Paleomagnetism, Rock Magnetism, and Magnetic Fabric of Upper Permian Sedimentary Rocks and Early Jurassic Intrusions, Karoo Basin, South Africa, and of a Highly Distorted Oligocene Ignimbrite, East-central New Mexico
Akatakpo, Solomon Omajuwa
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This dissertation includes three different projects that utilize paleomagnetic and rock magnetic techniques and principles to understand the thermal effect of magmatism on an older, host sedimentary rock sequence, its mineralogy and domain state, magnetization and remagnetization processes, to determine the magnetic polarity stratigraphy of Permian sedimentary rocks, and to evaluate the magnetic petrofabric of an Oligocene ignimbrite to understand its source and related transport history. Project #1 addresses the thermal effect of Early Jurassic Karoo large igneous province (LIPs) magmatism on a Permian sedimentary sequence cored in the Eastern Karoo Basin, South Africa. The study of the paleotemperature history of the host Permian sedimentary strata involved sampling the ~2320 m deep borecore intersected in the KARIN KWV-1 drilling project. The KWV-1 drilling experiment spudded about 10 km east of Willowvale town, Eastern Cape Province, and provides a unique natural laboratory for studying the effects of Karoo magmatism on the basin’s sedimentary sequence. Paleomagnetic and rock magnetic results from dolerite sills and host sedimentary strata reveal that single domain (SD) to multi-domain (MD) magnetite is the main magnetization carrier of the Karoo dolerite sills and host sedimentary strata sampled at distances greater than ½ the width of an adjacent sills. However, for host sedimentary strata sampled at distances < ½ the width of an adjacent sill, remanence is carried by both magnetite and SD pyrrhotite. Paleotemperature estimates based on the alteration index (A40) technique reveal that parts of the Ecca sedimentary sequence have remained at ambient, basinal burial temperatures, likely well below 250 ̊C. Thermal and alternating field (AF) demagnetization yield a well-defined interpretable response, with some data suggesting the preservation of early acquired Permian remanence in sections of the cored interval. Much of the sedimentary sequence has been subjected to a combination of thermoviscous/chemical remagnetization processes. Project #2 involves paleomagnetic and rock magnetic study of a ~50 m thick sequence (11 independent sites) of continental sedimentary rocks of the upper Permian/Lower Triassic Beaufort Group, lying above an Early Jurassic Karoo mafic sill (2 sites), exposed on Piersnaarbaken farm, Eastern Cape Province, to aid our understanding of the age of deposition of this section, which contains important palynologic information. Magnetic polarity results have been placed in a magnetostratigraphic context to infer that the age of deposition of these strata is part of the Illarawa mixed polarity zone (late Permian age), and in particular, part of the Elandsberg Member. Rock magnetic data show that magnetite is the main remanence carrier in these sedimentary rocks. Demagnetization experiments reveal that the natural remanent magnetization (NRM) of the host rocks has been partially overprinted by the Early Jurassic Karoo magmatism. Demagnetization data show that the NRM consists of two distinguishable components, referred to as component-1 and component-2. The component 1 is of NNW declination and moderate to steep negative inclination, unblocked in thermal demagnetization by ~ 460 ̊C and AF fields of << 90 mT. Component-2, on the other hand, is of dual polarity and is unblocked over a temperature range of ~ 460 ̊C to ~ 580 ̊C. The component-2 is of SSE declination at sites PBB4 and PBB5, and moderate to steep positive inclination, and is NNW declination and moderate to steep negative inclination at the remaining sites higher in the section. The dual polarity nature of Component-2 is interpreted to indicate that the sedimentary section was deposited after the Kiaman reverse polarity Superchron, and, possibly, during the latest Permian. Project #3 involves study of the anisotropy of magnetic susceptibility (AMS) and rock magnetism of the Palisades Tuff, Sierra Blanca, New Mexico. The Palisades tuff is of early Oligocene age and has been proposed to be the result of explosive eruption of quartz syenite magma that formed the nearby Three River Stock, based on geochemical similarity in terms of major and trace elements. Oriented block samples were collected from Palisades Tuff (PT) exposures at seven sites in the eastern Tularosa Basin. The magnetic fabric data derived from AMS measurements show that the PT exhibits both well-defined prolate and oblate fabrics. Rock magnetic experiments reveal that the magnetic mineralogy of the Palisades Tuff is dominated by pseudo-single to multi-domain magnetite, and hematite. The AMS data show that samples from some sites yield exclusively oblate fabrics, while samples from some yield exclusively prolate fabrics, and some are very mixed in behavior. For the sites with oblate fabrics, the inferred transport direction is east to northeast with few sample sites showing northwest and southwest directions. On the other hand, the inferred transport direction for sites with prolate grains and mixed prolate and oblate fabrics is west / northwest. The direction of transport inferred from the AMS data suggests that the Palisades tuff formed as a pyroclastic density current sourced from a caldera above Three River Stock, with varying degree of topographic modifications.