Distribution Of Trace Fossils Research Articles

Facies and sequence controls on the appearance of the Cambrian biota in southwestern Mongolia: implications for the Precambrian–Cambrian boundary

AbstractNeoproterozoic–Cambrian rocks of the Zavkhan Basin (Govi-Altay, western Mongolia) comprise large-scale alternations of siliciclastic- and carbonate-dominated units (cf. ‘Grand Cycles’). Analysis of such depositional sequences near the base of the Cambrian confirms that the distribution of trace fossils, small shelly fossils and calcimicrobial structures was strongly controlled by ecology and taphonomy, corresponding to specific points in a sea-level cycle. Evolution of the Cambrian biota is thus viewed through aseries of narrow time windows, once only for each depositional cycle. Correlation of the Precambrian–Cambrian boundary level on the basis of the first appearance of thePhycodes pedumassemblage is also fraught with difficulty, since stratigraphic resolution may be limited to a single sea-level cycle(c. 1–5 Ma). It is suggested that, in many cases, basin analysis will need to be undertaken before this boundary can be drawn.

Trace fossils and biofabrics at the Precambrian–Cambrian boundary interval in western Mongolia

AbstractA small suite of trace fossils from the Zavkhan Basin (Govi-Altay) includes many of the ichnotaxa typical of the Nemakit-Daldynian, Tommotian and younger Cambrian stages, and other indeterminate forms. The traces are almost entirely from the sandstone intervals of the large-scale alternations of carbonate and siliciclastic sediments, thus emphasizing the facies and taphonomic controls on trace fossil distribution, and hence the inherent difficulties in using trace fossils in detailed global correlation, especially in using first appearances. The assemblage of traces and biofabrics is seen as resulting from the partly non-uniformitarian (non-actualistic) sedimentary environments that pertained over the boundary interval, rather than as evolutionary failures and subsequently vacated environments.

Trace fossil zonation in Cretaceous turbidite facies, northern California

The differences between trace fossil assemblages of “proximal”; and “distal”; turbidites (terms used here in the broadest senses) are often attributed to varied preservation processes, although some workers have suspected that trace fossil distribution also represents regional deployment of benthic organisms. Thick‐bedded, thin‐bedded, and muddy turbidites (representing a proximal‐to‐distal environmental gradient) were studied at Point Saint George, northern coastal California, to determine which aspects of trace fossil zonation result from trace preservation and which aspects reflect the distribution of benthic tracemakers. Thin‐bedded turbidites contained nearly all of the trace fossils found in the sequence: predepositional traces (totalling 17 ichnotaxa and other distinctive biogenic structures) were preserved as casts on the soles of sandstone layers; postdepositional traces (9 forms) were preserved in full relief throughout beds. In contrast, muddy turbidites had only three ichnotaxa preserved in fl...

A generalized model for the development of ichnocoenoses in flysch deposits

A systematic survey of intrastratal trace fossil distribution in flysch deposits shows that the ichnocoenoses form along two radically different modes: 1. Post‐turbidite ichnocoenoses begin with synchronous colonization. Further development of the tiered structure depends primarily on sediment composition, textural profile and oxygen levels. Different species respond to these factors according to their oxygen tolerances, penetration potentials and nutritional requirements.2. Pre‐turbidite ichnocoenoses result from successional colonization, depending on the duration of background sedimentation. The depth of subsequent turbidite erosion determines which tier becomes preserved.

