Detrital and early chemical remanent magnetization in redbeds and their rock magnetic signature: Zicapa Formation, southern Mexico
Por:
Isabel Sierra-Rojas, Maria, Stanley Molina-Garza, Roberto
Publicada:
1 jun 2018
Resumen:
Poles from continental redbeds are a large fraction of the world's palaeomagnetic database. Nonetheless, the time of acquisition and origin of the remanent magnetization of redbeds has been long debated.We report palaeomagnetic data, rock magnetic data and microscope observations for Lower Cretaceous redbeds in southernMexico. These data allow us to discriminate between the hysteresis properties of remanent magnetizations of detrital and chemical origin, and to establish the early origin of a chemical remanence. Red sandstones of the Zicapa Formation contain a multicomponent remanence revealed by thermal demagnetization, and consisting of three stable components with partially overlapping laboratory unblocking temperatures of < 250 °C, ~300 to ~500 °C and > 600 °C, (low, intermediate and high temperature, respectively). They are interpreted as a viscous remanence residing in detrital magnetite, a chemical remanence residing in authigenic hematite and a depositional remanence residing in detrital hematite, respectively. The low-temperature component is nearly parallel to the recent dipole field. The tilt-corrected overall site means of the intermediate (chemical) and high temperature (depositional) components are indistinguishable (Dec = 282.0°, Inc = 12.4°, k = 13.33, a95 = 10.1°, N = 17, for the intermediate temperature; and Dec = 272.5°, Inc = 16.5°, k = 14.04, a95 = 11, N = 14, for the high temperature). Elongation/inclination analysis suggests that depositional and chemical components require applying an f = factor of approximately 0.4. Both of these components define a magnetic polarity zonation, but the polarity of the chemical and detrital components may or may not be the same. The chemical remanence coincides, more often than not, with the polarity of the depositional remanence of the overlying (younger) strata, suggesting a delay in remanence acquisition of tens to a few hundred ka for the chemical component. Pigmentary and detrital haematite were recognized with microscopic observations. The particle size of haematite ranges from approximately 10 to 300 µm for detrital haematite (martite, specularite and laterite), and from ca. 0.2 to 1 µm for pigmentary haematite flakes. The IRM of these rocks can be modelled with components of low coercivity (H1/2 between 5 and 10 mT interpreted as detrital magnetite), and components of a wide coercivity range (prevailing H1/2 from ~400 to 600 mT interpreted as haematite). Hysteresis ratios show a systematic correlation with demagnetization behaviour, with lower Hcr/Hc values and higher Mrs/Ms values for samples with a dominant chemical component, than form samples with a significant ( > 40 per cent) depositional component. © The Author(s) 2018.
Filiaciones:
Isabel Sierra-Rojas, Maria:
Univ Nacl Autonoma Mexico, Ctr Geociencias, Campus Juriquilla,Blvd Juriquilla 3001, Queretaro 76230, Mexico
Stanley Molina-Garza, Roberto:
Univ Nacl Autonoma Mexico, Ctr Geociencias, Campus Juriquilla,Blvd Juriquilla 3001, Queretaro 76230, Mexico
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