ANALYSIS OF UNUSUAL TUFA DEPOSITS IN THE SEVIER BASIN, UTAH

Most tufas can be easily identified and categorized in terms of textural characteristics and environment of formation. However, some unusual tufa deposits found on the western edge of the Sevier Basin in Utah were formed in association with a Lake Bonneville shoreline. These deposits outcrop at an elevation of approximately 1480 m, and are interpreted to have formed at the U2 shoreline of Lake Bonneville. They have a stratiform morphology, and were likely formed by sheet-like flow of groundwater into the lake. Four tufa facies are identified, all of which represent shoreline conditions, and all of which are distinct from the more mound-like tufa found in the nearby Tule Valley. A measured section shows considerable fluctuation of shoreline environmental conditions during the time of deposition. XRD patterns show that the primary mineral phase of these tufa deposits is dolomite, and textural evidence from SEM images and acetate peels suggests that the dolomite matrix may be primary. The stratiform morphology of the tufa and the possibility of primary dolomite precipitation suggest that the environment of formation was quite unique. It is therefore important to gain an understanding of the processes leading to the formation of the Sevier Basin tufa and the environmental conditions under which it was formed in order to understand more clearly the processes of tufa deposition in general, and to help characterize the paleoenvironment of Lake Bonneville at the time of deposition.