|
|
|
|
|
|
|
|
|
|
|
|
|
| Geology |
| Within the NAR there are four distinct geological provinces, each with different hydrological characteristics and
salinity issues. |
|
| The Yilgarn Craton
|
|
This province lies to the east of the Darling Fault and encompasses areas of the Yarra Yarra, Moore River and
Greenough sub-regions. It covers about 2.5 million hectares. |
|
Surface drainage through this area is typically east to west through reasonably incised drainage lines, many of which
remain substantially vegetated. However, salinity is aggressively spreading in this zone causing rapid decline of the
remaining vegetation, which is dominated by large trees. Drainage line stability is under serious threat from salinity
and the consequent death of large trees that play a crucial role in stabilising the banks and sediment of drainage
lines. The headwaters of these drainage lines further to the east are typically broad severely saline valley flats. So
for example, in the east branch of the Moore River, the most severe salinity presently occurs at the top of the
catchment. |
|
|
|
__________________________________________________________ |
|
| The Perth Basin
|
|
The Perth Basin underlies about 2.5 million hectares of the NAR and is almost totally confined to the West
Midlands sub-region. It is a deep trough of sedimentary layers with the total thickness of sedimentary rocks
possibly exceeding 15 000 m. |
|
Much of the West Midlands is considered to have moderate salinity risk with low to moderate salt storage. The
potential exists for widespread hydrological degradation particularly in the topographic lows. Groundwaters are
often very fresh, so initially degradation is induced by waterlogging, but as high evaporation rates drive the system,
land salinisation may be the long term result. |
|
|
|
_________________________________________________________ |
|
| The Irwin Sub-Basin
|
|
This area covers nearly 0.6 million hectares of the NAR. It is a narrow trough of sediments that exceed 5000 m
thickness. The sedimentary sequences are dominantly fine grained (siltstones, shales,
tillite) which have strongly
influenced subsequent soil development. |
|
This area is considered to have high salinity risk with very high salt storage and poorly drained soils. Groundwater
in the Irwin Sub-Basin is typically very saline and little used. Consequently, knowledge of groundwater processes
and trends is poor.
|
|
|
| __________________________________________________________ |
|
| The Northampton Block
|
|
The Northampton Block underlies 0.3 million hectares entirely in the Greenough Sub-Region. It is essentially a
large outcrop of crystalline granitic basement partially capped in the southern and western areas by thin sequences
of Mesozoic sediments. It is considered to have a moderate salinity risk with medium salt storage. Large parts of
the Block appear to be approaching or may very well have attained hydrological equilibrium. That is, groundwater
levels rise and fall in response to seasonal conditions but the longer term trend is stable, particularly in the southern
portion encompassing the Chapman Valley region. However, salinity continues to develop in the wet areas and
seeps by evaporative concentration.
|
(information
reproduced from: Northern
Agricultural Region Draft Regional Strategy: Natural Resource Management 2000 /
NAIMS)
|
|
|
