LATE TRIASSIC VOLCANISM OF THE IPSWICH BASIN

The initiation of the Late Triassic intra-montane Ipswich Basin in south-east Queensland is marked by the eruption of intra-cratonic, rift-related mafic and felsic volcanic sequences. Within the Ipswich area, the Sugars Basalt and Weirs Basalt, both basaltic-andesites, locally form the lowest unit in the basin. Younger, pyroclastic rocks occur interbedded within the overlying epiclastic rocks. The most significant of these are the ignimbrites and air-fall tuffs of the rhyodaciticHector Tuff, and the rhyolitictuffs of the Mount Crosby Formation. In the Brisbane area to the east, the rhyolitic Brisbane Tuff forms the base of the basin. The multiple ignimbrites that comprise most of the Brisbane Tuff were probably erupted from a single vent over a short time. To the south, theChillingham Volcanics crop out on either side of the NSW - Queensland border, and include rhyolitic lava flows and domes, pyroclastic rocks, and epiclastic sedimentary rocks. Other surface occurrences of volcanic rocks within the Ipswich Basin include the rhyolitic and andesitic lavas ofMoreton Island and the rhyolitic lavas of Stradbroke Island. In DDHGSQ26, the basal section comprises seven volcanic sub-units (1) ranging in composition from basaltic-andesite to rhyolite, and occurring as pyroclastic rocks and lava flows.

Introduction

The Ipswich Basin is one of several intermontane basins which formed within the New England Foldbelt during the Late Triassic. The basin is characterised by the thick accumulations of coal measures which were predominantly deposited on an alluvial plain (2). The commencement of the basin however is marked by the eruption of a basal sequence of mafic and felsic volcanic rocks. These volcanic rocks form part of a major silicic igneous province of Late Triassic age occurring throughout Southeast Queensland which also includes: the Agnes Water Volcanics (3); the Arrambanga Volcanics (3,4); the Mount Byron Volcanics (3); the Muncon Volcanics (3); the North Arm Volcanics (5, 3); and numerous plutons occurring on the eastern side of the New England Batholith. This paper focuses on the volcanic stratigraphy of the early basin forming volcanic rocks of the Ipswich Basin: the Weirs Basalt; Sugars Basalt; Tuff within the Mt Crosby Formation; Hector Tuff; Brisbane Tuff; Chillingham Volcanics; Stradbroke Island rhyolites; Moreton Island volcanics; and a suite of volcanic rocks form DDH GSQ 26.

Mafic Rocks of the Ipswich Area -The Weirs Basalt and Sugars Basalt

The Sugars Basalt and Weirs Basalt crop out in the Moggill and Mt Crosby areas respectively (see Figure 2). Both formations are of basaltic-andesite composition and, despite stratigraphic separation (6), are probable equivalents. Geochemically the basalts of the two units are indistinguishable.

The Weirs Basalt, which attains a maximum thickness of 30m in DDH NS295 (6) and 11m at it's type section (7), is defined as an altered porphyritic basalt (7). Underlying the unit both at the type section, and within DDH NS295, are members of the Blackwall Breccia, which is inferred to be a scree deposit derived from the local Palaeozoic basement. Although exposure of the Weirs Basalt is limited, at least four lava flows, including amygdaloidal and non-amygdaloidal, porphyritic and aphyric basalts comprise the formation.

The Sugars Basalt's maximum recorded thickness of 105 m is in DDH IC 256 (6), and has a thickness of approximately 37m at it's type section (7). The formation is defined by Cranfield (7) as an amygdaloidal basalt. At the type section of the Sugars Basalt, at least seven separate lava flows and an air-fall tuff occur. Houston's (6) drill log for DDH IC256 indicates the presence of two additional lava flows occurring above those at the type section. The lithologies of the flows within the type section include essentially non-porphyritic lavas, porphyritic lavas, and amygdaloidal versions of the aforementioned types. The development of cooling columns which are restricted to individual flows indicates that the thermal history of individual flows is unique and that at least small time gaps separated the individual flows. Given the comparatively small thickness of the flows, these breaks in activity may not have been significant.

