Magnetization acquisition in two Mesozoic red sandstones

Abstract

Paleomagnetic samples have been collected from folds and other sedimentary structures produced by soft-sediment deformation and from intra-formational conglomerates at several locations of the Chugwater formation of Wyoming and the Moenkopi formation of the Colorado Plateau, both of Triassic age. Most load casts and other small-scale folds were magnetized after deformation. Slump folds were magnetized before or during folding. Two of the four conglomerates contain clasts that were magnetized before they were ripped up. These observations indicate that most fine-grained sandstones are magnetized after deposition but before burial by a meter or so of sediment. Some sandstones and finer-grained samples can be magnetized very shortly after deposition if they are consolidated and dewatered by drying. Samples having more complicated magnetization histories can usually be identified by poor within-sample grouping of remanence directions and by the behavior of individual samples during demagnetization.