Ichnology of the Yeoman Formation 1

Transcription

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 , CD-ROM, Paper A-3, 16p. Abstract The recognition and classification of trace fossils in carbonates of the Upper Ordovician Yeoman Formation in southeastern Saskatchewan are hindered by the complex diagenetic history of these rocks. Since many primary characteristics of deposits greatly influence diagenesis, the distinction between sedimentary and diagenetic fabrics can 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 fossils represent Thalassinoides or sediment dolomitization around smaller, causative burrows. Most ichnological studies have been performed in clastics or chalks, and the methodology developed for these studies is difficult to apply to similar research in Paleozoic platform carbonates. The purpose of this paper is to describe the trace fossils of the Yeoman Formation, and to explore their potential usage in determining changes throughout the deposition of this thick platform carbonate sequence. In the examination of the biogenic sedimentary structures of the Yeoman Formation, nine discrete trace fossils are observed, 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 diverse benthic fauna, but the relatively uniform diameter of the feeding burrows suggests that a small group of organisms may have been responsible for the various forms. These burrowing organisms shifted their feeding behaviours in response to changes in paleoenvironmental conditions, such as water energy, depth, oxygenation, and nutrient availability. The association of more complex feeding structures with the larger, vegetative-state, disseminated B Gloeocapsomorpha prisca alginite indicates that harsher conditions, which accompany the algal blooms, forced infauna 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 southeastern Saskatchewan hampers the recognition and classification of contained trace fossils. Many primary characteristics of deposits greatly influence diagenesis, commonly making distinction between sedimentary and diagenetic fabrics difficult. An example of this problem is presented by the conspicuous dolomite mottling in rocks of the Yeoman Formation: 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 for them is commonly difficult to apply to the Yeoman strata. This paper describes the trace fossils of the Yeoman Formation and explores their potential usage in determining changes throughout the deposition of this thick platform carbonate. Although diverse feeding structures are represented in these sediments, it appears that the benthic 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 diagenetic analysis 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, Range 19-20W2) 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, George.Pemberton@ualberta.ca. Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 1

2 (1971). Fifteen of the wells were sampled for thin section, burrow fabric, and associated organic analysis. One hundred and fifteen thin sections were injected with blue epoxy for porosity determination and stained with Alizarin-Red S (Dickson, 1965) to differentiate calcite from dolomite. Subsequently, samples selected from different trace fossil associations were prepared for examination of organic matter contained within burrows using reflected light microscopy. These samples consisted of blocks measuring approximately 2 cm x 2 cm x 2 cm that were set in an epoxy resin and then polished using increasingly finer grit sizes and finally a slurry of 0.05 µm alumina suspended in water. A Zeiss Axioplan II microscope equipped with a 100 W UV light source was used for maceral 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 imaging of kukersite microscopic constituents. Digital images were captured using a Zeiss Axiocam system and Zeiss Vision software. 3. Recognizing and Classifying Trace Fossils in Carbonates Subject to Heavy Diagenesis Trace fossil classification primarily focuses on morphology. Secondary considerations include such factors as burrow lining, contrast between the burrow fill and matrix sediment, host sediment characteristics, and the nature of grain packing. Many trace fossils were originally described in clastic sedimentary rocks, where the original nature of the grains is much less influenced by recrystallization, replacement, and fabric-destructive diagenesis than in carbonates, 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, serves to enhance their appearance only to a point, beyond which destructive processes of recrystallization, replacement and, 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 sediment can make the discernment of discrete trace fossils difficult (Fürsich, 1975). Preservation is also skewed toward the latest 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 provided samples for many ichnological studies, are not available so the 3D shapes of the observed biogenic sedimentary structures must be inferred from repeated patterns and direct observations in core, where the horizontal expression of individual trace fossils is limited by core diameter. Bedding planes and contacts are rare in the Yeoman Formation where the overlying and underlying sediments are similar, and it is most commonly only firmground and hardground 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 the recognition 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 also hamper interpretation of the behaviour of the trace producer. Where the contrasting nature of the burrow fill and matrix determine the feeding behaviour of the trace producing organisms (Kotake, 1989), diagenesis can destroy this 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 to most closely resemble original fabric and mineralogy. Locally, present-day fabrics proved to be useful, since rocks of 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 burrow fill and the host matrix. Comparison with studies of other carbonates, especially age-equivalent rocks, is useful in noting how other researchers dealt with these complications. Due to these various difficulties, trace fossils are classified only at a generic level. Details required to identify them at species level are not readily available in this core study. Finally, it must be noted that, since freshly slabbed core surfaces and outcrops are not readily available, the more distinct 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 ecological implications of trace-fossil associations 4. Palaeophycus and Planolites These morphologically simple burrow types are discussed together for two reasons. First, they are the most abundant in the Yeoman, commonly occurring together in all the different fabrics of the formation. Second, the distinction between these two trace fossils has caused extensive debate even in examination of clastic sedimentary rocks (Pemberton and Frey, 1982; Keighley and Pickerill, 1995). Saskatchewan Geological Survey 2 Summary of Investigations 2003, Volume 1

