A basin in the hills

During the Triassic and into the Jurassic periods (Being part of the Mesozoic era) three major sedimentary basins formed in our region which are preserved today. The biggest, the one most people know about, and the youngest is the Clarence-Moreton Basin. This is a thick sequence of rocks which extends to Nymboida in the South up into southern Queensland. The Clarence-Moreton formed on top of, and with the Ipswich Basin. In southern Queensland it also begins grading into the Surat Basin.

The Ipswich Basin is smaller than the Clarence-Moreton as both basins are formally defined, but various sub-basins within the Clarence-Moreton actually formed at the same time as many of the parts of the Ipswich Basin and several units appear to conformably underlie (there is no time gap in deposition) or even inter-bed with the lower units of the Clarence-Moreton. The Ipswich Basin outcrops in a north-south line west of Murwillimbah and at Evans Head and Brooms Head. It is well known in southern Queensland for large actively mined coal deposits, it is not so well known south of the border and is often confused with being part of the Clarence-Moreton Basin.

The least known Triassic-Jurassic Sedimentary Basin is the Lorne Basin. This is further south than the Ipswich and Clarence-Moreton Basins and for the purposes of this blog will define the southern limit of the Northern Rivers. The Lorne Basin is the smallest of the three Basins. The middle of the basin is located at the village of Kew, it extends west almost to the village of Comboyne, south to Coppernook, almost to Wauchope in the North, and is present on the coast at Camden Haven and Diamond Head (Bob and Nancy have a tour of Diamond Head). The modern day Camden Haven River flows across the basin.

Unlike its contemporaries the Lorne Basin has rather poor pickings as far as coal deposits goes. This at first might be surprising given the thick units of coal formed further to the North and South at the roughly the same time as the Lorne basin was forming. In fact the coal seams found in the Lorne Basin are only of any significance in the units known as Camden Haven Group, and even then these are ‘thin coaly beds’ according to Pratt (2010) and earlier authors. What gives us a clue about the apparent absence of coal is the abundance of another rock type, conglomerate. According to Pratt (2010) there are several units of conglomerate which show that the sediments that were deposited in the basin traveled only a short distance and the river systems that transported these sediments was in a high energy environment (remember that for organic rich sediments to accumulate that will form coal the environment needs to be stable and swampy).

The clasts that make up the conglomerate in the Lorne Basin are derived from the Palaeozoic aged basement rock of the New England Orogen that surrounds the Basin. The clast composition reflects the slight variability in the New England Orogen Rock which is slightly different if the rock came from the north or the south side of the Basin. I will discuss the individual units of the Basin in future posts. But the whole picture of high energy deposition in the basin shows us that the Lorne Basin is a little unusual. It actually appears that the basin was elevated (not low lying like the Clarence-Moreton, Ipswich, Surat, Gunnedah, Sydney etc Basins) and situated in between large mountain ranges, this is known as an inter-montane basin. The well-known examples large active of inter-montane basins are in Asia in places such as Mongolia (these are much bigger than the Lorne Basin). Closer to home the McKenzie Basin near Mount Cook in New Zealand is a good example, although it is a bit smaller than the Lorne.

After the sediments had been consolidated there was a period of faulting through the Lorne Basin and during the Cenozoic era intrusions of granitic rock affected some parts of the basin and it was also partly obscured by lavas from the same era, though much of this lava has now been eroded away. The nearby Comboyne Volcano/volcanic centre was probably associated with these lavas and intrusions. Erosion of the lavas has caused a very attractive landscape including the Ellensborough Waterfall.

Since writing the above post Dylan reminded me that there is a theory that the Lorne Basin was initially formed during a meteorite impact (See comments below). I'll have to dig up some literature and discuss why this might be the case, however, for the time being it is worth noting that according to Tonkin (1998) the overall shape of the basin is very similar to other impact structures around the world. As Dylan points out: we have yet more unanswered questions!

References/bibliography:

Pratt, G.W. 2010. A Revised Stratigraphy for the Lorne Basin, NSW. NSW Geological Survey Quarterly Notes.
Tonkin, P.C. 1998. Lorne Basin, New South Wales: Evidence for a possible impact origin? Australian Journal of Earth Sciences. V45.