Distribution and Significance of Mesozoic Vertebrate Trace Fossils in Dinosaur National Monument

Search for new tracksites has now revealed a total of 25 localities in the older Mesozoic sedimentary rocks of Dinosaur National Monument (DNM). To date, the most productive units have proved to be the Late Triassic Chinle Group and the mainly Early Jurassic Glen Canyon Group in the western part of DNM. In addition several sites, that are an integral part of the overall paleontological picture, have been found just outside the boundaries of DNM. All these sites (and stratigraphic levels) add up to a total of about 45 tracksites in the DNM area. Chinle sites have now yielded dozens of trackways of theropod dinosaurs, ?prosauropod dinosaurs, mammal-like reptiles, ?phytosaurs, aetosaurs, lepidosaurs and tanystropheids, producing one of the most diverse Late Triassic track assemblages known anywhere. The ?prosauropod tracks are the first Late Triassic examples ever reported from the North American continent. The Lower Jurassic has also yielded theropod and prosauropod dinosaur tracks. These tracks are useful for correlation with other Early Mesozoic tracksites around the world and can be used to help construct a series of track zones (or Palichnostratigraphy) for the western United States. The discovery of prosauropod tracks (Pseudotetrasauropus) in the Chinle Group and in the lower part of Glen Canyon Group (Otozoum) points to a much wider distribution of prosauropod tracks in the western United States than previously supposed, and the need for a thorough study of these and similar track types. Further examination of the Jurassic Cannel Formation close to the DNM boundaries, reveals several tracksites that require further study.

Mixed Siliciclastic-Carbonate Deposition in an Early Cambrian Oxygen-Stratified Basin, Chapel Island Formation, Southeastern Newfoundland

ABSTRACT The upper portion of member 3 and member 4 of the uppermost Precambrian (Vendian)-Lower Cambrian Chapel Island Formation represents a large-scale shoaling-up deposit dominated by shelf siliciclastic mudstone. Medium-scale, higher-order shoaling-up cycles at the top of member 3 and in member 4 are capped by peritidal limestones that contain a wide variety of stromatolitic buildups and algal mud mounds. Sedimentation took place in an oxygen-stratified basin with accumulation of gray dysaerobic mudstone offshore and green and red mudstone in shallower, more oxygenated waters. Biofacies models for dysaerobic basins can be applied to this sequence and help in understanding the distribution of trace fossils, skeletal fossils (shells and pyritic steinkerns), and diagenetic features. These da a are important for the understanding of the stratigraphic distribution of shelly fossils within this lowest Cambrian unit. A mixed carbonate-siliciclastic facies model is presented in which carbonate sediments formed in intertidal areas along the shoreline of a siliciclastic muddy shelf. The carbonate shoreline prograded during periods of increasing rate of relative sea-level fall or decreasing rate of relative sea-level rise. Restriction of carbonate sediments to the peritidal environment may have been due to a combination of higher rates of carbonate sediment production and accumulation in the peritidal zone, seaward dilution by siliciclastic mud, and a paleogeographic position near the latitudinal climatic extreme for carbonate production.

Trace-fossil tiering in flysch sediments: examples from the Guipúzcoan flysch (Cretaceous-Paleogene), northern Spain

Distribution of trace fossils in flysch sediments is in great part conditioned by tiering; that is, the vertical partitioning of trace-producing organisms below the sediment-water interface. Ichnologic studies in the Guipúzcoan flysch (Upper Albian-Lower Eocene), northern Spain, show that tiering in flysch sediments is strongly controlled by sediment character, mode and rate of deposition, and degree of oxygenation of pore waters. Depending on the mode of deposition, trace-fossil tiering in flysch developed and was preserved in two radically different styles: (1) In event layers (chiefly turbidites), tiering is preserved without secondary distortion, owing to synchronous colonization and the lack of vertical ascent through time. Vertical tiering structure depends primarily on sediment composition, sequence and thicknesses of structural divisions, and oxygen profile. Variations in tiering are interpreted to reflect the particular oxygen tolerances, penetration potentials, and nutritional requirements of tracemakers; (2) In background sediments (hemipelagites and pelagites), tiering resulted from successive colonizations that were influenced by background sedimentation rates, sediment composition, and oxygen levels of pore waters. Tiering of actively or passively filled burrows is reflected by complex ichnofabrics and the superposition of different tiers by ascension. Tiering structure was frozen upon deposition of superjacent event layers. The original tiering of graphoglyptids (open burrows) may be reconstructed from their cross-cutting relationships on turbidite soles. These contrasting styles of tiering development are characteristic of flysch and other sediments composed of alternating event and background deposits.

