Sedimentary Fabrics Research Articles

Surface and subsurface sedimentary structures produced by salt crusts

The growth and subsequent dissolution of salts on or within sediment may alter sedimentary structures and textures to such an extent that it is difficult to identify the depositional origin of that sediment and, as a result, the sediment may be misinterpreted. To help to overcome such problems with investigating ancient successions, results are presented from a comprehensive study of the morphology and fabrics of three large areas of modern salt flats in SE Arabia: the Sabkhat Matti inland region and the At Taf coastal region, both in the Emirate of Abu Dhabi, and the Umm as Samim region in Oman. These salt flats are affected by tidal‐marine, alluvial and aeolian depositional processes and include both clastic‐ and carbonate‐dominated surficial sediments. The efflorescent and precipitated salt crusts in these areas can be grouped into two main types: thick crusts, with high relief (>10 cm) and a polygonal or blocky morphology; or thin crusts, with low relief (<10 cm) and a polygonal or blister‐like appearance. The thin crusts may assume the surface morphology of underlying features, such as ripples or biogenic mats. A variety of small‐scale textures were observed: pustular growths, hair‐like spikes and irregular wrinkles. Evolution of these crusts over time results in a variety of distinctive sedimentary fabrics produced by salt‐growth sediment deformation, salt‐solution sediment collapse, sediment aggradation and compound mixtures of these processes. Salt‐crust processes produce features that may be confused with aeolian adhesion structures. An example from the Lower Triassic Ormskirk Sandstone Formation of the Irish Sea Basin demonstrates how this knowledge of modern environments improves the interpretation of the rock record. A distinctive wavy‐laminated facies in this formation had previously been interpreted as the product of fluvial sheetfloods modified by soft‐sediment deformation and bioturbation. Close inspection of laminations seen in core reveals many of the same sedimentary fabrics seen in SE Arabia associated with salt crusts. This facies is the product of salt growth on aeolian sediment and is not of fluvial origin.

Observations of flow and grain movement near bed in the ripple and the dune regimes from high-speed video pictures.

The mode of grain movement is a significant factor in sedimentology because it influences the grain fabric of sediments. Laboratory studies regarding the relation between bedform and grain fabric have been carried out. However, grain movement and grain fabric of small bed waves formed immediately after the flow velocity decrease from the upper-plane-bed regime to the dune regime ("upper ripples") have not been investigated so far. This study investigated grain movement near the bed using high-speed video system in the ripple and dune regimes. Grains avalanched on lee slopes in the ripple regime. In the dune regime, however, grains moved as sheet flow everywhere on the surface of "upper ripples", which had wave heights comparable to that of ordinary ripples. These grain movements suggest that the dominant direction of long axes of grains in ripple deposits is similar to cross laminae, while the grain fabric of "upper ripples" shows imbrication with laminae as bases. Grain fabric can be a clue to judge whether small-scale cross-stratification was formed in the ripple regime or in early stage of the dune regime following the upper-plane-bed regime.

Diversity, growth forms and taphonomy: key factors controlling the fabric of coralline algae dominated shelf carbonates

Abstract The fabric of biogenic carbonate sediments can be differentiated (1) with respect to diversity of constituent components, (2) using features pertaining to their growth forms and (3) upon consideration of taphonomic aspects. These not only determine limestone fabrics, but also form the basis for facies differentiation and palaeoecological interpretation. This study is based on coralline algae dominated Lower Oligocene shelf carbonates from northern Slovenia from which seven facies (nummulitic, bivalve, foraminiferal-coralline algal, coralline algal, coralline algal-coral, coral and grainstone) are differentiated. The role of diversity, growth forms and taphonomy of coralline algae in each facies is discussed. Nine species from seven genera of coralline algae including geniculates and non-geniculates were recognized. Numerous different growth forms ranging from crusts, protuberances, lamellae to arborescent types are present. A wide range of taphonomic features including disarticulation, encrustation, fragmentation and abrasion can be observed. The determination of diversity is dependent on taxonomic identification using preserved diagnostic characters relevant to palaeontological and botanical systematics. Growth-form determination in thin section is influenced by orientation and sectioning effects. The taphonomy of red algae is highly dependent on initial growth form and the specific environment in which they are found. A number of taphonomic processes described in Recent environments (e.g. disease, shallow grazing) cannot be ascertained in fossil material, while others are readily observable. Some taphonomic processes are detrimental (e.g. fragmentation, abrasion) to the preservation and recognition of vegetative and growth-form features, while others have positive effects (e.g. encrustation).

