he deposits of Pleistocene Lake Tecopa cover about 90 square miles along the Amargosa River in southeastern Inyo Co'unty. These deposits consist chiefly of mudstone and inter-be~ded rhyolitic vitric tuffs that interfinger marginward with co~rser clastic sediments. A total of 12 tuffs, 2 inches to 13 fet=tt thick, were recognized in the 220 feet of exposed section. Tliey are mainly the result of ash falls directly into the lake, although the upper parts of some contain reworked material. This report summarizes the physical properties, chemistry, and areal distribution of those silicate minerals that formed in the tuffs during diagenesis.Zeolites, monoclinic potassium feldspar, searlesite, clay minerals, and opal occur in the altered tuffs. The zeolites are mainly phillipsite, clinoptilolite, erionite, and in trace to minor amounts, analcime and chabazite. Clay minerals are nearly ubiquitous and occur in trace to major amounts associated chiefly with zeolites and potassium feldspar. Most beds contain two or more zeolites, but some beds are monomineralic. Potassium feldspar also locally forms monomineralic beds.Zeolites are locally associated with relict glass; potasssium feldspar and searlesite are locally associated with zeolites, but nowhere has potassium feldspar oc searteslite been round associated with relict glass. Textural evidence shows that some po•tassium feldspar and searlesite replace phillipsite and suggests that all potassium feldspar and searlesite in the tuffs formed from phillipsite and other early diagenetic zeolites rather than directly from the rhyolitic glass.Three diagenetic facies recognized in the tuffs have fresh glass, zeolite, and potassium feldspar or searlesite as their diagnostic constituents. The fresh-glass facies is alrong the lake margin and is succeeded basinward by the zeolite facies and then by the potassium feldspar facies in the central part of the basin. The boundary between the zeolite and potassium feldspar facies bulges basinward at the inlets to Lake Tecopa. Original textures and sedimentary structures are generally preserved in tuffs of the zeolite and potassium feldspar facies. Preservation of these features in altered tuffs that are laterally equivalent to tuffs of the fresh-glass facies is convincing evidence that the present differences in composition and mineralogy are due to postdepositional processes. Experimental work by other mineralogists indicates that the formation of zeolites and feldspar is favored over clay minerals by a relatively high alkali ion to hydrogen ion activity ratio. The distribution and the gradational character of the facies are undoubtedly due to a chemical zonation of the pore water during diagenesis. The pore water was "fres'h" near the margin and inlets of the lake but increasingly alkaline and saline bas:inward. This zonation was probably inherited from the depositional environment. Water of Lake Tecopa was probably moderately to highly saline with a pH of 9 or higher, except near the lake margin.Solution of the 'rhyolitic glass by alkal...

Checklist of associated authigenic silicate minerals in tuffs of the Big Sandy Formation ___________________ _ Chemical analysis and composition of unit cell of analcime ______________________________-Chemical analysis and composition of unit cell of chabazite ________________________________-Semiquantitative spectrographic analyses of chabazite-rich and clinoptilolite-rich tuffs ____________________ _ Chemical analysis and composition of unit cell of clinoptilolite _______________________________-Chemical analysis and composition of unit cell of erionite _________________________________-Chemical analysis and composition of unit cell ofharmotome

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