Chillingham Volcanics
The Chillingham Volcanics crop out in a 2-5km wide strip extending for
approximately 100km, lying about 25km inland from the south-east Queensland/
north-eastern New South Wales coastline (see Figure 4). They crop out as a discontinuous
north-north westerly striking belt of gently westward dipping rocks. Both
pyroclastic and effusive rhyolitic rocks are preserved within the Chillingham
Volcanics, and most sites show evidence of two phases of activity - an early
pyroclastic phase and a later lava effusion phase.
Pyroclastic Phase
Pyroclastic rocks within the Chillingham Volcanics occur at the base of the
Chillingham Volcanics and mark the initiation of volcanic activity and a
depositional environment where ever the unit is preserved. Pyroclastic rocks are
volumetrically most significant in the northern occurrences of the Chillingham
Volcanics where only one eruptive cycle is preserved. Where two cycles of
activity are preserved, the initial pyroclastic rock deposits (prior to lava
effusion) are the thickest, with comparatively thin deposits occurring between
successive lava effusion cycles. The pyroclastic rocks preserved within the
Chillingham Volcanics include ignimbrites, air-fall tuffs (some with
accretionary lapilli), and pyroclastic surges, with the volumetrically most
significant being the ignimbrites and air-fall tuffs.
The thickest individual ignimbrites observed within the Chillingham Volcanics
are in the order of 200m thick, while the thinnest are less than 1m thick. Most
are several tens of metres thick. The ignimbrites of the Chillingham Volcanics
have a tremendous range in the lithologies of dense lithic clasts. Unlike the
ignimbrites of the Brisbane Tuff, which essentially only have
fragments of Palaeozoic Basement rocks as the common dense lithic component,
most dense lithic clasts in the ignimbrites of the Chillingham Volcanics are
rhyolite. Commonly most of these clasts are of flow banded lavas, with small
proportions of spherulitic lavas and re-worked ignimbrites.
Perhaps because the preservation of air-fall tuffs within the Chillingham
Volcanics would have been more dependant on the palaeo-environment than the
ignimbrites, the ignimbrite facies are quite common throughout the Chillingham
Volcanics, while air-fall tuffs are restricted to sections which also have
epiclastic sedimentary rocks, or in rare cases where air fall tuffs have been
preserved between ignimbrites. No air-fall tuffs have been observed preserved in
isolation from either sedimentary rocks or ignimbrites.
Lava Flow Phase
Lava flows volumetrically constitute most of the Chillingham Volcanics, and
probably mark the termination of volcanic activity within the unit, as no
evidence of late intrusions that might have fed still younger Triassic volcanism
has been identified within the outcropping examples of the Chillingham
Volcanics. All areas of the Chillingham Volcanics have at least one cycle of
effusive rhyolitic activity preserved.
Typically the lava flows of the Chillingham Volcanics possess a pervasive
flow banding fabric, which range in scale from the microscopic through to flow
bands (or domains) several metres in width. Commonly the larger bands of
decimetre scale or larger, segregate domains of finely flow banded rhyolitic
lava from more coarsely flow banded lava, spherulitic lava, or apparently
massive lava. The areas of near horizontal flow banding, which constitute most
flows, have been interpreted as being the distal parts of the flows/domes, while
the near vertical portions have been interpreted as either being the surface
manifestation of feeder dykes or the feeder dykes themselves. Studies of the
orientation of the originally vertical and near vertical flow band zones have
revealed that there is a consistent north-westerly strike possessed by the
majority of the flows. This strike is pervasive in all flows except for some
south of Mt. Warning which strike east-west. The pattern of the flow fabrics is
strongly suggestive of a fissure type eruption mechanism for the effusion of the
lava flows. The strike of these vertically oriented zones also closely parallels
the outcrop orientation of the Chillingham Volcanics, and the orientation of the
Ipswich Basin itself, inferring that the eruptive fissures may be related to
deep structures associated with the basin's formation.
