On the significance and utilization of secondary magnetizations in red beds

We document three cases of observed Quaternary or much older secondary magnetizations in red beds. A better than usual knowledge about past and present temperature conditions enables us to compare these secondary magnetizations with theoretical relationships between relaxation time and the temperature of acquisition of viscous partial thermo remanent magnetizations (VpTRM's). Arguments can be made for a viscous-thermal origin of the secondary magnetizations in two of the red bed collections, involving Beltian argillites from Montana and Late Precambrian red beds from the Michigan basin. In the third cases, involving Upper Keweenawan sediments from the Upper Peninsula of Michigan, a chemical origin must be assigned to the secondary magnetizations, because thermal and viscous cuases can be ruled out. Stability, blocking-temperature ranges, and discreteness of secondary and characteristics magnetizations are very similar for all three of the red bed collections, so that apparently no magnetic criteria exist to distinguish between partial remagnetizations of thermal or chemical origin in red beds. However, when thermal causes can be documented the theory of VpTRM acquisition offers possibilities for palaeo-heat-flow determinations.