Search for: All records

ABSTRACT Field and desktop mapping studies were conducted for the stable continental region in the Western Cape Province of South Africa to characterize fault activity of four fault systems, including the Worcester, Groenhof, Piketberg-Wellington, and Colenso faults. The geologic studies presented here were in support of a Probabilistic Seismic Hazard Analysis (PSHA) for a nearby nuclear power facility site. Previous studies performed by the South African Council for Geoscience in the region suggested evidence of near-surface co-seismic deformation (De Beer, 2004; De Beer et al., 2008). The goal of this study is to re-assess the prior interpretations of these four faults and gather the required data for including them in a seismic source model for use in a PSHA. The primary aspects to include in the characterization are the recency of movement, slip rate, kinematics, and geometry. To improve the interpretation and target sites, the study used a satellite-derived digital elevation model and aerial imagery for six areas, totaling over 900 km2 of data. Limited Quaternary cover, or other late Cenozoic deposits that overlie the Precambrian and Paleozoic bedrock structures, resulted in difficulty constraining the recency of faulting. The new observations presented in this study suggest that reactivation and surface rupture along pre-Cenozoic faults of the four fault systems have not occurred in at least the last 10 ka. Further, the lack of youthful tectonic geomorphology and deformation of Quaternary stratigraphy indicate that surface faulting has not occurred in the late to middle Quaternary along any of these four structures. 

Abstract Rock glaciers are common in alpine landscapes, but their evolution over time and their significance as agents of debris transport are not well‐understood. Here, we assess the movement of an ice‐cemented rock glacier over a range of timescales using GPS surveying, satellite‐based radar, and cosmogenic¹⁰Be surface‐exposure dating. GPS and InSAR measurements indicate that the rock glacier moved at an average rate of ∼10 cm yr⁻¹in recent years. Sampled boulders on the rock glacier have cosmogenic surface‐exposure ages from 1.2 to 10 ka, indicating that they have been exposed since the beginning of the Holocene. Exposure ages increase linearly with distance downslope, suggesting a slower long‐term mean surface velocity of 3 ± 0.3 cm yr⁻¹. Our findings suggest that the behavior of this rock glacier may be dominated by episodes of dormancy punctuated by intervals of relatively rapid movement over both short and long timescales. Our findings also show that the volume of the rock glacier corresponds to ∼10 m of material stripped from the headwall during the Holocene. These are the first cosmogenic surface‐exposure ages to constrain movement of a North American rock glacier, and together with the GPS and satellite radar measurements, they reveal that rock glaciers are effective geomorphic agents with dynamic multi‐millennial histories. 

Abstract We present new data from the debris-rich basal ice layers of the NEEM ice core (NW Greenland). Using mineralogical observations, SEM imagery, geochemical data from silicates (meteoric¹⁰Be, εNd,⁸⁷Sr/⁸⁶Sr) and organic material (C/N, δ¹³C), we characterize the source material, succession of previous glaciations and deglaciations and the paleoecological conditions during ice-free episodes. Meteoric¹⁰Be data and grain features indicate that the ice sheet interacted with paleosols and eroded fresh bedrock, leading to mixing in these debris-rich ice layers. Our analysis also identifies four successive stages in NW Greenland: (1) initial preglacial conditions, (2) glacial advance 1, (3) glacial retreat and interglacial conditions and (4) glacial advance 2 (current ice-sheet development). C/N and δ¹³C data suggest that deglacial environments favored the development of tundra and taiga ecosystems. These two successive glacial fluctuations observed at NEEM are consistent with those identified from the Camp Century core basal sediments over the last 3 Ma. Further inland, GRIP and GISP2 summit sites have remained glaciated more continuously than the western margin, with less intense ice-substratum interactions than those observed at NEEM. 

