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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Background
  5. Methodology
  6. Results
  7. Interpretations and Discussion
  8. Conclusions
  9. Acknowledgments
  10. References

Abstract– Ejecta from the large subsurface Tookoonooka impact structure have been found in the Lower Cretaceous strata of the extensive Eromanga Basin of central Australia. Observations from 31 wells spanning 400,000 km2 of the basin provide compelling evidence for the presence of a marine impact horizon of regional extent. Drill core was examined to determine the sedimentary context of the Tookoonooka impact event, the presence of ejecta, and the nature of the impact horizon. The base of the Wyandra Sandstone Member of the Cadna-owie Formation is an unconformity commonly overlain by very poorly sorted sediment with imbricated pebbles, exotic clasts, and occasional boulders. The basal Wyandra Sandstone Member is bimodal: a fine sand mode reflects an ambient sediment contribution and a coarse mode is interpreted to be impact-derived. Wells Thargomindah-1 and Eromanga-1, within four crater radii of Tookoonooka, contain distinctive clast-supported breccia-conglomerate beds at the base of the Wyandra Sandstone Member. Clasts in these beds include altered accretionary and melt impactoclasts, as well as lithic and mineral grains corresponding to the Tookoonooka target rock sequence, including basement. Petrographic evidence includes shock metamorphosed quartz and lithic grains with planar deformation features. These breccia-conglomerates are in stark contrast to the underlying, laterally persistent, unimodal Cadna-owie sediments and overlying shales deposited in an epeiric sea. The base of the Wyandra Sandstone Member is therefore interpreted to be the Tookoonooka impact horizon. The timing of the impact event is confirmed to be the Barremian-Aptian boundary, at 125 ± 1 Ma. The Wyandra Sandstone Member preserves both impact ejecta and postimpact marine sediments.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Background
  5. Methodology
  6. Results
  7. Interpretations and Discussion
  8. Conclusions
  9. Acknowledgments
  10. References

Tookoonooka and Talundilly are two subsurface anomalies in the Eromanga Basin of central Australia that were recognized by petroleum exploration in the 1980s (Fig. 1). Tookoonooka and Talundilly are some 300 km apart, buried under almost 1 km of flat-lying sediments near the center of the basin in southwest Queensland. The impact origin of subsurface structures is notoriously difficult to investigate due to inaccessibility and paucity of data. Tookoonooka was confirmed as an impact structure through the identification and measurement of shock metamorphic features in quartz grains (Gorter et al. 1989; Gostin and Therriault 1997) from the structure’s buried central uplift. However, Talundilly has not been adequately drilled to date, and remains a possible impact structure.

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Figure 1.  Map of the Eromanga Basin of central Australia with the locations of Tookoonooka impact structure, Talundilly structure, and wells studied. Modified after Bron (2010). Dashed concentric circles around Tookoonooka indicate theoretical 2Rc and 5Rc limits.

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Tookoonooka is the second-largest known impact structure in Australia, and possibly the tenth largest in the world (Earth Impact Database, 2011). It was estimated to have a final crater diameter of 66 km (Gostin and Therriault 1997), which is a recalculation of an earlier estimate of 55 km from seismic data (Gorter et al. 1989). Talundilly was estimated to have a 95 km diameter from seismic data (Longley 1989). A Talundilly crater diameter cited by Bevan (1996) and Gorter (1998) of 30 km, while it may be more accurate according to more recent crater-scaling techniques (cf. Turtle et al. 2005), was not thoroughly discussed by those authors, and no new data exist. Based on their similar stratigraphic positions, the two structures are thought to be the result of a binary impact event (Gorter 1998; Haines 2005). From palynology and seismic stratigraphic interpretations, the age of the impact event has been variously estimated at approximately 128 Ma between palynostratigraphic units PK2.1 and PK2.2 (Gorter et al. 1989; Gostin and Therriault 1997), and between 112 and 115 Ma (Gorter 1998), within the Lower Cretaceous (Fig. 2). Bron (2010) constrained the impact age to 125 ± 1 Ma based on a first presence of impactoclasts in the stratigraphy. The Tookoonooka structure was partially preserved by burial; on an erosion level scale of 1–7, where 1 is a largely uneroded crater and 7 is an eroded crater floor (cf. Grieve and Pilkington 1996), Gostin and Therriault (1997) estimated the crater erosion level at 5. Both events probably excavated basement lithologies (Gorter et al. 1989; Longley 1989; Gostin and Therriault 1997).

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Figure 2.  Stratigraphy of the Eromanga Basin. Modified after Bron (2010).

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The sedimentology of the Tookoonooka/Talundilly impact event has not been extensively investigated to date. No ejecta layer has previously been reported (Haines 2005), although impactoclasts originating from the Tookoonooka impact have recently been described (Bron 2010). This discovery prompted a wider search for impact-related sediments at the same stratigraphic horizon across the basin. This paper aims to: investigate the nature of the impact horizon, provide further evidence of the occurrence of Tookoonooka ejecta in the basin sediments, confirm stratigraphic age constraints of the impact, and better constrain the paleoenvironment at time of impact. As Talundilly’s impact origin is as yet unconfirmed, this article will essentially focus on Tookoonooka as the source of interpreted ejecta, although it is recognized that a Talundilly component may exist if the two causative events were coeval.

Background

  1. Top of page
  2. Abstract
  3. Introduction
  4. Background
  5. Methodology
  6. Results
  7. Interpretations and Discussion
  8. Conclusions
  9. Acknowledgments
  10. References

Eromanga Basin Sequence

The Eromanga Basin is a sedimentary superbasin of Jurassic-Cretaceous age covering 20% of the Australian continent (Krieg and Rogers 1995). The stratigraphy of the Eromanga Basin (Fig. 2) has been well documented: work has been done on the deeply weathered southern and southwestern basin margin outcrops, and knowledge of the buried sediments in the rest of the paleobasin comes mainly from petroleum exploration and widely spaced stratigraphic wells. The flat-lying nature of the sediments indicates that this paleobasin has experienced negligible tectonism since its inception. As the basin spans three Australian states and one territory (Fig. 1), a comprehensive literature review is not attempted here. The following paragraphs give reference to details of the formations investigated.

The stratigraphic context of the Tookoonooka impact is the Cadna-owie Formation (herein referred to as “the Cadna-owie”; Fig. 2). The Cadna-owie is comprised of very fine- to fine-grained, quartzose to sublabile sandstones with common siltstone interlaminae and thin interbeds (Senior et al. 1975; Exon and Senior 1976; Day et al. 1983). The sandstone matrix is calcareous in part and the siltstone may be carbonaceous (Draper 2002). The sedimentation of the Cadna-owie is laterally uniform, with a siltier lower unit and a sandier, slightly coarser upper unit (Draper 2002). The formation has an average thickness of 60 m (Senior et al. 1975; Exon and Senior 1976), but is more typically 75–100 m thick at the center of the basin (Moore and Pitt 1984, 1985). The Cadna-owie records the transition to a paralic or shallow marine environment as a very extensive, shallow epeiric sea transgressed the nonmarine basin; Eromanga Basin sediments predating the Cadna-owie are largely fluvio-lacustrine (e.g., Exon and Senior 1976). No volcanism has been reported in the basin from this geological time (Harrington and Korsch 1985; Wiltshire 1989; McDougall 2008). The presence of glauconite and marine palynomorphs (e.g., Day 1969; Senior et al. 1975; Day et al. 1983; Alley and Lemon 1988) is strongly evident of a marine influence during deposition. Glacial indications have been described from the southern basin margin areas (Frakes et al. 1995; De Lurio and Frakes 1999; Alley and Frakes 2003). The Cadna-owie was probably deposited in restricted marine cold-water conditions in a low-relief, high-latitude basin with poor sediment supply.

In contrast, the Wyandra Sandstone Member (herein referred to as “the Wyandra”; Fig. 2), stratigraphically assigned to the Cadna-owie Formation in central Queensland (e.g., Draper 2002), is a coarser grained, cleaner sandstone unit than the lower and upper Cadna-owie units as described above. The unit was first named by Senior et al. (1975), and defined as a thin, widespread, well-sorted, medium- to coarse-grained, quartzose sandstone with scattered carbonate cement and pebbles and no known fossils (Senior et al. 1975, 1978; Exon and Senior 1976). Although the Wyandra occupies the uppermost part of the Cadna-owie as originally defined, for clarity, all references to the Cadna-owie in this article will pertain to the Cadna-owie deposits underlying the Wyandra.