Thermodetrital and crystallodetrital magnetization in an Icelandic hyaloclastite

An Icelandic hyaloclastite, mostly composed of millimetric fragments of basaltic glass, that is fresh at the bottom of the unit but largely palagonitized in the upper part, has been studied by petrologic, mineralogical, and magnetic means, with the aim of determining the nature and characteristics of the natural remanent magnetization (NRM). The NRM was generally found to consist of two components: a thermodetrital remanent magnetization (thermo‐DRM) and a crystallodetrital remanent magnetization (crystallo‐DRM). Thermo DRM and crystallo‐DRM are defined here as the remanences acquired as a result of the deposition of magnetic particles of detrital origin individually carrying either a TRM (thermoremanent magnetization) or a CRM (crystallization remanent magnetization), respectively. Regardless of the chemistry and size and of these particles, Thellier experiments carried out on samples carrying a thermo‐DRM provide apparent paleointensities close to the expected geomagnetic paleointensity, which suggests that in the present case the fractional alignment of individual magnetic moments is similar for DRM and TRM. In the upper part of this outcrop, grain growth CRM was acquired by individual grains of magnetite which crystallized as a result of palagonitization of basaltic glass at low temperature (<100°C). No systematic difference could be observed between the directions of characteristic remanence in the layers richest in magmatic magnetic grains and the layer where secondary magnetite is the main remanence carrier. In both cases, the overall remanence exhibits a large inclination error (∼20°), and the samples have a marked anisotropy of magnetic susceptibility which is typical of sedimentary fabrics. Thus, secondary magnetite probably formed prior to the deposition of particles, and the bulk remanence in the palagonitized layers is a crystallo‐DRM rather than a CRM. Throughout the entire stratigraphic thickness, Thellier paleointensity data are of good or excellent quality regardless of the nature of the primary remanence. In agreement with previous theoretical inferences and experimental results the layers carrying a crystallo‐DRM provide a much lower (by a factor of 2) apparent paleointensity than the layers where the remanence is a thermo‐DRM. This suggests that palagonitized basaltic glasses should not be used for paleointensity determinations. Our study shows that application of the Thellier thermal paleointensity method to sedimentary rocks can be a useful tool for distinguishing crystalline versus thermal blocking of the magnetic moments of the individual particles which, after deposition, carry a DRM. More generally, discrepancies between relative paleointensities obtained from sediments can be expected if rocks with different proportions of thermo‐DRM, crystallo‐DRM, or CRM are compared.

Physical and Biological Influences on the Formation of Sedimentary Fabric in an Oxygen-Restricted Depositional Environment: Eckernforde Bay, Southwestern Baltic Sea

Sedimentological and radiochemical investigations of sediments in Eckernforde Bay were coordinated with observations of benthic biology and benthic-boundary-layer dynamics to identify processes influencing the formation of preserved sedimentary fabric. The bay is a trap for fine sediments because of its fjord-like basin geometry and hydrodynamic regime. Sediments are derived from both local sources (transported by winter storms) and sources in adjacent Kiel Bight (fair-weather transport of suspended sediment by internal waves). In the central basin of the bay, strata consist of pelletized, clay-rich beds and silty laminations, and reflect the interaction of bioturbation with alternating fair-weather and storm-associated sediment transport and deposition. Sediment-accumulation rates in the central basin of the bay are approximately 0.39 cm/yr (from 210 Pb geochronology). Results from a numerical model, as well as study of X-radiographs and accumulation rates, indicate that fair-weather processes contribute 0.15 cm/yr of time-averaged sediment accumulation, and the remaining 0.24 cm/yr is contributed by storm-generated deposition. Seasonal oxygen depletion restricts the benthic community to a pioneer assemblage of small-bodied, deposit-feeding polychaetes and bivalves. Excess 234 Th analyses indicate that bioturbation is restricted to the upper 1 cm of the seabed. Macrofaunal feeding activity produces abundant ovoid fecal pellets and biogenic microfabric in beds deposited under fair-weather conditions. In contrast, primary depositional fabric is preserved as laminations when storm deposits thicker than the depth of bioturbation disrupt biological communities.

