FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 12)

CHAPTER 6 (CONTINUED)
Excursion 6c 
Yalwal (40 miles return) 
This excursion covers the most interesting area of basement rocks in the Shoalhaven district. It has the added interest of being mineralised and of having been the source of $500,000 worth of gold. The new dam on Danjera Creek is included in this excursion.  
Proceed as in Excursions 6a and 6b. Take the Yalwal road 0.3 miles past Saltwater Creek. The road climbs steadily, reaching the base of the Nowra Sandstone 400 feet above sea level, or approximately the same height as The Pulpit. This illustrates the gradual dip of the strata to the east. Stop where gravel has been obtained on the right, 1.7 miles from the Burrier road. 
Stop 1. The Nowra Sandstone cliff is a short distance away to the right, from where there is a sharp drop of 600 feet to Barringella Creek, a small tributary of the Shoalhaven. 
The road gradually rises and in places small outliers of the lower Berry Formation cap the ridge. Excursion 6d turns off to the left 4.3 miles from the Burrier road. Cliffs of sandstone mark the edge of the narrow ridge along most of the road up to this point. Turn right along the Old Burrier fire road 1.7 miles further on. A further view is obtained on the left 1.3 miles along this road. 
Stop 2. The view below takes in the granite country around the junction of Yalwal and Bundundah Creeks. It is a three mile walk from here down to Yalwal Creek and upstream to meet the road again. The fire road deteriorates beyond this point, but may be followed down to Burrier. Return to the Yalwal road and proceed to Yalwal Gap 1.5 miles further on.
Yalwal Valley and
Yarramunmun Creek October 1964
  
Stop 3. The Nowra Sandstone displays all its usual features at this point and in the road cuttings towards Yalwal. There has been considerable fretting of the rock to form minor caves, and enlarged joints have added to the ruggedness of the scarp. 
Immediately below lies Yarramunmun 
Creek, the eastern branch of Yalwal Creek. It is a walk of 2 ½ miles to the road by way of the creek. The threefold subdivision of the Permian strata and the underlying Devonian sediments can be noted en route. 
Across Yarramunmun Creek, the cleared paddocks of Devonian basalt are obvious, beyond which is the mining area of Yalwal itself, on Danjera Creek, the western branch. Upstream from Yalwal, but still within the valley, are prominent cliffs of granite and rhyolite. Above all this lies the characteristic Nowra Sandstone surface, with outliers of the Berry Formation forming the Ettrema Tops, 8 miles beyond Yalwal. 
During the descent from Yalwal Gap, the following boundaries may be observed: base of the Nowra Sandstone, 0.2 miles; base of the Wandrawandian Siltstone, 0.7 miles; and the base of the Conjola Formation, 1.0 miles. Fossiliferous outcrops of the latter occur in an old quarry on the right just before the unconformity with the Devonian. 
Stop 1.0 miles from Yalwal Gap, where basement rocks first occur. It may prove difficult to locate the exact boundary except on foot. 
Stop 4. The Devonian strata are steeply dipping in contrast to the nearby horizontal Permian sediments. At this point, they consist of slaty siltstones and quartzite, apparently devoid of fossils. Permian marine fossils occur within inches of the unconformity. Below the base of the Permian, there is a marked change of the land surface and the vegetation. 
Continue to the bottom of the hill and stop at the creek crossing. 
Stop 5. Outcrops of weathered basalt occur on the Yalwal side of the creek. The rock is considerably altered, largely due to the hydration of the ferromagnesian minerals. This feature is common to all the Yalwal basalt exposures. The basalt is thought to be younger than the sediments observed earlier, and is preserved here in the core of a syncline whose axis runs northerly near the Danjera Creek Dam. 
The gravel of Yarramunmun Creek should be examined. It will be found to include quartzite, slate, and basalt, but no other volcanic rocks. A short distance below the bridge, Danjera Creek comes in from the left and the united stream forms Yalwal Creek. Its gravel includes flow banded rhyolite, spotted slate, and granite, brought down mainly from miles upstream. 
If the track on the bank is followed downstream for half a mile, the edge of the Carboniferous granite will be reached. It consists of quartz felspar porphyry at this point but the texture becomes more granitic further downstream. Nearby, the Conjola Formation descends to a lower level than elsewhere, suggesting that a small valley was filled in here in Permian time. 
Continue on to the dam. The reddish soil in the road cutting is the product of weathering of the underlying basalt. 
Stop 6. Danjera Creek Dam has a height of 108 feet 6 inches and a width of 1,322 feet. The underlying strata include siltstone as well as basalt and an old fault line was located during investigation work. It now lies beneath the dam but is not expected to cause any problems. 
The basalt exposed during construction was shown to contain a considerable amount of pyrite. Some cavities contained other sulphides as well. Fine alluvial gold may be washed in the creek below the dam but never occurred here in payable quantities. 
Much of the old town of Yalwal has been lost because of the building of this dam. Its waters are essential to maintain the flow of the Shoalhaven River down to the Burrier pumping station. It was built by the Shoalhaven Shire Council in view of current schemes to divert Shoalhaven River water to supply Sydney.  
The waters of the dam have not removed all traces of the gold mines, but access may be limited in view of possible contamination of the water supply. 
From the dam a strenuous climb of 1,000 feet leads up to the base of the Nowra Sandstone cliff. This is part of an outlier of these rocks and the edge of the cliff resembles a saw in the way it has been intersected by innumerable joint planes. To the right of the cliff, a saddle 700 feet above Yalwal gives access to Bundundah Creek. Here the valley floor is largely granite, but basalt and Devonian sediments outcrop upstream. Silver was mined from fissures in the granite at one time. From the dam across to Bundundah Creek is a little more than 3 miles. 
Take the highest road above the dam, which leads to the old Yalwal cemetery. 
Stop 7. Reference should be made to the map of the Yalwal mines area to locate the main features of interest. This is at the end
of the blog.