3 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 to elliptical in cross-section (Figures 1 and 2). Their diameter is constant along the burrow axis and ranges from less than 1 mm in some intervals to nearly 5 mm. Burrows are rarely branched, and the type of branching is not discernible. They have regular, smooth external ornamentation. The organic-rich lining of the burrows appears to have aided in both their preservation, which is generally good to excellent, and recognition. In intervals dominated by Planolites and Palaeophycus, burrow linings, which vary in thickness and locally appear annulated, comprise a variety 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 sensu Stasiuk and Osadetz, 1990), algal detrinite, and bitumen. Due to the lack of prepared samples, microfossils in the linings were not closely examined. Palaeophycus generally has a homogeneous internal burrow fill that may be recrystallized as saddle dolomite or coarser dolomite, or that may be dissolved, leaving the burrow core hollow. The fill, where unaltered, is similar to the host rock. However, it is rarely nondolomitized. Interestingly, nondolomitized Palaeophycus occurs in places amidst dolomitized Palaeophycus. Re-burrowing by Planolites is sometimes observed. 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 organic matter/kerogen and mud. Infills of Palaeophycus represent passive, gravity-induced sedimentation within open, lined burrows (Pemberton and Frey, 1982), although this cannot specifically be confirmed for Yeoman Formation occurrences. Palaeophycus is generally interpreted as the open dwelling burrows of suspension feeding or carnivorous animals (Pemberton and Frey, 1982). The presence of collapse structures and geopetal fills in the Yeoman burrows supports this interpretation. The primary objective in constructing dwelling burrows is protection, so intervals dominated by domichnia are thought to suggest well illuminated, shallow-water deposits where predation is greatest. In modern environments, dwelling burrows are generally restricted to shallow, well illuminated intervals (Schäfer, 1972). Possible trace makers may be sipunculid, enteropneust, or polychaete worms. Further examination Figure 1 - Palaeophycus (Pa) with thin dolomitic diagenetic haloes (Tri Link Tyvan 21/ W2, m). of scolecodonts in association with these burrows might help identify the behaviour of the trace-making organisms, since scolecodont shape reflects feeding behaviour (e.g., predation vs. scraping). The presence of scolecodonts is notable and merits further investigation as scolecodonts, due to their sensitivity to diagenesis, are rarely found in marine sediments with many trace fossils (Schäfer, 1972). Figure 2 - Palaeophycus with dolomitic halo. Photomicrograph taken under polarized light (Berkley et al Midale 41/ W2, m). c) Description of Ichnogenus Planolites Nicholson, 1843 Planolites is generally preserved as horizontal to subhorizontal, straight to curved (in places, meandering) intrastratal burrows (Figure 3), which are rarely branched, but crosscut and interpenetrate each other. Burrows are smooth walled and homogeneously filled. They rarely exhibit a meniscus structure and may contain secondary Planolites of equal or smaller diameter. Burrow diameter ranges from less than 1 mm (discernible only microscopically) to almost 10 mm and is unaltered by branching. Cross-sections are circular to elliptical, the latter resulting from compactional flattening or oblique Saskatchewan Geological Survey 3 Summary of Investigations 2003, Volume 1

4 Figure 3 - Planolites (Pl) with thin diagenetic haloes from a dolomitized mudstone interval (Husky Ceylon 41/ W2, m). sectioning of the burrow. Burrow thickness is generally constant along the observed burrow length (absolute lengths of burrows have not been determined). The fill differs from host rock in that it generally contains less dispersed organic matter and kerogen, but it may also be dolomitized, or contain a different dolomite type than that of the host rock. The fill may contain a contrast in allochem abundance (greater or smaller) or the burrow may contain oriented/structured allochems. These last few criteria generally require thin sections for recognition. Burrow density is dependent on facies, generally being higher in muddier substrates. d) Discussion Planolites is found in all the Yeoman facies in variable abundance, and may occur alone or as secondary burrows in Thalassinoides, Asterosoma, Rhizocorallium, or Trichophycus. It may form monospecific assemblages or be in association with Chondrites. Planolites is distinguished from Palaeophycus primarily by having unlined walls, and burrow fills that differ from the adjacent rock 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 Yeoman specimens, 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 is comparatively rare. Petrographic investigation is commonly required for identification, and that is not always possible. Since the Planolites organism re-burrowed numerous other kinds of pre-existing traces, the latter presumably contained enhanced nutrient levels such as dispersed organic matter and/or represented an easier path for new burrowing activities. The first of these two possible causes for Planolites re-burrowing seems to be valid as the fill of the last tier of burrows has the lowest content of organic matter. Planolites is generally interpreted to represent deposit-feeding behaviour. Preservation of mucous linings is unlikely in sediments subject to such extensive diagenesis as in the Yeoman Formation, so some burrows, due to an apparent lack of burrow lining, may be 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 required to 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 passing through 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 greater than 10 mm in thickness. Most burrowing organisms, when moving through sediment, secrete some mucus or other residue (Schäfer, 1972; Bromley, 1996). This mucus commonly acts as a catalyst for the formation of diagenetic haloes. Keighley and Pickerill (1995) suggested that a burrow lining might be inferred if the diagenetic halo extends from the burrow boundary into both the burrow fill and the host lining. In the Yeoman Formation, this criterion failed 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 Yeoman Formation, recognition of the burrow fill is problematic because the fill is diagenetically altered (difference of the inferred burrow fill from the matrix at the time of sedimentation, not their present-day difference, must be used as the ichnotaxonomic criterion). Where dissolution of the burrow core has occurred or dolomitization and recrystallization 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 textural state. g) Branching or Crosscutting Burrows? The crosscutting of burrows can easily be confused with true branching. Evidence to truly distinguish branching from crosscutting burrows may be enigmatic, and in some cases may not be interpretable (Keighley and Pickerill, Saskatchewan Geological Survey 4 Summary of Investigations 2003, Volume 1