Depositional chronology and fabric of Siwalik group sediments in Central Nepal from magnetostratigraphy and magnetic anisotropy

Magnetostratigraphic research, undertaken within the past 15 years in the Siwaliks distributed along 400 km of the Sub-Himalaya in central Nepal, has proved that the sediments possess highly reliable hematite-based primary detrital remanent magnetization suitable to determine depositional chronology. In order to bring out the polarity sequences in a common chronological frame, all available data are newly correlated to the latest global magnetic polarity time scale of Cande and Kent (S.C. Cande, D.V. Kent (1995) Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research 100, 6093–6095). Chronological data presented are referred, in relation to the diverse lithological nomenclature, to the formations whose ages are not constrained by isotopic or paleontologic ages. The age of the sections dated by magnetostratigraphy ranges between 14 and <2 Ma. Sediment accumulation rates average to 32–50 cm kyr −1. Rock-magnetic parameters, e.g. initial susceptibility and isothermal remanent magnetization ratios, allow correlation with an accuracy of up to a few hundred meters among several kilometers thick adjacent sections. Anisotropy of magnetic susceptibility (AMS) data reveal a well-defined fabric contributed to by paramagnetic ( k=10 −5 to 3×10 −4 SI) as well as ferromagnetic minerals ( k=3×10 −4 to 10 −2 SI). AMS ellipsoids are mainly oblate along with some prolate ones and the degree of anisotropy is mostly low ( P′<1.2). The magnetic fabric is of pre-folding origin with tilt-corrected sub-vertical magnetic foliation poles. The magnetic lineations do not show parallelism to the expected paleocurrent directions. Rather, sub-parallelism between the clusters of magnetic lineation and the fold axes/bedding strikes/thrust fronts is observed. A superimposed fabric consisting of a sedimentary-compactional and an overprint induced by a mild deformation process is suggested. The latter process was active during, and subsequent to, the deposition in the compressive tectonic setting of the foreland basin. The magnetic lineations for Tinau Khola and Surai Khola sections cluster around N80°W and N88°W respectively, whereas N27°W trend characterizes the Amiliya-Tui area south of Dang. The peak clusters in lineations are probably orthogonal to the true shortening axes. Their variation along the Sub-Himalaya, together with the fold axes or thrust front trends, may be used for accurate tectonic reconstruction. It is especially important when the orthogonality of the latter to the shortening axes may not hold true in the sectors with imbricate fold-and-thrust structures.

A new Variscan deformation map of England and Wales

Collections of bedding orientations from subareas of 550 km 2 are used to define bedding fabrics from the Variscan fold belt and foreland in England and Wales. The bedding fabrics, constructed from data taken from published geological maps (1 : 50,000 scale), are analysed in terms of the Scheidegger–Watson orientation tensor. The calculated eigenvalues of the orientation tensor define the average structural trends, including the best-fit fold axis, for each subarea, The eigenvectors of the tensor allow the original sedimentary fabrics to be distinguished from those modified by deformation. The progressive tectonic modification of these fabrics leads to the concept of fabric evolution paths, and the extent of the modification is expressed by a deformation parameter. Although it is emphasised that this parameter is not solely a function of finite strain magnitude, its variation is represented on a map which brings out different degrees of Variscan tectonic modification of bedding fabrics. This map delineates a Variscan bedding fabric front corresponding to the northern boundary of a zone of pervasive and strong deformation. In the foreland this new analysis allows the variation of Variscan trends to be mapped, and the significance of these trends to be quantified in terms of deformation magnitude.

Early diagenesis: impact of organic matter on mass physical properties and processes, California continental margin

High-resolution (scales of 2.5 mm to submicrometer), interdisciplinary studies of California continental slope sediments (0–80 cm) reveal the importance of marine organic material and mineralogy on mass physical properties in fine-grained sediments during very early diagenesis. The combination of organic material (≥2 wt.%) with smectite (≥5%) is associated with high depositional porosities, low effective stresses, low porosity reduction and dewatering, and the impedance of interstitial fluid flow. Living (microbiota) and nonliving (mucopolysaccharides) organic matter, affecting the physical properties, was revealed by transmission electron microscopy and carbon isotope studies. Calculations using the Redfield ratio show that sediments containing high organic carbon (≥5 wt.%) have `hydrated' organic material in the pore spaces that can occupy a volume that is up to 20% of the total sediment porosity. Pore-water chemistry and X-ray diffraction show evidence of early diagenetic processes that may significantly influence sediment fabric. An important geochemical process near the sediment–water interface that affects sediment fabric is the rapid dissolution of biogenic opal.

