LATE WISCONSINAN GLACIOMARINE DEPOSITION AND ISOSTATIC REBOUND, NORTHERN PUGET LOWLAND, WASHINGTON

The distribution and age of glaciomarine and marine sediment in the no rthern Puget Lowland, Washington, demonstrate that rapid retreat of co ntinental ice, the Everson marine incursion, and high rates of isostat ic rebound occurred between about 13 600 and 11 300 C-14 yr B.P. (11.3 ka). Glaciomarine and marine deposits are thickest in zones where ret reating ice lobes grounded, in the northeast Puget Lowland, and near l arge drainages. Glaciomarine sediment was deposited mainly from (1) su bmarine outwash in ice-proximal zones; (2) turbid underflows, disperse d melt water, icebergs, and resedimentation in transitional zones; and (3) dispersed melt water and currents in ice-distal zones, Marine, es tuarine, and emergence (intertidal and beach) facies accumulated in ar eas more than 10 km from ice margins, particularly near major rivers. Molluscan and foraminiferal assemblages in the glaciomarine and marine deposits indicate that turbid, cool, brackish water covered much of t he Puget Lowland during the Everson interval. Water was generally shal lower (<30 m) in the southern part of the area and deeper (15-60 m) to the north. Mineralogy and geochemical properties such as boron or sod ium content of the gravel-free fraction do not clearly distinguish gla ciomarine and marine deposits from terrestrial deposits. Isostatic reb ound rapidly lifted the glaciomarine and marine deposits through sea l evel between about 13.5 and 11.3 ka. The present altitudes of radiocar bon-dated shell and the marine limit show that initial rates of isosta tic rebound exceeded 10 cm yr(-1) in the northern Puget Lowland, but d ropped to 2 cm yr(-1) before 11 ka. The uplift gradient is about 0.6 m km(-1) to the north and steepens locally to at least 1.3 m km(-1). Th e pattern of emergence in the northern Puget Lowland is anomalous loca lly, perhaps as a result of complex isostatic effects near the glacier margin, rapid rise of sea level, or tectonic deformation.