5 1995). This holds true for the Yeoman Formation, where distinction between abundant Planolites and Chondrites is often arbitrary. 5. Composite Burrow Systems In ichnological studies, all biogenic sedimentary structures are commonly classified as discrete trace fossils. As such, 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 a single burrow type is impossible. Discrete burrows, which in addition to Planolites and Palaeophycus are commonly 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, which exhibit branching and crosscutting, evidence of U-shaped portions, spreite resulting from shifting of the burrow system, as well as spreite in the burrow fill (Figures 4 and 5). The fill is generally dolomitized mudstone, so the contrast between the host sediment and burrow fill may be a result only of diagenesis. The organic-rich linings may be thin or up to several millimetres thick, and commonly resemble concentric linings of Cylindrichnus concentricus described by Goldring (1996) and Fürsich (1974b). These linings mainly comprise organic matter that, where nondegraded, 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 in Palaeophycus linings are rarely observed in these burrow linings. Other portions of the burrow systems exhibit characteristics of Phycodes, Asterosoma, and Rhizocorallium (Figures 4 and 5). The burrow diameters of the original and later burrows are similar and do not change with branching. Rarely, a larger diameter Planolites crosscuts a smaller, lined burrow. b) Discussion The concentric linings are believed to have originated by the construction of a multi-layered wall whereby the organism 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 seems most applicable here because the material that lines the burrows is mostly planktonic, appears to have been introduced into the burrow, and is absent from the matrix sediment. These complex feeding systems may have initially served as the dwelling burrows of filter-feeding or carnivorous organisms. Once the environmental conditions changed, e.g., possibly less nutrient availability in the water column, these organisms altered their behaviour to exploit organic matter and organic detritus they had used earlier to line their burrows. The last tier of Planolites and/or Chondrites indicates deposit-feeding activities. Because organisms are known to change their feeding patterns diurnally and seasonally (Schäfer, 1972; Hummel, 1985), other changes in environment are not necessarily responsible for changes in feeding behaviour. Figure 4 - Horizontal section through composite burrows in the Yeoman Formation. Note repeated reworking of sediment (Husky Ceylon 41/ W2, m). Consistent burrow diameter throughout these systems suggests that all the tiers of these burrows were created by the same or similar organisms (Figure 5). Consequently, although diverse feeding patterns are preserved, these systems may have been produced by a single or small group of organisms. The inference that the suspensionfeeding components of these composite burrows and other suspension-feeding burrows described from the Yeoman Formation were formed by a single species is consistent with Turpaeva s hypotheses (1957 in Walker, 1972) concerning trophic relationships of benthic fauna. Based on an examination of feeding ecology of modern benthic animals, she believed that one trophic group that contains the most prevalent species generally dominates a community. This, she inferred, is characteristic of a stable community in which the organisms attain a noncompetitive feeding arrangement. These composite burrows, which generally occur in intervals believed to Saskatchewan Geological Survey 5 Summary of Investigations 2003, Volume 1

