ALBERT

Description: Silicified coniferous wood is commonly found in the Middle Jurassic (Bajocian) Lajas Formation of the Neuquen Basin, west-central Argentina. The wood is preserved in a succession of sandstones, siltstones, mudstones and minor conglomerates that represent deposition as part of tide-dominated deltas and fluvial plains across which large rivers meandered. Most of the wood occurs as dense accumulations in the tidal and fluvial channels. The wood fragments are worn, abraded, and lack both bark and branches, indicating that they were transported prior to deposition. The material is typically 20-30 cm long, with only infrequent examples of larger trunks (c. 80 cm in diameter, 5-6 m long). No trunks were found with root systems attached, and no stumps were found upright and in situ. The fossil wood genus Araucarioxylon dominates the assemblage. Growth rings are largely absent from the specimens, although one sample (from Rhea Gorge) displays highly diffuse and irregularly spaced rings, suggesting that it grew in different conditions from the others studied. Large-scale interpretations for southern Gondwana suggest a seasonally dry climate. However, these fossil wood specimens show no evidence of this, indicating that in this area at least the effects of any seasonal component to the climate may have been over-ridden by factors such as a locally plentiful supply of water and/or the possibility that growth was to some extent controlled by the taxonomic affinity of the trees.

Description: Ichnology is the study of trace fossils, which preserve the activity of animals as recorded by their tracks, trails, burrows and borings. Rather than giving information about the taxonomic affinities of a given type of organism, trace fossils yield information about an animal's behaviour in response to its environment. Trace fossils are almost always in situ, are commonly specific to a particular suite of environmental conditions, can be readily studied in core and may be common in strata devoid of body fossils. They are invaluable in thorough sedimentological analysis and are thus of great utility to petroleum geologists, sedimentologists and palaeontologists alike. Over the last 30 years or so, ichnology has been a rapidly developing branch of palaeontology that not only has important applications in classical palaeobiology (e.g. Donovan 1994; Bromley 1996), but is also of great value in the more applied disciplines of palaeoenvironmental and stratigraphical analysis. Much progress has been made in the development of this discipline, but there remain many fascinating and challenging issues, particularly in combining ichnology and sedimentology. This book aims to provide a summary of recent progress, with an up-to-date summary of most themes in modern ichnology. The volume stems from the 2003 Lyell Meeting sponsored by The Geological Society, The Palaeontological Association, BP, Shell, Exxon Mobil, Statoil, Total and Amerada Hess. The introductory paper by McIlroy (a) provides a condensed summary of some ichnological themes and frontiers, and outlines a practical approach for the description of trace fossils and identification of key stratigraphic ... This 250-word extract was created in the absence of an abstract.

Description: Ichnology straddles the boundary between palaeontology and sedimentology, and is becoming an increasingly important tool in both fields. For the palaeontologist, trace fossils allow insight into behaviour and biomechanics of animals that would otherwise be the subject of conjecture. For the sedimentologist, trace fossils have a marked impact on the interpretation of sedimentary rocks in that they destroy primary sedimentary structures, but can also reveal subtle palaeoenvironmental information beyond the resolution attainable by analysis of primary physical sedimentary structures. This contribution aims to review the major developments in the field of ichnology, and to highlight some of the tools and approaches currently used by ichnologists. A personal ethos for the study of trace fossils in core is outlined as a model ichnological protocol, and some of the frontiers of the science as a whole are briefly discussed.

Description: Tidal depositional systems are often interpreted as lowstand/transgressive estuarine deposits within sequences that are either wave or river dominated during highstand times. The Middle Jurassic Lajas Formation of the Neuquen Basin, Argentina, comprises 600 m of well-exposed tide-dominated facies deposited within four unconformitybounded sequences, spanning approximately 4.5 Ma. Facies associations include tidedominated deltas, sandy-heterolithic tidal channel fills and extensive progradational tidal-flat successions, which are locally cut by heterolithic tidal channel fills. Despite the narrow bathymetric depositional range and the complex facies variability, flooding surfaces can be defined and mapped along a 48 km-long outcrop belt. These flooding surfaces allow definition of three distinct types of parasequence that exhibit coarsening-upwards, finingupwards and coarsening- to fining-upwards motifs. Sequence boundaries are marked by widespread, but shallow, incision, and the juxtaposition of stacked fluvial/tidal channel fills on a variety of subtidal and intertidal facies. Unconventional grain-size changes at sequence boundaries can occur where basinward facies shifts are marked by juxtaposition of heterolithic-argillaceous intertidal/supratidal mudflat deposits on subtidal sandflat facies. The maintenance of macrotidal conditions through complete base-level cycles is interpreted as being due to the structural topography inherited from rifting, causing the whole sub-basin to behave as a structurally controlled embayment.

