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THE NATURE OF ZEOLITE OCCURRENCES IN DEPOSITS OF THE OLDUVAI BASIN, NORTHERN TANZANIA

¹ Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium
² Geobound International Ltd, Environmental Research Group and Consultants, Doornstraat 27, B-9550 Herzele, Belgium

View larger version (26K): [in a new window]   Figure 1. Location of the study area and the sampling sites. I–IV: Bed I to IV sites (I near site 45 of Hay, 1976; II to IV near site 85).

View larger version (47K): [in a new window]   Figure 2. Stratigraphy, lithology and mineralogy of the studied exposures, and the positions of intervals for which thin-sections were prepared (stratigraphy and lithology modified from logs with initial stratigraphical interpretations and paleosol identifications by R. Paepe and P. Kafumu, unpublished data). Stratigraphy : TIB, TIC, TIF, TIIA, TIIB–Tuffs IB, IC, IF, IIA and IIB (Hay, 1976); LAS – lower augitic sandstone (Hay, 1976); BII – Bed II interval in the exposure at the studied Bed III site; BIII – Bed III interval in the exposure at the studied Bed IV site. Mineralogy: An–analcime, Ch–chabazite, Ph –phillipsite (major occurrences indicated by full lines, minor occurrences by dotted lines), Cl –clinoptilolite, Er –erionite, Kn–kenyaite, Cb – intervals with high carbonate content. Sampling levels: lines and sample codes indicate the intervals for which thin-sections were prepared.

View larger version (162K): [in a new window]   Figure 3. Zeolites in lake-margin deposits of Bed I. (a) Partly altered vitric volcanic rock fragment, with chabazite coatings (arrow) and infillings [i] of vesicles (interval Z7; plain polarized light [PPL]). (b) Complete chabazite [c] infilling of a branching root gallery (Z9; PPL). (c) Phillipsite (elongated prismatic crystals) [p], along the sides of pores (left) and covering chabazite [c] infillings and coatings of vesicles in a completely altered vitric volcanic rock fragment (right) (chabazite phillipsite sequence) (Z3; PPL). (d) Thick chabazite [c] coating, with an intercalation of authigenic clay [ac] (chabazite authigenic clay chabazite); both materials, formed in a channel of which only a small central zone remains empty (bottom), are locally impregnated with Fe/Mn oxides (arrows) (ZK; PPL).

View larger version (145K): [in a new window]   Figure 4. Authigenic silicates in lake-margin deposits of Bed II. (a) Spherulitic kenyaite aggregate [k] in a pore with a chabazite infilling [c] (kenyaite chabazite sequence) (RS5; PPL). (b) Erionite crystals [e] in pores, in an interval containing nearly unaltered volcanic glass [g] (RS22; PPL).

View larger version (142K): [in a new window]   Figure 5. Zeolites in fluvial deposits of Bed II. (a) Analcime coating [a], overgrown by phillipsite crystals [p], followed by a fine clay coating, with strong Fe/Mn oxide impregnation of the sediment below the zeolite coating (fm) (Fe/Mn oxides analcime phillipsite fine clay sequence) (RS38; PPL). (b) Analcime (small equant crystals) (e.g. white arrow) and phillipsite (elongated prismatic crystals) (e.g. black arrow), occurring together in pores (right) and in cavities in an altered volcanic rock fragment (left) (analcime + phillipsite association) (RB3; PPL).

View larger version (150K): [in a new window]   Figure 6. Zeolites in fluvial deposits of Bed III. (a) Chabazite [c] around a corroded augite grain (right) and as a coating around a pore (top left) (RB15; PPL). (b) Subhedral natrolite crystal [n], covered by a fine clay coating [fc] and covering a thin analcime or chabazite coating (arrows) with some subhedral analcime crystals [a] (analcime/chabazite natrolite fine clay sequence) (RB21; PPL). (c) Analcime crystals [a] along the surface of a fine clay coating (labeled in the center) and in cracks within the coating (labeled at left) (fine clayanalcime) (MC1; PPL). (d) Coarse clay coating [cc], followed by a thin analcime [a] coating, followed by yellowish fine clay, followed by subhedral analcime [a] and sparitic carbonates [s], followed by orange fine clay (coarse clay analcime fine clay analcime + sparitic calcite fine clay) (RB24; PPL).

View larger version (152K): [in a new window]   Figure 7. Authigenic silicates in fluvial deposits of Bed IV. (a) Analcime infillings of narrow cracks (arrows) (MC18; PPL). (b) Volcanic glass fragment, completely altered to smectite (MC24; circular polarized light). (c) Phillipsite [p] and analcime [a] in an altered mineral grain (analcime + phillipsite association), in an interval with small phillipsite crystals along the sides of most pores (arrows), covering thin fine clay coatings (fine clayphillipsite sequence) (MC24; PPL). (d) Euhedral analcime crystals [a], enclosed by a sparitic carbonate cement [s] (analcime + sparitic calcite association) (MC11b; PPL).