Imagine a vast, submerged landscape off the coast of Western Australia, a place largely unseen and largely unknown. This is the Naturaliste Plateau, a colossal submarine feature spanning some 90,000 square kilometres, nestled in depths ranging from 2,000 to a staggering 5,000 metres. It’s a place that whispers tales of ancient geological drama, a story etched in rock and sediment far beneath the ocean's surface.
For years, scientists have been piecing together the puzzle of this enigmatic plateau. Early seismic surveys and drilling in the 1970s offered the first tantalizing glimpses. Deep Sea Drilling Project (DSDP) wells, specifically sites 258 and 264, unearthed a layered history of sedimentary rocks, stretching from the Cretaceous period all the way to the Miocene. While these wells didn't quite reach the plateau's foundational rock, samples from DSDP site 264, specifically volcaniclastic conglomerates, hinted at an oceanic origin. But then, a twist! Dredging expeditions in later years, like those by Marion Dufresne and Southern Surveyor, brought up samples of Cambrian granite and orthogneiss from the plateau's steep southern edge. This led to a significant shift in thinking: the current view is that the Naturaliste Plateau, or at least a substantial part of it, is built upon continental basement.
Looking closer at the seismic data, which, admittedly, is a bit sparse, we see that most of the plateau is blanketed by hundreds of metres of sediments, predominantly from the Cenozoic era. However, some seismic lines have revealed smaller rift basins, filled with up to 2.5 kilometres of sediment. Correlations with Ocean Drilling Program (ODP) wells suggest the upper layers in these basins are Cretaceous, but the age of the older, thicker sequences – potentially over a kilometre deep – remains a bit of a mystery, possibly dating back to the Jurassic to Early Cretaceous periods.
Structurally, the Naturaliste Plateau is quite complex. It's essentially a fragment of continental crust that was rifted apart during the Jurassic to Early Cretaceous, later modified by volcanic activity towards the end of the Early Cretaceous. The southern part of the plateau shows more faulting and erosion compared to its northern counterpart. Seismic profiles clearly image various rift basins, often shaped like half-grabens, bounded by steep, ENE-oriented normal faults. These basins can stretch for up to 120 kilometres and are typically 10 to 30 kilometres wide. The largest of these is found in the southeast, a series of en-echelon rift segments, while smaller, NE-trending basins dot the northern flank.
Interestingly, the northwestern part of the plateau is dominated by a large, circular topographic high. Without more detailed seismic data, this feature has been interpreted as a volcanic complex, adding another layer to the plateau's geological narrative.
When it comes to petroleum potential, the story is a bit more subdued. The small rift basins identified are thought to contain up to 2.5 kilometres of Late Jurassic to Early Cretaceous sediments. While this is a significant thickness, it's likely not enough for any potential source rocks to have become mature enough to generate hydrocarbons. So, for now, these basins aren't considered highly prospective. However, if more modern seismic data reveals significantly thicker sediment fills, this assessment might need a rethink. The eastern edge of the Naturaliste Plateau does brush up against the western Mentelle Basin, where recent assessments have indicated the presence of hydrocarbon-generating source rocks and potential traps. It’s a reminder of how interconnected these geological features can be.
The Naturaliste Plateau, therefore, isn't just a featureless expanse of seabed. It's a dynamic geological entity, a testament to the Earth's powerful forces, holding clues to our planet's ancient past, waiting for us to uncover more of its secrets.