Overlying the Wyandra, the Walumbilla Formation (also known as the Bulldog Shale in southern parts of the basin; Fig. 2) is a dark gray, fossiliferous, carbonaceous marine shale. Prior to deposition of the Walumbilla Formation, basin sediments were predominantly quartz-rich (with sublabile components) and mature (Exon and Senior 1976; Day et al. 1983). The Bulldog Shale in South Australia has a maximum known thickness of 340 m (Moore and Pitt 1985). The Walumbilla Formation is known to be greater than 450 m thick in the center of the basin, thinning to only 30 m in the far northwest of the basin in Queensland (Moore and Pitt 1985).

Impact Ejecta

Impact ejecta layers have been observed in sedimentary units spanning the geological record (cf. Montanari and Koeberl 2000). Only a fraction of the 178 confirmed terrestrial impact structures (Earth Impact Database 2011) have ejecta preserved; due to terrestrial erosion processes, ejecta are usually only observed at younger craters or where sedimentary burial has enabled preservation. Twenty-seven impact structures are currently categorized as having impacted marine environments, and distinct marine impact sedimentation processes have been recognized (Dypvik and Jansa 2003; Dypvik and Kalleson 2010). Due to sedimentation rates in marine environments, it is likely that ejecta may be more often preserved by burial in marine impact scenarios.

Classifications of impactites and terminology relating to ejecta have been proposed by a number of authors including French (1998), Stöffler and Grieve (2007), Melosh (1989), Montanari and Koeberl (2000), King and Petruny (2003), and Bron (2010); terms proposed by these authors will be applied herein. Melosh (1989) discussed the distribution of ejecta volumes based on radial limits. For a given crater, approximately 50% of the ejecta volume lies within a continuous ejecta blanket within two crater radii (2Rc) of the crater center (or 1Rc from the crater rim), and 90% lies within 5Rc. Between 2Rc and 5Rc lies a discontinuous ejecta blanket. Ejecta are considered proximal within 5Rc and distal beyond 5Rc. It must be noted that these theoretical distributions in the terrestrial realm are most applicable to fresh, nonaqueous impact sites where minimal erosion and reworking have occurred.

Diagnostic indicators of the impact origin of ejecta include geochemical anomalies and shock metamorphic features (cf. French 1998; French and Koeberl 2010). The latter may take the form of microscopic planar deformation in minerals, the presence of high-temperature/pressure mineral polymorphs, and diaplectic glasses. Microscopic planar deformation includes planar deformation features (PDFs) and planar fractures (PFs), although it is contested whether the latter can be considered an impact indicator in isolation (French and Koeberl 2010). The unique impact origin of “toasted” quartz, which is a brownish-colored textural variety of quartz reported from impact sites, has also been investigated by Whitehead et al. (2002). The effects of shock metamorphism have been recognized in numerous minerals in impact shocked and experimentally shocked rocks (reviews in Grieve et al. 1996; Stöffler 1972; Stöffler and Langenhorst 1994). Measurements of orientations of PDFs in quartz can be made microscopically on a universal stage; 15 crystallographic orientations are considered typical (cf. Stöffler and Langenhorst 1994; Ferrière et al. 2009). Occurrence of suites of PDF orientations have been further applied to impact shock pressure calibration (e.g., Grieve et al. 1996). It is recognized that different target materials, such as crystalline or unconsolidated/porous sedimentary rocks, may influence the suite of shock effects present at a given impact site. Hence, classifications have been proposed for the progressive stages of shock metamorphism in different target lithologies (Stöffler and Grieve 2007).

Tookoonooka Impactoclasts

Bron (2010) described the occurrence of accretionary and melt impactoclasts within the Wyandra. Similar to their volcanic equivalents (accretionary lapilli, melt lapilli, and bombs), a variety of melt morphologies and concentrically zoned accretionary clasts were recognized. Similar clasts have also been described from other impact sites around the world. Accretionary and armored impactoclasts up to 9 cm in diameter were observed in drill core across the Eromanga Basin (Bron 2010). Given their proximity to the Tookoonooka impact structure, their presence within the Tookoonooka crater fill, and the lack of contemporaneous volcanic evidence in the basin record, they were interpreted to be of Tookoonooka impact vapor plume origin. A new terminology was proposed to highlight the significance of these clasts to impact geology.

Methodology

  1. Top of page
  2. Abstract
  3. Introduction
  4. Background
  5. Methodology
  6. Results
  7. Interpretations and Discussion
  8. Conclusions
  9. Acknowledgments
  10. References

Drill cores from 31 wells in southwest Queensland and northern South Australia were examined in the search for a Tookoonooka ejecta layer (Fig. 1). Emphasis was first placed on the most proximal wells to the Tookoonooka structure within the theoretical continuous ejecta blanket limit, followed by an expanded search to more distal wells. The cores were studied in detail macroscopically across the formations of interest (the Cadna-owie, the Wyandra, and the lower Walumbilla) to investigate the presence, sedimentary context, distribution, and characteristics of possible ejecta across the basin. Characteristics such as grain size, sorting, clast composition, texture, and physical and biogenic sedimentary structures were recorded. Clast-supported breccia-conglomerate beds were observed in many of the Wyandra cores studied; as these were expected to contain discernible primary ejecta (Bron 2010), they were the focus of a more detailed investigation.

Cores from two Geological Survey of Queensland (GSQ) stratigraphic wells were chosen for microscopic study: Thargomindah-1 and Eromanga-1 (Fig. 1). They were chosen for their proximity to Tookoonooka and the presence of a clast-supported breccia-conglomerate (CSBC) bed at the base of the Wyandra. GSQ Thargomindah-1 is located about 64 km (1.9 Rc) east of the approximate Tookoonooka crater center, within the theoretical continuous ejecta blanket range of Tookoonooka, and 303 km from the center of Talundilly. GSQ Eromanga-1 is located 119 km northeast (3.6 Rc) of the Tookoonooka crater center and 219 km from the center of Talundilly. Both wells are within the theoretical proximal ejecta range of Tookoonooka. Petrographic microscopy was used to conduct point-counting of grains and search for shock metamorphic evidence within thin-sections of the CSBC beds. Data recorded were: detrital grain mineralogy, grain size, grain shape, optical properties, textures, and alteration. Standard point-counting procedures were used (cf. Tucker 1988). Over 300 grains were analyzed per thin section to obtain statistically significant results (Table 1). Grain size nomenclature utilizes the scales of Udden-Wentworth (as detailed in Folk 1980; Tucker 2001) and Blair and McPherson (1999). PDF orientations were measured using a four-axis universal stage and indexed (cf. Stöffler and Langenhorst 1994) utilizing the stereographic projection template of Ferrière et al. (2009). Thin sections of the Thargomindah-1 and Eromanga-1 CSBC beds were made at 705.08 m depth and 743.73 m depth, respectively.* For comparison with the ambient (i.e., interpreted preimpact) sedimentation, a thin section at 743.99 m depth in Eromanga-1 within the upper Cadna-owie was also made; results of analyses were compared with those of the two basal Wyandra CSBC beds.

Table 1.   Point-counting data sets for three thin sections analyzed.
Thin section from wellThargomindah-1Eromanga-1Eromanga-1
  1. Point-counting results are presented in Tables 4 and 5.

  2. aGrain sizes in thin section are uncorrected with respect to comparative sieved equivalents.

Stratigraphic intervalbasal Wyandrabasal Wyandraupper Cadna-owie
Total # points counted for volumetrics5731409308
Total # grains analyzed325316308
% volume grains––unresolved mineralogy∼1%∼1%∼3%
# detrital grains––mineralogy identified320309305
# detrital grains––grain size measureda301305294
# detrital grains––grain shape recorded301303282

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Background
  5. Methodology
  6. Results
  7. Interpretations and Discussion
  8. Conclusions
  9. Acknowledgments
  10. References

Sedimentological Observations

The Cadna-owie and Walumbilla Formations exhibit remarkably near-uniform character over vast distances (thousands of km; the breadth of the basin). They are fine- to very-fine-grained (sand or silt), well-sorted, and predominantly parallel-laminated. The dull-colored, beige-gray Cadna-owie (Fig. 3) is comprised of interlaminated sandstones and siltstones, and also displays ripple cross-lamination and pervasively burrowed, structureless beds. Rootlets, cm-scale coal beds, and wavy carbonaceous laminae also occur locally. Variations in the Cadna-owie across the study area are mainly in clay mineral content and degree of bioturbation. The ten meters of upper Cadna-owie directly underlying the Wyandra in Eromanga-1 are similar to the facies shown in Fig. 3B and are described in Table 2. Although interlaminations of siltstone and carbonaceous material are present, overall, the unit is sand-dominant. A sample was taken from this core interval to obtain an inventory of the grain mineralogies and microscopic characteristics present. The dark-gray, homogeneous Walumbilla shales exhibit a consistent character over hundreds of meters of depth, and are only interrupted by occasional shell hash deposits (usually molluscan).

