SEDIMENT DATING, PROVENANCE ANALYSIS AND SEQUENCE STRATIGRAPHY – TOOLS FOR REDUCING UNDERGROUND CONSTRUCTION RISKS IN COMPLEX GLACIAL TERRAIN

Quaternary sediments in the Puget Lowland present challenges to characterizing subsurface conditions. High-cost, high-risk underground construction projects benefit from detailed stratigraphic analysis to identify potential hazards. Determination of boundaries between glacial and nonglacial sequences significantly improves the geologist's ability to predict the distribution of troublesome deposits between explorations, the potential for erosional contacts and concentrated boulders, the presence of faults and discontinuities, and other hazards to underground construction.

Identifying sequence boundaries requires determining the provenance (glacial or nonglacial) and age of the deposit. Most of the glacial and nonglacial sequences are beyond the range of radiocarbon dating and extensive reworking of sediment can make distinguishing between glacial and nonglacial deposits difficult.

Tools that have been found useful for determining the provenance of Puget Lowland Quaternary deposits include bulk geochemical analysis to identify characteristic signatures of sediment source areas, identification of warm or cold climate fossil pollen, macrofossils, and visible differences in mineralogy, lithology, texture, rounding, and sorting of particles. Dating methods that have been successfully employed to extend beyond the limits of radiocarbon include thermo- and optically-stimulated luminescence, amino acid racemization of shells, and Argon/Argon and fission-track dating of tephra. Paleomagnetic determination of remnant polarity has been used to coarsely date Quaternary sediments in outcrops for correlation with borehole samples.

Sampling requirements, cost, analysis time, error ranges, and effectiveness are variable so a comprehensive program benefits from multiple methods. The value of implementing these analyses is high and may significantly reduce geologic uncertainty yet cost only 10% of standard geotechnical testing budgets for large projects.