School of Earth Sciences - Theses

Permanent URI for this collection

Search Results

Now showing 1 - 2 of 2
  • Item
    Thumbnail Image
    Neoproterozoic seas: ocean chemistry and marine carbonate mineralogy
    HOOD, ASHLEIGH ( 2014)
    The step-wise oxygenation of the ocean-atmosphere system is arguably one of the most profound processes in Earth history, affecting most surficial Earth processes. The last major oxygenation of the oceans is believed to have occurred in the Neoproterozoic Oxygenation Event (~800-540 Ma), and is implicated as a trigger for the rise of animal life. However, the timing of this event is not well constrained, both due to geochronological problems with Neoproterozoic stratigraphy; and because of the inherent uncertainty in ocean oxygenation proxies. Furthermore, there is now evidence for a more complex Neoproterozoic ocean chemical history, including return to strongly anoxic and ferruginous conditions. An additional complication in the understanding of Precambrian marine environments is the abundance of dolomite in Proterozoic successions. A recently discovered series of dolomitic reef complexes in the Neoproterozoic Adelaide Fold Belt, Australia, and Otavi Belt, Namibia, improve our understanding of Precambrian marine conditions. Stratigraphic and petrological analysis suggests that synsedimentary marine dolomite precipitation was pervasive within these reefs. Newly described dolomite cements have optical properties, chemical zonation and cathodoluminescent characteristics indicating that they were direct marine precipitates. Dolomite precipitation during marine diagenesis in these reef complexes suggests that the oceans of the Cryogenian were chemically different to those of the Phanerozoic. Marine dolomite precipitation appears to be linked to anoxic, magnesium-rich ocean conditions. These newly documented primary marine dolomite cements preserve information about conditions in the parent seawater via their petrographic properties and geochemistry. Being constrained by sedimentology, carbonate geochemistry provides a window into Cryogenian ocean chemistry and structure. Geochemical results reveal a pronounced chemical stratification where a thin veneer of oxic surface waters existed above a peritidal redoxcline with anoxic, strongly ferruginous seawater at depth. These conditions describe a ferro-sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late Precambrian. A return to Archean-like ocean conditions at this time suggests large-scale disruption of the ocean system during the Neoproterozoic. These conditions may be linked to extreme climatic fluctuations at this time, perhaps induced by ocean stratification in this Neoproterozoic ‘Stagnant Earth’. When analysed in stratigraphic framework, variations in carbonate mineralogy provide a record of ocean oxygenation during the Neoproterozoic. New sedimentological and stratigraphic constraints for the Namibian Otavi Belt provides a context for this variation and has also led to the discovery of new Cryogenian reef complexes. When correlated with the Adelaidian succession, the distribution of marine cements in these sequences reflects changing seawater conditions. Pre-Sturtian, Neoproterozoic oceans precipitated both dolomite and aragonite and developed widespread marine anoxia prior to glaciation. Interglacial Cryogenian oceans were extremely anoxic and ferruginous, with widespread dolomite precipitation. In contrast, late Cryogenian and Ediacaran oceans hosted abundant aragonite precipitation recording a gradual decline in marine dolomitisation. The deepening of the oceanic chemocline during this interval suggests that these seas were likely to have been moderately oxygenated, paving the way for the large-scale radiation of animal life.
  • Item
    Thumbnail Image
    Stratigraphy and sedimentology of Cryogenian carbonates, Flinders Ranges, South Australia
    Fromhold, Thomas Alexander ( 2011)
    The Adelaide Geosyncline of South Australia contains a Neoproterozoic-aged sedimentary succession consisting of a complex accumulation of sedimentary formations and units recording a diverse and unique depositional record. A detailed stratigraphic and sedimentological investigation of the interglacial period within the Cryogenian-aged Umberatana Group of the Northern and Central Flinders Ranges reveals a complex array of sedimentary successions lying between the Sturtian and Marinoan glacial deposits. In the Northern Flinders Ranges a major unconformity separates the Sturtian and Marinoan-aged sedimentary successions in the area. This forms a sub-aerial erosion surface with terrestrial and marginal marine sediments directly above the Angepena and Balcanoona formations in their respective localities. This exposure surface is here correlated with the previously documented submarine unconformity between the Yankaninna Formation and the underlying deep marine Tapley Hill Formation. This erosional event provides a chronostratigraphic marker horizon that coincides approximately with the previously defined Sturtian-Marinoan time series boundary in the Northern Flinders Ranges. These stratigraphic relationships also constrain lateral facies relationships between the Oodnaminta Reef Complex (Balcanoona Formation) and the Angepena Formation. Similarly, the shallow water Weetootla Dolomite is correlated with the deeper water carbonates of the Yankaninna Formation. In the Northern Flinders Ranges the Angepena Formation occurs as a marginal marine red-bed succession consisting of supratidal mudstones which are interbedded with subtidal and intertidal carbonates. The Angepena Formation is interpreted as a coastal mudflat succession that formed as a shoreward, laterally equivalent facies of the extensive carbonate platforms (reefs) of the Balcanoona Formation. Sedimentological and geochemical investigation of the Angepena Formation reveal that the unit contains a diverse accumulation of shallow marine carbonates including ooidal sands, tepee buckled algal mats, intraformational breccia (palaeo-caliche) and fenestral-bearing microbial deposits. The stratigraphic and sedimentological relationship within the interglacial successions of the Umberatana Group of the Northern Flinders Ranges are found to extend well over a hundred kilometres southwards into regions of the Central Flinders Ranges. The post-glacial Sturtian-aged Tapley Hill Formation records a near-identical depositional record to the Tapley Hill Formation of the Northern Flinders Ranges. In the Central regions, the Tapley Hill Formation is overlain by deep-marine carbonates and calcareous shales of the Wockerawirra Dolomite and Sunderland Formations respectively. The base of the Wockerawirra Dolomite is defined by an erosional surface, which is directly correlated to the unconformity found overlying the Tapley Hill Formation in the Northern Flinders Ranges (Sturtian-Marinoan series boundary). This stratigraphic relationship indicates the Wockerawirra Dolomite and Sunderland Formations of the Central Flinders Ranges are direct correlatives of the Yankaninna Formation of the Northern Flinders Ranges. The regionally widespread carbonate platform complexes of the Balcanoona Formation in the Northern Flinders Ranges preserve a unique history of the depositional record within the middle Umberatana Group of the Adelaide Geosyncline. Cessation of reef development coincides with a major regression event situated immediately below the Sturtian-Marinoan boundary. The regional consistency of the stratigraphic features found at the Sturtian-Marinoan boundary (i.e. unconformities) suggests that regional scale mechanisms, such as glacio-eustasy, were probably active during this otherwise ‘interglacial’ succession of the Cryogenian-aged Umberatana Group.