Sediment Budgets and Depositional Processes Influencing Submarine Canyon Systems, Equatorial Guinea, West Africa

Abstract

Sediment supply to the Equatorial Guinean continental margin consists predominantly of northbound littoral-drift derived mud and sand from the Ogooue River in Gabon. Locally, sediment is sourced from the Mitemele and Benito rivers, low discharge rivers ending in large estuaries. The sediment flux is low, and delivery of sand to the deep sea is limited to a few locations where canyon heads erode into the shelf edge. These canyons are erosive, sand rich, and terminate in extensive submarine aprons (Type I canyons). The vast majority of submarine canyons along the margin do not indent the shelf edge, are muddy and aggradational, and lack any downslope sediment apron/fan (Type II canyons). Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the Type II canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (>150 m), dilute, turbidity currents. This study attempts to reconstruct sediment budgets and routing systems for the Equatorial Guinean continental margin and determine the causal mechanisms for variations in temporal sediment flux. Planktic foraminifera from two cores from the modern seafloor, obtained in water depths of 268 and 497 m, provide radiocarbon ages which indicate an average sedimentation rate of ~30 cm/ky (centimeters per kiloyear) during the last ~40,000 years. This relatively slow accumulation rate does not seem to be influenced by changes in relative sea level, supporting the interpretation that hemipelagic deposition is the dominant process aggrading the Type II canyon system. A long term (8 Ma - Recent) sedimentation rate previously calculated in the study area (3.4 cm/ky) is 10x slower than the short term rate calculated in the present study. This discrepancy in rates may be a manifestation of the Sadler effect, a theory that predicts decreasing sedimentation rate as the measured time interval increases. Although the overall sedimentation rate during the last 40 ka averages ~30 cm/ky, there is an abrupt increase during the 15–10 ka time interval to 80–180 cm/ky. We investigate possible causal mechanisms for this increase using sea surface temperature and salinity proxies derived from foraminiferal trace metal data. Constraining temporal and spatial sediment flux to the study area will aid in understanding the development of low sediment supply continental margins with Type II canyons.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015