Stories of the Oldfield River According to local geologist, Darryl Sampson, the story of the Oldfield River really began about thirty-five to forty million years ago

"I think the river as we see it today has probably formed post those marine sediments so it's been during that fairly stable period. Probably from the mid to late Tertiary, which is about thirty-five million years ago. But then its course has probably changed quite a lot during those periods with the later fluctuations in sea level. What happens, as it's been flooded is that the sea comes in and the river has been flooded. Then with the following regression as the sea drops it could quite easily have changed its course as sediments have been deposited in the old channel.

During the drilling we did looking for groundwater around the Oldfield River we actually found old channels or sediments further to the west of the existing channel around Rockhole Road. So it has had a long history of movement, of migrating to the east, to the west with these variations. We could say that it is quite an old river. Some of the, what we term palaeochannels, in the eastern Goldfields were formed during the Tertiary around forty million years ago. They've since all been infilled by younger sediments. So I guess what we can say is that the Oldfield River as we see it today is probably quite young in geological terms but it has had a long history of development and redevelopment and migration.

All of the Ravensthorpe Range and the Bandalup Hill sequence and all of the granites to the north [of Munglinup] are part of the Yilgarn Block. The Yilgarn Block is essentially all the Archaean rocks, most of which date from 2.6 to 3 billion years old. [So the Oldfield River] starts in the Yilgarn, crosses the fault just north of Rockhole Road, it goes up through Tom Walker's place and cuts through his back paddocks, just north of his homestead. And so Tom is jumping about a billion years every day he drives across his farm (laughter).

That's the early history. The last movement on that fault was about 1.1 billion years ago as I understand it. Then there's a big gap, what happened between 1.1 billion years and the next record we've got in the geological history in this area is around about forty million years ago. So there's a big gap from 1.1 billion years to 40 million years. What's more than likely is that there were things deposited but they've all been eroded away, we don't see much of that history preserved.

A lot of the exciting fossil-rich outcrops that we see around the Munglinup area were formed when the Eocene sea forty million years ago was at its peak. Munglinup in those days would have been something like being in the Bahamas these days, a tropical environment with lovely warm waters with sponges. The diversity of fossils in those formations is quite large we have bivalves, gastropods, echinoids, bryozoans, and of course these sponges. Massive expanses of sponges, which would have formed on a shallow seafloor in very warm, tropical water.

The most common thing we see now is what everyone refers to as spongolite. The sponge spicules, the little silica needles from the sponges, are obviously well preserved because silica is quite resistant. In the weathering profile it will remain whereas a lot of the other more calcium carbonate-rich shells etc can break down over time and be absorbed through more acidic groundwaters. But even so we still see a lot of fossil-rich formations through the Munglinup area in the old Eocene sea sediments".
Darryl Sampson - geologist