Abstract How tectonic forcing, expressed as base level change, is encoded in the stratigraphic and geomorphic records of coupled source‐to‐sink systems remains uncertain. Using sedimentological, geochronological and geomorphic approaches, we describe the relationship between transient topographic change and sediment deposition for a low‐storage system forced by rapid rock uplift. We present five new luminescence ages and two terrestrial cosmogenic nuclide paleo‐erosion rates for the late Pleistocene Pagliara fan‐delta complex and we model corresponding base level fall history and erosion of the source catchment located on the Ionian flank of the Peloritani Mountains (NE‐Sicily, Italy). The Pagliara delta complex is part of the broader Messina Gravel‐and‐Sands lithostratigraphic unit that outcrops along the Peloritani coastal belt as extensional basins have been recently inverted by both normal faults and regional uplift at the Messina Straits. The deltas exposed at the mouth of the Pagliara River have constructional tops at ca. 300 m a.s.l. and onlap steeply east‐dipping bedrock at the coast to thickness between ca. 100 and 200 m. Five infrared‐stimulated luminescence (IRSL) ages collected from the delta range in age from ca. 327 to 208 ka and indicate a vertical long‐term sediment accumulation rate as rapid as ca. 2.2 cm/yr during MIS 7. Two cosmogenic¹⁰Be concentrations measured in samples of delta sediment indicate paleo‐erosion rates during MIS 8–7 near or slightly higher than the modern rates of ca. 1 mm/yr. Linear inversion of Pagliara fluvial topography indicates an unsteady base level fall history in phase with eustasy that is superimposed on a longer, tectonically driven trend that doubled in rate from ca. 0.95 to 1.8 mm/yr in the past 150 ky. The combination of footwall uplift rate and eustasy determines the accommodation space history to trap the fan‐deltas at the Peloritani coast in hanging wall basins, which are now inverted, uplifted and exposed hundreds of metres above the sea level. 

Abstract We review geochronological data relating to the timing and rate of Laurentide Ice Sheet recession in the northeastern United States and model ice margin movements in a Bayesian framework using compilations of previously published organic¹⁴C (n= 133) andin situcosmogenic¹⁰Be (n= 95) ages. We compare the resulting method‐specific chronologies with glacial varve records that serve as independent constraints on the pace of ice recession to: (1) construct a synthesis of deglacial chronology throughout the region; and (2) assess the accuracy of each chronometer for constraining the timing of deglaciation. Near the Last Glacial Maximum terminal moraine zone,¹⁰Be and organic¹⁴C ages disagree by thousands of years and limit determination of the initial recession to a date range of 24–20 ka. We infer that¹⁰Be inherited from pre‐glacial exposure adds 2–6 kyr to many exposure ages near the terminal moraines, whereas macrofossil¹⁴C ages are typically 4–8 kyr too young due to a substantial lag between ice recession and sufficient organic material accumulation for dating in some basins. Age discrepancies between these chronometers decrease with distance from the terminal moraine, due to less¹⁰Be inherited from prior exposure and a reduced lag between ice recession and organic material deposition.¹⁴C and¹⁰Be ages generally agree at locations more than 200 km distal from the terminal moraines and suggest a mostly continuous history of ice recession throughout the region from 18 to 13 ka with a variable pace best documented by varves. 

Interactions among tectonics, climate, and lithology shape the Earth’s surface. In regions dominated by tectonic quiescence and climate stability, the role of rock strength related to lithology, and its role in landscape evolution, can be most clearly perceived. We leverage these qualities in a unique field site by integrating topographic data, erosion rates, and a large suite of rock strength measurements to quantify the relationship between bedrock strength and erosion rates along a 200-km section of the southeast coast of Brazil where climatic and tectonic variability are minimal. Our findings reveal a 20-fold erosion rate variation best explained by differences in rock strength. We also demonstrate that incorporating lithological strength variability into the analysis of landscape incision substantially improves accuracy, with outcomes that better reproduce natural settings. Our results underscore the crucial role of rock strength in landscape evolution and emphasize the need for field studies to account for lithological variability and thus accurately interpret landscape dynamics. 

The emerging field of quartz luminescence properties in Earth-surface processes research shows promise, with optically stimulated luminescence (OSL) sensitivity proposed as a valuable tool for provenance or sediment history tracing. However, the geologic processes that lead to quartz sensitization remain under investigation. Here we study the impact of source rock and surface processes on the luminescence properties of quartz sand from bedrock and modern and Late Pleistocene alluvium generated from a mountainous catchment in northern Utah, USA. Continuous wave and linear modulated OSL are used to characterize the luminescence sensitivity and intensity of the fast-decay component. We compare the OSL sensitivity with sand-grain provenance and with proxies for surface processes such as topographic metrics, cosmogenic 10Be-derived erosion rates, chemical weathering indices, and magnetic susceptibility. Late Pleistocene sediment has low OSL sensitivity and a weak fast-decay component, similar to bedrock samples from the source area. In contrast, modern alluvium is dominated by the fast-decay component and has higher and more variable OSL sensitivity, with no clear relationship to bedrock sources in their prospective catchment areas. There is, however, an inverse relationship between OSL sensitivity and catchment-averaged erosion rates and a positive relationship with chemical weathering indices and magnetic susceptibility. These metrics suggest that the modern alluvium has experienced increased residence time in the shallow critical zone compared to the Late Pleistocene sediments. We suggest that changes in hillslope processes between the effectively wetter, cooler Pleistocene and the dryer, warmer conditions of the Holocene modulated the luminescence properties. The results suggest that climatic controls on rates and processes of chemical and mechanical weathering and sediment transport and residence within the critical zone are encoded in the luminescence properties of quartz sand. 

