Miocene shelf-edge deltas and their impact on deepwater slope progradation andmorphology, Northwest Shelf of Australia
C.M. Sanchez, C.S. Fulthorpe and R.J. Steel
Journal name: Basin Research
Issue: Vol 24, No 6, December 2012 pp. 683 - 698
Info: Article, PDF ( 1.61Mb )
Late-middle Miocene to Pliocene siliciclastics in the Northern Carnarvon Basin, Northwest Shelf of Australia, are interpreted as having been deposited by deltas. Some delta lobes deposited sediments near and at the shelf break (shelf-edge deltas), whereas other lobes did not reach the coeval shelf break before retreating landward or being abandoned. Shelf-margin mapview morphology changes from linear to convex-outward in the northern part of the study area where shelf-edge deltas were focused. Location and character of shelf-edge deltas also had significant impact on alongstrike variability of margin progradation and shelf-edge trajectory. Total late-middle and late Miocene margin progradation is ca. 13 km in the south, where there were no shelf-edge deltas, vs. ca. 34 km in the north where shelf-edge deltas were concentrated. In the central area, the deltas were arrested and accumulated a few kilometres landward of the shelf break, resulting in an aggradational shelf-edge trajectory, in contrast to the more progradational trajectory farther north. This illustrates a potential limitation of shelf-edge trajectory analysis: only where shelf-edge deltas occur, there is sufficient sediment available for the shelf-edge trajectory to record relative sea-level fluctuations reliably. Small-scale (ca. 400 m wide) incisions were already conspicuous on the coeval slope even before deltas reached the shelf break. However, slope gullies immediately downdip from active shelf-edge deltas display greater erosion of underlying strata and are wider and deeper (>1 km wide, ca. 100 m deep) than coeval incisions that are laterally offset from the deltaic depocenter (ca. 0.7 km wide, ca. 25 m deep). We interpret this change in slope-gully dimensions as the result of greater erosion by sediment gravity flows sourced from the immediately adjacent shelf-edge deltas. Similarly, gullies also incised further (up to 6 km) into the outer shelf in the region of active shelfedge deltas.