Upper Devonian microvertebrates from the Canning Basin, Western Australia

A diverse microvertebrate fauna is described from the Virgin Hills and Napier formations, Bugle Gap Limestone Canning Basin, Western Australia. Measured sections at horse Spring and Casey Falls (Virgin Hills Formation) and South Oscar Range (Napier Formation) comprise proximal to distal slope carbonates ranging in age from the Late Devonian Frasnian to middle Famennian. A total of 18 chondrichthyan taxa are identified based on teeth, including the first record of Thrinacodus tranquillus, Cladoides wildungensis, Protacrodus serra and Lissodus lusavorichi from the Canning Basin. A new species, Diademodus dominicus sp. nov. is also described and provided the first record of this genus outside of Laurussia. In addition, the upper range of Australolepis seddoni has been extended to Late Devonian conodont Zone 11, making it the youngest known occurrence for this species. The Virgin Hills and Napier formations microvertebrate faunas show close affinities to faunas recovered from other areas of Gondwana, including eastern Australia, Iran, Morocco and South China, which is consistent with known conodont and trilobite faunas of the same age

Changes of palaeoenvironmental conditions recorded in Late Devonian reef systems from the Canning Basin, Western Australia: A biomarker and stable isotope approach

Although the Late Devonian extinctions were amongst the largest mass extinction events in the Phanerozoic, the causes, nature and timing of these events remain poorly restrained. In addition to the most pronounced biodiversity loss at the Frasnian–Famennian (F–F) boundary and the end Famennian, there were also less extensively studied extinction pulses in the Middle to Late Givetian and the Frasnian. Here we used a combination of palynological, elemental, molecular and stable isotope analyses to investigate a sedimentary record of reef-systems from this time period in the Canning Basin, Western Australia. The acquired data generally showed distinct variations between sediments from (i) the time around the Givetian–Frasnian (G–F) boundary and (ii) later in the Frasnian and indicated a distinct interval of biotic stress, particularly for reef-builders, in the older sediments. Alterations of pristane/phytane ratios, gammacerane indices, Chlorobi biomarkers, δDkerogen and chroman ratios describe the change from a restricted marine palaeoenvironment with an anoxic/euxinic hypolimnion towards a presumably open marine setting with a vertically mixed oxic to suboxic water column. Simultaneous excursions in δ13C profiles of carbonates, organic matter (OM) and hydrocarbons in the older sediments reflect the stratification-induced enhancement of OM-recycling by sulfate reducing bacteria. Alterations in sterane distributions and elevated abundances of methyltrimethyltridecylchromans (MTTCs) and perylene indicate an increased terrigenous nutrient input via riverine influx, which would have promoted stratification, phytoplankton blooms and the development of lower water column anoxia.The detected palaeoenvironmental conditions around the G–F boundary may reflect a local or global extinction event. Our data furthermore suggest a contribution of the higher plant-expansion and photic zone euxinia to the Late Devonian extinctions, consistent with previous hypotheses. Furthermore, this work might contribute to the understanding of variations in Devonian reef margin and platform-top architecture, relevant for petroleum exploration and development in the global Devonian hydrocarbon resources

