GEOCHEMICAL CHARACTERISTICS OF GLACIOGENIC SEDIMENTS, PUGET LOWLAND, WASHINGTON

Quaternary sediments in the Puget Lowland comprise a complex succession of intercalated glacial, glaciofluvial, and glaciomarine sediments deposited during repetitive glacial and interglacial periods. The primary source regions include crystalline rocks from the southern Canadian Cordillera (SCC) and volcanic strata from the Cascade Range and Olympic Mountains, but the relative contribution from each source varied through time. In general, major glacial advances provided sediment from the SCC, and interglacial periods were dominated by locally derived Cascadian/Olympic detritus, but complex interfingering facies, diachronous sedimentation and intrabasinal reworking make basinwide correlation and comprehensive basin analysis difficult.

Major and trace element geochemistry from both Quaternary sediments and modern stream drainages from throughout the Puget Lowland suggest that it is possible to discriminate glacial and interglacial sedimentation events through bulk sediment composition. Major glacial advances, such as the Vashon advance, yield sediment that is high in SiO2, Ba, Sr, Cr, Ni, and Cr/V, and low in TiO2, Nb, Ce, V, La, Zr and Th/Sc. These values suggest derivation from exposed continental arc plutons in southern British Columbia, combined with detritus from ocean melanges such as the Bridge River terrane. Conversely, sediments deposited during interglacial periods were derived from Cascade volcanic rocks, and are correspondingly higher in TiO2, Nb, Ce, V, and La. However, simplistic interpretations are dangerous, as sediment mixing, elemental partitioning during weathering and transport and post-depositional elemental mobility can produce non-intuitive geochemical signatures (such as high SiO2 and Cr values in the same sample). However, geochemical provenance assessment based on multiple geochemical indicators permits discrimination between glacial and non-glacial sediments, and may be useful tool in stratigraphic correlations in complex Quaternary sediments of the Puget Lowland.