Holocene depositional evolution of a small active-margin estuary of the northwestern United States

Abstract

Seismic reflection data (Uniboom) and C 14-dated core samples from a small activemargin estuary, Alsea Bay, Oregon, indicate that Holocene sediment fills an ancestral river valley which has an axial depth of 55 m below present sea level. Holocene sediment in the estuarine basin overlies late Pleistocene and late Pliocene(?) valley-fill deposits indicating several sequences of valley down-cutting, submergence and deposition. Core samples of Holocene fill from six core sites in the estuary were analyzed for age, embayment salinity indicators, bulk density, grain-size distribution, source (mineralogy) composition and sedimentary structures. Deposition of Holocene fill has been dominated by lateral channel migration and overbank suspension deposition. Three stages of deposition during the Holocene transgression are indicated on the bases of sedimentary structures, grain-size distribution and embayment salinity. These stages represent periods of transition from: (1) a shallow fluvial environment (10 × 10 3–7.5 × 10 3 yrs B.P.), sedimentation rate between 0.4 and 0.7 cm yr −1; to (2) a deep-water estuarine environment (7.5 × 10 3–5 × 10 3 yrs B.P.), sedimentation rate approximately 1.1 cm yr −1; to (3) a shallow-water estuarine environment (5 × 10 3–0 yrs B.P.), sedimentation rate approximately 0.21 cm yr −1. A major decline in sedimentation rate at about 5 × 10 3 yrs B.P.. follows a decline in the rate of eustatic sea-level rise and a corresponding decrease in sediment accumulation rate. The Holocene fill is comprised of approximately 70% river sand (2.7 × 10 3 tonnes), 20% river silt and clay (7.5 × 10 7 tonnes) and 10% beach sand (2.9 × 10 7 tonnes). Comparisons of modern terrestrial-sediment supply rates (2.8 × 10 5–3.2 × 10 5 tonnes yr −1) and long-term retention rates of terrestrial sediment (3.45 × 10 4 tonnes yr −1, average for the last 1 × 10 4 yrs) imply that the Alsea Bay basin has probably trapped <15% of the terrestrial sediment supplied to it.