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Following sand deposition into southern Vulcan Sub-Basin areas during the Maastrictian stage, the North West Shelf was located sufficiently north for carbonate production to occur in areas without clastic input.

In the Vulcan Sub-Basin MegaSurvey interpretation, the Base Tertiary horizon is at the base of a marl sequence (Palaeocene and Grebe Formation) that grades upward into carbonates, it is on top of mostly marine shale with minor localised sandstone (Cretaceous Puffin Formation) and represents the end of regional sag and related depositional marine shale and sand progradation.

The Base Tertiary horizon in the Carnarvon Basin is an unconformity lying between the shaley Lambert formation above and the Miria Marl Formation below. In the southwest the horizon becomes very shallow and flat, and its exact position becomes unclear because well pick times become unreliable above the first checkshot, and the section is contaminated by high energy sea bed multiples that mask the underlying signal of the horizon reflector.