Abstract
Middle to upper Miocene siliciclastics (correlative with the Bare Formation) in the Northern Carnarvon Basin, Northwest Shelf of Australia, correspond to shelf and shelf-edge deltas that initially prograded across a preexisting carbonate shelf. Mapping of these deltas using 3D seismic data reveals lobes that display complex clinoform characteristics. Deltaic clinoform heights vary from ~40 m to 100 m. 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-edge trajectory analysis combined with the mapping of individual lobes suggest that the stratigraphy was built by a long-term (~2 m.y.) regressive phase, involving a basinward shift of deltaic deposition and producing shelf-edge deltas, followed by an aggradational episode that moved deltaic deposition landward relative to shelf edges. These trends appear to correlate with third-order global sea-level cycles.

Shelf-margin mapview morphology changes from linear to convex-outward in the northern area where shelf-edge deltas were focused. Incisions were already conspicuous on the coeval slope even before deltas reached the shelf-break. Nevertheless, slope gullies immediately downdip from the shelf-edge deltas display greater erosion of underlying strata and are wider and deeper (>1 km wide, ~100 m deep) than coeval incisions that are laterally displaced from the deltaic depocenter (~0.7 km wide, ~25 m deep). We interpret this change in slope-gully morphology as the result of greater erosion by sediment gravity flows sourced from deltas that had reached the shelf edge. Similarly, interpreted deep-water slope and basin sands are channelized in the northern area, where deltas were sited at the shelf-edge and slope gullies were larger and incised further (up to 6 km) into the outer shelf. In contrast, deep-water deposits along strike in the central area are represented by localized high-amplitude seismic facies (probable mass transport), where the slope was dominated by collapse and/or bypass. The wider and deeper slope gullies in the northern area were likely efficient at funneling sandy sediment flows from the deltas to the basin where they eroded channels. In the central area, the deltas were deposited a few kilometers landward of the shelf-break, contributing to shelf-edge aggradation rather than to slope progradation. Localized occurrence of shelf-edge deltas had significant impact on along-strike variability of margin progradation. Total late-middle and late Miocene margin progradation is ~13 km in the south where there are no shelf-edge deltas. In contrast, shelf-edge progradation is ~34 km in the north where shelf-edge deltas were concentrated.