What is a tessellated pavement?
Tessellation means ‘tile-like’ and all along the Gondwana Coast there are examples of tessellations of varying degrees of geometric perfection. The most regular jointing patterns are developed in fine sedimentary beds (e.g. siltstone) but where the fracturing is developed in coarser sandstone beds it
scarcely qualifies as a tessellated pavement.
Here, the fracturing has been so strong as to break straight through large hard drop stones lying in the paths of the joints. The 060o and 330o orientations of the 2 major joint directions throughout the Gondwana Coast are the same as those in the tessellated pavement near Port Arthur in Tasmania. It is noteworthy that all of these locations lie along the line of fracturing where NZ broke away from Australia progressively opening up the Tasman Sea from ~80 – 60Ma.
The jointing pattern is consistent with having been produced in the persistent resultant stress field and is likely to have developed over the 20 million years after initial rifting began at ~100Ma but, prior to drifting commencing (i.e. from 100Ma to 80Ma).
In an otherwise monotonously flat-lying sequence of sedimentary rock how can a 3m thick folded and faulted layer of siltstone be explained?
At the time the deformation occurred, the layer was lying on the sea floor in a semi-consolidated state and it must have been shaken violently by a significant earthquake.
Experimental laboratory studies demonstrate that these sediments experienced an earthquake of magnitude >6, [i.e. bigger than the Newcastle earthquake (1989) which caused several buildings to collapse and resulted in 12 deaths] was required to produce the scale of deformation found here.
The most likely source of such a major earthquake was the Pacific plate boundary which lay some 150km to the north east of here at ~270Ma.
How do spherical ‘cannon-ball’ concretions form in the siltstone?
Circulating groundwater carries with it dissolved minerals (such as calcium carbonate, iron carbonate and silica) and these may precipitate commonly in concentric layers around any kind of suitable nucleus such as a drop stone, glendonite cluster or fossil within the siltstone.
The concentric structure is responsible for the overall spherical shape. An 8 metre diameter sphere found in a Hunter Valley coal mine is the largest known concretion of this type to be found in the Sydney Basin.
What are Glendonites?
In addition to the fossils, several other significant and clearly visible features in the rocks have been used to determine more about the geological environment here, especially the palaeoclimate that prevailed on the Gondwana Coast at ~270Ma. Glendonites are unusual crystals and crystal clusters that only grow in organic rich mud on the seafloor when the water temperature is below 5oC.
They were first described by Dana, a famous American geologist who visited the town of Glendon, east of Singleton, in the Hunter Valley, more than 150 years ago. They were not, however, fully understood until similar crystals were found growing on the floor of the Ika Fiord in Greenland in the 1960s. They are now known to be crystals of the rare hydrated calcium carbonate mineral, Ikaite (CaCO3.6H2O).
Their presence throughout the Shoalhaven Group sediments provides conclusive evidence of the cold seawater temperature here on the Gondwana Coast as a result of its high latitude location in the Antarctic Circle from about 295Ma to ~270Ma.