The air-fall tuff within the Sugars Basalt at the type section is light green-grey coloured, and contains abundant feldspar and pyroxene phenocrysts. The mineralogy of the tuff is consistent with it being of basaltic-andesite composition and directly relate to the lava flows within which it is interbedded. Basaltic-andesite vents are not usually prone to extensive magmatic pyroclastic eruptions, the results commonly being a proximal splatter cone derived from fire-fountaining. Therefore the presence of the air-fall tuff within the upper sequence of the Sugars Basalt probably represents the product of a phreato-magmatic eruption. The extensiveness of this tuff indicates that the tuff is not the result of a secondary eruption caused by the lavas entering water. Instead, it implies that molten lava was emplaced directly into water.

Tuff within the Mt. Crosby Formation

The Mount Crosby Formation (see Figure 2) conformably overlies the Blackwall Breccia, disconformably overlies the Weirs Basalt and Sugars Basalt (7), and consists of polymictic conglomerates with comparatively minor quantities of mudstone and arenite. Minor amounts of lithic tuff have also been reported within the Mt. Crosby Formation (7). Surface occurrences of tuff are limited, however two tuffaceous sections separated by a small sequence of interbedded tuffaceous arenite and mudstone were observed near Pine Mountain. Within both DDH NS295 and DDH NS256 Houston (6) recorded the presence of Mt. Crosby formation tuffs. In both examples, two tuff horizons were present, once again separated by fine grained sedimentary rocks. Almond (1) reported the presence of a tuff within the Mt. Crosby Formation in DDH GSQ26, however he did not differentiate how many separate horizons of tuff were encountered. Hawkins (8) described a 25 m thick tuffaceous horizon within NS 93. This section has been subsequently reinterpreted as a lateral equivalent of the Mt. Crosby Formation, and has the two characteristic tuff horizons separated by shale beds.

The phenocryst content of both tuffs was made up of plagioclase feldspars, quartz, alkali-feldspars, and possible trace biotite. The approximate thickness of the upper tuff is approximately 1.3 m , while the lower tuff's thickness varies between 3.7 m (DDH 295) and 5.3 m (DDH 256). The upper, slightly coarser grained tuff contains abundant dense lithic clasts which were composed exclusively of black chert (average size 3mm). Small, comparatively unflattened pumice clasts were also observed within the upper tuff. This, together with the foliated appearance (parallel to bedding) of the bed when weathered, suggests that the upper tuff is ignimbritic in origin. The lower tuff contains chloritically altered glass shards, pumice clasts, and clay clots psudomorphing pumice. The lower sequence possesses an ignimbrite like texture and parallel clasts, indicating that it too may be ignimbritic in nature.

Commonly informally regarded as a lateral equivalent of theBrisbane Tuff, the two tuffs of the Mt. Crosby Formation, both of which are probably ignimbritic in origin, have no conclusive correlation with the Brisbane Tuff. Chemically the rocks are more dacitic than the Brisbane Tuff, and their dense lithic clast content is different from that of the Brisbane Tuff, being both depleted in comparative volume and dominated exclusively by black cherts where as the Brisbane Tuff contains chert and phyllitic clasts. Additionally, the Brisbane Tuff shows evidence of a greater degree of crystal enrichment than the Mt. Crosby Formation. Potentially the Mt. Crosby formation could be representative of a non-fines depleted, or possibly fines enhanced Brisbane Tuff, however chemical differences suggest a different eruption.

Hector Tuff

The Hector Tuff (See Figure 2) consists of air-fall tuffs, ignimbrites, mudstone, arenite, and minor amounts of conglomerate shale and coal. The formation, which is a member of the Kholo Subgroup, conformably overlies the Colleges Conglomerate and is conformably overlain by the Cribb Conglomerate. The formation occurs extensively throughout the Ipswich Area, however good outcrops occur only near the type section, where the unit is 22m thick (9, 7). The unit progressively thins eastward, and this thinning is accompanied by an apparent reduction in the amount of primary volcanic material preserved within the formation. The tuff is of rhyodacitic composition.

Most exposures of the formation consist of reworked, tuffaceous arenite or mudstone. Primary volcanic rocks observed within the formation include thin ignimbrite units, air-fall tuffs, and an accretionary lapilli tuff. The ignimbrites, which are usually grey~green in colour, show no evidence of welding. The clast content of the ignimbrites was difficult to determine due to the weathering of most outcrops, however small pumice clasts (up to 3 cm) and rare chert clasts were identified at some localities. The thickest individual ignimbrites observed in outcrop were only approximately 1.5m thick.

Air-fall tuffs are the volumetrically most abundant of the pyroclastic rocks within the formation. Although primary air-fall tuffs themselves are not as volumetrically significant as the ignimbrites, when the volumes of re-worked tuff (preserved in the form of tuffaceous arenites) are included, they take on greater importance. The air-fall tuffs are typically green in colour, are moderately fine grained, and rarely porphyritic. The thickness of observed primary occurrences of tuff seldom exceed 1.5 m, however re-worked tuffs (in the form of tuffaceous arenite) accumulated in thicknesses up to 5 m.

Brisbane Tuff

The Brisbane Tuff comprises rhyolitic ignimbrite and minor air-fall tuff, conglomerate, breccia, and volcano lithic arenite. It locally forms the base of the Ipswich Basin around Brisbane, but elsewhere is commonly deposited on thin breccia or rarely on a few metres of fluviatile sediment. The unit outcrops over a distance of approximately 50km, in a northwesterly striking band extending from Narangabah north of Brisbane, through to Woodridge in the south (see Figure 3a). The best outcrops of the unit are within the vicinity of the city of Brisbane, and it's inner city suburbs.

The Brisbane Tuff 's thickness ranges from less than a metre to more than 50 metres. Maximum thickness of the unit has been variously quoted as 300 feet (96 metres) (10), 700 feet (225 metres) (11), and 250 metres (9). The maximum observable thickness however is only approximately 40m, and occurs in the Kangaroo Point Cliffs.

Ground surges are frequently preserved at the disconfformal contacts with the underlying basement rocks. The surge layers are typically comparatively crystal rich, and contain abundant charcoal, but are otherwise essentially deficient in lithic clasts. The ground surge horizon is normally comparatively thin with poorly defined cross beds, but at a disused quarry at Windsor, prominent cross beds occurring in a thicker horizon indicate a local flow direction with a strike of 150~330§. Air-fall tuff deposits, often exhibiting mantle bedding, also underlay the main ignimbrite body. These air-fall tuffs are usually crystal rich, and are commonly silicified or stratified.

Multiple Ignimbrite Theory

Both direct and indirect evidence exists to support the theory that the Brisbane Tuff is constituted of multiple ignimbrites. The presence of interbeded fluviatile sediments (12, 9, 13), layers of accretionary lapilli air-fall tuff (14), weathered layers (13), and air-fall tuff within the ignimbrite provides the best direct evidence for multiple ignimbrites. Additionally three distinct types of pumice clasts have been recognised within different samples of the Brisbane Tuff:

1) A non-porphyritic pumice clast;

2) A porphyritic quartz and felspar rich pumice clast; and

3) A porphyritic quartz and biotite rich pumice clast.

The most common type of clast is the non-porphyritic clast. In any one outcrop only one of the above types of pumice clast is present. The presence of these three distinct types of pumice clasts indicates differing material being erupted at vent, and hence provides further evidence of multiple ignimbrite flows constituting the Brisbane Tuff.

Weathered horizons (13) within the Brisbane Tuff imply that a significant time break may have occurred between eruptions. However all outcrops studied indicate that the Brisbane Tuff occurs as a single cooling unit, therefore the break between eruptions must have been short enough for retention of heat within the ignimbrite. Additionally charcoal is conspicuous within the lower portions of the ignimbrite and surge layers, indicating the mass destruction of vegetation during the early eruptions. No conspicuous charcoal is visible within the upper ignimbrites suggesting that no significant re-vegetation occurred between eruptions.

Fossil Fumaroles

Fossil fumaroles are common within the Brisbane Tuff, and range in size from small pipes and joints of less than 1cm in width to large fumaroles of several metres in width. The smaller fumaroles are typically infilled with opal or goethite whereas the pronounced alteration halos of the larger fumaroles are composed of illite and muscovite. Where it is possible to establish stratigraphic control on the position of the fumaroles within the ignimbrite, they are usually restricted to the lower sequences of the ignimbrite. Some of the fumaroles are apparently rootless, but for many of the roots are not exposed.

Deposition Environment and Eruptive History

Much of the preserved extent of the Brisbane Tuff was believed to have been deposited as a valley fill ignimbrite. Most preserved contacts between the basement rocks and the Brisbane Tuff are steep (some almost sub-vertical) when compared to the orientation of bedding within the ignimbrite. Some localities have fossil scree slopes of brecciated basement material preserved under the overlying ignimbrite, which is consistent with there being steep valley walls. Prominent near horizontal cooling columns preserved near the contact with the basement at Windsor also indicate that the cooling surface (a valley wall) was near vertical. In addition to the flow lineations parallelling or sub parallelling the outcrop orientation, the presence of many fumaroles near the base of the ignimbrite showing evidence of water/ignimbrite interaction is also consistent with the ignimbrite being emplaced into a river valley.

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 theBrisbane 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.

Volcanics on Moreton Island

Moreton Island is essentially a sand island of Quaternary age except for the rocks which form the north-eastern point on the island (see Figure 3b). These rocks belong to three groups; undifferentiated Late Triassic volcanic rocks; Late Triassic Ipswich Basin sediments; and sediments of the Early Jurassic Woogooroo Subgroup of the Clarence Moreton Basin. Within the sequence of volcanic rocks, two compositional types are present - rhyolitic lavas in the northern most outcrops and a series of sub-aqueously emplaced mafic lavas on the eastern most outcrops on the island. At least two tuffaceous arenites, probably representing re-worked tuffs, were observed within the Ipswich Basin sediments overlying the mafic lava flows.

The most common facies preserved within the rhyolite lavas on Moreton Island is that of flow banded lava. The flow bands observed on Moreton Island are exclusively within the mm domain size, commonly highly contorted, and typically steeply dipping. Spheurilte growth within the lavas was largely restricted to the sub-mm scale, except for one narrow zone near North Point where they are up to 5 cm.

Emplacement History

At least two episodes of lava eruption are inferred as having occurred on Moreton Island. The first phase, which was a lava dome phase, is responsible for most of the rhyolitic lavas on the island. It is characterised by steeply dipping monotonous flow banded lava, with no observed spherulitic patches and little autobreccia. The younger lava, whose mode of emplacement could not be determined, contains more varied textures, including spherulitic and autobreccias. A significant time break occurred between the emplacement of the first lava dome, and subsequent emplacement of the second lava. At Honeymoon Bay, steeply inclined flow banded rhyolitic lava of the earlier dome is abruptly truncated by an erosive contact, and subsequently overlain by crumble breccias of the later lava. The break between flows was long enough to allow for the de-vitrification of lavas of the first dome, and for significant erosion to expose the (originally) glassy centre of the dome.

Rhyolite on Stradbroke Island

Stradbroke Island, likeMoreton Island, is essentially a sand island. Quaternary sand deposits have built up behind a rock barrier composed of undifferentiated rhyolitic lavas on the north-eastern corner of the island (see Figure 3c). The rhyolites which crop out on Stradbroke Island are typically flow banded on the mm to cm scale, have porphyritic textures, and contain abundant quartz phenocrysts.

