A geological unconformity is an unconformable contact between two different lithologies. In the case of this formation in Jubilee Creek in the Northern Flinders Ranges, stromatolitic limestone of the Balcanoona Formation overlays laminated mudstone of the Tapley Hill Formation.
These two formations followed the melting of an ice age dating 700 million years ago. “Black silt, rich in fine organic material derived from algae and bacteria, was deposited as laminations (each less that 0.5mm thick) in the calm shelf sea. The total thickness of this silt blanket (known as the Tapley Hill Formation) increased from a few tens of metres in the west to about 3km in the ranges east of Hawker. Here depth of water and rate of subsidence of the basin floor were greatest. As silt built up, the water around the margins of the basin became shallow enough for stromatolites and oolite banks to form; these are exposed near Port Augusta and Balcanoona [Balcanoona Formation].” – Excerpt from The Legacy of Time: The Story of the Flinders Ranges by the Royal Geographical Society of South Australia.
Stromatolitic limestone of the Balcanoona Formation:
Laminated mudstone, or ‘silt blanket’, of the Tapley Hill Formation:
