and the low-lying
Cumberland Plain. Minor igneous activity took place in the basin during the
Early Jurassic (i.e. 210 million years ago), Late
Mesozoic (i.e. 100–90 million years ago) and
Cenozoic eras (i.e. 65 million years ago). The Early Jurassic activity resulted in the formation of the
Prospect dolerite intrusion in
Prospect Hill. Although Jurassic sedimentation is not observed in the Sydney Basin, there are Jurassic volcanic
breccia pipes (
diatremes). The Sydney Basin is part of a major basin system that extends over from the
Bowen Basin in
Queensland through to the Basin in NSW. Onshore, the basin contains of Permo-Triassic clastic sediments, while the offshore basin contains of sediments. The basin overlies the
Lachlan Fold Belt and Late Carboniferous volcanoclastic sediments. The basin formed during extension in the
Early Permian, with
half-graben infilled with the Dalwood and Talaterang Groups. Foreland loading followed with the compression of the Currarong Orogen in the Early Permian.
Late Permian uplift associated with the
New England foreland loading phase resulted in the formation of depocentres with the northeast Sydney Basin with best preserved marine fossils. These depocentres filled with pyroclastic and alluvial-paludual sediments of the Newcastle Coal Measures. In the Triassic, uplift of the offshore basin resulted in reworking of Permian sediments in fluvial environments. The basin underwent a final phase of deformation (thrusting) in the
Middle Triassic where it was uplifted to become dry land, with an erosion occurring from this time to the present. Extension and breakup in the Tasman Sea beginning in the Late Cretaceous resulted in the current structural boundaries of the basin's eastern margin. In the south and west the Basin finishes in cliff lines formed on sandstones and
conglomerates of the primary Permian sediments, with waterfalls being widespread on all escarpments. •
Mid Triassic: 247–235 million years ago; A monolithic river with its beginnings to the south-west of
Broken Hill, in what was
Antarctica at that time, had its delta in what was the Sydney Basin. It is around five times bigger than the
Amazon River. There is predominance of
silica sand with minor lenses of
clay.
Plant fossils are scarce, but some fish fossils are found in the clay lenses. •
Late Triassic: 235–201 million years ago; As the river slowed with the erosion of the mountain range, finer shales were laid out. This strata is rich in
seed fern fossils. •
End of Triassic: 201 million years ago; Ascension and shifting at the Lapstone fault, with the Blue Mountains rising and the
western Sydney plain descending to a flat land and
Sydney CBD jousting upward. •
Jurassic: 201–145 million years ago; Erosion, with
Ashfield Shales remaining on top. Deep V-shaped valleys in the
Hawkesbury sandstone. Fracturing, volcanic intrusions form
Prospect Quarry,
Mount Tomah,
Mount Wilson and
Hornsby Quarry. •
Cainozoic (Tertiary and Quaternary): 66 million years to present; Gondwana broke up around 40 to 60 million years ago. That is when the Australian continent started to form where it drifted and rifted, where Sydney's rocks were elevated, canted and then later eroded by the weather. Sydney's
sedimentary rocks were shaped into a landscape that was defined by
bedrock valleys exposed into a raised plateau. Sydney's largest rivers, such as the
Hawkesbury,
Parramatta,
Georges and
Hacking Rivers eroded the region's deepest valleys. In this period, the Ashfield Shale got weathered to create a flatter landform with low, undulating topography and reasonably fertile soils, which heavily contrasted the plateaus, cliffs and gorges on the sandstone areas in the Sydney Region. ==Hydrology==