Anatomy of siliceous hot springs: examples from Yellowstone National Park, Wyoming, USA

Abstract

Numerous siliceous hot spring systems in the Norris and Lower Geyser Basins of Yellowstone National Park, Wyoming, provide insights into spring geometries, depositional facies, and lithofacies associated with modern hot springs. Analyses of active (Cistern Spring, Octopus Spring, Deerbone Spring, and Spindle Geyser) and inactive (Pork Chop Geyser) siliceous hot springs have facilitated the construction of a facies model for siliceous hot spring deposits at Yellowstone. Yellowstone's siliceous springs tend to group into four broad morphological categories: siliceous spires and cones, domal mounds, terraced mounds, and ponds. Siliceous spires/cones are subconical accumulations up to 5–7 m high and about 2 m in diameter, and are common deposits in Yellowstone Lake. Domal mounds are characterized by siliceous precipitates with a broad lens or shield geometry (2–3 m in vertical relief), discharge channels, and an areal accumulation of approximately 150 m 2. In contrast, terraced mounds have a stair-step morphology, a substantial pool (∼8–10 m in diameter), “shrubby” precipitates, and occupy areas of ∼2000 m 2. Siliceous ponds are variable in size, have little outflow, and exhibit low amounts of silica precipitation. Of these morphological varieties, domal mounds and terraced mounds are thought to have the best long-term preservation potential. The four spring morphotypes are composed of up to eight cumulative hot spring depositional facies: (1) vent (>95 °C), (2) proximal vent (<95 °C), (3) pool (∼80–90 °C), (4) pool margin (∼80 °C), (5) pool eddy (<80 °C), (6) discharge channel/flowpath (<80 °C to ambient), (7) debris apron (variable temperatures), and (8) geyser (variable temperatures). This facies model based on numerous springs facilitates our ability to interpret ancient hot spring deposits and to infer depositional conditions. Precipitation of siliceous sinter is the result of abiotic and biotic processes. Abiotic precipitational processes are dominant in the vent area, whereas biotic influences on the precipitate fabric become progressively more important downflow.