August 2005 LIP of the Month

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Possibly related to event #35 in LIP record database.

The Early Cretaceous Whitsunday Silicic Large Igneous Province of eastern Australia.

Scott Bryan*
Department of Geology & Geophysics, Yale University, New Haven CT USA
*From January 1, 2006: School of Earth Sciences and Geography, CEESR, Kingston University, Kingston‑upon‑Thames, Surrey, UK


Although large igneous provinces of basaltic composition are well-known, it has essentially only been recognised in the last 10 years that silicic-dominated igneous provinces exist with eruptive volumes comparable to those of the Continental Flood Basalt provinces, as well as being spatially and temporally related to continental break-up. The two main examples of silicic large igneous provinces are the Early Cretaceous volcanic rifted margin of eastern Australia (Bryan et al., 1997; 2000), and the Jurassic Chon Aike Province of South America and the Antarctic Peninsula (e.g., Pankhurst & Rapela, 1995; Pankhurst et al., 1998; 2000; Riley & Leat, 1999). Bryan et al. (2002) defined the term "Silicic Large Igneous Province" (SLIP) to describe these volcano-plutonic provinces that share many characteristics: 1) extents >105 km2; 2) extrusive volumes >105 km3; 3) volumetrically dominated (>75%) by dacite to rhyolite igneous rock that commonly have calc-alkaline I-type signatures; 4) lithologically dominated by silicic ignimbrite; 5) igneous activity over prolonged periods (up to 40 myrs); and 6) a spatial and temporal relationship to continental rifting, plate break-up and potentially, other mafic large igneous provinces.

The Whitsunday SLIP is the largest of the world's SLIPs. A perspective on the scale and magnitude of the Whitsunday SLIP is given in Table 1, which compares the province with the 1.6-0 Ma Taupo Volcanic Zone (TVZ) of New Zealand. The TVZ is described as the largest and most frequently erupting silicic magmatic system on Earth where ~90% of erupted material is rhyolitic (Wilson et al., 1995; Sutton et al., 1995). Table 1 demonstrates the Whitsunday SLIP is orders of magnitude bigger than the TVZ in all aspects. Furthermore, the eruptive output of the Whitsunday SLIP (>2.2 x 106 km3) is greater than that of several better-known continental flood basalt provinces. One of the unique features of this SLIP in eastern Australia is the huge volume of coeval volcanogenic sediment (>1.4 x 106 km3) that accumulated in adjacent sedimentary basins. The preserved areal extent of volcanism and its products is in excess of 3 x 106 km2.

The volcanic geology, igneous petrology/geochemistry and regional correlations of the Whitsunday SLIP have been detailed in previous publications (Ewart et al., 1992; Bryan et al., 1997; 2000), and more general aspects of SLIP magmatism in Bryan et al. (2002). The purpose of this contribution is to emphasise some important aspects of this LIP: 1) the close spatial-temporal relationship to continental rifting and passive margin formation, and how complexities in the rifting process have hindered an appreciation of the immensity of SLIP magmatism during the Early Cretaceous in eastern Australia; 2) the eruptive output of this province and to provide an upward revision of extrusive volume estimates following recent geophysical studies along the eastern Australian margin; 3) that a large proportion of the erupted products are preserved as huge volumes of coeval volcanogenic sediment in adjacent sedimentary basin systems; and 4) important long-term volcanic and compositional trends of magmatism.

Table 1: Comparison of the Whitsunday SLIP with the Taupo Volcanic Zone. Magma flux rate is averaged eruptive flux, based on known extrusive volumes for the provinces.


Age (Ma)

Volume (km3)

Dimensions (km)

Magma flux (km3 kyr‑1)


(Eastern Australia)


>2.2 x 106

>2500 x 200


Bryan et al. (1997; 2000)

Taupo Volcanic Zone
(New Zealand)


~2 x 104

300 x 60


Wilson et al. (1995); Houghton et al. (1995)

Geological Background

Early Cretaceous SLIP magmatism occurred as a within-plate, silicic-dominated pyroclastic volcanic belt >2,500 km long, roughly coincident with the present eastern Australian plate margin.

Figure 1: Continental reconstruction map of the SW Pacific at 100 Ma (Yan & Kroenke, 1993), showing the extent of the Whitsunday Silicic Large Igneous Province along the eastern Australian plate margin, which was the source of coeval, Aptian-Albian volcanogenic sedimentary rocks in the Great Artesian and Otway/Gippsland basin systems. Volcanogenic sediment was shed westwards (arrows) into the basin systems. The location of site 207, DSDP Leg 21 is shown, which bottomed in 96 Ma rhyolites on the Lord Howe Rise (McDougall & van der Lingen, 1974). PNG, Papua New Guinea; QP, Queensland Plateau; LHR, Lord Howe Rise; NR, Norfolk Ridge; NZ, New Zealand.

