ESTIMATING MIS 5 MARINE TERRACE INNER EDGE ELEVATIONS AT NORTHERN NEWPORT, OR

Accurate measurements of marine terrace inner edge elevations constrain Quaternary peak local sea-level high stands and allow local tectonic uplift rate estimation (Muhs et al., 1990; 1992; Simms et al, 2016). Marine terraces between Siletz Bay and Yaquina Bay, Oregon, are the northernmost expression of Marine Isotope Stage (MIS) 5a, 5c, and 5e high stands along the US Pacific coast (Kelsey et al., 1996), and help to constrain the southern margin of the North American ice complex since the Last Interglacial (Creveling et al., 2017). However, these MIS 5 terrace inner edges are obscured by colluvium and vegetation, leading to uncertainty in the local sea-level high stand elevation. Previous authors assumed average platform slopes to project a range of inner edge elevations (Kelsey et al. 1996; Bradley and Griggs, 1976).

We acquired Trimble R12 RTK GPS points along an exposed unconformity between the MIS 5a platform (‘Newport terrace’) and the overlying beach colluvium at locations near Yaquina Head where the modern sea cliff erodes perpendicular to the shoreline. We used a laser rangefinder where the platform was not directly accessible. Since there is no identifiable paleo sea cliff, we projected the slope angles computed between GPS point pairs to a minimum distance corresponding to the furthest upslope mapped MIS 5a deposits and a maximum distance corresponding to the furthest downslope mapped MIS 5c deposits. We assumed that there are no unmapped faults or changes in slope between the GPS points and the inner edge. After removing outliers above the modern topography (>70m), we constrained the average minimum and maximum MIS 5a inner edge elevation of 41.2 ± 6.58 m to 42.6 ± 7.87 m (1σ), respectively. To further support our projection for the MIS 5a inner margin, we projected near-coast GPS points to the inferred depth of the platform surface in upslope well logs. This yielded a mean minimum and maximum inner edge of 39.7 ± 0.18 m to 40.3 ± 0.25 m (1σ), respectively. The former and latter methods predict a local uplift rate for the north side of Yaquina Head of ~0.43-0.63 versus ~0.49-0.51 m/kyr, respectively for the last 80 kyrs. Finally, we present more speculative conclusions for MIS 5c and 5e inner edge elevations based on the assumption that they share the same platform slope and morphology as what we determined for the MIS 5a terrace.