DATING SINTER DEPOSITS IN NORTHERN DIXIE VALLEY, NEVADA- THE PALEOSEISMIC RECORD AND IMPLICATIONS FOR THE DIXIE VALLEY GEOTHERMAL SYSTEM

Abstract

A series of fossil spring deposits are exposed along the Dixie Valley fault just south of the producing geothermal field, in an area now characterized by active fumaroles and steaming ground. These deposits are composed of both travertine and sinter that have trapped pollen and other plant material during their formation. Radiocarbon dates on the organic material indicate that the youngest hot spring sinters range in age from about 3.4 ka to essentially modern. Banded travertine composed of calcite, dolomite, hematite, and barite may represent deposition from a warm spring at about 5040 ka. Older quartz-rich sinters are between 3.4 ka and Lake Dixie (11-12 ka) in age.

The mineralogy and texture of the opaline sinters are consistent with their young ages. One sinter (with a modern 14C age) consists of botryoidal heads of vitreous, opaline silica or "geyserite" that likely formed from actively spouting eruptions of boiling fluids along the fault zone. X-ray diffraction analyses indicate that the sinter is composed of original opal-A that has not undergone the transition to the more crystalline opal-CT or cristobalite (opal-C). Slightly older (2.2 to 3.4 ka) sinters appear to be admixtures of opal-C or opal-CT, microcrystalline quartz, and calcite. These sinters are predominantly thinly laminated to porous opal-CT, and contain abundant plant remains and clasts of even older microcrystalline quartz sinter.