USING SURFICIAL DEPOSITS TO CONSTRAIN QUATERNARY FAULT MOVEMENT IN COLORADO: INSIGHTS FROM RELATIVE DATING AND COSMOGENIC RADIONUCLIDE DATING (Invited Presentation)

In Colorado, the timing of Quaternary fault activity is determined primarily from relative dating methods, such as soils geomorphology and geomorphic relations. Surficial deposits are often assigned an age by correlation with different glacial advances. Fault activity is inferred from the age of the surface that it cuts. Differentiating glacial deposits associated with the Pinedale (MIS-2) or Bull Lake (MIS-6) glaciations, based on their landform morphology, is more straightforward than assigning ages for glacio-fluvial or fluvial deposits. As faults in Colorado cut across both glacio-fluvial fans and fluvial strath terraces, it is difficult to accurately constrain the timing of Quaternary fault activity. The deposition of glacio-fluvial fans is undoubtedly paced by glacial cycles, as great amounts of sediment are released during glaciations. However, the relation between glacial maxima and maximum fan aggradation rates is not clear. It is likely that climatically-driven changes in sediment supply also play a role in the cutting, deposition, and abandonment of Quaternary strath terraces. However, recent measurement of both ²⁶Al and ¹⁰Be profiles on strath terraces along the Colorado Front Range reveal long occupation of terrace surfaces, spanning several glacial-interglacial cycles. It is therefore difficult to assign a single age to a terrace surface or to correlate ages with glacial advances, as the “age” at which a terrace was abandoned can be much younger than the “age” at which the bedrock strath was cut. Furthermore, cosmogenic radionuclide dating of terraces demonstrates that river incision differs between basins, which challenges the use of terrace correlation across the region as a dating method. As the chronology of surficial deposits throughout much of the region lacks numeric age constraint, the geohazards associated with Quaternary faulting in the region are also poorly constrained.