Mineral reactions in the sedimentary deposits of the Lake Magadi region, Kenya RONALD C. SURD AM Department of Geology, University of Wyoming, Laramie, Wyoming 82071 HANS P. EUGSTER Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, Maryland 21218 ABSTRACT hand, the initial alkalic zeolite that will form in a specific alkaline lake environment cannot yet be predicted with confidence. Among The authigenic minerals, principally zeolites, in the Pleistocene to the many parameters controlling zeolite distribution, the compo- Holocene consolidated and unconsolidated sediments of the sitions of the volcanic glasses and of the alkaline solutions are cer- Magadi basin in the Eastern Rift Valley of Kenya have been inves- tainly of primary importance. tigated. Samples were available from outcrops as well as drill cores. Most zeolite studies are concerned with fossil alkaline lakes. The following reactions can be documented: (1) trachytic glass + While it is generally easy to establish the composition of the vol- H20 —» erionite, (2) trachytic glass + Na-rich brine —* Na-Al-Si gel, canic glass, the brines responsible for the authigenic reactions have + + (3) erionite + Na analcime + K + Si02 + H20, (4) Na-Al-Si long since disappeared. To overcome this handicap, it is desirable gelanalcime + H20, (5) calcite + F-rich brine—> fluorite + C03 , to investigate an alkaline lake environment for which glass as well (6) calcite + Na-rich brine —» gaylussite, and (7) magadiite —• as brine compositions can be determined — in other words, a + quartz + Na + H20. Erionite is the most common zeolite present, modern alkaline lake environment. Such an environment is Lake but minor amounts of chabazite, clinoptilolite, mordenite, and Magadi of Kenya. The Magadi basin is unique because the compo- phillipsite were also recognized. Erionite can form directly from sition and evolution of its waters have been very carefully trachytic glass by the addition of H20 only. It is characteristic of the documented (Eugster, 1970; Jones and others, 1976). Magadi basin because of the low content of alkaline earths in the This paper describes the authigenic mineral suite that is now volcanic glasses and in the solutions interacting with them. forming in Lake Magadi and that has formed in its Pleistocene pre- Analcime is common in outcrops of the High Magadi and cursors and interprets the reactions responsible for these as- Oloronga Beds. It forms from erionite by a reaction probably ini- semblages in terms of glass chemistry and brine chemistry. We tiated by a lowering of the silica activity, which results from the hope that this approach will result in a better understanding of the transformation of magadiite to chert. Analcime in the drill-core interdependence between authigenic silicates and aqueous samples grew at the expense of a Na-Al-Si gel. This gel forms at the geochemistry. lake shore and is washed into the lake during flooding conditions. Fluorite is common in the core samples and can be explained by STRATIGRAPHY AND LITHOLOGIES reaction of the fluoride-rich brines with calcium in the sediments, principally detrital calcite. Authigenic albite and potassium The Pleistocene and Holocene history of the Magadi basin sedi- feldspar were not recognized, probably for reasons of reaction ments was described in detail by Baker (1958, 1963) and Eugster kinetics. (1969). Lake Magadi is located in the lowest part of the Eastern The presence of authigenic minerals can be accounted for by Rift Valley in Kenya, just north of the Tanzania border. It is mainly considering the chemical compositions of the starting materials, a trona-precipitating saline lake, which is fed by alkaline hot mainly volcanic glasses, and the brines they come in contact with. springs issuing at the perimeter of the basin. The sediments ac- Lake Magadi represents a unique opportunity for studies of cumulating now belong to the Evaporite Series (see Table 1), and diagenesis because authigenic minerals are forming there at the present time and because the evolution of its waters is well known. TABLE 1. STRAT1GRAPHIC UNITS, TECTONIC EVENTS, AND ABSOLUTE AGES OF INTRODUCTION FORMATIONS FOUND IN THE MAGADI BASIN Zeolites are among the most common authigenic silicates of Units Age Lake sedimentary rocks (Hay, 1966). Among the most detailed studies (yr) are those of zeolites from saline alkaline lake environments (see Evaporite Series Sheppard and Gude, 1968, 1969a, 1973; Hay, 1970; Surdam and (slight tilting) 0 Magadi Parker, 1972). These zeolites generally formed by reaction of vol- canic glass with saline alkaline solutions. The alkalic zeolites that High Magadi Beds (minor faulting?) High Magadi result from such reactions are commonly phillipsite, erionite, 9,100 clinoptilolite, and, to a lesser extent, mordenite and chabazite. Trachyte flows (minor) These zeolites may subsequently be transformed to analcime (as in caliche 780,000 the Big Sandy, Barstow, and Green River Formations) or to potas- Oloronga Beds sium feldspar (as in Pleistocene Lake Tecopa). Analcime may also (extensive grid faulting) >780,000 Oloronga react to form potassium feldspar (as in the Big Sandy, Barstow, and Plateau trachyte flows -1,000,000 Green River Formations). (extensive) to 1,700,000 The paragenetic sequence of volcanic glass —» alkalic zeolite —* Note: Data from Eugster (in prep.). analcime —» potassium feldspar is well established. On the other Geological Society of America Bulletin, v. 87, p. 1739-1752, 11 figs., December 1976, Doc. no. 61208. 