Ages, distributions, and origins of upland coastal dune sheets in Oregon, USA

摘要

A total of ten upland dune sheets, totaling 245 km in combined length, have been investigated for their origin(s) along the Oregon coast (500 km in length). The ages of dune emplacement range from 0.1 to 103 ka based on radiocarbon (36 samples) and luminescence (46 samples) dating techniques. The majority of the emplacement dates fall into two periods of late-Pleistocene age (11-103 ka) and mid-late-Holocene age (0.1-8 ka) that correspond to marine low-stand and marine high-stand conditions, respectively. The distribution of both the late-Pleistocene dune sheets (516 km~2 total surface area) and the late-Holocene dune sheets (184 km~2) are concentrated (90% of total surface area) along a 100 km coastal reach of the south-central Oregon coast. This coastal reach lies directly landward of a major bight (Heceta-Perpetua-Stonewall Banks) on the continental shelf, at depths of 30-200 m below present mean sea level (MSL). The banks served to trap northward littoral drift during most of the late-Pleistocene conditions of lowered sea level (-50±20 m MSL). The emerged inner-shelf permitted cross-shelf, eolian sand transport (10-50 km distance) by onshore winds. The depocenter sand deposits were reworked by the Holocene marine transgression and carried landward by asymmetric wave transport during early- to mid-Holocene time. The earliest dated onset of Holocene dune accretion occurred at 8 ka in the central Oregon coast. A northward migration of Northeast Pacific storm tracks to the latitude of the shelf depocenter (Stonewall, Perpetua, Heceta Banks) in Holocene time resulted in eastward wave transport from the offshore depocenter. The complex interplay of coastal morphology, paleosea-level, and paleoclimate yielded the observed peak distribution of beach and dune sand observed along the south-central Oregon coast.