Postgraduate Profiles

Mercedes (Sadie) Belica

Mercedes (Sadie) Belica profile photo

Thesis: Early-Late Permian chronostratigraphic correlation and the Apparent Polar Wander Path of Gondwana: A magnetostratigraphic investigation of the Kiaman Reverse Superchron

Over the past 70 million years the Earth’s magnetic field has reversed its polarity more than 100 times, with each stable magnetic period, or chron, representing a unique and random event. Magnetostratigraphy is a geophysical dating technique that relies upon this reversal barcode of magnetism in sedimentary strata. The Earth’s magnetic field, generated by convection in the liquid iron outer core, behaves like a dipole aligned along Earth’s spin axis. Sedimentary rocks record a depositional remanent magnetization (DRM), while igneous rocks record a thermoremanent magnetization (TRM) that preserves the polarity and direction of the Earth’s magnetic field during the time of formation. The magnetic field reverses its polarity on average every 0.1-1 Ma, although two periods of stable field polarity have been observed (Cretaceous Long Normal and the Kiaman Reverse Superchron). The Global Polarity Timescale (GPTS) contains the reversal record for the Earth’s magnetic field for the past 160 Ma, while the pre-Jurassic record is pieced together from the surviving continental sections. Magnetostratigraphy is a central thread to the GPTS because it links the facets of geologic time using the global synchronicity of magnetic reversals, and is often available when absolute ages cannot be determined.

During the Permian, Australia and Africa formed part of the minor supercontinent Gondwana, along with India, South America, East Antarctica and Madagascar. Northward subduction of the Panthalassan plate beneath Gondwana initiated magmatism and extension along the southern margin of this supercontinent, which resulted in the formation of several intracratonic basins. The research here focuses on the Middle-Late Permian magnetostratigraphy and geochronology of the Sydney (East Gondwana) and Karoo Basins (West Gondwana). The sections span near the top of the Kiaman Reverse Superchron (KRS), a period from about 318 to ~265 Ma when the Earth’s magnetic field remained fixed in a reverse polarity. First classified by Irving and Parry (1963) from volcanic rocks near the town of Kiama, Australia, they noted that rocks of this age had the same (positive) polarity as others around the world, while further up in the section, a normal (negative) polarity was observed in the overlying Triassic rocks. The inferred polarity transition was named the Illawarra reversal, and despite its recognition, its stratigraphic position within the Sydney basin is still undetermined, while the age and locations of the reversal from other sections around the world are either incomplete or in dispute.

Why my research is important

Establishing a global chronological reference frame is crucial in the field of geology, and with the advances in U-Pb geochronology we are able to assign precise ages to sedimentary sections around the world for correlation. Two key Permian chronological markers include the Permian-Triassic boundary and the Kiaman Reverse Superchron. The Permian-Triassic boundary is the site of the largest mass extinction event in Earth’s history, while the Kiaman Reverse Superchron represents the longest lived stable geodynamo ever recorded. Determining the timing of these two events and their stratigraphic position within ancient depositional centers will help improve the GPTS and provide constraints on their potential causes.

This project will help resolve the following issues:

1. What is the age of the top of the Kiaman, and where does this boundary lie stratigraphically within the Sydney and Karoo basins? How does this affect pre-existing stratigraphic relationships between other global sections?

2. How do the paleomagnetic results affect the apparent polar wander path of Gondwana? Specifically, how do the results from the Sydney Basin volcanic rocks compare to the Late Paleozoic Australian pole path, as well as the sedimentary-calculated path from the Karoo Basin? How were the continents positioned during the Middle Permian?

3. Detailed magnetostratigraphy of a likely Permian section from the Karoo basin will help determine the correct stratigraphic position of the Permian-Triassic boundary, which is still poorly constrained here due to conflicting intra-basin age interpretations. The improved positioning of the boundary within the Karoo basin (well known for its abundant vertebrate fossil record) will allow for better chronologic, paleontologic, and tectonic comparison.

Funding

Field section "Pienaarsfontein" (Karoo Basin 2014)

Jul 2013

Jun 2017