Advances and emerging challenges in luminescence geochronology: Insights from the 2024 New World Luminescence Dating Workshop
No abstract available.
Quaternary Research↗
Geology topics
Shannon A. Mahan
Publications and source records attributed to Shannon A. Mahan.
Sediment transport, terrace incision, and sediment chemistry in subbasins of differing land use, Stinking Water Creek Basin near Rangely, Colorado
In many areas of western Colorado, Late Cretaceous Mancos Shale is a source of sediment, salinity, and selenium to surface water. Natural weathering processes and human activity can expose and move sediments from the Mancos Shale, leading to increased salinity and selenium concentrations in surface water. The U.S. Geological Survey, in cooperation with the Bureau of Land Management, studied four subbasins of Stinking Water Creek near Rangely, Colorado, to provide information regarding landscapes on Mancos Shale that have undergone land-use change. Rangeland grazing is present in all four subbasins. Subbasins A1 and A2 have land use related to energy development and subbasins B1 and B2 do not. Aerial photographs, sediment chemistry, optically stimulated luminescence (OSL) ages, cross-section surveys, and Watershed Erosion Prediction Project (WEPP) modeling were used to characterize the history of land-use change by estimating stream channel incision rates and sediment, salinity, and selenium loads. All subbasins had upper and lower terraces and a modern streambed channel, and these surfaces were dated using OSL. In subbasins A1, A2, and B1, the streambed age was older than the terraces, indicating the channels are incising and widening into the older alluvium. In subbasin B2, however, the streambed age was much younger than the terraces, indicating aggradation is the main process. Differences in channel morphology were not always related to land use but to the subbasin position (upstream compared to downstream). Channels in subbasins A1 and A2 were generally wider and shallower than channels in subbasins B1 and B2. Landscape disturbances related to energy development in subbasins A1 and A2 could have caused channel evolution to progress from incision to widening processes.
Colorado↗
Age reassignment using luminescence dating for the Mammoth Site, Black Hills, South Dakota, USA
The Mammoth Site (MS) at Hot Springs, South Dakota, provides a unique opportunity to reconstruct paleoenvironmental conditions on the Northern Great Plains of the United States. Previous radiocarbon analyses of mammoth bone apatite suggested the MS sinkhole was deposited ∼42–23 cal ka BP. However, two U-series ages and one thermoluminescence (TL) age ranged between 150 and 101 ka, suggesting the bones were considerably older. We analyzed six samples from the bone bed using infrared-stimulated luminescence (IRSL) on feldspars, because the quartz luminescence was saturated; the ages ranged from approximately 232 to 160 ka, showing better agreement with previous U-series and TL analyses. In addition, we dated three samples on two terraces that are at lower elevation than the MS. Our new IRSL ages show that the sediments of the MS were likely well bleached, derived from pond margin runoff events, and deposited as sandy-silt laminae couplets. Our chronology indicates that the 6-m-thick bone-bearing sediments accumulated at a rate of approximately 12 mm/yr and that deposition occurred during Marine Isotope Stage (MIS) 6 and MIS 7, not MIS 2 and MIS 3 as previously published.
South Dakota↗
Glacier fed deltas and new age constraints for glacial Lake Grand Gorge in the northern Catskill Mountains of New York State, USA
Ice-dammed lakes were common along the southern margin of the Laurentide Ice Sheet during deglaciation. In the Schoharie Valley of the northern Catskill Mountains, New York, a 171 km 2 lake known as glacial Lake Grand Gorge formed in an interlobate area. Previous researchers suggested that deltas developed into this glacial lake from meltwater supplied by the adjacent Hudson Lobe during deglaciation. This study investigates stratigraphy and sedimentology of Pleistocene deltas and lake deposits to determine the source of meltwater and sediment. Detailed examination of stratigraphy from quarries, stream exposures, and new exploration borings was combined with radiocarbon and OSL geochronology to establish event stratigraphy. Sedimentologic and geomorphic data from deltas in the Manor Kill and Platter Kill Valleys demonstrates that braided outwash was supplied directly from the Westerlo Sublobe that crossed the northeastern escarpment of the Catskill Mountains. New geochronologic data indicates that the lake was established by 28 ka and likely persisted to at least 22 ka. Insects and twigs recovered from lake sediments complement the OSL data and indicate: (1) that the lake was present significantly earlier than previous deglacial models predict, and (2) that it existed in an ice-free area of the northern Catskill Mountains during the last glacial maximum of the MIS 2 glaciation.
