Primary and Secondary Red Bed Magnetization Constrained by Fluvial Intraclasts

Abstract

The magnetization of hematite-bearing sedimentary rocks provides critical records of geomagnetic reversals and paleogeography. However, the timing of hematite remanent magnetization acquisition is typically difficult to constrain. While detrital hematite in sediment can lead to a primary depositional remanent magnetization, alteration of minerals through interaction with oxygen can lead to the postdepositional formation of hematite. In this study, we use exceptionally preserved fluvial sediments within the 1.1-billion-year-old Freda Formation to gain insight into the timing of hematite remanence acquisition and its magnetic properties. This deposit contains siltstone intraclasts that were eroded from a coexisting lithofacies and redeposited within channel sandstone. Thermal demagnetization, petrography, and rock magnetic experiments on these clasts reveal two generations of hematite. One population of hematite demagnetized at the highest unblocking temperatures and records directions that rotated along with the clasts. This component is a primary detrital remanent magnetization. The other component is removed at lower unblocking temperatures and has a consistent direction throughout the intraclasts. This component is held by finer-grained hematite that grew and acquired a chemical remanent magnetization following deposition resulting in a population that includes superparamagnetic nanoparticles in addition to remanence-carrying grains. The data support the interpretation that magnetizations of hematite-bearing sedimentary rocks held by >400-nm grains that unblock close to the Néel temperature are more likely to record magnetization from the time of deposition. This primary magnetization can be successfully isolated from cooccurring authigenic hematite through high-resolution thermal demagnetization.

Remagnetization of Red Beds on the Tibetan Plateau: Mechanism and Diagnosis

“…In samples retaining the DRM from both the Gongjue and Nangqian basins, DRM carried by detrital hematite unblocks between 600°C and 680°C (Figures 2a-2d, 2g, and 2h), corresponding to hematite grain sizes of >240 nm. These observations support that hematite carrying the CRM has a wide distribution of grain size and unblocking temperature spectra, while hematite carrying the DRM is usually coarse and has confined unblocking temperature spectrum due to hydrodynamic sorting (Jiang et al, 2015;Swanson-Hysell et al, 2019). The contribution of detrital (titano)magnetite to the NRM is low in the remagnetized red beds (Figure 4), and its signal is probably swamped by the authigenic hematite with a broad unblocking temperature spectra.…”

“…The concave and convex shape of the thermal decay curves is thus proposed to indicate CRM and DRM, respectively (Jiang et al, 2015). Uncertainty still exists, however, in the use of this criterion because paleomagnetic studies on natural samples reveal that the unblocking temperature spectra of the CRM can overlap with those of the DRM and extend up to the Néel temperature of hematite (Jiang et al, 2017; Swanson‐Hysell et al, 2019). In a recent study of the Paleogene red beds in the Nangqian Basin (eastern Tibetan Plateau), we have identified that remagnetized red beds contain large amounts of nanogoethite, whereas goethite is not present in red beds retaining the DRM (Huang, Jackson, Dekkers, Solheid, et al, 2019).…”

Rapid drift of the Tethyan Himalaya terrane before two-stage India-Asia collision

“…7). In these cases, it is possible that a chemical remanent magnetization, generated through oxidation of magnetite by orogenic hydrothermal fluids, has overprinted the primary detrital remanent magnetization completely [ 34 , 35 ]. The HTCs are considered the primary magnetization in all our sections; they are carried by detrital hematite and show dual polarities (Supplementary Figs 2 and 3).…”

Unraveling the Mineralogical Complexity of Sediment Iron Speciation Using Sequential Extractions

“…Due to the remanent coercivity on the lower end of hematite's range, we suggest that this hematite is either very fine grained (<1 μm) or very large grained (>50 μm) (Özdemir & Dunlop, ; Peters & Dekkers, ). Nanophase hematite has been shown to form in diagenetic processes either during early diagenesis through aging of ferrihydrite or through secondary diagenetic processes repartitioning iron, for example from iron‐bearing carbonates and silicates (Jiang et al, ; Swanson‐Hysell et al, ; Walker et al, ; Weil & Van der Voo, ).…”