ichnology of the yeoman formation€¦ · keywords: upper ordovician, yeoman formation, red river...

Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 1

Ichnology of the Yeoman Formation 1

Rozalia Pak 2 and S. George Pemberton 2

Pak, R. and Pemberton, S.G. (2003): Ichnology of the Yeoman Formation; in Summary of Investigations 2003, Volume 1,Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003-4.1, CD-ROM, Paper A-3, 16p.

Abstract

The recognition and classification of trace fossils in carbonates of the Upper Ordovician Yeoman Formation insoutheastern Saskatchewan are hindered by the complex diagenetic history of these rocks. Since many primarycharacteristics of deposits greatly influence diagenesis, the distinction between sedimentary and diagenetic fabricscan be difficult. This problem presents itself clearly in an examination of the trace fossils of the Yeoman Formation,which is characterized by conspicuous dolomite mottling. To date, it remains debatable whether these trace fossilsrepresent Thalassinoides or sediment dolomitization around smaller, causative burrows. Most ichnological studieshave been performed in clastics or chalks, and the methodology developed for these studies is difficult to apply tosimilar research in Paleozoic platform carbonates. The purpose of this paper is to describe the trace fossils of theYeoman Formation, and to explore their potential usage in determining changes throughout the deposition of thisthick platform carbonate sequence.

In the examination of the biogenic sedimentary structures of the Yeoman Formation, nine discrete trace fossils areobserved, many being part of composite burrow systems. Except for Trypanites, Trichophycus, and Palaeophycus,these trace fossils are indicative of feeding activities. Their diversity of form gives the impression of a diversebenthic fauna, but the relatively uniform diameter of the feeding burrows suggests that a small group of organismsmay have been responsible for the various forms. These burrowing organisms shifted their feeding behaviours inresponse to changes in paleoenvironmental conditions, such as water energy, depth, oxygenation, and nutrientavailability. The association of more complex feeding structures with the larger, vegetative-state, disseminated BGloeocapsomorpha prisca alginite indicates that harsher conditions, which accompany the algal blooms, forcedinfauna to adapt their feeding behaviours.

Keywords: Upper Ordovician, Yeoman Formation, Red River Formation, Asterosoma, Rhizocorallium,Thalassinoides, Trichophycus, composite burrows, biogenic sedimentary structures.

1. Introduction

The complex diagenetic history of carbonates of the Upper Ordovician Yeoman Formation in southeasternSaskatchewan hampers the recognition and classification of contained trace fossils. Many primary characteristics ofdeposits greatly influence diagenesis, commonly making distinction between sedimentary and diagenetic fabricsdifficult. An example of this problem is presented by the conspicuous dolomite mottling in rocks of the YeomanFormation: to date, whether the mottling represents Thalassinoides or sediment dolomitization around smaller,causative burrows remains debatable. Most ichnological studies have been carried out in clastics or chalks(Kennedy, 1975), sediments so different from Paleozoic platform carbonates that the methodology established forthem is commonly difficult to apply to the Yeoman strata. This paper describes the trace fossils of the YeomanFormation and explores their potential usage in determining changes throughout the deposition of this thickplatform carbonate. Although diverse feeding structures are represented in these sediments, it appears that thebenthic fauna was restricted, but capable of adapting behaviour to changes in sedimentation, water energy,oxygenation, and nutrient supply.

2. Methods

Cores, thin sections, and UV light petrography were utilized in the stratigraphic, sedimentologic and diageneticanalysis of the Yeoman Formation. Twenty-three wells (Pak et al., 2001) have been examined from the Midale(Townships 6 and 7, Range 11W2), Tyvan (Township 13, Range 13W2), and Ceylon (Townships 5 and 6, Range19-20W2) pools. Core was logged by identifying the biota, ichnology, and textural relationships in the rocks.Textures were described using Dunham’s (1962) classification scheme with modifications by Embry and Klovan

1 This project is funded by Husky Oil Limited.2 University of Alberta, Department of Earth and Atmospheric Sciences, Earth Sciences Building, Edmonton AB, T6G 2E3, E-mail: [email protected].


(1971). Fifteen of the wells were sampled for thin section, burrow fabric, and associated organic analysis. Onehundred and fifteen thin sections were injected with blue epoxy for porosity determination and stained withAlizarin-Red S (Dickson, 1965) to differentiate calcite from dolomite. Subsequently, samples selected fromdifferent trace fossil associations were prepared for examination of organic matter contained within burrows usingreflected light microscopy. These samples consisted of blocks measuring approximately 2 cm x 2 cm x 2 cm thatwere set in an epoxy resin and then polished using increasingly finer grit sizes and finally a slurry of 0.05 µmalumina suspended in water. A Zeiss Axioplan II microscope equipped with a 100 W UV light source was used formaceral analysis. Samples were observed under water immersion objectives (40X Apochromat, NA = 1.2;magnification range 400x to 1000x) using 365 nm excitation and 420 nm barrier-emission filter to optimize imagingof kukersite microscopic constituents. Digital images were captured using a Zeiss Axiocam system and Zeiss Visionsoftware.

3. Recognizing and Classifying Trace Fossils in Carbonates Subject to Heavy Diagenesis

Trace fossil classification primarily focuses on morphology. Secondary considerations include such factors asburrow lining, contrast between the burrow fill and matrix sediment, host sediment characteristics, and the nature ofgrain packing. Many trace fossils were originally described in clastic sedimentary rocks, where the original natureof the grains is much less influenced by recrystallization, replacement, and fabric-destructive diagenesis than incarbonates, where diagenetic alteration can effectively mask original sedimentary fabrics and primary mineralogy.Many trace fossils owe their preservation and distinctive features to diagenetic fabrics. Diagenesis, however, servesto enhance their appearance only to a point, beyond which destructive processes of recrystallization, replacementand, commonly, extensive dissolution take over, and burrows and all primary features fade from recognition.

