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: The Conception and St. John’s Groups of southeastern Newfoundland contain some of the oldest known fossils of the Ediacaran macrobiota. The Mistaken Point Ecological Reserve UNESCO World Heritage Site is an internationally recognized locality for such fossils and hosts early evidence for both total group metazoan body fossils and metazoan-style locomotion. The Mistaken Point Ecological Reserve sedimentary succession includes ∼1500 m of fossil-bearing strata containing numerous dateable volcanogenic horizons, and therefore offers a crucial window into the rise and diversification of early animals. Here we present six stratigraphically coherent radioisotopic ages derived from zircons from volcanic tuffites of the Conception and St. John’s Groups at Mistaken Point Ecological Reserve. The oldest architecturally complex macrofossils, from the upper Drook Formation, have an age of 574.17 ± 0.66 Ma (including tracer calibration and decay constant uncertainties). The youngest rangeomorph fossils from Mistaken Point Ecological Reserve, in the Fermeuse Formation, have a maximum age of 564.13 ± 0.65 Ma. Fossils of the famous “E” Surface are confirmed to be 565.00 ± 0.64 Ma, while exceptionally preserved specimens on the “Brasier” Surface in the Briscal Formation are dated at 567.63 ± 0.66 Ma. We use our new ages to construct an age-depth model for the sedimentary succession, constrain sedimentary accumulation rates, and convert stratigraphic fossil ranges into the time domain to facilitate integration with time-calibrated data from other successions. Combining this age model with compiled stratigraphic ranges for all named macrofossils within the Mistaken Point Ecological Reserve succession, spanning 76 discrete fossil-bearing horizons, enables recognition and interrogation of potential evolutionary signals. Peak taxonomic diversity is recognized within the Mistaken Point and Trepassey Formations, and uniterminal rangeomorphs with undisplayed branching architecture appear several million years before multiterminal, displayed forms. Together, our combined stratigraphic, paleontological, and geochronological approach offers a holistic, time-calibrated record of evolution during the mid−late Ediacaran Period and a framework within which to consider other geochemical, environmental, and evolutionary data sets.

Description: Beothukis mistakensis from the Ediacaran System of Newfoundland, Canada demonstrates complex fractal-like morphology through the development of primary-, secondary- and tertiary-order Rangea-like units. The primary-order rangeomorph units observed in B. mistakensis are tightly juxtaposed, show no evidence of being independent of one another and are made up of chamber-like secondary-order – probably mesoglea-filled – units. The growth of these rangeomorph units demonstrates that the frond developed from the tip towards the basal region through ontogeny. The tertiary-order units of Beothukis are considered to represent surface morphology on the secondary-order units. This is in contrast to palaeobiological reconstructions of Beothukis that invoke three-dimensional fractal-like branches with independent units, which has been used to infer an osmotrophic mode of life. It is considered here that the fractal-like morphology of the lower surface of B. mistakensis was an adaptation to increase surface area to volume ratio. The quilted morphology of Beothukis proposed here is consistent with a sessile, reclining, phagocytotic and/or chemosymbiotic mode of life similar to that invoked for the reclining rangeomorph Fractofusus.