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The Valanginian Unconformity marks the end of a period of continental breakup and the onset of Northwest Shelf subsidence and marine sedimentation. Breakup was evident in the Gascoyne and Cuvier Abyssal Plains whilst following the event, sedimentation of the Birdrong and Mardie Sandstones occurred in small deltas. This in turn was followed by deposition of the Muderong shale which acts as a regional seal.

Within the Vulcan Sub-basin, the Valanginian Unconformity represents the Base of the Bathurst Island Group, which provides the regional seal in the area. The unconformity is overlain by Lower Cretaceous shales and is underlain by variable age and variable lithology formations.

In the Carnarvon Basin the Intra Valanginian Unconformity horizon lies below the thick early Cretaceous Muderong Shale sequence. However, while the lithology immediately above and below the unconformity varies between sand and shale formations, frequently sand is juxtaposed with sand or shale with shale, so the acoustic impedance contrast is small. Analysis of synthetic seismograms shows that responses vary between peak and trough in equal numbers, which is consistent with variable lithologies. In places the upper and lower Flag Sands lie above and below the unconformity respectively, whereas in other areas the Lower Muderong Shale overlies the Forestier Claystone. The importance of the horizon is its stratigraphic proximity to the Flag sands and the Barrow group, as well as the Birdrong Sand and Mardie Greensand above the unconformity. Collectively these are one of the most important reservoir sequences in the basin.