Rhythmically Bedded Glaciomarine Sediments on Whidbey Island, Washington

Abstract

ABSTRACT Several facies of late Pleistocene glaciomarine sediments on Whidbey Island, Washington, are rhythmically bedded. The first consists of stratified silty sand which fines upward, transitionally, into pebbly mud. Several successions, each approximately 0.6 m thick, comprise a proximal meltwater facies thought to have been deposited in a shallow (< 10 m) submarine-fan environment. Both the positive (lobe shape) morphology of the exposure and the inferred depositional processes support this view. Underflow currents, either high-concentration turbidity currents or grain flows, are thought to have deposited the silty sand while flocculation, settling from turbid overflow and/or interflow plumes, and ice rafting were responsible for pebbly mud deposition. Such inferences are based on text ral analysis and observations of sedimentary structures. The rhythmic interbedding is explained by invoking seasonal fluctuations of meltwater discharge. This caused a variation of depositional process in areas proximal to meltwater input into the basin. Rhythmic interbedding is also observed within a channelized sequence of gravel, sand, and mud, which comprise a fining-upwards succession that includes turbidites. Laminations of sand and silt found in the upper portion of this section closely resemble the type of varved bedding commonly reported from glaciolacustrine sequences.