Few modern sediment dispersal pathways predate the breakup of Pangea. This suggests that river lifespan can be controlled by continental assembly and dispersal cycles, with the longest-lived river systems present during supercontinent regimes. Based on the strikingly similar age spectra and Hf isotopic array extracted from Paleozoic to early Mesozoic sedimentary sequences from the Paleo-Tethyan margin basins, we argue that a long-lived supercontinental-scale system, with headwaters originating in Antarctica, flowed northward to finally debouch on the margin with the Paleo-Tethys Ocean. Channel-belt thickness scaling relationships, which provide an estimate of drainage area, support the notion that this was a supercontinental-scale system. Sediments were eroded from Proterozoic orogenic belts and flanked resistant kernels of Archean cratons. Remnants of this system, which can still be traced today as topographic lows, controlled post-breakup drainage patterns in Gondwanan fragments in Western Australia. We conclude that supercontinental regimes allow sediment dispersal systems to be long-lived, as they provide both an abundant sediment supply, due to erosion of large-scale, collision-related internal mountain systems, and a stable, large-scale configuration that lasts until breakup.
Sediment dispersal pathways are ubiquitous systems that transverse continental domains, and as such, their evolution and longevity are likely associated with the Wilson cycle (Potter and Hambling, 2006; Gibling, 2017). For instance, the origin of large-scale drainage systems such as the Niger (West Africa), Orange (southern Africa), and Paraná (eastern South America) date back to the breakup of Pangea (Reijers et al., 1997; Dingle and Hendry, 1984). This observation poses the questions: what happens to sediment transport pathways during supercontinental regimes, and what, if any, is the relationship between pre- and post-breakup sediment dispersal patterns?
To explore the scale and lifespan of sediment transport pathways during supercontinental regimes and the controls involved, we investigate the sediment dispersal patterns of Ordovician–Triassic sequences that were deposited on the Paleo-Tethyan Gondwanan margin. These sequences were deposited during Gondwanan and Pangean regimes, and therefore the sediment sources and depocenters remained part of the same continental mass during the investigated time interval. Detrital zircon age spectra and Lu-Hf isotopes from these sequences document a long-lived transcontinental sediment dispersal system with its headwaters in Antarctica, draining through central Australia to final debouch off northwestern Australia. Additionally, we use channel-belt thickness scaling relationships to provide an independent estimate of drainage area of this river system during the Triassic. After the breakup of Gondwana, the ancestral river pathways of this system created a template for Cenozoic fluvial networks of Western Australia, in which low-lying topographic paleo-valleys can be traced.
METHODS, DATA, AND PROVENANCE FROM NORTHEASTERN GONDWANA
Previously dated detrital zircons from drill-core samples of Triassic rocks from the Northern Carnarvon Basin, Western Australia (Fig. 1), were analyzed for Lu-Hf isotopes by inductively coupled plasma–mass spectrometry (ICP-MS) (see the GSA Data Repository1). Results were compared with data from Paleozoic basins in Western Australia (Haines et al., 2013), Antarctica (Veevers, 2018), and Tibet (Li et al., 2010; Zhu et al., 2011) as well as with data from crystalline rocks in Antarctica (Morrissey et al., 2017) and central (Smithies et al., 2011; Hollis et al., 2013; Kirkland et al., 2013) and southwestern Australia (Smits et al., 2014). Comparison of detrital zircon age spectra from the Northern Carnarvon, Canning, and Officer Basins and Tibet (see locations in Figure 1, and in Figure DR1 in the Data Repository) show strikingly consistent threefold peaks in the ranges 1800–1500 Ma, 1250–1000 Ma, and 700–500 Ma (Figs. 2D–2L), suggesting derivation from similar sources throughout most of the Paleozoic. We are aware that age populations spanning these age ranges are related to major global orogenic events (Lawton, 2014); however, it is the distribution of these age populations in our sample set, as well as comparisons with other regions, that allow us to limit potential source areas. For example, other Paleozoic and Mesozoic basins in Australia contain only certain components of this threefold age assemblage (Figs. DR4F–DR4G, DR5A) despite being also part of East Gondwana, and in other cases none of the age assemblages are present (Fig. DR5B); thus we consider that this is a diagnostic ensemble of ages. We acknowledge that there are small age modes that vary with depositional age (Figs. 2D–2L) and that it is not possible to determine whether they are the result of genuine differences in provenance, intra-sample sorting during transport, or the moderate sample size (n ≈ 100). However, even if these small variations in age modes are genuine, they would be associated with changes in minor sediment transport pathways, not the large-scale dispersal patterns that are the focus of this study. Results of the Kolmogorov-Smirnov (KS) dissimilarity, described as the fractional difference between two distributions, show that in all possible age permutations, the dissimilarity between any two given spectra never exceeds 50% (Fig. DR7). Despite the fact that it has been argued that this non-parametric statistic is sensitive to sample size (Vermeesch, 2018), the difference between bootstrapped mean KS dissimilarities and the original ones are <4% (Fig. DR8), suggesting that in this case the KS dissimilarity provides a reliable indication of the similarities between the age spectra presented here.
