Quantifying the Dolomite problem and its impacts on Mg/Ca change through time

Opdyke, Dr Bradley1

1The Australian National University, Australia

Global dolomite deposition has declined globally throughout the Cenozoic.  While the volumes of other sedimentary rocks increase from the Paleocene to the Holocene.  This anomaly has been called the ‘dolomite problem’ and recognized since the time of Darwin.  Recently my team discovered that crustose coralline algae  (CCA) does precipitate calcium-magnesium carbonate with a dolomite chemistry.  CCA is not as abundant on immature reefs as mature reefs, in fact CCA ‘crusts’ only become thick and widespread once a significant portion of the reef flat has been located within the tidal zone for many thousands of years.  In a world where sea level is moving up and down like a Milankovitch driven yo-yo it is rare for these algal facies to become established, hence dolomite precipitation is rare. Recent sea level studies and compilations of stable isotope records from the Eocene to the Holocene allow us to model the probable trajectory of dolomite deposition over this time interval and calculate the probable Magnesium-Calcium ratio change in sea water.  At the present time we only have a few ‘tie points’ for the Mg/Ca over this time.  Using the stability of sea level as a driver of dolomite production we can predict more precisely how the Mg/Ca ratio has increased from 2.5 at the Eocene/Oligocene boundary to 5.1 in modern sea water.


Biography

AB Geochemistry Columbia University 1984

MS Geology The University of Michigan 1987

PhD Geology The University of Michigan 1990

Characteristics and diagenesis of the Upper Permian Beekeeper Formation from the Perth Basin, Western Australia

Adhari, Muhammad Ridha.1 Wilson, Moyra E. J.1

1School of Earth Sciences, The University of Western Australia, Perth, Australia

The Upper Permian Beekeeper Formation is a proven reservoir in the Woodada field, Perth Basin, Western Australia, yet the nature and sedimentary features of this formation are not well understood. The Beekeeper Formation is about 20 km wide and 70 km long and the thickness of this sedimentary unit is up to 134 m. This formation is interpreted to be deposited in a cool-water ramp setting and has been identified not only in the Woodada field, but also in the Beharra Spring field, Perth Basin. This study aims to better understand the characteristics of the Beekeeper platform and the diagenetic processes that occurred during its evolution. Sixty metres of conventional cores are available from five wells from the Perth Basin and 127 thin sections were made from those cores. Sedimentary logging, acetate peels, thin section petrography, and acid digestion have been conducted on the available dataset. Results from this study show that the Beekeeper Formation is a mixed carbonate-siliciclastic system, with both coeval and reciprocal mixing during sequence development. Bryozoas, brachiopods, and crinoids are the main bioclasts in packstones, grainstones, and rudstones. Primary matrix porosity is minimum in the Beekeeper Formation, but the secondary fracture porosity is of the highest quality. The fracture system is interpreted to be generated through tectonic activity on the basis of the relative timing of the paragenetic events, offset along the fractures, common sub-vertical fracture orientations, multi-size and multi-episode fracture development, and multi-phase fracture cements. The main diagenetic processes affecting the Beekeeper Formation include micritisation, boring, mechanical compaction, syntaxial overgrowth, granular-blocky calcite cementation, chemical compaction, dolomitisation, recrystalisation, and replacement, whereas bioclast and calcite vein dissolution are as minor features. These findings are expected to advance our understanding of this Upper Permian mixed carbonate-siliciclastic system and reservoir.

Keywords: Upper Permian, Mixed carbonate-siliciclastic, Beekeeper Formation, Fracture porosity


Biography

Muhammad Ridha Adhari is a student at the School of Earth Sciences, UWA sponsored by AAS. Adhari holds a bachelor degree from Institut Teknologi Bandung, Indonesia and a master degree from Curtin University, Australia. Currently, he is conducting a research on fractured carbonate-siliciclastic reservoir from the Perth Basin, Australia.

Deep water cuspate stromatolites in the Cryogenian Trezona Formation, South Australia

O’Connell, Brennan1, Wallace, Dr Malcolm W.1, Hood, Dr. Ashleigh v.s.1, Rebecchi,Luke1

1University of Melbourne, , Australia

Deep water stromatolite horizons are well developed in the Cryogenian Trezona Formation and were deposited in an open marine, mid- to outer-ramp setting. Stromatolite horizons predominantly occur in association with shales that contain intraclastic horizons interpreted as mass flow deposits, and are associated with iron oxides and irregular surfaces of erosion. These stromatolite horizons—developed as elongate structures with cuspate mm-scale laminae—are interpreted as condensed sections in sediment starved settings. Documentation of these deep water stromatolites adds to a small collection of deep water stromatolites, which appear to be largely represented by cuspate forms. Deep water stromatolites could be linked to specific redox environments such as anoxic/suboxic ferruginous waters, and/or may be related to carbonate saturation, sediment starvation, or other factors.


Biography

Brennan is a PhD candidate at the University of Melbourne.

Descending into the “snowball”: Improving interpretations of Tonian palaeoenvironments with multi-proxy elemental and isotopic geochemistry

Virgo, Georgina1,2, Collins, Alan2, Farkas, Juraj3, Blades, Morgan2, Amos, Kathryn1, Lloyd, Jarred2

1Australian School of Petroleum and Energy Resources, the University of Adelaide, SA 5005, Australia, 2Tectonics and Earth Systems (TES) and Mineral Exploration CRC, Department of Earth Sciences, the University of Adelaide, SA 5005, Australia, 3Metal Isotope Group (MIG), Department of Earth Sciences, the University of Adelaide, SA 5005, Australia

The Tonian–Cryogenian transition represents a period of significant physiochemical change in Earth history. It involved variations in oceanic and atmospheric oxygenation, significant changes in the biosphere, tectonic reorganisation, and the onset of the global ‘Sturtian’ glaciation. Tonian and Cryogenian sedimentary rocks in the Adelaide Superbasin, South Australia (SA), represent some of the most well-exposed, continuous and thick sections of this interval globally, recording major environmental shifts through distinct variations in lithology and isotope chemistry. Although this transition is geologically significant, it remains enigmatic due to a distinct lack of comprehensive, contemporary Tonian–Cryogenian research in South Australia.

