The character of magnetic records of deformed soft sediments with emphasis on liquefaction structures

Sammanfattning: In this thesis the characteristics of magnetic records in deformed soft sediments during postglacial time in Sweden are studied. Magnetic records refer in this study to magnetic remanence (NRM and ChRM) and susceptibility, including its anisotropy (AMS). Four types of soft sediment deformation were investigated in this study: (1) lake sediment of calm depositional conditions, (2) soft sediments of plastic deformation, (3) sediment which experienced earthquake shaking but remained varved and retained horizontal bedding, and (4) sediment with severe liquefaction deformation.In the 5 sites, the dominant carriers are magnetite multi-domain grains with minor pseudo single domain and even single domain grains.In the calm and undisturbed (or lightly disturbed) sediments (Kroktjärnen), the geomagnetic field is the dominant factor influencing the ChRM. ChRM is consistent with other records from regional sites and recorded the paleosecular variation. AMS is consistent but shows light dispersion in the core.In soft sediments of plastic deformation, ChRM is related to the fold correction ( Röbäck group 4). ChRM and AMS are locally consistent.In the sediments where varves remained intact, the ChRM declination of the Nykvarn profile possessed a systematic east swing upward whilst the inclination was very shallow.AMS was characterized by calm, current-controlled deposition. Both AMS and ChRM in the sand layers have different patterns from those in the silt-clay layers. The study of AMS confirms ChRM unusual pattern cannot be explained by crypt twist deformation but reflects the motion of fine magnetic grains independent of deformation. The ChRM of Mehedeby A reflected the geomagnetic field at the time of deposition. AMS indicates a calm deposition under current-controlled condition. The AMS of Sickla reflects similar deposition condition but with local disturbance in varves 7 and 8.In the sediments with liquefaction intrusion deformation (Röbäck groups 7 and 8, and Mehedeby B), the ChRM do not satisfy the fold test and the orientation dispersion is not related to deformation degree. Further study indicates that ChRM is consistent and oriented statistically along the geomagnetic field at the time of liquefaction intrusion. Meanwhile, AMS exhibit nearly random distribution. I interpret this as the re-alignment of fine magnetic particles to the geomagnetic field after the liquefaction intrusion and AMS reflects the disordered orientation of big magnetic grains, induced by the intrusion. These characteristics provide a magnetic method to distinguish between plastic deformation and liquefaction deformation.The study supports the assertion that fine magnetic particles play a special role in sedimentary magnetization (DRM/PDRM) and big magnetic grains mainly contribute to AMS.