Discontinuity surfaces in the Lower Cretaceous of the high Andes (Mendoza, Argentina): Trace fossils and environmental implications

The paleoecologic and paleoenvironmental significance of trace fossils related to discontinuity surfaces in the Lower Cretaceous marine deposits of the Aconcagua area are analysed here. Carbonate-evaporite shoaling-upward cycles, developed by high organic production in a shallow hypersaline restricted environment, make up the section. Two types of cycles are defined, being mainly distinguished by their subtidal unit. Cycle I begins with a highly dolomitized lower subtidal unit (Facies A), followed upward by an intensely bioturbated upper subtidal unit (Facies B). The nodular packstone facies (B 1) is capped by a discontinuity surface (firmground or hardground) and occasionally overlain by an oystreid bed (Facies C). Cycle II is characterized by a pelletoidal subtidal unit (Facies B 2) with an abnormal salinity impoverished fauna. Both cycles end with intertidal to supratidal evaporite deposits (Facies D and E, respectively). Attention is particularly focused on cycle I due to its ichologic content. The mode of preservation and the distribution of trace fossils in nodular packstone facies are controlled by original substrate consolidation. Thalassinoides paradoxicus (pre-omission suite) represents colonization in a soft bottom, while Thalassinoides suevicus (omission suite pre-lithification) is apparently restricted to firm substrates. When consolidation processes are interrupted early, only an embryonic hard-ground that represents a minor halt in sedimentation was developed. Sometimes, consolidation processes continued leading to an intraformational hardground. Colonization by Trypanites solitarius (omission suite post-lithification) and Exogyra-like oystreids possibly characterizes hard substrate stage. When two discontinuity surfaces follow closely, a post-omission suite may be defined in relation to the lower cemented surface. As trace fossils are so closely related to changes in the degree of bottom lithification, they prove to be very useful as indicators of substrate evolution. The presence of discontinuity surfaces, evidenced by trace fossil association, suggests changes of sedimentary rate and environmental conditions that should be taken into account in future studies seeking to erect depositional models for these Cretaceous deposits.

Distribution and Significance of Mesozoic Vertebrate Trace Fossils in Dinosaur National Monument

Dinosaur National Monument (DNM) encompasses a large area with surface exposures of Mesozoic rocks. Although only one isolated footprint (from an unknown locality) had previously been discovered at DNM, there is a high potential for preservation of fossil footprints in this area, as has been proven by discoveries in similar rocks in the region around DNM, and during research activity at DNM. As reported by Lockley et al. (1990), the purpose of this research is to seek, document and interpret vertebrate trace fossils in any of the ten potentially track-bearing Mesozoic stratigraphic units in which footprints might be discovered. During the course of the first year (6/90-6/91) the University of Colorado at Denver Research Group documented 11 localities where tracks were discovered at one or more stratigraphic levels (Lockley et al. 1990, 1991). In the first part of the second year of investigation (7/91-1 0/91) the research group discovered an additional five localities, and three additional sites were reported to us by Park Paleontologists and other researchers. This brings the total to 19 localities, of which three include at least two levels with tracks (Total = 22 localities). As discussed below, the tracks provide evidence of the activities of several dozen animals that represent distinct ancient animal communities.

