Geology and Soils of the Perth Region, Western Australia

Early Impressions of Western Australia

Upon leaving England, an observer in Albany in March 1836 described the country as a "dull, uninteresting time." From an elevated position, the landscape appeared as a woody plain, dotted with rounded, partly bare granite hills. The soil was consistently found to be sandy and very poor, supporting either coarse, thin, low brushwood and wiry grass, or stunted forests. While the bright green colour of the brushwood from a distance might suggest fertility, "a single walk, however, will dispel such an illusion." The observer concluded that anyone sharing this view would "never wish to walk again in so uninviting a country."

Landscape Evolution: Perth Area

The Perth area's landscape evolution is characterized by two primary structural units:

  1. Yilgarn Craton (Darling Range and Wheatbelt):

    • Geology: Composed of ancient igneous and metamorphic rocks.

    • Regolith: Predominantly laterite.

  2. Perth Basin (Swan Coastal Plain):

    • Geology: Underlain by a deep sequence of sedimentary rocks, mostly Cainozoic to Permian in age, with a maximum thickness of up to 15,000 ext{ m}. These sedimentary rocks are covered by superficial deposits closer to the surface.

    • Regolith: Consists of superficial deposits, primarily aeolian (wind-blown) and fluvial (river-deposited) sediments.

Influences of Fluctuating Sea Levels

The superficial deposits covering the Perth Basin are a significant legacy of past sea-level changes. Approximately 20,000 years ago, during the peak of the Last Glacial Maximum, the sea level was significantly lower than today.

Key changes in sea level include:

  • 18,000 Years Ago: Sea level was at its lowest, approximately -130 ext{ m} below current levels. This exposed a vast land area, and the ancestral Swan River would have extended much further west.

  • 7,000 Years Ago: Sea level had risen to approximately -10 ext{ m} below current levels.

  • 5,000 Years Ago: Sea level was at its highest point in recent geological history, about +2.4 ext{ m} above current levels. This resulted in an expanded modern shoreline compared to 18,000 years ago.

Evidence of these past conditions can be found in features like rhizo-concretions, root channels, and solution pipes, which formed during periods of lower sea level when the land surface was exposed to subaerial erosion and pedogenesis.

Geology of the Swan Coastal Plain

The Swan Coastal Plain's geology includes several key formations:

  • Tamala Limestone (Aeolianite): Composed of calcareous dunes and some reef complexes. The Spearwood sand is derived from this limestone.

  • Spearwood Dune System: Characterized by sands derived from the weathering of Tamala Limestone.

  • Bassendean Dune System: Consisting of modified aeolian sands.

  • Alluvial deposits: Comprising mixtures of clay and sand.

Today, the Swan Coastal Plain is situated between the Darling Scarp (part of the Pre-Cambrian Yilgarn Block) to the east and the Holocene interglacial coastline to the west. Subaerial erosion and pedogenesis have been active throughout the Late Cainozoic period. The geological units, from east to west, include the Darling Scarp (Pre-Cambrian Rocks), Mesozoic rocks of the Perth Basin, the Ridge Hill Sandstone, Guildford Formation, Bassendean Sands, Quindalup Dunes, and Modern Coastal Wetlands.

Major Geomorphic Elements (Swan Coastal Plain)

Four major geomorphic elements are recognized on the Swan Coastal Plain:

  1. Ridge Hill Shelf (Yoganup Formation): Located inland, often at the base of the Darling Scarp.

  2. Pinjarra Plain (Guildford Clay): Alluvial plains associated with rivers.

  3. Bassendean Dunes: Older, heavily leached aeolian dunes, generally further inland from the coast.

  4. Spearwood Dunes (Tamala Limestone): Younger, less leached aeolian dunes, closer to the current coastline.

  5. Quindalup Dunes: The youngest, active coastal dunes.

  6. Coastal Wetlands and Lakes: Interspersed throughout the plain, especially within dunal systems and along the coast.

Formation of Soils on the Swan Coastal Plain

Soil formation on the Swan Coastal Plain is a complex process involving:

  • Weathering of the Tamala Limestone: This process leads to the formation of structures like rhizoconcretions and solution pipes. As calcium carbonate (CaCO_3) is removed through dissolution and leaching, sand accumulates, and the underlying geology is exposed to erosional activity.

  • Modification of Sand Deposits: Superficial sands undergo further modification through:

    • Addition of organic matter: Accumulation at the surface, especially in vegetated areas.

    • Weathering: Further breakdown of mineral grains.

    • Translocation: Movement of materials (e.g., clay, organic matter, iron oxides) within the soil profile.

    • Action of groundwater: Influencing the chemistry and movement of dissolved substances.

  • Development of Soil Horizons and Podzol Formation: These processes lead to the development of distinct soil horizons and the widespread formation of Podzols, including