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Figure 3.  Core photos of the Cadna-owie formation. A) and B) Typical Cadna-owie facies: deposits generally show only slight variance in the thickness or color contrast of the alternating siltstone and sandstone layers or degree of bioturbation. Deposits are often very dull-colored and rich in clay minerals. Upper Cadna-owie facies are very similar to the lower Cadna-owie, but generally have more sand content. A) The lower Cadna owie in GSQ Maneroo-1, showing the obliteration of primary bedding structures by bioturbation. B) Unbioturbated, cross-laminated silty sandstone facies of the upper Cadna-owie in GSQ Thargomindah-1. C) and D) Deformation underlying the basal Wyandra contact in GSQ Jundah-1 well. C) Soft-sediment deformation. D) Soft-sediment deformation and microfaults in burrowed sediments.

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Table 2.   Summary of sedimentary observations for Cadna-owie and five basal Wyandra CSBC beds.
WellEromanga-1Thargomindah-1Eromanga-1Talgeberry-1Mirintu-1Quilpie-1
Stratigraphic intervalupper Cadna-owie formationbasal Wyandra sandstone CSBC beds
Thickness4.5 cm7 cm>14.5 cm8.5 cm4 cm
ColorMed gray-beigeLight gray-beigeLight gray matrixMed gray-beigeLight gray-beigeBeige-gray
Macroscopic textureFine- to very fine-grained, homogeneous, well sorted.Clast-supported breccia-conglomerate. Max apparent grain size coarse pebble. Predominant grain size VCGR sand. Poorly sorted. Clasts lenticular to blocky and angular.Max apparent grain size very coarse pebble. Poorly sorted. At base: matrix-supported. At top: very poorly sorted, clast-supported. Clasts very angular to irregular, blocky, and rounded shapes.Max apparent grain size cobble. Very poorly sorted, becoming slightly better-sorted upward. Abundant brown-beige, lenticular to platy clasts and gray granules.Max apparent grain size very coarse pebble. Poorly sorted. Clast shapes very angular and irregular.Bed of pebble-sized mud clasts and granular quartz grains. Max apparent grain size coarse to very coarse pebbles. Very poorly sorted. Overlain by med-grained, massive sandstone, underlain by massive siltstone.
CompositionMud-rich sandstone. Noncalcareous. Common plant fragments. Occasional coal clasts.Polymictic clasts incl. abundant quartz, accretionary, and melt impactoclasts. Calcareous cement.Polymictic clasts (mostly lithics), abundant calcareous cement.Polymictic clasts. Abundant lithics incl. metasedimentary, accretionary impactoclasts, laminated siltstone clasts.Polymictic clasts. Abundant lithics incl. metasedimentary, rip-ups, and possible melt impactoclasts.Granules mostly quartz or quartz-rich. Pebbles are mud-rich, gray, and massive with no internal structure. Bed is clay-mineral rich.
Sedimentary structuresParallel laminated. Ripple cross-lamination, thin carbonaceous laminations. Occasional rootlets. Burrowed.Crudely normal-graded (subtly fining upward, but becoming more clast-supported upward), weak high-angle laminations and cross-laminations. Subvertically imbricated clasts. Bi-directional imbrication.Overall reverse graded. At base: massive to very weakly laminated. At top: weakly imbricated.Parallel laminated, defined by abundant, imbricated pebble-sized clasts. Normal (coarse-tail) graded.High-angle bedding, imbrication of long angular clasts. Bi-directional imbrication.No apparent grading. Large lenticular mudclasts flat-lying.

The sedimentation style of the Wyandra is distinct from the underlying Cadna-owie and over lying Walumbilla Formations. It ranges from very poorly sorted beds containing common angular and irregular-shaped clasts, large rip-up clasts and imbricated pebbles to moderately well-sorted, very coarse-grained sandstone beds and laminated silt interbeds. Sedimentary structures such as massive bedding, graded bedding, trough cross-bedding, parallel lamination of coarse sediments, and possible hummocky and swaley cross-stratification (difficult to confirm in core width) are common in the Wyandra. Much coarser grained sediments distinguish the Wyandra: boulders and cobbles of amorphous- to aerodynamically shaped melt and accretionary impactoclasts (as described in Bron 2010) and metamorphic lithic fragments (e.g., in Talgeberry-1) occur at and above the base of the Wyandra across the basin. The Wyandra is frequently punctuated by clast-supported and matrix-supported breccia-conglomerate beds (CSBC and MSBC beds, respectively; Fig. 4), which are not present in the Cadna-owie or Walumbilla. Of the 31 wells studied, 18 had CSBC layers present in the Wyandra with clasts pebble-sized or larger (Fig. 1; Table 3). CSBC beds at the base of the Wyandra were observed in five wells (Tables 1 and 2; Figs. 4A–E).

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Figure 4.  Core photos of various breccia-conglomerate beds in the Wyandra from across the basin. Arrows indicate Cadna-owie-Wyandra contact. A)–E) Basal Wyandra CSBC beds. See Table 2 for details. F)–I) Wyandra CSBC and MSBC beds (nonbasal). A) and B) 4.5 cm thick bed at 705.08 m depth in GSQ Thargomindah-1 (2 core views). Bed overlies coal seam and erosional, angular basal Wyandra contact with flame structures. A) Tan-colored accretionary impactoclasts are prominent. Modified after Bron (2010). B) Note imbrication of pebble-sized lithic and probable altered melt clasts, indicating flow direction from left to right. C) 7 cm thick bed at 743.73 m depth in GSQ Eromanga-1. Bed is strikingly polymictic, with coarse, angular, exotic clasts. Basal Wyandra contact is undulose, and underlain by bedded sandstone. D) 8.5 cm thick bed at Mirintu-1 with irregular-shaped and angular clasts. Basal Wyandra contact is sharp and cross-cuts underlying flat laminations at a low angle. E) Bed at Talgeberry-1 location. Sharp, undulose basal Wyandra contact is overlain with lithic, quartz, and probable altered melt clasts. Large dark rip-up clasts are probably of Cadna-owie origin. White line on lower right side of core is a scratch. F) MSBC bed within Wyandra at GSQ Maneroo-1, with rip-up clasts and amorphously shaped clasts of likely melt origin. Vertical dark stripe on left is a rust-colored stain on the core. G) Part of a 31 cm thick CSBC bed within the Wyandra in GSQ Wyandra-1. Mixing with local sediments is evident here, as many of the red stratified clasts appear to be locally derived rip-up clasts not seen in other wells. Some clasts show internal deformation. Imbrication is evident elsewhere in this layer. H) CSBC bed within Wyandra at GSQ Bulloo-1, which is part of a 16 cm thick bed of seeming mud clasts, interpreted by Bron (2010) as a possible accretionary impactoclast bed (note large accretionary impactoclast with nucleus). Bed shows bidirectional imbrication. I) An MSBC bed with floating pebbles is overlain by a very coarse-grained, 7–15 cm thick CSBC bed exhibiting imbricated impactoclasts and a cobble-sized accretionary impactoclast described in Bron (2010), within the Wyandra at GSQ Eromanga-1. Beds overlie a dark, sandy siltstone bed deformed by microfaults.

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Table 3.   Observations at 31 well locations.
Well nameWell type, locationApprox. distance from Tookoonooka (Rc)aApprox. distance from Talundilly (Rc)aAccretionary impactoclasts present in Wyandra?CSBC beds present in Wyandra?cMaximum grain sized within Wyandra CSBC or MSBC bedsBasal Wyandra contactComments
  1. Drill cores are from stratigraphic, petroleum, and mineral wells from Queensland and South Australia government and Santos Ltd. core libraries; well locations are shown on Fig. 1. Stratigraphic well cores are 47.6 mm in diameter, petroleum well cores are usually 65 mm or 100 mm in diameter. “–” indicates not applicable. “n.d.” indicates not detected.

  2. aRc = Crater Radii, measured from crater center. Tookoonooka original crater diameter is most recently estimated at 66 km (Gostin and Therriault 1997), therefore 1 Rc = 33 km, or the distance from the crater center to the crater rim. Talundilly crater diameter is taken as 95 km as originally estimated, thus 1 Rc for Talundilly is 47.5 km.

  3. bObservations from Bron (2010).

  4. c“B” indicates that a basal Wyandra breccia-conglomerate layer is present.

  5. dMaximum grain-size is apparent grain-size within the context of drill core width. In stratigraphic well cores (4.76 cm in width), clast sizes larger than pebbles can often not be recognized; maximum clast size is restricted by the core width and may be larger than indicated.

  6. eDiscontinuous core, thus data may be incomplete.