Increased burrow densities, the absence of burrow linings and weak contrast between burrow fill and host sedimentcan make the discernment of discrete trace fossils difficult (Fürsich, 1975). Preservation is also skewed toward thelatest and deepest “tier” of burrowing, which has the highest preservational potential (Bromley and Ekdale, 1986).All things considered, the greatest limitation to examining the Yeoman trace fossils is the unavailability of data.Core description rarely allows a 3D view of trace fossil morphology. Weathered exposures, which have providedsamples for many ichnological studies, are not available so the 3D shapes of the observed biogenic sedimentarystructures must be inferred from repeated patterns and direct observations in core, where the horizontal expressionof individual trace fossils is limited by core diameter. Bedding planes and contacts are rare in the YeomanFormation where the overlying and underlying sediments are similar, and it is most commonly only firmground andhardground surfaces that are recognizable. Bedding plane recognition is also reduced by abundant pressure solution.Stylolites and solution seams commonly form at contacts between differing substrates, thus preventing therecognition of interface trace fossils, such as tracks and trails of benthic epifauna.

Taken together, the above factors not only make it difficult to accurately determine substrate controls, but alsohamper interpretation of the behaviour of the trace producer. Where the contrasting nature of the burrow fill andmatrix determine the feeding behaviour of the trace producing organisms (Kotake, 1989), diagenesis can destroythis crucial information.

Yeoman trace fossils are classified keeping the foregoing limitations in mind along with the following factors.Original fabrics are inferred by examining corresponding intervals. The zone least affected by diagenesis is taken tomost closely resemble original fabric and mineralogy. Locally, present-day fabrics proved to be useful, since rocksof differing original mineralogy and fabrics may have undergone different diagenetic pathways. For example,increased porosity or crystal size in a burrow fill may be indicative of a syndepositional contrast between the burrowfill and the host matrix. Comparison with studies of other carbonates, especially age-equivalent rocks, is useful innoting how other researchers dealt with these complications. Due to these various difficulties, trace fossils areclassified only at a generic level. Details required to identify them at species level are not readily available in thiscore study.

Finally, it must be noted that, since freshly slabbed core surfaces and outcrops are not readily available, the moredistinct trace fossils may skew observations concerning trace-fossil assemblages and abundances. Where possible,the trace-making behaviours are discussed in this paper as they are important in discussing the ecologicalimplications of trace-fossil associations

4. Palaeophycus and Planolites

These morphologically simple burrow types are discussed together for two reasons. First, they are the mostabundant in the Yeoman, commonly occurring together in all the different fabrics of the formation. Second, thedistinction between these two trace fossils has caused extensive debate even in examination of clastic sedimentaryrocks (Pemberton and Frey, 1982; Keighley and Pickerill, 1995).


a) Description of Ichnogenus Palaeophycus Hall, 1847

Burrow orientation is mainly horizontal with minor subhorizontal to (sub)vertical components (similar vertical,straight burrows that occur alone are classified as Skolithos). Burrows are straight to curved, and circular toelliptical in cross-section (Figures 1 and 2). Their diameter is constant along the burrow axis and ranges from lessthan 1 mm in some intervals to nearly 5 mm. Burrows are rarely branched, and the type of branching is notdiscernible. They have regular, smooth external ornamentation. The organic-rich lining of the burrows appears tohave aided in both their preservation, which is generally good to excellent, and recognition. In intervals dominatedby Planolites and Palaeophycus, burrow linings, which vary in thickness and locally appear annulated, comprise avariety of macerals (Gloeocapsomorpha prisca; spiny, acanthomorphic acritarchs; Leiosphaeridiai) and zooclasts(scolecodonts and chitinizoans). In intervals that also contain Asterosoma, Rhizocorallium, and composite burrows,however, the linings predominantly consist of G. prisca (notably the disseminated B G. prisca maceral variety sensuStasiuk and Osadetz, 1990), algal detrinite, and bitumen. Due to the lack of prepared samples, microfossils in thelinings were not closely examined. Palaeophycus generally has a homogeneous internal burrow fill that may berecrystallized as saddle dolomite or coarser dolomite, or that may be dissolved, leaving the burrow core hollow. Thefill, where unaltered, is similar to the host rock. However, it is rarely nondolomitized. Interestingly, nondolomitizedPalaeophycus occurs in places amidst dolomitized Palaeophycus. Re-burrowing by Planolites is sometimesobserved. Burrow density depends on facies.

b) Discussion

Palaeophycus is distinguished from Planolites primarily by wall linings and the character of the burrow fills.Although Palaeophycus is found in most of the Yeoman intervals, it is particularly abundant in those rich in

allochems but depleted in dispersed organicmatter/kerogen and mud. Infills of Palaeophycusrepresent passive, gravity-induced sedimentation withinopen, lined burrows (Pemberton and Frey, 1982),although this cannot specifically be confirmed forYeoman Formation occurrences. Palaeophycus isgenerally interpreted as the open dwelling burrows ofsuspension feeding or carnivorous animals (Pembertonand Frey, 1982). The presence of collapse structures andgeopetal fills in the Yeoman burrows supports thisinterpretation. The primary objective in constructingdwelling burrows is protection, so intervals dominated bydomichnia are thought to suggest well illuminated,shallow-water deposits where predation is greatest. Inmodern environments, dwelling burrows are generallyrestricted to shallow, well illuminated intervals (Schäfer,1972). Possible trace makers may be sipunculid,enteropneust, or polychaete worms. Further examinationof scolecodonts in association with these burrows mighthelp identify the behaviour of the trace-makingorganisms, since scolecodont shape reflects feedingbehaviour (e.g., predation vs. scraping). The presence ofscolecodonts is notable and merits further investigation asscolecodonts, due to their sensitivity to diagenesis, arerarely found in marine sediments with many trace fossils(Schäfer, 1972).

c) Description of Ichnogenus PlanolitesNicholson, 1843

Planolites is generally preserved as horizontal tosubhorizontal, straight to curved (in places, meandering)intrastratal burrows (Figure 3), which are rarely branched,but crosscut and interpenetrate each other. Burrows aresmooth walled and homogeneously filled. They rarelyexhibit a meniscus structure and may contain secondaryPlanolites of equal or smaller diameter. Burrow diameterranges from less than 1 mm (discernible onlymicroscopically) to almost 10 mm and is unaltered bybranching. Cross-sections are circular to elliptical, thelatter resulting from compactional flattening or oblique

Figure 1 - Palaeophycus (Pa) with thin dolomitic diagenetichaloes (Tri Link Tyvan 21/8-17-13-13W2, 2170.8 m).

