Fairfield Basin

Fairfield Basin
Fairfield Basin Syncline
View across the Cumberland Plain in the Fairfield Basin, with elevated sandstone terrain in the distance
Etymology	Fairfield
Location	Cumberland Plain
Region	Sydney, New South Wales
Country	Australia
State	New South Wales
Cities	Fairfield, Bankstown, Chullora
Characteristics
On/Offshore	Onshore
Part of	Sydney Basin
Hydrology
Rivers	Georges River, Parramatta River
Geology
Basin type	Synclinal structural basin
Plate	Australian Plate
Age	Triassic
Stratigraphy	Hawkesbury Sandstone, Mittagong Formation, Wianamatta Group
Faults	Homebush Bay Fault Zone

The Fairfield Basin, also known as the Fairfield Basin Syncline, is a major synclinal geological structure within the Sydney Basin of New South Wales, Australia. Situated beneath metropolitan Sydney, it forms part of the central structural region of the Sydney Basin and contains some of the greatest preserved thicknesses of its sedimentary succession. It is an open, broadly east–west-trending syncline expressed principally within the Triassic sedimentary rocks underlying Sydney.

The Fairfield Basin is one of several large-scale structural depressions recognised within the onshore Sydney Basin, alongside structures such as the Penrith Basin and Botany Basin. It should not be confused with the Sydney Basin itself, which is a much larger sedimentary basin extending across much of eastern New South Wales and offshore beneath the continental shelf. The Fairfield Basin instead represents a broad fold or structural low developed within the sedimentary rocks of the larger basin.

The Fairfield Basin affects rocks belonging principally to the upper part of the Triassic succession of the Sydney Basin. From oldest to youngest, the major units exposed or encountered around the structure include the Hawkesbury Sandstone, Mittagong Formation and Wianamatta Group.

Regional setting

The Fairfield Basin lies within the central portion of the Sydney Basin, a large Permian–Triassic sedimentary basin that extends from the Southern Highlands through the Sydney Metropolitan region and northwards towards the Hunter region, as well as offshore beneath the continental shelf. The Sydney Basin contains a sedimentary succession several kilometres thick, consisting predominantly of sandstone, shale, siltstone, conglomerate and coal-bearing rocks. The central part of the Sydney Basin is commonly identified with the Fairfield Basin because some of the greatest thicknesses of the sedimentary sequence have been encountered there. The Australian Museum similarly identifies the centre of the Sydney Basin as being around Fairfield, although the rocks exposed at the surface across metropolitan Sydney consist principally of the younger Triassic part of the basin succession.

View across the broad, gently undulating landscape of the Fairfield Basin, with Fairfield CBD in the middle distance and sandstone hills forming the horizon.

The Fairfield Basin occurs beneath the comparatively low-relief Cumberland Plain, although its geological structure is not equivalent to the modern topography of the plain. It is principally a subsurface structural feature recognised from the attitude and elevation of sedimentary strata rather than a discrete surface depression. The structure has influenced the preservation and distribution of the younger Wianamatta Group across the Cumberland Plain. In particular, the Bringelly Shale, which has been extensively removed by erosion elsewhere in central Sydney, is preserved within the Fairfield Basin and reaches a thickness of about 60 metres (200 ft) at Potts Hill. The structure has also been encountered or investigated during numerous major infrastructure projects, including the M5 and M8 tunnels and other road, railway and tunnelling projects across metropolitan Sydney.

Relationship to the Sydney Basin

Despite the similarity of their names, the Fairfield Basin and Sydney Basin describe geological features on very different scales. The Sydney Basin is a major sedimentary basin extending across tens of thousands of square kilometres of eastern New South Wales and containing a Permian–Triassic sedimentary succession several kilometres thick.

The Fairfield Basin is instead a structural subdivision within that larger succession. It represents a broad synclinal depression in which strata have been gently folded downward relative to surrounding structural highs. Consequently, rocks exposed at the surface around the Fairfield Basin remain part of the Sydney Basin sedimentary succession. The term Fairfield Basin is therefore broadly comparable to other named structural subdivisions of the Sydney Basin, including the Penrith Basin and Botany Basin, rather than representing a separate sedimentary basin with an independent depositional history. The distinction is particularly important because the Fairfield Basin is sometimes described as the "centre" of the Sydney Basin. In this context, centre refers to the central or depocentral structural region where particularly thick sections of the sedimentary succession are preserved, rather than implying that Fairfield represents the geographic centre of all rocks assigned to the Sydney Basin.

Structure

Schematic of an anticline and syncline. The Fairfield Basin forms a broad, open syncline, with strata on either limb dipping towards the basin's structural axis.

The Fairfield Basin is an open syncline with a broadly east–west-trending axis. Sedimentary strata on either side dip gently towards its structural axis. Like much of the Sydney Basin, the rocks have undergone comparatively little deformation and generally retain shallow dips. Early geological investigations recognised that the sedimentary rocks beneath western and central Sydney were not completely horizontal. Structure contours constructed on stratigraphic boundaries revealed a series of broad structural highs and lows, including the Fairfield Basin, Penrith Basin, Botany Basin and Mulgoa Dome.

