Geomorphic Reconstruction of Alki Point in West Seattle Based on Chronostratigraphic Relationships of Mid-Late Holocene-Aged Intertidal Deposits

Abstract

A tombolo may have tied a bedrock knoll at the western tip of Alki Point to the mainland before a large magnitude 7 or greater earthquake ruptured within the Seattle fault zone, resulting in ~23 ft of uplift at Alki Point and exposing shallow beach/intertidal deposits. The purpose of this study was to (1) reconstruct the geomorphology of Alki Point during the mid-late Holocene in order to understand the conditions under which marsh deposits at Alki Point were formed, and (2) partially characterize the spatial variability of marsh deposits at Alki Point. The motivation behind this work was to provide future paleoseismic studies a basis as to where reliable paleoenvironmental data within marsh deposits can be found. This data can be used by paleoseismologists to reconstruct the abrupt or gradual nature of environmental changes that may be related to coseismic uplift or other environmental factors. Data from three sites at Alki Point and one at a marsh near Restoration Point on Bainbridge Island include; (1) stratigraphic observations from borings and trench observations, (2) radiocarbon ages, and (3) elevation data from microfossil assemblages. Two-foot contours constructed over LiDAR data for Alki Point show low topographic relief trending northwest-southeast where a tombolo may have connected a bedrock knoll near the western tip of Alki Point to the mainland, with beach ridges shoreward of and parallel to a suspected tombolo. The direction of the shore-drift at Alki Point is from the south, driving the propagation of waves towards the southern shore of Alki Point. These factors would be the most important control on sediment transport, and would have assisted in shaping a tombolo along the southern margin of Alki Point. A sample taken 6 ft above MLLW from a tree log in the middle of a peat layer shoreward of the current beach ridges had a radiocarbon age of 6264 to 6529 cal yr B.P. This would have required that one or more beach ridge(s) were further offshore from where this peat would have deposited in a quiescent intertidal environment. As sea levels rose to near to the present level ~5500-6000 cal yr B.P., the beach ridge(s) would have transgressed to the current position. The formation of a tombolo with shoreward beach ridges may have allowed for one or more marshes to form on Alki Point since shore-drift derived from the south. Ages on beach deposits below the middle peat layer at Alki Playground are between 3078 to 4826 cal yr B.P., and the age on the upper peat layer at Alki Playground is modern (after 1950 AD) to 297 cal yr B.P. This suggests that the middle peat layer encountered at two of the Alki Point sites, was deposited sometime between 297 to 3078 cal yr B.P., after a tombolo would have formed.