Lake Bonneville
Lake Bonneville was the largest Late Pleistocene paleolake in the Great Basin of western North America. The Western Interior Seaway preceded Lake Bonneville. 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. 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. The Stansbury and Bonneville shorelines formed during the transgressive phase of Lake Bonneville; the Provo shoreline formed during the overflowing  phase. 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 did form 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. Changes in lake level were the result of changes in water balance caused by climate change (a simplified version of the water-balance equation is inputs equal outputs plus-or-minus storage changes). Storage changes are equal to volume changes, and changes in  volume are correlated with changes in lake level. When inputs (i.e., precipitation; runoff in rivers) were greater than outputs (i.e., 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, so that even if all the mountain glaciers in the basin  melted at once and the water flowed into the lake (this didn't happen—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. While Lake Bonneville existed the patterns of wave- and current-forming winds were not significantly affected by the Laurentide and Cordilleran ice sheets in northern North America. The name “Bonneville”. 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. 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. Geologic history. Lake Bonneville began to rise from elevations similar to those of modern Great Salt Lake about 30,000 years ago. During its transgressive phase in the closed basin (an endorheic basin), lake level oscillated because of changes in climate but the lake gradually rose until about 18,000 years ago when it reached its highest elevation, marked by the Bonneville shoreline. At that level the lake had risen to the lowest point on its basin rim and had begun to overflow  into the Snake River drainage 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. Groundwater sapping on the north slope of the Marsh Creek alluvial fan, which began long before the lake had reached its highest level, added to the instability and ultimate collapse of the fan -dam. The Bonneville flood probably lasted less than a year, during which time almost of water flowed out of the lake basin with a maximum discharge of about . Downcutting during the flood through the Marsh Creek alluvial-fan deposits and into the underlying Neogene sand, mud, and landslide debris, caused lake level to drop about . River flow from the lake across the Red Rock Pass threshold and out of the lake basin continued non-catastrophically for about 3000 years after the flood ended; the Provo shoreline formed during this overflowing phase. The Provo shoreline is distinguished from other shorelines of  Lake Bonneville by its topographic position, strong development, and thick accumulations of tufa. At the end of the overflowing phase, about 15,000 years ago, climate change and a shift to a negative water balance (more water evaporated off the surface of the lake than entered by rivers or direct precipitation) caused the lake to return to its closed-basin status as it declined to lower levels during the regressive phase. By 13,000 years ago the lake had fallen to an elevation similar to the average elevation of modern Great Salt Lake. During the regressive phase lake level declined approximately in  about 2000 years because of a change to warmer and drier climate ( is roughly 2/3 of the maximum depth of Lake Bonneville). Although Lake Bonneville and the Great Salt Lake are collectively one lake system, the name “Lake Bonneville” is applied to the lake during the period from 30,000 to 13,000 years ago, and the name “Great Salt Lake” since 13,000 years ago. 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 time. The conditions experienced in the basin today are typical of over 90% of the past 800,000 years: a dry desert basin with a few scattered low-elevation lakes, the largest of which (Great Salt Lake) was hypersaline. For most of the time between the end of the youngest of the deep pre-Bonneville lakes (the Little Valley lake cycle, about 150,000 years ago) and the initial rise of Lake Bonneville about 30,000 years ago, the lake would have resembled modern Great Salt Lake in surface area and depth. A short episode of slightly  higher lake levels during the Cutler Dam lake cycle occurred about 60,000 years ago; at this time a moderate-sized lake rose above the level of Great Salt Lake, but not as high as Lake Bonneville. In his monograph on Lake Bonneville, G.K. Gilbert called the offshore deposits of Lake Bonneville the “White Marl.” Although the name “White Marl” has not been used by the geologic community in a formal sense, the informal term “white marl” (or “Bonneville marl”) is frequently employed. 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. Most of this calcium carbonate is in the form of the mineral calcite, but aragonite is common in Bonneville marl in the Sevier basin and in the lower part of the Bonneville marl stratigraphic section in the main body. 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 Bonneville flood had catastrophic effects along the Snake River in what is now Idaho, but the influence of the flood can also be detected within the lake basin where a distinctive layer of sediment was deposited. The Bonneville flood bed can be identified in many surface exposures below the Provo shoreline and in sediment cores. The flood bed is characterized by an abrupt contact at its base between massive marl, which was deposited in the deepest water of Lake Bonneville, and finely laminated or ripple-laminated sandy marl, which was deposited by bottom currents during the flood. In  places the Bonneville flood bed is composed of reworked ostracode shells. The contact at the top of the flood bed is transitional to massive marl that was deposited in the lake floor during Provo time. The flood bed is best developed and most obvious in straits between partially submerged mountain ranges or in places where bottom currents were strong as lake water flowed toward its outlet at Red Rock Pass. Because the Bonneville flood bed was deposited in less than a year, it is useful as a well-dated (~18,000 years ago) stratigraphic marker within the Bonneville deposits. Previous publications , regarded the “Gilbert shoreline” as one of the prominent shorelines in the Bonneville basin, but this interpretation has been revised. The “Gilbert shoreline” consists of a line on a map that connects lacustrine shoreline features, such as barrier beaches, but there is no evidence that all the barriers formed at the same time. It is now clear that some of those barrier beaches are transgressive-phase Bonneville in age and some are regressive-phase Bonneville in age. The Gilbert episode was a rise of Great Salt Lake about higher than modern average levels, which culminated 11,600 years ago. But a  mappable shoreline of the Gilbert episode has not been recognized. Isostasy. The shorelines of Lake Bonneville have been warped by isostatic processes, as was recognized by Gilbert and extensively studied since Gilbert's day. 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. As an example of isostatic deformation of the shorelines, the elevation of the Bonneville shoreline near Salt Lake City is , but on Antelope Island in the Great Salt Lake, the elevation of the same shoreline is . Fossils, volcanic ashes, etc.. In addition to the abundant geological features produced by Lake Bonneville, such as shorelines and sediments, fossilized fish bones and scales reveal information about the physical and chemical characteristics of the paleolake. Pollen from plants that lived in the Bonneville basin is abundant in Bonneville marl. Invertebrate fossils in Lake Bonneville  deposits include mollusks and ostracodes, 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. Climate Oscillations, the Wasatch Fault and a Bonneville Flood theory. Lake Bonneville rose to its high stand during the final 20,000 years of the Pleistocene epoch, corresponding with the glacial maximum of the Laurentide and Cordillera Ice Sheets. This colder and wetter period in  conjunction with lake-effect snows from the expanding Lake Bonneville resulted in glaciers in the Wasatch.   At its maximum the glacier in Little Cottonwood canyon extended to the mouth of the canyon and was calving ice bergs into the lake. Lake Bonneville shoreline and strata evidence suggests that the lake experienced several dramatic short-term level oscillations during its life.   A long-standing and well-accepted theory attributes these level fluctuations to climate oscillations. The Wasatch Fault runs along the eastern edge of the Lake Bonneville basin and was submerged when the lake was at its high stand (see figure ). The fault naturally divides into segments and since the time of Lake Bonneville these segments have appeared to have slipped (failed) independently (e.g. an earthquake in the Salt Lake valley has not tended to occur at the same time as one in Weber or in Provo). A 2020 paper presented at the Geological Society of America Annual Meeting reported evidence of massive earthquake-induced surging in Lake Bonneville at the time of the Bonneville Flood.   The proposal in that presentation was that a multi-segment failure on the Wasatch Fault resulted in lake surging within the Bonneville Basin and, as  that surge moved up into the Cache Valley towards Red Rock Pass and the Zenda Threshold, the topography concentrated the energy into a wall of water which overtopped and broke the natural dam resulting in the Bonneville Flood. In 2021 additional evidence was presented in support of that theory.  In that same body of work, arguments were presented that the Lake Bonneville level oscillations previously attributed to climate episodes were instead due to multi-segment earthquake events on the Wasatch Fault which resulted in surging in the lake.  The known and suspected earthquake-induced lake level oscillations are:  the Hansel  Valley event, the Stansbury Oscillation, U1, U2, the Keg Mountain Oscillation, and the Provo Level Oscillation. (see figure). Shoreline evidence in the western deserts and a newly identified surge-debris boulder deposit in Little Cottonwood Canyon in the Wasatch Mountains (see photo) indicate that the lake surging at the eastern and western extremes was over 42 m (~140’) high for at least two of these events, including the one that corresponded with the Bonneville Flood. A more recent level fluctuation corresponding with the Gilbert shoreline was during the Younger Dryas and was a likely result of that climate event rather  than earthquake-induced surging. In the 2021 study, a correlation was hypothesized between basalt ash eruptions which formed deposits in the lake sediments and the earthquakes which resulted in at least three of these lake surging events (Stansbury, Keg Mountain and Provo). The outstanding question from these new findings is: were the multi-segment earthquake events a product of crust depression (isostasy) due to the weight of Lake Bonneville, or is there a longer-term cycle of multi-segment events on the Wasatch Fault which might place the Wasatch front communities at a higher risk today?