VOLCANIC AND HYPABYSSAL LITHOFACIES IN A MESOPROTEROZOIC VOLCANIC ARC SEQUENCE, BARBY FORMATION, SOUTHWEST NAMIBIA

The Barby Formation makes up part of a major Mesoproterozoic arc complex along the Kalahari craton margin in southern Africa. Previous workers interpreted the Barby Formation, which has a composite stratigraphic sequence of 8.5 km, to consist dominantly of basaltic to trachyandesitic lava flows and minor associated hypabyssal intrusions. Our initial mapping in a representative and well-exposed portion of the unit reveals a more complex lithofacies assemblage. Throughout the study area the volcanic rocks are intercalated with lacustrine deposits up to 54 m thick that consist of planar-laminated tuffaceous andesitic siltstone to sandstone and are inferred to have accumulated in distal settings relative to source vents. Lavas are present but occur in minor amounts and consist of compound basaltic flows, in which massive lava tongues grade on all sides into aa-type flow breccia. Basaltic to trachyandesitic pyroclastic deposits are much more abundant and form successions up to ~45 m thick that are intercalated with thinner intervals of lacustrine strata and occur over an area ≥ 20 km². The most common deposits consist of moderately agglutinated to densely welded spatter accumulations inferred to have formed along fissure vents erupting Hawaiian-style lava fountains. The spatter accumulations typically grade upward into phreatomagmatic deposits containing minor amounts of spatter and bombs mixed with poorly vesicular lapilli tuff and up to 30% disrupted lacustrine sediment. This change in eruptive style is inferred to record decreasing magma supply to the vents, which allowed external water to mix with magma in the right proportions to trigger phreatomagmatic explosions.

In other places farther from active vents, lacustrine sequences were intruded by sill packages consisting of 4-7 petrographically distinct basaltic to trachyandesitic sills, ranging from ~7 to 40 m thick. Zones of peperite occur along sill margins and thin sediment-filled fractures extend into sill interiors. Both features are consistent with intrusion into wet unlithified sediments, resulting in quench fragmentation of the magma and injection of fluidized sediment into thermal contraction cracks in the sills. We infer that the low-density sediments impeded upward rise of magma, causing it to be transported laterally as sills.