NEOGENE SEDIMENTATION IN THE WILLAMETTE VALLEY, OREGON

The Willamette Lowland consists of four major depositional basins. Three of the basins, the Portland Basin, the Tualatin Basin, and the central Willamette Valley, occur on the downwarped surface of mid-Miocene Columbia River Basalt Group lava. A fourth basin, the southern Willamette Valley, is largely erosional. The basin-fill sediment thickness locally exceeds 400 meters. Lithofacies mapping, based on over 3,000 well logs, provides insight into sediment sources, depositional history, and structural development.

The majority of basin-fill deposits are clay and silt with minor sand, but sand and gravel predominate where major drainages emerge into the basin forming broad alluvial fans. Deposits in the southern and central Willamette Valley include multiple large alluvial fans and appear to be primarily, but not exclusively, of Cascade Range origin. Deposits in the Portland Basin include fine- to coarse-grained sediment of both Cascade Range and Columbia River provenance. Deposits in the Tualatin Basin, in contrast, are almost entirely fine-grained and of Coast Range provenance.

Grain-size distribution indicates that the major drainages entering the Willamette Lowland from the Cascade Range have existed in their present locations during the entire period of deposition, and are the principal source of coarse-grained sediment. Coarse sediment dominates the entire stratigraphic section at the apices of the present alluvial fans. These proximal fan deposits rest directly on the underlying bedrock, and locally extend to below sea level. Coarse sediments grade into fine-grained deposits downstream and away from fans. The stratigraphic section includes at least two beds of coarse sediment that extend from the fans out into the basin suggesting episodic pulses of increased sediment from the Cascade Range.

Grain-size distribution in the vicinity of the Mount Angel fault in the central Willamette Valley suggests that the fault has been active during much of the history of the basin. A region of coarse sediment that extends to below sea level occurs on the downthrown side the fault, indicating local structural control of sedimentation for some period of time.