6 coincide with G. prisca algal blooms (Kent and Haidl, 1999), presumably resulted from a species of polychaete dominantly consuming this algal microfossil by a suspension-feeding mechanism. When conditions for successful filter feeding were interrupted, this worm-type organism might have adapted to a deposit-feeding behaviour that exploited the same algal microfossil that was readily available in the sediment. 6. Thalassinoides The characteristic dolomite mottling common in Lower Paleozoic platform carbonates has often been interpreted as large branching burrow networks of either the ichnogenus Spongeliomorpha or Thalassinoides (Bottjer et al., 1984; Zenger, 1992, 1996; Myrow, 1995; Zenger and LeMone, 1995). This interpretation has been favoured for the dolomite mottling of the Yeoman Formation (Kendall, 1976, 1977; Canter, 1998; Kissling, 1999), although others believed that the mottling represents dolomitic haloes, which formed around Planolites, Palaeophycus, and Chondrites (Carroll, 1978; Gingras, 2000). These two interpretations have different implications on the origin of the burrows found in the dolomite mottles. According to the former model, the burrows are secondary, but the latter model infers they are causative burrows, which facilitated dolomitization of the matrix. Due to this disparity in views regarding dolomite mottling, the mottling in the Yeoman Formation required close examination. According to Kendall (1977), three features remain in question if the mottles are interpreted as diagenetic haloes: Figure 5 - Composite burrows with many Rhizocoralliumlike components. Diameters of various burrow types are consistent, indicating that the same organism reworked these sediments. Planolites burrows in matrix are only faintly visible as their appearance is not enhanced by bitumen staining. As, Asterosoma; Ch, Chondrites; Pa, Palaeophycus; Pl, Planolites; Rh, Rhizocorallium; and Si, Silica nodule (Berkley et al Midale 11/ W2, m). 1) The bimodal distribution of mottle diameters (either uniformity or a wide range of mottle diameters would be expected if the mottles represent diagenetic alterations around burrow centres). 2) The otherwise uniformity in the size of dolomite mottling throughout the thickness of the Yeoman lower Red River interval across the whole Williston Basin (this uniformity suggests a burrow origin). 3) The mottles sometimes lack internal burrows, or they contain more than one burrow or burrows that are markedly eccentric (if dolomitization had proceeded uniformly outward from the burrow centres to generate cylindrical mottles, the mottles should always contain centrally located burrows). Gingras (2000), on the other hand, believed that, since the dolomite mottles do not exhibit regular sharply demarcated boundaries and constant burrow diameters throughout the network, they did not represent biogenic sedimentary structures. His study, however, focused on the surface Tyndall Limestone that, although it can be examined on a larger scale in outcrop, displays some different sedimentary characteristics than the subsurface Yeoman Formation. Thalassinoides-like burrows have been found in many Ordovician platform carbonates (Sheehan and Schiefelbein, 1984). Although the biological affinities of Saskatchewan Geological Survey 6 Summary of Investigations 2003, Volume 1

7 the organisms responsible for these networks have not yet been discovered, it seems plausible that at least some of the dolomite mottles in the Yeoman Formation represent Thalassinoides. Lack of Yeoman Formation outcrop prevents a diagnosis based on plan geometry, i.e., determination of vertical and horizontal extent of these large characteristically branching networks. The mottle networks found in outcrops of both the Tyndall Limestone and Lower Yeoman (Red River) Formation (Figures 6 and 7) greatly resemble other recorded Lower Paleozoic Thalassinoides (Sheehan and Schiefelbein, 1984; Watkins and Coorough, 1997). The proper taphonomic conditions may not have existed to preserve Thalassinoides in an identifiable form. This, combined with diagenetic destruction of many primary sedimentary structures, make the determination with absolute certainty of the presence of Thalassinoides 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-shaped branching with burrow diameter apparently remaining constant along branches. Burrows are generally straight or slightly curved. Their cross-section is circular or elliptical. Burrow walls are smooth, regular and unlined, but stylolites/pressure solution may in places give the appearance of thin organic-rich lining. Burrow fill is homogeneous, but may contain secondary Planolites and Chondrites. It is commonly dolomitized, occasionally silicified, and shows contrast in allochem abundance relative to the host rock. Burrow diameter ranges from 5 to 20 mm. Burrow length is indeterminable. The burrow density is variable from few discrete Thalassinoides to Thalassinoides comprising about 50 percent of the rock volume. Thalassinoides generally has poor preservation or recognition potential. b) Collective Indicators of Thalassinoides in the Yeoman Formation Although no single criterion in itself can be used to distinguish diagenetic haloes from burrows, a combination of criteria points to favouring one interpretation over another. In intervals where many of the following criteria are met, mottles are interpreted as Thalassinoides. Figure 6 - Thalassinoides (Th) parallel to bedding plane from outcrop of the lower Red River/Yeoman Formation. Preferential growth of fungi indicates burrow patterns. Located near Amisk Lake, Saskatchewan. Pencil 14 cm long. Dolomite mottles contain intraclasts made of the same material as the matrix. Such occurrences, some of which appear to represent collapse structures, are rare. This is probably the best evidence that what are now mottles were once open burrows. Note that the walls of these mottles are not sharp (Figure 8). Contrast in fabric, allochem abundance and types, and abundance of organic matter between mottle and matrix sediment. Since the dolomitization which formed the mottling is generally fabric destructive, an increased abundance or variety of allochems within the burrow is in itself a valid criterion to identify the structure as a burrow; however, the inverse cannot be claimed with such certainty (Figure 9). Absence of causative burrow from mottles over an interval. Absence is only certain when the burrow is seen in cross-section. An oblique section may miss the causative burrow. Although causative burrows may in places have been obliterated by the dolomitization that formed the diagenetic mottle, their widespread presence helps validate the assumption that, where absent, they never initially existed. Figure 7 - Thalassinoides-like burrows indicated by differential weathering of these dolomitized sediments and patterns of fungi growth. Outcrop from western shore of Amisk Lake, Saskatchewan. Pencil 14 cm long. The mottle wall is cut by burrows (i.e., the mottles do not strictly follow or completely contain smaller Saskatchewan Geological Survey 7 Summary of Investigations 2003, Volume 1

