SEDIMENTATION AND EARLY DIAGENETIC PROCESSES IN NASIKIE ENGIDA: A HOT-SPRING FED, PERENNIAL SALINE, ALKALINE LAKE IN THE MAGADI BASIN, KENYA RIFT VALLEY

Nasikie Engida (Little Magadi) is a saline, alkaline lake that lies in a narrow, steep-sided graben in Pleistocene trachytes 1.6 km north of Lake Magadi in the arid, southern Kenya Rift Valley. Although very shallow (< 1.2 m deep), Nasikie Engida is perennial because most inflow originates from about 35 alkaline hot springs (86°C) with very high discharge.

A preliminary study of the hydrochemistry and sediments of Nasikie Engida has shown a strong salinity gradient from the north, where the hot springs discharge, to the south, where bedded evaporites precipitate annually from evaporated brine. From north to south, a distance of 6 km, the lake water salinity increases from 30 g/l TDS to > 260 g/l TDS, and becomes depleted in Si, Na, alkalinity, and sulfate, while K, F, and Cl remain nearly constant. Silica is removed as authigenic silicates including gels in low-energy settings; Na and carbonate species are lost to evaporites (nahcolite, trona, gaylussite), and sulfate is probably reduced in the anoxic organic muds that underlie the lake floor. The salinity gradient is maintained for much of the year although some wind mixing occurs. Lake level fluctuates over a range of a few decimeters. Fish (Alcolapia grahami) and flamingos inhabit the hot northern lake waters. Thick microbial mats underlie efflorescent salt crusts around the shoreline and extend offshore.

The thermal waters transport almost no siliciclastic sediments to the lake, so modern sedimentation is predominantly chemical and organic. Crystal rafts that form in the southern part of the lake drift shoreward and sink, becoming interlayered with microbial mats and bottom nucleated salts. Much of the southern part of the lake is a saline pan with large polygons, but the salt layer (nahcolite, trona) is only about 12 cm thick. Ephemeral streams have produced two small salt-encrusted fan-deltas along the western lakeshore, but the remaining shoreline is a narrow (5-10 m) zone of siliciclastic boulders and gravels, some of which are undergoing corrosion in the alkaline waters. Small outcrops of Pleistocene sediments around the shoreline record former higher, more dilute lakes. Nasikie Engida provides clues to the earliest stages of sodium carbonate evaporite formation.