Problems encountered with the 14c dating of peat
8-26), aeromagnetic data (figure 3), and onshore geologic mapping in the Skagit delta (Whetten and others, 1988).With this information, we examined exposures (figure 27) and subsurface data (figure 28, table 1) to more precisely locate faults and constrain fault histories, lengths, and slip rates.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.
[back to contents] The approximate locations of the Devils Mountain, Strawberry Point, and Utsalady Point faults on Whidbey Island are inferred based on projections from offshore seismic-reflection data (figs.
Table 1 is a list of wells (including depth and location) used to draw stratigraphic correlation diagrams for this investigation.
The Devils Mountain fault zone is not exposed on Whidbey Island.
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.
Search for problems encountered with the 14c dating of peat:
Inland, the projected “basin-margin” fault strand is covered by vegetation and surficial deposits developed mainly on Vashon Drift (Pessl and others, 1989).