How the zebra rock got its stripes: Origin of hematite banding in East Kimberley siltstones

Coward, Andrew J1; Slim, Anja C1; Brugger, Joël1; Wilson, Siobhan A2; Pillans, Bradley J3; Williams, Tim4;

1School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC 3800, Australia, 2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2R3, Canada, 3Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia, 4Monash Centre for Electron Microscopy, Monash University, Clayton, VIC 3800, Australia

Zebra rock is an Ediacaran paleosol unique to the East Kimberley region of Western Australia, renowned globally for its highly regular and unusual red and white banding. The underlying mechanisms behind the self-organisation of zebra rock patterns has remained an enduring question ever since the first scientific studies of the ornamental stone in 1926 and continues to fascinate geologists to this day. Many theories have been proposed to explain the formation of these unique patterns, including the infilling of ripple beds, Liesegang banding, and ferronematic liquid crystals, but no definitive consensus has yet been reached, in part due to the lack of supporting chemical and mineralogical analysis.

Twenty-five zebra rock samples were analysed from five different outcrops across the Lake Argyle region using a variety of analytical techniques, including XRD, SEM, TEM, LA-ICP-MS and X-Ray CT imaging. The principle pigment of zebra rock was found to be exclusively hematite in all cases, a result consistent with previous studies. Within the dark banding, hematite manifested as large (1-10μm) aggregates of nanoscale (200-500nm) hexagonal platelets distributed within the interstitial spacing between larger quartz and kaolinite grains. Large (100 μm), polycrystalline hematite was also observed within the white bands, although at significantly reduced abundance (<1 wt%). Hexagonal platelets and needle-like crystals of hematite were observed in the overlying, iron-rich host shales, while the comparatively iron-poor, underlying shales exhibited a random arrangement of large iron-oxide nodules and dissolution voids.

Zebra rock mineralogy was found to be highly variable on a regional scale. Across the five deposits examined, four distinct mineralogical assemblages were identified, each defined by the absence or presence of one or more of alunite, muscovite and dickite as major phases. Each of these four assemblages contained key indicator minerals suggestive of argillic and advanced argillic hydrothermal alteration, most prominently dickite, kaolinite, alunite and, in low amounts, svanbergite and pyrophyllite. All four mineral assemblages can each be correlated with a different degree of argillic and advanced argillic alteration, suggesting a gradual neutralisation of acidic hydrothermal fluids as they progressed laterally along permeable bedding. On a local, outcrop scale, zebra rock mineralogy was found to be largely homogeneous, with no mineral phase other than hematite exhibiting consistent concentration gradients between the dark and light bands. Trace elements followed a similar pattern, with no consistent concentration gradients outside of significant enrichment in the dark banding of V51, Cr52, Mn55, Co59, Ni60 and Mo95, elements all known to strongly adsorb to hematite. Curiously, no HREE enrichment was detected, as might otherwise be expected in hydrothermal systems.

On the basis on the above findings, we propose two alternative hypotheses to explain the development of zebra rock patterns: 1) high temperature, argillic hydrothermal alteration in conjunction with the formation of the 510 Ma Kalkarindji large igneous province or 2) low temperature acid-sulphate weathering in anoxic, waterlogged soils during the Ediacaran. Dating techniques, such as paleomagnetic analysis and stable O isotopes, are required to determine which of these two mechanisms is responsible for the hematite patterns of zebra rock.


Biography

Andrew Jeremy Coward is a doctoral candidate at Monash University. Mr. Coward specializes in studying self-organization mechanisms in geochemical and biogeochemical systems with a specific interest in various banded siltstones and clays in north-western Australia.

Investigating the origin of organic matter in hydrothermal systems of the Archean

Zepeda, Vanessa1

1Queensland University Of Technology, South Brisbane, Australia

The origin of organic matter from the 3.43 billion-year-old Strelley Pool Formation (SPF), Pilbara Craton, Western Australia has been heavily debated. The region has an extensive history of diverse hydrothermalism induced by thrusting granitoid complexes which underlie the protocontinent. In consequence, hundreds of siliceous chert dike systems rich in carbonaceous material cross-cut Archean sedimentary rocks. Hydrothermal systems have the potential to synthesize organic molecules abiotically by a process called Fischer Tropsch Synthesis, particularly in the presence of certain metal catalysts. Conversely, a recently proposed hydrothermal pump hypothesis suggests that organic matter in anoxic waters could have been incorporated and redistributed to significant depths due to the higher geothermal gradients in hydrothermal systems. Differentiating between abiotic and biological sources becomes increasingly difficult because both processes induce fractionation effects on 13C that are significantly depleted. The author aims to address the questions regarding the origin of organic matter in the SPF by exploring the distribution of metal catalysts that could stimulate abiotic synthesis, as well as utilizing fine-scale geological and geochemical techniques to form paleoenvironmental interpretations of the ancient hydrothermal system. Developing new investigative methods and corroborative lines of evidence from ancient systems like the SPF is likely to yield new insights into the origin and early evolution of life on Earth, as well as other bodies in our solar system such as Mars.