New York↗
Preliminary geologic map of the Sparta East, Sparta West, and parts of the Glade Valley and Whitehead 7.5-minute quadrangles, North Carolina and Virginia, and the epicentral area of the August 9, 2020, Mw 5.1 earthquake near Sparta, North Carolina
Introduction New bedrock and surficial geologic mapping in the Sparta East, Sparta West, and parts of the Glade Valley and Whitehead 7.5-minute quadrangles, North Carolina and Virginia, investigates the geologic framework and causative mechanisms of the August 9, 2020, Mw 5.1 earthquake near Sparta, North Carolina. The mapping documents (1) the coseismic surface rupture from the 2020 earthquake and related brittle structures in the bedrock; (2) the fault contact between the western Blue Ridge and eastern Blue Ridge; (3) lithostratigraphy in the Lynchburg Group, Ashe Metamorphic Suite, and Alligator Back Metamorphic Suite; (4) the nature of the contact between the Lynchburg Group, Ashe Metamorphic Suite, and Alligator Back Metamorphic Suite; and (5) surficial deposits.
North Carolina, Virginia↗
Springtime formation of laminated soil carbonate rinds and changes in fluvial terrace soils on orbital timescales at Rio Mesa, Utah, USA
Laminated soil carbonate rinds are a Quaternary paleoclimate archive whose isotope composition is linked to soil formation conditions. At Rio Mesa, Utah (USA), we investigated the fidelity of rind records in a river terrace setting by determining the seasonal timing of rind formation and testing for inter-record replication. We infer soil carbonate formed in the spring season, contrasting with our prior inference of summer formation at Teasdale, Utah, ≈200 km distant. This apparent discrepancy occurs because of differences in the timing of the largest annual infiltration (spring vs. summer). At Rio Mesa, modern soil data show that soil carbonate δ 13 C would have high values (−2 to 2‰ VPDB) regardless of seasonal activity of C 3 versus C 4 plants because respiration rate is a strong control. We accordingly suggest reassessment of published records interpreting soil carbonate δ 13 C only via C 3 versus C 4 plant abundance. Three rind δ 13 C and δ 18 O records generally replicated. Intriguingly, rind δ 13 C may inversely correlate with summer insolation, evidence for global-scale influence on soils. Rind δ 18 O is not as clearly correlated with published western USA paleoclimate records, potentially due to regional differences in climate and because rinds record soil-specific processes. Our results support the fidelity of the soil carbonate rind paleoarchive and suggest that because rind formation seasonality is intimately tied to infiltration seasonality, spatial transects of rind records might be used to delineate boundaries between areas dominated by spring and summer infiltration, permitting reconstruction of the geographic extent of large-scale hydrologic phenomena such as the North American Monsoon.
Utah↗
Reconstructing the Quaternary depositional history using geologic mapping and three-dimensional modeling of the subsurface near Fort Morgan, northeastern Colorado
Centered on Fort Morgan, Colorado, this study is intended to build from previous work by adding a three-dimensional (3D) view of the subsurface to better understand the depositional history of Quaternary deposits. A 1:100,000 scale geologic map was made by combining previous geologic maps, regional soil maps, and recent field investigations. In addition to the geologic mapping, drill hole lithologic data from water wells and oil and gas exploration were compiled and lithologic units simplified to best represent the stratigraphy of the Quaternary deposits. From these subsurface data, a 3D subsurface model was constructed, trimmed at the surface by a digital elevation model, and a bedrock surface foundation gridded from drill hole data was added. The surface of the 3D model was then compared visually to the surficial geologic map. Cross sections were constructed from the 3D model and compared to site-specific drilling that was done as part of this project. Finally, the model was examined in detail to reconstruct the depositional history of the subsurface alluvial and eolian units. Alluvial and fluvial drainage basins exposed in the subsurface have a greater areal extent than the present-day narrow drainages. Older eolian sand in the subsurface tends to be interbedded with loess indicating coeval deposition. Holocene sand, both eroded from bedrock exposed at the surface north of the study area and reworked from the South Platte River, buries most of the interbedded older sand and loess.
