Abstract Geologists recognize a myriad of post-depositional agents, which invariably influence and alter any sediment as soon as it is deposited. Some of these agents are physical (e.g., oscillating waves, transportive currents and compaction phenomena); some are chemical (e.g., oxygen diffusion, mineral dissolution and cementation); and some are biological (e.g., sediment reworking by animals and plants). In many cases, the effects of biological agents in the sediment overshadow those of physical and chemical agents in producing recognizable features, such as textures, fabrics and sedimentary structures, that aid geologists in their attempts to understand the history of a sedimentary rock. It is very important for sedimentary geologists to recognize and understand primary textures, diagenetic fabrics and physical sedimentary structures; it is equally important for them to recognize and understand biogenic textures, biogenic fabrics and biogenic sedimentary structures. The study of post-depositional biological effects on sedimentary deposits is known as “ichnology” (from the Greek iknos , meaning “trace or track”, and logos , meaning “word or study”). The field encompasses those aspects of organism-substrate interrelationships that focus on how plants and animals leave a record of their activity in the sediment. Whether it be a trilobite that makes a footprint, a dinosaur that digs a nest, or a dense population of shrimp that thoroughly churns the sea-floor, its record in the rock is something to interest and excite the ichnologist. Ichnology is truly an interdisciplinary subject, involving important elements of paleontology, paleoecology, sedimentology, stratigraphy and even geochemistry. The subject has a history of inquiry extending

Abstract Classification is simply the orderly arrangement of voluminous amounts of information to aid in interpreting various kinds of relationships between items and also in communicating ideas about these items with other workers. Five different aspects of trace fossils have provided a basis for their classification, i.e., based on preservation (toponomy), taxononomy (systematics of presumed trace-makers), ichnotaxonomy (systematics of trace fossils), ethology (behavior) and paleoenvironment (bathymetry, etc.). The preservational and taxonomie classification schemes are primarily descriptive (i.e., objective); the behavioral and paleoenvironmental approaches to classification, on the other hand, are largely interpretive (i.e., subjective). Ichnology, sometimes divided into neoichnology and paleoichnology (palichnology), depending on whether you are dealing with modern or ancient settings, is centered on the study of biogenic (i.e., biologically produced) structures. These are categorized as follows (Table II-1). A number of specialized terms, most of which are listed in the Glossary at the end of this book, apply to various kinds of biogenic structures. For example, a “shaft” is a dominantly vertical burrow; a “tunnel” is a dominantly horizontal burrow (Fig. 2-l); a “U-shaped burrow” or “U-tube” is one comprising two shafts that join at depth in the sediment. If shafts and/or tunnels form a complex unit of highly branched burrows, the entire structure is a “burrow system”. A system of interconnected burrows is a “maze” or “gallery”, if dominantly horizontal, or “boxwork”, if both horizontal and vertical components are included (Fig. 2-2). Some burrows possess a “lining”, which is a distinct, thickened, burrow wall reinforced by mucus-like

Abstract Trace fossils are very peculiar things. More often than not, the greatest difficulty in dealing with them is simply recognizing that they are trace fossils in the first place! Many a spirited argument has arisen on the outcrop between geologists who take opposing views on the biogenic vs. non-biogenic origin of a particular structure in the rock, and a good share of these arguments go unresolved because of either poor preservation of the structure or lack of clear criteria for distinction between alternative solutions. We cannot do much about the preservation state, although there are some ways by which we can enhance the visibility of a structure or its component features (see Chapter 4, this volume). We also can seek to develop objective criteria to help us distinguish between biogenic structures and primary (mainly physical) or secondary (mainly chemical) non-biogenic structures. The pitfalls of recognition and identification are many, however, so workers always must proceed with caution and keep their minds open. If presented with a problematic structure, some criteria which may lead one to consider a biogenic origin include the following items (Table III-1). Be sure to note that the occurrence of one or more of these features may not assure a stamp of certainty on the identification, but at least its (their) occurrence may assist in offering an intelligent guess. (1) Obvious resemblance of the structure in question to the body form or a body part of an organism. Footprints are a good example of this, because the

