Australia: The Land Where Time Began

A biography of the Australian continent 

Cape York Peninsula

The landscapes of Cape York include dissected sandstone escarpments and plateaux, rugged mountains, massive bauxite cliffs, huge granite boulders, large sand dunes, perched lakes and, especially in the wet season, large rivers. The cape is the northernmost part of Australia in the Wet-Dry Tropic zone, where there are definite wet and dry seasons.

The climate is one of extremes, wet season deluges alternating with dry season fires and droughts lasting for about 7 months of the year. Fires are a major feature of the Top End in the dry season. The rocks of the cape formed from the Precambrian right through to the Cainozoic.

It is the most northerly point in Australia. Cape Weer-weer, is the most westerly part of the Cape (141 00 E), Cape York is the most northerly (10 41 S) and the most easterly point is Cape Flattery (145 00 E). It covers an area of 130,000 km2. The northern tip of the mainland is 160 km from the mainland of Papua New Guinea .

Geology

It consists of 2 major sedimentary basins, the Laura and Carpentaria. These 2 basins are separated by ancient rocks beneath the Great Dividing Range. The watershed in the range is formed by sandstone escarpments and undulating plains and plateaux on either side of the ranges. The ranges are cooler and wetter than the basins on either side. During the wet season very large volumes of water, with large quantities of sediment, are transported across the vast alluvial floodplains of the Channel Country, and in a good wet, on to Lake Eyre.

When the sea level dropped during the last ice age large amounts of sediments were deposited along the coast, forming large dune fields.

The wide range of parent rock types has meant there is a wide variety soil types. Soils of types from deep, highly weathered to shallow, stony, little developed soils. The combination of different soils, different climates, and different topography has resulted in a wide range of habitats. These include rainforests, heathlands, shrublands, grasslands, wetlands and woodlands. The complex biodiversity of the cape has made it a distinctive biogeographic region.

Biogeographical regions

Climate

There are 3 distinct rainfall regimes in tropical northern Australia, Wet Tropics - mean annual rainfall 1200-4000 mm, 60% of which falls in the wet season from December to March. The Wet-Dry Tropics - mean annual rainfall of 600 - 1600 mm , more than 90 % falling in the wet season between November and April. The east coast is an exception, the wet season accounting for 75 % of the annual rainfall. The Peninsula is in this climatic zone. The Dry Tropics, central and western tropical areas, has much less rain and it is unreliable, some parts receiving no rain for most of the year. The zone can receive showery rain brought in by southeasterly winds even in winter.

The winter months of May-July are cool and pleasant, but by October things are really heating up, and by November the first electrical storms begin, though not yet at the level of the wet season, which usually starts in about December. Usually some time in December or January, though it can be as late as in February,  the monsoon trough moves south over the Peninsula. This is when the real wet season starts. At this time the torrential downpours can continue for days at a time. There are often tropical cyclones in the area that can cross the coast, dumping huge amounts of rain in a short period of a few days or longer. It is normal to have seasonal flooding, which can be more severe when cyclones bring extra rain. Cyclones tend to affect some areas of the Peninsula more than others, such as between Princess Charlotte Bay and Cooktown, on the east coast and the southwestern corner of the Gulf of Carpentaria. On a local scale, squalls and thunderstorms that build during the later afternoon, can occur at any time of year. They are, however, more common at the times of seasonal change, March to May and October to November.

The rainfall mostly originates in the Gulf of Carpentaria or the Coral Sea, the actual pattern of rainfall being controlled by the distance from the sea and the topography of the area. Apart from drier areas around Princess Charlotte and Bathurst Bays, where the rainfall averages 1000-1250 mm/yr, rainfall decreases with distance inland. The rainfall  increases from south to north. The average for most areas receive above 1000 mm/year, but the average varies. It is much drier in southern inland areas, sometimes being closer to that of the Dry Tropics. Northern and eastern areas tend to have higher rainfall than the southern inland areas. The highest average rainfall recorded was at Lockhart River, though it is believed that some of the surrounding peaks may receive much higher rainfall. The seasonal rainfall at this part of the coast is more like that of the Wet Tropics. The mean relative humidity is usually higher than 80 % in the wet season, remaining high even in the drier winter months. The relative humidity in the southern inland ranges from about 60 to 70 % in the wet season, dropping to about 40 % in the dry. For most of the year southeasterly winds blow along the coast, averaging about 12-15 knots, the frequency and velocity increase during April and are blowing for 80 % of the time by May. Wind speeds average 29 knots in August. In November and December the prevailing winds are the northeasterlies.

Most areas are consistently hot with little variation diurnally or seasonally. Daily maxima are about 30-32 C along the east coast in the wet season, and 26-28 C in the winter part of the dry season, thanks to the southesasterlies. The temperature regime along the west coast, on the Gulf of Carpentaria, is different. There the winter dry-season temperatures are 30-32 C while in the wet season they rise to 35 C, rising occasionally to more than 38 C in November and December. On the rarer occasions when the temperature exceeds, it usually lasts for several days. In the high ranges it can get much cooler in winter, even down to about 10 C. In the most fire-prone inland areas it is normal to experience drought near the end of the dry season, and these droughts can last longer in dry spells.