Description: Tide-dominated deltas are poorly known from the stratigraphic record and are notoriously complex, owing to the wide spectrum of facies encountered and their spatial/temporal variability. The tide-dominated deltaic palaeoenvironment combines the ecological harshness of brackish-water settings with complex tidal channel/tidal-flat type facies architecture on the delta top, in association with more classic deltaic facies-stacking patterns. The Ile Formation is interpreted herein as a tide-dominated delta deposited in a microtidal setting. Its palaeoenvironments are interpreted based on a combination of ichnology, ichnofabric analysis and sedimentology. Ichnofabric stacking patterns are used to elucidate the internal architecture of the notoriously problematic aggradational multi-storey tidal channel units. The tide-dominated deltas of the Ile Formation have a distinctive ichnological signature that may be used to characterize tide-dominated deltas. In comparison to typical river-dominated deltas the Skolithos ichnofacies is less well developed and ichnodiversity is lower than expected in wave-dominated deltas. The ichnofabric model presented has potential to be used, with modification, in other tide-dominated deltaic settings.

Description: A new assemblage of frondose and filamentous Ediacaran macrofossils is reported from the upper Drook Formation of Pigeon Cove, Newfoundland. The frondose forms, all less than 3 cm in length, are considered to represent the juvenile growth stages of Ediacaran organisms including Charnia spp. and Trepassia spp. This is the first report of an assemblage wholly dominated by such small juvenile rangeomorph forms, and provides insights into the ontogeny and ecology of these earliest members of the Ediacara biota. The fronds occur alongside filamentous forms with similarities to microbial taxa, and both morphotypes are considered to postdate an assemblage of large ivesheadiomorphs on the same bedding plane. If so, the assemblage represents one of the oldest documented examples of secondary community succession. The new Pigeon Cove fossils also extend the stratigraphic ranges of several key frondose taxa (Charnia masoni, Charniodiscus spp.) back into some of the oldest known macrofossil-bearing strata. These revised ranges lend support to the suggestion that the previously observed low diversity within the Drook Formation may represent a combination of taphonomic and sampling artefacts. Furthermore, this assemblage implies that the diversification of architectural morphotypes within the Ediacara biota took place earlier than hitherto suspected.Supplementary material: A document containing figures of additional juvenile rangeomorphs and filamentous specimens, a table of specimen dimensions, and a complete digitized map of the Pigeon Cove bedding plane, is available at www.geolsoc.org.uk/SUP18529.

Description: Ichnology is commonly used in the palaeoenvironmental analysis and hydrocarbon reservoir characterization of bioturbated shallow marine facies, but is more rarely applied to deep marine and turbiditic successions. This work identifies the key trace fossils that are likely to be of use in ichnological analysis of drill cores, and in the application of ichnological data to the investigation of deep marine turbidite plays. The available data on trace fossil occurrences in turbidites have been analyzed for the whole of the Phanerozoic to identify the principal components of turbidite ichnofabrics at different times. It is found that a limited number of ichnofabric-forming ichnotaxa occur frequently in turbidite-bearing formations. In order of decreasing frequency of occurrence, the key ichnotaxa are Chondrites, Planolites, Scolicia, Zoophycos, Ophiomorpha, Phycosiphon, Thalassinoides and Nereites. Surprisingly, despite well-documented changes in deep marine benthic ecology through the Phanerozoic, ichnotaxonomic components of ichnofabrics show little behavioural change or evolution of new bioturbation styles. A number of ichnotaxa do show changes in their relative abundance through the Phanerozoic, possibly indicating ecological competition or niche replacement (e.g. Early Palaeozoic Dictyodora replaced by Mesozoic and Cenozoic Zoophycos). Ichnofabric-forming trace fossils need to be fully understood before they can be confidently applied to reservoir characterization of turbidite facies, but this study identifies the most important ichnotaxa in such settings and provides a framework for future research.