Date of Award
Program or Major
Doctor of Philosophy
Larry A Mayer
Seismic-reflection data, bathymetric data, and sediment cores were used to map the seafloor geology of eastern Juan de Fuca Strait, and interpret the stratigraphy in terms of the latest deglacial episode and associated sealevel change. The surficial geologic units comprise bedrock (unit 1), ice-contact diamicton (unit 2), glacial-marine sediments (unit 3), and post-glacial sediments (unit 4). Bedrock crops out near Vancouver Island, and diamicton crops out in the numerous morainal banks. A series of banks running roughly north-south in the middle of the strait divides it into two areas based on the main surficial units; post-glacial sediments dominate to the east and glacial-marine sediments to the west. Subunits within the glacial-marine sediments suggest progression from an ice-proximal to ice-distal depositional environment during glacier retreat. There is currently little sediment input to the strait, so most modern sediments consist of reworked glacial deposits that occur in banks and coastal exposures.
The strait was deglaciated rapidly; in about 100 years at a rate of 475 m/yr. Ice retreat was probably episodic, however, with times of rapid calving retreat separated by periods when retreat paused on morainal banks.
After the period of maximum marine submergence following deglaciation, isostatic rebound caused relative sealevel to fall to a level lower than present. A number of drowned features indicate sealevel fell to 55 m below present sealevel (-55 m) by 11,280 yr B.P., then reached the low stand maximum of around -60 m by 10,720 yr B.P. During regression, sealevel fell 150.4 m at an average rate of 59.0 mm/yr; meanwhile the eustatic rise was 37.3 m, yielding an average rate of crustal uplift of 73.6 mm/yr. During the subsequent transgression, sealevel rose to a depth of -55 m by 10,630 yr B.P., then a depth of -44 m by 9,880 yr B.P., and -33 m by 8,910 yr B.P. by 10,700 yr B.P. most isostatic adjustment was complete and eustatic rise dominated, resulting in a gradual transgression toward the present. Relative sealevel rose 60.4 m during transgression at 5.6 mm/yr, while the eustatic sealevel rise was 61.4 m, giving an average rate of crustal uplift of less than 1 mm/yr.
Hewitt, Antony Trevor, "Seafloor geology, deglacial history, and early post-glacial evolution of eastern Juan de Fuca Strait" (2002). Doctoral Dissertations. 66.