Sedimentary

 Today we went to two more fossil localities on our journey northwards in the North Island. First up we turned off the main highway about 16 kms north of Wanganui city and travelled along a gravel road beside the Whanganui river for a few kilometres. We parked up beneath some cliffs alongside the road, known to geologists as a great place to see the “Wilkies Shellbed”. It consists of a thick sequence of densely packed fossil oysters, that were living about 2.5 million years ago. There are also other fossils to be found, but the oysters predominate and must have been part of an enormous oyster bed. They would have grown on top of earlier, dead shells, gradually building up the thick sequence into a large oyster reef. The same layer can be found over an area about 50kms across! The photo shows Marianna next to a rock made up almost entirely of the giant oysters. top number two was further north again. We followed state highway 4 all the way up past the volcanoes to Taumaranui, where we turned off to meet the Whanganui river again at Paparoa Rapids. Right beside the river, about 50m downstream from a waterfall on the opposite side, we found some beautiful large fossil shells. The whitish colour stood out from the surrounding grey rock, making them relatively easy to find. These rocks are a bit older than the Wilkies Shellbed, about 11 to 13 million years old. They were laid down in fairly shallow water perhaps 20 – 50 metres deep, that covered a large part of the central North Island at the time, well before the volcanoes existed. In the photo here, you can see some shells as well as a dark tube shaped fossil which may be a piece of wood or the lining of an animal burrow. We weren’t quite sure from a first inspection. Tomorrow we head further north again to hunt more fossils.

Today I am on a fossil hunting tour to check out some of the best fossil hunting sites in the North Island, along with James Crampton and Marianna Terezow. They have been writing a book for New Zealand fossil hunters and we are giving the locations a final check to make sure the details are correct in the book chapters. Our first location is Ototoka Beach, about 20kms west of Wanganui. This location is great for fossil collecting. The cliffs are jam packed with shells of different types. They are relatively young, about 2.4 to 1.5 million years old, and most of the species can still be found alive today. What is particularly interesting about this locality is that you can see changes in the sediments as you look up the cliffs. The fossils and rock types indicate changes in water depth occurring in cycles. These are sea level changes that have been shown to be due to climate cycles (ice ages) and reflect the accumulation of ice on land (low sea levels) alternating with with deeper seas when the ice melted during the interglacials. over 45 of these cycles have been recognised from the rocks in this part of New Zealand. As we looked over the rock beds we could see some thin layers of lignite (poor quality coal) about 5cm thick. (see as a thin dark horizontal band about 3/4 of the way up this photo) These would have formed in a coastal swamp environment. Further up the cliff there were beds of shells showing that the sea level had gone up, and above them were mudstones indicating a water depth of perhaps 50 – 100 metres. Scientists are able to make these estimates because similar shells are still living and are therefore able to be used for direct comparison. This has made the Wanganui area a world class geological site for studying global sea level change.