Evolution of magnetic fabrics during incipient deformation of mudrocks (Pyrenees, northern Spain)

The anisotropy of magnetic susceptibility (AMS) of Eocene mudrocks from the Southern Pyrenean Foreland Basin documents the progressive development of tectonic fabrics in sediments that mesoscopically show no evidence for deformation. Two end members are observed: (1) a strong oblate fabric, with strong clustering of the minimum axes of susceptibility that is characteristic of depositional and compaction processes; and (2) a prolate magnetic ellipsoid with moderate to strong minimum axes girdle that reflects weak cleavage in the mudrocks. Low-temperature experiments and anhysteretic remanent magnetization reveal that the paramagnetic fraction dominates the AMS signal and thus, represent the preferred orientation of phyllosilicates. Characteristic stages of magnetic fabrics in mudrocks can be established and thus the AMS can be used as an indicator of cleavage development and intensity in mudrocks. The three stages of fabric development (early deformation, pencil structure and weak cleavage) reflect a process of increasingly preferred crystallographic orientation of the phyllosilicates that is evidenced by the change of distribution of the minimum susceptibility axes. This is particularly evident in the eigenvalues, which we plot on a Woodcock diagram that describes both shape and fabric strength, and allows different stages of deformation to be distinguished. Because appreciable depth during deformation (elevated temperatures and high confining pressures) can be ruled out, we propose that the sediments were wet and only partly lithified when the phyllosilicates became progressively reoriented. Thus, the observed fabric is explained as a result of the close interaction of deformation and diagenetic processes. Tectonically induced deformational fabrics formed while diagenesis progressed. Although the study area is a foreland basin, it offers a valuable analogue to active accretionary wedges. The application of AMS in these settings, where sediments are progressively dewatered through the development of fabrics, permits a similar quantification of fabric development and, eventually, of deformation.

Characterizing pore fabrics in sediments by anisotropy of magnetic susceptibility analyses

Characterizing pore fabrics in sediments by anisotropy of magnetic susceptibility analyses

When the worm turned: Concordance of Early Cambrian ichnofabric and trace-fossil record in siliciclastic rocks of South Australia

Studies of sedimentary fabrics and trace fossils across the Precambrian-Cambrian interval in South Australia demonstrate that the appearance of Treptichnus pedum , which defines the base of the Cambrian, also marks the initial development of preservable infaunal activity. There is a significant shift in the amount and nature of bioturbation at this time. These results support the broad-scale utility of T. pedum as the indicator of the base of the Cambrian.

Influence of benthic macrofauna on the geotechnical and geophysical properties of surficial sediment, North Sea

Spatial and temporal variations in the structure of an Amphiura-Echinocardium macrobenthic community were studied in relation to geotechnical and geophysical properties of the seabed at a muddy-sand site in the seasonally stratified region of the southern North Sea. Vertical profiles of geotechnical properties were recorded in sediments collected by box corer. Maxima in water, organic matter, and fine particle contents coincided with the presence of the burrowing brittle star Amphiura filiformis and the mud shrimp Callianassa subterranea in the upper and lower parts, respectively, of the cores. A significant relationship existed between the abundance of A. filiformis and the water content of the upper 0.05 m of the bed. There were important temporal variations in rigidity modulus, derived from acoustic shear wave propagation in freshly recovered cores, of the upper 0.06 m of the sediment. The rigidity modulus was 45% greater in January than in May and this has been related to the burrowing/feeding activity of the macrobenthic community; there was an inverse relationship between A. filiformis abundance and bed rigidity. Thus the bed had a lower bulk density and lower rigidity in summer due to biological modification of the sediment fabric. This implies that the bed was less resistant to erosion in the summer. Such an effect may be important during summer storms in the shallowest parts of the seasonally stratified zone (ie close to shelf fronts).

