LINKING TEMPERATURE AND MECHANISM OF LACUSTRINE CARBONATE GROWTH: PYRAMID LAKE, NEVADA

Lacustrine carbonates are exposed along cliffs and shorelines of Pyramid Lake and Winnemucca Dry Lake, Nevada. These lakes were part of the larger Pleistocene Lake Lahontan, an intermountain pluvial system. They are considered an analogue to some important lacustrine hydrocarbon reservoirs. Existing research has mostly focused on the growth of carbonate towers/mounds that dot the Pyramid Lake landscape. However, carbonate shoreline attached systems make up the largest volume of carbonate and are understudied. Their volume and potential reservoir connectivity make them of greater interest to hydrocarbon exploration.

Three types of carbonate deposits were characterized through mapping and petrographic analyses: inner tufa towers/mounds, outer thrombolites that surround the mounds, and shoreline tufas. ⁸⁷Sr/⁸⁶Sr, clumped (D₄₇) and stable isotopes of carbon (d¹³C) and oxygen (d¹⁸O) were evaluated for each group and paired with paragenetic interpretations from petrographic analysis to determine environmental conditions and temperatures for the different phases of carbonate precipitation, and evaluate potential mechanisms associated with their growth. Seismic data were interpreted across Pyramid Lake to link tufa growth to mechanisms of formation and depositional history.

Outer thrombolite and shoreline tufas have similar textures and mineralogy that form from a combination of rapid (~1-1.5 mm/yr) biotic and abiotic processes at ambient temperatures (0-25°C). The data are consistent with tufa year around growth, at equilibrium, and within a closed system. Mounds and towers formed where groundwater was discharged along faults and mixed with lake water. Clumped isotope signatures of these carbonates yield non-geologic apparent temperatures, which we interpret to arise via kinetic fractionation from disequilibrium processes (e.g. rapid degassing of CO₂). All ⁸⁷Sr/⁸⁶Sr values are consistent with a hydrologically closed system such that fault fed tufa cannot be distinguished from shoreline tufa. Further, all tufa samples are enriched relative to internal present-day water sources and underlying basalt. We propose that elevated Sr values are sourced externally from the Humboldt River consistent with the majority of shoreline tufas at Pyramid Lake deposited during the Lake Lahontan high stand (~10 ka) when the river was a more dominant source.