STRATIGRAPHY OF A TIDAL DELTA NEAR MBWENI, TANZANIA

We mapped the subsurface of a tidal delta near Mbweni, Tanzania, using a gouge auger in order to estimate the recent geologic history of the area. Our coring suggests that the shoreline has been retreating for at least the last 1000 years, and that at least one sedimentologically unusual event occurred in our record.

The delta we studied is spatially small (~500 m along shore, and ~500 m cross shore), and has mud and trunks associated with an ancient mangrove forest outcropping on the beach. We drove 11 cores, each about 1 meter deep, in the delta in order to determine subsurface stratigraphy. In most cases the modern delta is relatively thin (less than 50 cm), although it is possible that thicker sections exist. Beneath the modern delta lies 30-40 cm of mud, becoming thicker landward. The mud is underlain by sand compositionally indistinguishable from that on the modern delta.

We interpret the mud, which contains rotted tree trunks rooted in growth position, as representing an ancient mangrove swamp. Macro- and microfossils found within the mud support this interpretation; together they suggest that the shoreline has been eroding back during the last 1000 years, despite long-term uplift along the Tanzanian coastline.

Within the mud layer lies a thin (~1 cm) layer of sand that can be traced through most of the cores we took. Sediment analysis of this layer suggests it fines landward, and contains sediment finer than currently available on the modern shoreface (though likely available on the delta front below low tide).