Lake Bonneville

Lake Bonneville was the largest Late Pleistocene paleolake in the Great Basin of western North America. It was a pluvial lake that formed in response to an increase in precipitation and a decrease in evaporation as a result of cooler temperatures. The lake covered much of what is now western Utah and at its highest level extended into present-day Idaho and Nevada. Many other hydrographically closed basins in the Great Basin contained expanded lakes during the Late Pleistocene, including Lake Lahontan in northwestern Nevada.

Geologic description

thumb|Lake Bonneville and other Late Pleistocene paleolakes in the Great Basin during the [[Last Glacial Maximum|last major global glaciation. Lake Bonneville is shown in the context of western North America and the southern margins of the Laurentide and Cordilleran ice sheets. Note that some of the red arrows show floods in eastern Washington (from Lake Missoula) that were unrelated to the Bonneville flood.|alt=]]

Shorelines of Lake Bonneville are visible above Salt Lake City along the western front of the Wasatch Mountains and on other mountains throughout the Bonneville basin. These shorelines appear as shelves or benches that protrude from the mountainside above the valley floor, are visible on the ground from long distances and on satellite images, and have both depositional and erosional segments along their lengths. Three shorelines of Lake Bonneville that can be traced throughout the basin have been given names: Stansbury, Bonneville, and Provo. Numerous other unnamed shorelines, which cannot be mapped everywhere in the basin, some of which formed during the transgressive phase and some during the regressive phase, are also present on piedmont slopes and alluvial fans. At its maximum, when Lake Bonneville was more than deep and almost in surface area, it covered almost as much area as modern Lake Michigan, although its shoreline was more complex with many islands and peninsulas. Great Salt Lake, Utah Lake, and Sevier Lake are the largest post-Bonneville lakes in the Bonneville basin.

Causes of lake expansion and contraction

Lake Bonneville was not a proglacial lake although it formed between about 30,000 and 13,000 years ago, when glaciers at many places on Earth were expanded relative to today during the last major glaciation. For most of its existence (that is, during the transgressive plus regressive phases) Lake Bonneville had no river outlet and occupied a hydrographically closed basin. Storage changes are equal to volume changes, and changes in volume are correlated with changes in lake level. When inputs (e.g., precipitation; runoff in rivers) were greater than outputs (e.g., evaporation from the lake surface; evapotranspiration in the basin), lake level rose, and when outputs were greater than inputs, lake level fell. Changes in global atmospheric circulation led to changes in the water budget of Lake Bonneville and other lakes in the Great Basin of western North America. Mountain glaciers in the Bonneville drainage basin stored less than 5% of the water that Lake Bonneville held at its maximum and so even if all of the mountain glaciers in the basin melted at once and the water flowed into the lake (that did not happen since it took thousands of years for the mountain glaciers to melt, and Lake Bonneville was falling by that time), it would have had little effect on lake level. Lake Bonneville had no river connection with the huge North American ice sheets.

The name "Bonneville" and its discovery

Lake Bonneville was named by the geologist G.K. Gilbert after Benjamin Louis Eulalie de Bonneville (1796–1878), a French-born officer in the United States Army who was also a fur trapper and explorer in the American West. Bonneville's adventures were popularized by Washington Irving in the 1800s, but Captain Bonneville probably never saw Great Salt Lake or the Great Basin. G.K. Gilbert was one of the greatest geologists of the 19th Century, and his monumental work on Lake Bonneville, published in 1890, set the stage for scientific research on the paleolake that continues today. Gilbert was the first person to describe the major features of Lake Bonneville, however, many other early European and American explorers in the region recognized the shorelines of the ancient lake, such as Captain John C. Frémont in 1843 Although a general description and understanding of Lake Bonneville has been established by the work of many people, details of the paleolake, including its history and connections to global environmental systems, will be pursued for many years to come.

thumb|Map of Pleistocene lakes in the Great Basin of western North America.

[[File:Chronology figure.png|alt=Chronology of Lake Bonneville. "Calibrated ages" are approximate calendar years before present (present regarded as A.D. 1950). Elevations are adjusted for differential isostatic rebound in the basin.[3]|thumb|Chronology of Lake Bonneville. "Calibrated ages" are approximate calendar years before present (A.D. 1950). Elevations are adjusted for differential isostatic rebound in the basin.]]

Geologic history

Lake Bonneville began to rise from elevations similar to those of modern Great Salt Lake about 30,000 years ago. During its early transgressive phase the lake fluctuated within a few 10s of meters of the level of modern Great Salt Lake, but after about 24,000 years ago it began a rapid rise to higher elevations, During its transgressive phase in the closed basin (an endorheic basin), lake level oscillated because of changes in climate. At its highest level the lake had risen to the lowest point on its basin rim and had begun to overflow into the Snake River drainage basin near Red Rock Pass in what is now southeastern Idaho. The overflow, which would have begun as a trickle across the dam formed by the Marsh Creek alluvial fan, quickly evolved into a tremendous flood, the Bonneville flood, which charged down the Marsh Creek valley to the Portneuf River, into the Snake River and then into the Columbia River and Pacific Ocean. River flow from the lake across the Red Rock Pass threshold and out of the lake basin continued non-catastrophically for at least 1000 years after the flood ended; the Provo shoreline formed during this overflowing phase. climate change and a shift to a negative water balance (more water evaporated from the surface of the lake than entered the lake by rivers, direct precipitation, or groundwater) caused the lake to return to its closed-basin status as it declined to lower levels during the regressive phase.

Lake Bonneville was anomalous in the long-term history of the basin. As the largest of four deep lakes in the basin during the past 800,000 years, Lake Bonneville plus the other three deep Pleistocene lakes, persisted for less than 10% of the period. at this time a moderate-sized lake rose above the level of Great Salt Lake, but not as high as Lake Bonneville.

thumb|Bonneville flood bed in Lake Bonneville marl at an exposure in northern Utah. The base of the flood bed is at the level of the shovel blade. For scale, the shovel handle is about in length.

In his monograph on Lake Bonneville, G.K. Gilbert called the offshore deposits of Lake Bonneville the "White Marl". The Bonneville marl at locations far from sources of clastic sediment (gravel, sand, and silt), such as river deltas or active wave zones, is dominated by clay-sized particles of calcium carbonate that precipitated chemically from the lake water. Aragonite is the dominant carbonate mineral in sediments of post-Bonneville Great Salt Lake. Dropstones, probably mostly derived from shore ice, but possibly also from floating root balls, are common in the marl, and consist of granule- to boulder-sized clasts. the "Gilbert shoreline" was regarded as one of the prominent shorelines in the Bonneville basin, but this interpretation has been revised. The Gilbert episode (now referred to as the Currey cycle of Great Salt Lake Earth's crust subsided beneath the weight of the water while the lake existed, but when the lake evaporated and the water load was considerably reduced, the crust beneath the lake basin rebounded. As a result, the elevation of the Bonneville shoreline is higher in the Lakeside Mountains, elevation , west of the Great Salt Lake near the center of the Lake Bonneville water load, than at Red Rock Pass, , where the lake was very shallow. Pollen from plants that lived in the Bonneville basin is abundant in Bonneville marl. and bones of extinct mammals are found in Pleistocene deposits in the Bonneville basin. Volcanic ashes in sediments of Lake Bonneville help with correlations and aid in deciphering lake history. Lake Bonneville shorelines, and those of other paleolakes on Earth, are good analogs for shorelines on other planets, such as Mars.

References

External links

Brigham Young University - Geology - maps of Lake Bonneville
Utah Geological Survey- maps of Lake Bonneville, and additional information on Lake Bonneville and Great Salt Lake