We present robust palaeoenvironmental interpretations for a complete pre- to post- Sturtian glacial succession near Copley in the northern Flinders Ranges, SA. During fieldwork, a ~3km sedimentary log was measured for facies and sequence stratigraphic analyses, and 350 samples were collected for elemental and isotopic geochemical analyses. Our study reveals multiple regressive-transgressive cycles, recorded by deltaic rippled and cross-stratified sandstones, through lagoonal intraclastic magnesite and stromatolitic carbonates, to subtidal laminated siltstone and platform carbonates. These pre-glacial formations are unconformably overlain by subglacial to ice contact pebbly diamictites with quarzitic and dolomitic interbeds, which grade into proglacial laminated mudstone and sandstone with dropstones. We suggest that these facies reflect glaciomarine conditions. The post-glacial formation consists of subtidal laminated shales and carbonates, reflecting widespread transgression after the glaciation.

Elemental chemistry, along with C- and Sr-isotope signatures were analysed to determine primary basin water chemistries or palaeo-seawater compositions, and to further constrain the depositional setting. Results demonstrate a nearshore/restricted, dysoxic setting with indications for moderate hydrothermal input, which supports the sedimentological data. Furthermore, there is an inverse relationship between 13C and 87Sr/86Sr data, ranging from 7.37‰ to -6.68‰ and 0.7088 to 0.7182, respectively. In addition, the studied carbonates also exhibited relatively light 88Sr values (≤0.211‰). These isotopic observations could reflect a drop in relative sea level, increased weathering of carbonates and sufficient input of continental material, which is consistent with sequence stratigraphic interpretations. Such settings might be analogous to the modern Coorong depositional environment, SA, where interaction of seawater with brackish continental waters facilitate precipitation of primary dolomite and magnesite. Insights into the water chemistry and isotope signatures of these primary Mg-rich carbonates will assist with interpretations of isotope data collected from the studied Tonian dolomites and magnesites. This multi-proxy study presents new palaeoenvironmental insights into a key Tonian–Cryogenian succession, which sheds light in our understanding of how the world descended into one of the most severe glaciations ever recorded.


Biography

Third year Phd student, research focused on sedimentology and geochemistry of Neoproterozoic sediments in South Australia

Neoarchean carbonates as archives of paleo-redox conditions on early Earth: Insights from metal isotope analyses of the Tumbiana Formation, Pilbara, WA

Farkas, Juraj1; Klaebe, Robert1, Scarabotti, Liam1, Stormberg, Jessica2, Spinks, Sam2

1Metal Isotope Group (MIG), Department of Earth Sciences, University of Adelaide, SA, Australia, 2CSIRO, Division of Mineral Resources, Perth, WA, Australia

The redox conditions of the earth’s surface environments are intimately linked to past changes in the atmospheric O2 levels, and thus to the long-term evolution of photosynthetic life on our planet. It is generally believed and accepted that Archean Eon is characterised by extremely low levels of atmospheric O2 concentrations with predominantly anoxic / euxinic marine and terrestrial environments; where the first significant rise in atmospheric O2 levels is documented much later during the Paleo-Proterozoic period at around 2.4 to 2.1 billion years (Byr) ago, the latter also called the Great Oxidation Event (GOE). This study aims to further constrain paleo-redox conditions during Neoarchean times by analysing selected metal isotope tracers and elemental concentrations in carbonate-rich samples from the Tumbiana Formation of the Fortescue Group in the Northern Pilbara Craton in WA, dated at ~2.7 Byr, which thus represent unique archives of paleo-environmental and redox conditions on early Earth. Here we analysed stable chromium isotopes (d53/52Cr variations), coupled with REEs and elemental Zn/Fe ratios, to infer past changes in redox conditions during the deposition of the Tumbiana Formation; and radiogenic strontium isotopes (87Sr/86Sr ratios) were used to test marine versus continental / lacustrine origin of the studied Neoarchean carbonates from the Fortescue Group. Interestingly, acquired results strongly support non-marine and thus likely lacustrine origin of the Tumbiana Formation carbonates; and more importantly our pilot data from redox-sensitive proxies (d53/52Cr and Zn/Fe ratios) point to an active redox cycling of Cr isotopes during the deposition of the Tumbiana Formation at ~2.7 Byr, thus challenging the prevalent views of strictly anoxic conditions during the Neoarchean. In addition, the above geochemical/isotope evidence for an active redox cycling (i.e., oxidation-reduction) of chromium and iron is documented in horizons with abundant stromatolitic carbonates, perhaps suggesting that local production of O2 via photosynthesizing microbial communities in restricted lacustrine settings could be partly responsible for the observed paleo-redox signals recorded in d53/52Cr and Zn/Fe tracers in carbonates from the Tumbiana Formation. We will discuss broader implications of the above findings for paleo-redox studies in deep times, including possibilities that predominantly anoxic and O2 depleted Archean earth’s surface environments could also harbor localised ‘oxygenated oasis’ in very specific depositional settings.


Biography

My research interest is focused on the application of metal isotopes and novel analytical techniques to low-temperature geochemistry, including earth system evolution studies, and isotope tracing of metals in near-surface environments. I completed my PhD at Uni Ottawa, Postdoctoral training at Harvard, and recently established Metal Isotope Group at UniAdelaide. 

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