8 burrows), or the mottle wall crosscuts another burrow (Figure 10). Mottles generally have sharp walls; however this criterion is not always the best (see discussion below). Large mottles and small Planolites exhibit the same diagenetic fabrics and the walls of both have the same sharpness. Mottles exhibit Y- and T-branching, characteristic of Thalassinoides (Figure 11). Mottle diameter appears constant along the burrow axis throughout an interval. Mottles exhibit mechanical compaction and may in places be broken, squashed, or flattened. The presence of mechanically compacted mottles suggests mottle formation occurred prior to sediment lithification. Re-burrowing by Planolites; the re-burrowing of large burrows by deposit feeders is common, especially where the fill of the large burrow may provide a better food source for the later burrowers (Bromley, 1996). Figure 8 - Roof collapse as evidence of formerly open burrow systems. H, molds of halite hopper crystals with void filling saddle dolomite cement; Pa, Palaeophycus; Pl, Planolites; and Th, Thalassinoides (Berkley et al Midale 31/ W2, m). Mottle occurrence in intervals that contain firmgrounds with Thalassinoides. This is merely circumstantial evidence if organisms existed that were large enough to create Thalassinoides at the firmground intervals, would they not have existed before and after? Figure 9 - Wackestone fill of Thalassinoides (Th) in a mudstone matrix (Husky Ceylon 11/ W2, m). Figure 10 - Thalassinoides (Th) re-burrowed by Palaeophycus (Pa), with Palaeophycus crosscutting Thalassinoides wall (Husky Ceylon 41/ W2, m). Saskatchewan Geological Survey 8 Summary of Investigations 2003, Volume 1

9 Figure 11 - Thalassinoides (Th) with Y-shaped branching in the horizontal plane. Re-burrowed by Planolites (Pl). Note Palaeophycus (Pa) without diagenetic haloes (Husky Ceylon 11/ W2, m). One of the criteria noted by Bromley (1996) and cited by Gingras (2000) to determine if a structure is a burrow or not is the presence of a regular, sharply demarcated boundary. Here, this criterion is regarded to be insufficient evidence for differentiating between a diagenetic fabric and biogenic sedimentary structures because many indisputable Planolites have walls that are not sharp as they have apparently been obscured by dolomitization. If the dolomitization process can blur a Planolites wall, it should also be able to obscure a Thalassinoides wall. Furthermore, some of the mottles containing intraclasts did not have sharp walls. Lack of burrow wall sharpness may also result from factors other than diagenetic blurring. If the biogenic sedimentary structures were formed in a soupy or soft substrate, and little evidence is preserved to suggest that the precursor was a coarser grain texture, mechanical compaction of the mottles can blur the walls of large Planolites or Thalassinoides. Added evidence in these intervals indicating that the mottles formed before sediment lithification is the presence of molds after halite hoppers that must have developed in a void space or in sediment soft enough for them to displace material as they grew. These hopper crystals, now infilled with saddle dolomite or, less commonly, anhydrite, are frequently (but not exclusively) found in dolomite mottles that fit the other criteria 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 diagenetic halo 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 in abundance of allochems and organic matter (Figure 12). Dolomitization of the halo appears fabric destructive, and allochems are faintly preserved or gradually decrease in abundance towards the causative burrow. Diagenetic front of the dolomite halo terminates against a macrofossil, but in other places continues farther from the causative burrow. Palaeophycus burrows, when not centrally located within dolomite mottles, may be causative structures with diagenetic haloes. Mottles do not exhibit much mechanical compaction (i.e., no squashed mottles over an interval). The distinction between Thalassinoides and diagenetic haloes remains locally uncertain, especially in intervals that are intensively affected by diagenesis. In nodular zones and stylo-mottled zones, the 3D configuration is impossible to determine. Stylolites have further obliterated the nature of the mottle walls. The origin of many nodular and rubbly textures in limestones and dolostones has been attributed to burrowing (Fürsich, 1972), so these textures in the Yeoman may, in places, have originated as large burrow systems that are now impossible to distinguish. Figure 12 - Dolomitic diagenetic haloes commonly form around Palaeophycus and Planolites. Polarized light (Husky Ceylon 11/ W2, m). Saskatchewan Geological Survey 9 Summary of Investigations 2003, Volume 1

