Greigite related complex polarity records unraveled through an integrated rock magnetic analysis: A case study on Neogene sections in East Timor (Indonesian Archipelago)

Abstract

Inconsistent polarity patterns in greigite-bearing marine sediments is a common problem in magnetostratigraphic and paleomagnetic research. It is caused by the diagenetic nature of greigite, which can result in multiple generations of this mineral being present in a sedimentary record. The associated remanence `haystack' can be filtered to some extent by paleomagnetic field tests, not all rock records are suitable for such an approach however. The use of end-member modelling of IRM-acquisition curves as a reference frame for an integrated rock magnetic and microscopic research is proposed in this thesis as an alternative. This approach aims to identify all magnetic mineral suites, evaluate the nature of the magnetic remanences and afterwards apply this knowledge to the paleomagnetic data. To test this approach, a case study is performed on two late Miocene/Pliocene syn-tectonic marine sections in East Timor (Timor Leste), which have been sampled by {Bakker2011} to constrain the uplift of the young Timor-orogen by using a magnetostratigraphic age model but showed an inconsistent and complex polarity record. The Viqueque Type section (stratigraphic thickness: 240 m) is located in the hinterland, the Cailaco River section (stratigraphic thickness: 500 m) is deposited in the foreland of the orogen. For each section two end-members are constrained and analysed further. By using microscopy the end-members are further specified into three and two magnetic mineral suites for the Viqueque Type and Cailaco River section respectively. The first suite is present in both sections and in all lithologies, it consists mainly of detrital magnetite holding a viscous remanence magnetization, caused by partial reductive dissolution of stable magnetite grains after deposition. The second and third suite of the Viqueque Type section both consist of interacting single domain greigite grains with a chemical remanent magnetization. The second suite is present in almost all shale levels of the Viqueque Type section and is characterized by its early diagenetic nature, therefore deemed to hold a reliable nearly syn-depositional signal. The third suite, residing only in a few shale levels in the Viqueque Type section, consists of late diagenetic greigite intergrown between sheet silicates, resulting in anomalously high magnetizations. The second suite of the Cailaco River section consists of interacting single domain greigite as well and occurs only in a specific interval (260-380 m from the base of the section). This suite consists of late diagenetic iron sulphide nodules and carries a post-depositional remanence which is discarded for magnetostratigraphic use. The magnetostratigraphy of the Viqueque Type section shows a consistent polarity pattern after revision; it is correlated unequivocally to the Geomagnetic Polarity Time Scale with biostratigraphic age constraints. The minimum age for the top of the section is equal to the top of Kaena subchron (C2An.1r), followed down-section by the older subchrons C2An.2n and C2An.2r (Mammoth) in the Gauss polarity chron down to 43 meters from the base of the section. This correlation implies reasonably constant sedimentation rates, which are now constrained to ~80 cm/ky. A clear tectonic rotation in this hinterland section is not observed. Although a reliable magnetostratigraphy of the Cailaco River section appears to be impossible, a significant tectonic rotation is determined. The reversed signal residing in the late diagenetic greigite nodules indicates an anticlockwise rotation of the section of ~65°. The results from this case study show that a well designed rock magnetic research combined with microscopical observations can unravel complex and seemingly inconsistent polarity patterns related to greigite. End-member modelling of IRM-acquisition curves cannot distinguish the subtle differences between the various greigite populations but is nonetheless crucial to the integrated approach as a framework besides its `first-order' discriminative power.