Geology of portion of the Grand Unconformity north of Broken Hill, N.S.W.

Abstract

The unconformity between the Torrowangee series and the Willyama complex was first recognised by Mawson and described by him in his memoir of 1912. This was followed by Andrews comprehensive report on the geology of the Barrier Ranges which was published ten years later. No further attempt was made to remap or reinterpret the regional geology of the area until 1950 when Zinc Corporation geologists made a broad study on a regional basis. An important outcome of this recent work was that a post Torrowangee age was proposed for the Paps granite 'dyke' which outcrops parallel to the unconformity from Yanco Glen to the Paps and then swings westward, still parallel to the unconformity towards Brewery Creek. A post Torrowangee age was likewise proposed for the Brewery Creek Pluton and other similar granite masses in the area. These granites are a phase of the newer or Protogine granites of Mawson (1912) and the Mundi Mundi granites of Andrews and Brown (1922) and have previously been regarded as pre-Torrowangee in age. Zinc Corporation geologists have also reported that the pegmatites which are so abundant in the older Willyama rocks have been intruded into the base of the Torrowangee series in certain localities. The post Torrowangee age of the granites raised an important problem, in that the tillites, which occur near the base of the Torrowangee series, contain abundant large granite boulders, seemingly identical with the supposedly younger granite. The exact nature of the unconformity itself has also been much debated in recent years. Some have gone so far as to suggest that this structure has been entirely obliterated by Post Torrowangee thrusting and metamorphism. Another problem was the occurrence, in the Brewery Creek area, of quite a considerable thickness of highly folded glacigene sediments straigraphically below the basal quartzite horizon which occurs along the Yanco Glen - Paps line. These problems together with the possibility of some post Torrowangee pegmatization prompted the present investigation.