Australia: The Land Where Time Began

A biography of the Australian continent 

The Lopingian of Australasia - A Review

According to the authors1 this paper is an overview of the distributions in time and space of the Lopingian strata and biotas in Australia, New Zealand, Timor and New Caledonia. The authors1 propose "a new schematic Late Permian (Lopingian) regional palaeogeographical reconstruction and marine palaeobiogeographical synthesis"1 , based on this review. Some key features of the patterns of the circulation of the ocean around southeastern Gondwana, that has primarily been inferred from the distribution of regional marine faunas dating to the Lopingian.

The Lopingian successions were deposited in a wide range of settings - tectonic, palaeoenvironmental and palaeolatitudinal, throughout Australasia. Strata of Lopingian age are present in New Zealand in a number of tectonostratigraphical terranes, such as the Brook Street, Dun Mountain-Maitai, Caples, Waipapa and Torless terranes, each of which originated in a distinctive basin, island arc or ocean ridge. These successions mostly represent segments of volcanic arcs, clastic aprons flanking arcs, intra- and fore-arc basins, and accretionary complexes, that have been displaced. The tracing of some into New Caledonia and eastern Australia has been by stratigraphical, biostratigraphical or petrological correlations. In New Zealand the spectrum of tectono-sedimentary environments of Lopingian age is interpreted, according to the authors1 to have been situated on a convergent continental margin near northeastern Australia, on the Panthalassan margin of southeastern Gondwana. Throughout the Lopingian age successions of New Zealand faunas of brachiopod and bivalves occur widely, and they have been studied closely. Ranging from the mid-late Wuchiapingian to the end of the Changhsingian 5 faunal zones have been recognised.

Non-marine successions dominate the deposits dating to the Lopingian in Australia in many sedimentary basins that are widely distributed across the continent. The Gympie Terrane of eastern Australia and the Canning Basin and the Bonaparte Basin in Western Australia, are, according to the authors1, the only onshore exposures of marine deposits known in Australia. Eastern Australia has the most extensive terrestrial sedimentary deposits dating from the Lopingian, that are within a large meridional foreland basin complex, comprised of the Tasmania, Sydney, Gunnedah and Bowen Basins. The dynamics of the New England Orogen, which occurred along the southeastern margin of Gondwana on the Panthalassan edge of the supercontinent, governed the evolution of these basins through the Lopingian and Early Triassic. In eastern Australia, rich coal measures, plant fossils, and locally, fossils of non-marine vertebrates and invertebrates are well preserved in most successions that are of non-marine origin. It has been recognised that a succession of 6 palynostratigraphical zones are present in eastern Australia that span the Lopingian through the Early Triassic. The discrimination and correlation of deposits of onshore and offshore successions has been allowed by the finding of several of these palynozones in Western Australia. In the marine successions of Lopingian age in western Australia and Timor, unlike those in eastern Australia and New Zealand, carbonates that are interbedded only locally with siliciclastic sediments and sparse volcaniclastics dominate the successions of Western Australia. On a passive continental margin that was actively rifting these successions were deposited in a very large sedimentary basin. Evidence has been found that throughout the Lopingian many species of shallow-marine invertebrate were shared, with biotic interchanges occurring freely and frequently between the Western Australian Superbasin (that included Timor) and Himalayan region of Nepal, Southern Tibet and northern India.

 

The authors1 synthesis and discussion

According to the authors1 in the Lopingian (Late Permian) a sequence is present from deposits around the world that displays evidence of biotic and environmental changes that appear to culminate in the greatest mass extinction event in the entire history of the Earth at the Permian-Triassic boundary. According to the authors1 the Lopingian sequences in Australia show, to some extent, evidence of this event, though there remains some uncertainty as a result of some apparent conflict between the evidence provided by conodonts, bivalves and earlier assumptions on the ranges of plant species based on stratigraphy. The authors1 suggest that the start of the Lopingian in eastern Australia appears to coincide with the initiation of the Hunter-Bowen Orogeny, at a time when the evidence indicates there was a complete withdrawal of marine conditions from the eastern portion of mainland Australia, and significantly increased volcanism in the New England Fold Belt. The authors1 have suggested that the continuing compression and subduction of the island arc systems, that were offshore from the Palaeopacific Plate, caused the rising of the New England Fold Belt and this provided the increased sediment for a pulse of volcanogenic sedimentation that overfilled the extensive meridional Bowen-Gunnedah-Sydney foreland basin system, and in this system many large fluvial systems developed that were braided to slightly sinuous. At this time, or possibly slightly after the start of Lopingian, in Western Australia a major marine transgression occurred during which carbonates were deposited in northwestern basins, the Lower Hardman Formation in the Canning Basin and the Lower Dombey Formation in the Bonaparte basin.

