MAGNETIZATION ACQUISITION AND REMAGNETIZATION PROCESSES IN RED BEDS: WHAT MAKES A RED BED PRONE TO REMAGNETIZATION?

Red beds, hematite-cemented, detrital sedimentary rocks dominated by medium sand to silt grain sizes, are well-known to often contain geologically stable magnetizations (RMs)and have been the subject of numerous paleomagnetic studies (e.g., primary paleomagnetic poles, magnetostratigraphy, structure/tectonics). Despite decades of paleomagnetic and rock magnetic work on red beds, magnetization acquisition and remagnetization processes are not well-understood, with the exception that,ostensibly, the principal RM carrier is hematite. Possible contributors to the source of geologically stable remanence in red beds include(1) detrital hematite or specularite,(2) detrital magnetite that has experienced in situ oxidation,and (3) fine pigment hematite, or other form of non-detrital "secondary" hematite,with a wide range of grain sizes and therefore laboratory unblocking temperatures. Red beds could therefore vary from quartz arenites with only hematite pigment, to arkoses with detrital hematite, magnetite (oxidized), ferromagnesian silicates (oxidized), and hematite pigment. Red beds with a sizeable concentration of fine detrital hematite probably have a greater likelihood of acting as a high-fidelity recorder of an ancient field than those with abundant oxidized ferromagnesian silicates and coarse pigment, which may integrate more field time in remanence acquisition. Red beds are by no means immune to remagnetization; many cases, where thermoviscous processes appear implausible, document the isolation of a secondary RM to maximum hematite unblocking temperatures. It is likely that red beds with little detrital hematite and oxidized ferromagnesian silicates have a greater likelihood of remagnetization through low temperature chemical processes. Several examples from current paleomagnetic studies on rocks ranging from Neoproterozoic to Eocene, including observations using high-resolution scanning electron microscopy, reveal that these hypotheses are well-substantiated.