High-relief microbial boundstone platforms

Carbonate platforms with high-relief, steep depositional slopes and margins dominated by microbial boundstone do not occur in present-day tropical marine sedimentary environments. These types of platforms (defined as M-type carbonate factory by Schlager, 2003) are, however, well-represented in the Phanerozoic geologic record. They accumulated following mass extinction events of reef-building marine organisms with a rigid carbonate skeleton, in particular during the late Paleozoic (post Frasnian) and Mesozoic. This study aims to outline some common characteristics of high-relief microbial platforms with respect to platform geometry, facies belts and lithofacies types based on the evaluation of several case studies from outcrops and published literature of Late Devonian, Carboniferous, Permian, Middle Triassic and Early Jurassic age. The term microbial boundstone is here meant to include those reef lithofacies consisting of variable proportions of: 1) microbial micrite and microspar, 2) early marine cement, and 3) skeletal biota. 1) Laminated and clotted peloidal micrite and microsparitic fabrics are precipitated in situ by microbially mediated processes such as biologically induced and influenced carbonate precipitation in association with cyanobacteria, heterotrophic bacteria and their EPS-rich (extracellular polymeric substances) biofilms. These fabrics are gravity defying and build a rigid framework that isolates primary cavities up to centimeters in size. 2) Early marine cement fills primary boundstone voids and generally consists of botryoidal aragonite and/or radial and radiaxial fibrous cement. Marine cement can constitute more than 50% of the boundstone lithofacies. 3) Skeletal biota can range from scarce to abundant and vary in composition according to the geological period; they do not act as rigid framework builders although they might contribute to it. Common biota include calcareous and siliceous sponges, calcareous green and red algae, bryozoans, brachiopods, mollusks, foraminifers, echinoderms and corals. Microbially mediated precipitation can take place through various organomineralic processes, which are mostly light independent and associated with degradation of organic matter from biofilms and sponge tissues in dysoxic conditions. Thus, the microbial boundstone carbonate producing factory is not limited by light penetration within the water column and can extend well beyond the depth of the photic zone. This constitutes a fundamental difference with respect to the coralgal reef-building facies of the modern tropical carbonate realms. Observations from multiple examples show that the microbial platforms are characterized by high-relief (several hundred meters) geometry with steep slope (> 30\u2070), lack of a raised rim at the margin, a flat platform interior and a gradual transition from the platform-top interior into the slope through a set of outermost platform beds, slightly dipping basinward. The slope clinoforms vary from planar to exponential and the cement-rich microbial boundstone can accumulate in situ on the slope contributing to the stability of the steep clinoforms. In terms of lithofacies belts, the well-bedded meter-scale cyclic platform interior is basinward transitional to subtidal facies. In such outer platform subtidal facies, cyclicity is less pronounced and microbial boundstone lenses can often alternate with coated grain and skeletal grainstone and packstone. The outer platform facies belt can be a few hundred meters or only tens of meters wide. At the platform break and along the upper slope, microbial boundstone lithofacies dominate and can accumulate in situ on the slope to depths of 300-400 m. At these depths, corresponding to the upper to lower slope transition, microbial boundstone tongues interfinger with detrital breccias consisting mostly of slope-derived resedimented microbial boundstone clasts. At the toe of slope, horizontally bedded basinal deposits interfinger with platform-derived resedimented rudstone, grainstone and packstone, including slope-derived boundstone clasts and platform- and slope-derived coated and skeletal grains. Platform-derived resedimented grains generally by-pass the steep upper slope through grain and turbidity flows and accumulate at the toe of slope. The analyzed microbial carbonate platforms seem to export less platform-derived sediment relative to the amount of microbial boundstone precipitated on the slope. Some of these platform examples, but not all, are characterized by matrix-free clast-supported lower slope breccias cemented by radiaxial fibrous calcite transported downslope via avalanches and non cohesive debris flows. At the toe of slope, clast-supported breccias may lack cementation and be compacted with sutured grain contacts. Detrital carbonate mud along the slope and at the toe of slope is scarce explaining the clast-supported matrix-free character of the breccia deposits. This might be attributed to the fact that most of the micrite was precipitated in situ to form the boundstone rather than being detrital carbonate mud resedimented downslope. A key element of microbial platforms is that their steep slopes consist of a large percentage (>50%) of sediment produced directly on the slope down to subphotic depths rather than being the site of prevalent accumulation of platform-derived detrital material. This characteristic has significant implications for the style of growth of the depositional system (progradation vs. aggradation), the evolution of its geometry and the responses of the carbonate factory to changes in accommodation space and eustatic sea level (cf. Kenter et al. 2005). In terms of rates of carbonate production, accumulation and progradation, microbial platforms are estimated to be equivalent to, if not more productive than, present-day fast-growing carbonate systems. The development of microbial platforms seems favored in, but not exclusive of, confined basins with starved basinal sediment that turns anoxic to dysoxic due to limited bottom current circulation rather than by open ocean upwelling settings. Confined basins might have provided the ideal chemical, physical and biological conditions to sustain microbial boundstone and early marine cement precipitation for a time scale of million years; such conditions might include high supersaturation of seawater with respect to carbonate minerals, nutrient levels, weak competition by skeletal metazoans and availability of stable substrates for microbial life. In terms of reservoir potential microbial boundstone facies at platform margins and along slopes of high-relief platforms are characterized by large size primary porosity and they might be associated with adjacent anoxic organic-rich basinal deposits (potential source rocks). Microbial boundstone depositional pore network might, however, be poorly connected or occluded by early marine cementation unless secondary diagenetic dissolution and fractures enhance the connectivity of the pore system. Reference Kenter, J. A. M., Harris P. M., and Della Porta, G., 2005, Steep microbial boundstone-dominated platform margins \u2013 examples and implications: Sedimentary Geology, v. 178, p. 5-30. Schlager, W., 2003, Benthic carbonate factories on the Phanerozoic: International Journal of Earth Sciences, v. 92, p. 445-464

