Geomorphologic indicators of sea level and lowstand paleo-shelf exposure on early–middle Miocene sequence boundaries

Relationships between clinoformal sequence boundaries and relative sea level change, in particular locations of lowstand shorelines and amounts of paleo-shelf exposure, represent a key topic in sequence stratigraphy. This paper uses 3D seismic data to analyze morphologies of two sequence boundaries in the early–middle Miocene section of the Northern Carnarvon Basin of the Australian Northwest Shelf. A step-like discontinuity is recognized on the outer paleo-shelf of sequence boundary DLS4 based on reflection truncation and seismic facies change. Vertical offset at this step varies from ~ 7 to 65 m, increasing along strike from SW to NE. The trace of this discontinuity comprises both linear and arcuate segments in coherence time slices. Isolated mounds and small platforms also occur on this sequence boundary, basinward of the discontinuity. Underlying sub-sequence boundary DLS3.1 displays a smaller step-like discontinuity, together with U- and V-shaped incisions as well as numerous, isolated depressions, the latter occurring within two large arcuate, concave-basinward features on coherence slices. We discuss several possible origins for these discontinuities. We conclude that step-like discontinuities on DLS4 and DLS3.1 represent buried wave-cut terraces or sea cliffs. Erosion is presumed to have occurred at or near ambient sea level. The incisions of DLS3.1 are karst. Together, these interpretations imply significant lowstand paleo-shelf exposure of early–middle Miocene sequence boundaries.