1739 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/12/1739/3418402/i0016-7606-87-12-1739.pdf by guest on 27 September 2021 1740 SURDAM AND EUGSTER the area of the active lake covers about 60 km2. There is evidence High Magadi and Oloronga Beds are discussed separately. The drill for the existence of two larger precursor lakes in the basin. The cores penetrated a very much thicker section of lake sediments than shoreline of the lake from which the High Magadi Beds were de- is available in the outcrops at the margin of the basin. Exact corre- posited is still visible, and the High Magadi Beds have been dated lation of the stratigraphic units from outcrop to drill core is as 9,100 yr. old (Butzer and others, 1972). The older lake is repre- difficult. Therefore, the drill-core mineralogy is tabulated sepa- sented by the Oloronga Beds, which have been dated by K-A meas- rately from the outcrop mineralogy. A tentative correlation will be urements on a glassy trachyte flow as at least 780,000 yr. old discussed later. (Fairhead and others, 1972; Eugster, in prep.). Below the Oloronga Beds are the extensive plateau trachyte flows that cover the Rift MINERALOGY AND CHEMISTRY OF Valley floor and are as old as 1.7 m.y. (Baker, 1958; Baker and HIGH MAGADI AND OLORONGA BEDS others, 1971). Outcrops of High Magadi and Oloronga Beds occur abundantly High Magadi Beds, Outcrop Samples in the alluvial flats and at the foot of the hills surrounding Lake Magadi (for sample locations, see Fig. 1). The detailed distribution The mineralogy of the High Magadi Beds is characterized by the of the two sets of lake beds is rather complex, because there is a following groups of minerals: (1) detrital silicates, (2) saline miner- considerable region of overlap. However, good Oloronga outcrops als, (3) calcite, (4) sodium silicates and quartz, and (5) authigenic are most common southwest, northwest, and northeast of Lake zeolites. The most abundant detrital silicate is anorthoclase, which Magadi, whereas the High Magadi Beds can best be studied east occurs as fragments of single crystals. These are generally fresh or and southeast of the lake. A detailed account of the stratigraphy, slightly altered to clay minerals and (or) calcite. These fragments lithologies, and sedimentary history will be published elsewhere are identical in mineralogy with the phenocrysts in the trachytic (Eugster, in prep.), and a brief summary will suffice here. lavas and were derived either from pyroclastic ash falls or directly The sedimentary rocks that constitute the High Magadi and from the lavas. Similar anorthoclase fragments are abundant in the Oloronga Beds are very similar; however, the Oloronga Beds are modern muds. Another common detrital mineral is amphibole. The commonly more indurated, and they are capped by a thick caliche amphibole also occurs as crystal fragments, many of which are al- (Kunkar) limestone (50 to 75 cm thick). This caliche is an im- portant stratigraphic marker because it occurs throughout the ba- sin, whereas the post-Oloronga lava flows are restricted to the northwest area (loc. 665 and 696). The sedimentary rocks are prin- LITTLE cipally bedded chert and tuffaceous material. The High Magadi MAGADI cherts have clearly been derived from magadiite horizons (Eugster, 1969), and the Oloronga cherts probably had a similar origin. The tuffaceous material varies greatly. Some tuffs must have formed from pyroclastic debris associated with individual volcanic events. Figure 1. Index and sample-locality map Such tuffs may contain pumice fragments as much as 1 cm in of Lake Magadi region, Kenya, East Africa. diameter. Others also contain detrital material and range from coarse, cross-bedded sandstone to finely laminated mudstone. However, even the clearly detrital components are of volcanic deri- vation. In fact, the sediments of the lower sequence of the High Magadi Beds (Eugster, 1969) may consist principally of eroded Oloronga material. There are no perennial rivers flowing into the Magadi Basin at present, and this condition probably also applied to High Magadi and, perhaps, even Oloronga time. Hence, the sediments in the basin were either derived from within the basin or were carried in by wind. Since the sediments are all underlain by thick sequences of trachytic flows, most of the sediments, except for the cherts, calcite, and organic fractions, are obviously of volcanic origin. EXPLANATION SAMPLING AND SAMPLE LOCATIONS OLORONGA BEDS Samples were obtained from outcrops, shallow pits, and drill HIGH MAGADI BEDS cores.