Figure 2 - Palaeophycus with dolomitic halo.Photomicrograph taken under polarized light (Berkley et alMidale 41/2-10-7-11W2, 2592.9 m).


sectioning of the burrow. Burrow thickness is generallyconstant along the observed burrow length (absolutelengths of burrows have not been determined). The filldiffers from host rock in that it generally contains lessdispersed organic matter and kerogen, but it may also bedolomitized, or contain a different dolomite type than thatof the host rock. The fill may contain a contrast inallochem abundance (greater or smaller) or the burrowmay contain oriented/structured allochems. These last fewcriteria generally require thin sections for recognition.Burrow density is dependent on facies, generally beinghigher in muddier substrates.

d) Discussion

Planolites is found in all the Yeoman facies in variableabundance, and may occur alone or as secondary burrows in Thalassinoides, Asterosoma, Rhizocorallium, orTrichophycus. It may form monospecific assemblages or be in association with Chondrites. Planolites isdistinguished from Palaeophycus primarily by having unlined walls, and burrow fills that differ from the adjacentrock in texture fabric, composition, and colour. These fills represent sediment processed by the trace maker,especially through deposit-feeding activities of mobile endobionts (Pemberton and Frey, 1982). In the Yeomanspecimens, the local development of weak menisci further supports an active back-fill interpretation.Interpenetrations and re-burrowed segments of Planolites are easily confused with true branching, which iscomparatively rare. Petrographic investigation is commonly required for identification, and that is not alwayspossible. Since the Planolites organism re-burrowed numerous other kinds of pre-existing traces, the latterpresumably contained enhanced nutrient levels such as dispersed organic matter and/or represented an easier pathfor new burrowing activities. The first of these two possible causes for Planolites re-burrowing seems to be valid asthe fill of the last tier of burrows has the lowest content of organic matter. Planolites is generally interpreted torepresent deposit-feeding behaviour. Preservation of mucous linings is unlikely in sediments subject to suchextensive diagenesis as in the Yeoman Formation, so some burrows, due to an apparent lack of burrow lining, maybe incorrectly classified as Planolites.

e) Burrow Lining or Diagenetic Halo?

Central to the diagnosis of these forms is the presence or absence of wall linings (Pemberton and Frey, 1982).Diagenetic haloes can readily be mistaken for linings in a megascopic examination, so thin sections may be requiredto differentiate between the two. Burrow lining is indicated by a concentration of organic matter and/or bitumen,evidence of spreite at burrow walls or, where there is a thick wall, by contrasting abundance of organic matter.Increased porosity along burrow walls is taken to suggest a dissolved lining or mucous layer. A zone of “arranged”allochems found around burrows commonly indicates a zone of deformation caused by the organism’s passingthrough a semi-consolidated (plastic) substrate, rather than a burrow lining (Rhoads, 1970).

Diagenetic haloes are, in fact, very common in the Yeoman Formation and range from less than 1 mm to greaterthan 10 mm in thickness. Most burrowing organisms, when moving through sediment, secrete some mucus or otherresidue (Schäfer, 1972; Bromley, 1996). This mucus commonly acts as a catalyst for the formation of diagenetichaloes. Keighley and Pickerill (1995) suggested that a burrow lining might be inferred if the diagenetic halo extendsfrom the burrow boundary into both the burrow fill and the host lining. In the Yeoman Formation, this criterionfailed to prove useful, as the fills of both lined and unlined burrows are commonly dolomitized.

f) Determining the Nature of Original Burrow Fill

The secondary ichnotaxobase is the burrow fill. In carbonate sedimentary rocks like those of the YeomanFormation, recognition of the burrow fill is problematic because the fill is diagenetically altered (difference of theinferred burrow fill from the matrix at the time of sedimentation, not their present-day difference, must be used asthe ichnotaxonomic criterion). Where dissolution of the burrow core has occurred or dolomitization andrecrystallization have been fabric destructive, burrow fill can be misinterpreted, leading to incorrect classifications.This approach to identification is viable only if the burrow fill and matrix presently reflect their primary texturalstate.

g) Branching or Crosscutting Burrows?

The crosscutting of burrows can easily be confused with true branching. Evidence to truly distinguish branchingfrom crosscutting burrows may be enigmatic, and in some cases may not be interpretable (Keighley and Pickerill,

Figure 3 - Planolites (Pl) with thin diagenetic haloes from adolomitized mudstone interval (Husky Ceylon 41/6-5-6-19W2, 2746.3 m).


1995). This holds true for the Yeoman Formation, where distinction between abundant Planolites and Chondrites isoften arbitrary.