Distribution and Significance of Mesozoic Vertebrate Trace Fossils in Dinosaur National Monument

Dinosaur National Monument (DINO) encompasses an area that has rocks with a high potential for preservation of vertebrate trace fossils, especially dinosaur tracks. The purpose of this research is to document the presence, type, and distribution of vertebrate trace fossils in Mesozoic rocks exposed in DINO. These rocks include the Moenkopi Formation, Chinle/Popo Agie Formation, Glen Canyon Sandstone, Carmel Formation, Entrada Sandstone, Morrison Formation, Cedar Mountain Formation, Dakota Formation, and Frontier Formation. This study will increase our knowledge of the stratigraphic and geographic distribution of vertebrate tracks as well as provide taxonomic, behavioral, and paleoenvironmental data. During the 1990 field season, reconnaissance of the western part of DINO revealed the presence of vertebrate trace fossils in the Chinle/Popo Agie Formation. In addition, our examination of the Moenkopi Formation suggests that vertebrate tracks are probably present in this unit. Locality information was also obtained for probable track-sites in the Carmel Formation, Entrada Sandstone, and Morrison Formation.

Shallow and Marginal Marine Triassic Trace Fossils and Ichnofabric from Northwest Australia (Ocean Drilling Program Leg 122): ABSTRACT

The ichnofabric index method of ranking amount of bioturbation was used for the first time in conjunction with discrete trace fossils to examine shallow-water marine cores. Previous ichnological studies on cores have focused primarily on outer shelf and deep-sea discrete trace fossils. Upper Triassic cores examined in this study were recovered off northwest Australia during ODP Leg 122. These sediments were deposited in a shallow-water and continental shelf setting, which included swamp and prodelta environments. The most common lithology is siltstone with interbedded mudstone and sandstone. Sediments deposited in a swamp setting have rootlets and coal beds with an ichnological record consisting primarily of mottled bedding rather than discrete trace fossils. Ichnofabric indices 1 through 5 were recorded. Marginal marine/lagoonal facies have a low trace fossil diversity with common Chondrites, Planolites, and Teichichnus. Recorded ichnofabric indices include 1, 2, and 3. Laminated mudstones and siltstones (ii1) are most common. Fully marine open shelf strata are thoroughly bioturbated (ii5 and ii6) with Thalassinoides, Zoophycos, Teichichnus, and Planolites. Wackestone and packstone occur in discrete uppermost Triassic intervals and have ii1 through ii6 represented. In part due to the drilling process, sandstones and reefal limestones were poorly recovered and ichnofabric is not wellmore » preserved. Physical sedimentary structures and lateral facies relationships can be difficult to discern in core. In shallow marine deposits, the distribution of ichnofabric indices and discrete trace fossils within these strata provide an additional important data base to evaluate depositional environments.« less

Trace Fossils and Bioturbation in Peritidal Facies of the Potsdam-Theresa Formations (Cambrian-Ordovician), Northwest Adirondacks

The Cambrian-Ordovician Potsdam Sandstone, Theresa Formation, and Canadian correlatives in the St. Lawrence Lowlands preserve tide-dominated facies during the basal Cambrian transgression. Low intertidal sand flats in the upper Potsdam contain a Skolithos Ichnofacies dominated by Diplocraterion parallelum in clean, herringbone cross-bedded sandstones indicative of high tidal current energy. Wind-wave-driven longshore and tidal currents along a macrotidal coastline were funneled northeast-southwest by Precambrian topographic relief of up to 65 m. This relief is now expressed as the Thousand Islands of New York and Canada. The conformably overlying Theresa Formation preserves a shoaling-upward sequence of mixed clastic-carbonate facies. Shallow subtidal and peritidal facies contain a mixed Skolithos-Cruziana Ichnofacies in sharply alternating lithofacies consisting of gray, intensely bioturbated, poorly sorted calcareous sandstone, and meter-thick, white cross-bedded sandstone. The parallelism between ichnofacies and lithofacies indicates that environmental energy level and persistence rather than water depth controlled trace fossil distribution. Bioturbated sandstones contain a Cruziana ichnofacies of abundant deposit feeders including: Fustiglyphus , Gyrochorte , Neonereites uniserialis , Phycodes flabellum, Planolites beverlyensis, Rosselia socialis, and Teichichnus. Suspension feeders are represented by D. habichi, D. parallelum, Skolithos, Monocraterion, and possibly Palaeophycus tubularis. Scavenging or deposit-feeding arthropods are represented by rare Cruziana furrows. Cross-bedded sandstones containmore » a Skolithos Ichnofacies of shallow Skolithos and Monocraterion burrows, and an undescribed large epistratal eurypterid( ) trail.« less