Abstract Biogenic structures are manifestations of the activities of benthic organisms and, as such, can be viewed not only as biologic entities but also as sedimentary structures. Consequently, special methods may be necessary in order to study them properly. Farrow (1975) provided an excellent summary of the techniques presently used in studying fossil and recent traces; these are summarized in Tables IV-1 and IV-2. (1) Field observations. Perhaps the most fundamental technique used in studying trace fossils is simple detailed observation. Trace fossils in some cases are very difficult to collect, because they require the recovery of large rock slabs or, when dealing with cores, sampling may be prohibited. Therefore, it becomes imperative to describe the fossils in the field with as much accuracy and precision as possible. Various field forms or checklists have been devised by some workers to ensure that important details are not overlooked. Accurate sketches and/or good photographs are virtually mandatory. Simple field sketches have the advantage of being easy, inexpensive and not dependent upon good lighting; moreover, a sketch always contains the pertinent features that may not show up in a photograph because of low contrast, small size or poor photography. Disadvantages, of course, are that sketching is a time-consuming, practiced skill that varies considerably from one person to another. Photographs, on the other hand, are very quick to produce (a fraction of second!) and are objective in the final result. However, important details often cannot be captured effectively on film if the lighting is imperfect

Abstract Trace fossils typically are good facies indicators, and for that very reason they typically are poor index fossils for biostratigraphy. That is, trace fossils generally are characterized by long time ranges and restriction to particular benthic habitats. Body fossils of rapidly evolving, universally distributed taxa are much more useful for stratigraphie correlation and age-dating. When such body fossils are not present, however, trace fossils may prove helpful in accomplishing gross stratigraphy. In some lithologies, such as coarse siliciclastics, or in some environments where the inhabitants are mainly soft-bodied, such as lakes or deep-sea basins, trace fossils often are the only game in town. The following examples demonstrate the utility of traces in solving stratigraphie problems. In most parts of the world, the Precambrian-Cambrian boundary is defined at the horizon where the first fossils of shelled organisms appear, and almost invariably these are trilobites. The question arises, should be boundary be picked at the lowest trilobite fossils (i.e., either body fossils or trace fossils), or must one find actual body fossils of the animals? We suggest that the latter is an incomplete approach to biostratigraphy; if trace fossils testify that trilobites were present earlier than the body fossil record along would indicate, that important paleontologic evidence should be considered in constructing evolutionary chronologies and establishing stratigraphie boundaries. In many places (e.g., northern Norway, central Sweden, Greenland, northern Spain, southern Australia, southwestern Canada and eastern California), tracks which may have been made by trilobites occur in rocks that lie stratigraphically below the

Abstract If geologists wish to make accurate paleoenvironmental reconstructions based on trace fossil evidence, a clear understanding of the factors that limit trace fossil distribution is essential. Too often we read unqualified statements, such as, “the presence of Ophiomorpha indicates the environment was a beach”, or “the water depth must have been abyssal because of the occurrence of Nereites .” Such interpretations may not be incorrect, but they do imply rather simplistic application of some complex relationships. Trace fossils represent the presence of particular kinds of behavior patterns in particular habitats, and as such they reflect the paleoecology of the trace-makers. Just as the distribution of animals and plants is influenced directly by a wide variety of physical environmental parameters, so is the trace fossil record they produce (e.g., see papers in Frey, 1975b). Perhaps the most fundamental aspect to understand about a sedimentary rock is whether it was deposited on land or under water. This is not always easy to discern, and the literature is full of controversies over the origin of specific rock units, which may be interpreted by some workers as subaerial dune deposits and by others as tidal flat deposits, or by some workers as paleosols and by others as lake beds. Perhaps trace fossils can help in such situations. Paleosols are preserved remnants of ancient soil horizons, which provided stable, although chemically evolving, substrates for organisms. The soils commonly are recognized by characteristic lithic and geochemical sequences, but usually they have a characteristic ichnologic signature as well.