Thargomindah-3strat, QLD<1>5Wyandra not present
Aros-2Petroleum, QLD<2>5YesNoCobbleSharp, low angle
Ipundu 6Petroleum, QLD<2>5YesbYesCobbleSharp, angular, unduloseWyandra not fully corede
Ipundu North-1Petroleum, QLD<2>5YesbNoPebbleSharp, deformation, flame structures, angularWyandra not fully corede
Kercummurra-1Petroleum, QLD<2>5YesbNoPebblen.d.Base Wyandra not corede
Kooroopa-1Petroleum, QLD<2>5Wyandra not corede
Talgeberry-1Petroleum, QLD<2>5YesbYes, BboulderSharp, undulose, erosional, cross-cuttingWyandra not fully corede Fig. 4E, Table 2
Talgeberry-2Petroleum, QLD<2>5YesbYesCobbleSharp, flame structuresFig. 5D
Tarbat-6Petroleum, QLD<2>5YesbYesbouldern.d.Base Wyandra not corede
Tarbat-8Petroleum, QLD<2>5YesbYesCobblen.d.Base Wyandra not corede
Thargomindah-1Strat, QLD<2>5YesbYes, BCobbleSharp, erosional, flame structures in coal, deformationFigs. 4A and 4B, Table 2
Thargomindah-2Strat, QLD<2>5YesbYesCobbleSharp, unduloseFig. 5B
Tintaburra-2Petroleum, QLD<2>5YesbNoAngular, undulose, flame structures, deformationWyandra not fully corede
Toby-1Petroleum, QLD2>5Wyandra not corede
Eromanga-1Strat, QLD<4<5YesbYes, BCobbleUndulose, erosionalFigs. 4C and 4I, Table 2
Tickalara-1Strat, QLD<4>5YesbYesPebbleSharp, undulose, erosional
Bulloo-1Strat, QLD5>5YesbYesPebbleSharp, erosional, cross-cuts rootletsFigs. 4H and 5E
Connemara-1Strat, QLD>5>5YesbYesPebbleSharp, angular, erosionalFig. 5C
Dullingari-1Petroleum, SA>5>10n.d.Non.d.Wyandra not fully cored; base Wyandra not corede
Jundah-1Strat, QLD>5<4YesbYesPebbleSharp
Mirintu-1Petroleum, QLD>5>5YesbYes, BPebbleSharp, erosional, cross-cuts rootlets, cross-cuts laminations at low angleWyandra not fully corede Fig. 4D, Table 2
Quilpie-1Strat, QLD>5<5YesbYes, BCobbleSharpTable 2
Wyandra-1Strat, QLD>5>5YesbYesboulderSharp, cross-cuts paleosolFigs. 4G and 5A
Augathella-2-3RStrat, QLD>10<4n.d.NoSharp
CBH-2Petroleum, SA>10>10YesbYesPebbleSharp
Cootabarlow-2Petroleum, SA>10>10n.d.NoSharpWyandra may not be fully corede
Machattie-1Strat, QLD>10>5YesbYesPebbleSharp, erosionalFig. 5F
Maneroo-1Strat, QLD>10<4Yes––rareYesPebbleSharpFig. 4F
SPH-1Mineral, SA>10>10Wyandra not present
Skeleton-2Strat, SA>10>10n.d.Non.d.Wyandra not fully corede
Tambo-2Strat, QLD>10<4YesbNoPebbleSharp

Basal Wyandra CSBC beds at Thargomindah-1 (Figs. 4A and 4B) and Eromanga-1 (Fig. 4C) are similar to the character of other basal Wyandra CSBC beds (Table 2). Both are poorly sorted and composed of a range of highly angular fragments to rounded clasts. The basal Wyandra CSBC bed at Thargomindah-1 exhibits crude normal grading and directly overlies a thin black coal seam within the Cadna-owie. The basal Wyandra CSBC bed at Eromanga-1 is reverse-graded overall, with maximum grain sizes of 1 cm near the base to 3.9 cm near the top, and overlies bedded fine-grained sandstone of the Cadna-owie. Exotic pebbles are the most distinctive feature of the beds in hand sample. In the Thargomindah-1 bed imbricated, polymictic clasts range from fractured and veined metamorphics that are blocky and angular to light-brown, clay-rich impactoclasts that are lenticular to irregular in shape. Accretionary impactoclasts up to 2.2 cm in length in this layer were geochemically analyzed by Bron (2010). The polymictic nature of the Eromanga-1 bed is also distinctive, with red, greenish, black, gray, and brown colors of sedimentary and volcanic lithic clasts. These beds are the lowermost of many breccia-conglomerate beds in the Wyandra at these two well locations. Shallower (younger) breccia-conglomerate beds in Eromanga-1 (e.g., Fig. 4I) contain some of the largest accretionary and melt impactoclasts discovered in drill core in the basin thus far.

Basal Contact of the Wyandra

The basal contact of the Wyandra in the wells studied is consistently sharp, accompanied by a significant, sudden jump in grain size across the contact with the Cadna-owie (Table 3). Minimum average grain sizes above the contact are usually medium- to very coarse-grained sand, whereas the maximum observed grain size in Cadna-owie beds underlying the contact was fine-grained sand. The contact is erosional, angular, cross-cutting, and undulose at various locations (Figs. 4 and 5), and is often defined by flame structures (e.g., Figs. 4B, 5B, and 5D) or scours (e.g., Figs. 4E, 5C, and 5F). Rare occurrences of sandy paleosol and thin, minor coal seams in the Cadna-owie also underlie the Wyandra (e.g., Figs. 4B and 5A). In places, the contact truncates rootlets (Fig. 5E). The contact in Thargomindah-1 is erosional: it cross-cuts underlying coal laminations of the Cadna-owie and variably exhibits cm-scale flame structures to incompletely ripped-up bedding (Figs. 4A and 4B). Apparent clasts within the coal are composed of the same sandstone matrix present above the contact, and may be filled scours, filled burrows, or minor sand injectites. Below the coal lies siltstone with rootlets, churned (highly burrowed) bedding, and soft-sediment deformation features. The contact in Eromanga-1 (Fig. 4C) appears more gradational than at Thargomindah-1, but this may be due to the similarity between the matrix material of the basal Wyandra CSBC bed and the underlying Cadna-owie sandstone, which is slightly coarser and cleaner at this location.

image

Figure 5.  Core photos showing the abrupt nature of the basal Wyandra contact (indicated by arrows). A) At GSQ Wyandra-1, gray Wyandra planar-bedded sandstone with floating pebbles sharply overlies red, convoluted sandstone bedding or sandy paleosol of the Cadna-owie. B) GSQ Thargomindah-2. Large rip-up clasts floating in sandstone above sharp undulose contact defined by minor flame structures and microfaults. The Cadna-owie below the contact is burrowed interbedded sandstone and siltstone. C) GSQ Connemara-1 (2 views of core). Scours filled with coarse lithic material define the erosional contact; dark, sandy, bioturbated Cadna-owie siltstone was partially ripped up by the scouring. D) Contact at Talgeberry-2 (2 views of core). Beige-gray Wyandra sandstone with floating, angular clasts (sedimentary lithics), overlying dark brown, silty, homogeneous mudstone of the Cadna-owie. Contact is sharp, and exhibits flame structures. E) GSQ Bulloo-1. Contact cross-cuts mudstone unit of the Cadna-owie with rootlets and burrows. Sparse, altered accretionary impactoclasts float in the sandstone above the contact. F) GSQ Machattie-1 (2 views of core). Fine-grained, cross-laminated sandstone of Cadna-owie underlies coarser, calcite-cemented Wyandra sandstone with floating pebbles and rip-up clasts. Contact is scoured, and cross-cuts underlying bedding, which exhibits minor deformation structures.

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Deformation

Throughout the basin, microfaults and soft sediment deformation structures commonly underlie the Wyandra in the Hooray and Cadna-owie Formations (Figs. 3C and 3D). Convoluted bedding appears to coincide with younger, more mud-rich sediments, and microfaults with older, more lithified sediments. These structures are, by contrast, very rare within the Wyandra; indeed, in places, their occurrence stops abruptly at the base of the Wyandra (e.g., Figs. 5A and 5F). It was also observed that some rip-up clasts within the Wyandra exhibit this deformation. In GSQ Thargomindah-3, located in the Tookoonooka outer crater structure, similar deformation is widespread; microfaults, sedimentary injectites (sandstone dykes), and soft sediment deformation structures are abundant in the crater fill.

Bioturbation

The Cadna-owie is frequently highly bioturbated, with churned bedding characteristic of abundant biological activity coupled with slow sedimentation rate (e.g., Figs. 3A and 5B). The Wyandra, in contrast, is largely devoid of bioturbation; both sandstone beds and siltstone interbeds appear unaltered by burrowing activity. Across the study area, the base of the Wyandra typically marks an abrupt cessation of bioturbation and rootlets (e.g., Figs. 5B, 5C, and 5E).

