Late Wisconsinan glaciomarine deposition and isostatic rebound, northern Puget Lowland, Washington

Abstract

The distribution and age of glaciomarine and marine sediment in the northern Puget Lowland, Washington, demonstrate that rapidretreatofcontinentalice,theEverson marineincursion,andhighratesofisostatic reboundoccurredbetweenabout13 600and 11 300 14 C yr B.P. (11.3 ka). Glaciomarine and marine deposits are thickest in zones where retreating ice lobes grounded, in the northeast Puget Lowland, and near large drainages. Glaciomarine sediment was depositedmainlyfrom(1)submarineoutwash in ice-proximal zones; (2) turbid underflows, dispersed melt water, icebergs, and resedimentation in transitional zones; and (3)dispersedmeltwaterandcurrentsinicedistal zones. Marine, estuarine, and emergence(intertidalandbeach)faciesaccumulated in areas more than 10 km from ice margins, particularly near major rivers. Molluscan and foraminiferal assemblages intheglaciomarineandmarinedepositsindicate that turbid, cool, brackish water covered much of the Puget Lowland during the Everson interval. Water was generally shallower (<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 sodium content of the gravel-free fraction do not clearly distinguish glaciomarine and marine deposits from terrestrial deposits. Isostatic rebound rapidly lifted the glaciomarine and marine deposits through sea level between about 13.5 and 11.3 ka. The presentaltitudesofradiocarbon-datedshell and the marine limit show that initial rates of isostatic rebound exceeded 10 cm yr 21 in thenorthernPugetLowland,butdroppedto 2cmyr 21 before 11 ka. The uplift gradient is about 0.6 m km 21 to the north and steepens locally to at least 1.3 m km 21 . The pattern of emergence in the northern Puget Lowland is anomalous locally, perhaps as a result of complex isostatic effects near the glacier margin, rapid rise of sea level, or tectonic deformation.