GSA Annual Meeting, November 5-8, 2001

Paper No. 0
Presentation Time: 1:30 PM-5:30 PM


TERRY, Rebecca C., ROGERS, Raymond R. and WIRTH, Karl R., Geology Dept, Macalester College, 1600 Grand Ave, St. Paul, MN 55105,

Karoo-equivalent strata, namely the Fulton's Drift, Gushu, Mpandi, and Samkoto Formations, crop out in the Limpopo Valley of southern Zimbabwe. A previously unrecognized unconformity and associated silcrete mark the contact between the Mpandi Formation and superjacent Samkoto Formation. The unconformity, which can be traced throughout the outcrop belt, displays up to 1 m of erosional relief and is overlain by a dispersed lag of chert clasts presumably derived from the underlying Mpandi silcrete. The 2 m thick silcrete is characterized by a globular base that passes upward to a crystalline pseudobrecciated top. The silcrete displays a GS to M-Fabric matrix, with no glaebules or colloform features. Framework grains primarily consist of fine-grained, sub-angular quartz, although rare oxide minerals and microcline grains also occur. Complex vug and void fills consist of microcrystalline quartz, length fast chalcedony, and macrocrystalline quartz. Length slow chalcedony is also present in the silcrete, but not as a void fill. XRD analyses reveal that clays are not present in the silcrete, but illite and montmorillonite do occur in low abundance (1-7 wt. % clay) in underlying strata. XRF analyses indicate that the silcrete is ~97 wt. % SiO2. Aside from SiO2, most elemental concentrations (e.g., TiO2, Al2O3) within the silcrete are markedly low relative to underlying sediments. A zone of accumulation of mobile and immobile elements (major and trace) occurs approximately 10 m below the silcrete. The most stable element in the profile is Sc.

The Mpandi silcrete apparently formed near the surface via pedogenic processes with multiple periods of silicification during long term weathering as a relict soil. Residual concentration of silica reflects the removal (or partial removal) of all other elements by weathering, with the possible addition of some silica from a nearby source. Mobilized elements were transported downsection, and the silcrete was subsequently exposed by erosion.