Petrographic Results

Petrography of the Cadna-Owie

In the upper Cadna-owie sample from Eromanga-1, microscopic analyses (Table 1) revealed that clast compositions are dominated by feldspar (40%), lithics (19%), and quartz (14%); data and analyses from point-counting are presented in Tables 4 and 5, and Figs. 6–7. Feldspars commonly display dissolution textures and heavy alteration. Of the lithic component, cherts and volcanic grains are prominent, whereas metamorphic, hydrothermal, melt, and accretionary clasts, and nonchert sedimentary lithic clasts are absent (Fig. 7D). Of the quartz grains, igneous grains are most common, few metamorphic quartz grains were observed, and no hydrothermal (vein) quartz is present (Fig. 7C). Organic material (plant debris), mica, altered amphibole, pyroxene, opaques, and bioclasts are present in decreasing amounts, in addition to accessory minerals (Table 5). The Cadna-owie is compositionally an arkosic arenite (Fig. 7A).

Table 4.   Summary of analyses and interpretations for three samples.
WellThargomindah-1Eromanga-1Eromanga-1
  1. Note: Grain sizes are abbreviated as follows: VFGR = very fine-grained; FGR = fine-grained; MGR = medium-grained; CGR = coarse-grained; VCGR = very coarse-grained.

  2. aMedian and skewness are not useful metrics for bimodal samples but are shown here for comparative purposes.

  3. bM9 method was used to calculate mean grain size.

  4. cGrain size analyses by thin section give a value of apparent sorting; sediments may not be as poorly sorted visually as they appear in thin section (Tucker 2001). Values for CSBC beds indicate that they would probably still be poorly sorted visually, but Cadna-owie sample would be moderately well-sorted visually.

  5. eThe application of maturity concepts is only moderately relevant here, as it applies mainly to sandstones and conventional sedimentation processes; it is used here for the sole purpose of contrasting the CSBC layers to the background sedimentation. Compositional maturity formula after Pettijohn et al. (1987): Q+Lch:F+L-Lch; volumetric data used in calculation.

  6. fSee Table 5 for mineral abbreviations.

Stratigraphic intervalbasal Wyandra CSBCbasal Wyandra CSBCupper Cadna-owie
Depositional environmentHigh energyHigh energyLow energy, cold
MarineMarineMarine
MetricsMedian grain sizea (phi)0.17 (CGR sand)a1.43 (MGR sand)a2.46 (FGR sand)
Mean grain sizeb (phi)0.03 (CGR sand)0.85 (CGR sand)2.41 (FGR sand)
Mode or modesBimodal: 1) VCGR sand 2) FGR sandBimodal: 1) FGR sand  2) GranuleUnimodal: 1) VFGR sand
Apparent sortingc (phi)1.74; poorly sorted1.85; poorly sorted0.74; moderately sorted
Skewnessa (phi)−0.11; coarse-skeweda−0.45; strongly coarse-skeweda−0.03; near-symmetrical
ClassificationdRock typeTexturalSandy breccia-conglomerateBreccia-conglomerateMuddy sandstone
CompositionalLithic areniteLithic areniteArkosic arenite
MaturityeTexturalImmature: angular and coarse clast content; poorly sorted; bimodalSubmature-mature: persistently FGR and well sorted
Compositional51:49 (mod mature)22:78 (immature)30:70 (immature)
CSBC content comparisonfQuartz contentProminent Qm and Qh volumesQ less prominent than Thargomindah-1
Lithic contentSimilar to Eromanga-1 r.e. L variety. Twice L volume of Cadna-owie.Greater L content than Thargomindah-1 r.e. volume & coarseness of Ls, Lch, and Li. Four times L volume of Cadna-owie.
Impactoclast contentBasal CSBC bed has significantly greater volume and grain size range of Limp clasts than Eromanga-1 basal CSBC bed.Accretionary impactoclasts not prominent in basal CSBC bed. Other CSBC beds coarser than Thargomindah-1.
Table 5.   Petrographic results for three thin sections.
WellEromanga-1Thargomindah-1Eromanga-1CSBC coarse grain fractionaCSBC very angular grain fractiona
Stratigraphic intervalCadna-owieWyandra CSBCWyandra CSBC
Grain size data (%)
 Very coarse pebblen.d.n.d.n.d.
 Coarse pebblen.d.0.70.3
 Medium pebblen.d.1.31.3
 Fine pebblen.d.2.32.3
 Granulen.d.9.010.2
 Very coarse sandn.d.26.24.9
 Coarse sandn.d.9.03.9
 Medium sand2.019.612.1
 Fine sand22.824.642.0
 Very fine sand59.26.019.0
 Coarse silt12.21.33.6
 Medium silt3.1n.d.0.3
 Fine silt0.7n.d.n.d.
 Very fine siltn.d.n.d.n.d.
Total100100100
Grain shape data (%)
 Well–rounded17.05.09.2
 Rounded12.820.916.8
 Subrounded28.025.319.5
 Subangular32.323.918.8
 Angular9.916.921.5
 Very angularn.d.8.014.2
Total100100100
 Count or volumecCountVolumeCountVolume  
  1. Note: “n.d.” = not detected.

  2. aVery angular clast shapes and coarse-grained sand-coarse pebble grain sizes are present in CSBC beds, but absent in the Cadna-owie. 1 = observed in Thargomindah-1 CSBC bed; 2 = observed in Eromanga-1 CSBC bed.

  3. bDetrital grain mineralogy only; e.g., feldspar grains altered to clays, fine-grained mica, or carbonate are recorded as “F.” Abbreviations are as follows: “undiff” = mineralogy undifferentiated due to grain size or excessive alteration, etc.; “alt” = heavily altered; “Q” = quartz; “Qi” = igneous or common quartz, with “Qiv” (volcanic source) and “Qip” (plutonic source) distinguishable; “Qm” = metamorphic quartz; “Qh” = hydrothermal or vein quartz; “F” = feldspar; “Fp” = plagioclase; “Fk” = K-feldspar; “L” = rock fragment (lithic); “Lch” = chert of sedimentary or volcanic origin; “Ls” = sedimentary rock clast; “Lm” = metamorphic rock clast; “Li” = igneous rock clast; “Lh” = hydrothermal (vein) rock clast; “Limp” = melt and accretionary impactoclasts; “Bio” = carbonaceous material, organic debris, plant fragments, or bioclasts; “O and goeth” = opaques and iron oxy-hydroxides; “Amph” = amphibole. Heavy minerals include apatite, garnet, zircon, epidote.

  4. cFor Cadna-owie thin section, due to small grain-sizes, % count values are equivalent to % volume values.

Grain mineralogy datab (%)
 Q (undiff)2.64.63.31.60.41n.d.
 Qi (undiff)3.610.57.77.61.71n.d.
 Qiv6.23.12.38.22.11,21,2
 Qipn.d.3.43.02.20.91,21,2
 Qm1.615.420.83.26.01,21
 Qhn.d.10.810.33.52.51,21
Total quartz14.047.747.326.313.5
 F (undiff or alt)24.013.97.916.13.62n.d.
 Fp12.04.32.42.80.6n.d.1,2
 Fk3.90.30.20.60.1n.d.n.d.
Total feldspar39.918.510.519.64.4
 L (undiff)2.9n.d.n.d.1.60.4n.d.n.d.
 Lch7.82.21.29.87.51,22
 Ls othern.d.5.24.22.253.31,21,2
 Lmn.d.0.61.21.34.01,2n.d.
 Li8.47.14.05.48.91,2n.d.
 Lhn.d.0.90.7n.d.n.d.1n.d.
 Limpn.d.8.325.56.31.41,21
Total lithics19.224.336.826.675.5
 Bio10.13.11.77.61.81,2 minor2
 O and goeth1.92.51.48.51.9n.d.n.d.
 Mica6.20.90.51.90.41,2 minor1
 Amph and pyroxene3.9n.d.n.d.1.90.4n.d.n.d.
 Glauconite0.30.60.40.60.1n.d.n.d.
 Chlorite0.60.60.40.30.1n.d.n.d.
 Heavy minerals0.6n.d.n.d.1.60.722
 Other3.21.81.05.11.1n.d.n.d.
Total100100100100100
image

Figure 6.  Grain size distributions from thin section observations. See Table 5 for mineralogy abbreviations. A) Frequency (%) distribution versus grain size for 3 thin sections: Thargomindah-1 (Th-1) and Eromanga-1 (E-1) basal Wyandra CSBC beds, and Eromanga-1 Cadna-owie (C-o). See Table 1 for data set details. B–D) Frequency (%) distribution curves of grain sizes with stacked mineralogy for 3 thin sections. B) Eromanga-1 Cadna-owie. C) Thargomindah-1 basal Wyandra CSBC bed. D) Eromanga-1 basal Wyandra CSBC bed.

