The Role of Diagenesis in Exfoliation of Submarine Canyons

On the lower slope of the U.S. continental margin offshore New Jersey, submarine canyons incise carbonate sediments, and the canyon morphology is characterized by steep, nearly vertical walls, linear and flat canyon floors, stepped terraces, ''U''-shaped cross-sectional profiles, and the lack of well-developed tributary gullies. Canyon morphology contrasts sharply with that of canyons in the upper and middle slope that are cut into siliciclastic sediments and have ''V''-shaped profiles, narrow sinuous thalwegs, and dendritic tributary gullies.A research program was undertaken to study the role that diagenesis and the physical properties of bedrock have in formation and growth of submarine canyons in a carbonate setting. Alvin dives were conducted on the New Jersey lower slope where canyons incise siliceous chalks or chalk rocks and siliceous porcellanitic chalks. Morphological canyon components such as steep walls, flat floors and terraces, linear chutes, and joint surfaces were observed and sampled. Volume reduction resulting from fluid expulsion due to the opal-A to opal-CT diagenetic transformation, which occurs with progressive burial of the silica-rich chalks, contributes greatly to fracturing of the bedrock. Fracturing at all scales is oriented subparallel and vertical to bedding in the porcellanites, and vertical in the chalk rocks. Unroofing of overburden and canyon excavation by mass-wasting causes exfoliation and diagenetically generated fractures to expand. Once canyon formation begins, it continues development by positive feedback since there is a continued stress-release fracturing: loss of support leads to failure, and erosion continues by spalling and sliding of blocks from the extensively fractured rocks.Results obtained in this study provide a timing for fracture formation in deep-sea, silica-rich chalks. Visual observations and sampling provide an understanding of fracture orientation, scale, and network communication. Observations of modem sedimentary processes in a canyon/slope carbonate setting help to understand ancient environments.