Biography

Vanessa was born in Arizona and moved to Hawaii for her bachelors and masters degrees. She completed two internships at the Jet Propulsion Laboratory where she worked with molecular biosignatures preserved in rocks. During this internship she was introduced to Dr. David Flannery and applied to QUT for her PhD.

Ediacaran stratigraphy in the western part of the Northern Territory Amadeus Basin, central Australia

Verdel, Charles1, Donnellan, Nigel1, Normington, Verity1, Simmons, Jack1

1Northern Territory Geological Survey, Alice Springs and Darwin, Australia

Neoproterozoic stratigraphic nomenclature has historically differed between the eastern and central to western parts of the Amadeus Basin, leading to issues in basin-wide correlation and evaluation of potential lateral facies changes. In the more extensively described eastern part of the basin, the major Ediacaran lithostratigraphic units are (in ascending order) the basal Ediacaran cap carbonate of the Olympic Formation, Pertatataka Formation, Julie Formation, and latest Ediacaran lower Arumbera Sandstone. New mapping has revealed that most of these units correspond with parts of the informal and historical “Winnall beds” of the central and western Amadeus Basin. The new, formally defined Winnall Group spans Ediacaran to perhaps Cambrian time and consists of five formations: in ascending order, the Breaden, Gloaming, Froud, Liddle, and Puna Kura Kura formations. In the westernmost part of the Northern Territory Amadeus Basin (i.e., the region stretching from approximately Watarrka (Kings Canyon) National Park to the Western Australia border), the main part of the Winnall Group that is exposed is the Liddle Formation, a succession of cross-bedded sandstone that reaches a thickness of approximately 500 meters. Lithostratigraphic comparison with the eastern Amadeus Basin suggests that the Liddle Formation is partly or entirely correlative with carbonate of the Julie Formation, and extensive detrital zircon U-Pb data from the Liddle Formation are permissive of a mid to late Ediacaran age assignment. The Liddle Formation may be a stratigraphic counterpart of the late Ediacaran Bonney Sandstone in the Adelaide Superbasin of South Australia.


Biography

Charlie Verdel is a project geologist with the Northern Territory Geological Survey in Alice Springs.

High-resolution rapid thermal neutron tomographic imaging of fossiliferous cave breccias from Southeast Asia

Smith, Holly1, Bevitt, Dr Joseph2, Garbe,Dr Ulf 2, Zaim,Professor Yahdi3, Rizal, Dr Yan3, Aswan, Dr3, Puspaningrum, Dr Mika Rizki3, Trihiscaryo, Dr Agus3, Price, Dr Gilbert4, Webb,Dr Gregg4

1Griffith University, Nathan, Australia, 2Australia Nuclear Science and Technology Organisation, Sydney, Australia, 3Institut Teknologi Bandung, Bandung, Indonesia, 4University of Queensland, Brisbane, Australia

Tomographic imaging is gaining importance amongst palaeontologists as a non-invasive approach to studying fossil remains. Traditional methods of preparing a fossil risks damage to the specimen and destroys contextual evidence in the surrounding matrix. CT imaging can reveal the internal composition and structure of buried fossils and consolidated sediment matrices before any destructive mechanical or chemical preparation. Neutron tomography (NT) discerns denser matrices impenetrable to CT imaging; however, this approach remains relatively underutilised in palaeontology. We employ high-resolution rapid thermal neutron tomographic imaging to visualise internal diagnostic features of dense fossiliferous breccia from three Pleistocene cave localities in Sumatra, Indonesia. We demonstrate that these seemingly homogeneous breccia are an excellent source of data to aid in determining taphonomic and depositional histories of complex depositional sites such as tropical caves. The breccia subsamples retain excellent details of site formation history and results suggest the primary agents in the formation of the breccia and concentration of incorporated vertebrate remains are several rapid depositional phases of water and sediment gravity flow. This study highlights the potential for analyses of breccia deposits in palaeontological studies in Southeast Asian caves in the future.

Keywords: neutron tomography; fossil; cave; breccia; taphonomy, Pleistocene


Biography

Holly is a British Palaeontologist studying her PhD at Griffith University, Brisbane, focusing on the taphonomic histories of cave breccia in Southeast Asia.

All models of mangrove phenology are wrong, but some are useful

Younes, Nicolas 1, Joyce, Karen1, Maier, Stefan1,2

1Centre for Tropical Environmental and Sustainability Science and School of Science and Engineering, James Cook University, Townsville/Cairns, Australia, 2Maitec, P.O. Box U19, Charles Darwin University, Darwin, Australia