Abstract The texture, composition and stability of sediment may be altered significantly as a result of burrowing activities of organisms. Sediment texture can be changed by (A) mixing together sedimentary layers with different textural characteristics, (B) deposit-feeding, either by sorting the sediment durinq ingestion or by breakdown or aggregation of grains during digestion and excretion, (C) sorting of grains during burrow construction, either by selecting certain size classes for the wall structure or by simply moving certain size fractions of the sediment out of the way, and (D) sediment production from bioerosion of rock. Sediment composition can be changed with respect to its (A) organic content, (B) trace element concentration and (C) redox potential. Sediment stability can be increased by the baffling effect of rigid dwelling tubes, or it can be decreased by the disturbance of grain packing (and thus increasing the porosity, permeability and volume of interstitial water) caused by organisms churning the sediment. Biogenic graded bedding is a vertical gradient in the sediment texture resulting directly from bioturbation. For example, Rhoads and Stanley (1964) studied the sedimentologic effects of burrowing by the deposit-feeding polychaete Clymenella torquata in intertidal habitats in Barnstable Harbor, Cape Cod Bay, Massachusetts. The animal lives in a vertical, 20 cm-long, agglutinated tube and feeds with its head down and anus up. At the lower end of the tube it ingests mud and fine sand, which makes its way through the worm's gut and is excreted as uncompacted feces at the sediment surface. Because grains larger

Abstract Preferred orientation or alignment of traces is seen occasionally, but it is not really a common phenomenon. When observed, however, oriented or aligned burrows can be important pieces of evidence to indicate current strength and direction; usually such patterns occur in fairly high-energy conditions, because there seemingly is no need for an animal to respond directly to the azimuth of low-energy currents. The agglutinated tube of the polychaete Diopatra cuprea , for example, typically exhibits a current-influenced orientation (Myers, 1970, 1972). The tube itself is vertical in the sediment, but the short tube cap that protrudes above the water-sediment interface usually is bent over in a horizontal position, so that the shape of the whole strucutre is an upside-down “L”. The animal inhabits shifting substrates in sandy tidal channels and estuarine point bars, where current strengths can be rather high. The tube cap is oriented perpendicular to the dominant current direction (i.e., neither directly into nor away from the current), so that the worm may sit in its burrow and grab food particles or construction materials that pass by. Preferred orientation of Diopatra tubes enabled Kern (1974) to interpret paleocurrent directions in Middle Eocene marginal marine deposits in southern California. Not only do individual Diopatra tubes exhibit special orientation, but also dense populations of the tubes may be spatially arranged in linear patterns. Along the eastern U.S. coast it is not uncommon to see large numbers of Diopatra tubes lined up in rows on point bars, the rows being oriented perpendicular

Abstract Organisms that produce traces occupy such a wide range of substrates that it can be said that as long as a sediment is exposed, it may be occupied. This is especially the case in marine settings, where a succession of infaunal communities may inhabit a deposit as it progresses through the successive stages of soupground, softground, firmground and hardground/rockground. The transition of soupy sediment to indurated rock occurs by compaction and diagenesis and thus accompanies the succession of ichnocoenoses in the same deposit. Complex ichnofabrics result from the juxtaposition of trace fossils produced in an ever-lithifying substrate. For example, borings of the Trypanites ichnofacies (i.e., in a rockground) may be superimposed on burrows of the Glossifungites ichnofacies (i.e., in a firmground), which in turn may be juxtaposed upon burrows of the Skolithos ichnofacies (i.e., in a softground), which themselves may have been emplaced in an already thoroughly bioturbated sediment (i.e., in a soupground). An excellent example of this parallel succession of diagenetic and ichnologic events affecting a sediment can be seen in the Upper Cretaceous chalk of northern Europe. Compaction deformation of trace fossils and sharpness of burrow margins provide information about the character of the original chalk substrate. Zoophycos and Chondrites , for example, occur only in situations where the sediment was firm but uncemented, even though they commonly are overprinted on a background ichnofabric containing indistinct, highly compacted and deformed burrows. Thalassinoides , on the other hand, apparently was produced in a variety of substrate conditions, including soupgrounds, softground, firmgrounds

