Daniel M. Deocampo

Sedimentary Structures Generated by Hippopotamus amphibius in a Lake-margin Wetland, Ngorongoro Crater, Tanzania

Abstract Large mammals, especially Hippopotamus amphibius, have created a distinctive set of traces in a spring-fed freshwater wetland on the margin of the saline-alkaline crater lake in the Ngorongoro Crater, Tanzania. It is comprised of a ∼30 m diameter zone of deep (<2m) bioturbation due to hippo wallowing, surrounded by dendritic to radial hippo trails 1–5 m wide and <0.5 m deep, infilled with organic-rich mud. These trails narrow and thin as they grade into trackways on the lake flat. Tracks and trackways of more terrestrial mammals, such as bovids and equids, are found in the lake flat muds surrounding the hippo-dominated area. Associations of sedimentary structures such as these are important indicators of paleoenvironmental conditions where they are preserved in the sedimentary record, due to the strong affinity of Hippopotamidae for freshwater environments.

A Mineralogical and Geochemical Analysis of Bed I in Olduvai Gorge, Tanzania

Olduvai Gorge is located in northern Tanzania (3°S, 35.35°E) on the Serengeti Plain and on the margin of the East Africa Rift System. Seventy-nine sediment samples were collected during the 2015 field season from a 6-meter high section of lacustrine sediment (~1.9-1.86 Ma). Samples were collected at ~15 cm intervals from 50 cm to 580 cm. The purpose of these analyses are to determine the chemistry of clay minerals and whether, or not, authigenic clays can be used as paleoenvironmental proxies. Bulk minerology was obtained by XRD analyses of random-oriented powder. Samples were analyzed for 15 minutes from 10 to 65 degrees 2θ. An abundance of K feldspars were found and a few samples contained zeolites (phillipsite). Calcite is common, whereas dolomite is rare. XRD analyses of oriented clays were conducted on additional Bed I sediments, located ~ 300 meters to the east. The samples were analyzed air-dried and ethylene glycol solvated from 3 to 45 degrees 2θ for 15 minutes in order to identify the major clay mineral phases. Previous work has shown that the major clay phases are authigenic illite, smectite and illite/smectite. Preliminary results from this study are consistent with these findings. Samples were also analyzed from 59 to 63 degrees 2θ for 30 minutes. The 060 peak positions varied from 1.510 Å to 1.522 Å. This likely corresponds to variations in octahedral Mg from 1.5 to 2.5 atoms per half formula unit. This variation in Mg suggests fluctuation in salinity of paleo Lake Olduvai over the ~35 kyr period represented by the ~5 m thick lake section. To conclude, preliminary results do suggest that clay chemistry and authigenic clay minerals can be used as paleoenvironmental proxies.