Problems encountered with the 14c dating of peat
Based on projection of seismic-reflection data (figs.
8, 10), the southern fault of the zone (inferred to be the most active strand and the northern margin of the Everett basin) extends through an area characterized by large landslides and variable exposure on Whidbey Island’s east coast and a marshy lowland on the island’s west coast.
At c, the Whidbey Formation is overlain by crudely stratified gravel assigned to the Possession Drift and interbedded sand (parallel bedded and crossbedded) and silt assigned to the nonglacial Olympia beds (figure 6). Peat from these strata is too old ( 48,480 14C yr B. Dragovich (written commun., 2000) obtained four radiocarbon dates from the same unit at four localities within 2.5 km north of location c (figure 27) along Whidbey Island’s northeast coast.
Moreover, an aeromagnetic anomaly that defines the basin margin (figure 3) in Skagit Bay lies just south of these islands, on the southern margin of a small, prominent, shallow (43,510 14C yr B. About 200 m farther north (b in figure 27), similar sand and gravel facies exposed in a landslide scarp strike 70° and dip 17° NW.Based on their sedimentology, these strata are interpreted as interglacial alluvial-plain deposits. These strata, and the peat-bearing strata exposed at a, are correlated with the Whidbey Formation, the oldest known interglacial alluvial-plain deposits recognized on Whidbey Island and in the northern Puget Lowland (Easterbrook, 1994a, b). The lithology and sedimentology of these strata are identical to those of well-documented Whidbey Formation localities elsewhere on Whidbey Island (see, for example, Easterbrook, 1968, 1994b; Stoffel, 1981). Subsurface data (figure 30, see next section) from this local northern Whidbey Island area show that strata occurring at similar elevations and with similar lithology are overlain by two units of interbedded “hardpan” (the term drillers use for dense clay-rich diamict), gravel, and sand interpreted as glacial drift.The peat yielded radiocarbon dates of 40,130 14C yr B. The Whidbey Formation predates both the Possession and Vashon Drifts, whereas the younger nonglacial Olympia beds predate only the Vashon Drift (figure 6).In contrast, younger (marine isotope stage 3) nonglacial Olympia beds (figure 6) are absent (due to nondeposition or erosion) in several wells and in most outcrops on Whidbey Island.Sea level at this time was much lower than at present and in stage 5 (Pillans and others, 1998), significantly limiting both stratigraphic accommodation space and preservation potential. As an alluvial-plain deposit, the top of the Whidbey Formation was a nearly horizontal surface across northern Whidbey Island following deposition.