5. Composite Burrow Systems

In ichnological studies, all biogenic sedimentary structures are commonly classified as discrete trace fossils. Assuch, a false impression of a diverse benthic community may result. In many intervals of the Yeoman Formation,composite burrow systems are identified which reflect a combination of organism behaviours, and classification as asingle burrow type is impossible. Discrete burrows, which in addition to Planolites and Palaeophycus arecommonly part of these systems, will be discussed in subsequent sections.

a) Description of Composite Burrows

These complex biogenic sedimentary structures are simply or multiply lined, anastomosing burrow systems, whichexhibit branching and crosscutting, evidence of U-shaped portions, spreite resulting from shifting of the burrowsystem, as well as spreite in the burrow fill (Figures 4 and 5). The fill is generally dolomitized mudstone, so thecontrast between the host sediment and burrow fill may be a result only of diagenesis. The organic-rich linings maybe thin or up to several millimetres thick, and commonly resemble concentric linings of Cylindrichnus concentricusdescribed by Goldring (1996) and Fürsich (1974b). These linings mainly comprise organic matter that, where non-degraded, is dominantly made up of disseminated A and B G. prisca alginite (sensu Stasiuk and Osadetz, 1990).Amorphous organic matter and algal detrinite are also common in burrow linings. Macerals and zooplankton seen inPalaeophycus linings are rarely observed in these burrow linings. Other portions of the burrow systems exhibitcharacteristics of Phycodes, Asterosoma, and Rhizocorallium (Figures 4 and 5). The burrow diameters of theoriginal and later burrows are similar and do not change with branching. Rarely, a larger diameter Planolitescrosscuts a smaller, lined burrow.

b) Discussion

The concentric linings are believed to have originated by the construction of a multi-layered wall whereby theorganism added successive walls pushing early-formed layers outwards (Aller and Yingst, 1978, in Goldring, 1996).Of Goldring’s three hypotheses for formation of concentrically lined burrows, Aller and Yingst’s method seemsmost applicable here because the material that lines the burrows is mostly planktonic, appears to have beenintroduced into the burrow, and is absent from the matrix sediment. These complex feeding systems may have

initially served as the dwelling burrows of filter-feedingor carnivorous organisms. Once the environmentalconditions changed, e.g., possibly less nutrient availabilityin the water column, these organisms altered theirbehaviour to exploit organic matter and organic detritusthey had used earlier to line their burrows. The last tier ofPlanolites and/or Chondrites indicates deposit-feedingactivities. Because organisms are known to change theirfeeding patterns diurnally and seasonally (Schäfer, 1972;Hummel, 1985), other changes in environment are notnecessarily responsible for changes in feeding behaviour.

Consistent burrow diameter throughout these systemssuggests that all the tiers of these burrows were created bythe same or similar organisms (Figure 5). Consequently,although diverse feeding patterns are preserved, thesesystems may have been produced by a single or smallgroup of organisms. The inference that the suspension-feeding components of these composite burrows and othersuspension-feeding burrows described from the YeomanFormation were formed by a single species is consistentwith Turpaeva’s hypotheses (1957 in Walker, 1972)concerning trophic relationships of benthic fauna. Basedon an examination of feeding ecology of modern benthicanimals, she believed that one trophic group that containsthe most prevalent species generally dominates acommunity. This, she inferred, is characteristic of a stablecommunity in which the organisms attain anoncompetitive feeding arrangement. These compositeburrows, which generally occur in intervals believed to

Figure 4 - Horizontal section through composite burrows inthe Yeoman Formation. Note repeated reworking ofsediment (Husky Ceylon 41/10-18-5-20W2, 2771.5 m).


coincide with G. prisca algal blooms (Kent and Haidl,1999), presumably resulted from a species of polychaetedominantly consuming this algal microfossil by asuspension-feeding mechanism. When conditions forsuccessful filter feeding were interrupted, this worm-typeorganism might have adapted to a deposit-feedingbehaviour that exploited the same algal microfossil thatwas readily available in the sediment.

6. Thalassinoides

The characteristic dolomite mottling common in LowerPaleozoic platform carbonates has often been interpretedas large branching burrow networks of either theichnogenus Spongeliomorpha or Thalassinoides (Bottjeret al., 1984; Zenger, 1992, 1996; Myrow, 1995; Zengerand LeMone, 1995). This interpretation has beenfavoured for the dolomite mottling of the YeomanFormation (Kendall, 1976, 1977; Canter, 1998; Kissling,1999), although others believed that the mottlingrepresents dolomitic haloes, which formed aroundPlanolites, Palaeophycus, and Chondrites (Carroll, 1978;Gingras, 2000). These two interpretations have differentimplications on the origin of the burrows found in thedolomite mottles. According to the former model, theburrows are secondary, but the latter model infers theyare causative burrows, which facilitated dolomitization ofthe matrix.

Due to this disparity in views regarding dolomitemottling, the mottling in the Yeoman Formation requiredclose examination. According to Kendall (1977), threefeatures remain in question if the mottles are interpretedas diagenetic haloes:

1) The bimodal distribution of mottle diameters (eitheruniformity or a wide range of mottle diameters wouldbe expected if the mottles represent diageneticalterations around burrow centres).

2) The otherwise uniformity in the size of dolomitemottling throughout the thickness of the Yeoman–lower Red River interval across the whole WillistonBasin (this uniformity suggests a burrow origin).

3) The mottles sometimes lack internal burrows, or theycontain more than one burrow or burrows that aremarkedly eccentric (if dolomitization had proceededuniformly outward from the burrow centres togenerate cylindrical mottles, the mottles shouldalways contain centrally located burrows).

Gingras (2000), on the other hand, believed that, since thedolomite mottles do not exhibit regular sharplydemarcated boundaries and constant burrow diametersthroughout the network, they did not represent biogenicsedimentary structures. His study, however, focused onthe surface Tyndall Limestone that, although it can beexamined on a larger scale in outcrop, displays somedifferent sedimentary characteristics than the subsurfaceYeoman Formation.

