Remanent magnetization in penecontemporaneous structures of the Pleistocene Scarborough Formation, Ontario, Canada

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Abstract
Detrital remanent magnetization (DRM) of sediments comprising two penecon-temporaneous structures in the deltaic facies of the upper Pleistocene Scarborough Formation, near Toronto, Ontario, displays a concentrated orientation that suggests remagnetization after deformation. The two sampled sedimentary structures consist of (1) a load deformation of silty sands in a coarser-grained silty sand matrix, and (2) slump folds of thinly layered silty sands in a fine sand bed. The sediments from both structures possess a bimodal grain size distribution. Remagnetization is ascribed to the free rotation of detrital magnetic grains under high porewater pressure conditions that developed as a consequence of rapid burial by younger sediments. Liquefaction of the sediments under the effect of this high fluid pressure was achieved by the reverse gradient of hulk density and accentuated by differential overloading, as is shown by the presence of the load deformation. Elsewhere, the high porewater pressure reduced the strength of the sediments enough to cause them to slump down the gentle delta slope. Associated sedimentary features, such as a clay-silt sealing bed and a sand dyke, suggest that the subsequent locking in of DRM by dewatering took place at a time significantly later than that of initial deposition. The tight clustering of DRM orientations in both sedimentary structures point to rapid acquisition of DRM once the sediments had been drained.