Distribution and genesis of authigenic silicate minerals in tuffs of Pleistocene Lake Tecopa, Inyo County, California

Abstract

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...

Clay mineralogy of Pleistocene Lake Tecopa, Inyo County, California

“…Identifications and con clusions were based on physical and chemical proper ties and upon the commercial uses to which the clays could be put. Sheppard and Gude (1968), in their study of the authigenic silicate minerals in the tuffs of Lake Tecopa, described those tuffs in detail and we will use them as stratigraphic markers for the present study. They reported the occurrence of sepiolite in two of their samples, but they noted that neither Droste (1961) nor Kerr and Langer (1965) had reported sepiolite in their studies of other playa clays of the Mojave Desert.…”

“…The reports of these investigations have also described the general geology of the area surrounding Lake Tecopa. Sheppard and Gude (1968) gave a detailed report on the mineralogy of the tuffs associated with the mudstones, and an excellent review of the previous geologic work in the area. Therefore, only a brief description will be given here to acquaint the reader with the mudstones in the basin.…”

Uranium ore in the Mariano Lake-Lake Valley cores and the genesis of coffinite in the Grants uranium region, New Mexico

“…The concentric pattern formed by these facies is characteristic of sediments deposited in a closed salinealkaline lake basin (Sheppard and Gude, 1968). The alluvial facies is predominantly smectitic sandstone and mudstone; the mudflat facies consists of smectitic mudstone and claystone, and the lacustrine facies is characterized by smectitic claystone interbedded with discrete tuff beds, which have been silicified and/or altered to zeolites.…”

“…At pH values greater than nine, not uncommon in lacustrine facies of a saline-alkaline lake environment (Sheppard and Gude, 1968;Hay, 1966), the dissolution rate of volcanic ash increases dramatically (Zielinski, 1982) as does the solubility of silica (Krauskopf, 1959). Thus, escaping Brushy Basin pore fluids may have had higher silica contents than fluids closer to the basin margin.…”

“…These lithologic and mineralogic changes are interpreted to represent a transition from mudflat to playa margin facies in the Brushy Basin Member, similar to facies changes that Bell (1986) recognized in the southeastern part of the Grants uranium region. Rapid facies changes are typical of a saline-alkaline lake environment (Sheppard and Gude, 1968).…”