Thalassinoides-like burrows have been found in manyOrdovician platform carbonates (Sheehan andSchiefelbein, 1984). Although the biological affinities of

Figure 5 - Composite burrows with many Rhizocorallium-like components. Diameters of various burrow types areconsistent, indicating that the same organism reworkedthese sediments. Planolites burrows in matrix are onlyfaintly visible as their appearance is not enhanced bybitumen staining. As, Asterosoma; Ch, Chondrites; Pa,Palaeophycus; Pl, Planolites; Rh, Rhizocorallium; and Si,Silica nodule (Berkley et al Midale 11/7-3-7-11W2,2576.8 m).


the organisms responsible for these networks have not yet been discovered, it seems plausible that at least some ofthe dolomite mottles in the Yeoman Formation represent Thalassinoides. Lack of Yeoman Formation outcropprevents a diagnosis based on plan geometry, i.e., determination of vertical and horizontal extent of these largecharacteristically branching networks. The mottle networks found in outcrops of both the Tyndall Limestone andLower Yeoman (Red River) Formation (Figures 6 and 7) greatly resemble other recorded Lower PaleozoicThalassinoides (Sheehan and Schiefelbein, 1984; Watkins and Coorough, 1997). The proper taphonomic conditionsmay not have existed to preserve Thalassinoides in an identifiable form. This, combined with diagenetic destructionof many primary sedimentary structures, make the determination with absolute certainty of the presence ofThalassinoides in the Yeoman Formation impossible.

a) Description of Thalassinoides Ehrenberg, 1944

Burrows are preserved intrastratally in vertical section. The orientation of their limbs is vertical and horizontal,although locally some are inclined, or portions of some are inclined. These burrows exhibit both Y and T-shapedbranching with burrow diameter apparently remaining constant along branches. Burrows are generally straight orslightly curved. Their cross-section is circular or elliptical. Burrow walls are smooth, regular and unlined, butstylolites/pressure solution may in places give the appearance of thin organic-rich lining. Burrow fill ishomogeneous, but may contain secondary Planolites and Chondrites. It is commonly dolomitized, occasionallysilicified, and shows contrast in allochem abundance relative to the host rock. Burrow diameter ranges from 5 to20 mm. Burrow length is indeterminable. The burrow density is variable from few discrete Thalassinoides toThalassinoides comprising about 50 percent of the rock volume. Thalassinoides generally has poor preservation or

recognition potential.

b) Collective Indicators of Thalassinoides in theYeoman Formation

Although no single criterion in itself can be used todistinguish diagenetic haloes from burrows, acombination of criteria points to favouring oneinterpretation over another. In intervals where many ofthe following criteria are met, mottles are interpreted asThalassinoides.

• Dolomite mottles contain intraclasts made of thesame material as the matrix. Such occurrences, someof which appear to represent collapse structures, arerare. This is probably the best evidence that what arenow mottles were once open burrows. Note that thewalls of these mottles are not sharp (Figure 8).

• Contrast in fabric, allochem abundance and types,and abundance of organic matter between mottle andmatrix sediment. Since the dolomitization whichformed the mottling is generally fabric destructive,an increased abundance or variety of allochemswithin the burrow is in itself a valid criterion toidentify the structure as a burrow; however, theinverse cannot be claimed with such certainty (Figure9).

• Absence of causative burrow from mottles over aninterval. Absence is only certain when the burrow isseen in cross-section. An oblique section may missthe causative burrow. Although causative burrowsmay in places have been obliterated by thedolomitization that formed the diagenetic mottle,their widespread presence helps validate theassumption that, where absent, they never initiallyexisted.

• The mottle wall is cut by burrows (i.e., the mottles donot strictly follow or completely contain smaller

Figure 6 - Thalassinoides (Th) parallel to bedding planefrom outcrop of the lower Red River/Yeoman Formation.Preferential growth of fungi indicates burrow patterns.Located near Amisk Lake, Saskatchewan. Pencil 14 cmlong.

Figure 7 - Thalassinoides-like burrows indicated bydifferential weathering of these dolomitized sediments andpatterns of fungi growth. Outcrop from western shore ofAmisk Lake, Saskatchewan. Pencil 14 cm long.


burrows), or the mottle wall crosscuts anotherburrow (Figure 10).

• Mottles generally have sharp walls; however thiscriterion is not always the best (see discussionbelow).

• Large mottles and small Planolites exhibit thesame diagenetic fabrics and the walls of both havethe same “sharpness”.

• Mottles exhibit Y- and T-branching, characteristicof Thalassinoides (Figure 11).

• Mottle diameter appears constant along the burrowaxis throughout an interval.

• Mottles exhibit mechanical compaction and may inplaces be “broken”, squashed, or flattened. Thepresence of mechanically compacted mottlessuggests mottle formation occurred prior tosediment lithification.

• Re-burrowing by Planolites; the re-burrowing oflarge burrows by deposit feeders is common,especially where the fill of the large burrow mayprovide a better food source for the laterburrowers (Bromley, 1996).

• Mottle occurrence in intervals that containfirmgrounds with Thalassinoides. This is merelycircumstantial evidence – if organisms existed thatwere large enough to create Thalassinoides at thefirmground intervals, would they not have existedbefore and after?

Figure 8 - Roof collapse as evidence of formerly openburrow systems. H, molds of halite hopper crystals with voidfilling saddle dolomite cement; Pa, Palaeophycus; Pl,Planolites; and Th, Thalassinoides (Berkley et al Midale31/11-34-6-11W2, 2606.2 m).

Figure 9 - Wackestone fill of Thalassinoides (Th) in amudstone matrix (Husky Ceylon 11/5-8-6-19W2, 2713.3 m).

Figure 10 - Thalassinoides (Th) re-burrowed byPalaeophycus (Pa), with Palaeophycus crosscuttingThalassinoides wall (Husky Ceylon 41/10-18-5-20W2,2778.6 m).