A quick and inexpensive sample preparation method for AMS microfabric analysis in fine-grained sediment

Unless disturbed by postdepositional processes, sedimentary grain fabric reflects the physical conditions at the time of sediment deposition. Anisotropy of magnetic susceptibility, or AMS, is a sophisticated yet simple method for measuring grain fabric. In most cases this technique requires a self-supporting, right-cylindrical sample, and as a result unlithified sediment needs first to be lithified. Generally this is done with expensive, time-consuming resin impregnation procedures. In consolidated, mud-rich, clast-poor sediment, however, a new technique of cyclical steaming and (steam) soaking proved to be highly effective in softening the sediment sample enough so that it could be cored for AMS-sample preparation. Using this new method, samples were prepared in about 90% less time and for only 10% of the cost of standard impregnation techniques.

The Ichnologic Record of the Continental Invertebrate Invasion: Evolutionary Trends in Environmental Expansion, Ecospace Utilization, and Behavioral Complexity

The combined study of continental trace fossils and associated sedimentary facies provides valuable evidence of colonization trends and events throughout the Phanerozoic. Colonization of continental environments was linked to the exploitation of empty or under-utilized ecospace. Although the nonmarine trace fossil record probably begins during the Late Ordovician, significant invasion of nonmarine biotopes began close to the Silurian-Devonian transition with the establishment of a mobile arthropod epifauna (Diplichnites ichnoguild) in coastal marine to alluvial plain settings. Additionally, the presence of vertical burrows in Devonian high-energy fluvial deposits reflects the establishment of a stationary, deep suspension-feeding infauna of the Skolithos ichnoguild. The earliest evidence of plant-arthropod interaction occurred close to the Silurian-Devonian boundary, but widespread and varied feeding patterns are known from the Carboniferous. During the Carboniferous, permanent subaqueous lacustrine settings were colonized by a diverse, mobile detritus-feeding epifauna of the Mermia ichnoguild, which reflects a significant paleoenvironmental expansion of trace fossils. Paleozoic ichnologic evidence supports direct routes to the land from marginal marine environments, and migration to lakes from land settings. All nonmarine sedimentary environments were colonized by the Carboniferous, and subsequent patterns indicate an increase in ecospace utilization within already colonized depositional settings. During the Permian, back-filled traces of the Scoyenia ichnoguild record the establishment of a mobile, intermediate-depth, deposit-feeding infauna in alluvial and transitional alluvial-lacustrine sediments. Diversification of land plants and the establishment of ecologically diverse plant communities through time provided new niches to be exploited by arthropods. Nevertheless, most of the evolutionary feeding innovations took place relatively early, during the Late Paleozoic or early Mesozoic. A stationary deep infauna, the Camborygma ichnoguild, was developed in Triassic transitional alluvial-lacustrine deposits. Terrestrial environments hosted the rise of complex social behavioral patterns, as suggested by the probable presence of hymenopteran and isopteran nests in Triassic paleosols. An increase in diversity of trace fossils is detected in Triassic-Jurassic eolian deposits, where the ichnofauna displays more varied behavioral patterns than their Paleozoic counterparts. Also, a mobile, intermediate-depth, deposit-feeding infauna, the Vagorichnus ichnoguild, was established in deep lake environments during the Jurassic. In contrast to Paleozoic permanent subaqueous assemblages typified by surface trails, Jurassic ichnocoenoses are dominated by infaunal burrows. High density of infaunal deposit-feeding traces of the Planolites ichnoguild caused major disruption of lacustrine sedimentary fabrics during the Cretaceous. Most insect mouthpart classes, functional feeding groups, and dietary guilds were established by the end of the Cretaceous. Diversification of modern insects is recorded by the abundance and complexity of structures produced by wasps, bees, dung-beetles, and termites in Cretaceous-Tertiary paleosols. The increase in bioturbation migrated from fluvial and lake-margin settings to permanent subaqueous lacustrine environments through time.