Pitfalls of Paleobathymetric Interpretations Based on Trace Fossil Assemblages

Trace fossils have been used widely as indicators of original water depth in paleoenvironmental studies, but most paleobathymetric interpretations in ichnology have been based on only a small number of standard ichnofacies. In addition to bathymetry, it is important to recognize that salinity, oxygen concentration, and substrate character also play a major role in controlling the distribution of trace fossils. Several pitfalls of paleobathymetric interpretation based on trace fossils can lead to erroneous paleoenvironmental reconstructions. The most common of these pitfalls, of which historical geologists should be wary, include the following: 1) occurrence of a particular ichnogenus does not necessarily indicate the presence of the ichnofacies of the same name, and a standard ichnofacies may be identified without the presence of the namesake trace fossil; 2) environmental shifts of certain trace fossils have occurred through time, so the paleoenvironmental significance of a particular ichnotaxon may have changed during its histoty; 3) the nine standard ichnofacies that have enjoyed wide use by ichnologists have broad environmental significance that may include, but certainly extends beyond, bathymetry; 4) not all environmental situations are represented in the nine standard ichnofacies, so many trace fossil associations in the real world cannot be easily fit into those categories.

Multivariate Analyses of Trace Fossil Distribution from an Early Mississippian Oxygen-Deficient Basin, Central Appalachians

Multivariate Analyses of Trace Fossil Distribution from an Early Mississippian Oxygen-Deficient Basin, Central Appalachians

Characteristic trace-fossil associations in oxygen-poor sedimentary environments

Research Article| August 01, 1988 Characteristic trace-fossil associations in oxygen-poor sedimentary environments A. A. Ekdale; A. A. Ekdale 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Search for other works by this author on: GSW Google Scholar T. R. Mason T. R. Mason 2Department of Geology and Applied Geology, University of Natal, Durban 4001, South Africa Search for other works by this author on: GSW Google Scholar Geology (1988) 16 (8): 720–723. https://doi.org/10.1130/0091-7613(1988)016<0720:CTFAIO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation A. A. Ekdale, T. R. Mason; Characteristic trace-fossil associations in oxygen-poor sedimentary environments. Geology 1988;; 16 (8): 720–723. doi: https://doi.org/10.1130/0091-7613(1988)016<0720:CTFAIO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The sedimentologic record of oxygen-poor depositional environments typically includes trace fossils produced by deposit-feeding organisms. We propose an oxygen-controlled trace-fossil model in which increasing oxygen concentration of interstitial water in sediment parallels a transition from fodinichnia-dominated through pascichnia-dominated to domichnia-dominated trace-fossil associations. In certain places, such as parts of the late Paleozoic Oquirrh Basin in Utah, this model provides a reasonable alternative to the more traditional, depth-controlled model of trace-fossil distribution. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A Sedimentologic Description of a Microtidal, Flood-Tidal Delta, San Luis Pass, Texas

