Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Animal-Sediment Interactions and Trace Assemblages on the Asymmetrical Fraser River Delta Front and Prodelta, British Columbia, Canada
(1) Earth Sciences, Simon Fraser University, Burnaby, BC, Canada.
Neoichnological and sedimentological characteristics of the Fraser River Delta front differ updrift (north) and downdrift (south) of the main river distributary. Sediments on the updrift side of the delta front comprise heterolithic sand and mud interbeds with typically low bioturbation intensities (BI 1-4). Sands are dominant at shallow depths (25 to 75 m), below which mud beds are increasingly common. Sediments on the downdrift side of the delta front constitute nearly 100% mud from 25 m to 150 m water depth with higher and more variable bioturbation intensities (BI 0-6). Bioturbation increases with depth everywhere on the delta front. In the prodelta (150 m to 300 m water depth), the updrift and downdrift regions exhibit similar sedimentological and ichnological characteristics, and are mainly intensely bioturbated (BI 4 to 6). The distinct differences in the sedimentological, and neoichnological characteristics of the updrift and downdrift delta front result from asymmetrical tidal flow across the delta. In turn, tidal currents are the dominant control on the sedimentation rate and distribution of sediments (sand versus mud) . As such, the Fraser River Delta is considered to be a tide-influenced asymmetrical delta.
Trace assemblages are also compared to infaunal diversities across the delta front and prodelta. The largest burrowing invertebrates are the holothurians, irregular (heart) urchins, and crustaceans. These infauna are commonly found in association with other burrowing animals, including polychaetes, bivalves, nemerteans and sipunculids. The polychaetes, heart urchins and holothurians are the dominant infaunal groups. Based on the morphology of the traces and the ethology of the infauna, several traces-tracemaker associations are made. Irregular urchins produce traces akin to Scolicia and Bichordites. Holothurian burrows are morphologically similar to Teichichnus, Asteresoma, Artichnus, Arenicolites and Diplocraterion. Crustaceans produce Thalassinoides and Ophiomorpha. Bivalves produce Siphonichnus, and some polychaetes and nemerteans produce traces similar to Skolithos and Cylindrichnus. Infaunal diversity is generally higher and more variable on the updrift delta front, while infaunal diversity on the downdrift delta front is low and more consistent both alongshore and across-shore.