Satellite-derived phenology is frequently used to illustrate changes in plant phenology and the effects of climate forcing. However, each study uses a different method to detect phenology. Plant phenology refers to the relationship between the life cycle of plants and weather and climate events. Phenology is often studied in the field, but recently studies have transitioned towards using satellite images to monitor phenology at the plot, country, and continental scales. The problem with this approach is that there is an ever-increasing variety of earth observation satellites collecting data with different spatial, spectral, and temporal characteristics. In this paper we ask if studies that detect phenology using different sensors over the same site produce comparable results. We hypothesize that apparent phenology changes with: 1) areal extent; 2) site location; 3) frequency of observation; 4) spatial resolution; 5) temporal coverage; and 6) the number of cloud contaminated observations. We used Landsat and Sentinel 2 imagery over the Darwin Harbor (Northern Territory, Australia) as case study, and found that apparent phenology does change with the sensor, site, and cloud contamination.  Importantly, the apparent phenology is comparable between Landsat and Sentinel 2 sensors, but it is not comparable to phenology derived from MODIS. This is due to differences in the spatial resolution of the sensors. Cloud contamination also significantly changes the apparent phenology of vegetation. In this paper we expose the complexity of modelling phenology with remote sensing and help guide future phenology investigations.


Biography

Nicolas is an Environmental Engineer from Colombia, and has just finished his PhD at James Cook University. Recently, he focused on plant phenology, and is currently looking al Live Fuel Moisture Content for bushfire prediction and prevention.

Paleoflora and environment of the Surat Basin at the time of the JK transition

Cooling, Jennifer1; Esterle, Joan1; McKellar, John2

1University Of Queensland, St Lucia, Australia, 2Department of Natural Resources, Mines and Energy, Brisbane, Australia

The Westbourne Formation to Mooga Sandstone interval of the Surat Basin contains the most complete record of deposition from Queensland from the time of the Jurassic–Cretaceous transition. As relatively few botanical macrofossils have been reported from this interval, and few of those have been well described, palynological studies, such as the one presented here, provide the best window into the region’s flora just before the arrival of the angiosperms. Palynological samples taken from three stratigraphic boreholes that intersected this interval produced a diverse, but relatively stable, microflora of 218 taxa. Fern spores are the most abundant component of assemblages, averaging 51 percent of all samples, in particular those of the Osmundaceae and Matoniaceae. Spores produced by the Dicksoniaceae, Gleicheniaceae, Horsetails, Marattiaceae, Schizaeaceae, Pteridaceae and Polypodiaceae are also represented. Averaging 20 percent of all assemblages the spores of the lycopods were the second most abundant group and were the most taxonomically diverse. Conifer pollen averaged 17 percent of all assemblages with that produced by the Araucariaceae being the most common of these. Pollen from the Cheirolepidaceae, Podocarpaceae, Pinaceae and Taxodiaceae is also recorded in these samples. Pollen produced by the seed ferns averaged five percent of all samples, while spores produced by bryophytes averaged less than two percent and the monocolpate pollen that may have been produced by some combination of cycads, ginkgoes and gnetales less than one percent of all assemblages. While the link between palynofloral and macro- floral abundances is not a direct one, being influenced by sedimentological and taphonomic effects, some broad conclusions about the parent flora can be made. The Surat Basin flora of the mid- Tithonian to early Hauterivian was a diverse one, with a floodplain (from where these samples were taken) flora dominated by members of the Matoniaceae and Osmundaceae along with numerous other species of ferns, lycopods and bryophytes. The somewhat drier upland flora is represented by the Araucariacean conifers with the seed ferns and the Podocarpacean and Cheirolepidacean conifers producing much of the remaining upland palynofloral assemblage.

With their high proportion of fern spores to conifer pollen, the majority of the samples from this project can be recognized as coming from a backswamp or floodplain facies. This paleoenvironmental interpretation supports the view that the Westbourne Formation to Mooga Sandstone sequence was, like much of the Australian Late Jurassic sequence, deposited in a highly fluvial environment of meandering and braided streams, shallow lakes and swampy floodplains. This interpretation is further supported when the contributions of different palynomorph ecogroups (PEGs) to the assemblages are considered and the results of polytopic vector analysis (PVA) of the assemblages.


Biography

Jennifer Cooling is a PhD candidate from the University of Queensland due to complete her project on the Palynology of the Jurassic-Cretaceous of the Surat Basin this year.

Allocyclic controls on shoreface sedimentation from the Late Cretaceous to Miocene, Gippsland Basin, SE Australia

Mahon, Elizabeth1, Wallace, Malcolm1

1University of Melbourne, Melbourne, Australia

Shoreface deposits can act as excellent proxies for basinal conditions, recording relative sea level fluctuations and tectonic episodes as changes in sediment type, and depositional architecture. The Gippsland Basin contains a continuous succession of amalgamated clastic shoreline deposits from the Cretaceous to present day. This basin has experienced a number of major tectonic, eustatic, climatic and oceanographic events during the Cenozoic. From the Late Cretaceous to the Miocene, stacked shoreface deposits display progradational to transgressive geometries. Despite a range of significant allocyclic events, these shoreface deposits are predominantly transgressive, and strong basinal subsidence appears to be one of the major driving forces controlling shoreline behavior.


Biography

Elizabeth Mahon is a PhD candidate at the University of Melbourne, researching the structural and stratigraphic history of the Gippsland Basin. Prior to beginning her PhD she worked for Chevron in the Australian Business Unit.

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