Australia: The Land Where Time Began

A biography of the Australian continent 

Flood Basalt Provinces Containing Mafic Volcaniclastic Deposits - Review

According to the authors¹ it is generally assumed that thick lavas and intrusive rocks, mostly in the form of sills, comprise exclusively flood volcanic provinces, any pyroclastic rocks being limited to silicic composition. In many provinces there are mafic volcaniclastic deposits (MVDs), eruptions forming such are potentially meaningful in terms of their potential for having impacts on the atmosphere and their links to mass extinction events. The Siberian Traps, the province where MVDs are most voluminous, is also the one that is temporally associated with the greatest mass extinction of the Phanerozoic. Knowledge about these deposits and eruptions remains incomplete, much more research being required before a more convincing link can be established, this contribution by the authors¹ reviews, in some detail, the current knowledge of MVDs for the provinces in which they are better known, such as the North Atlantic Igneous Province, which includes Greenland, the Faeroe islands, the British Islas, as well as tephra layers that have been found in the North Sea Basin and its vicinity. Other provinces are the Ontong Java Plateau, the Ferrar and the Karoo. A brief overview is also provided by the authors¹ of the current knowledge about MVDs in other provinces, including the Columbia River Basalts, the Afro-Arabian Province, the Deccan Traps, the Siberian traps, the Emeishan and from the Australia, the Archaean example.

MVDs occur at their thickest accumulations in flood basalt provinces underlying the lava pile such as the Faeroe Islands more than 1 km, Ferrar Province ≥ 400 m and the Siberian Traps 700 m. The greater thickness of MVDs in the case of the Faeroes can be attributed to accumulation in a local sedimentary basin, but in the Ferrar and Siberian Provinces the deposits are widespread, more than 3 x 10⁵ sq km for the latter. More than 300 m of MVDs have been found in 1 drill hole on the Ontong Java Plateau, and there are no overlying lavas. The thickness of volcaniclastic deposits is much less where they have been sandwiched between lavas.

Primary MVDs are predominantly of phreatomagmatic origin, in most cases the authors¹ reviewed, as indicated  by the clast assemblage that generally consists of basaltic clasts in which the vesicularity varies, dominantly non-vesicular to poorly-vesicular, and mixed with abundant debris of country rock. Loose quartz particles that derived from sandstone that was poorly consolidated in underlying sedimentary basins (East Greenland, Ferrar and Karoo), are included among the accidental lithic components. Aquifers supplying water to fuel phreatomagmatic activity were also a source of debris. The climate appears to have been very dry in the Paraná-Etendeka at the time of emplacement of the lavas (aeolian sand dunes) and there are no reports of MVDs.

In several provinces, Deccan, North Atlantic, Ferrar, Karoo, volcanic vents have been reported that were filled with mafic volcaniclastic material, with diameters ranging from a few 10s of m to about 5 km. It is believed these were excavated in country rocks, that were relatively soft, and rarely in flood lavas, by phreatomagmatic activity in a similar manner to diatreme formation.

Sources & Further reading

1. Ross, P. S., I. Ukstins Peate, M. K. McClintock, Y. G. Xu, I. P. Skilling, J. D. L. White, and B. F. Houghton. "Mafic Volcaniclastic Deposits in Flood Basalt Provinces: A Review." Journal of Volcanology and Geothermal Research 145, no. 3–4 (7/30/ 2005): 281-314.