Bedding types in Holocene tidal channel sequences, Knik Arm, Upper Cook Inlet, Alaska

Abstract

Uplifted convoluted and horizontal to subhorizontal beds of varying thickness in intertidal silt as old as 3,280 + or - 90 yr B.P. are exposed in the banks of tidal channels of unknown depth in the intertidal zone in Knik Arm of Upper Cook Inlet. Internal discordances may occur both within convoluted beds and between convoluted and horizontal to subhorizontal beds. At the base of many convoluted beds, there is a rapid gradation upward into laminae which are severely deformed; that is, in some places, the contortions appear to have originated along a single bedding plane. Where the convoluted sequences are truncated by nearly horizontal sequences, the distortion must have resulted from syndepositional or postdepositional events prior to their burial by the overlying beds. Various forms of gravitational and tidal processes caused the deformation of the Knik Arm deposits; events triggering the movement of the sediment include any or all of the following: (1) seismic activity, (2) sediment or ice loading, (3) wave-induced liquefaction, (4) pore-pressure changes brought on by changing levels of tidewater, (5) undercutting of sediments by channel migration, and (6) freezing and thawing.