GEOTECHNICAL DRILLING IN A BURIED FJORD PROVIDES EVIDENCE OF MULTIPLE ISOSTATIC MOATS MARGINAL TO COAST MOUNTAINS, SW BRITISH COLUMBIA, CANADA DURING CORDILLERAN ICE SHEET GROWTH, MARINE ISOTOPE STAGES 4 TO 2

Geotechnical test drilling for a 2 km LRT tunnel east of Burnaby Mountain in the greater Vancouver area, SW British Columbia, Canada discovered that the tunnel follows the long axis of a buried fjord (Evergreen fjord (EF)). The sedimentary fill in EF has a maximum thickness of more than 160 m and it extends more than 37 m below present sea level (extant). EF is in the Fraser Lowland, marginal to the Coast Mountains. Sediments within it record growth and decay of the Cordilleran Ice Sheet and their effects on isostatic depression and rebound.

The oldest EF sediments (> 54.5 ky BP) are tentatively assigned to MIS 4 (Semiahmoo Glaciation). Till, muddy gravel and silt with dropstones indicate that glacial ice twice entered the EF during MIS 4: 75 m of generally stoneless silt was deposited between the two incursions when the area was distal from glaciers. Relative sea level rose at least 80 m above extant during this glaciation.

Woody peat, tree roots and pedogenic soil in cores mark MIS 3. ¹⁴C ages between 54.5-26.8 ky BP were determined on these sediments. After ca. 26.8 ¹⁴C ky BP, subaerial peat 48 m above extant was inundated by fine silty sand which coarsened upward into ice-contact gravel and diamicton. This marks the onset of the Coquitlam Stade of the MIS 2 Fraser Glaciation (dated elsewhere between ca. 20 and 18.5 ky BP). Terrain 68 m above extant became emergent and vegetated during the ca. 18.5 ky BP Port Moody Interstade (PMI). Relative sea level then rose rapidly with onset of the Vashon Stade of Fraser Glaciation, burying the PMI organic deposits beneath slackwater sediments. Thick sand deposited in the fore of the advancing Cordilleran Ice Sheet reached at least 115 m above extant prior to the deposition of the Vashon Till. This sand contains dropstones so its upper limit gives a minimum estimate of relative sea level.

The record supports the existence of a Coast Mountains-marginal isostatic moat in this area: it flexed with waxing and waning of the Cordilleran Ice Sheet through at least two glacial cycles. Referred to estimates of global eustatic sea level over MIS 4-2, moat depression was at least 160 m during MIS 4 and the onset of MIS 2. Assuming a maximum eustatic draw-down of 120 m during MIS 2, it may have exceeded 200 m immediately after PMI. They reflect low mantle viscosity and rapid ice sheet buildup and wasting.