The nature of inclined heterolithic stratification in a mixed tidal-fluvial setting: depositional processes, sedimentology and ichnology, Middle Arm, Fraser River, Canada

The dynamic interplay of hydrodynamic processes in the subequally mixed tidal-fluvial Middle Arm, lower Fraser River dictates the nature of inclined heterolithic stratification (IHS) developed on channel bars. Intertidal zone and upper subtidal zone sediments are deposited in mm- to cm-scale, rhythmically alternating sand and mud beds, and this trend appears to continue to the lower subtidal part of the bar. Rhythmic laminasets are interbedded with dm-scale sand beds that reflect seasonal changes in river discharge. Surface samples from the intertidal zone are mud-dominated on the upstream and downstream ends of each bar, and the center of each bar is sandy such that bars exhibit a mud-sand-mud profile. Muddy bedsets in the intertidal zone are laterally extensive for up to 1 km in the along-strike direction, and muddy bedsets in the subtidal zone appear to extend for similar distances. Ichnological trends reflect the relative input and persistence of brackish-water in the channel. Burrow diversity, burrow density, and trace size increases in the seaward direction.