One of the criteria noted by Bromley (1996) and cited byGingras (2000) to determine if a structure is a burrow ornot is the presence of a regular, sharply demarcatedboundary. Here, this criterion is regarded to beinsufficient evidence for differentiating between adiagenetic fabric and biogenic sedimentary structuresbecause many indisputable Planolites have walls that arenot sharp as they have apparently been obscured bydolomitization. If the dolomitization process can ‘blur’ aPlanolites wall, it should also be able to obscure aThalassinoides wall. Furthermore, some of the mottlescontaining intraclasts did not have sharp walls. Lack ofburrow wall sharpness may also result from factors otherthan diagenetic blurring. If the biogenic sedimentarystructures were formed in a soupy or soft substrate, andlittle evidence is preserved to suggest that the precursorwas a coarser grain texture, mechanical compaction of themottles can blur the walls of large Planolites orThalassinoides. Added evidence in these intervalsindicating that the mottles formed before sedimentlithification is the presence of molds after halite hoppersthat must have developed in a void space or in sedimentsoft enough for them to displace material as they grew.These hopper crystals, now infilled with saddle dolomiteor, less commonly, anhydrite, are frequently (but notexclusively) found in dolomite mottles that fit the othercriteria for Thalassinoides.

c) Collective Indicators of Diagenetic Haloes in the Yeoman Formation

The following criteria, when found together within an interval, are here taken to collectively favour a diagenetichalo interpretation.

• No crosscutting observed between smaller burrows and the dolomite mottles over the interval.

• Contrast between mottle and matrix at the outer boundary of the halo is gradational petrographically, and inabundance of allochems and organic matter (Figure 12).

• Dolomitization of the halo appears fabric destructive, and allochems are faintly preserved or gradually decreasein abundance towards the causative burrow.

• Diagenetic front of the dolomite halo terminates against a macrofossil, but in other places continues fartherfrom the causative burrow.

• Palaeophycus burrows, when not centrally locatedwithin dolomite mottles, may be causative structureswith diagenetic haloes.

• Mottles do not exhibit much mechanical compaction(i.e., no squashed mottles over an interval).

The distinction between Thalassinoides and diagenetichaloes remains locally uncertain, especially in intervalsthat are intensively affected by diagenesis. In nodularzones and stylo-mottled zones, the 3D configuration isimpossible to determine. Stylolites have furtherobliterated the nature of the mottle walls. The origin ofmany nodular and rubbly textures in limestones anddolostones has been attributed to burrowing (Fürsich,1972), so these textures in the Yeoman may, in places,have originated as large burrow systems that are nowimpossible to distinguish.

Figure 11 - Thalassinoides (Th) with Y-shaped branching inthe horizontal plane. Re-burrowed by Planolites (Pl). NotePalaeophycus (Pa) without diagenetic haloes (HuskyCeylon 11/5-8-6-19W2, 2719.2 m).

Figure 12 - Dolomitic diagenetic haloes commonly formaround Palaeophycus and Planolites. Polarized light (HuskyCeylon 11/5-8-6-19W2, 2710.0 m).


d) Discussion

Those burrows identifiable with greatest certainty as Thalassinoides generally occur in the same carbonate intervalsthat contain kukersites. Thalassinoides is common at the firmground contacts between carbonate and superjacentkukersite. Many burrows are re-burrowed by Planolites. Some examples of Thalassinoides are found in otherYeoman intervals. Thalassinoides most likely represents the fodinichnia and domichnia of arthropods or largeworms. Phyllopods were suggested by Bottjer et al. (1984). Thalassinoides burrows are generally interpreted asdwelling or combined feeding/dwelling structures. The interpretation generally applied to Thalassinoides behaviourcannot be applied because the arthropods (e.g., shrimps) responsible for making these burrows had not yet evolvedin the Ordovician. Until a possible trace producer is identified, the Lower Paleozoic Thalassinoides may remain anenigma (Myrow, 1995). No organism has been observedin any Thalassinoides burrow, and until an example isfound the trace maker cannot be identified with certainty(it may never be found if the trace was produced by asoft-bodied organism with very low preservationpotential).

The Lower Paleozoic dolomite mottling with diffusedolomite walls may represent burrows formed inthixotropic sediment. Although the walls of these mottlesresemble diagenetic fronts, they also fit criteria listed byRhoads (1970) for burrows formed in a soupy(thixotropic) substrate. It may be possible that someunknown animal of Lower Paleozoic age mined thesediment prior to any dewatering and lithification, andthat the fill of its burrows was more susceptible todolomitization. The burrow walls remained blurry due tolack of substrate consistency.

7. Chondrites

a) Description of Chondrites Sternberg, 1833

Burrows are smooth-walled and are found in vertical andhorizontal section (Figure 13). Branches are subvertical tohorizontal; however, some zones are dominated byhorizontal burrows. Locally, the branching is dendritic,but generally the orders and nature of branching areindeterminable. Cross-sections are elliptical to circularand range from less than 1 mm to 3 mm in diameter. Theinternal sediment is homogeneous, and through itspaucity of organic matter/kerogen, generally contrastswith the matrix sediment (though locally Chondrites hasfill that is richer in organic matter/kerogen than thematrix, see Figure 14). Preservation of these burrowsranges from good to poor.

b) Discussion

Chondrites is common in muddy, organic-rich substrates,and as secondary burrows of Thalassinoides andcomposite burrows systems. It is also common in organic-rich kukersites. Chondrites burrows may occurexclusively with Planolites, or as part of a compositeburrow system. They are generally interpreted asfodinichnia, but despite their common occurrence insediments throughout the Phanerozoic, the trace makerhas not yet been identified. The vertical upper portions,suggested by Kotake (1991) to be domichnia, are notobserved. Chondrites indicates a lack of oxygenation ofthe substrate only when occurring as a monospecific suite(Bromley and Ekdale, 1984). However, since itcommonly occurs as the deepest tier of trace fossils(Bromley and Ekdale, 1986), it may appear to be a

Figure 13 - Chondrites (Ch) in an organic-rich mudstone(kukersite) (Berkley et al Midale 41/2-10-7-11W2,2576.5 m).

Figure 14 - Chondrites (Ch) with fills rich in organicmatter (dominantly Gloeocapsomorpha prisca alginite) (TriLink Tyvan 21/8-17-13-13W2, 2165.0 m; scale bar is 1 cm).


monospecific suite because its high density has obliterated all other burrows. Chondrites is found alone in some, butnot all, kukersites. The trace-making organism may have colonized the substrate after the deposition of thekukersite, so Chondrites does not necessarily indicate a low-oxygen environment for kukersite deposition.

