Zircon Records a Shift in Sediment Provenance in Triassic Australia
How can we tell where a river flowed from more than 200 million years after it disappeared? Its channels may have long since been buried, while the surrounding mountains have undergone prolonged erosion and deformation, but the sand carried by the river can preserve clues to its history. Triassic strata in the Bowen Basin of eastern Australia contain just such a sedimentary record. By analysing detrital zircons and the composition of sandstone fragments, researchers sought to reconstruct where these ancient rivers obtained their sediment and how drainage systems along eastern Gondwana were reorganised through time.
The study focused on the Rewan Group and the younger Clematis Group in the northern Bowen Basin of Queensland. The Bowen Basin lay inland of the orogenic belt along the eastern margin of Gondwana, adjacent to the Tasmanides and New England Orogen to the east and opening toward the continental interior to the west. From the Late Permian into the Triassic, the Hunter–Bowen Orogeny continually modified topography, patterns of basin subsidence, and river gradients. As a result, rivers could receive sediment from very different source regions at different times.
The researchers' main clue was detrital zircon. Zircon commonly crystallises in igneous rocks and contains uranium that gradually decays to lead, allowing its crystallisation age to be determined by U–Pb dating. When a rock is eroded, zircon grains can enter a river and eventually become incorporated into sandstone while retaining the age of their original crystallisation. If a sandstone contains abundant zircons about 245 million years old, its catchment likely had access to rocks formed around that time. If much older zircons become abundant, the sediment may instead have been derived from older basement rocks or from pre-existing sandstones that had already been deposited, eroded, and transported again.
The study analysed 12 sandstone samples from the Rewan and Clematis groups, producing data from more than 2,000 zircon grains that passed the filtering criteria, and examined 35 sandstone thin sections. The thin sections allowed the researchers to determine whether the sand was composed mainly of quartz, feldspar, or different types of lithic fragments. These observations were then compared within a common framework with previously published palaeocurrent data and detrital zircon datasets from the neighbouring Galilee Basin and Gympie Terrane. This multi-proxy approach reduced the risk of assigning provenance solely from a particular zircon age population.
In the lower part of the Rewan Group, the Sagittarius Sandstone contains a prominent zircon peak at about 246 Ma, together with components around 303 Ma and 353 Ma. The overlying Arcadia Formation has a dominant peak near 245 Ma, along with substantial populations around 315 Ma and 330 Ma. Their proportions are not identical, but their overall zircon age distributions are very similar. Statistical comparisons likewise place most Rewan Group samples close together, indicating a broadly coherent mixture of sediment sources across different depocentres.
Rewan Group sandstones generally contain abundant lithic fragments, many of them volcanic in origin. Some samples have very low quartz contents and contain relatively high proportions of feldspar and volcanic rock fragments. Many zircon grains also have crystallisation ages only slightly older than the depositional ages of the sandstones. Taken together, these observations led the researchers to infer that, during Rewan Group deposition, the Bowen Basin continued to receive volcanic and igneous material from the active continental margin to the east. The New England Orogen and associated magmatic systems represent plausible major sources. Published palaeocurrent data are consistent with this interpretation, suggesting that much of the sediment entered the basin from the east and was then transported along structurally confined lowlands into the basin interior.
By the time the younger Clematis Group was deposited, sediment composition had changed. Samples from the Taroom Trough lack the conspicuous 240–250 Ma zircon peak characteristic of the Rewan Group and instead show major peaks around 294 Ma, 373 Ma, and 512 Ma. Only 6.5% of zircons in these samples fall between 220 and 285 Ma, whereas 55.8% are older than 400 Ma. The change is less pronounced in the Denison Trough, where the dominant population lies around 350 Ma but a small component near 236 Ma remains. This indicates that the magnitude of the provenance shift differed between parts of the Clematis Group. Although the Clematis Group was deposited later than the Rewan Group, it contains a greater contribution from older zircons, pointing to a substantial change in sediment supply.
Clematis Group sandstones are generally richer in quartz and contain little feldspar, while the time gaps between zircon crystallisation and deposition are typically longer. This combination is consistent with a reduction in young arc-derived material from the east and an increased contribution from older rocks in the continental interior. An increase in older zircons, however, does not necessarily mean that more sediment was transported directly from inland source terranes. Those zircons could also have been recycled from older strata within the basin. The Bowen Basin experienced structural uplift, erosion, and stratigraphic hiatuses, which could have exposed previously deposited Permian sedimentary rocks to renewed erosion. Because those older strata already contained ancient zircon populations, their recycling could generate a similar age signature. The researchers therefore retained both possibilities: the Clematis Group may have received more sediment from the continental interior, experienced stronger sediment recycling, or undergone both processes at the same time.
The zircon age distributions of the Rewan Group are more similar to those of the Gympie Terrane and the older Bandanna Formation, all of which carry a stronger active-margin signature. In contrast, the Clematis Group of the Taroom Trough more closely resembles the Warang Sandstone and Porcupine Gorge Formation of the Galilee Basin, which contain larger proportions of older zircons.
Younger strata of the Moolayember Formation once again contain a greater proportion of young zircons, shifting their provenance signature back toward a Rewan Group-like pattern. If this regional comparison is correct, the provenance change recorded by the Clematis Group may have been restricted to only part of the Triassic, after which the drainage system was reorganised again. Current stratigraphic age constraints and sample coverage are still insufficient to determine precisely how long this interval lasted.
The sandstones therefore preserve a record of a continually reorganising landscape in Triassic eastern Australia. During deposition of the Rewan Group, the Bowen Basin was broadly part of an integrated drainage system dominated by sediment supplied from the active continental margin to the east. During Clematis Group deposition, contributions from the continental interior and the recycling of older sedimentary rocks became more important, with the magnitude of that change varying among depocentres. The uplift of mountains, subsidence of basins, and rerouting of rivers left no landscape that can still be observed directly today, yet each reorganisation of the drainage system remains recorded in sandstone through zircon grains that still carry the ages of the rocks from which they came.
Author: Shui-Ye You
Reference:
Scipione M et al. (2026). Integrated Triassic sediment routing along eastern Gondwana (Australia). EarthArXiv.
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