10 d) Discussion Those burrows identifiable with greatest certainty as Thalassinoides generally occur in the same carbonate intervals that contain kukersites. Thalassinoides is common at the firmground contacts between carbonate and superjacent kukersite. Many burrows are re-burrowed by Planolites. Some examples of Thalassinoides are found in other Yeoman intervals. Thalassinoides most likely represents the fodinichnia and domichnia of arthropods or large worms. Phyllopods were suggested by Bottjer et al. (1984). Thalassinoides burrows are generally interpreted as dwelling or combined feeding/dwelling structures. The interpretation generally applied to Thalassinoides behaviour cannot be applied because the arthropods (e.g., shrimps) responsible for making these burrows had not yet evolved in the Ordovician. Until a possible trace producer is identified, the Lower Paleozoic Thalassinoides may remain an enigma (Myrow, 1995). No organism has been observed in any Thalassinoides burrow, and until an example is found the trace maker cannot be identified with certainty (it may never be found if the trace was produced by a soft-bodied organism with very low preservation potential). The Lower Paleozoic dolomite mottling with diffuse dolomite walls may represent burrows formed in thixotropic sediment. Although the walls of these mottles resemble diagenetic fronts, they also fit criteria listed by Rhoads (1970) for burrows formed in a soupy (thixotropic) substrate. It may be possible that some unknown animal of Lower Paleozoic age mined the sediment prior to any dewatering and lithification, and that the fill of its burrows was more susceptible to dolomitization. The burrow walls remained blurry due to lack of substrate consistency. 7. Chondrites a) Description of Chondrites Sternberg, 1833 Burrows are smooth-walled and are found in vertical and horizontal section (Figure 13). Branches are subvertical to horizontal; however, some zones are dominated by horizontal burrows. Locally, the branching is dendritic, but generally the orders and nature of branching are indeterminable. Cross-sections are elliptical to circular and range from less than 1 mm to 3 mm in diameter. The internal sediment is homogeneous, and through its paucity of organic matter/kerogen, generally contrasts with the matrix sediment (though locally Chondrites has fill that is richer in organic matter/kerogen than the matrix, see Figure 14). Preservation of these burrows ranges from good to poor. b) Discussion Chondrites is common in muddy, organic-rich substrates, and as secondary burrows of Thalassinoides and composite burrows systems. It is also common in organicrich kukersites. Chondrites burrows may occur exclusively with Planolites, or as part of a composite burrow system. They are generally interpreted as fodinichnia, but despite their common occurrence in sediments throughout the Phanerozoic, the trace maker has not yet been identified. The vertical upper portions, suggested by Kotake (1991) to be domichnia, are not observed. Chondrites indicates a lack of oxygenation of the substrate only when occurring as a monospecific suite (Bromley and Ekdale, 1984). However, since it commonly 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/ W2, m). Figure 14 - Chondrites (Ch) with fills rich in organic matter (dominantly Gloeocapsomorpha prisca alginite) (Tri Link Tyvan 21/ W2, m; scale bar is 1 cm). Saskatchewan Geological Survey 10 Summary of Investigations 2003, Volume 1

11 monospecific suite because its high density has obliterated all other burrows. Chondrites is found alone in some, but not all, kukersites. The trace-making organism may have colonized the substrate after the deposition of the kukersite, so Chondrites does not necessarily indicate a low-oxygen environment for kukersite deposition. 8. Trichophycus a) Description of Ichnogenus Trichophycus Miller and Dyer, 1878 Burrows are found in subhorizontal to horizontal section (Figure 15). Trichophycus generally occurs as straight burrows that are circular to elliptical in cross-section and have diameters which range from one to several centimetres (one observed chamber had a diameter of 10 cm) and which locally vary along burrow axes. There is little evidence of branching. Walls are irregular, locally containing ridges or grooves, which suggest scratchmarks. Stylolites and bitumen concentrated at burrow walls give the impression of thin organic-rich lining. Burrow fill is homogeneous, and, partly through having a higher organic content, distinctly differs from the host rock. Usually dolomitized, the fill commonly contains secondary Planolites and Chondrites whose fills have lower organic contents. Allochems are better preserved and occur in greater variety than in the host sediment. The fill is richer in bryozoans, trilobites, and ostracods. Burrow preservation is excellent due to lithologic contrast and to high organic content of the fill. b) Discussion The contrast between Trichophycus fill and the host matrix may be due to better preservation in the sheltered burrow environment. Alternatively, the burrow fills may represent remnants of an eroded bed. Trichophycus burrows 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 size observed in core have not been observed in Yeoman rocks. These burrows are distinguished from Thalassinoides by their lack of branching. Furthermore, their sharp walls and distinctive nature suggest a different origin than that of Yeoman Formation Thalassinoides. Figure 15 - Horizontal section through Trichophycus (Tri) (Berkley et al Midale 11/ W2, m). 9. Skolithos a) Description of Ichnogenus Skolithos Haldeman, 1840 Skolithos burrows are vertical or subvertical (Figure 16) and are intrastratally preserved. Diameters range from 1 to 3 mm. They are generally straight, but are locally curved. They are several centimetres in length, but their overall length cannot be ascertained. Two types of Skolithos are observed. The first is lined by allochems and is visible only on freshly slabbed surfaces. The second type is more common and generally more visible due to organic matter in its fills and linings. Burrow fills are found to be the same as the host sediment except for those that are lined with organic matter, which are commonly dolomitized. Figure 16 - Skolithos (Sk) in the Yeoman Formation (Berkley et al Midale 41/ W2, m). Saskatchewan Geological Survey 11 Summary of Investigations 2003, Volume 1