Fabric, Texture and Anisotropy of Magnetic Susceptibility in High-Grade Iron Ores from the Quadrilátero Ferrifero, Minas Gerais, Brazil

The iron protores of the Quadrilatero Ferrifero in Minas Gerais, Brazil, are quartz- or carbonate-banded laminated rocks (itabirite) which still display primary features similar to those encountered in non-metamorphic jaspilites, such as sedimentary layering, slump structures and porous granoblastic martite fabric with relictic kenomagnetite. Synmetamorphic deformation initiated recrystallization of the platy hematite variety (specularite) and by this gave rise to a variety of secondary grain fabrics and textures portrayed by the intensity of magnetic (low-field) susceptibility and by the geometry of corresponding anisotropy ellipsoids. Folded ores usually have a foliation roughly parallel to the compositional banding, an axial plane schistosity, lepidogranoblastic fabric and textures dominantly resulting from flexural slip. The stereogram of the corresponding (11.0) texture shows a single maximum parallel to the intersection line of banding and schistosity. The AMS is generally low; its ellipsoid is triaxial. Increasing strain and shearing obliterates the primary layering and induced the formation of ore mylonites with lepidoblastic (S-tectonites) or nematoblastic (L-tectonites) grain fabrics which results in varying shapes of the AMS ellipsoids. Schistose ores, finally, have girdle arrangements of hematite-(11.0) poles, high susceptibilities and oblate shapes of the AMS ellipsoids, whereas strongly lineated ores display a single maximum parallel to the stretching direction, low to moderate susceptibilities and AMS ellipsoids with prolate shape. Secondary recrystallization and grain growth does not affect the lattice-preferred orientation.

Shallow geotechnical profiles, acoustic character and depositional history in glacially influenced sediments from the Hebrides and West Shetland Slopes

Abstract Surficial sediments from the Hebrides and West Shetland Slopes are mostly matrix-supported diamictons or laminated muds with subordinate sand. Those from the outer shelf and upper slope have water content and undrained shear strength profiles that suggest variable light to moderate overconsolidation, whereas those from the lower slope and basin floor have water content and undrained shear strength profiles that suggest normal consolidation. We believe that these geotechnical differences are genetic and reflect the processes operative in former glacial and glaciomarine palaeoenvironments along the continental margin: variably overconsolidated sediments are the products of marginal or proximal glaciomarine environments in which ice loading has occurred, whereas normally consolidated sediments are found in distal glaciomarine to basin plain settings. The depositional setting also controls the void index of the sediment via its sedimentary fabric and we find a sensible relationship between the compression curve of a sediment and its mode of deposition. It is usual to find that individual values lie scattered about this curve as a result of small variations in the packing. These variations correspond to minor sediment layers from which acoustic reflections may interfere constructively to give a multilayered acoustic texture, whose appearance is thus ultimately related to the magnitude of the scatter.

Magnetic fabric of clay sediments from the external northern Apennines (Italy)

We report on the anisotropy of magnetic susceptibility (AMS) analyses of fine-grained sediments deposited during the Messinian in foredeep basins at the front of the northern Apenninic chain. The data refer to 32 sampling sites, mostly distributed in the fine-grained intervals of the Laga and Colombacci formations, extending along the belt for a total length of about 300 km. Rock magnetism analyses indicate that the magnetic susceptibility and its anisotropy are in most cases dominated by the paramagnetic minerals of the clay matrix. In order to delineate the contribution of the ferrimagnetic fraction to the overall susceptibility fabric, the anisotropy of the anhysteretic remanent magnetisation was investigated at some representative sites. The magnetic fabric of the studied sediments mostly reflects the effects of compaction, showing a predominant magnetic foliation parallel to the bedding plane. At all the sites a well distinct magnetic lineation was also found, which is parallel to the fold axes and thrust fronts, both at local and regional scales. This feature is maintained in sequences that differ for sedimentological character and age, implying that the magnetic lineation was produced by a mild tectonic overprint of the primary sedimentary-compactional fabric. The relationship between the magnetic lineation trends and the vertical axis rotations detected by Speranza et al. [Speranza, F., Sagnotti, L., Mattei, M., 1997. J. Geophys. Res. 102, 3153–3166] indicates that the magnetic lineation formed during the compressive phases of the Messinian-early Pliocene, when the Apenninic front was almost rectilinear and oriented N320°.

Research Methods Paper A Quick and Inexpensive Sample Preparation Method for AMS Microfabric Analysis in Fine-Grained Sediment

ABSTRACT Unless disturbed by postdepositional processes, sedimentary grain fabric reflects the physical conditions at the time of sediment deposition. Anisotropy of magnetic susceptibility, or AMS, is a sophisticated yet simple method for measuring grain fabric. In most cases this technique requires a self-supporting, right-cylindrical sample, and as a result unlithified sediment needs first to be lithified. Generally this is done with expensive, time-consuming resin impregnation procedures. In consolidated, mud-rich, clast-poor sediment, however, a new technique of cyclical steaming and (steam) soaking proved to be highly effective in softening the sediment sample enough so that it could be cored for AMS-sample preparation. Using this new method, samples were prepared in about 90% less time and for only 10% of the cost of standard impregnation techniques.

