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Pockmarks on the Modern Seafloor as Indicators of Submarine Canyon Abandonment, Offshore Equatorial

Z. R. Jobe and D. R. Lowe
Stanford University, Stanford, CA, [email protected]

Pockmarks are cone-shaped depressions found on the modern seafloor that have been interpreted to form by pore fluid (gas and/or water) expulsion from underlying sediments. Pockmarks range in size from 10 m in diameter and 1 m in depth to over 4 km in diameter and 200 m in depth. Pockmark locations can be seemingly random or related to various subsurface features (e.g. faults, submarine channels). Pockmarks were discovered almost 40 years ago and are well-known seafloor features, but their formation and evolution still remains elusive.

A high-resolution 3D seismic reflection dataset offshore Equatorial Guinea (1.6° N) in west Africa sheds light on pockmark formation, evolution, and association with submarine canyons. Pockmark morphology is circular, averaging 400 m in diameter and 40 m in depth, and the pockmarks are organized into linear trains. It is unclear whether gas or water is responsible for these pockmarks. Pockmarks are created when a submarine canyon infills with fine-grained sediment due to abandonment by turbidity flows. Pockmark evolution commences along an abandoned canyon that develops cross-canyon ridges. These ridges evolve into elongate, peanut-shaped double pockmarks and finally into discrete pockmarks that are aligned in a train. This evolution produces long-lived (>1 m.y.) features that are predominantly aggradational.

Six pockmark trains, in various stages of pockmark evolution, are seen on the modern seafloor and are shown to be related to previously abandoned canyons in the subsurface. The pockmark trains directly overlie and follow the map pattern of abandoned canyons. Due to their association with abandoned submarine canyons, pockmarks on the modern seafloor can assist in locating possible hydrocarbon exploration targets. The longevity of pockmarks suggests they can be indicators of not only shallow subsurface features, but also deeper features associated with hydrocarbon accumulations.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009