Cyclic fluvial sedimentation in a rapidly subsiding basin, northwest Washington

Abstract

The Eocene Chuckanut Formation of northwest Washington comprises as much as 6000 m of alluvial strata and is one of the thickest nonmarine sequences in North America. The Bellingham Bay Member forms approximately the lower half of the Chuckanut Formation; it is characterized by repetitive fining-upward cycles consisting of a lower coarse-grained and an upper fine-grained member. Two types of cycle have similar vertical profiles but can be distinguished by the nature of their coarse member. Coarse members of the first type are thick (> 2–3 m), are commonly multi-storey, have apparent sheet geometry and are interpreted as the deposits of meandering-river channels and point bars. Coarse members of the second type are thinner (< 2–3 m), have apparent ribbon geometry and are interpreted as floodplain (crevasse?) channel deposits. Down-basin changes in the Bellingham Bay Member include a major decrease in grain size and an increase in fine-member thickness and cycle-recurrence interval. These changes reflect decreased flow competence and increased stability of channel banks. Similar trends might be found in other ancient alluvial sequences and could be useful in studies of both basin geometry and coal exploration. Sediment-accumulation rates are a minimum of 50 cm 1000 yrs −1, implying rapid subsidence. This subsidence probably had a major control on the sedimentation style of the Bellingham Bay Member, which resembles computer-generated models of alluvial stratigraphy at high subsidence rates (Allen, 1978).