Abstract:
Undetected depositional and/or compaction-caused inclination errors may result in an overestimation of tectonically caused latitudinal offset. Hodych & Buchan used a single-component isothermal remanent magnetization (IRM) acquired in a DC field at a 45° angle to the bedding to test for inclination errors in Silurian red beds. This approach was criticized by Stamatakos et al. We produced synthetic depositional and compaction-caused inclination errors to test Hodych and Buchan's approach. Red bed samples collected from the Eocene Suweiyi Formation, Tarim Basin (northwest China) were disaggregated using an ultrasonic cleaner and the sediments were mixed with distilled water to make a sediment slurry. The sediment slurry spontaneously separated into silt-dominated and clay-sized parts, so deposition and compaction experiments were conducted with three categories of slurries: coarse grained, the fine grained and a 1:1 volume ratio mixture of the coarse- and fine-grained slurries. During compaction clay-sized sediments experienced 17°-19° inclination shallowing at 58° magnetic field inclination, while coarser sediments showed little laboratory compaction-caused inclination error. Acquisition of IRM, coercivity spectra and unblocking temperature spectra reveal that the magnetic carrier for the fine-grained sample is dominated by pigmentary haematite. A deposition experiment was also conducted with the coarse-grained sediments, which showed a range of depositional inclination error, ~0°-30°, carried by larger high unblocking temperature/coercivity particles. The intermediate unblocking temperature (or coercivity) component carried presumably by pigmentary haematite is an accurate record of the ambient magnetic field direction. Our data indicate that the single-component IRM method of Hodych & Buchan can be used to identify and correct for the compaction-caused inclination error of the fine-grained samples, while it failed to detect any depositional inclination error in the coarse-grained samples. Therefore, to better quantify the inclination error in red beds, it is suggested that multiple IRMs be measured in various directions.
