Mineral precipitation and diagenesis in the sediments of the Lake Bogoria basin, Kenya Rift Valley

Abstract

Lake Bogoria is a perennial, saline, alkaline closed lake fed by hot springs and ephemeral runoff. Detailed studies of the sedimentology, mineralogy and chemistry of the late Quaternary sediments, both from exposures around the shoreline and from lake bottom cores, have provided abundant evidence for early diagenesis and mineral precipitation. Marginal fan-deltaic clastics exhibit carbonate (calcite, dolomite), iron oxide, opaline silica and zeolite (analcime, natrolite) cements, especially where groundwater is shallow and subject to evaporative concentration. Travertines associated with microbial activity around hydrothermal springs are predominantly calcite; high temperature, inorganically precipitated travertines are aragonitic. Most are relic deposits. In places, spring carbonates have been silicified (opal-A) or altered to fluorite in response to changing groundwater composition. Fan-delta conglomerates have been hydrothermally altered by alkaline spring waters yielding zeolites (analcime, thomsonite, clinoptilolite, mordenite), clay minerals, opaline silica and magadiite. The lacustrine sediments include prodeltaic clastics, organic oozes and sodium carbonate evaporites. Trona and nahcolite predominate in the central sub-basin. The local occurrence of thermonatrite may be associated with sub-lacustrine hydrothermal activity. In the north subbasin, evaporites are exclusively gaylussite. Sodium silicates (magadiite, kanemite) occur in black organic oozes and formed in an anoxic environment rich in sodium carbonate and dissolved SiO2. Other authigenic minerals in the lacustrine sediments include pyrite, siderite, fluorite, zeolites (analcime, mordenite, clinoptilolite), opaline silica (formed from a gel precursor around sub-lacustrine springs?) and vivianite.Modern and ancient saline, alkaline lake deposits are well known for their abundant and varied mineral suites. These include minerals precipitated directly from solution, such as many salts and alkaline-earth carbonates, and authigenic minerals such as zeolites and sodium silicates, formed by the interaction of interstitial brines with sediment or fresher waters (Eugster & Hardie 1978;Eugster 1980a). Much of this knowledge stems from the detailed studies by Hay (1970), Eugster (1980b) and others working in the rift valley lake basins of East Africa.Of the East African lakes, Magadi is often regarded as a 'type-model' for saline, alkaline rift lakes, having been studied intensively by Eugster and colleagues for nearly two decades (Surdam & Eugster 1976;Jones et al. 1977; Eugster 1980b, this volume). It is today essentially an ephemeral salt pan, only holding surface water briefly during the rainy seasons.Since 1976, research has been conducted at Lake Bogoria, another saline, alkaline rift lake some 220 km to the N (Fig. 1, inset B). Unlike Magadi, which is mostly hot-spring fed and lacks significant surface inflow, Bogoria is perennial, being fed both by hot springs and seasonal streams. Furthermore, since Lake Magadi itself has little marginal relief, cl...

Geochemical evidence of hydrothermal recharge in Lake Baringo, central Kenya Rift Valley

“…Such linear relationships are seen for Cl versus Na, and F versus Na (Figure 7). Conservative mixing of two components requires linear relationships for every pair of species (Jones et al, 1977;Renaut et al, 1986;Albarède, 1995;Jones and Deocampo, 2003;Deocampo, 2004a,b). The estimate of the positive gradient for Cl versus Na gives 0Ð245 with a coefficient of determination R 2 D 0Ð94.…”

Tracks, Trails and Trampling by Large Vertebrates in a Rift Valley Paleo-Wetland, Lowermost Bed II, Olduvai Gorge, Tanzania

“…Although vertebrate bioturbation has been recognized previously as an important process, the paleoenvironmental interpretations were associated with sedimentation in depositional systems such as volcaniclastic (Leakey and Hay, 1979), lake margin (Laporte and Behrensmeyer, 1980;Renaut et al, 1986;Hay et al, 1986;Cohen et al, 1993;Deocampo and Ashley, 1999), fluvial (Haynes, 1985;Holliday, 1995), deltaic (Behrensmeyer and Laporte, 1981), and coastal (Van der Lingen and Andrews, 1969) environments. There is now a need to extend the documentation of bioturbation to groundwater-fed wetland systems.…”

Diagenesis in Okavango fan and adjacent dune deposits with implications for the record of palaeo-environmental change in Makgadikgadi–Okavango–Zambezi basin, northern Botswana

“…Diagenetic change in rift sediments is often associated with the adjacent basins below the fault scarp (e.g. Renaut et al, 1986) while variability in sediments in rifts with little or no topographic change has been overlooked. This work describes diagenesis in mapped Okavango alluvial fan and adjacent dune siliclastic sediments in the western MOZ (MakgadikgadiOkavango-Zambezi) basin in northern Botswana.…”