A Paleo-Lake and wetland paleoecology associated with human use of the distal Old River Bed Delta at the Pleistocene-Holocene transition in the Bonneville Basin, Utah, USA

Mollusk and ostracode assemblages from the distal Old River Bed delta (ORBD) contribute to our understanding of the Lake Bonneville basin Pleistocene-Holocene transition (PHT) wetland and human presence on the ORBD (ca. 13,000–7500 cal yr BP). Located on U.S. Air Force-managed lands of the Great Salt Lake Desert (GSLD) in western Utah, USA, the area provided 30 samples from 12 localities. The biological assemblages and the potential water sources using 87Sr/86Sr analyses showed wetland expansion and contraction across the PHT, including the Younger-Dryas Chronozone (YDC). The record reflects cold, freshwater conditions, which is uncharacteristic of the Great Salt Lake Desert, after recession of Lake Bonneville. Lymnaea stagnalis jugularis, Cytherissa lacustris, and possibly Candona sp. cf. C. adunca, an endemic and extinct species only reported from Lake Bonneville, suggest cold-water environments. Between 13,000–12,400 cal yr BP, a shallow lake formed, referred to as the Old River Bed delta lake, fed by Lake Gunnison, as shown by 87Sr/86Sr ratios of 0.71024–0.71063 in mollusk fossils collected at the ORBD, characteristic of the Sevier basin. These findings add paleoenvironmental context to the long-term use of the ORBD by humans in constantly changing wetland habitats between 13,000–9500 cal yr BP.

GSA in the Field in 2020

COVID-19 made for a highly unusual year as it affected almost every facet of life. The pandemic made gathering and visiting the field nearly impossible as we quarantined and moved into virtual spaces. Three groups submitted guides for publication during the height of the pandemic: two for trips that would have taken place during the GSA Annual Meeting in Montréal, Canada, and one from the Rocky Mountain Section Meeting in Provo, Utah, USA. Readers will enjoy these journeys to the Ottawa aulacogen/graben on the Northeast U.S.–Canadian border; the southern Québec Appalachians; and Lake Bonneville, the Wasatch Range, and Great Salt Lake in Utah.

A field trip to observe features of Lake Bonneville, mountain glaciation, and Great Salt Lake near Salt Lake City, Utah, USA

ABSTRACT On this field trip we visit three sites in the Salt Lake Valley, Utah, USA, where we examine the geomorphology of the Bonneville shoreline, the history of glaciation in the Wasatch Range, and shorezone geomorphology of Great Salt Lake. Stop 1 is at Steep Mountain bench, adjacent to Point of the Mountain in the Traverse Mountains, where the Bonneville shoreline is well developed and we can examine geomorphic evidence for the behavior of Lake Bonneville at its highest levels. At Stop 2 at the mouths of Little Cottonwood and Bells Canyons in the Wasatch Range, we examine geochronologic and geomorphic evidence for the interaction of mountain glaciers with Lake Bonneville. At the Great Salt Lake at Stop 3, we can examine modern processes and evidence of the Holocene history of the lake, and appreciate how Lake Bonneville and Great Salt Lake are two end members of a long-lived lacustrine system in one of the tectonically generated basins of the Great Basin.

G.K. Gilbert and the Bonneville shoreline

The Bonneville shoreline, the highest, and second-most prominent shoreline of Pleistocene Lake Bonneville in Utah, Nevada, and Idaho, has been thought for many years to have formed during a period ofprolonged overflow (500 to 1000+ years) and lake-level stability prior to the Bonneville flood. That traditional idea was initially promoted by G.K. Gilbert during the 1870s before he spent over a decade on field work related to Lake Bonneville. During Gilbert’s field work, his observations led him to a different interpretation of how the Bonneville shoreline developed, and by the time his final report on Lake Bonneville was published in 1890, he was no longer promoting the idea of prolonged overflow. Instead he thought of the Bonneville shoreline as a geomorphic record of the highest level attained by the transgressing lake in the closed basin; the shoreline marks the boundary between lacustrine-dominated landforms below and fluvial-dominated landforms above. For over 120 years after Gilbert’s (1890) monograph was published, researchers ignored his interpretation, and assumed (but did not present supporting evidence), that Lake Bonneville had overflowed for a prolonged period prior to the Bonneville flood while the Bonneville shoreline developed. Re-examination of the geomorphology of the Bonneville shoreline, the stratigraphy of Lake Bonneville deposits, the geomorphology of the overflow area, and the history of Lake Bonneville, shows that Gilbert’s 1890 interpretation is consistent with observations. Considering this, to accurately interpret the history of Lake Bonneville the Bonneville shoreline should be viewed as the level the lake had reached in the closed basin when its transgression ceased and it began to spill into the Snake River drainage basin.

