COMPARATIVE EVALUATION OF DEFORMATION STRUCTURES EXPOSED IN THE DOUBLE BLUFF STRATIGRAPHY OF WHIDBEY ISLAND,WA

Comparative study of two sequences of deformation structures exposed in the stratigraphic record at Double Bluff beach on south Whidbey Island, WA indicates that their genetic origin is dissimilar.

Deformation structures found near the top of section in the glacial lacustrine clay units have distinct fold patterns with their respective axes aligned east-west and perpendicular to the inferred N-S ice flow direction of Puget Lobe over this location. These structures are typically found closely beneath the Vashon Till and are inferred to have a glaciotectonic origin.

In the lower and middle sections of the exposed stratigraphy at Double Bluff beach, a more extensive sequence of deformation structures are observed for almost 1.5 km in alluvial silts and sands described as part of the non-glacial Whidbey formation. These deformation structures have been interpreted by others as being related to paleoseisimicity, but have not been closely studied.

Some important lines of evidence that support a liquefaction origin for the lower deformation structures follows:

1. Liquefaction and water escape structures are most common in silt to medium-grained sand sized particles, which is the case for the deformed features observed in the lower section.

2. A combination of liquefaction and gravitational loading is more likely to occur in a stratigraphic setting where silty-sandy material (i.e., alluvium) is overlain by an impermeable layer such as glacial lacustrine silts or clays.

3. Unlike the glaciotectonic fold structures observed in the clay units in the upper section, the deformation structures studied in the lower section do not possess orientations consistent with shear stress of overriding Vashon ice.

It is our contention that the extensive sequence of deformation structures was not related to glaciotectonic processes during ice loading, but more likely was related to liquefaction processes including fluidization and loading during a paleoseismic event, predating the last glaciation.