Glacio-isostatic Response in the Puget Lowland By : Robert M. Thornson Presentation : Esten King

Background Goal : Extend the marine based glacio-isostatic record farther south into the Puget Sound. Examines proglacial lake shorelines to differentiate isostatic rebound from possible tectonic activity. Focuses on the areas covered by the Puget Lobe.

Puget Lobe Confined by the Cascade Mountains and Olympic Mountains. At maximum extent 14ka, the Puget Lobe was 100km wide and 1,200m thick. Proglacial lakes filled all of the troughs of the Puget Lowland created by ice dams. Flow direction is very well known for Puget Lobe because of many streamlined features in lowland. Shoreline features from the Proglacial lakes are used as control points for post glacial warping. More on that later.

Loading History Reconstruction of the Puget Lobe based on; boundary of Vashon till, orientation of basal streamlined features, stratigraphic and topographic evidence, and comparison with modern analogs. Map of maximum glacier thickness was prepared by subtracting a smoothed present topography from the reconstructed Puget Lobe Stratigraphic and topographic evidence is maximum limit along constraining mountains. The map of maximum thickness relies on modern topography which is a fair assumption since Vashon till layer is relatively thin (10m) and the limited volume of recessional outwash.

Isopach Thickness Map Reconstruction of the ice thicknesses of the Puget Lobe. Assumption that basal and surface flowlines were the same are justified because nearly all of the motion occurred through basal sliding. Surface of the lobe was broadly convex.

Postglacial Retreat 14,000ka – Sedro Woolley 13,650ka and 13,430ka – Lake Washington 13,100ka – Vancouver Island These dates are interpreted as a rapid deglaciation of the Puget Lobe around 13.5ka. Dates are from Postglacial peats found directly after deglaciation and give a good proxy for time of deglaciation.

Receding Puget Lobe Proglacial lakes occupying one or more of the Puget Sound troughs grew as the ice sheet retreated. The lakes grew as long as the lake was north of its controlling spillway. Shorelines were diachronic since they would have been exposed progressively as the ice sheet retreated. Rapid recession of the Puget Lobe decreased the diachronicity.

Reconstruction of Shorelines Proglacial lakes shorelines are used to reconstruct water plane levels. Present differences in elevation of features that formed at the same water plane must have experienced subsidence or uplift. Only works for shoreline features that can be assigned to water planes independent of their present elevation.

Requirements for Reconstruction 1. Controlling spillway cannot be directed over glacial ice. 2. Potential erosion must be minimal 3. Shoreline features must be assigned to a specific water plane. 4. Shoreline features must be separated so that tilt can be detected Only glacial lakes Russell, Hood and Bretz meet all 4 conditions.

The different lakes are controlled by the different spillways The different lakes are controlled by the different spillways. Lake russell through the black lake spillway and lake bretz through the leland creek spillway.

Lacustrine Delta Reconstruction The other method of assigning water planes used ice proximal gravel fan deltas prograding into recessional lakes or the sea. Relative ages are clear because of proximity to ice. The deltas do not correspond to exactly lake level, but 1-4m below. There were fan deltas coming off of the cascades and olympics but its harder to give relative ages to them. The contacts between the topset and forset beds are well exposed. The delta corresponds to the level at which stream flow could no longer transport gravel by bed shear.

Marine Delta Reconstruction Marine limit rises northward from 40m at Leland Creek to 140m at the Canadian Border. Three marine fan deltas in close proximity (15km) to the Leland Creek Spillway. Used to estimate the marine limit at the same time as Lake Bretz.

Black Lake Spillway Drains west from the southern Puget Sound through the Chehalis Valley. Served as base level for Lake Hood, Lake Russell, and smaller lakes. The general shallowness of the spillway channel (<10m) reduces error associated with erosion. Flow depth and spillway erosion did not exceed 4m in the spillway channel. The channel had the capacity to carry the discharge during the 4 month melting season.

Leland Creek Spillway Drains north in between Quilcine Bay and Discovery Bay. At its modern peak the Leland Creek Spillway (68.6m) sits above the Black Lake Spillway (40.1m). Uplift must have occurred at Leland Creek or else drainage would have continued through the lower Black Lake Spillway. Drainage from Lake Hood reversed direction to the Leland Creek Spillway creating a new water plane level and Lake Bretz.

Triangles represent deltas Triangles represent deltas. Open triangles and vertical arrows represent minimum estimates.

Contour map showing net vertical uplift Contour map showing net vertical uplift. Assumes a -100m global sea level. Lacustrine deltas (triangles), spillways (circles), marine deltas (squares).

Cross section of uplift along A-A’ Cross section of uplift along A-A’. Dashed lines (maximum error) stippled (realistic error).

Cross section from B-B’ Cross section from B-B’. Shows broad convexity due to thickness of the ice sheet.

Conclusion As the Puget Lobe ice sheet retreated glacial lakes grew recording water plane levels. Glacial lake shorelines and gravel fan deltas were used to reconstruct water plane levels. Glacial lakes were defined by their spillway. Isostatic rebound height depends on ice thickness.