Colorado↗
Geochronologic data reveal Late Pleistocene to Holocene debris-flow history and wildfire association within Whiskeytown National Recreation Area, Klamath Mountains, northern California
Understanding the local to regional history of extreme events such as debris flows and floods provides context to plan for and mitigate these hazards to life, property, and infrastructure. The Klamath Mountains of northwestern California have experienced both debris flows and devastating wildfires. Whiskeytown National Recreation Area (WHIS) is at the heart of this range and has a wealth of debris flow–related landforms. Gaining an understanding of prehistoric flows and their relationship with fire or other potential triggers can help mitigate future problems. Optically stimulated luminescence and radiocarbon analyses from sediment and entrained organics in undisturbed facies, including beneath partially buried boulders, establishes a chronology of paleo-events in WHIS. The levee deposits indicate a repetition of debris flows during the latest Holocene, every 125–150 years, since 850 yr. Larger flows occurred, with a record elucidated from debris-flow deposits along Clear Creek, with Middle Holocene ages, ca. 2600 to 5500 yr, most of which have sufficient concentrations of charcoal to indicate origins as postfire debris flows. Deposits at higher elevations show events from the latest Pleistocene ca. 13,000 yr. This geochronology indicates that these are not singular events but are relatively common and inherent to the geomorphic processes shaping this landscape.
California↗
Spatiotemporal variations in strain release and seismic rupture in multifault systems: An example from Panamint Valley, southeastern California
Geometrically complex, multifault ruptures have been observed in recent, damaging earthquakes in southeastern California, sparking renewed efforts to identify physical conditions that promote or inhibit fault discontinuity-spanning coseismic ruptures. The likelihood of ruptures propagating across fault discontinuities is thought to be partly controlled by fault geometries, rupture direction, and the history of strain release. However, these parameters vary in space and time over multiple earthquake cycles, making it difficult to forecast the likelihood that an earthquake on one fault will trigger rupture on a nearby fault. Here we use tectono-geomorphic mapping of a geometrically complex fault zone in Panamint Valley, southeastern California, to assess spatiotemporal variations of paleo-rupture patterns and geometries of fault discontinuities over multiple earthquake cycles. First, we identify ten generations of late Pleistocene to Holocene alluvium using geomorphic parameters and luminescence dating to constrain ages of alluvium and bracket late Holocene earthquake timing. Then, we quantify slip kinematics using high-resolution structure from motion digital surface models. We find the Panamint Valley transtensional relay (PVTR) hosted four late Holocene earthquakes, bracketed to ~5.8–3.4 ka, ~3.8–2.2 ka, ~2.4–0.6 ka, and ~0.64–0.16 ka, with ~0.6–1.1 m of slip per event, correlative to Mw ≈ 6.7–6.9 earthquakes. Additionally, we find similarities in earthquake timing on the Ash Hill, PVTR, and Panamint Valley faults and similarities in the slip magnitude and slip kinematics between the Ash Hill and PVTR faults, implying that the PVTR may co-rupture with nearby faults. Paleo-rupture patterns indicate that seismogenic strain transfer may occur through the PVTR, along different combinations of fault segments and jump distances, over multiple earthquake cycles. These data highlight the utility of tectono-geomorphic mapping in evaluating paleo-rupture patterns and suggest that the PVTR may act to propagate and/or arrest rupture between the Ash Hill and Panamint Valley faults.
California↗
Luminescence dating of stone structures in northeastern United States
There is no consensus on who built the numerous stone structures that dot the archaeological landscape in the northeastern United States. Professional archaeologists traditionally have attributed them to colonial farmers, but increasing numbers of archaeologists have joined many nonprofessional groups and Native Americans in arguing for Indigenous origins. Better understanding of these structures can be obtained by determining how old they are. This article reviews nearly 60 luminescence ages, on both sediments and rocks, that have been obtained in recent years. Many of the derived ages fall in the sixteenth century, between initial European contact and substantial colonial settlement. A few ages are significantly older, suggesting that this technology has a deeper origin. The results warrant more research into these structures and rethinking their significance.
