A PALEOGEOGRAPHIC CONTROL ON THE FORMATION OF THICK, DOWNWARD-GROWING SILL PACKAGES EMPLACED INTO WET SEDIMENTS WITHIN A MESOPROTEROZOIC VOLCANIC ARC IN SOUTH-CENTRAL NAMIBIA

On the edge of the Kalahari Craton, the Mesoproterozoic (~1.2 Ga) Barby Formation of south-central Namibia preserves an extensive, ~8500-m-thick volcanic arc sequence. Previously described to consist dominantly of intertonguing rhyolitic to basaltic lava flows, detailed mapping indicates the facies architecture is much more complex. Our new mapping reveals three main facies: Hawaiian, Strombolian and phreatomagmatic pyroclastic deposits; lava flows; and packages of petrographically distinct basaltic to andesitic sills. The sills intrude sequences of fine-grained lacustrine sediment up to ~54 m thick that accumulated in major, long-lived, possibly fault-controlled lakes.

Several sill packages have been identified, each of which contains 2-6 compositionally discrete sills with sill thicknesses ranging from ~7 m to 40 m. All sill packages share two significant features: zones of blocky to fluidal peperite up to 1 m thick between the sills and surrounding fine-grained lacustrine sediments, and tendrils of sediment up to 14 m long are injected throughout the sills. These features indicate that the sills were emplaced into wet, unconsolidated sediment, and fluidization of the host sediments likely helped generate the required space for magma intrusion.

We infer that the low density of the wet lake sediments produced a shallow crustal barrier to uprising magma, resulting in lateral transport of magma into the mechanically weak sedimentary sequences. Intrusion of the first sill in each package created an effective trap for subsequent batches of uprising magma, leading to downward growth of each sill package. Composite sills contain zones of magma mingling where enclaves of early-emplaced plagioclase-phyric andesite are contained within more mafic rock indicating repeated injection of diverse magma batches during dynamic growth of the sills. Basaltic to andesitic pyroclastic deposits and lava flows elsewhere in the Barby Formation were emplaced in areas where lakes were absent or present in minor amounts, perhaps in areas of higher relief. This indicates a strong paleogeographic control on the occurrence of effusive or pyroclastic eruptions versus lateral transport of magma in sills at shallow depths beneath the surface.