Collaborative Research: Explaining the Eruption of Huge Volumes of Magma-Sediment Mixtures Preceding Flood Basalt Effusion, Karoo Lip, South Africa

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HoughtonEAR-0106237The onset of flood basaltic volcanism is frequently explosive but this explosive volcanism is often overlooked with respect to the voluminous flood basalts that follow. We document here unusual voluminous basaltic breccias that predate and are interbedded with the earliest flood basalts of the Karoo Large Igneous Province (LIP), South Africa. No detailed studies or facies models for these deposits exist, nor can existing models for explosive volcanism explain them. The Karoo breccias have an outcrop area of 1000km2, a total volume of at least 500km3, which can be divided into flow units with volumes which may be as high as 27km3. The breccias consist of juvenile vitric clasts in a matrix dominated by sedimentary sand grains. These sediment-magma mixtures infill large caldera-like collapse structures from which they were probably erupted, and also form outflow sheets interbedded with the earliest Karoo lavas. Juvenile clasts in the breccias are very similar to those of associated fluidal peperites suggesting a continuum of mixing processes from the subsurface to surface. The collapse structures contain bodies of dolerite juxtaposed with large blocks of fused sandstone, suggesting prolonged intervals of efficient heat transfer.This study will look at pre-eruptive, eruptive and emplacement mechanisms that led to the production of the Karoo lavas, by studying and comparing infill and outflow facies, and by looking at proximal to distal variations within a representative outflow sheet. We will develop and test the hypothesis that mixing was preceded by underplating of large areas of wet unconsolidated sand by sustained and constantly replenished high-level sills of basalt. Mixing and eruption may have been accompanied by upwelling of gravitationally unstable slurries, or jetting of magma-sediment mixtures issuing from close-spaced, but ephemeral vents. We will use detailed geological mapping and facies analysis to infer emplacement processes and SEM and image analysis of juvenile clasts to constrain pre-eruptive degassing and fragmentation mechanisms. In year 1, we will study the infill sequence of the Sterkspruit Complex, selected because, in addition to excellent exposure, it contains an unusual near-vent deposit on its margin. In year 2, we will extend the study beyond the Complex to characterize outflow deposition and place the complex in a regional context. We will use chemical stratigraphy to construct the interplay between effusive and explosive phase, and will constrain this work by Ar/Ar age determinations.The study has implications for the onset of eruption in all flood basalt provinces. Similar deposits are currently under study, by White and students, for the stratigraphically equivalent Ferrar LIP of Antarctica. Little studied voluminous breccias also occur at the base of the flood basalts in the East Greenland area of the North Atlantic Tertiary LIP, and in the Siberian Traps LIP. Models for the global impact of flood basalts are predicated only on the effusive component. This study will add data that will enable refinement of these models.