Laboratory depositional and compaction-caused inclination errors carried by haematite and their implications in identifying inclination error of natural remanence in red beds

Abstract

Summary 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.

Is there a normal magnetic-polarity event during the Palaeocene-Eocene thermal maximum (∼55 Ma)? Insights from the palaeomagnetic record of the Belluno Basin (Italy)

“…This extreme degree of shallowing is speculatively due to the combination of depositional inclination flattening, typical of the detrital haematite, and post‐depositional compaction of clays, which are particularly abundant in the carbonate‐depleted CMU. Tan et al (2002) found experimentally that during shallow (<100 m) burial compaction, clay‐size sediments containing detrital haematite particles can experience a magnetic inclination shallowing of 17°–19° starting from an induced inclination of 58°. For increasingly higher burial depths, which are usually associated with increasingly older sediments, compaction‐derived inclination flattening tends to increase accordingly (Tan et al 2002).…”

“…Tan et al (2002) found experimentally that during shallow (<100 m) burial compaction, clay‐size sediments containing detrital haematite particles can experience a magnetic inclination shallowing of 17°–19° starting from an induced inclination of 58°. For increasingly higher burial depths, which are usually associated with increasingly older sediments, compaction‐derived inclination flattening tends to increase accordingly (Tan et al 2002). The difference in degree of flattening between the Forada CMU ( f = 0.35) and the Cicogna CMU ( f = 0.14) can be attributed to different compaction degrees of sediments with slightly different composition.…”

Inclination variation in the Late Jurassic to Eocene red beds from southeast Asia: lithological to locality scale approach

“…In this connection three different approaches have been recognized; (1) no inclination shallowing in coarse‐grained sediments but shallowing in clay‐sized sediments; (2) no inclination shallowing in fine‐grain sediments but shallowing in coarse‐grained sediments and (3) similar trend of inclination shallowing both in coarse and fine‐grained sediments. According to their experimental work, Tan et al . (2002) have claimed that hematite bearing clayey sediments underwent a compaction related inclination shallowing of 17°–19°, while coarse‐grained sediments received very little inclination shallowing.…”

Paleomagnetic versus GPS determined tectonic rotation around eastern Himalayan syntaxis in East Asia

“…Almost all the compiled data sets have been extracted from the red beds, where the affects of secular variation have been sufficiently averaged out from the characteristic remanent magnetizations (which include depositional, post-depositional, and chemical magnetizations). Although, an anomalously shallow inclinations have been observed in some of the paleomagnetic directions, their respective declinations are considered as true image of the paleogeomagnetic field directions (Chauvin et al, 1996;Cogné et al, 1999;Bazhenov and Mikolaichuk, 2002;Tan et al, 2002;Narumoto et al, 2006;Tauxe et al, 2006). As shown in Fig.…”