UNUSUAL BOMB-RICH STROMBOLIAN PYROCLASTIC DEPOSITS IN A MESOPROTEROZOIC ARC SETTING, BARBY FORMATION, NAMIBIA

Extensive basaltic andesites and shoshonites in the 1.2 Ga Barby Formation in SW Namibia record development of a major continental margin arc within the 1.4-1.0 Ga Namaqua-Natal orogenic belt along the southern margin of the Kalahari craton in southern Africa. Our ongoing work shows that important parts of the ~8-km-thick arc succession consist of proximal pyroclastic deposits erupted from small monogenetic volcanoes and intercalated with sequences of lacustrine sediment, suggesting that volcanism occurred within a basinal setting in an extensional part of the arc where lakes repeatedly formed between eruptions.

Here we report results of detailed mapping of a well exposed area that provides a cross-sectional view of a typical part of the arc succession. Massive, cliff-forming, unusually bomb-rich pyroclastic fall deposits up to 80 m thick occur along with planar-bedded lapilli-tuff and lapillistone produced by Hawaiian style lava fountains and explosive phreatomagmatic eruptions. Ellipsoidal bombs in the massive deposits are tightly packed or separated by varying amounts of droplet-like or angular, nonvesicular lapilli and small amounts of spatter. The abundant bombs in the massive deposits indicate dominantly Strombolian style eruptions, in which explosive bursts from large gas slugs rising in the conduit ejected coarse, fluidal pyroclasts. However, typical Strombolian scoriaceous lapilli are rare. The large amounts of bombs indicate sustained explosivity driven by repeated bursting of uprising gas slugs at a rate not typically seen in Strombolian events.

Interbedded lacustrine and pyroclastic deposits are truncated along strike by a chaotic mass of typically closely packed bombs (up to 2 m long) that extends over an area of at least 1000 by 600 m and surrounds smaller masses of coalesced spatter and tilted lacustrine sediment. The large volume of bombs suggests high mass eruption rates over a significant time period, which we infer triggered collapse of parts of the source vent or vents, possibly along with some of the substrate. Similar collapse events associated with relatively violent eruptions have been observed in modern eruptions from Strombolian volcanoes, although generally not on this scale. Such collapse events may add to the hazards associated with explosive Strombolian eruptions.