American Antiquity↗
Geologic map of Scoggins Dam, Henry Hagg Lake, and Scoggins Valley, Washington County, Oregon
New geologic mapping (Wells and others, 2020b) and geophysical mapping (Blakely and others, 2000; McPhee and others, 2014; Wells and others, 2020a) document kilometers of Cenozoic right-lateral offset along the Gales Creek Fault Zone, a major, northwest-striking fault zone forming the boundary between the Tualatin Valley and the Coast Range. The Bureau of Reclamation’s (Reclamation) Scoggins Dam (fig. 1), in the Coast Range foothills west of Forest Grove, Oregon, lies within the Gales Creek Fault Zone as mapped by Wells and others (2020a, 2020b; fig. 2). Active faults of the Gales Creek Fault Zone defined by paleoseismic trenching (Redwine and others, 2017, 2019b, Horst and others, 2018, 2019, 2021, and Wells and others, 2020a) are presently mapped as projecting through the existing dam. The Pacific Northwest Region of Reclamation requested assistance with geologic studies around Scoggins Dam to provide better understanding of fault locations and their activity, which are needed to design a modification of the dam (Maguire, 2019a, b). The scope of this project includes detailed geology of the existing Scoggins Dam site, Henry Hagg Lake, the reservoir behind the dam, and Scoggins Valley downstream of the existing dam, particularly around a potential new dam site, where Scoggins Creek cuts through a narrow gap formed by a resistant felsic tuff bed that crosses the valley.
Oregon↗
Timing and geometry of the Chemehuevi Formation reveal a late Pleistocene sediment pulse into the Lower Colorado River
The Chemehuevi Formation is a distinctive 50−150-m-thick wedge-shaped Pleistocene sedimentary unit deposited by the Colorado River. It lines the perimeters of the river’s floodplains and bedrock canyons for more than 600 km between the mouth of the Grand Canyon and the delta region in the Gulf of California. The formation is composed of a basal tan to light-yellowish-brown and pale-orange mud-dominated facies overlain and interbedded by a light-yellow-brown sand-dominated facies. The unit is one of two extensively exposed aggradational packages in the Lower Colorado River corridor, in addition to a series of other smaller alluvial terrace deposits. The Chemehuevi Formation appears to represent the response of a fully integrated Colorado River system to a significant perturbation, in contrast to the Bullhead Alluvium, which is likely a unique result of Pliocene river integration. The aggradation of the Chemehuevi Formation in the Lower Colorado River corridor may be similarly due to a unique event in the Colorado River system, or it may instead be a well-preserved sedimentary sequence recording typical behavior of the Colorado River below the Grand Canyon in the late Pleistocene. As such, multiple causal mechanisms have been proposed, but no study to date has conclusively explained the Chemehuevi Formation. To help resolve its timing, duration, and origin, we applied post-infrared infrared stimulated luminescence, carbonate U-Th series, and zircon sensitive high-resolution ion microprobe U-Th series geochronology to determine the ages of key exposures of the unit over a wide spatial area. These new data demonstrate that the Chemehuevi Formation was deposited ca. 110−90 ka. The depositional ages collectively overlap, suggesting that deposition occurred rapidly relative to the resolution of the geochronometers. The new depositional timing coincides with a shift from glacial to interglacial conditions after the marine isotope stage 5-6 transition. This observation is consistent with a climate-induced sediment pulse as a causal mechanism, yet correlations with similar deposits in the Colorado River headwaters or in neighboring catchments appear elusive. Potentially, climate transitions between glacial and interglacial periods induced a sediment pulse from hillslopes of the Colorado River system that resulted in the Chemehuevi Formation. An alternative or additional explanation is that the Chemehuevi Formation represents release of lava dam−impounded sediment in the Grand Canyon. The surface geometry of the Chemehuevi Formation projects upstream to the approximate location of lava dams, and the largest possible lava dam impoundment (the Upper Prospect dam) is comparable in volume to the formation. The lava dam hypothesis appears to be a possible explanation for the Chemehuevi Formation. However, tying deposition to a specific lava dam or series of lava dams remains challenging due to discrepancies in timing and volume. The combined effects of a series of lava dams may have led to the Chemehuevi Formation, as the last Pleistocene lava dam eruption coincides with the onset of deposition. Alternatively, the formation may result from the combined effects of both regional climate transitions and the lava dams that created a transient reservoir to compound a climate transition−driven sediment pulse. The geochronologic data presented here do not allow us to distinguish between the lava dam or climate transition hypotheses but will need to be reconciled with any future proposed depositional model.
