129-39 The “Petrified Waterfall” at Floracliff Nature Sanctuary: New Insights on Tufa Development in Central Kentucky.

Abstract:

Floracliff Nature Sanctuary (FNS) was founded in 1987 to protect an iconic landscape along the Kentucky River and provide community space for environmental education in the Inner Bluegrass region of central Kentucky. One of the most important geological features of FNS is an 18 m high tufa deposit on Elk Lick Creek, a tributary of the Kentucky River fed by two springs whose chemistries are dominated by Ca2+, HCO3−, and SO4−. Tufas are generally classified as terrestrial carbonates that form from cold spring waters and interactions with plant life. In the 1950s, several Kentucky geologists noted the tufa on their maps and reported a “petrified waterfall” of freshwater carbonate. Apart from these observations, little is known about the geo-environmental processes that influence the development of this tufa or the timeline of its formation. We studied the tufa on Elk Lick Creek in 2026 to address these unknowns, using an approach that combined field observations and direct sampling. At Elk Lick Creek, turbulent spring waters flowing down a series of waterfalls drive CO2 degassing and increase pH, allowing carbonate to precipitate. Curtains of bryophyte mosses act as nucleation sites for carbonate encrustations, which progressively grow downward, coalesce, and lithify. This set of processes occurs at different stages at the study site, such that we observe the coexistence of young and weakly calcified bryophyte curtains along with much older and thicker tufa at the base of Elk Lick Falls with surficial plant growth. A core extracted from the base of the falls was analyzed using X-ray computed tomography, X-ray fluorescence, X-ray diffraction, and conventional petrography. The data revealed a highly porous internal structure dominated by calcite, with trace amounts of accessory minerals such as quartz and illite. Tufa calcite exhibits a microcrystalline texture with clotted micrite fabric in thin section, and large pores are filled by bladed calcite cement; microscopic features suggest multiple cycles of calcite crystallization and dissolution probably characterized the formation of the tufa. Our results indicate that this tufa is a composite feature that has developed over several stages, with the oldest material potentially at the base of the waterfall near the bedrock contact. Ongoing research aims to resolve the age of this tufa and build an inventory of central Kentucky tufas for geoheritage.

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