Lutetium-hafnium (Lu-Hf) analysis of dated zircon grains allowed us to refine the criteria to discriminate source areas of similar age. The threefold age assemblage displays distinctive initial Hf isotopic ratios (Figs. 2A and 2B). Integration of U-Pb and Hf detrital zircon isotopic data for Western Australian basins can be closely linked to crystalline source regions in Australia and Antarctica within Gondwana. Zircons in the range 1800–1500 Ma have similar ages and Hf-isotopic values (176Hf/177Hf = 0.2815–0.2820) as igneous zircons from the Albany-Fraser and Arunta orogens (Western Australia; Fig. 2A; Hollis et al., 2013; Smits et al., 2014). Detrital zircons spanning 1300–1000 Ma correspond in age range and Hf-isotopic values (176Hf/177Hf = 0.2817–0.2822) with the Albany-Fraser orogen (Smits et al., 2014), the Musgrave orogen (central Australia; Smithies et al., 2011; Kirkland et al., 2013), and magmatic rocks from East Antarctica (Windmill Islands; Morrissey et al., 2017). Ages between 700 and 500 Ma show the largest spread in the Hf-isotopic signal, with 176Hf/177Hf ratios ranging from 0.2813 to 0.2827. The origin of this detritus is controversial. One hypothesis is that the Paterson orogen (Western Australia; Fig. DR1), which contains granites dated at 650–630 Ma (Martin et al., 2017), is a possible source for some of this detritus; however, the Hf-isotopic array extracted from the zircons presented here is lower than 176Hf/177Hf = 0.2820 (Fig. DR10), arguing against it acting as a significant sedimentary source. Another hypothesis suggests that the 700–500 Ma population is originally from Antarctica and entered via eastern Australia but underwent several phases of sediment recycling in Neoproterozoic–late Paleozoic basins in central Australia (cf. Haines et al., 2013). An alternative hypothesis is that the 700–500 Ma zircon population is directly derived from Antarctica (Cawood and Nemchin, 2000; Veevers, 2018). A comparison of the Hf-isotopic array for similar-age detrital zircons in Permian–Triassic sandstones (Veevers, 2018) in the Prince Charles Mountains and offshore Prydz Bay (Antarctica) supports this conclusion (Fig. 2A). The lack of 700–500 Ma ages (Fig. DR4) in sedimentary units deposited after the breakup from Antarctica in Western Australia substantiates this idea.
The common provenance record for the Paleozoic–Mesozoic strata in Western Australia argue for a long-lived and stable sediment dispersal system. Furthermore, there is no change in provenance associated with the end-Carboniferous to early Permian glaciations (Gzhelian–Sakmarian, ca. 305–285 Ma; Mory et al., 2008), suggesting that sediment transport pathways were not significantly affected by glacial activity.
Our data indicate that detritus in the basins along the Paleo-Tethyan margin of northern Gondwana were fed by two main sediment transport pathways: a south-to-north system with headwaters in mountains in East Antarctica (cf. Cawood and Nemchin, 2000; Veevers, 2018), and a subsidiary network starting in central Australia. Previously published isopach maps from Paleozoic sequences (Mory and Haines, 2013; Fig. DR11), paleocurrent data from onshore Permo-Carboniferous units (Martin, 2008; Fig. DR11B), and 700 seismically mapped Triassic channel belts (Martin et al., 2018; Fig. DR11A) from offshore Western Australia provide additional physical evidence for sediment transport pathways. Our interpretation refutes the idea that basins along the Paleo-Tethyan margin of northern Gondwana were fed by only Australia-wide sediment dispersal systems, derived from eastern Australia (Wang et al., 2016). The euhedral nature of the Triassic zircons (see the Triassic zircon population in Figs. 2D and 2E; Fig DR12), which requires derivation from local sources, and the lack of Permian ages (e.g., Shaanan and Rosenbaum, 2018) argue against eastern Australian sources. Furthermore, channel-belt thickness scaling relationships, which provide an estimate of drainage area, suggest a continental-scale system, at least during the Triassic (Fig. 3). Channel-belt thickness may be used as a proxy for paleo-flow depth, which scales to the contributing drainage area. Channel-belt thickness data of Triassic sedimentary sequences in the Northern Carnarvon Basin (Martin et al., 2018) and the scaling relationship of Milliken et al. (2018) provide a first-order estimate for the scale of the paleo-catchment area (see the Data Repository for details). Although a lack of data prevents us from applying this approach to older parts of the successions onshore, the common provenance record throughout is consistent with a similar continental-scale drainage system existing since the Ordovician, while rates of sediment supply may have varied over time.
