Inherited clast dispersal patterns: Implications for palaeoglaciology of the SE Keewatin Sector of the Laurentide Ice Sheet

The net effect of ice‐flow shifts resulting in the dilution or reworking of clasts on a single preserved till sheet is often unknown yet has major implications for palaeoglaciology and mineral exploration. Herein, we analyse variations in till clast lithologies from a single till sheet, within palimpsest‐type Glacial Terrain Zones in NE Manitoba, Canada, to better understand sediment–landform relationships in this area of high landform inheritance. This near‐ice‐divide area is known to consist of a highly fragmented subglacial landscape, resulting from spatio‐temporal variations in intensity of reworking and inheritance throughout multiple glacial events (subglacial bed mosaic). We show that a seemingly homogenous ‘Keewatin’ till sheet is composed of local (>15 km) and continental‐scale (～100‐km‐long carbonate train and 350–600 km long Dubawnt red erratic train) fan, irregular (amoeboid) or lobate palimpsest dispersal patterns. Local dispersal is more complex than the preserved local landform flowset(s) record, but appears consistent with the overall glacial history reconstructed from regional flowset and striation analyses. The resultant surface till is a spatial mosaic interpreted to reflect variable intensities in modification (overprinting) and preservation (inheritance) of a predominately pre‐existing till sheet. A multi‐faceted approach integrating till composition, regional landforms, ice‐flow indicators, and stratigraphic knowledge is used to map relative spatio‐temporal erosion/reworking intensity.     

Growth rate of the preserved continental crust: II. Constraints from Hf and O isotopes in detrital zircons from Greater Russian Rivers

Detrital zircons from the Ob, Yenisey, Lena, Amur, Volga, Dnieper, Don and Pechora rivers have been analyzed for U-Th-Pb, O and Lu-Hf isotopes to constrain the growth rate of the preserved continental crust in Greater Russia. Four major periods of zircon crystallization, 0.1-0.55, 0.95-1.3, 1.45-2.0 and 2.5-2.9 Ga, were resolved from a compilation of 1366 zircon U/Pb ages. The Archean zircons have δ¹⁸O values lying between 4.53‰ and 7.33‰, whereas Proterozoic and Phanerozoic zircons have a larger range of δ¹⁸O values in each of the recognized U/Pb time intervals with maximum δ¹⁸O values up to 12‰. We interpret the zircons with δ¹⁸O between 4.5‰ and 6.5‰ to have been derived from a magmatic precursor that contains little or no sedimentary component. The variable δ¹⁸O values of the zircons were used to constrain the ¹⁷⁶Lu/¹⁷⁷Hf ratios of the crustal source region of the zircons, which, in turn, were used to calculate Hf model ages (T_DM^V). The crustal incubation time, the time difference between primitive crust formation (dated by T_DM^V) and crustal melting (dated by zircon U/Pb age), varies between 300 to 1000 Myr for the majority of analyzed zircon grains, but can be up to 2500 Myr. The average T_DM^V Hf model age weighted by the fraction of zircons in the river load is 2.12 Ga, which is in reasonable agreement with the area-weighted average of 2.25 Ga. The T_DM^V Hf model age crustal growth curve for zircons with mantle-like δ¹⁸O values (4.5-6.5‰), weighted by area, shows that growth of the Great Russian continental crust started at 4.2 Ga, and that there are two principal periods of crustal growth, 3.6-3.3 Ga and 0.8-0.6 Ga, which are separated by an interval of low but more or less continuous growth. An alternative interpretation, in which the average ¹⁷⁶Lu/¹⁷⁷Hf ratio (0.0115) of the continental crust is used for the Paleoproterozoic zircons from the Lena River, lowers the average T_DM^V age of these grains by about 500 Myr and delays the onset of significant crustal growth to 3.5 Ga.The two principal growth periods recognized in Greater Russia differ from those identified from the Gondwana and the Mississippi river basin, which show peaks at 1.7-1.9 and 2.9-3.1 Ga (Hawkesworth and Kemp, 2006a) and 1.6-2.2 and 2.9-3.4 Ga (Wang et al., 2009), respectively. The older 3.6-3.3 Ga or 3.5-3.3 Ga peak for Greater Russia is slightly older than the older Gondwana-Mississippi peaks, whereas the younger 0.8-0.6 Ga peak is distinctly younger than the youngest peak in either Gondwana or the Mississippi river basin. This suggests that the two major peaks of crustal growth identified in Gondwana and the Mississippi river basin may not be global periods of enhanced continental growth and that the major periods of crustal growth may differ from continent to continent.     

Temporal salinity variation in salt evaporation basins in south-eastern Australia

Salt evaporation basins in south-eastern Australia, in contrast with natural saline lakes in this region, were not highly saline and exhibited little seasonal pattern in water depth and salinity over a 2-year sampling period. Lack of seasonality arose from either constant inflow (from continuous groundwater pumping) or erratic inflow (from unpredictable irrigation demands). Differences in zooplankton species composition between the salt evaporation basins and natural saline lakes might reflect the differences in temporal salinity patterns. Some typical saline lake zooplankton were not found in the evaporation basins. Salt evaporation basins therefore may represent additions to the inland water habitats of semi-arid Australia.     

Error in polygon overlay processing of geomorphic data

Predictive models for calculating sediment yield and discharge require accurate areal data. Such models may be unrealistic when using digitized data given the potential error involved in compiling and digitizing thematic polygon maps. The estimation of boundary variability for digitized polygon maps of a 0·34 km² area of badlands in Dinosaur Provincial Park, Alberta shows the effects of positional errors introduced during mapping and digitizing processes. Polygon overlay of maps of surface features and slopes produced high frequencies of very small polygons and some unlikely combinations of slopes and surface features, and decreased reliability in areal measurements in the composite map. At an epsilon band width of 0·7 m, a reasonable estimate of boundary variability, 31·7 per cent of the resultant overlaid map can be considered unreliable.     

Geomorphological development of the eastern margin of the Australian Craton

The denudation chronologies of five uplands of contrasting geological structure located at the eastern margin of the Australian Craton are examined. They are the Isa Highlands, MacDonnell Ranges and Flinders Ranges (fold mountains, orogenic setting); the Arcoona Plateau (dissected plateau, platform setting); and the Gawler Ranges (massif of old silicic volcanics, cratonic setting). In each, surfaces of Mesozoic age, many of them exhumed and of pre-Cretaceous age, are preserved. Each also appears to have been uplifted recurrently. Each was either overwhelmed or bordered by the Early Cretaceous (Neocomian-Aptian) sea. Tectonism associated with the break-up of Gondwana probably allowed this important marine transgression. Thereafter, thalassostatic and erosional/depositional isostatic responses have maintained the uplands as uplands and the intervening basins as negative topographic and structural units; the pattern of topography has been constant for the last 60–100 Ma.     

Power function for interpolating dates in recent sediment

Many paleolimnological studies deal with short cores; the upper 20 to 30 centimeters of lake sediment are often poorly compacted and it may be perilous to interpolate dates using straight lines. A power function (AGE=a+b*DEPTH^c) provides a good approach for interpolating dates where only a few dated horizons are available in a core. This may be particularly useful where multiple short cores are taken from a single site, and dated by correlation of key horizons with a master core which is ²¹⁰Pb dated. This approach assumes a constant rate of dry sediment deposition.     

Comments on Allison's (1990) developments in a non-destructive method of determining rock strength

In Developments in a non-destructive method of determining rock strength, Allison (1990) compares data collected using an ultrasonic apparatus with data obtained from Schmidt hammer tests. He concludes that the Schmidt hammer data shows a wide degree of scatter and that its accuracy as field technique is questionable. No discussion is made of how the Schmidt hammer was used or of the total number of readings taken and the range of values. The graphs presented comparing data derived from some samples using ultrasonic equipment do not appear markedly at variance from the Schmidt hammer-derived data but true comparison is not possible because the graphs use different measurement criteria. No information is given on comparative time and financial costs, which must be significantly different for the two techniques.     

Introduction to deep earth electrical conductivity

Pure and Applied Geophysics -     

Safe separation distance score: a new metric for evaluating wildland firefighter safety zones using lidar

Safety zones are areas where firefighters can retreat to in order to avoid bodily harm when threatened by burnover or entrapment from wildland fire. At present, safety zones are primarily designated by firefighting personnel as part of daily fire management activities. Though critical to safety zone assessment, the effectiveness of this approach is inherently limited by the individual firefighter’s or crew boss’s ability to accurately and consistently interpret vegetation conditions, topography, and spatial characteristics of potential safety zones (e.g. area and geometry of a forest clearing). In order to facilitate the safety zone identification and characterization process, this study introduces a new metric for safety zone evaluation: the Safe Separation Distance Score (SSDS). The SSDS is a numerical representation of the relative suitability of a given area as a safety zone according to its size, geometry, and surrounding vegetation height. This paper describes an algorithm for calculating pixel-based and polygon-based SSDS from lidar data. SSDS is calculated for every potential safety zone within a lidar dataset covering Tahoe National Forest, California, USA. A total of 2367 potential safety zones with an SSDS ≥1 were mapped, representing areas that are suitable for fires burning in low wind and low slope conditions. The highest SSDS calculated within the study area was 9.65, a score that represents suitability in the highest wind-steepest slope conditions. Potential safety zones were clustered in space, with areas in the northern and eastern portions of the National Forest containing an abundance of safety zones while areas to the south and west were completely devoid of them. SSDS can be calculated for potential safety zones in advance of firefighting, and can allow firefighters to carefully compare and select safety zones based on their location, terrain, and wind conditions. This technique shows promise as a standard method for objectively identifying and ranking safety zones on a spatial basis.