Paleomagnetism and Rock Magnetism of Permian sedimentary rocks and Early Jurassic Karoo (Large Igneous Province) sills intersected in the KWV-1 borecore, Eastern Cape Province, South Africa

Abstract

The nearly 100 Ma record of almost continuous shallow marine to non-marine sedimentation from the Late Carboniferous to the Early Jurassic recorded in the Karoo Basin, South Africa, includes coal deposits and potentially significant shale gas occurrences. As such, numerous studies have addressed the thermal history of the Karoo Basin, using a range of approaches. We use paleomagnetic and rock magnetic data to evaluate the paleotemperature history of the host Permian strata (Ecca Group and lowermost Beaufort Group) intersected in the ∼2320 m deep KARIN KWV -1 drilling project core, obtained in 2015. Where cored, the Karoo sedimentary sequence has been intruded by several Early Jurassic mafic sills, belonging to the Karoo Large Igneous Province, of thickness ranging from less than 10 cm to ∼150 m. The continuous core provides a unique natural laboratory for studying the effects of Karoo magmatism on the basin's sedimentary sequence. The KWV-1 drilling project was spudded ∼10 km east of Willowvale town, Eastern Cape Province.The spatial variation in thermal effects in host sedimentary strata, relative to proximity to Karoo sill contacts, is well-defined on the basis of anisotropy of magnetic susceptibility (AMS) data, paleomagnetic demagnetization response, and thermal alteration index (A40) values based on temperature vs. magnetic susceptibility tests. A zone of pervasive remagnetization of host sedimentary strata characterizes the contact aureoles, the spatial extents of which are to a first order dictated by the thickness of each Karoo sill. A40 data are interpreted to show that ambient paleotemperature estimates increase from ∼150 °C in lower Beaufort/upper Ecca strata to ∼285° in Lower Ecca strata that are sufficiently removed from Karoo sills. However, lower Ecca Group rocks in close proximity to Karoo sills were likely heated to ∼500 °C. Natural remanent magnetization (NRM) intensities of rocks in proximity to sills range from ∼17 to ∼ 438 mA/m, and susceptibilities range from 1.3 to 3.2 × 10−3 SI volume. In contrast, for strata sampled at distances greater than the width of the nearest sill, NRM intensities range from 0.056 to ∼4 mA/m, and susceptibility values range between 193 and 488 X 10−6 SI volume. For host sedimentary rocks, remanence is completely unblocked at temperatures ranging from ∼350 °C to ∼630 °C and acquisition and backfield DC demagnetization of isothermal remanent magnetization (IRM) indicate that specimens are fully saturated in fields of ∼200–600 mT, with coercivity of remanence of ∼30–100 mT. All of the eleven Karoo sills examined in this study yield remanence inclinations that are moderate to steep negative, and are inferred to have recorded a normal polarity field, possibly during a single chron. Our results are interpreted to suggest that substantial thermal effects of Karoo sills on the host sedimentary strata are largely confined to the contact aureoles, with a width that is dependent on sill thickness. We hypothesize that parts of the Ecca sedimentary sequence have remained at ambient, basinal burial temperatures below about 150 °C, and thus that an early-acquired Permian remanence may possibly be preserved in these sedimentary rocks at sufficient distance from thermal contact aureoles. Alternatively, much of the sedimentary section may have been subject to a combination of thermoviscous/chemical remagnetization.