|North Solitary Island is also considered part of the Coramba Beds|
The Solitary Islands (and the South Solitary Island in particular) is known to be rock comprised of turbidites (marine mass wasting derived sediments) derived from volcanic parent rock and ash-fall tuff (Korsch 1993). This same assemblage is present on the mainland throughout the area called the Coffs Harbour Block or Coffs Harbour Association and is considered Carboniferous in age. The stratigraphic unit is probably the Coramba beds which mean there is also the possibility that chert, jasper and metabasalt are present as they are elsewhere on the mainland.
I had no idea about the geology of South Solitary Island until I read Korsch (1993) in which he was permitted to visit all of the solitary islands to determine whether the concept of a giant fold (called an orocline) was present off the coast. If the orientation of the rock strata was right it would demonstrate that the area between Brooms Head and Coffs Harbour and then inland up through the Orara region and eventually looping back up into Queensland was a giant fold in the earth. Korsch (1993) did observe just such features and this has resulted in much further interest and research (including papers published in the last 12 months) about the tectonic history of the New England and Northern Rivers. I will go into more details about the extraordinary folding and tectonic history in future posts as there is an incredible amount of detail and unknowns when it comes to our area.
Oddly, Weber et al (1978) mentioned that a report from 1945 that there is an area of molybdenum mineral deposit on the South Solitary Island. The size of the island (and being a national park) is such that it could never be mined but it is such an unknown curiosity. Webber et al (1978) describes the deposits:
Worthy of passing mention is an occurrence of molybdenite at the eastern extremity of the Demon Block. Narrow, molybdenite-bearing quartz veins have been reported from South Solitary Island, 16.5km northeast of Coffs Harbour, by Fisher (1945, p10). The host rock is unknown.The reason this is a little odd in my mind is because molybdenite is not very common in the Coffs Harbour Block. Some molybdenum formed in areas related to specific types of intrusions to the south in the nearby Nambucca Block (e.g. see my earlier post on the Valla Monzogranite) but to my knowledge this has not occurred to any significant extent in the Coffs Harbour Block. Just another slightly out of place geological feature in our region.
Korsch, R.J. (1993) Reconnaissance geology of the Solitary Islands: constraints on the geometry of the Coffs Harbour Orocline. New England Orogen Conference 1993, University of New England.
Weber, C.R., Paterson, I.B.L & Townsend, D.J. (1978) Molybdenum in New South Wales. Geological Survey of New South Wales 43.