Correcting the bias toward shallow paleomagnetic inclinations in hematite-bearing sedimentary rocks: Theory, experiments, and applications

The deflection of remanent magnetization from the magnetic field is due to the anisotropy of magnetization in a sample. Inclination shallowing was simulated by theoretical analyses of preferred distributions of anisotropic magnetic particles, and equations were derived to correct red bed inclination shallowing. Laboratory redeposition and compaction experiments on disaggregated Suweiyi Formation sediments were conducted to study the characteristics of compaction-caused inclination shallowing of fine-grained red beds and depositional inclination error of silt and sand-sized red beds. It is demonstrated that compaction can cause ∼20° of inclination shallowing in a 60° inclination magnetic field. Depositional inclination error depends on the depositional rate and hematite particle size, and a multi-component remanence may result. The remanence carried by low coercivity and low unblocking temperature fine-grained hematite particles is an accurate record of the Earth's magnetic field direction, while the high coercivity and high unblocking temperature component of remanence carried by larger hematite particles may suffer from 10° to 30° of inclination shallowing in a 60° inclination field. These synthetic samples were used to test a single component IRM approach proposed by Hodych and Buchan (1994) to identify and correct inclination shallowing in red beds. This approach was able to detect and correct the compaction-caused inclination shallowing, but it failed to detect any depositional inclination error. The Mauch Chunk Formation (Mississippian) and the Kapusaliang Group (Cretaceous) red beds were collected to study the magnetic anisotropy effect on the inclinations of natural sedimentary rocks. Both formations showed depositional and compaction magnetic fabrics for both susceptibility and remanence. An inclination correction applied to the Cretaceous red beds steepened the inclination by 20° and reduced the amount of northward movement of the Tarim basin (NW China) to a few hundred kilometers, which is consistent with geologic observations. Application of inclination correction to the Mauch Chunk Formation indicated that its inclination had been shallowed by 38°. A corrected paleopole position is identical to the poles of volcanic rocks from North America and Europe. This study suggests that inclination shallowing of red beds is more important than previously thought.