8. Trichophycus

a) Description of Ichnogenus TrichophycusMiller and Dyer, 1878

Burrows are found in subhorizontal to horizontal section(Figure 15). Trichophycus generally occurs as straightburrows that are circular to elliptical in cross-section andhave diameters which range from one to severalcentimetres (one observed chamber had a diameter of10 cm) and which locally vary along burrow axes. Thereis little evidence of branching. Walls are irregular, locallycontaining ridges or grooves, which suggest scratch-marks. Stylolites and bitumen concentrated at burrowwalls give the impression of thin organic-rich lining.Burrow fill is homogeneous, and, partly through having ahigher organic content, distinctly differs from the hostrock. Usually dolomitized, the fill commonly containssecondary Planolites and Chondrites whose fills havelower organic contents. Allochems are better preservedand occur in greater variety than in the host sediment. Thefill is richer in bryozoans, trilobites, and ostracods.Burrow preservation is excellent due to lithologic contrastand to high organic content of the fill.

b) Discussion

The contrast between Trichophycus fill and the hostmatrix may be due to better preservation in the shelteredburrow environment. Alternatively, the burrow fills mayrepresent remnants of an eroded bed. Trichophycusburrows were interpreted by Seilacher and Crimes (1969)to be feeding structures of small trilobites. In this study,trilobites large enough to build Trichophycus of the sizeobserved in core have not been observed in Yeomanrocks. These burrows are distinguished fromThalassinoides by their lack of branching. Furthermore,their sharp walls and distinctive nature suggest a differentorigin than that of Yeoman Formation Thalassinoides.

9. Skolithos

a) Description of Ichnogenus SkolithosHaldeman, 1840

Skolithos burrows are vertical or subvertical (Figure 16)and are intrastratally preserved. Diameters range from 1to 3 mm. They are generally straight, but are locallycurved. They are several centimetres in length, but theiroverall length cannot be ascertained. Two types ofSkolithos are observed. The first is lined by allochemsand is visible only on freshly slabbed surfaces. Thesecond type is more common and generally more visibledue to organic matter in its fills and linings. Burrow fillsare found to be the same as the host sediment except forthose that are lined with organic matter, which arecommonly dolomitized.

Figure 15 - Horizontal section through Trichophycus (Tri)(Berkley et al Midale 11/7-3-7-11W2, 2613.4 m).

Figure 16 - Skolithos (Sk) in the Yeoman Formation(Berkley et al Midale 41/2-10-7-11W2, 2575.2 m).


b) Discussion

The presence of Skolithos indicates enough water agitation on a regular basis for filter-feeding organisms tocolonize the substrate. They are rare and are generally found in association with Palaeophycus. They are thought tobe the vertical dwelling tubes of polychaetes; however, vertical-tube components of many other trace fossils may bemistaken for Skolithos in core analysis.

10. Rhizocorallium

a) Description of Ichnogenus Rhizocorallium Zenker, 1836

Rhizocorallium burrows are preserved intrastratally and occur vertically to horizontally in random orientationsthroughout given intervals (Figure 17). The U-shaped burrows show no evidence of branching except where theyare associated with composite burrow systems. Diameters are from 1 to 5 mm, and the lining thickness ranges from5 µm to several millimetres. The burrow length is indeterminable. Their lining generally has much greater organiccontent than the host sediment, and exhibits spreiten. If the organic matter is not degraded, it is dominantlydisseminated A and B G. prisca alginite. The internal sediment of the latest burrow is generally carbonate with loworganic content. The burrow fill may contrast in bothmineralogy and organic content with the host matrix.Rhizocorallium may contain secondary Planolites orChondrites. It is occasionally observed in kukersiticintervals (Figure 17) where fill is depleted in organicmatter. Burrow density is variable, but these sedimentsare never fully bioturbated by Rhizocorallium.

b) Discussion

Rhizocorallium burrows have been interpreted torepresent both suspension- and deposit-feeding activities.R. jenense, represented by more or less straight, short,and commonly oblique spreiten burrows, is interpreted asdwelling burrows of filter feeders (Fürsich, 1974a). Theabundance of G. prisca in burrow linings, combined withits absence in burrow fills and host matrix, suggests thatthe organisms incorporated these maceral varieties intothe burrow lining. This further suggests thatRhizocorallium was produced by filter-feeding or surfacedetritus-feeding behaviour.

11. Asterosoma

a) Description of Ichnogenus Asterosoma vonOtto, 1854

Asterosoma-like structures are found in both horizontalcut and vertical section. They are preserved intrastratally(Figure 18). They are commonly associated withcomposite burrow systems. Their orientation is horizontalto subhorizontal; no related vertical shafts are observed.They are curved, bulbous burrows with lobes. Noevidence of branching is found. Burrow diameter incross-section ranges from 1 to 6 mm. It is variable alongthe burrow length, which, in turn, is often indeterminabledue to lobe curvature. Walls are annulated, containingseveral concentric layers of organic matter. If the organicmatter is not degraded, it is dominantly disseminated Aand B G. prisca alginite. The internal fill usually consistsof low organic content carbonate sediment (Figure 19),sometimes with meniscate structures. Carbonate fill iscommonly dolomitized, as is the carbonate component ofthe burrow lining. Preservation of lobes is good, and theability to recognize them is enhanced by their organiclining.

