Sedimentology and neoichnology of a muddy translating point bar in the fluvio-tidal transition, Serpentine River, BC, Canada

Abstract

Point bars are common in the fluvio-tidal transition of rivers, yet detailed characterization of their sedimentology and neoichnology is rarely reported. Herein, we present sedimentological and ichnological descriptions for a seaward translating point bar located in the fluvio-tidal transition of the Serpentine River, BC, Canada. This bar accumulates in mainly polyhaline water, and channel flow is tide-dominated. The landward bar has steep channel banks and grain size fines from medium sand in the thalweg to mud in the upper intertidal zone. The seaward side of the bar forms a bench-like morphology that is steeply inclined in the intertidal zone, has a wide, gently dipping platform in the lower intertidal/uppermost subtidal zone, and a steep lower subtidal channel bank. Sediment sizes fine from coarse- to medium-grained sand in the thalweg, to mud in the lower intertidal zone. Medium-grained sand is deposited on the middle- and upper-intertidal zone of the seaward-side of the bar as a result of flow reversal and impingement of the flood tidal current against the seaward side of the translating point bar.The neoichnological assemblage of the intertidal part of the translating point bar is characteristic of brackish-water environments, and comprises low diversities of diminutive structures that occur locally in high densities (Bioturbation Indices 3–4). The neoichnological distribution correlates closely with the macrobenthos distribution, which in turn indicates marine-nutrient distribution. Marine nutrient supply is likely controlled by the bar's morphology. Biogenic structures attributable to Arenicolites, Skolithos, Polykladichnus and Siphonichnus mark the middle and upper intertidal zones, as well as the lower intertidal zone along the landward side of the bar. The trace makers that produce this trace assemblage are resilient to prolonged subaerial exposure and employ diverse feeding strategies. The lower intertidal zone along the seaward side of the bar is affected by decreasing current velocities across the wide bench-like morphology, which permits regular deposition of marine nutrients. Here, population counts are high and trace makers specialize in surface deposit- and filter-feeding. Traces include Gyrolithes-like structures and Siphonichnus. The Gyrolithes and Gyrolithes-like burrows that are produced by S. kowalevskii at the study site are hypothesized to potentially reflect the trace maker's anatomical predisposition, and not represent some spatial or anchoring advantage.This work represents the first published study on the neoichnology of a tide-dominated, translating point bar in the fluvio-tidal transition. The model we present is intended as an analog to show the variability in fluvio-tidal point bars and how they might be manifested in the sedimentary record.