tufa - display specimen of thrombolite tufa from the paleoshoreline of ice age Lake Lahontan

Tufa is a calcium carbonate deposit that forms around the outlet of a hot spring that emerges below the surface of a body of water. These examples were formed during the Pleistocene high-stand of Lake Lahontan, which filled a connected grouping of basins in Nevada east of Reno. It was similar to ice age Lake Bonneville in Utah, and now is greatly reduced in size. Its peak was approximately 12,700 years ago and its largest remnant is Pyramid Lake, currently around 500 feet deep. The water depth there during the Pleistocene was 900 feet, and ancient shorelines can be seen far above the current lake suface, as strand lines on the surrounding mountains. 

With the climate drying at the end of the Pleistocene, endorheic lakes in the Basin and Range Province, which are lakes with no outlet to the the sea, gradually evaporated, leaving salt-flat playas in the bottoms of the basins. 

This tufa was formed where springs emerged from a limestone block below Lake Lahontan's surface and near the lake shore. Whether or not tufa is purely a chemical precipitation, where calcium-rich spring water meets carbonate-rich lake water to precipitate calcium carbonate, or where there is also a biological component as well, is a topic still under discussion. Both are likely.

This tufa was collected in Churchill County, Nevada, roughly 15 miles south of Fallon and just west of US Highway 95. It is an example of thrombolite tufa, which forms as irregular mesoclots of carbonate. This specimen shows the characteristic popcorn-like nodular exterior of this tufa type.

 The field photos show textures of the tufa draped over limestone.

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