Diagenetic Alteration of Silicic Ash in Searles Lake, California

Richard L. Hay1 and Sandra G. Guldman2
1 Department of Geology, University of Illinois 1301 West Green Street, Urbana, Illinois 61801
2 Environmental Science Associates, 760 Harrison Street San Francisco, California 94107

Abstract: Ash layers from Searles Lake, California, were sampled in core of drill hole KM-3, which penetrated 693 m of lacustrine sediment deposited in a playa-lake complex over the past 3.2 my. Lake water changed from moderately saline and slightly alkaline (pH ∼7.5) to highly saline 2.04 my ago and to highly saline and alkaline (pH ∼9.5) 1.28 my ago. As a result of brines flushing downward, the upper 291 m of sediment, spanning the past 1.28 my, contain highly saline, alkaline pore fluid. Silicic ash layers in contact with highly saline, alkaline pore fluid were first altered to phillipsite and merlinoite and then to K-feldspar and searlesite. The transformation of phillipsite and/or merlinoite to K-feldspar required more than 45,000 years and was largely completed in 140,000 years. Tephra layers in contact with moderately saline, slightly alkaline pore fluid vary from uncemented vitric ash containing minor smectite to bentonites in which glass is wholly altered to smectite, clinoptilolite, analcime, and opal. Layers with much fine tephra are more altered than the coarser, better-sorted layers. Alteration is attributed to hydrolysis in essentially a closed hydrologic system, in which the alteration of glass to smectite raised the pH, aSiO2, and (Na+ + K+)/H+ activity ratio to the level where clinoptilolite formed. Some diffusion and/or fluid flow is, however, indicated by the loss of SiO2 during the alteration of ash layers to smectite and by anhydrite deposited during and after clinoptilolite in some tuffaceous sandstones.

Key Words: Alkaline • Saline lake • Anacime • Clinoptilolite • Diagenesis • K-feldspar • Merlinoite • Phillipsite • Volcanic ash • Zeolite

Clays and Clay Minerals; December 1987 v. 35; no. 6; p. 449-457; DOI: 10.1346/CCMN.1987.0350605
© 1987, The Clay Minerals Society
Clay Minerals Society (www.clays.org)