DEPOSITIONAL MODELS FOR TUFA DEVELOPMENT IN PLEISTOCENE LAKE BONNEVILLE, UTAH

Tufa deposits in the shorezones of late Pleistocene Lake Bonneville are a common occurrence but little is known about the depositional regimes associated with their formation. Tufa of the regressive phase Provo level stillstand, about 14,500 ¹⁴C years ago, is found around the Bonneville basin in three forms:(1) encrusting tufa on bedrock or large boulders (2) carbonate cemented beachrock with sand, pebble, gravel, and boulder size clasts (3) cemented beachrock with an encrusting tufa cap.

Encrusting tufa can reach thicknesses of up to 1 meter and forms in sediment starved localities. Encrusting tufa is observed as a coating on exposed bedrock or boulders located on headlands and outer edges of shorelines. On the other hand, beachrock exists in localities with ample sediment supply and represents a secondary depositional regime that often overlies a primary erosional feature. Beachrock with an encrusting tufa cap represents a sediment rich environment transitioning into a sediment starved environment. Underlying beachrock provides an appropriately firm substrate for encrusting tufa to form.

Carbonate deposition in the shorezone of Lake Bonneville has similarities to marine carbonate reef systems. As in marine environments, these lacustrine tufas are found in areas with low clastic input and high wave energy. Observations at 13 field localities in the Bonneville basin demonstrate that encrusting tufas are typically found in sediment starved areas and in erosional shoreline regimes. In areas such as headlands that are exposed to high wave energies and have low sediment incorporation, tufa production is plentiful during the closed basin episodes of the Provo stillstand. Tufa has an affinity for waters that are turbulent (i.e., fluvial waterfall and cascade tufas), warm, and calcium rich; therefore these lacustrine tufa deposits suggest parallel conditions for Lake Bonneville during the regressive Provo stillstand.