The Fairfield Basin is particularly evident in structure contours drawn on the base of the Wianamatta Group, corresponding approximately with the top of the Hawkesbury Sandstone. These contours show the younger strata descending towards the structural low before rising again on the opposite limb. The orientation of the structure can consequently influence local bedding directions over a large part of Sydney. North of the structural axis, strata generally dip towards the basin from the northern limb, while rocks south of the axis form its southern limb. Geotechnical investigations south of the structure at Moorebank, for example, record strata dipping north-northwest towards the mapped synclinal axis. The fold is broad compared with the numerous smaller faults, joints and local warps affecting the Sydney Basin. Consequently, its geometry is more readily recognised from regional geological mapping, borehole correlations and tunnel exposures than from an individual surface outcrop.

Stratigraphic relationships

The Hawkesbury Sandstone forms the principal older bedrock beneath the Fairfield Basin and is approximately 250 metres (820 ft) thick in the Sydney region. One of the depocentral areas of its preserved distribution occurs within the Fairfield Basin, placing the structure within an important central part of the Triassic sedimentary succession. Around the margins and limbs of the syncline, Hawkesbury Sandstone occurs closer to the surface as younger strata thin or have been removed by erosion. Towards the structural low it passes beneath the Mittagong Formation and Wianamatta Group. This relationship has been particularly well documented along the southern limb, where the M5 and M8 tunnels have provided extensive subsurface exposures through the regional structure.

The Mittagong Formation forms the relatively thin transitional unit between the Hawkesbury Sandstone and overlying Wianamatta Group. Within the Fairfield Basin it is significant as a stratigraphic boundary between the sandstone-dominated older succession and the younger shale-dominated rocks preserved within the synclinal depression. Its position along the limbs of the basin has been encountered in tunnelling and geotechnical investigations, including the M8 tunnels, where it marks the transition from Hawkesbury Sandstone into Ashfield Shale.

The Wianamatta Group has a particularly close relationship with the Fairfield Basin. The broad downfolding of the syncline preserved these younger Triassic strata while erosion removed them from many surrounding structural highs. Consequently, the principal Wianamatta outcrop in the Sydney region occupies the Fairfield Basin, and the greatest preserved thickness of the group within the Sydney 1:100,000 geological sheet occurs within the structure. The group comprises the Ashfield Shale, Minchinbury Sandstone and Bringelly Shale. Ashfield Shale is widespread across the basin and forms the lowest major Wianamatta unit, while the thin Minchinbury Sandstone provides a stratigraphic marker between the Ashfield and Bringelly shales. Their distribution reflects the synclinal structure, with progressively younger strata generally preserved towards the structural low.

The Bringelly Shale demonstrates particularly clearly the influence of the Fairfield Basin on the preservation of Sydney's younger Triassic rocks. Post-Triassic erosion has removed much of the formation elsewhere in the Parramatta–Sydney region, leaving it largely restricted there to the synclinal Fairfield Basin. Its present distribution therefore reflects the protection afforded by the structural depression. At Potts Hill, approximately 60 metres (200 ft) of Bringelly Shale is preserved within the basin. The formation overlies the Minchinbury Sandstone and represents some of the youngest extensively preserved strata within the Fairfield Basin.

Extent

South-western Sydney across the Cumberland Plain, encompassing part of the western Fairfield Basin.

The Fairfield Basin extends considerably beyond the suburb of Fairfield from which it takes its name. Geological mapping shows the synclinal structure continuing eastward beneath substantial parts of metropolitan Sydney. Its axis has been identified beneath the Chullora area, where investigations at the Chullora Railway Workshops describe a west-northwest to east-southeast-trending structural axis forming part of the Fairfield Basin. Farther east, geological mapping identifies the Fairfield Basin syncline beneath the inner-western suburbs. Investigations for the Cooks River to Iron Cove GreenWay show its east–west-trending axis passing near Constitution Road. The structure therefore should not be interpreted as a small basin lying directly beneath Fairfield itself. Rather, the name refers to a regional geological structure extending through a substantial portion of the central Sydney Basin.

The northern limb of the Fairfield Basin extends beneath central and inner Sydney. Geological information from basement excavations, tunnels and other underground works around the Sydney central business district and Sydney Harbour has provided much of the historical understanding of the bedrock on this side of the syncline. The Sydney CBD is predominantly founded on Hawkesbury Sandstone, while progressively younger rocks including the Mittagong Formation and Ashfield Shale occur away from the harbour and towards the structural low. Regional bedding dips are generally shallow. Engineering investigations in northern Sydney commonly describe strata dipping towards the central Fairfield Basin, illustrating the regional influence of the structure even at considerable distances from its axis.

