Date of Award

May 2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Christopher A. Scholz

Keywords

Carbonate rock, Lake, Lake Lahontan, Microbialite, Tufa, Winnemucca Dry Lake

Subject Categories

Physical Sciences and Mathematics

Abstract

Carbonate mineral formation in lakes is a consequence of the interactions of lake water chemistry, physical processes at several scales, and biological influences, all of which are greatly affected by individual lake basin attributes such as basin size and physiography, bedrock geology, catchment vegetation, hydroclimate, and resident biota. Because of this high degree of variability in lacustrine depositional environments, lacustrine carbonate rocks exhibit large differences with respect to texture, morphology, and distribution within the lake basin. Lacustrine carbonate rocks are often important sedimentary archives of lake conditions and past terrestrial climates via their chemical composition, particularly with respect to the stable isotopes of carbon and oxygen, and trace elements such as Mg and Sr. However, in order to best interpret these geochemical proxies, we must first understand the geochemical conditions under which deposition occurs. In addition to providing paleoenvironmental histories, lacustrine carbonates are also important hydrocarbon reservoirs. Enormous quantities of oil and gas are stored in such reservoirs in South America, Africa, and China, and exploration continues for new lacustrine plays. Accordingly, understanding the depositional constraints is important for predicting carbonate facies distribution and reservoir properties. This research examines the lacustrine carbonate depositional system of the Pyramid Lake and Winnemucca Dry Lake basins in order to better constrain the geochemical, physical, and biological influences on carbonate deposition in large lakes.

Large deposits of lacustrine carbonate rocks, known as “tufas”, are exposed along the margins of Pyramid Lake and Winnemucca Dry Lake, adjacent lake basins located in the Basin and Range Province of western Nevada (USA). Tufas are in-situ accretionary carbonate deposits that build upward from the lake floor, and the largest of these structures rises more than 100 m above base level in the Pyramid Lake basin. Tufas were deposited mainly during times when lake levels were higher than the present. In the Late Pleistocene and Early Holocene, Pyramid and Winnemucca Lakes, along with five other basins, filled and coalesced to form the pluvial Lake Lahontan, reaching its peak highstand ~15,500 years before present (ybp). Lake levels rose and fell several times over the time period of tufa deposition; Pyramid Lake is presently at a relative lowstand, with a maximum depth of 109 m, while Winnemucca Dry Lake desiccated in the 1930s. These modern low lake levels expose numerous tufa deposits, enabling outcrop and basin-scale examination of tufa depositional patterns.

Tufas in this study were classified based on their mesoscale texture and exposure morphology using a combination of satellite imagery, digital outcrop models created from aerial imagery, and field mapping. Tufas were also examined petrographically, and analyzed for trace element geochemistry, stable isotope geochemistry, and organic geochemistry. Textural variations were found to result primarily from variations in water temperature and microbial influence. Early deposition of tufas occurred in cold, relatively lowstand lake waters, with minimal microbial influence, and tufa volumes were relatively small during this growth stage, with these facies composing <25% of the tufa volume at studied exposures. Rates of deposition increased as lake water temperatures and levels increased (linear growth rate of 0.005 cm/yr to 0.03 cm/yr), and microbial influences came to dominate the depositional process. Photosynthetic microbes were the dominant builders in the tufa depositional system at this time and produced large volumes of tufa across the basin, with these facies composing 60 – 95% of the tufa volume at studied exposures. Morphological variations of tufas at the outcrop scale are largely related to position along the basin margin and their relationship to underlying basin geology and structure. Tufa deposition, in general, is strongly linked to groundwater influx into the basin; the rate of groundwater inflow, along with the temperature, has a strong influence on the tufa morphology, and also affects the geochemical composition of the tufa carbonate. This groundwater overprint must be considered when evaluating tufas as paleoenvironmental indicators, as the trace element and stable isotope geochemistry of groundwater-influenced carbonate does not directly reflect conditions in the overlying lake water column. The results of this study show that understanding the paleotopography and paleohydrology of the basin, which are commonly controlled by basin structure, are the most significant factors for predicting tufa distributions at the basin scale. However, for paleoenvironmental proxy analysis, it is critical to untangle the overprint of groundwater and the degree of microbial influence during deposition.

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