At the end of the Permian the vast, cold-climate peatlands that formed the Australian black coal deposits started to dry up, and a similar situation existed elsewhere in Gondwana. This warming phase of Australian geological history can be separated into two parts. The older and longer part lasted through the Triassic and Jurassic (252–145 Ma). Ice had begun to diminish by about 275 Ma, and, a little later in Australia than in other areas of Gondwana, a great warming followed. This resulted in arid landscapes, compared to the Permian wet peatlands, dominated by inland rivers and lakes and leading to a wide span of inland plains accumulating sediment. The span reached its maximum in the early Jurassic (around 180 Ma ago). The second part of the warming lasted for most of the early Cretaceous (135–105 Ma). A rapid sea level rise was driven by a global pulse of seafloor spreading. Development of young, shallow ocean at the expense of old, deep ocean (see Figure 2.2) had direct consequences for ocean volume, with widespread flooding of the continents. Shallow, cool-water inland seas developed across much of Australia. However, eastern Australia was more expansive than at present, having been built progressively by accretion during the successive orogenies described in Chapter 5. The presence of the magmatic arc, with active volcanism, continued as a feature of eastern Australia. Due to subsequent events only the inboard fringe of the magmatic arc now remains onshore. The seas withdrew from 105 Ma ago, leaving an essentially emergent continent. The breakup which led to the final disintegration of Gondwana followed. Rifting along the eastern margin of Australia trimmed its extent essentially to what exists today.

WARMING OF THE INLAND PLAINS

Interpretation of the climate is hampered because the sizes of the old mountain belts are not exactly known, although the sedimentary rock and fossil records do provide many clues to the Mesozoic course of events.