AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Two Fundamentally Different Types of Submarine Canyons: Modern and Ancient Examples from the Continental Margin of Equatorial Guinea, West Africa
(1) Stanford University (*now with Shell Oil Company), Stanford, CA.
(2) Stanford University, Stanford, CA.
(3) Hess Corp, Houston, TX.
Individual submarine canyons on the continental margin of Equatorial Guinea, West Africa have been active for more than 80 million years. During the late Cretaceous, a ‘classic,’ erosional, sand-rich, submarine canyon system dominated the margin. This system was abandoned during the Paleogene, but the relict topography was re-activated in the Miocene during submarine erosion associated with tectonic uplift. After this erosion, a decrease in sediment supply resulted in a drastic transformation in canyon morphology and activity, initiating the ‘Benito’ canyon system. This non-typical canyon system is aggradational, does not indent the shelf edge and has no downslope sediment apron. Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (> 150 m), dilute, turbidity currents. There is little evidence for erosion, mass wasting, or sand-rich deposition in the Benito canyon system. When a canyon loses flow access, usually due to piracy, it is abandoned and eventually filled. Fluid escape causes the successive formation of ‘cross-canyon ridges’ and pockmark trains along buried canyon axes during canyon abandonment.
The modern seafloor south of the study area is cut by the shelf indenting, erosive, sandy ‘Ceiba’ canyon that is morphologically similar to the Cretaceous canyon system. The Ceiba canyon also has an associated downslope submarine fan. Based on comparison of the Cretaceous, Benito, and Ceiba canyon systems, we recognize two main types of submarine canyons: ‘Type I’ canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand-rich fill, and large downslope submarine fans/aprons. ‘Type II’ canyons do not indent the shelf edge and exhibit smooth, aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass wasting, whereas hemipelagic deposition and dilute, sluggish turbidity currents are the main depositional processes sculpting Type II canyons. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and petroleum prospectivity of any submarine canyon.