Biodiversity Strategy - Gosford City Council - NSW Government

Gosford soils occur predominantly on Triassic age sediments of the Wianamatta and Narrabeen
Groups. These lie in the north-east part of a major tectonic unit known as the Sydney Basin.
Middle Triassic Hawkesbury Sandstone occurs extensively on the Somersby Plateau and
Macdonald Ranges. This overlies the Narrabeen Group and consists of medium to coarse
grained quartz sandstones with minor shales and laminite lenses (Murphy 1993). It also
outcrops on the Boudii peninsular, Kincumba Mountain and Mount Elliot.
The eastern part of the LGA is predominantly the Terrigal (formerly Gosford) Subgroup of the
Triassic Narrabeen Group. It consists of massive and coarse bedded lenticular beds of lithicquartz
sandstone, siltstone and claystone (Murphy 1993). An example of the Terrigal formation
can be seen at the Skillion in Terrigal which displays the distinctive fine sandstone with shale
lenses (Fisher 1997).
Small post Triassic diatremes outcrop at Peats Ridge and Kulnura on the Somersby Plateau.
These are basalt intrusions that were forced through the overlying Hawkesbury sandstone
during the Jurassic period (Fisher 1997). Quartenary alluvium occurs along major watercourses
such as Brisbane Water and around the coastal estuaries. It consists of unconsolidated sands,
silts, clays and gravels. Large deposits of Quartenary marine sediments are found along the
coast and include barrier beach systems that enclose Tuggerah and Munmorah Lakes as well
as the Umina/Ettalong/Woy Woy peninsular (see also 1:100, 000 geology sheet
http://www.geoscience.gov.au). The Umina Woy Woy peninsular consists of a large sandplain
with the remnants of its parallel dunes still evident in some places. Perched sand dunes occur
on cliff tops within Bouddi National Park and adjacent areas (Strom et al.1980).
A - 4.3
Geomorphology
The broad physiographic (or geomorphological) regions of the Gosford region are described in
Chapman and Murphy 1989 and Murphy 1993. These are:
• McDonalds Ranges incorporating Mangrove Dam Creek catchment area and
Kulnura in the north and Dharug National Park to the west. It consists of deeply
dissected rugged steep hills on Hawkesbury sandstone and Narrabeen Group
sediments.
• Somersby Plateau covering the Central Mangrove area, Mangrove Mountain and
Peats Ridge down to Somersby and Kariong on Hawkesbury sandstones. The
Somersby Plateau forms a northern extension of the Hornsby Plateau and is
described as an undulating plateau on Hawkesbury sandstone (Chapman and
Murphy (1989) and Murphy (1993). It includes the Woy Woy peninsular but this
would appear to be better described as per Bell(2004) as Coastal Floodplains and
Estuaries incorporating the major floodplains of Erina and Narara Creeks that feed
into Brisbane Water, the coastal estuaries and the Umina sandplain.
• Hawkesbury Valley covering the drowned river valleys of the Hawkesbury River
and its tributaries which dissect the rugged and distinctive Hawkesbury sandstone
and softer Narrabeen Group sediments, and
• Erina Hills covering the eastern parts of the LGA described as a low and undulating
topography below 30 metres above sea level on Narrabeen Sandstone geology. The
coastal plain has a low relief with a undulating to flat topography.
• The Watagan Mountains region covers a small section in the north of the LGA at
Niagara Park including Strickland State Forest.
Streams dissect the plateau area exposing the softer interbedded siltstone and Sydney
sandstone. Most flow in a southerly direction toward the Hawkesbury River and Broken Bay via
Biodiversity - Technical Report Page 83