Soils and Geology

Cape York Peninsula soils are, in general, nutrient deficient, having been described as the poorest of the poor, considered as poor in a continent of poor soils. With such impoverished soils, fire becomes an important source of phosphorus and nitrogen.

The rock of the Peninsula is a stable shield of Precambrian metamorphics, Middle Palaeozoic granite and Late Palaeozoic volcanics. These rocks are buried beneath the sediments from the Mesozoic and Cainozoic in the Carpentaria Basin and Laura Basin. The division of the Peninsula into 4 distinct areas is based on these rock types. The north-south trending axis of very old rugged ranges and plateaux - the Peninsula Ridge, the Hodgkinson Basin, the Carpentaria Sedimentary Basin and Laura sedimentary basin, and the coastal flood plain and dunefields of marine origin.

The Peninsula Ridge (the Coen Inlier)

This is the backbone of the Peninsula. It is the oldest part of the Peninsula. It extends from the Olive River Area on the eastern coast of the Peninsula down to 15o30'S. For most of the length of the Great Dividing Range it is underlain by these ancient rocks. The lowest part of the ridge is in the south, reaching its highest point of about 800 m east of Coen, sloping down to about 250 m to the north. North of Olive River the ridge continues along the coast beneath the sea as far as the tip of Cape York.

The ridge is composed of Precambrian metamorphic rocks and Palaeozoic igneous granite. The 3 main areas of metamorphics on the Peninsula Ridge are to the south and west of the ridge, the Holroyd Metamorphics, to the east, the Coen Metaporphics, and to the north, the Sefton Metamorphics of the Iron Range and Temple Bay areas. In the Palaeozoic, granites were extensively intruded into the older metamorphics, resulting in the highest and most rugged land on the Peninsula. The Cape York Peninsula Batholith formed along the length of the ridge as a result of the intrusion of massive granite rocks in the Devonian. Further intrusion of granite occurred in the Carboniferous and the Permian in the area around Iron Range and near Coen. Around this time sediments were being deposited in the lower areas to form the Pascoe River Beds.

In the Palaeozoic, the Janet Range Volcanics, the Kangaroo River Volcanics and the Cape Grenville Volcanics, as well as some smaller volcanic outcrops, some near Iron Range and second Red Rocky Point,  were formed by eruptions of acid (quartz-rich) lava near Iron Range and Cape Grenville.

Red and yellow podzolic soils are the most common soil type formed from the Precambrian and Palaeozoic Ridge rocks, of these the reed soils are better drained. Podzolics are low nutrient, heavily leached, acidic soils. Lithosols, partly- or unweathered rock debris, with not much soil formation, are the dominant soil type on steeper escarpments.

The Yambo Inlier is an ancient shield in the Einasleigh Uplands. The Silurian Palmerville Fault separates the Yambo Inlier from the Hodgkinson Basin. This basin formed when sediments were deposited above Precambrian basement metamorphics. The Hodgkinson Basin also contains some volcanic and granite outcrops. The most spectacular rocks are the huge boulders at Cape Melville.

Sediments from the ranges were deposited in the Laura sedimentary basin and the Carpentaria sedimentary basin at a time of an extensive marine incursion during the Mesozoic and Early Cainozoic. They were then uplifted, forming vast areas of plains and plateaux. These sediments consist of coarse sandstones that were overlain by finer shale, siltstone and mudstone. The sediment types include sand, sandstone, sandy claystone, silt and clay.

Red, yellow, grey and brown earths are all found in in these basins. These soils are deep, very permeable, loamy, and relatively uniform, with a high clay content at lower depths. In some areas, the podzolic soils have massive ironstone or ironstone gravel in the surface layers over extensive areas. They are lateritic podzolics. Deep lateritic podzolics are usually covered by deep red earths. The laterite is exposed, with the upper levels of bauxite, around Weipa and Ussher Point.

Marine sediments were deposited as extensive floodplains around the coastal and subcoastal areas of both basins during the Cainozoic. These areas are inundated both by high tides and seasonal flooding that covers them for long periods. The soils here are black and grey (cracking clay soils) and loamy or silty-surfaced solodic soils. The deeper levels of floodplain soils with a high clay content are very alkaline. They shrink and swell a lot, and are infertile and finely textured. When they dry out the cracks are wide and deep, but are sticky and waterlogged when wet. The shallower and even more alkaline solodic soils become very hard when dry, becoming oozy when wet.

A period of ice ages began less than 2 million years ago. Cold climates were more widespread than at any time since the Permian, 250 Ma. High sand dunes were piled up along the coast during these glacial periods when the sea level was lowered, exposing large areas of seabed that were exposed to wind erosion.

Sources & Further reading

Dawn W. Frith & Clifford B. Frith, Cape York Peninsula: A Natural History, Reed, 1995

 

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading