A quantitative assessment of the neoichnology and biology of a delta front and prodelta, and implications for delta ichnology
Abstract The Fraser River delta front exhibits asymmetry with respect to its neoichnological, biological, and sedimentological characteristics on opposing sides of the main distributary channel, as well as the submarine canyon seaward of the channel. This phenomenon is attributed to asymmetric tidal flow, wherein there is a net northward tidal flow along the delta front. The southern delta front lying updrift of the Main Channel, is primarily composed of sandy beds and bedsets that exhibit low bioturbation intensities (BI 0–3) and limited biodiversity (H` 0.9–1.4). Tracemakers in the updrift delta front predominantly consist of polychaetes and bivalves that produce suites dominated by simple vertical and horizontal traces attributable to the Skolithos Ichnofacies. The suites consist of traces comparable to Thalassinoides, Planolites, Skolithos, rare Conichnus, and Cylindrichnus. The northern delta front lying downdrift of the Main Channel, and the prodelta consist of muddy beds and bedsets that display high bioturbation intensities (BI 3–6) and diverse infaunal populations (H` 1.4–2.3). Polychaetes, bivalves, and echinoderms comprise the main tracemakers, and these infauna produce complex and robust ichnological suites typical of the Cruziana Ichnofacies. The downdrift delta front and prodelta trace assemblage includes traces comparable to Skolithos, Planolites, Thalassinoides, Artichnus, Palaeophycus, Conichnus, Asterosoma, Rosselia, Cylindrichnus, Gyrolithes, Teichichnus, Phycosiphon, Arenicolites, Polykladichnus, and Scolicia. The spatial distribution (area and volume) of biogenically induced sediment disruption indicates that the updrift delta front is significantly less bioturbated than its downdrift counterpart. Additionally, the prodelta does not exhibit evidence of asymmetry with respect to its neoichnological character; all spatial variability is recorded in delta front positions.