Abstract “Bioerosion”, a term coined by Conrad Neumann in 1966 as an abbreviation of “biologic erosion”, encompasses the processes of wasting of indurated rock by organisms. The term has been used to describe every form of biologic penetration into hard substrates (i.e., lithic, skeletal or woody) together with destruction of the substrate and creation of preservable trace fossils that arise from the process. An extremely wide array of organisms are and have been involved in bioerosion (e.g., see Bromley, 1970; Warme, 1970, 1975), and their work has progressed at all scales from the microscopic to the gigantic. The minute scar etched on a shell by a brachiopod's pedicle has great paleoecologic significance. On the other hand, the sapping of cliffs by innumerable bivalves and sponges brings about the isolation of islands from continents; according to natural historians in Britain, the boring bivalve Zirfaea crispata is said to have played a leading role in the separation of England from the European continent! Had the Great Armada sent against England by Philip II of Spain in 1588 not been annihilated by the activities of the shipworm (a wood-boring bivalve), with the help of the unbored English fleet, the language of this textbook probably would have been Spanish! The product of bioerosion that usually concerns geologists most is the development of a characteristic sculpture of the substrate, producing a bioerosion fabric, which is more or less equivalent to bioturbation fabric of unlithified sediments. Within a bioerosion fabric the individual structures produced by a bioeroding

Abstract Bioerosion of carbonate substrates by microorganisms more or less constitutes a subdiscipline of its own. This distinction from macroscopic bioerosion is due to the fact that the organisms involved are dominantly plants, fungi and prokaryotes. Because the borings themselves are so small, they demand special techniques for their study. Consequently, microscopic bioerosion is a field for specialists that is rarely entered by workers in related fields. Several animal groups bioerode on a microscopic scale. Radula traces produced by chitons and snails can barely be seen with the naked eye. The borings produced by phoronids and bryozoans also are so small that they can only be studied using the same casting techniques that are used for microscopic bioerosion. This chapter deals with endolithic plants, fungi and prokaryotes, which, although taxonomically diverse, form an ecologically homogeneous group. The organisms involved are listed in Table XI-1. The chiorophytes comprise a number of genera and species that actively bore in carbonate substrates. They produce branching networks of tunnels varying greatly in size. Some may be as little as 2μ m in diameter. In some species such fine filaments may collect together in main canals as wide as 25 μ m, but including sack-like chambers of greater size (Golubic, Perkins and Lukas, 1975, p. 243). A shell or light-colored rock that has been thoroughly bored by green algae takes on a pale green color. Two genera of rhodophytes, Porphyra and Bangia , are endolithic in early stages of their life cycle, after which they become epilithic.

Abstract Continental subaerial (i.e., non-aquatic) deposits can be conveniently grouped into two main categories: eolianites and soil zones. A common misconception regarding such deposits is that they are devoid of fossils. Although their paleontologic record is indeed meager, distinct suites of trace fossils are present that ultimately may prove useful as diagnostic tools in identifying these environments in the rock record. By far the most extensive geologic record of continental non-aquatic environments is that characterized by eolianites (i.e., sand dune deposits). Modern eolian sediments are associated with two major areas: (1) sandy deserts (or ergs) and (2) coastal dunes. Their identification depends primarily on the recognition of features that can be attributed to the transportation, deposition and erosion of sediment by wind. Recent reviews on the physical characteristics of eolianites (Bigarella, 1972; Collinson, 1978c; Walker and Middleton, 1977; Ahlbrandt and Fryberger, 1982 have suggested that the following features may be diagnostic of wind-derived deposits: (a) large-scale, high-angle cross-strata (up to 35 m thick), which commonly are planar-tabular or trough to wedge-shaped; (b) high ripple indices with large bedforms showing a consistent ripple index from base to top; (e) sedimentary structures related to the process of sand avalanching down dune slip faces; (d) minor sedimentary features, including raindrop imprints, vertebrate tracks and deformation of lee-side laminae; (e) intercalated interdune deposits and/or poorly sorted lag deposits along erosional bounding surfaces; (f) frosting of sand grains; (g) large and small-scale deformation features; (h) characteristic light and heavy mineral separation ratios. Although such sedimentologie features

