SYDNEY- HUNTER REGION 

 

The below map (which is after Bembrick et al., 1973 and Branagan, 1976) conveys a general structural understanding of the Sydney to Lower Hunter region.  This region is part of the Sydney Basin. 

 

Southern Coalfield map (Moffit 1999).  The strong NNE structural grain is evident, the more

northerly trend of the Lapstone Monocline zone is less evident, and the ENE trend (as for

 the Nepean Monocline zone) is seen in the distribution of basalt outcrops along it.

 

The Sydney Basin is bounded on the west by the older rocks of the Lachlan Fold Belt, and on the northeast by the New England Fold Belt.

Beyond Sydney the basin is terminated to the south by the continent edge.  Due north, however, it has a vast continuity, extending through into Queensland.  Hence it is also sometimes referred to as the Sydney-Bowen Basin.  The Sydney Basin (or Sydney-Bowen Basin) is famous for its Permian coal measures.

Major centres of development include Newcastle, Maitland, Lithgow, Wollongong and Sydney (NB: Maitland is not labelled on the map here but is located just below the word 'thrust' in the "HUNTER THRUST SYSTEM" label).  Coal has been mined extensively in the Wollongong, Lithgow, Newcastle-Maitland and Sydney-Newcastle districts.  It was once mined to a small extent at Sydney itself (Balmain Colliery).  However, Sydney is located near the centre of the basin and the depth of coal, plus the extent and density of urban development, now makes the Sydney area itself a most unfavourable place for coal mining.

One on us (L. Sherwin) is co-author of "Geology of the Wollongong and Port Hacking 1:100,000 sheets", covering an area extending from Port Kembla in the south northwards over the southern parts of the Sydney metropolis (Camden, Campbelltown, Sutherland districts).

 

One well-known, early recognised, and very prominent structural zone is the Lapstone structural zone (containing the Lapstone Monocline and Nepean Fault elements [faulting is known as the Kurrajong Fault at its northern end] shown on the map).   As shown on the abover map the direct continuity of the northerly-trending Lapstone structural zone (monocline/fault) is lost near Bargo.  Some investigators consider that a vague "Nepean River Lineament" may continue in the same direction for further south of there, however it has been generally considered that the northerly-trending structural zone must terminate thereabouts since to the south of Bargo the more prominent feature becomes a set of parallel northwesterly trending monoclines (Nepean, Balmoral, Alpine and Mt Murray Monoclines (see Fig. 11 in Sherwin & Holmes 1986 for greater detail).

A separate webpage will be developed to further discuss the Lapstone structural zone in greater detail.  since there has been considerable later work to what is shown in this map and not all interpretations agree.  If you have any photos we could use to show structural details along the zone please send to LachlanHunter.  In places along the zone the beds of Hawkesbury Sandstone are seen to be strongly dipping to the east, and they sometimes carry prominent slickenside markings.  The greater significance of the Lapstone structural zone, and why it exists at all,  is not clear.  It has been postulated that it may reflect some much older feature, such as a former marine shelf edge of 'hinge'.  As the map shows, a reconstructed hinge line for Permian marine deposition time is of similar northerly trend.  A swing eastward at its southern end of that hinge line is also reminiscent of current structural patterns.  

The next image below (via Google Earth) shows the western edge of the Cumberland Basin, looking west. 

This shows the eastern edge of the Blue Mountains Plateau (called simply the "Blue Mountains" by Sydneysiders), and a river (Nepean River which downstream is renamed the Hawkesbury River) running along the foot of the escarpment of the plateau front.  The plateau edge corresponds with the Lapstone Monocline / Nepean Fault structural zone.  Except for the development seen along the main road heading west across the Blue Mountains, the Blue Mountains plateau is largely undisturbed vegetation and contains very large areas dedicated as national park status.  The "lakes" seen in this picture ('Penrith Lakes') are artificial and fill the voids of extensive quarrying on the Castlereagh Plain that has served as the major sand source for the greater Sydney area.  A little off to the northeast of there is seen the large Castlereagh State Forest which is likely the largest unbroken extent of native vegetation still left anywhere in the Cumberland Basin.  Much of the land of the Cumberland Basin has been fully cleared or else cleared to the stage of much thinned vegetation.  Much of it has been used, and still is used, for market garden scale agriculture, the growing of flowers, or for running livestock.  Close housing development has generally spread westwards from Sydney.  This "urban sprawl" continues vigorously.  The entire Fairfield Basin is now covered by close housing but much of the Penrith Basin remains of a rural aspect.  The division between the Fairfield Basin and the Penrith Basin is marked by a N-S zone of more hilly country (running through Horsley Park, Cecil Hill and West Hoxton).  Although the topography along that zone is not anywhere of true dip slope aspect the hilly zone does have the Ashfield Shale subcropping along its eastern flank.

