaishan section at Xuzhou City, North China.A. Scanned photograph of the polished slab, yellow arrows showing the boring and white arrows pointing to the reworked burrows with a halo.B. Enlarged area of white dashed rectangle in A, showing boring in pebble and fragmentary burrows with a halo.C. Interpretative drawings of B, showing fragmentary burrow with homogeneous micritic halo and boring in pebble with differently colored laminae, with yellow representing the burrow fills, red representing the boring, blue representing the micritic diagenetic halo, and gray representing the matrix.

Fig. 7 .
Fig. 7. Micro-X-ray fluorescence (XRF) mapping of broken burrows with a halo and in situ preserved burrow with a halo, co-occurring with true borings from upper part of the Chaomidian Formation in the Zhaishan section, Xuzhou City. A. Micro-XRF mapping results of Mg with high concentration of Mg in the matrix.B. Micro-XRF mapping results for Ca showing higher concentration of Ca in the burrow fills.C. Micro-XRF mapping results, indicating where high concentrations of K occur.D. Micro-XRF mapping results, indicating where high concentrations of Al occur.E. Micro-XRF mapping results with high contents of Si in matrix.F. Micro-XRF mapping results, showing results for Si and Al.G. Micro-XRF mapping results for Fe, with high contents of Fe in the matrix and higher contents in the halo.H. Micro-XRF mapping results of P, with a homogeneous distribution in halo and burrow fills and low concentrations of P in the matrix.White arrows indicate burrows with a halo, both in situ and reworked.

Fig. 8 .
Fig. 8. Micro-X-ray fluorescence (XRF) mapping of polished slab containing borings and truncated burrow system from the Chaomidian Formation in the Mangshan section, North China.A. Polished slab used for micro-XRF test, yellow arrows showing the borings overlapping the truncated thick halo burrow system, green dashed rectangle showing the area mapped using micro-XRF.B. Enlarged area from dashed rectangle in Fig. A, yellow arrows indicating encrusting pyrite.C. Micro-XRF mapping results with high concentration of Ca in host rock.D. Micro-XRF mapping results showing high concentration of S in the surface of boring and in the truncated surface.E. Micro-XRF mapping results with high concentration of Fe in borings, truncated surface and burrow halos.F. Micro-XRF mapping results indicating high concentration of Si in borings, truncated surface and halos around burrows.G. Micro-XRF mapping results with high concentration of Al in borings, truncated surface and halos around burrows.Yellow arrows indicating enrichment of Fe and S on the pyrite encrusted surface.White arrows point to the truncated burrow (with halo) system, yellow arrows indicate encrusted pyrite, yellow dashed line indicates the truncated surface.

Fig. 9 .
Fig. 9. Micro-X-ray fluorescence (XRF) mapping of polished slab of flat pebble conglomerate with borings from the upper part of the Chaomidian Formation at Dananzhuang section, North China.A. Polished slab for micro-XRF test.B. Micro-XRF mapping results with high concentration of Ca in pebbles.C. Micro-XRF test showing high concentration of K in matrix.D. Micro-XRF tests showing where high concentration of Al occurs.E. Micro-XRF tests displaying higher concentrations of Si.F. Micro-XRF tests with high concentrations of Fe in matrix and the margin of pebbles.G. Micro-XRF tests showing a relative homogeneous distribution of P. H. Micro-XRF tests with a relative homogeneous distribution of S in the whole section.Yellow arrows show the borings with red arrows indicating the synsedimentary structure of the flat pebble conglomerate.I. Enlargement of the pebble in Fig. A, displaying two borings, penetrating the laminae in the pebble.J. Interpretative line drawing of Fig. I, yellow color represents the hosting pebble, with green color representing the glauconite encrustation on the boring and hardground surface.

Fig. 10 .
Fig. 10.Histograms of relative volume values for fragmentary burrows with a halo and borings occurring in breccias.A. Histogram of relative volume of burrow occupation in intraclasts from late Cambrian, with added normal distribution curve.B Histogram of relative volume of trace fossils occupation in intraclasts from Lee et al. (2018), with the normal distribution curve added.C. Histogram of relative volume of trace fossils occupation in intraclasts including data from Lee et al. (2018) and data from A, showing a similar pattern in both studies.D. Histogram of relative volume of boring occupation in intraclasts, with the distribution curve different to the curve generated for burrows.E. Histogram of number of borings per clast.F. Histogram of number of burrows per clast.Data and references shown in TableS1.

Fig. 11 .
Fig. 11.Line diagram showing branching burrow with a halo occurring in softground fractured at different angles.A. Oval shaped fragmentary burrow with a halo.B. I-shaped reworked burrow with a halo.C. Clavate fragmentary burrow with a halo; D. Intraclast containing two burrows with a halo.

Fig. 12 .
Fig. 12.A decision tree for distinguishing burrows with a halo and borings in flat-pebble conglomerates (FPC).