ABSTRACT San Luis Pass is a microtidal tidal inlet located at the southwest end of Galveston Island, Texas. Continuous cores taken with a portable vibracoring rig, and surface grab samples provide data for constructing vertical sequences and three-dimensional sedimentologic descriptions of the flood-tidal delta complex. These descriptions are based on the type and vertical and lateral distribution of lithofacies, sedimentary structures, textures, and trace fossils. A vertical sequence through the flood-tidal-delta complex consists, from base to top, of: highly bioturbated bay muds and associated oyster reefs; highly bioturbated clayey sand/sandy clay of the tidal delta margin; burrowed sand and shelly sand of the tidal delta; and rooted or burrowed mud of the marsh or mud flat. Washover shell hash deposits may occur at random intervals throughout this sequence. A vertical sequence in the vicinity of San Luis Pass consists of a basal tidal inlet deposit of graded layers of sand and shell overlain by burrowed to shelly sand of the barrier spit. This description of a microtidal, flood-tidal delta differs significantly from descriptions presented for mesotidal, flood-tidal-delta systems in the 1) general lack of large-scale, high-angle sedimentary s ructures; 2) presence of intense bioturbation; 3) presence of washover deposits; and 4) general coarsening-upward nature of the vertical sequence.

Trace fossils from the Precambrian–Cambrian boundary candidate at Meishucun, Jinning, Yunnan, China

AbstractThe Precambrian–Cambrian boundary candidate section at Meishucun, China, has yielded trace fossils which are abundant at some horizons. The earliest occur in Unit 3 of the Zhongyicun Member approximately 8 m above the lower selected stratotype reference point for the boundary and includeArenicolitessp.,Asteriacitessp.,Neonereites biserialis, N. uniserialisandSellaulichnus meishacunensis. The next trace-fossil-bearing horizon is in Unit 6 of the Zhongyicun Member whereCochlichnussp.,Monomorphichnussp.,Neonereites biserialisandN. uniserialisoccur. Immediately above, in Unit 7, areCruzianasp.,Didymaulichnus miettensis, Monomorphichnussp. andRusophycussp. In the Badaowan Member at the top of the section there areDidymaulichnussp. andTaphrhelminthopsis circularisin Unit 9,Arenicolitessp.,Diplocraterionsp.,Gordia molassica, Skolithossp. andT. circularisin Unit 11, andGordia meandria, ?Plagiogmussp.,Skolithossp. andT. circularisin Unit 12.Comparison of this trace-fossil distribution with that in key Precambrian–Cambrian boundary sections in other countries indicates that the ranges of a few trace fossils cross the boundary (e.g.Didymaulichnus, Neonereites, Planolites) but most appear only in the Cambrian. Different ichnogenera seem to appear at various levels above the boundary.ArenicolitesandAsteriacitesare among the first, whileTaphrhelminthopsis circularisis only encountered higher in all sequences. Some have only been recorded at much higher levels and relatively close to the first appearance of trilobites (e.g.Cruziana, Diplocraterion, Rusophycus). This suggests that the first appearance of specific trace fossils or groups of trace fossils may be valuable for locating the boundary in some sections and for correlating late Precambrian and early Cambrian strata.

Precambrian‐Cambrian trace fossils from western New South Wales

The distribution of trace fossils in the upper part of the Adelaidean succession (Farnell Group) of the Barrier Range near Fowlers Gap is reviewed. Of the main, stratigraphically distinct assemblages, the older from the Fowlers Gap Formation, contains only a few, simple, non‐diagnostic, sinuous and branching trails, and is assumed to be of latest Precambrian age. The assemblage in the succeeding Lintiss Vale Formation has a high diversity, mainly of simple behavioural types produced by worm‐like animals and molluscs, but lacking arthropod traces. It therefore represents a latest Precambrian or an earliest Cambrian assemblage. A third ichnofauna with simple, sinuous and branching trails is recorded from the Wonominta Beds of the Warratta inlier west of Tibooburra. The occurrence of these trails indicates that at least in the northern areas of ‘basement’ Wonominta outcrop the rocks are no older than latest Precambrian (i.e. the age of the earliest indubitable traces of animal activity) and, judging from the...

A Sedimentologic Model for a Microtidal Flood-Tidal Delta--San Luis Pass, Texas: ABSTRACT

San Luis Pass is a microtidal inlet located at the southwest end of Galveston Island, Texas. Continuous cores taken with a portable vibracoring rig, and surface grab samples provide data for developing a 3-dimensional sedimentologic model of the flood-tidal delta complex located landward of the tidal inlet. This model is based on the type, and vertical and lateral distribution of lithologic units, sedimentary structures, textures, and trace fossils. A complete bayward vertical sequence in the flood-tidal delta complex consists (from base to top) of highly bioturbated bay clays and associated oyster reefs, highly bioturbated clayey sands/sandy clays of the delta margin, variably burrowed sand to shelly sand of the delta, and rooted or burrowed muds of the marsh or mud flat. Washover shell-hash deposits may occur at random intervals throughout this sequence. A more seaward sequence in the vicinity of San Luis Pass consists of a basal tidal inlet deposit of graded layers of sand and shell overlain by burrowed to shelly sand of the barrier spit. This model for microtidal flood-tidal deltas differs significantly from models presented for mesotidal flood-tidal delta systems in the general lack of large-scale, high-angle sedimentary structures; the presence of intense bioturbation; the presence of washover deposits; and the general upward-coarsening nature of the vertical sequence. Mesotidal flood-tidal deltas with clean, coarse to medium-grained sands may make good petroleum End_Page 490------------------------------ reservoirs. Microtidal flood-tidal deltas with highly bioturbated clayey sands to sandy clays would undoubtedly prove to be poor petroleum reservoirs. End_of_Article - Last_Page 491------------

Trace Fossils Within Limestone Interbeds, Oak Grove Member, Carbondale Formation (Pennsylvanian, Desmoesian), Northwestern Illinois: ABSTRACT

Most trace fossils have been described from terrigenous rocks, commonly sandstones or sandstone/shale interbeds. The prevailing opinion appears to be that carbonate rocks rarely contain trace fossils or that ichnofossils are more difficult to study in them. Neither is true. Methodology is especially important in these studies, and the peculiarities of carbonates make them particularly rewarding subjects for trace fossil analysis. At Wolf Covered Bridge in Knox County, Illinois, the Desmoinesian Oak Grove Member is more than 5 m (16 ft) thick and consists mostly of shale with several thin carbonate (limestone and siderite) interbeds. The two thickest of these carbonates are a lower gray septarian (or Marginifera) limestone about 0.2 m (8 in.) thick, and about 1.5 m (5 ft) higher a 0.3 m (1 ft) thick Linoproductus limestone. Both are sparse to packed mixed biomicrites with diverse and abundant fossils. The depositional environments of these limestones were similar: nearshore, quiet, delta-influenced, somewhat brackish, shallow water deposits interpreted by Merrill (in 1975) to have formed in water less than 20 m (66 ft) deep. Limestones were sliced perpendicular to bedding in the normal fashion and large slabs were also cut parallel to bedding with a wire saw. Serial sections cut perpendicular to bedding were photographed by X-radiography permitting three-dimensional reconstruction of some burrows. Large slabs cut parallel to bedding were etched and acetate peels prepared in the convention manner, but of unconventional size (some more than 1.0 m, 39 in., long). The polished surfaces were later gridded, coated, and the distribution of body and trace fossils mapped both megascopically and microscopically from the peels. The level of bioturbation is exceedingly high, especially in the Marginifera limestone. Several generations of truncating trace makers are evident. Recognizable ichnogenera include a spectacular Rhizocorallium 40 cm (16 in.) long with waves of spreite outlined by calcitornellid (pseudopthalmid) foraminifers, numerous Chondrites up to 10 cm (4 in.) high, and common Planolites. Lithologic differences among burrow types are striking and many vague, relict, earlier generations of traces remain, traced primarily by allochem distribution. Substrate stabilities differed between the pair of limestones and bearing strength was probably a major factor controlling community structure. There are suggestions of ghost biota and lack of significant compaction of the micrite in the lower carbonat interbed. End_of_Article - Last_Page 513------------