PESA SA : Evening Technical Talk

Three-Dimensional Architecture of Wave-Dominated Delta Deposits

Brian Willis^1, Tao Sun² and Bruce Ainsworth³

1. Presenter, Willis Geoscience, Consultant, Longmont, CO, USA; 2. Chevron Technology Center, Research Fellow, Houston, TX, USA; 3. Adjunct Professor, University of Adelaide, Australia.

Sep. 29th 2025

Join PESA SA/NT for an Exclusive Distinguished Lecturer Event Featuring Brian Willis 

PESA SA/NT is proud to host Brian Willis, renowned fluvial sedimentologist and stratigrapher with decades of experience at Chevron and BP, as part of our Distinguished Lecturer Tour. This PESA Distinguished Lecture is made possible with the generous support of Santos Ltd.

Overview:

New numerical wave-influenced delta depositional models are challenging long-held rational for interpreting sea level variations recorded by wave-dominated shallow-marine successions. Shallow-marine, wave-dominated deposits (parasequences) are generally inferred to exhibit a decrease in wave energy and grain size with increasing water depth and to occur in facies belts that are laterally continuous for long distances along strike. Bedding geometry and vertical facies successions within these parasequences are interpreted in relation to a prograding equilibrium shoreface profile (cf. Bruun rule) and a gradual upward-coarsening facies progression (cf. Walther’s law). Sea-level fall is commonly inferred to generate a sharp-based shoreface succession, characterized by an abrupt vertical transition from heterolithic lower shoreface to sandy upper shoreface deposits across a marine erosion surface. A truncated vertical shoreface succession, capped by a marine erosion surface, is inferred to record significant wave ravinement during sea level rise and transgression.

Three-dimensional, process-physics-based, coupled hydrodynamic-morphodynamic wave-influenced delta models suggest that wave-dominated deltas will develop a sandy shoreface inner clinoform dipping from the subaerial delta plain to a relatively flat wave-scoured subaqueous delta top, which is laterally separated from a muddier delta front outer clinoform that dips from the subaqueous delta top edge to the shelf floor. As these systems prograde, deposits of these dual-clinoforms will become vertically stacked and will be separated by a regressive surface of marine erosion formed on the subaqueous delta top. Grain-size contrasts between these vertically stacked clinoform deposits reflect differences in sediment-transport directions and sorting under river- and wave-driven littoral currents along the coast, and cannot be uniquely related to sea-level changes. Gradual vertical facies successions develop where waves are less effective at reworking river-supplied sediment alongshore. In contrast, sharp-based shoreface deposits record more effective wave separation of sands onto the shoreface clinoform as muds are preferentially transported offshore onto the subaqueous delta clinoform.

The continuity of a regressive surface of marine erosion over many tens to hundreds of kilometres across mid-shelf regions of some stratigraphic sequences reflects a gradual lateral shift in the position of littoral current erosion on a subaqueous delta top. Timelines cross such vertical lithic discontinuities throughout the extent of a prograding deposit, and the regressive surface of marine erosion thus has little chronostratigraphic significance. The model results are used to suggest: 1) Characteristic strongly asymmetric wave-dominated parasequences suggest common river avulsion at the start of transgression, 2) Observed down-dip transitions from “gradual-based” to “sharp-based” shoreface deposits might record reduced sediment supply to the coast relative to rates of longshore drift as the system expands toward its auto-retreat limit, rather than transitions from normal to forced regression, and 3) Regional “top-truncated” shoreface successions might record progradation of dual-clinoform shorelines with wide deviation in net direction of regional sediment supply and shallow-water wave transport, rather than significant regional transgressive ravinement. The results of these models suggest caution in inferring sea-level changes from the character of vertical facies changes observed in individual well logs and isolated outcrop exposures. The models suggest new facies relationships that require testing in modern systems, outcrops, and subsurface examples.

Timings for the event:

5:30 to 6:00 – pre-talk drinks and canapes
6:00 to 7:00 – evening talk and Q&A
7:00 to 7:30 – post-talk drinks in the bar

PESA SA/NT will provide a selection of canapes. Beer, wine and soft drinks are available and can be purchased from the bar. 

Ticket prices: Students Free, Members $30, Non-Members $40, Fluvial Stratigraphy Workshop attendees Free

Please note that ticket sales will close on Thursday September 25th at 5:00 PM.