Near the Yalwal cemetery November 1967
The cemetery is situated on the south west side of a syncline which has folded the Devonian strata. At this point, the basalt gives way to sandstone and siltstone, in which Upper Devonian plant fossils occur. These may be collected beside the track 200 yards down the hill towards Danjera Creek. Beneath these sediments lie conglomerates, which outcrop prominently where the creek emerges from its narrow gorge upstream from the dam. Older sediments, including quartzite and slate, occupy most of the area where the mines are situated. These are faulted against all the younger strata and it is thought that they may represent part of the Ordovician basement. The whole of these strata are folded along a roughly north-south axis. 
The gold mineralisation appears to be associated with the faults mentioned above. The gold is found with pyrite and arsenopyrite and the minerals have been introduced along zones of weakness in the sediments, being deposited as impregnations in the altered rock. They appear to have had their origin in the underlying granite, outcrops of which occur a short distance north of Danjera Creek. The nature of the mineralisation is such that large scale quarrying rather than normal mining has been necessary to make operations payable. In view of the limited testing that has been carried out in some areas and the fact that the productive zone is concealed beneath the overlying Permian sediments in the south, it seems certain that large quantities of gold remain undiscovered. It should also be noted that gold could only be mined profitably from the oxidised zone, down to a depth of 100 feet. The primary sulphide ore beneath that is comparatively low in gold values. 
The main mining claims and some of their important features are listed below. 
The Poor Man. Situated to the north of most of the larger mines, its main feature is a large open cut in quartzite and slate. 
The Eclipse and Pinnacle mines were worked together after 1901 and are the site of recent prospecting work. The workings include several tunnels and an open cut. At least 8,000 ounces of gold were extracted. 
The Golden Crown. Conglomerate outcrops prominently near this mine, which has sizeable underground workings in addition to an open cut. 
Sawpit Gully was the only real alluvial area on the field, although some fine gold was washed in the main creek near the present dam site. Coarse gold, but in limited quantities, was found here. 
The Albion and Hidden Treasure workings each consist essentially of a single tunnel cut through quartzite. 
The Fountainhead property consists of a few shallow excavations. 
The Pioneer has been one of the leading producers and at least 12,000 ounces of gold have been produced. Several large open cuts were operated and ore was removed by way of the long Pioneer tunnel underneath. 
The Homeward Bound adjoins the Pioneer claim and the two were worked together from around 1927. Something like 30,000 ounces have been produced and the large excavation indicates the enormous tonnage of rock that has been quarried. Recent attempts towards further production have been unsuccessful.
Remains of the Homeward Bound treatment works January 1967
The Star workings adjoin the Permian capping and have not proved to very economical. 
The Caledonian, although it produced 20,000 of gold before 1900, has not been worked much since. Little is known about the underground workings. The gold occurred in rich quartz veins, unlike most of the other mines. This seems to be another form of secondary enrichment as the values did not persist at depth. 
The Golden Quarry workings consist of a number of small cuttings adjoining the Caledonian, in an area which is worthy of further prospecting.  
Geology field trip Danjera Creek April 1966
Basement rocks outcrop in Danjera Creek for 10 miles upstream from Yalwal.  The rock types are the same as those seen around the mine. Great cliffs of rhyolite and granite are very prominent and their level above the creek suggests that high land existed here in Permian time. Some sulphide mineralisation has been discovered around the margins of the granite where it intrudes dolerite, 3 miles upstream.

