DON J. EASTERBROOK Department of Geology, Western Washington State College, Bellingfiam, Washington 98225
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DON J. EASTERBROOK Department of Geology, Western Washington State College, Bellingfiam, Washington 98225 Pleistocene Chronology of the Puget Lowland and San Juan Islands, Washington ABSTRACT The Puget Lowland and San Juan Islands, lying tion of Russia, the Paudorf Interval of Europe and in the Puget Trough between the Cascade Range the Farmdalian of the midwestern United States. and the Olympic Mountains, were in the path of Esperance Sand, deposited by meltwater streams Pleistocene ice sheets which flowed southward in front of advancing ice during the Fraser Glacia- from Canada. tion and subsequently overridden during the Floodplain, silt, sand, and peat of the Whidbey Vashon Stade, was radiocarbon dated at 18,000 + Formation were deposited prior to 40,000 years 400 years. During deglaciation, the ice sheet ago, during an interglaciation believed to be thinned until it floated in marine water and poorly equivalent to the Puyallup and Sangamon Inter- sorted, fossiliferous glaciomarine drift was de- glaciations. Possession Drift, which overlies sedi- posited in the Everson Interstade. Shell-bearing ments of the Whidbey Formation, is radiocarbon sediment at altitudes up to 400 feet above present sea level and unfossiliferous glaciomarine drift up dated at 34,000 j^OOO years B'P' in "S Upper to 600 feet indicate that relative sea level at that part and is limited by a radiocarbon age of > 39,900 time was 500 to 700 feet higher. Eighteen radio- in its lower part. The Possession is correlated with carbon dates from marine shells in Everson glacio- late phases of the Zyriansk Glaciation in Russia marine drift vary from 10,370 ± 300 to 13,010 ± and the early Wiirm in Europe. 170 years. The Everson Interstade ended with Peat lying on Possession Drift yielded radio- lowering of relative sea level to approximately its present position and the disappearance of floating carbon dates between 27,200 + ^ and 22,700 ± ice. The following Sumas Stade ended shortly 550, representing the Olympia Interglaciation before 9,920 + 760 years ago. which is correlated with the Karginsk Interglacia- INTRODUCTION A large part of the region is directly under- lain by Pleistocene unconsolidated deposits. The Puget Lowland and San Juan Islands This paper deals with radioisotope chronology lie in the Puget Trough between the Cascade and correlation of Pleistocene deposits between Range on the east and the Olympic Mountains Seattle and the Canadian boundary. and Vancouver Island on the west (Fig. 1). Lying directly in the path of Pleistocene ice The Lowland is an elongate structural trough, lobes which extended southward from British modified by Pleistocene deposition and erosion, Columbia, the Puget Lowland and the San lying mainly between sea level and 500 feet. Juan Islands were glaciated several times. Ex- Marine water enters the region through the cellent sea cliff exposures along the many miles Strait of Juan de Fuca and extends about 100 of shoreline provide abundant evidence of the miles southward in Puget Sound and Hood Pleistocene stratigraphy of the region, and Canal. The San Juan Islands are part of a radiocarbon dating of peat, wood, and marine structural arch which transects the structural shells in many of the deposits allows firm estab- depression of the Puget Lowland and brings lishment of late Pleistocene absolute chro- to the surface Paleozoic and Mesozoic sedimen- nology. tary rocks and pre-Devonian crystalline rocks. Study of glacial deposits in the Puget Low- Much of the larger islands and parts of the land and San Juan Islands was supported by other islands are composed of these rocks, research grants from the National Science thinly mantled with glacial sediments. Thicker Foundation as part of an investigation of glacial and more extensive Pleistocene deposits are problems in northwest Washington. The field found at low elevations on many of the islands. seasons of 1960-62 were spent in the lowland Geological Society of America Bulletin, v. 80, p. 2273-2286, 2 figs., 2 pis., November 1969 2273 2274 D. J. EASTERBROOK—PLEISTOCENE CHRONOLOGY, WASHINGTON Maximum extent of glacier in Fraser Lowland during Sumas Stade Maximum extent of Cordil- leran glacier in Washington during Vashon Stade Boundary of physiographic subdivision Figure 1. Index map of the Puget Lowland and San Juan Islands. Region discussed in this paper is outlined with a heavy black line. between the Canadian border and Bellingham, DOUBLE BLUFF DRIFT the seasons of 1963-65 on Whidbey and Camano Islands, and parts of the summers of The oldest glacial deposits recognized in the 1964-66 in the San Juan Islands. The 1966-67 Puget Lowland between Seattle and British field seasons were spent studying deglaciation Columbia belong to the Double Bluff Drift near the Canadian boundary and alpine glacia- (Easterbrook and others, 1967; Easterbrook, tion