 

As can be appreciated from the above image, the edge of the Blue Mountains Plateau (very early inferred to be a monocline) appeared to the early settlers to be like an impenetrable wall.

Early explorers tried, but failed, in attempts to 'cross' this apparent mountain range seen west of Sydney.  What lay beyond these 'Blue' mountains was of much interest during the first 25 years of European colonisation.  In 1788, the First Fleet from England had arrived at Sydney Harbour.  Following that settlers had taken up portions of land right  across the Cumberland Plains and into the foothills of the Blue Mountains.  But no one who tried to press further west could find anywhere habitable.  The mountains seemed impenetrable and limitless.  The Sydney colony feared it was hemmed in, unable to expand west.  By 1813 there had been many failed attempts:

• William Paterson, a soldier-explorer-botanist set out westwards in 1793 and followed up the Grose River (at the northern end of the above view) until he had to give up.
• George Bass in 1796 ventured via the Lower Burragorang Valley to near the Kanangra Plateau.
• Various escaped convicts are believed to have fled west in search of freedom on the other side of the range, but their fates are unknown.
• John Wilson and party in 1798 attempted a crossing to the southwest, with no success although they did collect the first lyrebird specimen and made the first written reports of the wombat and koala.
• Francis Barrallier in 1802, like Bass in 1796, almost reached the Kanangra Plateau.
• George Caley in 1804 reached the basalt covered prominences of Mount Tomah and Mount Banks and from there looked west.  He reported that it seemed the mountains went on forever. 

In 1813 a party financed by local graziers, with four servants, five dogs and four packhorses, set out from St Marys and met with final success.  They had followed the mountain ridge-tops, instead of the valleys, and quite by chance chose the main ridge of the Blue Mountains (where the railway and Great Western Highway now run as seen towards the top of the above view).  Within months of their discovery of a route across the mountains, government surveyor George Evans reached the site where the town of Bathurst would later be developed.  The following year, 1814, a convict gang of just 28 men pushed 160 km of new road across the Blue Mountains wilderness in only six months – an amazing feat, in return for which they were given their freedom.  When the road was completed, Governor Macquarie made the nine days trip westwards and proclaimed the site of Bathurst.

In 1932 it was proposed that all Crown lands of the Greater Blue Mountains region should be set aside as Blue Mountains National Park to preserve the mountains' outstanding bushland and to protect wildlife.  And thus was fixed the strong contrast still seen today between the Blue Mountains plateau and the urbanised region of the Cumberland Plains and Greater Sydney region to the east of it. 

As shown on the above map after Bembrick et al. (1973), it became generally thought that west of Sydney the Lapstone Monocline is truncated by the southward convergence with the Nepean Fault (also known at its northern end as the Kurrajong Fault).  Sherwin and Holmes (1986), however, resumed use of the name Lapstone Monocline southwards towards Bargo.   

West of Sydney at Wallacia the Nepean River does a westerly bend into and out of the escarpment (where the Warragamba River joins the Nepean, traditionally regarded as a site of classic river capture) and the monocline is well exposed there. The idea of it cutting out further south came in part from subsurface data from the coal mining industry. Since 1973, however, there have been several further studies into the complex nature of the whole 'Lapstone Monocline' structural zone [full references are yet to be added here].

REFERENCES

Bembrick, C.S., Herbert, C., Scheibner, E. and Stuntz, J., 1973. Structural subdivision of the New South Wales portion of the Sydney-Bowen Basin. Quarterly Notes of the Geological Survey of New South Wales, 11, pp. 1-13.

Branagan, D. (Ed.), 1976. An Outline of the Geology and Geomorphology of the Sydney Basin. Excursion Guide, 25th International Geological Congress, Sydney. 60 pp.

Moffit R.S., 1999.  Southern Coalfield Regional Geology 1:100 000 [Map, 1st edition]. Geological Survey of New South Wales, Sydney.

Sherwin, L. and Holmes, G.G., eds. 1986. Geology of the Wollongong and Port Hacking 1:100 000 sheets 9029, 9129. Geological Survey of New South Wales, Sydney. 179 pp.