Water Provenance at the Old River Bed Inland Delta and Ground Water Flow from the Sevier Basin of Central Utah during the Pleistocene-Holocene Transition

To ascertain the provenance of water reaching wetlands in an area sustaining a population of Pleistocene–Holocene foragers, 87-strontium/86-strontium isotopic ratios (87Sr/86Sr) of mollusks from channels of the Old River Bed inland delta of central Utah were measured. Potential provenances examined included overflow from Pleistocene–Holocene Lake Gunnison, ground water flow from the Sevier basin, ground water discharge from piedmont aquifers infiltrated by Lake Bonneville, and ground waters from local regional aquifers. Old River Bed inland delta channels active from ~13.2 cal ka BP until ~11.2 cal ka BP have 87Sr/86Sr values of 0.70930–0.71049 that are consistent with water sourced from Lake Gunnison in the Sevier basin. Inland delta channels active from ~11.2 cal ka BP until shortly after ~9.3 cal ka BP have 87Sr/86Sr values of 0.70977–0.71033, suggesting ground water flowed from the Sevier basin during the early Holocene. Ratios of 87Sr/86Sr did not match known values for Lake Bonneville, but the youngest Old River Bed inland delta channel system has an 87Sr/86Sr ratio consistent with a local ground water source, perhaps Government Creek. Consistent ground water discharge may explain the persistence of foragers in the region despite the increasingly arid climate of the Great Basin.

Shallow groundwater flow and inverted fresh/saline-water interface in a hypersaline endorheic basin (Great Basin, USA)

Pilot Valley is an 828-km2 arid-region endorheic basin in western USA. Bounding mountain ranges rise as much as 1,900 m above the nearly flat 379-km2 playa floor. Up to 3.8 m of Pleistocene Lake Bonneville mud and thin oolitic sand layers form the surface layer of the basin floor. Groundwater conditions were evaluated using data from shallow monitoring wells and borings, springs, infiltrometer measurements, slug and dilution tests, geophysical transects, and precision elevation surveys. Alluvial fan groundwater discharges at fan/playa interface springs and underflows to the shallow basin sediments along the western side of the basin; the groundwater only underflows along the eastern side. Precision surveying established a Lake Bonneville shore-line break in slope as the cause of the spring discharges. Tectonic tilting causes groundwater to flow from east to west and to the topographic low. Monthly measured and pressure transducer data established seasonal pressure responses and upward groundwater gradients. All basin groundwater is lost to evapotranspiration at the topographic low, where a thin salt pan has developed. Groundwater evolves from fresh to hypersaline near the alluvial fan/playa interface where there is an inverted salinity gradient and a groundwater pressure ridge. The pressure ridge and inverted salinity interface are due to: (1) osmotic pressure established between the oolitic sand of high hydraulic conductivity and the overlying low-hydraulic-conductivity lake mud at the fan/playa interface, and (2) the collision between fresh groundwater flow driven by a steep hydraulic head and hypersaline groundwater flow driven by a nearly flat hydraulic head.

Rock Canyon near Provo, Utah County: A Geologic Field Laboratory

Rock Canyon near Provo, Utah is an ideal outdoor laboratory. The canyon has been known and explored for many years by scientists and students for its fascinating geology, biology, and botany. It is also a favorite location for rock climbers, hikers, and other outdoor enthusiasts. Facilities near the mouth of the canyon including parking, restrooms, a lecture amphitheater, and a covered pavilion with picnic tables provide an ideal location for visitors. Geology is the focal point of this beautiful canyon with a history that stretches from the Precambrian (about 700 million years ago) to the Wasatch fault and Lake Bonneville, which covered much of western Utah at its peak roughly 18,000 years ago. Excellent exposures of the rocks allow visitors to see features clearly and piece together the history of the canyon. The oldest rocks are glacial deposits of the Mineral Fork Tillite. The tillite is overlain by a thick section of Paleozoic rocks of Cambrian to Permian age, all of which have been deformed into an asymmetric, overturned fold formed during the Sevier orogeny, a roughly 140 to 50 million year old mountain building event. The upper reaches of the canyon were sculpted by glaciers during the Pleistocene and deposits of the Provo and Bonneville levels of Lake Bonneville are found at the mouth of the canyon, now cut by a recent alluvial fan. Also, at the mouth of the canyon are excellent exposures of features associated with the Wasatch fault.