Spring-related carbonate rocks, Mg clays, and associated minerals in Pliocene deposits of the Amargosa Desert, Nevada and California

摘要

The Amargosa Desert of Nevada-California is a spring-fed structural basin containing large amounts of Pliocene carbonate rocks and Mg clays deposited in playas, marshland, ponds, and flood plains. Pliocene basin-fill deposits dated at 2.4 to 3.2 Ma are subdivided into three lithofacies, designated "Tpa, Tld, and Tpl." Tpa is chiefly limestone and was deposited, in marshland and ponds. Tld consists chiefly of limestone and montmorillonite claystone that were deposited in varied environments, including ponds and flood plains. Tpl is almost entirely Mg clays, limestone, and dolomite, which were deposited in playas and associated marshland. The largest area of playa sediments, termed East Playa, is in the east-central part of the basin. Limestone and dolomite seepage mounds and caliche-breccia musses were formed along zones of ground-water leakage within East Playa. Chemically precipitated Mg smectite and sepiolite form large deposits in Tpl. Sepiolite was formed in water of low salinity, whereas Mg smectite was formed in water of higher salinity. Authigenic celadonitic illite and K-feldspar are commonly associated with Mg smectite and are very likely reaction products of montmorillonite in saline, alkaline water. Silicic vitric ash layers were locally altered to phillipsite and K-feldspar in saline, alkaline water and to montmorillonite as a result of leaching by water of low salinity. Carbonate rocks are locally silicified along seepage zones. Carbonate minerals of the Pliocene deposits have stable isotopic compositions that vary with their lithofacies. Tpa limestones deposited in or near areas of spring discharge have low 18O values averaging +16.6. Tld limestones recrystallized in the vadose zone have higher 18O values, averaging +18.1. Carbonate minerals of Tpl are the most 18O-rich, with calcite averaging +20.3 and dolomite +24.3, reflecting evaporative concentration in playas. Much of the dolomite is of replacement origin yet is in approximate isotopic equilibrium with the calcite it replaces. Relatively pure sepiolite has lower 18O values than do Mg smectite clays, which fits with the lower salinity inferred for sepiolite precipitation. Sepiolite and Mg smectite disseminated in limestone fall outside the oxygen isotopic limits of the relatively pure clays, possibly because of diagenetic recrystallization. The Pliocene climate was wetter than that of the present, and springs were more widespread in the Amargosa basin. This dolomite may have formed in water isotopically and chemically about the same as that of the calcite, and dolomitization may have been chiefly a result of the differing crystallization kinetics of calcite and dolomite and of fluctuating water chemistry in a playa environment. The saline, alkaline water of East Playa resulted from evaporation of a mixture of alkali bicarbonate runoff from the volcanic terrane to the north with Ca-Mg bicarbonate water discharged in springs from the Ash Meadows ground-water system, whose recharge area is principally in Paleozoic carbonate rocks to the east and northeast. Seepage along fault zones in East Playa was initially of Ash Meadows type, low in silica, and changed to siliceous water, derived from volcanic rocks, resulting in silicification of carbonate deposits. Caliche breccias along the western margin of the basin record a similar change in the composition of ground-water seepage, which may be attributed to a period of decreased precipitation about 2.5 Ma which reduced recharge from nearby carbonate rocks relative to that from more distant volcanic rocks to the north and northwest.