Zeolitic diagenesis of late Quaternary fluviolacustrine sediments and associated calcrete formation in the Lake Bogoria Basin, Kenya Rift Valley

Abstract

Late Quaternary fluviolacustrine siltstones, mudstones and claystones (Loboi Silts) on the northern margins of the saline, alkaline Lake Bogoria in the Kenya Rift Valley contain up to c. 40% authigenic analcime and minor natrolite. The zeolitic sediments are reddish brown and up to 1 m thick. The amount of analcime increases upward in the profile, but decreases with distance from the lake. The altered sediments show many pedogenic features including zeolitic root mats, rootmarks, concretions and carbonate rhizoliths. Residual patches of calcrete locally cap the zeolitic rocks. The profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake. The analcime occurs as submicroscopic (0–5–2–5 μ.m) subhedral and euhedral crystals, which have an average Si/A 1 ratio of 2–33 (as determined by X‐ray microanalysis) or 2–18 (d‐value of 639 analcime peak). The analcime formed in lake marginal sediments (soils) by reaction of silicate detritus with Na2CO3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration. Poorly ordered clay minerals were probably the main reactants. Authigenic illite may have been a by‐product of the reactions. Chemical analyses suggest that pore waters supplied some of Na+, and possibly K+ and SiO2. The associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration. The Ca2+ may have originated from infiltrating dilute runoff and groundwater. Authigenic smectite was precipitated in open porosity following analcime formation. The zeolitic alteration at Lake Bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline, alkaline lake sediments.

Differentiating paleowetland subenvironments using a multi-disciplinary approach: An example from the Morrison formation, South Central Wyoming, USA

“…As fine-grained, rhyolitic or dacitic ash altered in aqueous Morrison environments, zeolites and quartz precipitated out of porewater similar to deposits in PleistoceneHolocene systems in Africa (Tiercelin et al, 1987;Ashley and Driese, 2000). Interstitial pore water became more alkaline in lakes and fens with restricted drainage (Hanor, 2000;Jennings, 2005) and excess silica and concentrated ions combined to form early diagenetic zeolites in carbonaceous material accumulating in the fen (Hay, 1970;Taylor and Surdam, 1981;Gottardi and Galli, 1985;Renaut, 1993). The abundance of zeolites and the paucity of plant material and silt-and sand-sized sediments in the Btg horizons at the base of Pedotype H implies that these environments received little or no overland flow from streams, but rather experienced a rise in the groundwater table.…”

“…In modern hydric soils, similar residual aggregates of organic material, also called organic bodies, have been documented in mucky mineral O horizons (Vepraskas, 2001). Restricted alkaline conditions indicated by illite clays and abundant early authigenic analcime in more basinward deposits suggest that evapotranspiration was a dominant process in this fen, similar to Quaternary deposits in tropical semi-arid regions (Surdam and Eugster, 1976;Renaut, 1993;Hay and Kyser, 2001).…”

“…Pedotype H is interpreted as a fen deposit that accumulated during widespread flooding after a long period of landscape stability. Analcime is well documented in organic-rich deposits as a product of altering volcanic ash in low-temperature aqueous environments (Hay, 1970;Renaut, 1993). Highly reactive airfall-ash deposited in water quickly alters to smectite clay and siliceous gel (Chamley, 1989;Hillier, 1995).…”

Primary dolostone related to the Cretaceous lacustrine hydrothermal sedimentation in Qingxi sag, Jiuquan Basin on the northern Tibetan Plateau: Discussion

“…Analcime with low Si/Al ratio (2.0-2.2) is generally interpreted as "…the product of direct precipitation from highly alkaline water or by reaction of such water with clay minerals or other sedimentary materials" (Coombs and Whetten, 1967). This theory is widely accepted (Remy and Ferrell, 1989;Renaut, 1993;English, 2001;Mees et al, 2005;Do Campo et al, 2007;Karakaya et al, 2013), whereas none of the researchers related this type of analcime to hydrothermal fluid and most of them tended to support the mode of clay mineral transformation (Remy and Ferrell, 1989;Renaut, 1993;Karakaya et al, 2013). According to the mean Si/Al ratio, clay minerals seem to be an important role in the formation of analcime and it is improper to draw the conclusion of direct crystallization from hot water.…”

Regional to global correlation of lower Cretaceous (Hauterivian–Barremian) shallow-water carbonates of the southern Apennines (Italy) and Dinarides (Montenegro), southern Tethyan Margin

“…3C) whose thickness ranges from a few cm to 80 cm (Raspini, 1996;Sandulli, 2000). Calcrete horizons are also characterized by a dark microcrystalline matrix locally containing small ( V 200 Am) rounded problematic grains dissolved and occluded by drusy mosaic cement, light-brown microsparitic patches, floating peloids and vacuolar structures of irregular shape looking like small root molds (Renaut, 1993). Rare rounded structures with diameter up to 1 mm or rare groups of irregular pores frequently filled by calcite and separated by micritic anastomised partitions are interpreted as rhizocretions (Fig.…”