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image

Figure 7.  Ternary plots for 3 thin sections, using volumetric data. Solid data points represent bulk volumes, white-filled data points represent Wyandra CSBC bed coarse fractions only (grain sizes not present in Cadna-owie, i.e., CGR sand to pebble size fraction). Classifications after Pettijohn et al. (1987) and Tucker (2001). See Table 5 for mineralogy abbreviations. A) Q-F-L plot for sandstone composition classification. Chert is included with lithic grains. B) Grain size plot for rock texture classification. C) Plot of quartz clast compositions. D) Plot of lithic clast compositions. Does not include melt and accretionary impactoclast grain population.

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The Cadna-owie is unimodal, with a narrow, near-symmetrical grain size spread (Fig. 6A, Table 4). Ninety-eight percent of grains are less than medium-grained sand size, and approximately 5% of the grains are medium silt size or smaller, which implies that the rock is matrix-poor (e.g., Pettijohn et al. 1987). Texturally, the Cadna-owie is a muddy sandstone (Fig. 7B). Ten percent of the grains are angular (Table 5). This angular fraction is all fine-grained sand size or smaller, and is mainly comprised of feldspar.

Petrography of Basal Wyandra CSBC Beds

Point-counting revealed that the Thargomindah-1 and Eromanga-1 CSBC beds at the base of the Wyandra contain a similar variety of detrital clast compositions (Fig. 6, Tables 2 and 5). Volumetrically, the most common components are quartz and lithics, with lesser feldspar. Of the quartz grains, metamorphic species are predominant, followed by igneous and hydrothermal grains (Fig. 7C). Of the lithics in the Thargomindah-1 bed, altered melt and accretionary clasts, sedimentary clasts, volcanic clasts, chert, and metamorphic clasts are present in decreasing order (Fig. 7D). Minor amounts of hydrothermal lithics are also present. Clasts interpreted to be devitrified and altered melt material, including rare microcrystalline quartz spherules, and accretionary impactoclasts account for about 8% of the grains (25% volumetrically). Some melt clasts exhibit flow-banding textures and aerodynamic shapes, and are described in more detail by Bron (2010). The melt clasts are relatively less altered than the volcanic lithics; the latter are often greenish in color and have a texture defined by feldspar laths. Accretionary impactoclasts in thin section often show distinctive concentric zoning, very fine-grained internal texture, and are usually tan to light gray in color. Of the lithics in the Eromanga-1 bed, nonchert sedimentary rock varieties are predominant, followed by volcanic, chert, and metamorphic grains (Fig. 7D). Altered melt clasts are also present. The CSBC beds have experienced significant diagenetic alteration, particularly the feldspar, volcanic lithic, and melt and accretionary components.

Grain size and grain shape analyses show that the two Wyandra CSBC beds are poorly sorted and distinctly bimodal (Table 4, Fig. 6). Clast sizes span from medium silt to coarse pebble. Coarse-grained sand to pebble-sized clasts account for 23–49% of the grains; this coarse fraction is dominated by lithics and quartz, particularly metamorphic and hydrothermal quartz, chert, accretionary impactoclasts, and sedimentary rock fragments. The angular to very angular fraction of the clasts (comprising 25–36% of the grains; Table 5) is dominated by quartz, feldspar, and lithic fragments. Amongst the very angular grains (8–14% of the grains), a grain shape that is not present in the Cadna-owie data set, quartz is predominant in both beds. These CSBC beds can be classified texturally as sandy conglomerate and conglomerate, respectively, and compositionally as lithic arenites (Figs. 7A and 7B).

Shock-Metamorphosed Grains

Petrographic observations in the Thargomindah-1 and Eromanga-1 CSBC beds at the base of the Wyandra include shock-metamorphosed grains. Monocrystalline and polycrystalline quartz grains as well as quartz within lithic clasts exhibit PDFs and PFs. A brownish “toasted” appearance of quartz was also commonly observed. Shock data will be presented in a separate, forthcoming publication. Two shocked polycrystalline grains are illustrated in Fig. 8. Most shocked grains lie in the coarse-grained sand to fine pebble size fraction. No shock-metamorphosed grains were observed in the Cadna-owie analysis.

image

Figure 8.  Petrographic photos of two shocked grains within the Eromanga-1 basal Wyandra CSBC bed. Each grain is shown under both plain light (left) and crossed polars (right). A and B) A toasted, polycrystalline metamorphic quartz grain, exhibiting multiple sets of planar microstructures under the petrographic microscope. Universal stage measurements in various subcrystals indicate that up to three sets of PDFs are present per subcrystal, and include the following orientations: {inline image}, {inline image}, {inline image}, and {inline image}, the latter indicated by an arrow in the largest subcrystal. At least two sets of PFs are also present, with orientations of {inline image} and {inline image}. C and D) Very irregularly shaped lithic clast with toasted appearance. Universal stage measurements in quartz crystals within this clast indicate that up to six sets of PDFs are present per crystal, and include only higher angle PDF plane orientations {inline image} and above, consistent with previous measurements made at Tookoonooka (Gostin and Therriault 1997). No basal (0001) or {inline image} PDF orientations were observed in these two grains, which is consistent with observations at other sedimentary target impact sites (Grieve et al. 1996).

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Interpretations and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Background
  5. Methodology
  6. Results
  7. Interpretations and Discussion
  8. Conclusions
  9. Acknowledgments
  10. References

All sedimentary and petrographic observations suggest that the onset of Wyandra deposition indicates a sudden, regional change in sedimentation style, depositional energy, and sediment source. The consistency of the Cadna-owie sedimentation over time and area suggests a depositional environment of little change. The unimodality and mean grain size of the Cadna-owie suggest a degree of maturity and grain recycling concomitant with poor sediment supply in a low-energy environment of regional extent. Similarly, in the Walumbilla, dark shales indicate a quiescent depositional environment, although in deeper marine conditions than the Cadna-owie. Shell hash indicates redeposition of broken shell into deeper water by occasional storm events. In contrast, a high-energy depositional environment of the Wyandra is indicated by scours, imbricated pebbles, and the predominance of high flow regime sedimentary structures. The Wyandra records the sudden, basin-wide appearance of a coarse and angular grain fraction absent in the Cadna-owie; the bimodality of CSBC beds at the base of the Wyandra confirms the presence of two grain populations (and thus two sources) when compared with the unimodal Cadna-owie (Fig. 6). The finer-grained mode of the CSBC beds mirrors the Cadna-owie mode, whereas the abrupt appearance of the coarser mode befits a catastrophic emplacement mechanism. In the absence of volcanism and significant tectonism in the flat-lying basin, this coarser mode is interpreted to derive from the Tookoonooka impact event. This coarser “impact” mode is more pronounced at Thargomindah-1 than at Eromanga-1: the differences in the skewness, median, and mean grain sizes of the CSBC beds suggest that sediment transport occurred from Thargomindah-1 to Eromanga-1 (cf. Tucker 2001) and that dilution of ballistic ejecta (with ambient sediments) occurred distally; i.e., toward Eromanga-1 (cf. Melosh 1989). This is consistent with Tookoonooka as the possible source. As basal Wyandra CSBC beds record the first presence of this coarse material, they probably represent the least reworked Tookoonooka ejecta beds in the basin, and may record immediate postimpact sedimentary processes.

The abrupt Cadna-owie-Wyandra contact appears to be an unconformity across the entire basin. Additional observations by Dettman (1985) and Gallagher et al. (2008) in South Australia and at the Aros-2 well location (Fig. 1), respectively, support this view. Our observations of the erosional character of this contact and the sudden shift from a heavily burrowed Cadna-owie to a virtually unbioturbated Wyandra are basin-wide. Flame structures at the contact imply that rapidly deposited volumes of sediment caused sediment loading and water escape. Water escape structures are unexpected in the Cadna-owie’s depositional context, and thus evidence seismicity or the higher sedimentation rate of the Wyandra. Scouring observed at the contact may explain the absence of basal Wyandra CSBC beds at many of the locations studied. Although a regional Tookoonooka ejecta blanket may initially have existed, it is likely that at many locations, it was eroded by scouring or reworking subsequent to emplacement. Deformed rip-up clasts of likely Cadna-owie origin within the Wyandra are further evidence of this scouring. From these observations, we interpret that the Cadna-owie-Wyandra contact represents an event horizon of regional significance: the Tookoonooka event horizon.

