Shorelines rose as much as 7 meters along southern Puget Sound and Hood Canal between 500 and 1700 years ago. Evidence for this uplift consists of elevated wave-cut shore platforms near Seattle and emerged, peat-covered tidal flats as much as 60 kilometers to the southwest. The uplift was too rapid for waves to leave intermediate shorelines on even the best preserved platform. The tidal flats also emerged abruptly; they changed into freshwater swamps and meadows without first becoming tidal marshes. Where uplift was greatest, it adjoined an inferred fault that crosses Puget Sound at Seattle and it probably accompanied reverse slip on that fault 1000 to 1100 years ago. The uplift and probable fault slip show that the crust of the North America plate contains potential sources of damaging earthquakes in the Puget Sound region.

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I but some paleobotanical evidence indicates that the Homerian The nonmarine sedimentary rocks of Tertiary age in the Cbok Inlet region, once thought to be entirely of Eocene age, are shown by paleobotanical evidence to be mostly of Paleocene, Miocene, and Pliocene age. Our study of Clhickaloon floras confirms the Paleocene age of the Chickaloon Formation, as suggested by Barnes and Payne (1956). The Wishbone Formation has not yielded fossil plants, but its conformable and gradational relationship to the underlying Chickaloon Formation indicates that it is a t least partly of Paleocene age, although some rocks of Eocene age may be included. Our study of Kenai floras, shown to be mostly of Miocene and probable Pliocene age, confirms the suggestion of Barnes and Payne that two different coal-bearing rock sequences of disparate age may be represented by the Chickaloon Formation of the Matanuska Valley and the Kenai Formation of the Cook Inlet-Susitna Lowlands. The Tsadaka Formation, which rests unconformably upon the Chickaloon and Wishbone Formations, represents a marginal conglomeratic facies of the Kenai Formation; the fossil floras indicate that the Tsadaka Formation was deposited during the first half of the Miocene Epwh. Three new provincial time-stratigraphic units-the Seldovian, Homerian, and Clamgulchian Stages-are proposed. These units encompass all plant-bearing strata in Alaska and in adjoining parts of the same ancient floristic provinces that are of approximately the same age as those parts of the Kenai Formation represented in the type and reference sections designated in this report. Rocks belonging to these three stages are recognized and distinguished from one another primarily by fossil plants. The (Seldovian Stage is characterized by a rich and diversified warm-temperate flora containing many elements that are now exotic to Alaska but that were widespread during the Miocene. At least 23 fossil plant species appear to be restricted to the Seldovian Stage. Comparisons of different floras suggest that lower and upper subdivisions of the Seldovian Stage may be recognized. Paleobotanical correlations indicate that the Seldovian (Stage corresponds approximately to the lower half of the Miocene Series as recognized in northwestern conterminous United States and Japan, but some upper Oligocene rocks may also be included. The Homerian Stage is characterized by a less diversified and relatively provincial flora in which many of the exotic elements are lacking. At least 11 fossil plant species appear to be restricted to the Homerian Stage. The provincialim of the flora makes correlation in traditional Qmch-Series terms difficult, Stage corresponds a t least in part to the upper half of the Miocene Series ; some lower Pliocene rocks may also be included. m e Clamgulchian Stage is characterized by an extremely provincial flora that is depauperate in species of woody plants. Nearly all the warm-temperate exotic genera have disappeared. Three species of willow and one species of alder seem to be restricted to t...

At a marsh on the hanging wall of the Seattle fault, fossil brackish water diatom and plant seed assemblages show that the marsh lay near sea level between ∼7500 and 1000 cal yr B.P. This marsh is uniquely situated for recording environmental changes associated with past earthquakes on the Seattle fault. Since 7500 cal yr B.P., changes in fossil diatoms and seeds record several rapid environmental changes. In the earliest of these, brackish conditions changed to freshwater ∼6900 cal yr B.P., possibly because of coseismic uplift or beach berm accretion. If coseismic uplift produced the freshening ∼6900 cal yr B.P., that uplift probably did not exceed 2 m. During another event about 1700 cal yr B.P., brackish plant and diatom assemblages changed rapidly to a tidal flat assemblage because of either tectonic subsidence or berm erosion. The site then remained a tideflat until the most recent event, when an abrupt shift from tideflat diatoms to freshwater taxa resulted from ∼7 m of uplift during an earthquake on the Seattle fault ∼1000 cal yr B.P. Regardless of the earlier events, no Seattle fault earthquake similar to the one ∼1000 cal yr B.P. occurred at any other time in the past 7500 years.