Gondwanaland breaks up | Plains of quartz |
Jurassic soils | Effect of incursion |
Essentially there were no major geological events in northern
Australia from the mid-Proterozoic (1000 million years ago) to the
Jurassic (150 million years ago). The savannas continued to erode
for all that time. Meanwhile, life had developed in the oceans,
grown legs and lungs and emerged into the sunshine. Gondwanaland
broke up into the land masses we know today.
During the Jurassic, the ancient eroded plain was covered with
quartz pebbles (quartz tends to indicate extremely eroded/
weathered rock) which were derived from the quartz veins
criss-crossing the landscape.
There was little soil development as grass had not yet evolved.
Soil is created when rock is broken down in situ. Without grass or
other forms of land-covering vegetation, the rock will be
constantly eroded and moved about.
However, with grass for example, the rock is held in place while
it breaks down or decays. Some elements of the rock will be more
soluble than others and so soil horizons develop. The acids
leaching from the leaf litter also play an important role in
chemically breaking down the rock. Note that the longer soil is
weathered in this way, the poorer in nutrients it becomes. That is,
over time the mineral elements get leached out.
The soil that did exist during the Jurassic was derived from
nutrient-poor marine sediments. In the same way that soil loses its
nutrient value over time, marine sediments, which have been washing
into the ocean down rivers and streams, also lose their mineral
value.
There was a great incursion of the sea around 100 million years
ago — during the Cretaceous — and directly after the
time of the dinosaurs. This covered the low quartz pebble-strewn
landscape with fine clay sea sediments. Over time these layers
accumulated to be tens of metres thick, thus covering many ancient
landforms.
Localised uplift of land (isostacy again) resulted in local
landscapes looking like miniatures of the Kakadu escarpment. Sea
cliffs and road cuttings around Darwin expose the non-conformity:
the quartz pebble-strewn landscape upon which the dinosaurs used to
roam appears quite clearly between the ancient folded rock below
and the horizontal beds of marine fossil-rich Cretaceous claystone
above. The cretaceous sea extended into central Australia, and the
same claystones that form the local relief of the Darwin landscape
are the host-rocks for the Coober Pedy opal fields.