The SLIP was bordered by two major continental sedimentary basin systems along its western margin: the Great Artesian Basin and Otway-Gippsland-Bass basins in northeastern and southeastern Australia, respectively.

Figure 2: Location of the Whitsunday Volcanic Province (132-95 Ma) and Early Cretaceous sedimentary basins of eastern Australia that contain >1.4 x106 km3 of coeval LIP-derived volcanogenic sediment (Bryan et al., 1997). Red squares are locations of dated igneous rocks (ages in italics) along the southeast margin of Australia (Jones & Veevers, 1983; Smith et al., 1988; Middlemost et al., 1992; Hubble et al., 1992). LIP magmatism was followed by: 1) km-scale uplift of the eastern margin of Australia beginning ~100-95 Ma; 2) sea-floor spreading in the Tasman Basin-Cato Trough-Coral Sea Basin occurring between 84-52 Ma; and 3) intraplate alkaline volcanism (80-0 Ma, shown in black) that was partly synchronous with sea floor-spreading, and mostly erupted along the passive margin mountain range of eastern Australia. QLD, Queensland; N.S.W., New South Wales; Vic., Victoria, Tas., Tasmania; S.A., South Australia.

These pre-existing basin systems became major repositories for huge volumes of volcaniclastic material erupted from the SLIP (Bryan et al., 1997). In contrast, the eastern margin of the SLIP was likely bordered by rift basin complexes developed coincident with SLIP magmatism. Compiled geophysical data indicate that several offshore basins (Central & Western Rift Provinces of the Lord Howe Rise, New Caledonia, Queensland & Townsville) began opening in the Early Cretaceous (Wellman et al., 1997; Willcox & Sayers, 2001; Willcox et al., 2001; Lafoy et al. 2005).

An important point is that SLIP magmatism immediately preceded large-scale continental rifting that began in the middle Cretaceous and led to: 1) the opening of a series of basins, some floored by oceanic crust (Tasman Basin-Cato Trough-Coral Sea Basin system, South Loyalty Basin) and others by extended continental crust (New Caledonia, Lord Howe, Middleton, Queensland, Townsville and Capricorn Basins), and 2) the dispersion of rifted microcontinents (e.g., Lord Howe Rise, Dampier Ridge, New Caledonia-Norfolk Ridge, Queensland Plateau) away from the eastern margin of Australia.

Figure 3: Topographic-bathymetric map of the southwest Pacific with the tectonic elements referred to in text labelled. Offshore rift basin systems with Early Cretaceous rift fill are indicated by orange dashed lines, and continental sedimentary basins with Early Cretaceous volcanogenic sedimentary rocks in pink. Red lines define preserved limits of the Whitsunday Silicic Large Igneous Province in northeast Australia. The location of DSDP site 207 on the southern Lord Howe Rise is also shown. Abbreviations: CWRP, Central and Western Rift Provinces of the Lord Howe Rise (LHR); DR, Dampier Ridge; FB, Fairway Basin; LB, Laura Basin; LoB, Loyalty Basin; MB, Maryborough Basin; MP, Marion Plateau; NC, New Caledonia; NCB, New Caledonia Basin; NR, Norfolk Ridge; OGB, Otway-Gippsland-Bass Basins; QB, Queensland Basin; QP, Queensland Plateau; TB, Townsville Basin; WVP, Whitsunday Volcanic Province.

The Tasman-Cato Trough-Coral Sea Basin system is the largest of the basin systems, with sea floor-spreading occurring between the Cretaceous and Palaeocene (chron 33 to 24, 84-52 Ma; Lafoy et al., 2005). The Tasman Basin is triangular in outline, the result of a sea floor-spreading system that propagated northward in a zipper-like fashion over time, resulting in the separation of the Lord Howe Rise microcontinent (Fig. 3) from eastern Australia (Willcox et al., 2001).

The end-result of Cretaceous-Tertiary rifting of eastern Gondwanaland is that the SLIP is now largely dismembered and only part of the province remains intact (Fig. 3). This is primarily because break-up and sea floor-spreading processes were complex, involving the movement of several microplates, the failure of several rifts and consequent ridge jumps by the sea floor-spreading system (Gaina et al. 1998, 2003; Willcox et al., 2001). Of interest is that initial half spreading rates of the Tasman Basin from 83 to 79 Ma are indicated to have been unusually slow (4 mm a-1) causing the basin to only open by ~30 km during this time (Gaina et al., 1998). The initially slow rates of rifting may be an important factor in explaining the extended period between SLIP magmatism (~130 to 95 Ma) and the first appearance of new oceanic crust in the Early Campanian (~84 Ma). Understanding the complexities of the rifting processes and dismembering of the SLIP are important because they have been major factors in hindering the recognition of: 1) the scale and magnitude of Early Cretaceous silicic magmatism; and 2) the tectonic setting and relationship of separated but coeval igneous and volcanosedimentary terranes in eastern Gondwanaland. Past interpretations of individual terranes in isolation have led to contradictory tectonic models for the Early Cretaceous of eastern Gondwanaland.

Numerous apatite fission track thermochronology studies along the eastern Australian margin have recorded a major Early Cretaceous heating event (coincident with SLIP magmatism), with increases in maximum Cretaceous palaeotemperatures toward the margin, followed by mid- Cretaceous cooling beginning ~100-95 Ma (e.g., O'Sullivan et al., 1995; 1996; 1999; Marshallsea et al., 2000; Kohn et al., 2002; 2003). These studies have generally concluded that: 1) Cretaceous heating was due to a greater depth of burial and increased palaeogeothermal gradient (crustal heat flow), and 2) mid-Cretaceous cooling occurred in response to kilometre-scale denudation associated with rifting along the eastern Australian margin, leading to the formation of a passive margin mountain range (the Great Dividing Range of eastern Australia). A widely recognised palaeomagnetic overprint affecting the crust of southern and eastern Australia is also attributed to this Early to mid‑Cretaceous thermal event (e.g., Thomas et al., 2000).

The Whitsunday SLIP is like many other large igneous provinces in being followed by asthenospheric-derived or "hotspot"-style mafic volcanism (see review in Johnson, 1989). This intraplate alkali basaltic volcanism shows a clear spatial association with the passive margin mountain range, forming a broken belt 4,400 km long along the 'highlands' of eastern Australia (Fig. 2). Intraplate alkaline volcanism has continued to the Holocene, occurring within 500 km of the coastline, and has an extrusive volume of >20,000 km3 (Johnson, 1989). However, this volcanism is part of a much broader, mostly basaltic and alkaline igneous province emplaced across continental and oceanic lithosphere in the southwest Pacific Ocean region during the Cainozoic (Finn et al., 2005). For eastern Australia, several features of note are that: 1) a 15 myr hiatus occurred between the terminal phases of Whitsunday SLIP magmatism and the first expressions of intraplate mafic volcanism, which correlates with a period of uplift, erosion and abrupt crustal cooling of the eastern margin, based on the apatite fission track data; 2) the widespread eruption of intraplate alkali basalts overlapped in time with sea floor-spreading in the Tasman Basin, and in particular, that the onset of intraplate mafic volcanism at ~80 Ma (Sutherland et al., 1996) coincided with a more than five-fold increase in the spreading rate to 22 mm a-1 in the Tasman Basin at 79 Ma (Gaina et al., 1998; Willcox et al., 2001); 3) some of the youngest intraplate mafic volcanism has occurred in northern and southern Australia and at the extremities of the intraplate volcanic belt; and 4) the most primitive basalt geochemical signatures from the Whitsunday SLIP overlap those of the younger within‑plate alkaline basalts of eastern Australia (see Ewart et al., 1992), indicating a 'geochemical connection' between pre‑break‑up SLIP magmatism and syn‑ to post‑break‑up intraplate volcanism (Bryan et al., 2000). In summary, the time-space relationships between magmatism, highlands uplift and sea floor-spreading are most readily explained by detachment models where eastern Australia is interpreted as an upper-plate passive margin (e.g., Lister & Etheridge, 1989).

The Whitsunday Silicic Large Igneous Province

The Whitsunday SLIP is defined by the following igneous, volcanosedimentary and tectonic elements of Early Cretaceous age: 1) the Whitsunday Volcanic Province of northeast Australia; 2) scattered igneous intrusions and volcanic rocks along the southeast Australian margin; 3) the Great Artesian and Otway-Gippsland-Bass Basin systems of northeast and southeast Australia, respectively; and 4) submerged volcanic and rift-fill sequences on marginal continental plateaus and troughs.

1. Whitsunday Volcanic Province

The Early Cretaceous Whitsunday Volcanic Province is the northern and relatively intact extension of the SLIP that extended along the eastern Australian margin (Fig. 2). It was preserved on the Australian continental margin following an eastward ridge jump of the Tasman spreading ridge in to the Cato Trough at ~65 Ma (Falvey & Mutter, 1981). The Whitsunday Volcanic Province (s.s.) was defined by Bryan et al. (1997) as a >300 km long and ~100 km wide belt of Early Cretaceous volcanic and intrusive rocks exposed in the Whitsunday, Cumberland and Northumberland Island groups, and onshore exposures to the east of Proserpine. (Fig. 4)

Figure 4: Generalised geology of the northern Whitsunday Volcanic Province. Inset shows rose diagram of dyke orientations (number of dykes is 320; from Bryan et al., 2003).

Physiographically, the province now forms a drowned mountain range along the central Queensland coast. The Whitsunday Volcanic Province does, however, form part of a larger silicic pyroclastic volcanic belt, first recognised by Clarke et al. (1971) that extends for >900 km long, ≥100 km wide and over 1 km thick along the Queensland coast (Figs 2, 3). Important exposures of Early Cretaceous volcanic rocks to the south of the province include the Shoalwater Bay Volcanics (Kirkegaard et al., 1970), and Grahams Creek Formation in the Maryborough Basin (Ellis, 1968), but these have not been studied to the same extent as the rocks of the Whitsunday Volcanic Province (Fig. 1).

Volcanic Geology

Lithologically, the Whitsunday Volcanic Province is volumetrically dominated by welded dacitic-rhyolitic lithic-rich ignimbrite (Fig. 4), and most exposures often present monotonous sequences of stacked, welded ignimbrite units forming km-thick sections. The eruptive powerhouses of the province were several, relatively large (~10-20 km diameter) calderas that form a northwest-trending belt through the province. Some intracaldera ignimbrite units are up to 1 km thick (Clarke et al., 1971; Ewart et al., 1992; Bryan et al., 2000), whereas extracaldera units are 10's to less commonly, 100's metres thick. Coarse lithic lag breccias containing clasts up to 6 m diameter (Ewart et al., 1992) cap the ignimbrites in proximal sections and record caldera collapse episodes. Exhumed caldera sections (e.g., Whitsunday Island) reveal multiple caldera-forming ignimbrite eruptions producing caldera fill successions of at least 3-4 km thick. Several calderas were flooded, being occupied by shallow lakes and rhyolite domes during eruptive hiatuses. Phreatomagmatic eruptive phases often characterised the caldera-forming eruptions as a result of explosive magma interaction with the caldera lakes. Intercalated with the silicic pyroclastic rocks are basaltic and silicic lavas, with andesitic lavas minor. Basalt lavas are uncommon in island (eastern) exposures but are volumetrically more abundant in younger mainland (western) exposures (Fig. 4). Associated with the volcanics are locally significant thicknesses of coarse volcanogenic conglomerate and sandstone (Clarke et al., 1971; Bryan et al., 2000). The sedimentary rocks are texturally and compositionally immature, reflecting the local volcanic provenance, and sedimentation appears to have been in poorly confined, high-energy alluvial environments. Dyke swarms and sills are an integral feature of the volcanic sequences (Fig. 5), with individual dykes ranging up to 50 m in thickness, and have a strong N-S orientation (Fig. 4; Ewart et al., 1992; Bryan et al., 2000; 2003). Silicic dykes predominate in the exhumed caldera sections, whereas mafic dykes and sills are abundant in western exposures where basaltic lavas are also more common. Early Cretaceous granites also intrude the volcanic sequences (Fig. 4). Overall, the volcanic sequences record a multiple vent, but caldera-dominated, low relief volcanic region (Bryan et al., 2000).

Figure 5: Coastal exposure of a mafic dyke swarm, Scawfell Island, Whitsunday Volcanic Province.


The complete whole-rock geochemical dataset (344 analyses) can be accessed via the PETLAB database of the Institute of Geological and Nuclear Sciences (New Zealand) at:

Chemically, the igneous suite ranges continuously from basalt to high-silica rhyolite (Fig. 6), with calc-alkali to high-K affinities (Ewart et al., 1992). In detail, however, this range is defined by dyke compositions (Fig. 7); lavas are bimodal (basalt-andesite and rhyolite to high-silica rhyolite), and ignimbrites dominantly rhyolitic in composition. It should be noted that the geochemical data set is biased towards the volumetrically minor lavas and dykes (N = 243 out of 344 analyses), and when volume considerations are taken into account, igneous rock compositions within the province are overwhelmingly silicic.

Figure 6: A) Total alkalis versus silica diagram showing the range and continuity of volcanic and intrusive compositions of the Whitsunday Volcanic Province, which is represented at the small scale by individual island sequences (South Molle). The diagram contains 315 X-ray fluorescence analyses. B) Hf-Ta-Th relationships for the Whitsunday Volcanic Province from Ewart et al. (1992). Note the projection of mafic compositions into the E-MORB or within-plate field, whereas the crust-derived rhyolites plot in the destructive plate margin field.

Figure 7: Silica histogram showing the distribution of compositions within dykes, lavas, pyroclastic rocks, and coeval granites.

The range of compositions has been generated by two-component magma mixing and fractional crystallization superimposed to produce the rhyolites (Ewart et al., 1992). The two magma components are: 1) a volumetrically dominant partial melt of relatively young (Mesozoic to Palaeozoic), non-radiogenic calc-alkaline crust; and 2) a within-plate tholeiitic basalt of E-MORB affinity (Fig. 6B), and similar to the Tertiary intraplate basalts of eastern Australia (Ewart et al., 1992; Stephens et al., 1995). A critical point to emphasise is that the calc-alkaline and arc-like signatures have been inherited from the crustal source, and do not provide any constraints on the (Early Cretaceous) tectonic setting in which the magmas were produced (e.g., Roberts & Clemens, 1993). The volumetric dominance of silicic igneous compositions reinforces the point that Early Cretaceous magmatism reflected a massive partial melting event of the continental crust.


Relatively limited K/Ar and Rb/Sr isotopic dating of the Whitsunday Volcanic Province has established an age range of 132 to 95 Ma, but with a main period of igneous activity between 120 and 105 Ma (Ewart et al. 1992; Bryan et al., 1997). Other K/Ar age data from eastern Queensland (e.g., Green & Webb, 1974; Allen et al., 1998) suggest precursory magmatic activity may have begun as early as 145 Ma, although U/Pb zircon dating has as yet, not duplicated these older ages. However, it is being increasingly recognised that some Late Jurassic and Early Cretaceous K/Ar ages are reset ages (e.g., Allen et al., 1998; Uysal et al., 2001) due to the major thermal event associated with SLIP magmatism (e.g., as evidenced by Early Cretaceous heating in the apatite fission track data for eastern Australia). Preliminary LA-ICP-MS U/Pb zircon dating of ignimbrites from the Whitsunday Volcanic Province (S Bryan & C Allen, unpublished data) supports the notion of a main period of activity between 120 and 105 Ma. In addition, zircon xenocrysts with ages of ~490 and 787 Ma U/Pb ages dated from two silicic ignimbrites are consistent with the Nd model TDM ages of 200-600 Ma calculated for the magma crustal sources (Ewart et al., 1992). Considerably more geochronological work is required to understand the eruptive history of the province, in particular to constrain better, melt production rates, and eruptive volume and composition relationships with time.

2. Igneous rocks of southeast Australia

Minor volumes of Early Cretaceous igneous rocks are preserved along the southeast margin of Australia (Fig. 2) with the most prominent example being the Mount Dromedary ring complex (e.g., Smith et al., 1988; Nott & Purvis, 1995). Early Cretaceous intrusions, most likely related to the Mount Dromedary igneous complex have also been dredged from the continental slope off southern New South Wales (Hubble et al., 1992). The rocks along the southeast Australian margin tend toward bimodal compositions, occurring as basalt-basanite lavas and dykes and monzonite-syenite intrusions and trachytic to rhyolitic volcanics. Overall, igneous compositions are slightly more alkaline than coeval igneous rocks from the Whitsunday Volcanic Province to the north (Smith et al., 1988; Middlemost et al., 1992). Igneous ages are mostly between ~110 and 90 Ma (Jones & Veevers, 1983, Middlemost et al., 1992; Hubble et al., 1992). More substantial volumes of Early Cretaceous igneous rock existed along the southeast margin, but have subsequently been rifted away and are now located in submerged rift basins on the Lord Howe Rise on the eastern side of the Tasman Basin. The strong increase in maximum Cretaceous palaeotemperatures towards the southeast margin as evidenced by apatite fission track data (e.g., Kohn et al., 2002) provide indirect evidence that a major igneous belt was located close to the southeast margin during the Early Cretaceous.

3a. Great Artesian Basin system

In addition to voluminous silicic volcanism, the Cretaceous geological history of eastern Australia is marked by the development of extensive sedimentary basins, most notably the Great Artesian Basin (Figs 1-3). The latest Triassic to Cretaceous Great Artesian Basin (GAB) covers ~22% of Australia, but the preserved remnants of this intracratonic basin system and the stratigraphy of other, now isolated areas of Jurassic‑Cretaceous outcrop suggest the GAB may have originally covered a much larger area of eastern Australia (Fielding et al., 1996). The various components of the GAB (Eromanga, Surat, Carpentaria, Laura and Clarence‑Moreton basins) preserve Early Cretaceous (Aptian-Albian), volcanogenic sedimentary rocks that cover >2 x 106 km2 to an average thickness of 500 m. The Aptian-Albian volcanogenic sedimentary rocks are dominantly mudstone, siltstone and sandstone that were deposited in environments ranging from fluvial/lacustrine to coastal plain and shallow marine (Smart & Senior, 1980; Hawlader, 1990; Fielding et al., 1992). This volcanogenic sedimentation represents an abrupt and fundamental change in sediment provenance from underlying (Neocomian), basement-derived quartzose sandstones. The volcanogenic sandstones are feldspathic-lithic, with an average Q:F:L ratio of 15:41:44, and volcanic lithic grains represent more than 90% of the total lithic component (e.g., Hawlader, 1990). Palaeocurrent data indicate an easterly source (the Whitsunday SLIP) for sediment. The sheet‑like external and internal geometry of formations within the GAB suggests that predominantly passive, thermal subsidence (Fielding et al., 1996) coincided with volcanism along its eastern margin.

3b. Otway-Gippsland-Bass Basin system

The WNW‑trending Otway, Gippsland and Bass basins in southeast Australia (Figs 1-3) are transtensional rift basins that form the eastern extremity of a complex rift system (Southern Rift System of Willcox & Stagg, 1990), which extended along the length of the southern margin of Australia during the Late Jurassic ‑ Early Cretaceous. Their formation represents a precursor stage to the post‑Middle Cretaceous break‑up of Australia and Antarctica (e.g., Falvey & Mutter, 1981). These basins are significant for containing >2.5 km thickness of Aptian-Albian age volcanogenic sandstone and mudstone.

Figure 8: Stratigraphic subdivision of the Otway and Strzelecki Groups, Otway and Gippsland Basins. Outcrop age ranges are illustrated by vertical black bars beside stratigraphic columns, and age range of volcanism in the Whitsunday Volcanic Province is illustrated by the vertical red bar beside fission track age data (from Gleadow & Duddy, 1980). Palaeocurrent measurements are from channel sandstone facies for the P. notensis zone (Wonthaggi Formation) and C. striatus - P. pannosus zones (Eumeralla Formation); N is number of measurements. Figure modified from Bryan et al. (1997).

As in the GAB, this volcanogenic sedimentation represents a fundamental change in provenance from underlying basement-derived quartz-rich sandstones, such that volcanogenic sedimentation was almost to the exclusion of basement-derived material (Gleadow & Duddy, 1980). Palaeocurrent data (Fig. 8) indicate the source of the volcaniclastic sediment lay outside and to the east of the Gippsland Basin (the easternmost basin; Bryan et al., 1997). The volcanogenic sandstones are dominantly volcanic lithic, with detrital minerals of predominantly plagioclase, with lesser quartz, hornblende, pyroxene (augite), apatite, titanite, and zircon. Fission track dating of apatite, titanite, and zircon (Fig. 8) supports a contemporaneous volcanic source for the sediment (Gleadow & Duddy, 1980), and is further supported by palynological dating indicating the volcanogenic sandstones are Aptian ‑ Albian in age.

4. Submerged volcanic and rift-fill sequences on marginal continental plateaus and troughs

It has previously been suspected by several workers that a substantial volume of Early Cretaceous igneous and volcanogenic sedimentary material may be present in submerged rift basins along the eastern margin of Australia. This was based largely on seismic studies (e.g., Falvey & Mutter, 1981), and the existence of ~96 Ma rhyolites recovered from deep sea drilling of the Lord Howe Rise (site 207, DSDP Leg 21; McDougall et al., 1974). The amount of geophysical data has greatly increased over the last 10 years, permitting a more detailed regional evaluation of these offshore regions. Three major rift basin systems are indicated to contain Early Cretaceous rift fill offshore from eastern Australia: the Central and Western Rift Provinces (Stagg et al., 1999) of the Lord Howe Rise off southeast Australia, and the Queensland and Townsville Basins off northeast Australia (Fig. 3).

Lord Howe Rise

The Lord Howe Rise (LHR) is a continental 'ribbon' or microcontinent 1600 km long and 400-500 km wide, extending southward from the eastern Coral Sea to the western margin of New Zealand (Fig. 3; Willcox et al., 2001). It became separated from the southeast margin of Australia following sea floor-spreading in the Tasman Basin (e.g., Gaina et al., 1998). It has been divided into several basins and blocks by Stagg et al. (1999) based on satellite gravity and limited seismic profiles. On the western side of the LHR is an extensive rift basin system 150-250 km wide (Central & Western Rift Provinces, Fig. 3) that in plate reconstructions of the LHR against eastern Australia, would have extended along the southeastern margin of Australia, and adjacent to the Gippsland Basin (Willcox et al., 2001). Several seismic megasequences have been recognised and correlated with known megasequences of basins on the southeast Australian margin (Willcox & Sayers, 2001). Significantly, the seismic data indicate north-south-trending graben development along the western LHR during the ?latest Jurassic/Early Cretaceous. Infill is inferred to have been initially by volcanics and then syn-rift sediments that average 1.5-3 km in thickness, but reach a maximum of 4+ km thickness in the deepest basins. This phase of syn-rift sedimentation has been correlated with the Early Cretaceous Otway/Strzelecki Groups (Fig. 8) and volcanogenic sandstones of the Otway-Gippsland basins (Willcox & Sayers, 2001). Furthermore, volcanic bodies are interpreted to form significant parts of the rift fill in some basins (Willcox et al., 2001).

Queensland and Townsville Basins

The Queensland and Townsville Basins are two major rift basins within a large, submerged extensional continental terrane off the northeast margin of Australia (Fig. 3). Prior to Late Cretaceous-Tertiary break-up, a continental mass of ~700 km width existed adjacent to the margin (Draper et al., 1997). The Queensland and Townsville Basins are part of a complex rift system that probably began to form in the ?Late Jurassic to Early Cretaceous, and now coincide with major bathymetric troughs where water depths vary from ~1000 to 2000 m (Wellman et al., 1997). The Queensland Basin represents a northern strike continuation of the Whitsunday Volcanic Province. In contrast, the Townsville Basin is distinctive in striking ~east-west across the margin (Fig. 3), and is thought to have formed through overall oblique northwest-southeast extension (Wellman et al., 1997). These rift basins are ~100 km wide, and on the basis of seismic and gravity data contain rift valley sequences up to 3 km thick, of which ≥1 km is Cretaceous in age (Taylor & Falvey, 1977; Falvey & Mutter, 1981). Presently, there is no information available on the character of the Cretaceous rift fills to the basins, but are likely to be volcanic and volcanogenic sedimentary rocks as in the adjacent Whitsunday Volcanic Province and Great Artesian Basin system.


Revised volume estimates

An extrusive volume of >1.5 x 106 km3 was estimated for Whitsunday SLIP magmatism by Bryan et al. (1997). This was based on the preserved volumes of volcanogenic sediment in the Great Artesian Basin system (>1 x 106 km3), the Otway-Gippsland Basins (>4 x 105 km3) and the Whitsunday Volcanic Province (>1 x 105 km3). A variety of new data are now available permitting a reassessment of extrusive volumes from the Whitsunday SLIP. Revised volume estimates are given in Table 2.

Initial volume estimates for the Whitsunday Volcanic Province were based on an average thickness of 1 km, but further stratigraphic work indicates preserved thicknesses to be in the order of 2-4+ km (Bryan et al., 2000). Of note is that Smart & Senior (1980) estimated that a volcanic belt of 3000 km long by 130 km wide and 2 km thick was required to account for the volume of detritus preserved in the Great Artesian Basin. The major addition to the revised volume estimate is that of volcaniclastic rock preserved in the ~1500 km x 150-200 km wide rift basin system of the Lord Howe Rise (Stagg et al., 1999; Willcox et al., 2001; Willcox & Sayers, 2001). A similar volume to that preserved along the northeast Australian margin is indicated. In summary, minimum extrusive volumes of the Whitsunday SLIP, including volcanic and coeval volcanogenic sedimentary rock, preserved along the eastern Australian margin are estimated to be >2.2 x 106 km3.

Importance of the sedimentary record

The Whitsunday SLIP is unusual in that a large proportion of its products are preserved as huge volumes (>1.4 x 106 km3) of coeval volcanogenic sediment in adjacent continental sedimentary basins (Fig. 1; Bryan et al., 1997). The additional preservation of >5.5 x 105 km3 of volcaniclastic material is now indicated for offshore rift basins (Fig. 3). Such substantial volumes of coeval volcanogenic sediment are not characteristic of other LIPs. Age dating of volcanic mineral grains (e.g., Gleadow & Duddy, 1980), the fresh nature of the detrital mineral grains and the sheer volume of volcanogenic sediment in the Otway/Gippsland and Great Artesian Basin systems preclude any arguments for an igneous basement-derived source for the sediment. An important issue then is how such huge volumes of predominantly sand-grade volcanogenic sediment were rapidly generated and transported over large distances with limited weathering (Smart & Senior, 1980) to fundamentally alter the basin fill history and reservoir potential of at least two widely separate and tectonically unrelated, major continental sedimentary basin systems.

In a comparison of volcanic phenocryst compositions from the Whitsunday Volcanic Province with detrital mineral grains from the coeval volcanogenic sedimentary formations of the Great Artesian and Otway-Gippsland Basin systems, Bryan et al. (1997) demonstrated that it was phenocryst compositions from the volumetrically dominant dacitic to silicic ignimbrites that matched the detrital volcanic mineral grain compositions. This overlap indicated that silicic pyroclastic volcanism was the major expression of Whitsunday SLIP magmatism in eastern Australia. Furthermore, the overlap implied there was a remarkable consistency in mineral composition, and consequently, whole rock chemistry for Whitsunday SLIP magmatism, particularly considering that volcanism and sedimentation were occurring over a distance of >2500 km along the eastern Australian margin.

For the Whitsunday SLIP, there were two important reasons why there was such an important sedimentary record of magmatism. The first was the fortuitous circumstance that two major pre-existing continental sedimentary basins were adjacent to the province, and able to accommodate huge volumes of volcanogenic sediment. A marine transgression during the Aptian-Albian further facilitated sediment accumulation in the Great Artesian Basin. The intersection of the newly developing ~N-S volcanic rift system along eastern Australia with the ~E-W Antarctic-Australian rift to the east of the Gippsland Basin, allowed the shedding of volcanic material towards the west into the Otway/Gippsland basin system that was entering a sag phase of basin development. The second was that the pyroclastic mode of fragmentation and dispersal was a critical factor in producing large volumes of sand-grade sediment to be rapidly delivered into the continental basin systems. Large volumes of easily erodible, nonwelded pyroclastic debris were likely present along the flanks of the province that could be remobilised by efficient west-draining fluvial systems. In contrast, only minor volumes of coarse fragmentary material (e.g., a' a' lava breccias, scoria deposits) tend to be produced during flood basalt eruptions in the continental flood basalt provinces. The quick burial of brecciated material by succeeding lava flows combined with the generally coherent (unfragmented) nature of the flood basalt lavas are important factors for why continental flood basalt provinces have a limited sedimentary response.

Volcanic and compositional trends

Volcanic stratigraphic studies of the Whitsunday Volcanic Province (Bryan et al., 2000) highlight some important longer term trends in volcanism and erupted compositions. Although a range of igneous compositions are present (Fig. 6A), stratigraphic constraints indicate volcanism evolved towards more bimodal compositions with time, which is a more common characteristic of magmatism associated with continental rifting. Early phases of volcanism (~130-115 Ma) were dominantly explosive, erupting dacitic to rhyolitic magmas, whereas volcanism during later stages (<115 Ma) was both effusive and explosive, when mainly basaltic and rhyolitic lavas were erupted contemporaneously with predominantly rhyolitic to high-silica rhyolite ignimbrites. Mafic magma compositions became less contaminated with time, with intrusion of primitive E-MORB gabbros occurring late in the igneous history. Some indication for a trend toward more mafic and alkaline compositions is evident from the ~110-90 Ma igneous rocks along the southeast Australian margin, and then the eruption of intraplate alkali volcanics beginning ~80 Ma.

Table 2: Revised volume estimates for the Whitsunday Silicic Large Igneous Province.

Tectonic element

Preserved volume of volcanic material (km3)

Whitsunday Volcanic Province1

>2.7 x 105

Southeast Australia


Otway-Gippsland Basins

>4 x 105

Great Artesian Basin

>1 x 106

Townsville-Queensland Basins2

>1 x 105

Lord Howe Rise3

>4.5 x 105



  1. An average thickness of 2 km is used to estimate preserved extrusive volumes for the >900 x ~100 km wide silicic volcanic belt along the Queensland coast.
  2. Volume is based on basin extents of 1000 x 100 km and an Early Cretaceous rift fill of 1 km.
  3. Volume is calculated for the ~1500 km x 150-200 km wide rift basin system (Central and Western Rift Provinces of Stagg et al., 1999) that contains on average, 1.5-3 km thickness (2.25 to 6.75 x 105 km3) of Early Cretaceous rift fill (Willcox et al., 2001). A conservative thickness of 2 km is used in volume calculations.


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