Quantitative seismic geomorphology of sediment conduits on an evolving Miocene slope in Taranaki Basin (New Zealand): The influence of increasing slope gradient through time

Abstract

Submarine canyons, channels and gullies are conduits that transport sediments across shelf-slope margins to deep water. In South Taranaki Basin, an increase in sediment supply through the Miocene resulted in progradation and significant steepening of the slope system. Previous studies have identified numerous sediment conduits developed within this system, however their morphology and morphometric relationships with the depositional slope have not been considered. Here we apply seismic geomorphology to establish the statistical relationships between the metrics of sediment conduits at three stratigraphic intervals between which the slope gradient progressively increased. In the early-Middle Miocene, sinuous upper Moki Formation channel complexes with an average width of 1.1 km developed on a slope with an average gradient of 0.2o, routing sediment from south to north. By the late-Middle Miocene, the slope began to prograde rapidly, concurrent with a regional reorientation of the slope to the northwest, on which the lower Mount Messenger Formation canyon networks developed with a slope gradient of 0.4–1.0o. At shallow slopes of less than 0.5o, canyon morphometrics (mean width 6.6 km) are 1.8–4.7 times larger than on related upper slopes with gradients steeper than 0.5o (mean canyon width of 2.7 km). This significant shift in morphometrics occurs abruptly across the clinoform toe line. Rapid Late Miocene slope progradation resulted in the development of steep clinoform slope surfaces up to 9o, into which linear upper Mount Messenger Formation gully complexes incised. The mean gully width throughout the Middle to Late Miocene interval decreased from 1.3 km to 1.0 km as the slope gradient became steeper. This study documents how the morphology and morphometrics of sediment conduits on the South Taranaki Basin slope system changed through time in relation to changes in depositional slope gradients.