Duncan McIlroy

The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis

It has been increasingly realized by sedimentologists in the petroleum industry and academia that integration of ichnological information into sedimentological models, and vice versa, is one of the main means by which we can improve our understanding of ancient depositional environments. This volume aims to provide an analytical review of the ichnology of all major depositional environments and the use of ichnology in biostratigraphic and sequence stratigraphic analysis, as well as highly refined palaeoenvironmental studies. The remit of the book is achieved through a combination of review articles and novel research papers that outline methodologies and protocols for improving our understanding of ancient palaeoenvironments. Trace fossils from microscopic borings to dinosaur footprints are considered. The comprehensive coverage of ancient depositional environments means that the book will be of use as a course text for undergraduates as well as an invaluable reference text for all students of ichnology whether they are ichnologists, sedimentologists or petroleum geologists by trade.

First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point formation, Newfoundland

Evidence for locomotion in the Precambrian fossil record is scant. Reliable Ediacaran trace fossils are all younger than 560 Ma, and consist of relatively simple horizontal burrows and trails from shallow-water deposits. Here we describe an assemblage of macroscopic locomotory traces from deep-water environments at Mistaken Point, southeastern Newfoundland, Canada, dated to ca. 565 Ma. These trails extend the record of complex trace fossils back into the earliest Avalonian biota. Our new evidence for large motile organisms on the seafloor at this time suggests that at least some of these early Ediacaran organisms, whose biological affinities are widely debated, could have been muscular and of metazoan grade.

Some ichnological concepts, methodologies, applications and frontiers

Abstract 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.

Palaeobiology and evolution of the earliest agglutinated Foraminifera: Platysolenites, Spirosolenites and related forms

Tubular agglutinated fossils of Platysolenites antiquissimus Eichwald, 1860, P. cooperi n.sp. and Spirosolenites spiralis Glaessner, 1979 are examined from selected lower Cambrian successions in Avalonia (England; Wales; Newfoundland) and Baltica (Russia and Estonia). Platysolenites cooperi n.sp. is shown to extend below the first appearance of P. antiquissimus in the Neoproterozoic-Cambrian boundary stratotype region, SE Newfoundland, and to occur at higher stratigraphic levels in Wales and Finnmark. Taphonomic, teleological and ultrastructural studies on well-preserved material are consistent with a similar grade of organization and mode of life for P. antiquissimus and the living astrorhizacean foraminiferid Bathysiphon. However, agglutinated proloculi are here described from both rectilinear P. antiquissimus and coiled S. spiralis, which suggests that neither were typical astrorhizaceans.

Ichnofabrics and sedimentary facies of a tide-dominated delta: Jurassic Ile Formation of Kristin Field, Haltenbanken, Offshore Mid-Norway

Abstract 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.

Lateral variability in shallow marine ichnofabrics: implications for the ichnofabric analysis method

Ichnological analysis, commonly undertaken both in the field and in core studies, is generally carried out in vertical logged successions. This work documents the results of a detailed ichnofabric study of two highly bioturbated sandstone beds separated by a mudstone, within the thick, Lower Jurassic succession of the Yorkshire coast, UK. The two sandstone beds contain relicts of hummocky cross-stratification and rare bioclasts of bivalves and belemnites. Bioturbation of the beds is intense and the diversity of trace fossils is considerable. Trace fossils present include Teichichnus rectus, Teichichnus isp. A, Thalassinoides isp., Palaeophycus heberti, Diplocraterion parallelum, Rhizocorallium, Rosselia socialis, Schaubcylindrichnus isp., Chondrites isp., Planolites isp. and Phycosiphon incertum. Ichnofabrics were studied at 5 m intervals parallel to the modern cliff-line, which is parallel to the inferred Jurassic palaeo-coastline. The results demonstrate that there is considerable patchiness to ichnofabric even in a comparatively simple depositional setting such as a storm-influenced shelf. This observation is in line with studies of the modern sea floor. The implications of patchiness in ichnofabric are broad-ranging, but most importantly suggest that caution is needed in the use of ichnofabric analysis for intra-regional correlations.

Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma)

Muscle tissue is a fundamentally eumetazoan attribute. The oldest evidence for fossilized muscular tissue before the Early Cambrian has hitherto remained moot, being reliant upon indirect evidence in the form of Late Ediacaran ichnofossils. We here report a candidate muscle-bearing organism, Haootia quadriformis n. gen., n. sp., from approximately 560 Ma strata in Newfoundland, Canada. This taxon exhibits sediment moulds of twisted, superimposed fibrous bundles arranged quadrilaterally, extending into four prominent bifurcating corner branches. Haootia is distinct from all previously published contemporaneous Ediacaran macrofossils in its symmetrically fibrous, rather than frondose, architecture. Its bundled fibres, morphology, and taphonomy compare well with the muscle fibres of fossil and extant Cnidaria, particularly the benthic Staurozoa. Haootia quadriformis thus potentially provides the earliest body fossil evidence for both metazoan musculature, and for Eumetazoa, in the geological record.

