On the Evolution of Submarine Canyons; A Case Study from the Taranaki Basin, New Zealand

The Plio-Pleistocene section of the Taranaki basin is characterized by numerous submarine canyons associated with a series of northeast progradational wedges. These depositional episodes, known as “The Great Foresets” are characterized by clinoforms that dip at approximately 2.8 degrees. The relief of these foresets is approximately 400 m. Multiple deeply-incised submarine canyons are observed with each set of progradational wedges. These canyons can be mapped from where they initiate with tributive drainage patterns inboard of the shelf edge, all the way to the basin floor. In some instances the tributive networks initiate at apparent slump scars associated with broad and shallow slope failure. Typical canyon widths range from 1.5 to 2.0 m.

The canyons commonly contain turbidite channel-fill deposits at their base only, with no apparent sand-prone deposits associated with subsequent canyon fill. These basal turbidites are characterized by meandering channels that show evidence for both swing and sweep within the canyons. Some canyons are flat-floored with channel deposits impinging on both sides, whereas others are v-shaped in transverse profile and are narrower. It is inferred that the flat-floored canyons had achieved a graded or equilibrium profile such that downcutting erosion and deposition were in balance and only lateral erosion by channel sweep was occurring. The axial slope of such canyons is typically 1.4 degrees. In contrast, the v-shaped canyons are steeper, with axial profiles ranging from 1.7 to 2.0 degrees. These canyons are inferred to have not reached their ultimate graded profile before they were abandoned.

Common to all the canyons observed was that at their up-system limit each tributary canyon arm was characterized by a straight channel segment extending approximately 1.0 to 1.5 km downsystem. In every instance the straight-channel segments grade into moderate to high-sinuosity channel patterns. The presence of straight-channel segments at their up-dip limits suggests that sediments initially were associated with mass transport processes before giving way to turbulent flow downsystem.

Also common to nearly all canyons observed was the fact that the only apparent coarse-grained fill was located at the base of each canyon fill. The absence of coarse grained (i.e., channel-fill) deposits within the rest of the canyon fill suggests that the coarse-grained supply to the canyons shut down abruptly in each instance.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California