The Chillingham Volcanics crop out in a 2-5km wide strip extending for
approximately 100km, lying about 25km inland from the south-east Queensland/
north-eastern New South Wales coastline (see Figure 4). They crop out as a discontinuous
north-north westerly striking belt of gently westward dipping rocks. Both
pyroclastic and effusive rhyolitic rocks are preserved within the Chillingham
Volcanics, and most sites show evidence of two phases of activity - an early
pyroclastic phase and a later lava effusion phase.
Pyroclastic Phase
Pyroclastic rocks within the Chillingham Volcanics occur at the base of the
Chillingham Volcanics and mark the initiation of volcanic activity and a
depositional environment where ever the unit is preserved. Pyroclastic rocks are
volumetrically most significant in the northern occurrences of the Chillingham
Volcanics where only one eruptive cycle is preserved. Where two cycles of
activity are preserved, the initial pyroclastic rock deposits (prior to lava
effusion) are the thickest, with comparatively thin deposits occurring between
successive lava effusion cycles. The pyroclastic rocks preserved within the
Chillingham Volcanics include ignimbrites, air-fall tuffs (some with
accretionary lapilli), and pyroclastic surges, with the volumetrically most
significant being the ignimbrites and air-fall tuffs.
The thickest individual ignimbrites observed within the Chillingham Volcanics
are in the order of 200m thick, while the thinnest are less than 1m thick. Most
are several tens of metres thick. The ignimbrites of the Chillingham Volcanics
have a tremendous range in the lithologies of dense lithic clasts. Unlike the
ignimbrites of the Brisbane Tuff, which essentially only have
fragments of Palaeozoic Basement rocks as the common dense lithic component,
most dense lithic clasts in the ignimbrites of the Chillingham Volcanics are
rhyolite. Commonly most of these clasts are of flow banded lavas, with small
proportions of spherulitic lavas and re-worked ignimbrites.
Perhaps because the preservation of air-fall tuffs within the Chillingham
Volcanics would have been more dependant on the palaeo-environment than the
ignimbrites, the ignimbrite facies are quite common throughout the Chillingham
Volcanics, while air-fall tuffs are restricted to sections which also have
epiclastic sedimentary rocks, or in rare cases where air fall tuffs have been
preserved between ignimbrites. No air-fall tuffs have been observed preserved in
isolation from either sedimentary rocks or ignimbrites.
Lava Flow Phase
Lava flows volumetrically constitute most of the Chillingham Volcanics, and
probably mark the termination of volcanic activity within the unit, as no
evidence of late intrusions that might have fed still younger Triassic volcanism
has been identified within the outcropping examples of the Chillingham
Volcanics. All areas of the Chillingham Volcanics have at least one cycle of
effusive rhyolitic activity preserved.
Typically the lava flows of the Chillingham Volcanics possess a pervasive
flow banding fabric, which range in scale from the microscopic through to flow
bands (or domains) several metres in width. Commonly the larger bands of
decimetre scale or larger, segregate domains of finely flow banded rhyolitic
lava from more coarsely flow banded lava, spherulitic lava, or apparently
massive lava. The areas of near horizontal flow banding, which constitute most
flows, have been interpreted as being the distal parts of the flows/domes, while
the near vertical portions have been interpreted as either being the surface
manifestation of feeder dykes or the feeder dykes themselves. Studies of the
orientation of the originally vertical and near vertical flow band zones have
revealed that there is a consistent north-westerly strike possessed by the
majority of the flows. This strike is pervasive in all flows except for some
south of Mt. Warning which strike east-west. The pattern of the flow fabrics is
strongly suggestive of a fissure type eruption mechanism for the effusion of the
lava flows. The strike of these vertically oriented zones also closely parallels
the outcrop orientation of the Chillingham Volcanics, and the orientation of the
Ipswich Basin itself, inferring that the eruptive fissures may be related to
deep structures associated with the basin's formation.