The southern limb has been particularly well exposed by major tunnelling projects beneath southern Sydney. The M8 Motorway provided an extensive subsurface section through the Hawkesbury Sandstone, Mittagong Formation and Ashfield Shale along the southern side of the Fairfield Basin Syncline. The tunnel alignment was geologically valuable because it crossed the regional structure while simultaneously intersecting major fault and dyke systems. This allowed engineers and geologists to observe variations in lithology and structural deformation over long underground exposures that are rarely available at the surface in metropolitan Sydney. The M8 excavations confirmed that the Fairfield Basin is an open syncline and provided detailed information on the lithological character of the southern limb. Hawkesbury Sandstone dominates much of the tunnel alignment, transitioning through the Mittagong Formation into Ashfield Shale towards the eastern parts of the works. At Moorebank, geological structure contours indicate that the base of the Wianamatta Group dips north-northwest towards the Fairfield Basin axis, which has been mapped approximately 5 kilometres (3.1 mi) north of the investigated area.

Faulting and igneous activity

The Fairfield Basin is a fold and should not itself be confused with a fault zone. The sedimentary rocks of the region are nevertheless cut by numerous joints, faults, fracture zones and igneous intrusions formed during later phases of Sydney's geological history. Sydney's major north-northeast-trending fault zones cut across the broader regional structure. These faults commonly occur as zones of closely spaced joints, brecciation and fault gouge. The major systems include the Homebush Bay Fault Zone, Luna Park Fault Zone, Woolloomooloo Fault Zone and Watsons Bay Fault Zone.

The Fairfield Basin is also crossed by Jurassic igneous intrusions. Sydney contains numerous basaltic and doleritic dykes, generally trending approximately east–west, which cut the older Triassic sedimentary rocks. These include the Great Sydney Dyke and numerous smaller intrusions. The combination of the east–west regional fold, north-northeast fault systems and approximately east–west dyke systems produces a complex structural pattern beneath parts of central and southern Sydney. Excavation of the M8 tunnels was particularly useful for examining these relationships because the tunnel alignment intersected both major fault systems and regional dyke sets while passing through the southern limb of the Fairfield Basin.

Geomorphology and hydrogeology

Although primarily a subsurface geological structure, the Fairfield Basin has influenced the distribution of rock types exposed at the surface and consequently aspects of Sydney's landscape. Resistant Hawkesbury Sandstone typically produces rugged ridges, cliffs and dissected terrain, whereas the shale-dominated Wianamatta Group generally weathers to lower, more gently undulating landscapes. The preservation of Wianamatta Group rocks within the structural depression contributed to the extensive shale landscapes of the Cumberland Plain. Weathering of these rocks produces the clay-rich soils characteristic of much of western Sydney. The structure may also have influenced local drainage. At Chullora, geological investigations have suggested that the Fairfield Basin and the southeastward thinning of the Bringelly Shale influenced development of the Cooks River valley. The orientation of the bedrock may also affect deeper groundwater movement beneath the area.

Groundwater conditions within the Fairfield Basin vary substantially with lithology, weathering, joints and faults. The shale-rich Wianamatta Group generally has relatively low primary permeability, although groundwater can occur along bedding surfaces, sandstone interbeds, joints, fractures and weathered zones. The orientation of strata within the Fairfield Basin can influence groundwater movement. At Chullora, deeper groundwater flow has been interpreted as being affected by the orientation of the bedrock associated with the basin and by the Cooks River catchment. Groundwater within the Wianamatta Group may also be naturally saline. Investigations at Chullora attributed poor regional groundwater quality partly to saline connate water retained within the shale sequence. Faults and joint swarms cutting the basin can locally provide more permeable pathways through otherwise relatively low-permeability rock, while clay-rich weathered zones may impede groundwater movement. Consequently, local hydrogeological conditions cannot be inferred from the regional fold structure alone.

Engineering geology

The M5 Motorway, whose tunnels pass through the eastern part of the Fairfield Basin

The Fairfield Basin is important to the engineering geology of metropolitan Sydney because its structure controls the depth, distribution and orientation of several major bedrock units. Large infrastructure projects crossing the basin may encounter substantial transitions between strong Hawkesbury Sandstone and the generally weaker shale, siltstone and claystone of the Wianamatta Group.

Knowledge of the position of the synclinal axis and the dip of its limbs assists in predicting which formations will be encountered during tunnelling and deep excavation. The regional structure has consequently been considered during investigations for motorway tunnels, railway projects and major developments throughout Sydney. The M5 motorway tunnels were driven through the eastern part of the Fairfield Basin. Excavation showed gently undulating bedding, with strata locally dipping towards the northwest, north and northeast.

The later M8 tunnels provided substantially more information on the southern limb of the Fairfield Basin. Their long underground exposures allowed correlations between the Hawkesbury Sandstone, Mittagong Formation and Ashfield Shale and provided observations of fault zones, joint swarms and igneous dykes crossing the regional structure. The Fairfield Basin has also been incorporated into regional geotechnical models for other Sydney infrastructure. Investigations for the North West Rail Link identified the Fairfield Basin as the central part of the Sydney Basin in which the greatest sedimentary thicknesses have been encountered and used its location to explain the general south-westward regional dip along the project corridor.

See also

• Sydney Basin
• Geology of New South Wales
• Cumberland Plain
• Wianamatta Group
• Hawkesbury Sandstone
• Bringelly Shale
• Homebush Bay Fault Zone
• Great Sydney Dyke