Abstract: X-Ray powder diffraction and thin section analyses indicate that marginal lacustrine mudstones of the Green River Formation in the south-central Uinta basin, Utah, contain abundant analcime. The analcime has a low Si/Al ratio (<2.31) and occurs as very fine grained disseminated crystals and, to a lesser extent, as coarser-grained pore-filling cement. Analcime-rich mudstones and associated sandstones, siltstones, and carbonates lack volcanic detritus and zeolites other than analcime, thus making it difficult to support the concept that the analcime formed from precursor zeolites derived from volcanic glass altered in saline, alkaline-lake water. Abundant dolomite, syneresis cracks, and the absence of freshwater pelecypods and gastropods suggest that the lake (Lake Uinta) was moderately saline and alkaline. The restricted illite-illite/smectite clay mineral suite in the analcime-rich mudstones suggests that detrital clays significantly altered in a moderately saline and alkaline environment, thereby providing a source of Si and Al for the formation of analcime.
Red mudstones contain twice as much analcime as green mudstones (14 vs. 7 wt. %). Green mudstones have a day mineral suite consisting of illite (44 wt. %), mixed-layer illite/smectite (35 wt. %), smectite (12 wt. %), and minor kaolinite (4 wt. %) and chlorite (5 wt. %), whereas red mudstones have a more restricted day mineral suite consisting ofillite (68 wt. %) and mixed-layer illite/smectite (26 wt. %) with very minor smectite, chlorite, and kaolinite. Periodic minor fluctuations in lake level probably exposed large areas of shallow lacustrine-interdistributary green mud. Evaporative pumping on the exposed mudflats concentrated the moderately saline and alkaline-lake water, thereby producing Na-rich brines that enhanced the formation of analcime by accelerating the alteration of detrital clays and, perhaps, other minerals. Oxidation of iron from altered iron-bearing minerals stained the analcime-rich mud red with iron hydroxide or oxide (perhaps hematite). The overall reaction from green to red mud (mudstones) was probably: detrital phyllosilicates + Na-brine + iron-bearing minerals + oxygen → analcime + iron hydroxide or iron oxide.