in the Cascade Range adjacent to the 1968), which consists of till, glaciomarine drift, lowland. Unless otherwise indicated, radio- gravel and sand. The Double Bluff Drift is carbon dates were made by Isotopes Inc. of exposed in several bluffs near sea level on New Jersey from samples collected by the Whidbey and Camano Islands and on the writer. northwest corner of the Olympic Peninsula at For a discussion of previous investigations of Point Wilson. Pleistocene geology in the Puget Lowland, the There is little or no weathering of pebbles reader is referred to Armstrong and others in the drift, and no soils or weathering profiles (1965), Easterbrook and others (1967), and have been found at the top of the unit. In view Easterbrook (1968). Table 1 shows the se- of the lack of weathering, Easterbrook and quence of climatic events now recognized and others (1967), concluded that the drift on a suggested correlation with radiocarbon-dated Whidbey Island was not of great antiquity, sediments in the northern and central Puget but they were uncertain as to whether it Lowland and San Juan Islands. correlated with the Stuck Drift or one of the WHIDBEY FORMATION 2275 two units belonging to the Salmon Springs estimated. The upper surface of the Whidbey Drift of the southern Puget Lowland (Crandell Formation is typically somewhat irregular, and others, 1958). It now seems unlikely that varying from about 35 feet above sea level to the Double Bluff Drift correlates with the below sea level. In some places, beds are trun- Salmon Springs Drift, but a definite correlation cated by the overlying units, but how much with the Stuck Drift remains to be demon- of the Whidbey has thus been cut out by strated. erosion is difficult to estimate. Farther south, No deposits of the Double Bluff or older the Whidbey is, in places, as much as 250 feet glacial intervals have been recognized in the thick, suggesting that a significant portion of San Juan Islands, although the ice of each the Whidbey in the San Juan Islands has been glacial episode recognized farther south must removed by erosion. have passed over the islands. What may be the At its type locality and at many other sea oldest glacial drift in the islands is a strongly cliff exposures, the Whidbey Formation lies indurated till which occurs on the south shore upon Double Bluff Drift and is overlain by of Cypress Island and on Allan and Burrows Esperance sand. In a few places, however, Islands (Fig. 2). Possession Drift lies between the Whidbey and Esperance. WHIDBEY FORMATION The sediments of the Whidbey Formation The Whidbey Formation is named from suggest a floodplain environment (Hansen and deposits at Double Bluff on Whidbey Island, Mackin, 1949; Easterbrook and others, 1967). where it is represented by sand, silt, clay, and The silt and sand was deposited on extensive peat totaling several hundred feet in thickness flood plains and deltas. Peat beds interbedded (Easterbrook and others, 1967). Outcrops of with fine-grained sediment represent organic the Whidbey are widespread on Whidbey and material deposited in swampy areas on the Camano Islands and on the northwest corner flood plains. of the Olympic Peninsula near Point Wilson. The best exposures of the Whidbey Forma- Age and Correlation tion in the San Juans occur in sea cliffs along Twelve radiocarbon age determinations have the west shore of Guemes Island (Fig. 2) where been obtained from samples of peat and wood about 35 feet of interbedded silt, sand, and in the Whidbey Formation, but all were beyond peat is exposed above sea level (PL 1, fig. 1). the limits of conventional C14 dating. The ages The base of the Whidbey is not exposed above are listed in Table 2 and the sample localities sea level so the true thickness there cannot be are shown on Figure 2. Although correlation of the Whidbey with See Plate Section for all plates. stratigraphic units in the southern part of the TABLE 1. PLEISTOCENE SEQUENCE IN THE NORTHERN AND CENTRAL PUGET LOWLAND AND SAN JUAN ISLANDS, WASHINGTON GEOLOGIC CLIMATE UNITS STRATIGRAPHIC UNITS C14 AGE* Sumas Stade Sumas Drift -10,000 -11,000 Everson Interstade Everson Glaciomanne Drift FRASER GLACIATION -13,030 Vashon till Vashon Stade Esperance Sand -18,000 -23,000 OLYMPIA INTERGLACIATION Quadra Sediments -27,000 -35,000 SALMON SPRINGS GLACIATION Possession Drift -40,000 PUYALLUP INTERGLACIATION Whidbey Formation ? STUCK (?) GLACIATION Double Bluff Drift * Because most of the units are time-transgressive the ages shown are approximate. 2276 D. J. EASTERBROOK—PLEISTOCENE CHRONOLOGY, WASHINGTON Puget Lowland is not at present clearly de- Seattle. On the basis of the nature of the sedi- fined, some tentative conclusions may be ments, their thickness, and their distribution, reached. The nature and distribution of Whid- it is apparent that the Whidbey represents a bey sediments indicate that they once formed significant interval during which glacial ice part of an extensive floodplain that covered was absent from the lowland.