The abundant deformation observed throughout the greater basin below the Cadna-owie-Wyandra contact in the Cadna-owie and Murta (Hooray) Formations (also observed by John and Almond 1987) may be another impact indication. Significant paleoslope, a common requirement for some of the deformation structures observed, is suggested by neither the flat-lying sediments of the Cadna-owie nor the scale of their lateral continuity in the basin. Rather, the soft-sedimentary deformation and microfaults, in addition to deformed rip-up clasts, suggest that the quiescent sedimentation of the Cadna-owie was disrupted by a widespread seismic event that preceded or coincided with the onset of Wyandra deposition. They are interpreted to be impact seismites: the response of water-rich, ductile soft sediments, and partly lithified sediments to the passage of impact seismic waves through the sediment (cf. Shiki et al. 2000). We also interpret the presence of similar, likely time-equivalent deformation structures in the sediments of Tookoonooka’s outer crater structure at Thargomindah-3, also observed by Young et al. (1989), to evidence impact-related fluidization and deformation.

Sediment Provenance

Clear differences in the composition of the Wyandra CSBC beds and the Cadna-owie sediments give provenance indications that are interpreted to signal the influx of Tookoonooka impact ejecta (Figs. 6 and 7; Table 4). The Cadna-owie’s sandstone composition suggests in part a mafic volcanogenic source. Volcanic lithics could only derive from recycled basement rock (Harrington and Korsch 1985) or contemporaneously from the distal eastern margin of Australia (Wiltshire 1989; McDougall 2008), the latter requiring an aeolian transport indication in the Cadna-owie given the basin’s low relief and the ex-basin source. The Cadna-owie’s slight negative skewness (Table 4) may suggest an aeolian component (cf. Tucker 2001). In contrast to the underlying Cadna-owie, the Wyandra CSBC beds are interpreted to be laden with impact ejecta.

The most compelling evidence of impact in the Wyandra CSBC beds is the presence of significant volumes of melt and accretionary clasts and shock metamorphosed grains. The melt and accretionary clasts clearly have a different origin than the volcanics discussed above. They are thought to provide substantive evidence of impact, and are thus termed melt and accretionary impactoclasts (cf. Bron 2010). The diagenetically altered nature of these clasts suggests chemical instability, which, in combination with their high energy mode of deposition, suggests that they experienced little reworking and quick burial. The nearby Tookoonooka structure is the likely source of these clasts. The presence of shocked grains in the Wyandra CSBC beds compared with an absence of shocked grains in the Cadna-owie represents diagnostic evidence of the arrival of impact-derived material at the base of the Wyandra. The association of the shocked grains with the coarser grain size mode of the Wyandra CSBC beds also indicates that CSBC material is from a different source than the Cadna-owie sediments.

The high proportion of lithics and quartz in the Wyandra CSBC beds clearly implies that the Wyandra CSBC beds are compositionally unrelated to the source of the considerable underlying thickness of Cadna-owie sediments. The Wyandra CSBC beds, particularly the coarse realm, are richer than the Cadna-owie in metamorphic, hydrothermal, chert, and sedimentary lithic clasts as well as metamorphic and vein quartz species (Figs. 7C and 7D). Many of the coarse lithic clasts observed in the Wyandra at numerous locations, particularly light green and greenish-blue metasedimentary clasts and veined grayish quartzite clasts, appear identical to basement lithologies in the Tookoonooka central uplift (at Tookoonooka-1) and Thargomindah Shelf basement rocks at depth in Thargomindah-1. At least 7 wells from the Tookoonooka structure encountered similar basement lithologies at depth (QPED, 2009). The Tookoonooka impact event excavated Jurassic-Cretaceous Eromanga Basin sediments, metasedimentary basement rock of the Thargomindah Shelf, and possibly Permo-Triassic Cooper Basin sediments (Gorter et al. 1989; Longley 1989; Gostin and Therriault 1997). Thus, much of the lithic and quartz content in the Wyandra is interpreted to derive from this “fresh” source in the form of ejecta, and includes Tookoonooka basement material. Nearby wells beyond the crater structure indicate that, at present-day compaction, the average thickness of strata between the base of the Wyandra and the basement is approximately 450 m (QPED, 2009). Thus, the impact would have excavated this thickness of preimpact strata (target sequence) at a minimum. The enhanced quartz content is responsible for the compositional maturity of the Thargomindah-1 basal Wyandra CSBC bed relative to the Cadna-owie. The different proportion of quartz and lithics in the two basal Wyandra CSBC beds analyzed could be due to heterogeneous ejecta distribution.

As a result of our analyses, two distinct clast assemblages have emerged: a Cadna-owie (ambient sedimentation) clast assemblage and an interpreted impact assemblage (Table 6, Figs. 6B–D). It is important to note that as the Cadna-owie sediments were part of the Tookoonooka target sequence and they were probably locally mixed with the ejecta (e.g., by ballistic sedimentation processes cf. Melosh 1989; impact tsunami scour, or postimpact sedimentary reworking), some overlap in the assemblages would be expected. From impactite terminology (cf. Stöffler and Grieve 2007), the basal Wyandra CSBC beds at Thargomindah-1 and Eromanga-1 could be classified as proximal, allochthonous, polymict impact breccia and basal Wyandra CSBC beds in wells beyond 5Rc could be considered distal air fall beds (with shocked and unshocked clasts). However, this classification refers to initial ejecta deposits and does not take into account reworking of ejecta by postimpact processes, or dispersal of ejecta by waves and currents in a marine impact scenario.

Table 6.   Interpreted clast provenance from petrographic results: differentiation of an impact assemblage from the ambient sediments.
Clast mineralogyaCadna-owie “ambient” assemblage“Impact” assemblageComments on source
  1. Notes: Y = present; n.d. = not detected.

  2. aSee Table 5 for mineral abbreviations.

QivYYBoth
Qipn.d.Y
QmYYBoth
Qhn.d.Y
FpYYBoth
FkYn.d.
LchYYBoth sedimentary and volcanic origin. Minor in Thargomindah-1
LsMinorY
Lmn.d.Y
LiYYBoth. Minor in Thargomindah-1
Lhn.d.Y
Limpn.d.YMinor in Eromanga-1
BioYn.d.Organics of diagenetic origin not included here
MicaYMinor
Amph and pyroxeneYn.d.
GlauconiteYn.d.
Heavy minerals (e.g., apatite, garnet, zircon, barite)YYBoth

Distribution

The Wyandra breccia-conglomerate beds described appear concurrently across the basin at the same stratigraphic level. Their distribution corresponds to a spatial extent of about 400,000 km2, and suggests a regionally instantaneous mechanism of deposition. This distribution correlates roughly with the presumed extent of the Eromanga Basin in the late Lower Cretaceous, and correlates with the occurrence of accretionary and melt impactoclasts within the Wyandra observed by Bron (2010). It is interpreted to be the known limit of the Tookoonooka impact deposits.

The Age of the Tookoonooka Impact Event

From the many indications discussed above, the basal contact of the Wyandra is considered to be the Tookoonooka impact horizon, corroborating the findings of Bron (2010). The base of the Wyandra is correlated with the base of the PK3 palynological zone and the Barremian-Aptian boundary by Gallagher et al. (2008). The timing of the Tookoonooka impact event is thus constrained to approximately 125 ± 1 Ma (Fig. 2). This provides a better constraint on the age of the Tookoonooka impact event than earlier estimates, and lies within the gross age range given by them.

Implications for the Impact Paleoenvironment and the Origin of the Wyandra Sandstone

Our observations of glauconite and bioturbation support previous work that indicates that Cadna-owie sedimentation occurred in a paralic to restricted shallow marine setting (summaries in Draper 2002; Krieg and Rogers 1995). These depositional environments appear to have predominated prior to Wyandra deposition in the basin. However, our observations of occasional nonmarine sediments in the Cadna-owie, sometimes directly underlying the Wyandra, indicate the shallow nature of the epicontinental sea at time of impact, where shallow marine, marginal marine, swampy, and lagoonal environments coexisted. Paleogeographical reconstructions of the early to mid-Cretaceous imply that Tookoonooka was centrally located in the Eromanga Basin (Veevers 1984; Frakes et al. 1987; BMR, 1990), and that the Cretaceous Eromanga Sea covered a large part of central Australia by the time of impact. Interpretations by Bron (2010) regarding the origin of specific types of accretionary impactoclasts, which resemble hydroclastic volcanic accretionary lapilli, support a marine impact hypothesis. Thus, Tookoonooka was most likely a marine impact, and ejecta were deposited into a shallow sea and reworked in a marine environment before burial. Although the Wyandra has previously been interpreted to represent transgressive deposits (Senior et al. 1975; Exon and Senior 1976; Draper 2002), transgression cannot account for the sudden presence of the observed volumes of coarse sediments in the persistently fine-grained basin deposits of the time. As the Wyandra records a high energy of deposition and marine signature and occasionally overlies nonmarine sediments, this may indicate that the Wyandra was at least partially emplaced by impact tsunamis. Base surge mechanisms may have contributed to this deposition proximal to the crater.