Arizona, California, Nevada↗
Neotectonic mapping of Puerto Rico
Puerto Rico is part of the Puerto Rico-Virgin Islands microplate, along the Caribbean–North American plate boundary between the Puerto Rico trench subduction zone and the Muertos Trough incipient subduction zone. Despite recent seismicity and geodetically constrained deformation of ~3 mm/yr of left-lateral shear across the island, Quaternary fault locations remain largely uncertain. Preservation of recent faulting in the landscape is masked by distributed faulting, high weathering rates of the tropical climate, steep topography, frequent landsliding, and extensive agriculture and urbanization along coastlines and river valleys. We present remote neotectonic mapping of historical aerial imagery from the 1930s-60s and <1-m lidar topography, integrated with field observations, to create an updated active fault map of Puerto Rico. We focus on faults that offset younger geomorphic surfaces, with ages estimated by geologic mapping, OSL and radiocarbon dating, and morphologic interpretations. We present new evidence for Quaternary activity on seven faults, including the South Lajas, Salinas, Punta Montalva, Great Southern Puerto Rico, Cerro Goden, Parguera, and San Marcos faults. We find that active faulting occurs preferentially along the southern and western coasts and does not always spatially coincide with preexisting bedrock faults, possibly reflecting partial reactivation of older faults in the modern strain field.
Puerto Rico↗
Paleoseismology of the Sawtooth fault and implications for fault behavior in the epicentral region of the 2020 Mw 6.5 Stanley, Idaho, earthquake
The 2020 moment magnitude ( M w ) 6.5 Stanley, Idaho, earthquake raised questions about the history and extent of complex faulting in the northwestern Centennial Tectonic Belt (CTB) and its relation to the Sawtooth normal fault and Eocene Trans‐Challis fault system (TCFS). To explore faulting in this area, we excavated a paleoseismic trench across the Sawtooth fault along the western margin of the CTB, and compared an early Holocene (9.1 ± 2.1 ka, 1δ ) rupture at the site with lacustrine paleoseismic data and fault mapping in the 2020 epicentral region. We find: (1) a history of partial to full rupture of the Sawtooth fault ( M w 6.8–7.4), (2) that shorter ruptures ( M w ≤ 6.9 ) are likely along distributed and discontinuous faults in the epicentral region, (3) that this complex system that hosted the 2020 earthquake is not directly linked to the Sawtooth fault, (4) that the northeast‐trending TCFS likely plays a role in controlling fault length and rupture continuity for adjacent faults, and (5) that parts of the TCFS may facilitate displacement transfer between normal faults that accommodate crustal extension and rotation. Our results help unravel complex faulting in the CTB and imply that relict structures can help inform regional seismic hazard assessments.
Idaho↗
Surficial geology and Quaternary fault map of the Las Vegas Valley, Clark County, Nevada
The Surficial Geology and Quaternary Fault Map of the Las Vegas Valley, Clark County, Nevada is a 1:50,000-scale compilation of published 1:24,000-scale geologic maps integrated with new field and desktop mapping. This geologic map compilation and GIS database are part of a broader study on the Quaternary faults in the Las Vegas Valley, the results of which are summarized in this report. The map compilation utilized a variety of digital base maps to evaluate the characteristics of surficial deposits and Quaternary fault scarps including lidar data in undeveloped parts of the valley and orthorectified historical aerial photos in the urbanized center of the valley. The map distinguishes twenty surficial map units including alluvial-fan and ephemeral-wash deposits, groundwater discharge deposits of the Las Vegas Formation, and alluvium deposits of Las Vegas Wash. Quaternary faults include the Quaternary Las Vegas Valley fault system in the center of the valley and the Frenchman Mountain fault system along the eastern margin. All Quaternary fault traces in the valley were evaluated, and the previously published fault mapping was modified as needed. The new mapping and uniform correlation of surficial deposits displaced by Quaternary faults yields a consistent valley-wide characterization of the recency of fault activity. The map and GIS database include a Quaternary geochronology compilation of 119 previously published surficial deposit ages of various types, 35 new luminescence ages, and 3 new radiocarbon ages. The new age data further bracket the ages of the Las Vegas basin Quaternary stratigraphy and provide new constraints on the timing of Quaternary fault activity.