Most of the rocks which comprise Stradbroke Island are quartz phenocryst rich rhyolitic lavas and autobreccias. Some rocks occurring at Adder Rocks and Point Lookout have an ignimbritic looking texture; however such occurrences are isolated to small zones, and the absence of recognisable fiamme or pyroclastic flow structures probably precludes the possibility that they are truly ignimbritic. These ignimbrite like rocks at Point Lookout contain clasts of rhyolite, and are overlain by quartz rich porphyritic flow banded rhyolitic lavas identical to those exposed elsewhere on the island. The rhyolite clasts are altered, probably metamorphosed, dark grey coloured, non-porphyritic, and (unlike their host) are quartz phenocryst poor. It is likely these rocks represent lenticular zones within the lava flows, containing small blocks of older lava flows incorporated as xenolith material.

Eruptive History

At least three eruptive periods are represented on Stradbroke Island. The earliest phase of effusive activity produced a non-porphyritic, flow banded rhyolite which is only preserved as metamorphosed xenoliths within other flows on the island. At least three generations of porphyritic lavas were then emplaced from at least two different domes located at Adder Rock and South Headland. Age relationships between the two domes were impossible to determine as no cross-cutting relationships were observed. Texturally these rocks bear a close resemblance to the ignimbrites of the Brisbane Tuff, and given the similar stratigraphic horizon the units occupy, it is possible that the two units may be related.

Volcanic Rocks in GSQ 26

Thick accumulations of volcanic rocks of assumed Late Triassic age have been intersected in many drill holes. The thickest suite uncovered so far were those in DDH GSQ 26 (see Figure 1). This drill hold intersected 350 m of undifferentiated volcanic rocks which ranged in composition from rhyolites and dacites through to andesites. Almond(1) identified a total of seven previously undescribed sequences of volcanic rocks. Almond (1) correlated two of these undescribed units with theChillingham Volcanics (Subunit E) and the Sugars basalt (Subunit F), however neither of these units show any chemical affinity with either of these units, suggesting no genetic link. Additionally, none of the volcanic rocks of GSQ 26 show chemical affinities with any other volcanic or plutonic rocks occurring in the Ipswich Basin, suggesting they formed from a unique vent (15).

Conclusions

The volcanic history preserved within the Ipswich Basin suggests two distinct phases of activity, an older phase of mafic volcanism followed later by a phase of felsic volcanism. The mafic volcanic rocks within the basin are basaltic-andesites and andesites. Although only stratigraphically recognised as theWeirs Basalt and Sugars Basalt in the Ipswich area, other mafic rocks have been identified within drill holes and onMoreton Island. These rocks are highly altered and a small disconformity exists between these rocks and other Ipswich Basin rocks, suggesting a brief pause between their effusion and the deposition of the rest of the basin. Chemically these basalts have signatures of rift-related basalts and andesites (15), which is in character with the extensional environment inferred for the Ipswich Basin in the Late Triassic.

The second phase of volcanic activity, which is better preserved, apparently occurred sporadically though much of the Late Triassic. The volcanic rocks produced during this phase were dominantly rhyolitic in composition, as evident from theChillingham Volcanics,Brisbane Tuff, and the Moreton andStradbroke Island rhyolitic lavas. Rhyodacitic rocks were also produced, although they are volumetrically insignificant compared to the rhyolites, and occurred predominantly within the Ipswich Area. Multiple centres of activity have been inferred for this period, with most units possessing distinctive chemical signatures (15). All rocks, however, are continental and rift-related.

References

Citation Details

ROACH, A.F., 1996. Late Triassic Volcanism of the Ipswich Basin, in, Mesozoic Geology of the Eastern Australia Plate Conference, Geological Society of Australia, Extended Abstracts No. 43, p 476-484

Author Details Andrew Roach School of Earth Sciences Macquarie University North Ryde 2109 Australia aroach@oocities.com

Acknowlegments This project was supported by a Macquarie University Post-Graduate Award and an ARC Special Investigator Award to John Veevers. I kindly thank Richard Flood and Karen Jennings for reviewing the original manuscript.

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