Figure 17 - Rhizocorallium (Rh) in organic-rich mudstones(kukersites) indicative of deposit-feeding behaviours. SomeAsterosoma (As)-like behaviour is also indicated by thesebiogenic sedimentary structures. Their fill is devoid oforganic matter. Abundant spreite indicate repeatedreworking and probing of the sediment. Nature of contacthas been destroyed by pressure solution. Palaeophycus (Pa)in carbonate sediment below contains the same type oforganic matter as that within the matrix of the kukersite(Husky Ceylon 11/5-8-6-19W2, 2710.0 m).


b) Discussion

In horizontal section, evidence of multiple bulbs islimited to two. Vertical sections are, however, indicativeof Asterosoma-type behaviour. Generally these are part ofcomposite burrow systems, which also exhibit othersimilar feeding behaviours. Farrow (1966) found thatpresent-day Asterosoma burrows occur in finer, moreargillaceous sediments deposited farther from theshoreline. Although he inferred that they might be ofeither crustacean or annelid origin, Asterosoma aregenerally considered to be the feeding (fodinichnia)structures of worms. The organism is believed to haveprobed repeatedly into the sediment to enlarge the galleryand work more and more sediment vertically and laterally(Chamberlain, 1971). Asterosoma found in kukersites isapparently formed by this mechanism (Figures 17 and19). However, Asterosoma-like components of compositeburrow systems, where they are not secondary burrowsthat exploited the linings of earlier burrows, are thoughtto have resulted from waste-stowage type behaviours(Bromley, 1991; Kotake, 1991).

12. Trypanites

a) Description of Ichnogenus Trypanites Mägdefrau, 1932

Trypanites borings are preserved across stratal contacts (Figures 19 and 20). They are visible as ‘piped’ zones,introducing sediment from an overlying layer into an underlying layer, most commonly at the base of kukersiticintervals (Figure 19). They have, however, also been observed at the upper surface of a kukersite bed and betweentwo adjacent carbonate layers. Their fill therefore contrasts to that of the host rock, so burrows lying betweenkukersite and carbonate sediment are the most easily recognized. The internal sediment is homogeneous except forcommon secondary burrows of Asterosoma, Planolites, or Chondrites that are evident from the contrastingly lowerorganic content of their fill. Trypanites borings have vertical openings and may remain vertical, or curve to ahorizontal chamber. They range in length from one centimetre to indeterminable. The borings are rarely branchedand commonly have lobate or bulbous shapes. Boring diameter, which ranges from a couple of millimetres to morethan one centimetre, generally changes if the burrow is branched. Walls are generally sharp and appear to besmooth. Preservation is excellent except where stylolites have formed along the hardground and have destroyed thecontact. In such occurrences, a hardground is commonly inferred from the presence of Trypanites in the carbonatesediment underlying the stylolitic contact.

Figure 18 - Asterosoma (As), Palaeophycus (Pa), andPlanolites (Pl). Asterosoma and Palaeophycus containabundant organic matter within their linings, but the fill ofPlanolites is devoid of organic matter (Tri Link Tyvan 21/8-17-13-13W2, 2157.4 m).

Figure 19 - Asterosoma (As) as a secondary burrow within aTrypanites (Try) at the hardground base of an organic-richmudstone (kukersite). Borings also contain secondaryPlanolites (Pl) and lithoclasts (L). Lithoclasts suggest thatthe sediment was cemented prior to deposition of theorganic-rich mudstone (Berkley et al Midale 21/11-35-6-11W2, 2584.8 m; scale bar is 1 cm).


b) Discussion

Most commonly Trypanites occurs below kukersites, butburrows have been observed between adjacent carbonatebeds. Where contacts have been destroyed by pressuresolution, their former presence is inferred from boringsfound below a thick stylolite, rich in organic matter andkerogen. Thin sections are commonly required todifferentiate Trypanites from Thalassinoides at afirmground. Commonly no evidence (e.g., suturedallochems and cut fabrics) can be found to determinewhether the substrate was cemented or not at the time offormation of these biogenic sedimentary structures. Apossible hypothesis for the development of these“borings” is as burrows at omission surfaces. Palmer(1978) suggested that sediment around burrows becamelithified while the burrows remained open, and the surfacesubsequently became a hardground during sedimentbypassing. This hypothesis, suggesting that these are pre-omission suites burrows (Bromley, 1975), may explainthe presence of Thalassinoides-type burrow morphologiesobserved at hardgrounds. Further evidence for thishypothesis is the absence of encrusting organisms onmany of these hardground surfaces. Sea-floor conditionsat time of cementation were possibly adverse toencrusting fauna – for example, sea-floor anoxia mayhave existed when organic-rich kukersites were depositedover hardgrounds.

13. Conclusions

Description and classification of trace fossils in the Yeoman Formation are necessary as they are related to the mostconspicuous characteristic of these sediments, namely, the dolomite mottles. Their presence has importantimplications on both sedimentology and diagenesis. Detailed examination of the dolomite mottles indicates thatmany indeed represent Thalassinoides, but until the trace-making organism is identified, this will remain debatable.In this examination of the biogenic sedimentary structures of the Yeoman Formation, nine discrete trace fossils havebeen observed, many being part of composite burrow systems. Except Trypanites, Trichophycus, and Palaeophycus,they are indicative of feeding activities. The diversity of forms gives the impression of a diverse benthic fauna. Therelatively uniform diameter of the feeding burrows suggests that a small group of organisms may have beenresponsible for the various forms. These burrowing organisms shifted their feeding behaviour in response tochanges in paleoenvironmental conditions, such as water energy, depth, oxygenation, and nutrient availability. Theassociation of more complex feeding structures with the larger, vegetative-state, disseminated B G. prisca alginiteindicates that harsher conditions, which accompany the algal blooms, forced infauna to adapt their feedingbehaviours.

14. Acknowledgments

Personnel at Saskatchewan Industry and Resources, Subsurface Geological Laboratory in Regina, were of greatassistance in core logging and the collection of samples. I am grateful to Kim Dunn (GSC, Calgary) for preparingsamples for UV microscopy, and to L.D. Stasiuk (GSC Calgary) for help in logging organic matter contained withinburrow linings. Members of the Ichnology Research Group, University of Alberta, are greatly thanked fordiscussion of ideas, especially Eric Hanson and Tom Saunders for editing and insightful comments on this paper.

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