A magnetic study conducted on reversed polarity, Plio-Pleistocene marine sediments from the Rio Dell formation suggests that maghemitization occurring in the source area may have enhanced their paleomagnetic reliability. In a section characterized by variable amounts of magnetic overprinting, some of the reliable horizons had natural remanent magnetization (NRM) intensities nearly 1 order of magnitude greater than other reliable and all the unreliable horizons from the Rio Dell. These high-NRM horizons had high anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) intensities, suggesting a greater concentration of magnetic minerals. Thermal demagnetization data indicate that maghemite is the magnetic carrier in the high-NRM horizons. ARM versus &khgr; and Lowrie-Fuller tests show that these horizons have magnetic grains dramatically smaller than magnetic-bearing horizons. Contraction cracking during maghemitization may have reduced the effective grain size of the meghemite-bearing horizons. The small magnetic grain size of the maghemite increases these horizons' coercivity and viscous acquisiton coefficient but not their JVRM. Sediment grain size analyses show that the maghemite-bearing horizons have the smallest sediment grains and unreliable magnetite-bearing horizons have the largest grains. Assuming hydrodynamic sorting of the magnetic oxides, these data suggest that the finest-grained magnetic minerals are the most susceptible to oxidation. Maghemite occurs in horizons 1.5--0.9 m.y. old. Reduced chemical and increased mechanical weathering during the Pleistocene glaciations could have increased the concentration of magnetic iron oxides in these sediments and caused their maghemitization in the provenance area.