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

Jobe, Zane R.; McGann, Mary M.; and Lowe, Donald R.
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Sediment supply to the Equatorial Guinean continental margin consists predominantly of northbound littoral-drift derived mud and sand derived 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, Jobe et al., 2010, doi: 10.1016/j.marpetgeo.2010.07.012). 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.

Two cores from the modern seafloor, in water depths of 400 and 800 m, were sampled, sieved, and picked for Globigerinoides ruber, a planktonic foraminifer preferred for radiocarbon dating. The samples were analyzed at the Center for Accelerated Mass Spectrometry (CAMS); errors were consistently less than 1%. The obtained radiocarbon ages indicate a stable sedimentation rate of 30-40 cm/ky (centimeters per kiloyear) through the last 20,000 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. Jobe et al. 2010 calculated a sedimentation rate of 3.4 cm/ky for the last 8 million years (mid Miocene to present), a value an order of magnitude smaller than those 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. Future work aims to extend this study to a sand-rich Type I canyon system in the study area in order to compare the efficacy of different sediment routing systems and the active depositional processes along the west African coast.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013