Australia: The Land Where Time Began

A biography of the Australian continent 

LIPs - Central Atlantic Magmatic Province

Extensive volcanism marked the breakup of the central Atlantic region in the Early Jurassic, of which the best known are the flood basalts of the Newark Basin, NE of the US (Olsen et al., 1997). The original volume of these extrusives is believed to have been a relatively modest 50,000 km³ (McHone, 1996), though Deckart (Deckart et al., 1997) was led by the study of scattered outcrops in French Guyana, Surinam and Guinea to speculate that a major CFBP may have formed before the rifting of the central Atlantic. Further support for this hypothesis came from the work of Marzoli (Marzoli et al., 1999), who discovered additional flood basalts in northern and central Brazil that were contemporaneous. It has therefore been proposed that a Central Atlantic Magmatic Province (Marzoli et al., 1999), that had scattered outcrops that are now centripetally located around the periphery of the central Atlantic region. Estimates of the original area of the province, that were based on the assumption that a continuous entity was formed by the flood basalts, range up to 7 x 10⁶ km², and a volume of at least 2 x 10⁶  km ³ (Marzoli et al., 1999). The province may have originally been one of the largest of its kind if this is the case (Olsen, 1999).

The dates for the Central Atlantic Magmatic Province that are best constrained are found in the Newark Basin, where the onset of volcanism began at 201 ± 1 Ma is suggested by several U-Pb ages (Dunning & Hodych, 1990; Weems & Olsen, 1997) and lasted only 580 ± 100 ka (Olsen et al., 1997). In the southeastern USA extensive dykes have yielded ⁴⁰Ar-³⁹Ar ages, which have external errors of 199.5 ± 2.0 Ma (Hames et al., 2000). It is suggested by lavas and intrusives of South America that are well-dated with U-Pb ages ranging from 204 to 197 Ma (Marzoli et al., 1999). The eruptions are placed by the most precise dates within the Hettangian Stage of the basal Jurassic and this is confirmed, in the Newark Basin, by the presence of the lowest lava 30 m above the base of the palynologically  defined Triassic-Jurassic (T-J) boundary ( Fowell & Olsen, 1993. The loss of 60% of palynospecies at this boundary, as well as with a proliferation of fern spores, suggest there was a sudden and severe crisis in the terrestrial flora at the boundary. An even more severe loss, >95% of species extinction, has been reported for leaf species in northern Europe (McElwain et al., 1999). This floral extinction has however, not yet been recognised beyond the North Atlantic region (Hallam & Wignall, 1997).

The close, though not perfect, temporal link between the onset of flood basalt eruptions and a terrestrial crisis has led most workers to infer that there is a cause and effect relationship (Rampino & Struthers, 1988; Courtillot et al., 1999; Pálfy et al., 2000). Most of the climatic changes have been reported from the T-J boundary interval. Therefore, an analysis (McElwain et al., 1999) of the stomatal density in fossil leaves from east Greenland and the Baltic suggests there was a major increase in atmospheric CO₂ in the basal Jurassic which Wignall suggests may be attributable to central Atlantic volcanism.

High precision U-Pb dating of an ash layer from a marine boundary section in the Queen Charlotte Islands, Canada, where a date of 199.6 ± 0.3 Ma was obtained from a level that was 5 m from the top of the highest radiolarian zone of the Triassic, provided further data on the mass extinction event at the J-T boundary. According to Wignall this date appears to be somewhat younger than the terrestrial T-J boundary in the Newark Basin, which led to the suggestion (Pálfy et al., 2000) that marine extinctions may have post-dated the terrestrial by approximately 700 ka. The precise dating of the mass extinction event in marine sections at the end of the Triassic, however, is not well established (Hallam & Wignall, 1997). Amongst the most prominent victims of this crises were the bivalves and ammonoids, and it is suggested by their record that there was a gradual decline in the Rheatian Stage of the latest Triassic, which may be partially due to changes in local facies, with a final coup-de-grace somewhat below the T-J boundary (e.g. Hallam & Wignall, 2000). The latest Rheatian marine extinction probably coincides with the floral crisis of the end-Rheatian, if the marine T-J boundary is indeed marginally younger than the terrestrial boundary.

Sources & Further reading

Wignall, P. B. (2001). "Large igneous provinces and mass extinctions." Earth-Sci. Rev. 53: 1-33.