The ichnologic record of the continental invertebrate invasion; evolutionary trends in environmental expansion, ecospace utilization, and behavioral complexity

The combined study of continental trace fossils and associated sedimentary facies provides valuable evidence of colonization trends and events throughout the Phanerozoic. Colonization of continental environments was linked to the exploitation of empty or under-utilized ecospace. Although the nonmarine trace fossil record probably begins during the Late Ordovician, significant invasion of nonmarine biotopes began close to the Silurian-Devonian transition with the establishment of a mobile arthropod epifauna (Diplichnites ichnoguild) in coastal marine to alluvial plain settings. Additionally, the presence of vertical burrows in Devonian high-energy fluvial deposits reflects the establishment of a stationary, deep suspension-feeding infauna of the Skolithos ichnoguild. The earliest evidence of plant-arthropod interaction occurred close to the Silurian-Devonian boundary, but widespread and varied feeding patterns are known from the Carboniferous. During the Carboniferous, permanent subaqueous lacustrine settings were colonized by a diverse, mobile detritus-feeding epifauna of the Mermia ichnoguild, which reflects a significant paleoenvironmental expansion of trace fossils. Paleozoic ichnologic evidence supports direct routes to the land from marginal marine environments, and migration to lakes from land settings. All nonmarine sedimentary environments were colonized by the Carboniferous, and subsequent patterns indicate an increase in ecospace utilization within already colonized depositional settings. During the Permian, back-filled traces of the Scoyenia ichnoguild record the establishment of a mobile, intermediate-depth, deposit-feeding infauna in alluvial and transitional alluvial-lacustrine sediments. Diversification of land plants and the establishment of ecologically diverse plant communities through time provided new niches to be exploited by arthropods. Nevertheless, most of the evolutionary feeding innovations took place relatively early, during the Late Paleozoic or early Mesozoic. A stationary deep infauna, the Camborygma ichnoguild, was developed in Triassic transitional alluvial-lacustrine deposits. Terrestrial environments hosted the rise of complex social behavioral patterns, as suggested by the probable presence of hymenopteran and isopteran nests in Triassic paleosols. An increase in diversity of trace fossils is detected in Triassic-Jurassic eolian deposits, where the ichnofauna displays more varied behavioral patterns than their Paleozoic counterparts. Also, a mobile, intermediate-depth, deposit-feeding infauna, the Vagorichnus ichnoguild, was established in deep lake environments during the Jurassic. In contrast to Paleozoic permanent subaqueous assemblages typified by surface trails, Jurassic ichnocoenoses are dominated by infaunal burrows. High density of infaunal deposit-feeding traces of the Planolites ichnoguild caused major disruption of lacustrine sedimentary fabrics during the Cretaceous. Most insect mouthpart classes, functional feeding groups, and dietary guilds were established by the end of the Cretaceous. Diversification of modern insects is recorded by the abundance and complexity of structures produced by wasps, bees, dung-beetles, and termites in Cretaceous-Tertiary paleosols. The increase in bioturbation migrated from fluvial and lake-margin settings to permanent subaqueous lacustrine environments through time.

Environmental influences on the formation of sedimentary fabric in a fine-grained carbonate-shelf environment: Dry Tortugas, Florida Keys

Sedimentological studies were undertaken in the Dry Tortugas to examine environmental influences on the formation of sedimentary fabric. 234Th, 210Pb, grain size, porosity, and fabric analyses reveal the presence of a soft, fine-grained, well mixed surface layer underlain by compact, shelly, intensely bioturbated carbonate muds. Vertical zonation of biogenic structures indicates that deep bioturbation is advective and results in transport of fine material to the seabed surface. The impact of physical oceanographic processes appears to be restricted to the surface layer; however, potential for preservation of this layer in the sediment record is low due to intense mixing in deeper sediments. Thus, preserved sediments retain an incomplete record of the dominant benthic processes at the study area.