Strange concretions at Katherine NT

Strange fossils like concretions on fossil pavement behind a caravan park near Katherine in the Northern Territory are probably concretions formed from microbial degradation of organic matter, according to Professor Martin Kennedy, head of department at Macquarie University's Department of Earth and Planetary Sciences.

We found the concretions near the river behind the Manballu Caravan Park. Prof Kennedy says it is always hard to tell from a photograph, but the oval crescent shape, dark colour, decimeter size and fine grained texture of the structures lead him to believe this is a “concretion” which is typically an early cemented area that typically forms from microbial degradation of organic matter.

"They most commonly form early after sediment deposition and before burial. Sometimes within them you will find a fossil- it is commonly suggested that the organic matter in that fossil served as the initial nucleation point of cement formation- the cements are partially made from the carbon within the organic matter. Once initial precipitation of cement starts with a few crystals, the crystals serve as substrate for further crystal growth and the process continues outward. Some of these can become large, like the Moleki boulders on the beach in New Zealand, but your image is more the typical size of tens of centimetres. An X-ray of this material should reveal that it is comprised of calcium carbonate, maybe siderite, if it is a concretion. Also its carbon isotopic value would be < -5 per mill parts per billion and a thin section petrographic image would show the calcium carbonate isolated to the pores of grains."