12 b) Discussion The presence of Skolithos indicates enough water agitation on a regular basis for filter-feeding organisms to colonize the substrate. They are rare and are generally found in association with Palaeophycus. They are thought to be the vertical dwelling tubes of polychaetes; however, vertical-tube components of many other trace fossils may be mistaken 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 orientations throughout given intervals (Figure 17). The U-shaped burrows show no evidence of branching except where they are associated with composite burrow systems. Diameters are from 1 to 5 mm, and the lining thickness ranges from 5 µm to several millimetres. The burrow length is indeterminable. Their lining generally has much greater organic content than the host sediment, and exhibits spreiten. If the organic matter is not degraded, it is dominantly disseminated A and B G. prisca alginite. The internal sediment of the latest burrow is generally carbonate with low organic content. The burrow fill may contrast in both mineralogy and organic content with the host matrix. Rhizocorallium may contain secondary Planolites or Chondrites. It is occasionally observed in kukersitic intervals (Figure 17) where fill is depleted in organic matter. Burrow density is variable, but these sediments are never fully bioturbated by Rhizocorallium. b) Discussion Rhizocorallium burrows have been interpreted to represent both suspension- and deposit-feeding activities. R. jenense, represented by more or less straight, short, and commonly oblique spreiten burrows, is interpreted as dwelling burrows of filter feeders (Fürsich, 1974a). The abundance of G. prisca in burrow linings, combined with its absence in burrow fills and host matrix, suggests that the organisms incorporated these maceral varieties into the burrow lining. This further suggests that Rhizocorallium was produced by filter-feeding or surface detritus-feeding behaviour. 11. Asterosoma a) Description of Ichnogenus Asterosoma von Otto, 1854 Asterosoma-like structures are found in both horizontal cut and vertical section. They are preserved intrastratally (Figure 18). They are commonly associated with composite burrow systems. Their orientation is horizontal to subhorizontal; no related vertical shafts are observed. They are curved, bulbous burrows with lobes. No evidence of branching is found. Burrow diameter in cross-section ranges from 1 to 6 mm. It is variable along the burrow length, which, in turn, is often indeterminable due to lobe curvature. Walls are annulated, containing several concentric layers of organic matter. If the organic matter is not degraded, it is dominantly disseminated A and B G. prisca alginite. The internal fill usually consists of low organic content carbonate sediment (Figure 19), sometimes with meniscate structures. Carbonate fill is commonly dolomitized, as is the carbonate component of the burrow lining. Preservation of lobes is good, and the ability to recognize them is enhanced by their organic lining. Figure 17 - Rhizocorallium (Rh) in organic-rich mudstones (kukersites) indicative of deposit-feeding behaviours. Some Asterosoma (As)-like behaviour is also indicated by these biogenic sedimentary structures. Their fill is devoid of organic matter. Abundant spreite indicate repeated reworking and probing of the sediment. Nature of contact has been destroyed by pressure solution. Palaeophycus (Pa) in carbonate sediment below contains the same type of organic matter as that within the matrix of the kukersite (Husky Ceylon 11/ W2, m). Saskatchewan Geological Survey 12 Summary of Investigations 2003, Volume 1