Abstract Aquatic nonmarine (i.e., fresh water) deposits can be conveniently subdivided into lotic (running water) and lentic (standing water) environments. Rivers, streams and wave-swept beaches of lakes are mostly lotie; while ponds, lakes and pools are lentic. Such environments often are characterized mistakenly as being devoid of trace-making organisms, and, in many cases, this trait has been utilized to distinguish fresh water from marine deposits in the rock record. However, contrary to this popular belief, modern fluvial and lacustrine environments commonly are inhabited by a diverse assemblage of infaunal and epifaunal organisms, which can and do create preservable biogenic structures. Little ichnologic work has been done in modern aquatic nonmarine deposits. Recently, however, significant strides have been made to better understand the animal-sediment relationships of such environments (e.g., see Chamberlain, 1975a; Meichior and Ericson, 1979; Fisher, Lick, McCall and Robbins, 1980; Ratcliffe and Fagerstrom, 1980; McCall and Tevesz, 1982; Fisher, 1982; Cohen, 1984). The physical characteristics of freshwater environments are fairly well known. Fluvial deposits have received in-depth study, and a number of facies models, including characteristic vertical sequences, have been erected (e.g., see Collinson, 1978; Cant, 1982). Lacustrine environments, on the other hand, are much more enigmatic; however, criteria for their recognition have been summarized by several workers (e.g., see Picard and High, 1972; Collinson, 1978b; Pouch and Dean, 1982. As is the case with marine environments, ichnologic investigations of fresh water deposits have proceeded from two main directions: (1) study of Holocene traces, the organisms that produce them, and

Abstract Marginal marine zones comprise a multitude of depositional environments, including salt marshes, tidal flats, washover fans, lagoons, bays, estuaries, tidal deltas and tidal inlets and channels. The facies of many of these environments are intimately related and may differ from one another only in subtle ways. Considerable work has been accomplished on the physical and biogenic characteristics of such facies along modern coastlines, particularly the coasts of Georgia (Howard and Frey, 1980a, 1980b, 1980c; Frey and Howard, 1980, California (Warme, 1967,1971; Ronan, Miller and Farmer, 1981 and Germany (Schafer, 1962; Reineck and Singh, 1973. From a biological standpoint marginal marine zones are environmentally very stressful, because they are subject to extreme short-term variations in temperature, salinity, subaerial exposure, energy level and food supply. For these reasons they are inhabited only by organisms that are well-suited to withstand such rigors. Trace-making organisms constitute an integral part of the assemblage, and the distribution of their traces aids in recognizing marginal marine environments in the rock record. Washover fans are formed from wind-generated storm surges that cross over or cut through barrier islands. Such deposits generally range in thickness from a few centimeters to several meters, and they consist of lobate to sheet-like sand bodies, which extend into the marsh or back-barrier lagoon (Andrews, 1970; Schwartz, 1975). Under transgressive conditions, the development of washover fans constitutes an important mechanism for the landward migration of barrier island complexes (Reinson, 1979). Their recognition in the rock record, therefore, may be critical in interpreting marginal marine

Abstract In marine settings seven recurring ichnofacies are recognized, each named for a representative ichnogenus: Trypanites , Teredolites , Glossifungites , Skolithos , Cruiziana, Zoophycos and Nereites . These trace fossil associations reflect adaptations of benthic trace-makers to such environmental parameters as substrate consistency, hydrodynamic energy level, depositional processes, hydrography and food supply. The traces in the marine softground ichnofacies (i.e., Skolithos , Cruziana , Zoophycos and Nereites) are distributed according to numerous environmental factors, including especially bathymetry; the traces in the hardground ( Trypanites ), firmground ( Glossifungites ) and woodground ( Teredolites ) ichnofacies are distributed on the basis of substrate type and consistency (Fig. 15-1). The bathymetric zonation of softground ichnofacies is imperfect. Particular combinations of trace fossils vary according to local conditions and age relationships, and there are numerous examples of bathymetric displacement of many of supposed facies-characteristic ichnotaxa (e.g., shallow-water occurrences of Zoophycos and deep-water examples of Skolithos ). Therefore, extreme caution must be exercised when applying bathymetric generalizations to rock sequences based solely on individual trace fossils. However, as Howard (1978) has pointed out, in spite of these limitations, zonations based on the energy-depth ichnofacies model (Fig. 15-2) are real. Seilacher's (1967a) model continues to stand as an excellent indicator of general depositional conditions. One of the best examples of the energy-depth zonation of trace fossils is the classic beach-to-offshore sequence, which is characterized by a relatively simple energy gradient. Shallow, nearshore zones are typified by high-energy conditions and are dominated by physical sedimentary structures. Deeper, lower-energy, offshore environments display increasing biogenic influence (e.g., see Howard and Reineck, 1972

