Abstract

The transportation and deposition of sediment in upper Turnagain Arm is mainly the result of strong tidal currents that develop in response to the maximum spring tidal range of 11.4 m. Mineralogical evidence showing an extra-arm source for much of the intertidal sediment indicates that the effects of the flood tide dominate the sediment dispersal system. Flooding usually commences with a tidal bore that travels at velocities of 300 cm/sec or more.

The intertidal sediment is mainly very well sorted very fine sand and coarse silt of quartzo-feldspathic composition. Throughout much of the area, sediment accumulates in bars oriented parallel to the axis of the estuary. Textural patterns reflect the interaction of bar geometry and the decrease of current energy toward the periphery of the estuary. Sedimentary structure facies also correlate with current energy levels and with velocity controlled bedform sequences demonstrated in flume experiments. Progradation would produce the following fining-upward sequence: parallel laminated sand → mixed parallel and cross laminated silty sand → cross laminated silty sand → organic-rich parallel laminated silt and sandy silt. The “Paleocurrent” pattern deduced from measurements of bedform orientations at low tide show strong maxima about 90 degrees apart and bipolar plots for cross lamination. The pattern reflects changing current direction controls as a function of tidal stage.

Identifying a deposition setting similar to Turnagain Arm should be possible through the recognition of the assemblage of process-related features described in this study.