Coal accumulation abruptly ceased in all basins of eastern Australia, and in central Australia the Cooper Basin, in the closing phase of the Changhsingian, though the precise timing of this termination is uncertain. The persistence of active braided river systems is indicated by the presence immediately above the uppermost coals of sedimentary successions dominated by thick sandstone, though a wholesale change in the vegetation of the region is indicated by the plant fossils in the sedimentary rocks that display evidence of a climate that is drying and/or warming (Retallack, 2002). Typical redbed facies are developed extensively in all basins of eastern Australia, that are slightly higher in the succession at a level that is assumed to be the boundary between the Permian and Triassic (Jensen, 1975; Herbert, 1997), and these generally persist through the Early Triassic interspersed with quartzose sands that had been derived from the craton to the west of the deposits. In eastern Australia sedimentation essentially occurred in settings that were non-marine through the transition from the Permian to the Triassic, though it has been suggested that in beds in the Sydney Basin around the Permian-Triassic boundary there was an estuarine influence (Uren, 1980), and there have also been reports of marine influences on sedimentation in that area that were based on ichnology and sequence stratigraphical analysis (Naing, 1990; Herbert, 1997).  In Western Australia towards the close of the Changhsingian a significant marine transgression coincided with the start of the Protohaploxypinus microcorpus Palynozone.

The biotic turnover events matche to some degree the sequence of palaeoenvironmental changes of Lopingian age that have previously been outlined above. In the Sydney and Bowen Basins (from Coal Cliff Sandstone, Widden Brook Conglomerate,  Caley Formation, and Dooralong Shale in the lower Narrabeen Group)1 the Permian-Triassic boundary has traditionally been believed to be approximately the time of the last appearance in the fossil record of glossopterids, that have been correlated with the last appearance of coal. At a several locations in the Sydney Basin and the Gunnedah Basin shales that are present immediately above the uppermost coal from the Permian, and according to the authors1  contain a flora dominated by the peltasperm Lepidopteris callipteroides (Carpentier) (Retallack, 2002) and its associated reproductive structures, but also including conifers (Voltziopsis africana Seward, V. walganensis Townrow, Podozamites-like leaves), ginkgophytes (Ginkgophytopsus sp.), sphenophytes (Paracalamites australis Rigby Schizoneura gondwanensis Feistmantel), ferns (Cladophlebis carnei Holmes and Ash, Merianopteris sp.), lycophytes (Isoestes beestonii Retallack) and large winged seeds (Retallack, 2002).

The flora of the post-coal measure period, that has been attributed to Lepidopteris callipteroides Oppelzone (see Retallack, 1977,2002) contains a representation of gymnosperms that is fundamentally different from the assemblages found in the youngest coal measures, though a few genera of ferns and sphenophytes from the coal measures persisted into the strata that immediately overlies the coal measures. According to the authors1 small, scale- or needle-like leaves or pinnules that had a cuticle that was thick and papillate and stomata that were strongly protected were characteristic of many of the gymnosperms in the L. callipteroides zone. At this time there was a consistent pattern in which there was abrupt environmental and floristic changes, that are seen across eastern Gondwana, with plants adopting typical drought-resistant features as they adapted to the drought-stressed environments they then inhabited (McLoughlin et al., 1997; Lindström & McLoughlin, 2007). Following this there was a period of recovery in which the diversity of the vegetation increased over a prolonged interval (Vajda & McLoughlin, 2007). The L. callipteroides zone has been considered to be equivalent to the Protohaploxypinus microcorpus Palynozone (Retallack, 2002), which is from the latest Permian, based on the correlation between spore-pollen zones of eastern Australia, fossil sites of marine origin in the Perth Basin, and around the margin of Tethys, a mixed fossil assemblage (Helby et al., 1987; Foster & Archbold, 2001; Metcalf et al., 2008). In the Sydney Basin the rise of the Dicroidium (corystosperm) flora (Retallack, 2002), that was dominant throughout Gondwana, is marked by the replacement of the assemblages of the L. callipteroides zone by a mixed Lepidopteris-Dicroidium flora, the Dicroidium zuberi Oppelzone, in the Stanwell Park Claystone, of approximately Induan age, that is generally about 70 m higher in the Narrabeen Group.

According to the authors1 in Australia the Permian-Triassic boundary is clearly above the P. crenulata Zone, in terms of palaenostratigraphy, as the latter is tied to the early Changhsingian of the Salt Range (Balme; 1970; Foster, 1982). According to the authors1 the presence of productid, athyrid, elythid and spiriferid brachiopods, though they had been identified only to generic level, were the basis for the putative correlation of the lower part of the P. microcorpus Zone with the early to middle Changhsingian (Thomas et al., 2004). In the Perth Basin there is no diagnostic fauna in the upper part of the P. crenulata Zone, though there is a strong negative departure in δ13C values that continues through this interval in both marine and terrestrial facies suggests a possible correlation with the biotic crisis of the latest Changhsingian. The authors1 suggest that, assuming the ranges that have been proposed for the species of conodont that have been identified (Metcalf et al., 2003) from the Hovea Member are valid, the basal 3 m of the 'sapropelic interval' of this unit also dates to the latest Changhsingian. In the Hovea Member levels higher than the upper 8 m of the sapropelic interval contain conodonts that have ranges from the early Dienerian to early late Smithian. As a result the Permian-Triassic boundary should be placed within an interval of about 9 m, at a depth of from 2216-2225 m in the Corybas #1 well within the lower part of the sapropelic interval.

The authors1 suggest that within the succession in Australia, it is clear there are still uncertainties, though the resolution of the Permian-Triassic boundary has apparently improved.

 

Sources & Further reading

  1. Shi, G. R., J. B. Waterhouse, and S. McLoughlin. "The Lopingian of Australasia: A Review of Biostratigraphy, Correlations, Palaeogeography and Palaeobiogeography." Geological Journal 45, no. 2-3 (2010): 230-63.
Author: M. H. Monroe
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Last Updated 22/02/2013

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading