Charles G. Oviatt

Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years

Abstract During the past 35,000 years, Lake Bonneville, Lake Russell, and Lake Searles underwent a major period of lake-level change. The lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15,000 and 13,500 yr B.P. The rise of Lake Lahontan was gradual but not continuous, in part because of topographic constraints (intrabasin spill). Lake Lahontan also had an oscillation in lake level at 15,500 yr B.P. Radiocarbon-age estimations for materials that were deposited in the lake basins indicate that Lake Bonneville rose more or less gradually from 32,000 yr B.P., and had major oscillations in level between 23,000 and 21,000 yr B.P. and between 15,250 and 14,500 yr B.P. Lake Russell and Lake Searles had several major oscillations in lake level between 35,000 and 14,000 yr B.P. The timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states. All four lakes may have had nearly synchronous recessions between about 14,000 and 13,500 yr B.P. After the recessions, the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11,500 and 10,000 yr B.P. These data provide circumstantial evidence that the Younger Dryas Event affected climate on at least a hemispheric scale. During the Holocene, the four lakes remained at low levels, and small oscillations in lake level occurred. An important aspect of the lake-level data is the accompanying expansion of lake-surface area at the time of the last highstand. Lake Bonneville and Lake Lahontan had surface areas about 10 times larger than their mean-historical reconstructed areas whereas Lake Russell and Lake Searles had surface areas about 5 times larger than their mean-historical reconstructed areas. Differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream, or they may have resulted from lake/atmosphere feedback processes.

Radiocarbon chronology of Lake Bonneville, Eastern Great Basin, USA

Abstract Lake Bonneville occupied a series of connected topographically-closed structural basins in the eastern Great Basin from about 30 ka to 12 ka. The following synthesis of Lake Bonneville history is based on a critical evaluation of the stratigraphic and geomorphic contexts of 83 radiocarbon ages of a variety of samples, including wood, charcoal, dispersed organic matter, mollusk shells, and tufa. The lake began to rise from levels close to average Holocene levels after about 28 ka. By 22 ka it had transgressed approximately 100 m; between 22 and 20 ka it regressed about 45 m in the Stansbury oscillation and the Stansbury shoreline was formed. Transgression after 20 ka proceeded in two phases—a rapid phase from 20 to 18 ka, and a slower phase from 18 to 15 ka. The lake overflowed intermittently at its highest level (the Bonneville shoreline) from about 15 to 14.5 ka, then catastrophically dropped 100 m during the Bonneville Flood to the level of the Provo shoreline, which it occupied until about 14 ka. Subsequent closed-basin regression was rapid and complete by 12 ka, and was followed by a modest transgression to form the Gilbert shoreline between 10.9 and 10.3 ka. The Lake Bonneville record is an accurate proxy of the changing water balance in the Bonneville basin during the late Pleistocene, although the nature of the climatic changes during this period are still uncertain.

Lake Bonneville fluctuations and global climate change

Lake Bonneville, the largest late Pleistocene closed-basin lake in the North American Great Basin, fluctuated widely in response to changes in climate. The geochemistry and mineralogy of endogenic calcium carbonate deposited in deep water, and stratigraphic studies of shore-zone deposits, provide evidence of millennial-scale lake-level fluctuations that had amplitudes of about 50 m between 30 and 10 ka. Falling-lake events occurred at 21, 18.5–19, 17.5, 16–15.5, 14–13, and 10 ka (radiocarbon years) synchronously with the terminations of Heinrich events H1 and H2 and other smaller scale iceberg-rafting events (a, b, c, and Younger Dryas) in the North Atlantic Ocean. The Lake Bonneville results thus support other climate records that suggest that late Pleistocene millennial-scale climate change was global in extent. The size and shape of the Northern Hemisphere ice sheets, which determined the mean positions of storm tracks, may have been the primary control on late Pleistocene water budgets of Great Basin lakes.

Late Pleistocene and early Holocene rivers and wetlands in the Bonneville basin of western North America

Abstract Field investigations at Dugway Proving Ground in western Utah have produced new data on the chronology and human occupation of late Pleistocene and early Holocene lakes, rivers, and wetlands in the Lake Bonneville basin. We have classified paleo-river channels of these ages as “gravel channels” and “sand channels.” Gravel channels are straight to curved, digitate, and have abrupt bulbous ends. They are composed of fine gravel and coarse sand, and are topographically inverted (i.e., they stand higher than the surrounding mudflats). Sand channels are younger and sand filled, with well-developed meander-scroll morphology that is truncated by deflated mudflat surfaces. Gravel channels were formed by a river that originated as overflow from the Sevier basin along the Old River Bed during the late regressive phases of Lake Bonneville (after 12,500 and prior to 11,000 14C yr B.P.). Dated samples from sand channels and associated fluvial overbank and wetland deposits range in age from 11,000 to 8800 14C yr B.P., and are probably related to continued Sevier-basin overflow and to groundwater discharge. Paleoarchaic foragers occupied numerous sites on gravel-channel landforms and adjacent to sand channels in the extensive early Holocene wetland habitats. Reworking of tools and limited toolstone diversity is consistent with theoretical models suggesting Paleoarchaic foragers in the Old River Bed delta were less mobile than elsewhere in the Great Basin.

The 87Sr/86Sr ratios of lacustrine carbonates and lake-level history of the Bonneville paleolake system

Lakes in the Bonneville basin have fluctuated dramatically in response to changes in rainfall, temperature, and drainage diversion during the Quaternary. We analyzed tufas and shells from shorelines of known ages in orderto develop a relation between 8 7 Sr/ 8 6 Sr ratio of carbonates and lake level, which then can be used as a basis for constraining lake level from similar analyses on carbonates in cores. Carbonates from the late Quaternary shorelines yield the following average 8 7 Sr/ 8 6 Sr ratios: 0.71173 for the Stansbury shoreline (22-20 1 4 C ka; 1350 m), 0.71153 for the Bonneville shoreline (15.5-14.5 1 4 C ka; 1550 m), 0.71175 for the Provo shoreline (14.4-14.0 1 4 C ka; 1450 m), 0.71244 for the Gilbert shoreline (∼10.3-10.9 1 4 C ka; 1300 m), and 0.71469 for the modern Great Salt Lake (1280 m). These analyses show that the 8 7 Sr/ 8 6 Sr ratio of lacustrine carbonates changes substantially at low- to mid-lake levels but is invariant at mid- to high-lake levels. Sr-isotope mixing models of Great Salt Lake and the Bonneville paleolake system were constructed to explain these variations in 8 7 Sr/ 8 6 Sr ratios with change in lake level. Our model of the Bonneville system produced a 8 7 Sr/ 8 6 Sr ratio of 0.71193, very close to the observed ratios from high-shoreline tufa and shell. The model verifies that the integration of the southern Sevier and Beaver rivers with the Bear and others rivers in the north is responsible for the lower 8 7 Sr/ 8 6 Sr ratios in Lake Bonneville compared to the modern Great Salt Lake. We also modeled the 8 7 Sr/ 8 6 Sr ratio of Lake Bonneville with the upper Bear River diverted into the Snake River basin and obtained an 8 7 Sr/ 8 6 Sr ratio of 0.71414. Coincidentally, this ratio is close to the observed ratio for Great Salt Lake of 0.71469. This means that 8 7 Sr/ 8 6 Sr ratios of >0.714 for carbonate can be produced by climatically induced low-lake conditions or by diversion of the upper Bear River out of the Bonneville basin. This model result also demonstrates that the upper Bear River had to be flowing into the Bonneville basin during highstands of other late Quaternary lake cycles: carbonates from the Little Valley (130-160 ka) and Cutler Dam (59 ′ 5 ka) lake cycles returned 8 7 Sr/ 8 6 Sr ratios of 0.71166 and 0.71207, respectively, and are too low to be produced by a lake without the upper Bear River input.

Age and paleoclimatic significance of the Stansbury shoreline of Lake Bonneville, Northeastern Great Basin

Abstract The Stansbury shoreline, one of the conspicuous late Pleistocene shorelines of Lake Bonneville, consists of tufa-cemented gravel and barrier beaches within a vertical zone of about 45 m, the lower limit of which is 70 m above the modern average level of Great Salt Lake. Stratigraphic evidence at a number of localities, including new evidence from Crater Island on the west side of the Great Salt Lake Desert, shows that the Stansbury shoreline formed during the transgressive phase of late Pleistocene Lake bonneville (sometime between about 22,000 and 20,000 yr B.P.). Tufa-cemented gravel and barrier beaches were deposited in the Stansbury shorezone during one or more fluctuations in water level with a maximum total amplitude of 45 m. We refer to the fluctuations as the Stansbury oscillation. The Stansbury oscillation cannot have been caused by basin-hypsometric factors, such as stabilization of lake level at an external overflow threshold or by expansion into an interior subbasin, or by changes in drainage basin size. Therefore, changes in climate must have caused the lake level to reverse its general rise, to drop about 45 m in altitude (reducing its surface area by about 18%, 5000 km2), and later to resume its rise. If the sizes of Great Basin lakes are controlled by the mean position of storm tracks and the jetstream, which as recently postulated may be controlled by the size of the continental ice sheets, the Stansbury oscillation may have been caused by a shift in the jetstream during a major interstade of the Laurentide ice sheet.

Late Pleistocene and Holocene lake fluctuations in the Sevier Lake basin, Utah, USA

Sevier Lake is the modern lake in the topographically closed Sevier Lake basin, and is fed primarily by the Sevier River. During the last 12 000 years, the Beaver River also was a major tributary to the lake. Lake Bonneville occupied the Sevier Desert until late in its regressive phase when it dropped to the Old River Bed threshold, which is the low point on the drainage divide between the Sevier Lake basin and the Great Salt Lake basin. Lake Gunnison, a shallow freshwater lake at 1390 m in the Sevier Desert, overflowed continuously from about 12 000 to 10 000 yr B.P., into the saline lake in the Great Salt Lake basin, which continued to contract. This contrast in hydrologic histories between the two basins may have been caused by a northward shift of monsoon circulation into the Sevier Lake basin, but not as far north as the Great Salt Lake basin. Increased summer precipitation and cloudiness could have kept the Sevier Lake basin relatively wet.By shortly after 10 000 yr B.P. Lake Gunnison had stopped overflowing and the Sevier and Beaver Rivers had begun depositing fine-grained alluvium across the lake bed. Sevier Lake remained at an altitude below 1381 m during the early and middle Holocene. Between 3000 and 2000 yr B.P. the lake expanded slightly to an altitude of about 1382.3 m. A second expansion, probably in the last 500 years, culminated at about 1379.8 m. In the mid 1800s the lake had a surface altitude of 1379.5 m. Sevier Lake was essentially dry (1376 m) from 1880 until 1982. In 1984–1985 the lake expanded to a 20th-century high of 1378.9 m in response to abnormally high snow-melt runoff in the Sevier River. The late Holocene high stands of Sevier Lake were most likely related to increased precipitation derived from westerly air masses.

Sequence stratigraphy of lacustrine deposits: A Quaternary example from the Bonneville basin, Utah

The late Quaternary lacustrine sedimentary record in the Bonneville lake basin in the eastern Great Basin provides an excellent example of sequence stratigraphy. Two sequences, referred to as the Little Valley and Bonneville Alloformations, are exposed in the bluffs of the Sevier River where it has entrenched its Pleistocene delta between Leamington and Delta, Utah. Both alloformations contain offshore marl units and fine-grained deltaic or underflow-fan deposits. They can be identified and mapped by tracing the unconformity separating them and employing a number of geochronometric tools, including amino acid epimerization in fossil gastropods, radiocarbon and thorium-230 ages, and basaltic tephrochronology. Thin transgressive sand of the Little Valley Alloformation is overlain by deeper-water marl and down-lapping regressive-phase deltaic silt. The Bonneville Alloformation lies unconformably above the Little Valley deposits. Fine-grained deltaic sediments deposited during the transgressive phase of Lake Bonneville fill the entrenched Sevier River valley that was eroded subsequent to the Little Valley lake cycle. Marl deposited during the deep-water phase is overlain by down-lapping deposits of the regressive phase below the Provo shoreline but is the uppermost unit in the altitudinal range where the lake was lowered catastrophically during the Bonneville Flood. The sequence stratigraphic interpretation leads to the conclusion that the Sevier River delta as a whole is probably made up of a number of sediment sequences, each composed of several facies. Recognition of this complexity could be important in potential applications of the stratigraphic model.

Variation in the composition of Lake Bonneville marl: a potential key to lake-level fluctuations and paleoclimate

Lake Bonneville marl provides a stratigraphic record of lake history preserved in its carbonate minerals and stable isotopes. We have analyzed the marl in shallow cores taken at three localities in the Bonneville basin. Chronology for the cores is provided by dated volcanic ashes, ostracode biostratigraphy, and a distinctive lithologic unit believed to have been deposited during and immediately after the Bonneville Flood.A core taken at Monument Point at the north shore of Great Salt Lake encompasses virtually the entire Bonneville lake cycle, including the 26.5 ka ‘Thiokol’ basaltic ash at the base and deposits representing the overflowing stage at the Provo shoreline at the top of the core. Two cores from the Old River Bed area near the threshold between the Sevier basin and the Great Salt Lake basin (the main body of Lake Bonneville) represent deposition from the end of the Stansbury oscillation (≈ 20 ka) to post-Provo time (≈ 13 ka), and one core from near Sunstone Knoll in the Sevier basin provides a nearly complete record of the period when Lake Bonneville flooded the Sevier basin (≈20–13 ka).In all cores, percent calcium carbonate, the aragonite to calcite ratio, and percent sand were measured at approximately 2-cm intervals, and δ18O and δ13C were determined in one core from the Old River Bed area. The transgressive period from about 20 ka to 15 ka is represented in all cores, but the general trends and the details of the records are different, probably as a result of water chemistry and water balance differences between the main body and the Sevier basin because they were fed by different rivers and had different hypsometries. The Old River Bed marl sections are intermediate in position and composition between the Monument Point and Sunstone Knoll sections. Variations in marl composition at the Old River Bed, which are correlated with lake-level changes, were probably caused by changes in the relative proportions of water from the two basins, which were caused by shifts in water balance in the lake.

Evidence for a shallow early or middle Wisconsin-age lake in the Bonneville Basin, Utah

Abstract Relatively complete stratigraphic records of the Bonneville cycle and of at least one and probably two earlier lacustrine are exposed along the Bear River below Cutler Dam in northern Utah between altitudes of 1290 and 1365 m. In most exposures the unconformity between the Bonneville Alloformation and the underlying unit, herein named the Cutler Dam Alloformation, is marked by slight erosional relief and by a weakly to moderately developed buried soil, herein named the Fielding Geosol. In truncated profiles, the Fielding Geosol reaches a maximum of stage II carbonate morphology. Wood from near the base of the Cutler Dam Alloformation yielded a 14 C date of >36,000 yr B.P. (Beta-9845). Alloisoleucine/isoleucine (aIle/Ile) ratios of Sphaerium shells from the Cutler Dam beds average 0.15 ± 0.01 in the total hydrolysate, which is significantly greater than the average for Sphaerium shells of Bonneville age elsewhere in the basin. Therefore, the Cutler Dam Alloformation is older than 36,000 yr B.P., but much younger than deposits of the Little Valley lake cycle (140,000 yr B.P.?) which bear shells having significantly higher aIle/Ile ratios. The Cutler Dam Alloformation along the Bear River may be broadly correlative with marine oxygen-isotope stages 4 or 3. Fine-grained, fossiliferous, marginal-lacustrine facies of the Cutler Dam Alloformation are exposed at altitudes near 1340 m, and are probably the highest exposures of sediments deposited in the early or middle Wisconsin lake in the Bonneville basin.