13 Figure 18 - Asterosoma (As), Palaeophycus (Pa), and Planolites (Pl). Asterosoma and Palaeophycus contain abundant organic matter within their linings, but the fill of Planolites is devoid of organic matter (Tri Link Tyvan 21/ W2, m). b) Discussion In horizontal section, evidence of multiple bulbs is limited to two. Vertical sections are, however, indicative of Asterosoma-type behaviour. Generally these are part of composite burrow systems, which also exhibit other similar feeding behaviours. Farrow (1966) found that present-day Asterosoma burrows occur in finer, more argillaceous sediments deposited farther from the shoreline. Although he inferred that they might be of either crustacean or annelid origin, Asterosoma are generally considered to be the feeding (fodinichnia) structures of worms. The organism is believed to have probed repeatedly into the sediment to enlarge the gallery and work more and more sediment vertically and laterally (Chamberlain, 1971). Asterosoma found in kukersites is apparently formed by this mechanism (Figures 17 and 19). However, Asterosoma-like components of composite burrow systems, where they are not secondary burrows that exploited the linings of earlier burrows, are thought to have resulted from waste-stowage type behaviours (Bromley, 1991; Kotake, 1991). 12. Trypanites Figure 19 - Asterosoma (As) as a secondary burrow within a Trypanites (Try) at the hardground base of an organic-rich mudstone (kukersite). Borings also contain secondary Planolites (Pl) and lithoclasts (L). Lithoclasts suggest that the sediment was cemented prior to deposition of the organic-rich mudstone (Berkley et al Midale 21/ W2, m; scale bar is 1 cm). 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 kukersitic intervals (Figure 19). They have, however, also been observed at the upper surface of a kukersite bed and between two adjacent carbonate layers. Their fill therefore contrasts to that of the host rock, so burrows lying between kukersite and carbonate sediment are the most easily recognized. The internal sediment is homogeneous except for common secondary burrows of Asterosoma, Planolites, or Chondrites that are evident from the contrastingly lower organic content of their fill. Trypanites borings have vertical openings and may remain vertical, or curve to a horizontal chamber. They range in length from one centimetre to indeterminable. The borings are rarely branched and commonly have lobate or bulbous shapes. Boring diameter, which ranges from a couple of millimetres to more than one centimetre, generally changes if the burrow is branched. Walls are generally sharp and appear to be smooth. Preservation is excellent except where stylolites have formed along the hardground and have destroyed the contact. In such occurrences, a hardground is commonly inferred from the presence of Trypanites in the carbonate sediment underlying the stylolitic contact. Saskatchewan Geological Survey 13 Summary of Investigations 2003, Volume 1

14 Figure 20 - Trypanites (Try) containing lithoclasts. Sediment filling the boring is slightly richer in organic matter (Berkley et al Hume N 41/ W2, m). b) Discussion Most commonly Trypanites occurs below kukersites, but burrows have been observed between adjacent carbonate beds. Where contacts have been destroyed by pressure solution, their former presence is inferred from borings found below a thick stylolite, rich in organic matter and kerogen. Thin sections are commonly required to differentiate Trypanites from Thalassinoides at a firmground. Commonly no evidence (e.g., sutured allochems and cut fabrics) can be found to determine whether the substrate was cemented or not at the time of formation of these biogenic sedimentary structures. A possible hypothesis for the development of these borings is as burrows at omission surfaces. Palmer (1978) suggested that sediment around burrows became lithified while the burrows remained open, and the surface subsequently became a hardground during sediment bypassing. This hypothesis, suggesting that these are preomission suites burrows (Bromley, 1975), may explain the presence of Thalassinoides-type burrow morphologies observed at hardgrounds. Further evidence for this hypothesis is the absence of encrusting organisms on many of these hardground surfaces. Sea-floor conditions at time of cementation were possibly adverse to encrusting fauna for example, sea-floor anoxia may have existed when organic-rich kukersites were deposited over hardgrounds. 13. Conclusions Description and classification of trace fossils in the Yeoman Formation are necessary as they are related to the most conspicuous characteristic of these sediments, namely, the dolomite mottles. Their presence has important implications on both sedimentology and diagenesis. Detailed examination of the dolomite mottles indicates that many 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 have been 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. The relatively uniform diameter of the feeding burrows suggests that a small group of organisms may have been responsible for the various forms. These burrowing organisms shifted their feeding behaviour in response to changes in paleoenvironmental conditions, such as water energy, depth, oxygenation, and nutrient availability. The association of more complex feeding structures with the larger, vegetative-state, disseminated B G. prisca alginite indicates that harsher conditions, which accompany the algal blooms, forced infauna to adapt their feeding behaviours. 14. Acknowledgments Personnel at Saskatchewan Industry and Resources, Subsurface Geological Laboratory in Regina, were of great assistance in core logging and the collection of samples. I am grateful to Kim Dunn (GSC, Calgary) for preparing samples for UV microscopy, and to L.D. Stasiuk (GSC Calgary) for help in logging organic matter contained within burrow linings. Members of the Ichnology Research Group, University of Alberta, are greatly thanked for discussion of ideas, especially Eric Hanson and Tom Saunders for editing and insightful comments on this paper. 15. References Bottjer, D.J., Sheehan, P.M., Miller, M.F., Byers, C.W., and Hicks, D.O. (1984): Thalassinoides in the Paleozoic; Geol. Soc. Amer. Bull., Abstr. with Prog., v16, p451. Bromley, R.G. (1975): Trace fossils at omission surfaces; in Frey, R.W. (ed.), The Study of Trace Fossils, Springer- Verlag, New York, p (1991): Zoophycos: Strip mine, refuse dump, cache or sewage farm?; Lethaia, v24, p Saskatchewan Geological Survey 14 Summary of Investigations 2003, Volume 1