Excursion 6d. 
Yarramunmun and Calymea Creeks (30 miles) 
Tianjara Falls April 1967
These creeks flow northwards from the ridge along which the Braidwood road runs. The first plunges over the Tianjara Falls and joins Danjera Creek at Yalwal. The second flows through the Yerriyong Valley to become Saltwater Creek before joining the Shoalhaven. As remote places they are of interest to bushwalkers, but less so to geologists. The remarks below explain the places of greatest interest, accessible in these valleys. 
The Dean’s Gap fire road leaves the Yalwal road on the left at a point 4.3 miles from the Burrier road. See Excursion 6c, Stop 1. Dean’s Gap is reached 3 ½ miles south along this road. 
Stop 1. At this point the ridge reaches its narrowest point and it is possible to descend into the valleys on either side. The road to the right leads into the valley of Yarramunmun Creek. There are many fire and logging roads in this valley. Take the branch on the left after 400 yards. This leads down to the valley floor, a total drop of 800 feet, descending to a point at the junction of Yarramunmun and Boolijong Creeks. The typical subdivision of the Permian strata may be noted on the way down. Although all published geological maps of the area show basement rocks at creek level, outcrops of Permian sandstone certainly occur for ½ mile downstream from the junction. It is quite possible that basement rocks do outcrop upstream and that there was a valley here in Permian time, now filled with sandstone. From the Boolijong Creek junction, it is a 7 mile walk downstream to the Yalwal road at Danjera Creek. 
Continue south towards the Braidwood road. About 2 ½ miles further on, turn right along Hyland’s Lookout fire road. 
Stop 2. This provides a fine view of the valley of Boolijong Creek 750 feet below. In the distance, from right to left, are the Ettrema Tops (outliers of the Berry Formation), the basalt hills of Sassafras, and the Tianjara mesas. 
Some 2 ½ miles further on, the Dean’s Gap road joins the Braidwood road at Turpentine. The last few miles are over the Berry Formation and at Turpentine the vegetation is much richer than that seen along the Yalwal road. Turn left, and keep left towards Nowra. After a short distance, the Nowra Sandstone is met again and the change in vegetation is remarkable. Shortly afterwards, gravel pits are to be seen on the right. These are numerous between Turpentine and the aerodrome. About 3 ½ miles from Turpentine, a fire road to the right leads to the Parma Creek fire road (see Excursion 8a). One mile further on, a view of Calymea Creek may be had on the left a short distance from the road. At 6.5 miles from Turpentine Corner, a road branches off on the left to Yerriyong, on Calymea Creek. There is little of geological interest in the valley. Shortly afterwards, a view of Nowra Hill opens up and the road crosses the Berry Formation all the way to Nowra. 
Questions 
1. Discuss the scenic effects of the Nowra Sandstone. 2. In what way has the distribution of Permian rocks hindered mining in the Yalwal-Grassy Gully region? 
3. Discuss the merits of the sites for the Danjera Creek and proposed Shoalhaven River dams. 4. Give a brief account of the Devonian and Carboniferous history of the Yalwal gold field. 5. What unusual features are noticeable about the earliest Permian sediments west of Grassy Gully? Suggest possible causes for these. 6. Discuss the evidence which supports the existence of folds in the Cabbage Tree Creek- Flat Rock Creek area. 7. The high country west of Nowra presents a uniformly smooth surface from a distance. Suggest reasons why this is so. 8. Give an account, based on field observations, of the nature and origins of the igneous rocks of the Yalwal district. 


Comments

Post a Comment

Popular posts from this blog

Image
FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 6) Chapter 3: The Nowra District   Nowra  is strategically built on the first flood-free site upstream on the south bank of the Shoalhaven River. It was not an important settlement until about 1880, when Terara was extensively flooded. Soon after, the coming of the railway and the construction of the present road bridge assured its development as a rural centre. Today, it is a rapidly growing town, with developing industry, tourism, and defence establishments.  Several factors dominate the geology of the area. The Nowra Sandstone has provided a resistant surface, broken only by the Shoalhaven River and a few of its tributaries. Where this sandstone has dipped below sea level at Nowra, an extensive flood plain has developed, the origin of which is of considerable interest and will be explained later.  Coolangatta Mountain  is the only unusual feature in the area. It is really an outlying part of the Cambewarra Range,...
Read more
Image
FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT  NSW AUSTRALIA (Part 1)   John G Paix 1970 (Reviewed 2013) ©  Australian Field Geology Club visit to the Cambewarra Range February 1965   A Word or Two of Explanation This book was written in the years 1968-1970. At the close of my time as a Science teacher at Nowra High School in 1967, I undertook to write a report titled " The Geology of the Shoalhaven Shire " for the then Shoalhaven Shire Council, which was published by them early in 1968.  It soon became apparent that there was much additional information, especially relevant to the Geology excursions I had conducted during my 5 years in Nowra, which was not included in that book.  What you see in front of you today was prepared for publication over the next three years and I fully expected to see it published in 1970. For reasons that do not matter at this distance in time, this never happened. I was left with a typed, proof-read copy, all the maps, drawings, p...
Read more
Image
FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 23) CHAPTER 15: THE CLYDE RIVER VALLEY T aking its rise at an elevation of 2,500 feet near Sassafras, this small river flow south and descends by way of a great canyon to a level of 200 feet at Yadboro. It does this in less than 20 miles, yet it is a further 40 miles to the sea downstream. This is a peculiar result of the dip of the Shoalhaven group sediments. At one time, these must have extended further south than at present. When the area was uplifted, it was tilted to the north, so that the ancestral Clyde River had to erode its bed against this trend.  This undoubtedly led to the formation of many waterfalls and the evolution of the canyon upstream from Yadboro.  The southern edge of the sandstone scarp presents a bold face and is broken into numerous outliers. Some such as The Castle and the Pigeon House are well known to bushwalkers, but there are many others. They all have a similar structure, being capped by cliffs of the...
Read more