Previous work implies that the geographical position of Tookoonooka in the Lower Cretaceous was ±65° latitude (Day 1969; Veevers 1984; Frakes et al. 1995; Klootwijk 2009). Near-freezing temperature mineralization in the Cadna-owie has been recorded from basin margin sites with an equivalent paleolatitude to Tookoonooka (De Lurio and Frakes 1999), and glacial lonestones in the Lower Cretaceous strata have been reported in the southwest of the basin (Frakes et al. 1995). Ice covering may have contributed to the low energy of the environment, and the potential for aeolian transport. In combination with a possibly semiarid climate (Fig. 7A), (cf. Tucker 2001), a suppressed weathering regime likely existed, evidenced by the presence of fresh feldspar in the Cadna-owie. Considerations of this paleoenvironment lead to new questions regarding how ejecta distribution was affected in a potentially ice-covered sea. For example, although much of the coarse material in the Wyandra is clearly impact-related, could air fall ejecta littered on distal ice have contributed to postimpact dropstones in the basin? Other considerations include how the impact affected the ice covering, the duration of impact-induced regional warming, and how impact mechanisms may have been affected by the presence of ice or frozen sediments, although the thickness of the latter would have been minor relative to impact excavation depth.

Implications for Talundilly

A Talundilly ejecta contribution to the Wyandra will need to be considered if Talundilly is proven to be an impact coeval with Tookoonooka as has been speculated. However, there are at least four factors that could make the resolution of a Talundilly ejecta component in the CSBC beds problematic:

  • 1
    Heterogeneous distribution of ejecta lithologies almost always occurs at impact sites. There are two main reasons for this. First, heterogeneity in ejecta occurs with respect to depth of excavation of target lithologies: the innermost and uppermost material in the crater is more highly shocked and more distally deposited, while target material from the deeper and outer crater is deposited proximally (Melosh 1989). Second, for larger craters, lateral heterogeneity of the target material would be expected.
  • 2
    The original morphology of Tookoonooka’s ejecta blanket cannot be precisely determined, as the ejecta were probably deposited and reworked in a marine environment. Thus, a Talundilly contribution would be difficult to resolve on ejecta distribution patterns alone.
  • 3
    Mixing of ballistic ejecta with pre-existing surface materials occurs to a greater extent at distance from the rim of a crater, resulting in distal dilution of the primary ejecta (Melosh 1989).
  • 4
    Precise impact target lithologies remain unknown. As a target sequence at point of impact is excavated by the cratering process and much of the crater rim (for Tookoonooka, and also likely for Talundilly) was eroded prior to burial (Gostin and Therriault 1997), we can only speculate on the true target sequence by correlation with surrounding rocks at depth.

Subtle differences in clast compositions between the Wyandra CSBC beds in Thargomindah-1 and Eromanga-1 (Table 4) may in part represent the different target sequences of Tookoonooka and Talundilly, respectively, based on crater proximity. Talundilly probably excavated Permo-Triassic Galilee Basin sediments, Devonian-Carboniferous Adavale Basin sediments, and various basement lithologies of the Maneroo Platform (granite, volcanics, and metasediments) in addition to Eromanga Basin sediments (Longley 1989; QPED 2009). The Maneroo Platform basement does not extend south to the Tookoonooka area (Longley 1989), thus differences between it and the Thargomindah Shelf basement may be a key factor in differentiating Talundilly lithic ejecta. For example, a distinct red granule in the basal Wyandra CSBC bed at Eromanga-1 is similar to red volcanics of the Maneroo Platform at depth in Maneroo-1, whereas prominent metamorphic and vein quartz volumes in the basal Wyandra CSBC bed at Thargomindah-1 may represent a proportionally greater volume of metasedimentary rocks in the basement at Tookoonooka. The much greater volume of coarse sedimentary lithics in the Eromanga-1 bed are unlike the sedimentary lithics in the Thargomindah-1 bed and may originate from Adavale and Galilee Basin sedimentary rocks in Talundilly’s target sequence. Adavale and Galilee Basin rocks predate the Eromanga Basin and are not known in the subsurface of the Tookoonooka area. Similarly, the population of coarse chert and volcanic lithics in the Eromanga-1 bed, insignificant in the Thargomindah-1 bed, may also reflect Talundilly’s target sequence. Thus, a Talundilly signature is intimated, and may be resolved with future work. As impactoclasts are distributed beyond the location of Talundilly, itself about 10 Rc from Tookoonooka, and some distal Wyandra CSBC beds beyond Talundilly have thicknesses comparable to wells proximal to Tookoonooka, it seems likely that Talundilly could have contributed to this volume of ejecta.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Background
  5. Methodology
  6. Results
  7. Interpretations and Discussion
  8. Conclusions
  9. Acknowledgments
  10. References

For an impact crater the size of Tookoonooka, significant volumes of proximal and distal ejecta would be expected. Observations from 31 wells across the Eromanga Basin, covering an area of 400,000 km2, imply that the Tookoonooka ejecta are indeed present in the basin, from proximal locations to well beyond 10 crater radii from the impact site. Evidence in the rock record shows:

  • 1
    The ambient (preimpact) sediments of the Cadna-owie Formation record a very low-relief, low-energy, low-sediment supply depositional setting with a marine signature and laterally persistent character.
  • 2
    Hundreds of meters of fine-grained Cadna-owie deposits are punctuated by an interval of relatively high-energy, high-sedimentation rate, coarse-grained and exotic lithic-rich deposition (the Wyandra Sandstone Member) before returning to quiescent, low-energy marine deposition.
  • 3
    Coarse breccia-conglomerate beds in the Wyandra Sandstone Member contain clasts of basement rock, shock metamorphosed grains, and accretionary and melt impactoclasts. An impact clast assemblage can be differentiated based on petrographic results.
  • 4
    The upper Cadna-owie contact with the base of the Wyandra is unconformable, and is accompanied by evidence of erosion, water escape, and underlying seismites.

We conclude that the first appearance of Tookoonooka ejecta in the sediment record most likely occurs at the base of the Wyandra Sandstone Member in the Lower Cretaceous, equivalent to 125 ± 1 Ma. The Cadna-owie contact with the base of the Wyandra represents an event horizon of regional significance. The presence of basement clasts in sediments overlying this horizon is consistent with interpretations of Tookoonooka’s transient crater excavation down to basement depths. Tookoonooka was a marine impact; CSBC beds at the base of the Wyandra probably record immediate postimpact sedimentary processes as ejecta were emplaced into a largely shallow, but potentially icy, sea. The low-energy setting in a transgressing epeiric sea presented an ideal opportunity for the preservation of these ejecta by burial.

Footnotes
  • *

    For both GSQ Thargomindah-1 and GSQ Eromanga-1 wells, the depths inferred here for the base of the Wyandra Sandstone Member are different from those documented in the Stratigraphic Drilling Reports (Almond 1983, 1986), namely 732.74 m and 773.7 m, respectively. As the Wyandra was occasionally misinterpreted to be synonymous with the upper Cadna-owie in early wells (Draper 2010, personal communication), the base of the Wyandra was revised for GSQ Thargomindah-1 to approximately 705 m (QPED, 2009). We have placed the base at 705.08 m. As the depth pick for the base of the Wyandra in GSQ Eromanga-1 has not yet been revised (QPED, 2009) and the stratigraphic drilling reports for both wells have the same author, we adopt a similar interpretation for GSQ Eromanga-1 and place the base of the Wyandra at 743.73 m.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Background
  5. Methodology
  6. Results
  7. Interpretations and Discussion
  8. Conclusions
  9. Acknowledgments
  10. References

Acknowledgments–– This article is part of the Ph.D. thesis of Katherine Bron, and was completed while in receipt of scholarships from the University of Adelaide and the Australian School of Petroleum. The authors thank the Geological Survey of Queensland, PIRSA (Primary Industries and Resources South Australia), and Santos Ltd. for providing core and/or data access for this study. This work was supported in part by the Barringer Family Fund. We thank Peter Haines, Ric Daniel, and Kathryn Amos for their assistance. We extend our gratitude to Elin Kalleson, David King, and the AE (Gordon Osinski), who provided helpful comments on manuscript improvement.

Editorial Handling–– Dr. Gordon Osinski

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  6. Results
  7. Interpretations and Discussion
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  9. Acknowledgments
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