GEOLOGIC QUADRANGLE MAPPING AND STUDIES OF QUATERNARY STRATIGRAPHY IN THE PUGET LOWLAND NEAR EDMONDS, WASHINGTON

The Edmonds East 7.5’ quadrangle and adjoining parts of the Edmonds West quadrangle north of Seattle, Washington, were mapped in cooperation with the University of Washington as part of regional geologic studies of the Puget Lowland. Data were acquired through fieldwork; carbon-14, thermoluminescence, and infrared stimulated luminescence dating; analyses of drilling records and laser terrain imagery (LIDAR); and evaluation of existing geologic maps. Late Pleistocene age strata underlie the entire map area and mainly consist of lodgment till, sandy outwash, and lake sediments deposited during the last major advance (Vashon Stade) of the Cordilleran Ice Sheet. Vashon till is particularly extensive, capping broad uplands and draping obliquely across underlying units near the shorelines of Puget Sound and Lake Washington. Pre-Vashon glacial and interglacial sediments are exposed in stream valleys and coastal bluffs. Holocene deposits are sparse, except for widespread landslides along the coast.

Our key findings are: (1) Vashon recessional outwash is more abundant than shown on previous maps, forming fluvial and deltaic deposits graded to ice-marginal lakes in Puget Sound and Lake Washington. New LIDAR imagery greatly facilitated mapping of these deposits in the southern half of the area. (2) LIDAR also revealed gross inaccuracies in existing topographic base maps in densely forested areas. (3) Till exposed in cliffs south of Richmond Beach previously was correlated incorrectly with the pre-Vashon Possession Drift; it actually consists of Vashon till that drapes westward into Puget Sound. (4) Some of the oldest strata in the map area, which are exposed near the base of bluffs south of Edwards Point, yield luminescence minimum ages of about 100 and 200 ka; these ages correlate with the Whidbey Formation and Double Bluff Drift, respectively. (5) An undated lacustrine unit exposed discontinuously in the southeastern part of the map area apparently dips southward about 0.5 degrees between Scriber Creek and Lake Washington; this exceeds the inclination expected from postglacial rebound and thus possibly reflects tectonic tilting. Further study is warranted, because the outcrops lie near the eastward projection of the Kingston arch and slightly south of the southeastward projection of the south Whidbey Island fault.