Abstract. Since the 1990s, analysis of cosmogenic nuclides, primarily 10Be, in quartz-bearing river sand, has allowed for quantitative determination of erosion rates at a basin scale. Paired measurements of in situ cosmogenic 26Al and 10Be in sediment are less common but offers insight into the history of riverine sediment moving down slopes and through drainage basins. Prolonged sediment burial (>105 years), a violation of assumptions underlying erosion rate calculations, is indicated by higher 26Al-based than 10Be-based erosion rates due to preferential loss of shorter-lived 26Al by decay when quartz is shielded from cosmic rays. Here, we use a global compilation of 26Al and 10Be data generated from quartz-bearing fluvial sediment samples (n = 624, including 121 new measurements) and calculate the discordance between erosion rates derived from each nuclide. We test for correlations between such discordance and topographic metrics for drainage basins, allowing us to infer the likelihood of sediment burial during transport in different geomorphic settings. We find that nearly half of samples (n = 276) exhibit discordance (> 1σ uncertainty) between erosion rates derived from 10Be and 26Al, indicating sediment histories that must include extended burial during residence on hillslopes and/or in the fluvial system after or during initial near-surface exposure. Physical basin parameters such as basin area, slope, and tectonic activity exhibit significant correlation with erosion rate discordance whereas climatic parameters have little correlation. Our analysis suggests that 26Al/10Be erosion rate discordance occurs more regularly in basins larger than 1,000 km2, particularly when such basins have low average slopes and are in tectonically quiescent terrains. Sediment sourced from smaller, steeper basins in tectonically active regions is more likely to have similar 10Be and 26Al erosion rates indicative of limited storage and limited burial during residence in the hillslope and fluvial sediment system. The data and analysis we present demonstrate that paired 26Al and 10Be analyses in detrital fluvial samples can provide a window into watershed processes, elucidating landscape behavior at different spatial scales and allowing a deeper understanding of both sediment routing systems and whether erosion rate assumptions are violated. Large lowland basins are more likely to transport detrital sediment that has experienced prolonged sediment storage and burial either on hillslopes and/or in fluvial networks; thus, erosion rates from such basins are lower limits due to nuclide decay during storage. Conversely, samples from smaller upland basins are more likely to provide reliable erosion rates. 

Abstract. Measurements of multiple cosmogenic nuclides in a single sample are valuable for various applications of cosmogenic nuclide exposure dating and allow for correcting exposure ages for surface weathering and erosion and establishing exposure–burial history. Here we provide advances in the measurement of cosmogenic 10Be in pyroxene and constraints on the production rate that provide new opportunities for measurements of multi-nuclide systems, such as 10Be/3He, in pyroxene-bearing samples. We extracted and measured cosmogenic 10Be in pyroxene from two sets of Ferrar Dolerite samples collected from the Transantarctic Mountains in Antarctica. One set of samples has 10Be concentrations close to saturation, which allows for the production rate calibration of 10Be in pyroxene by assuming production–decay equilibrium. The other set of samples, which has a more recent exposure history, is used to determine if a rapid fusion method can be successfully applied to samples with Holocene to Last Glacial Maximum exposure ages. From measured 10Be concentrations in the near-saturation sample set we find the production rate of 10Be in pyroxene to be 3.74 ± 0.10 atoms g−1 yr−1, which is consistent with 10Be/3He paired nuclide ratios from samples assumed to have simple exposure. Given the high 10Be concentration measured in this sample set, a sample mass of ∼ 0.5 g of pyroxene is sufficient for the extraction of cosmogenic 10Be from pyroxene using a rapid fusion method. However, for the set of samples that have low 10Be concentrations, measured concentrations were higher than expected. We attribute spuriously high 10Be concentrations to failure in removing all meteoric 10Be and/or a highly variable and poorly quantified procedural blank background correction.