Late Devonian carbonate magnetostratigraphy from the Oscar and Horse Spring Ranges, Lennard Shelf, Canning Basin, Western Australia

The Late Devonian was a time of major evolutionary change encompassing the fifth largest mass extinction, the Frasnian–Famennian event. In order to establish a chronological framework for global correlation before, during, and following the Frasnian–Famennian mass extinction, we carried out a coupled magnetostratigraphic and biostratigraphic study of two stratigraphic sections in the Upper Devonian carbonate reef complexes of the Lennard Shelf, in the Canning Basin, Western Australia. Magnetostratigraphy from these rocks provides the first high-resolution definition of the Late Devonian magnetic polarity timescale. A 581-m-reference section and an 82-m overlapping section through the marginal slope facies (Napier Formation) of the Oscar Range as well as a 117-m section at Horse Spring (Virgin Hills Formation) were sampled at decimeter to meter scale for magnetostratigraphy. Conodont biostratigraphy was used to correlate both sections, and link magnetostratigraphic polarity zones to a globally established biostratigraphy. A stable, Characteristic Remanent Magnetization (ChRM) with dual polarities (NE, shallowly upward and SW, shallowly downward) is recovered from ∼60%∼60% of all samples, with magnetite inferred to be the chief magnetic carrier from thermal demagnetization characteristics. These directions define a geomagnetic pole at 49.5°S/285.8°E and α95=2.4α95=2.4 (n=501n=501), placing the Canning Basin at 9.9°S during the Late Devonian, consistent with carbonate reef development at this time.A conservative interpretation of the magnetostratigraphy shows the recovery of multiple reversals from both sections, not including possible cryptochrons and short duration magnetozones. Field tests for primary remanence include positive reversal tests and matching magnetozones from an overlapping section in the Oscar Range. A strong correlation was found between magnetic polarity stratigraphies of the Oscar Range and Horse Spring sections, and we correlate 12 magnetostratigraphic packages. The relative stratigraphic thicknesses of the isochronous sediments from these two sections indicate that carbonate accumulation was ∼4.5× faster in the middle slope deposits at Oscar Range than in the more distal, lower slope Horse Spring deposits for the middle Frasnian through Famennian. The magnetic field during the Late Devonian underwent a relatively high reversal frequency with good potential for regional and global correlation, and should prove useful in deciphering a high-resolution chronostratigraphy across the Lennard Shelf to enable higher confidence examination of reef development across a major biotic crisis

Development of a Regional Stratigraphic Framework For Upper Devonian Reef Complexes Using Integrated Chronostratigraphy: Lennard Shelf, Canning Basin, Western Australia

Questions regarding heterogeneity and architecture of reefal carbonate platform systems may be resolved by well-constrained chronostratigraphic frameworks, developed from the integration of multiple independent signals in the rock record. This makes possible a meaningful comparison of coeval stratigraphy and facies in different settings. For the Canning Basin Chronostratigraphy Project (CBCP), key outcrop transects and shallow cores were logged for lithofacies and sampled at sub-meter scale for magnetostratigraphy, stable isotope chemostratigraphy, conodont-fish biostratigraphy, biomarker geochemistry, and elemental chemostratigraphy. The dataset entails nearly 4 km of measured stratigraphy and 6800 samples of Middle and Upper Devonian (Givetian, Frasnian and Famennian) carbonate platform-top, reef, foreslope, and basinal deposits along the Lennard Shelf, Canning Basin, Western Australia. The extracted rock signals were integrated in conjunction with sequence stratigraphic concepts to generate a multifacetted, regional chronostratigraphy and predictive lithofacies model across 250 km of the exposed Devonian reef complexes.Final results from the ongoing project will include: high-resolution, high-confidence correlations across different carbonate settings that were not achievable before using traditional biostratigraphy or sequence stratigraphic concepts; unprecedented examination of Lennard Shelf carbonate heterogeneity and architecture within the constrained framework; an integrated workflow for establishing robust chronostratigraphic frameworks that can be tailored for the subsurface; a magnetostratigraphic framework for parts of the Late Devonian (a period of uncertainty in the polarity reversal record), calibrated to biostratigraphic zones; and geochemical and elemental profiles that reflect sea level fluctuations, water column stratification/circulation, and biotic crises during deposition of the reef complexes. The integrated framework and predictive depositional model for the Devonian reef complexes will be a comprehensive product that serves as an analogue for carbonate researchers and petroleum geoscientists worldwide