Problems encountered with the 14c dating of peat
At the end of Whidbey deposition, paleo-relief on this surface in local areas like this small part of northern Whidbey Island would have been at most a few meters, reflecting erosion and deposition associated with fluvial channels and levees.Modern alluvial-delta plains of the Puget Lowland (such as the Skagit delta a few kilometers east of Whidbey Island) provide a useful analog. In both correlation diagrams, the top of the Whidbey Formation is interpreted as a gently undulating though relatively horizontal surface; thus erosion into the Whidbey Formation by advance outwash facies of the Possession Drift appears to have been fairly limited and (or) uniform across this small area of northern Whidbey Island.Additionally, virtually all coastal-bluff exposures on Whidbey Island and on the northeastern Olympic Peninsula have been examined for this and previous investigations (Johnson and others, 1996, among others) to provide a framework for recognizing structural deformation in Quaternary strata.Subsurface information comes from lithologic logs of water wells (figure 28; table 1).The Whidbey Formation is relatively thick and widespread on Whidbey Island (Easterbrook, 1968), and is present in every well that reaches its stratigraphic level.This occurrence reflects its deposition during a period of relatively high sea level (marine isotope stage 5; Muhs and others, 1994; Pillans and others, 1998), a condition which generates significant stratigraphic “accommodation space” (Jervey, 1988; Posamentier and others, 1988) in which sediments can accumulate and have high potential for preservation in the stratigraphic record.Inland, the projected “basin-margin” fault strand is covered by vegetation and surficial deposits developed mainly on Vashon Drift (Pessl and others, 1989).
For this study, we used a data base, maintained by the Island County Health Department, that includes logs for more than 3,500 water wells.
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.
P.), whereas an organic silty sand sample yielded a finite age of 41,380± 2,150 14C yr B. and organic material in silt yielded a finite age of 40,590 ± 530 14C yr B. These two finite ages from organic sediment are very close to the practical limit of radiocarbon dating, and they may have been contaminated by minute amounts of younger carbon.
East-west stratigraphic correlation diagrams on the north and south sides of the inferred trace of the Devils Mountain fault on northern Whidbey Island (locations, figs. The diagram north of the fault includes 19 wells within 1.4 km of the inferred fault trace and one surface outcrop (figs. Stratigraphic units inferred on the correlation diagram include pre-Whidbey Formation Pleistocene deposits, the Whidbey Formation, Possession Drift, Olympia beds, Esperance Sand Member, main body of Vashon Drift, and Everson Drift (figure 6).