Fossils and matgrounds from the Neoproterozoic Longmyndian Supergroup, Shropshire, UK

Body and trace fossils of Ediacaran affinity are described from strata of the late Neoproterozoic Longmyndian Supergroup exposed near Church Stretton, Shropshire, UK. The almost spherical soft-bodied Ediacaran fossil Beltanelliformis brunsae Menner occurs rarely in the Burway Formation, but much smaller, simpler, discoidal structures are common in both the Burway and Synalds formations and are referred to Beltanelliformis minutae sp. nov. Similar discoidal structures, but with a distinct central depression, are included in Intrites punctatus Fedonkin and are common at several horizons. Two blocks with numerous examples of Medusinites aff. asteroides (Sprigg) Glaessner & Wade were recovered from the Burway Formation. The purported Ediacaran body fossil ‘Arumberia’ Glaessner & Walter is common at several horizons but its biogenicity is not accepted herein. ‘Arumberia’ is thus treated along with evidence for microbially bound sediment surfaces or matgrounds that have been suggested by several authors to be necessary for some types of Ediacaran preservation. The assemblage of simple trace and body fossils along with matgrounds is typical of latest Neoproterozoic time, though some elements range into the Phanerozoic.

Re-evaluation of the petroleum potential of the Kufra Basin (SE Libya, NE Chad) : does the source rock barrier fall ?

Abstract The Kufra Basin is a large, underexplored, Palaeozoic intracratonic sag basin in SE Libya and NE Chad with extensions into NW Sudan and SW Egypt. The basin fill consists of shallow marine to fluvial deposits ranging in age from infracambrian to Cretaceous. Geologically, the basin is very similar to the Murzuq Basin in SW Libya which recently presented Libya with its largest oil discovery for over a decade. Most of the hydrocarbon play elements known from the Murzuq Basin also occur in the Kufra Basin: thick, porous Cambro-Ordovician sandstones are present and would form good reservoirs, lower Silurian shales may act as effective seals, and there are potential structural traps in seismically defined fault blocks. However, the source rock availability in the Kufra Basin is currently unclear. One of the two main source rock candidates in the basin is a lower Silurian shale unit (Tanezzuft Formation). The Tanezzuft shales have been described as being up to 130 m thick in outcrops at the basin margins, but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by AGIP between 1978 and 1981. Hot shales developed at the base of this widespread Silurian shale unit form important source rocks in many areas of North Africa and Arabia. These hot shales are interpreted to have been deposited in palaeodepressions, such as incised valleys of the preceding lowstand, or intrashelf basins, during the initial transgression after the melting of the late Ordovician ice cap. The areal distribution of the organic-rich unit is, therefore, discontinuous. Fieldwork in the Kufra Basin has shown that the basal Tanezzuft horizon is not exposed on the northern and eastern margins of the basin. Deep infracambrian rift grabens have been interpreted on seismic lines from the Kufra Basin and, in analogy to Oman and Algeria, could contain organic-rich infracambrian deposits. The infracambrian succession in the Kufra Basin may contain a second major potential source rock and warrants further investigation.

A biota of Ediacaran aspect from lower Cambrian strata on the Digermul Peninsula, Arctic Norway

Three elements of the ‘Ediacara fauna’ are described from lower Cambrian strata on the Digermul Peninsula, Norway. Nimbia occlusa Fedonkin, 1980 and Tirasiana sp. occur approximately 80 m above the base of the Lower Breivik Member, which approximately coincides with the Neoproterozoic–Cambrian boundary. A specimen of Cyclomedusa sp. has also been found in the Lower Duolbasgaissa Member about 600 m above the boundary, in rocks of trilobite-bearing age. These discoveries add to a growing body of evidence that some elements of the dominantly Neoproterozoic Ediacara fauna continue into the Phanerozoic, thereby diminishing the scope of a possible late Neoproterozoic mass-extinction event. The taxa described here, particularly Nimbia and Cyclomedusa , also occur at many other localities within Neoproterozoic strata and, in common with other elements of the Ediacara fauna, display remarkable morphological variation. Some of this diversity in form is probably caused by environmental and preservational factors. The possibility that it may, at least in part, reflect an inability of these early life forms to replicate faithfully their genes during reproduction should, however, not be overlooked.