THE ROLE OF SUPERCONTINENTAL-SCALE TECTONICS IN CONTROLLING TRANSCONTINENTAL SEDIMENT TRANSPORT PATHWAYS
We suggest that the scale and duration of the sediment transport pathways supplying detritus to the Western Australian basins require both a large sediment supply and a stable plate-tectonic configuration. Thermochronology data support the notion that East Antarctic terrains within Gondwana may have delivered a significant volume of sediments to generate and maintain large sediment dispersal systems. Apatite fission-track data from the northern Prince Charles Mountains and the Mawson Escarpment in Antarctica indicate that as much as 5 km of denudation took place from the Carboniferous until the Jurassic (Lisker et al., 2003). Additionally, models based on radar, gravity, and magnetic data support the idea that significant uplift took place in the Gamburtsev Mountains, East Antarctica (Fig. 1), during the Permian. Modeling of these geophysical data sets suggests that uplift was due to the combination of rift-flank uplift, root buoyancy, and the isostatic response to fluvial and glacial erosion, and these would have provided sufficient relief to account for significant amounts of denudation (Ferraccioli et al., 2011). Furthermore, the intra-continental deformation event associated with the Alice Springs orogeny (ca. 450–300 Ma; Fig. 1; Haines et al., 2013) in central Australia generated an additional source of sediments, which then fed into the main south-to-north sediment transport pathway. Large volumes of sediment being transported over great distances during supercontinent regimes have also been inferred based on zircon chronology of Cambrian to Ordovician sedimentary sequences from the Tethyan Himalaya (Myrow et al., 2010).
Our data show that the sediment transport system presented here developed in an intercontinental setting (Fig. 1), similar to modern large-scale drainage systems (e.g., Amazon [South America] and Mississippi [USA] rivers; Fig. 3). The intercontinental setting and the supercontinent regime in which these sediment transport pathways developed were likely key factors for controlling the longevity of this system (Fig. 1). Furthermore, this sediment transport system developed in a setting flanked by resistant cratons (Fig. 1), establishing a stable configuration that allowed the pathways to be entrenched in adjoining Proterozoic orogens and resilient to change.
The lack of post-Hauterivian (ca. <130 Ma) clastic deposits (Lewis and Sircombe, 2013) in the Northern Carnarvon Basin indicates that this system ceased delivering sediments from Antarctica to the North West Shelf of Australia after the breakup of Gondwana. However, while this system decreased in scale and was no longer active, it created a template for some of the Cenozoic drainage systems in Western Australia, whose paleo-valleys have been preserved (e.g., Magee, 2009). This study showcases how supercontinent regimes allow sediment transport pathways to be long-lived and at a scale proportional to continental area. This means that supercontinents supply both sufficient sediment volume, through the generation of large-scale internal mountain systems associated with their collisional assembly, and a stable, large-scale configuration that lasts until breakup. This suggests that despite the fact that most modern drainage systems postdate the breakup of Pangea (Potter and Hamblin, 2006; Gibling, 2017), ancestral sediment transport pathways can still be traced to origins within the Gondwanan supercontinent cycle.
This research was supported by Australian Research Council’s Industrial Transformation Research Hubs project IH130200012 and grant FL160100168. Morón acknowledges support from the University of Melbourne Early Career Researcher grant. We thank Geoscience Australia for providing access to the zircons used for further Hf analyses, and J.D. Woodhead and R. Maas for their help with the Hf isotopic analysis. Morón thanks M.R. Gibling for his helpful discussions about the longevity of sediment dispersal systems and M. Blum for his comments about the scaling analysis. We thank three anonymous reviewers for their constructive comments on earlier versions of this manuscript. Haines publishes with the permission of the Executive Director of the Geological Survey of Western Australia. Lewis publishes with the permission of the Chief Executive Officer of Geoscience Australia.