Abstract Justification for a separate chapter on the subject of ichnology of marine carbonate - as opposed to terrigeneous - sediments lies in the nature of carbonate grains and the environments in which they are produced, cemented and destroyed. The special properties of carbonate substrates and carbonate depositional environments are reflected by the characteristic trace fossils in several ways. In most respects, however, the ichnology of the shallow marine carbonate environments resembles that of their physical equivalents for terrigeneous sediments (see Chapter 15). The emphasis of this chapter is on carbonate systems of the warmer seas; those of temperate regions are more elastic in nature than their warm-water equivalents, and their ichnology has received little attention. Aspects of ichnology that are peculiar to carbonate environments belong in three general categories: (i) bioturbation and burrows; (2) pellet formation; (3) bioerosion of carbonate grains (including miente envelope formation), of beachrock and coasts, and of hardgrounds and reefs. Unbioturbated sediments of beach and restricted subtidal environments tend to be strongly laminated. Thus, the presence of any burrows within the fabric is conspicuous, as also is the complete destruction of the lamination by total bioturbation when it occurs. Absence of bioturbation is much more common, as it is in the same environments in the terrigeneous realm, and usually this may be accounted for by either physical reworking of the sediments or lack of benthos. In high-energy settings, physical reworking of sediment and the concomitant deposition of thick, single-event units, are frequent and tend to obliterate

Abstract Pelagic carbonate oozes have been major sediment of continental slopes and the abyssal plain above calcite compensation depth since at least Jurassic times. The deposit owes its properties to the domination of nannofossil and microfossil calcite skeletons and a paucity of terrigenous material. In shallower, shelf settings, normally the pelagic constituents are diluted by land-derived muds and coarser materials and the purer carbonate oozes are restricted to local importance. During the late Cretaceous, however, world wide transgression drowned large areas of shelf and land under epicontinental seas and extensive pelagic oozes were deposited under unusually shallower conditions to produce the well known on-shore occurrences of Cretaceous chalk. It is these rocks that form the basis of the present chapter. Their importance as reservoir rocks in several areas, not least in the North Sea, has led to the intensive investigation of the properties of chalks in recent years (e.g., Scholle, 1977a; 1977b); detailed ichnologic studies of chalks, however, have rather lagged behind. We define “chalk” in more or less the same terms as in general use in the Deep Sea Drilling Project reports (Gealy, Winterer and Moberly, 1971, p. 17; Benson, Sheridan et al., 1978, p. 14), as a firm, partly indurated calcareous ooze or friable limestone. Its fully lithified equivalent has been termed “chalkstone” (Bromley and Gale, 1982; Scholle, Arthur and Ekdale, 1983, p. 623), in harmony with the nomenclature of terrigenous rocks (clay, claystone; silt, siltstone; etc.). At first sight, chalks represent a monotonous lithology comprising firm, pale-colored micrites.

Abstract Continental margins, whether “active” or “passive” in the tectonic sense, usually are very dynamic environments of deposition. Deep-water slope environments in the marine realm, taken in the broad sense to include the outer continental shelf, continental slope and continental rise, exhibit considerable variation in sedimentary facies (Fig. 18-1). Deposition may be by sliding (e.g., glide or slump), gravity transport (e.g., grain flow or debris flow) or gravity-induced currents (e.g., turbidity currents). Sedimentation is spasmodic, with long periods of non-deposition punctuated by short bursts of rapid sediment influx. Turbidites, or flysch deposits in general, are spasmodically deposited slope sediments (Bouma, 1962; Mutti and Ricci Lucchi, 1972; Middleton and Hampton, 1973; Walker and Mutti, 1973; Walker, 1979b). They may occur along oceanic continental margins, within isolated marine basins or even in large lakes, although it is only the marine deposits that contain well-known ichnofaunas. In the strict sense, turbidites are packages of sediment deposited by gravity-driven, fluidized turbidity currents. Typically, but certainly not invariably, such deposition occurs in fairly deep water (i.e., bathyal and abyssal depths). Turbidite packages usually exhibit a characteristic cyclicity of microfacies, known widely as the “Bouma sequence” (Fig. 18-2). An ideal Bouma sequence contains the following succession: T a , massive or graded sand layer; T b , planar parallel-laminated sand layer; T c , ripple cross-laminated or convolute-laminated fine sand or silt layer; T d , planar parallel-laminated silt layer; T e , hemipelagic mud layer, which may be finefy laminated, bioturbated or apparently structureless. Many turbidites do not contain such an idealized sequence at all, and