Nevada↗
Using geologic mapping to understand temporal and spatial relations of closely clustered to concurrent latest Holocene surface ruptures on two intersecting faults, south-central Mojave Desert, California
The Pinto Mountain Fault Zone (PMFZ) marks a major structural boundary between east-oriented sinistral faults of the eastern Transverse Ranges (to the south) and northwest-oriented dextral faults of the south-central Mojave Desert (to the north). These structural fault systems comprise sinistral and dextral deformational domains of the Eastern California Shear Zone (ECSZ) that intersect one another in the Copper Mountain and Twentynine Palms areas. The U.S. Geological Survey (USGS) is conducting detailed geologic mapping and geochronologic investigations designed to clarify geometric, kinematic, and temporal relations among the two domains, that are focused on the central portion of the left-lateral PMFZ near its intersection with the right-lateral Copper Mountain Fault (CMF) and Mesquite Lake Fault Zone (MLFZ).
California↗
High-resolution geophysical and geochronological analysis of a relict shoreface deposit offshore central California: Implications for slip rate along the Hosgri fault
The Cross-Hosgri slope is a bathymetric lineament that crosses the main strand of the Hosgri fault offshore Point Estero, central California. Recently collected chirp seismic reflection profiles and sediment cores provide the basis for a reassessment of Cross-Hosgri slope origin and the lateral slip rate of the Hosgri fault based on offset of the lower slope break of the Cross-Hosgri slope. The Cross-Hosgri slope is comprised of two distinct stratigraphic units. The lower unit (unit 1) overlies the post–Last Glacial Maximum transgressive erosion surface and is interpreted as a Younger Dryas (ca. 12.85–11.65 ka) shoreface deposit based on radiocarbon and optically stimulated luminescence (OSL) ages, Bayesian age modeling, seismic facies, sediment texture, sediment infauna, and heavy mineral component. The shoreface was abandoned and partly eroded during rapid sea-level rise from ca. 11.5 to 7 ka. Unit 2 consists of fine sand and silt deposited in a midshelf environment when the rate of sea-level rise slowed between ca. 7 ka and the present. Although unit 2 provides a thin, relatively uniform cover over the lower slope break of the older shoreface, this feature still represents a valuable piercing point, providing a Hosgri fault slip rate of 2.6 ± 0.8 mm/yr. Full-waveform processing of chirp data resulted in significantly higher resolution in coarser-grained strata, which are typically difficult to interpret with more traditional envelope processing. Our novel combination of offshore radiocarbon and OSL dating is the first application to offshore paleoseismic studies, and our results indicate the utility of this approach for future marine neotectonic investigations.
California↗
Pliocene–Pleistocene hydrology and pluvial lake during Marine Isotope Stages 5a and 4, Deep Springs Valley, western Great Basin, Inyo County, California
Deep Springs Valley (DSV) is a hydrologically isolated valley between the White and Inyo mountains that is commonly excluded from regional paleohydrology and paleoclimatology. Previous studies showed that uplift of Deep Springs ridge (informal name) by the Deep Springs fault defeated streams crossing DSV and hydrologically isolated the valley sometime after eruption of the Pleistocene Bishop Tuff (0.772 Ma). Here, we present tephrochronology and clast counts that reaffirms interruption of the Pliocene–Pleistocene hydrology and formation of DSV during the Pleistocene. Paleontology and infrared stimulated luminescence (IRSL) dates indicate a freshwater lake inundated Deep Springs Valley from ca. 83–61 ka or during Late Pleistocene Marine Isotope Stages 5a (MIS 5a; ca. 82 ka peak) and 4 (MIS 4; ca. 71–57 ka). The age of pluvial Deep Springs Lake coincides with pluvial lakes in Owens Valley and Columbus Salt Marsh and documents greater effective precipitation in southwestern North America during MIS 5a and MIS 4. In addition, we hypothesize that Deep Springs Lake was a balanced-fill lake that overflowed into Eureka Valley via the Soldier Pass wind gap during MIS 5a and MIS 4. DSV hydrology has implications for dispersal and endemism of the Deep Springs black toad ( Anaxyrus exsul ).
California↗