Current crescents from the Herreria Formation (Lower Cambrian) of northern Spain

Current crescents are well preserved on the upper surfaces of quartzite beds in the Lower Cambrian Herreria Formation of the Cantabrian Mountains in northern Spain. Within a sequence of nearshore deposits, the current crescents are interpreted as having formed by flow separation of wave backwash around obstacles on a beach. The obstacles are not preserved and are inferred to have been perishable organic material, such as stranded algae or jellyfish, which subsequently decayed.     

ENDOLITHIC LICHENS,RAPID BIOLOGICAL WEATHERING AND SCHMIDT HAMMER R‐VALUES ON RECENTLY EXPOSED ROCK SURFACES: STORBREEN GLACIER FORELAND,JOTUNHEIMEN, NORWAY

The endolithic lichen Lecidea auriculata is known to enhance rock surface weathering on the Little Ice Age moraines of the glacier Storbreen in Jotunheimen, central southern Norway. This study demonstrates the reduction in Schmidt hammer Rvalues that followed the rapid colonization by this lichen of pyroxene‐granulite boulders on terrain deglaciated over the last 88 years. In the absence of this lichen, the characteristic mean R‐value of boulder surfaces is 61.0 ± 0.3; where this lichen is present, R‐values are lower by at least 20 units on surfaces exposed for 30–40 years. A similar reduction in rock hardness on rock surfaces without a lichen cover requires about 10 ka. The rapid initial weakening of the rock surfaces is indicative of rates of biological weathering by endolithic lichens that may be two orders of magnitude (200–300 times) faster than rates of physico‐chemical weathering alone. If not avoided, the effects of this type of lichen are likely to negate the effectiveness of the Schmidt hammer and other methods for exposure‐age dating, including cosmogenic‐nuclide dating, in severe alpine and polar periglacial environments. The results also suggest a new method for dating rock surfaces exposed for <50 years.     

Late Quaternary glaciation and equilibrium line altitude variations of the McKinley River region,central Alaska Range

Dortch, J. M., Owen, L. A., Caffee, M. W. & Brease, P. 2009: Late Quaternary glaciation and equilibrium line altitude variations of the McKinley River region, central Alaska Range. Boreas, 10.1111/j.1502‐3885.2009.00121.x. ISSN 0300‐9483 Glacial deposits and landforms produced by the Muldrow and Peters glaciers in the McKinley River region of Alaska were examined using geomorphic and ¹⁰Be terrestrial cosmogenic nuclide (TCN) surface exposure dating (SED) methods to assess the timing and nature of late Quaternary glaciation and moraine stabilization. In addition to the oldest glacial deposits (McLeod Creek Drift), a group of four late Pleistocene moraines (MP‐I, II, III and IV) and three late Holocene till deposits (‘X’, ‘Y’ and ‘Z’ drifts) are present in the region, representing at least eight glacial advances. The ¹⁰Be TCN ages for the MP‐I moraine ranged from 2.5 kyr to 146 kyr, which highlights the problems of defining the ages of late Quaternary moraines using SED methods in central Alaska. The Muldrow ‘X’ drift has a ¹⁰Be TCN age of ～0.54 kyr, which is ～1.3 kyr younger than the independent minimum lichen age of ～1.8 kyr. This age difference probably represents the minimum time between formation and early stabilization of the moraine. Contemporary and former equilibrium line altitudes (ELAs) were determined. The ELA depressions for the Muldrow glacial system were 560, 400, 350 and 190 m and for the Peters glacial system 560, 360, 150 and 10 m, based on MP‐I through MP‐IV moraines, respectively. The difference between ELA depressions for the Muldrow and Peters glaciers likely reflects differences in supraglacial debris‐cover, glacier hypsometry and topographic controls on glacier mass balance.     

Magmatic Differentiation and Alteration in Isofacial Greenschists and Blueschists, Shuksan Suite, Washington: Statistical Analysis of Major-Element Variation

A multivariate statistical technique separates compositionalvariability in metaigneous rocks attributed to magmatic differentiationfrom compositional effects of alteration. Magmatic differentiationin metabasalts from the Shuksan Suite, Washington, is intermediate(mean MD index=2.6) between fresh ocean-floor basalts from theFAMOUS area (MD=0.0) and eastern Galapagos-rift (MD=3.8). Identificationand removal of a magmatic differentiation trend from the Shuksansamples reveals the effects of sea-floor weathering and hydrothermalalteration, which caused significant decreases in Si and Ca,increases in Na and K, and oxidation of iron. Comparison tomodern altered MORBs indicates that the Shuksan protolith rockswere probably hydrothermally altered at a sea-water/rock ratioless than 15. The intercalated metabasaltic greenschists and blueschists fromthe Shuksan Suite differ in composition. Stepwise discriminantanalysis finds Fe₂O₃ and CaO to be the only variables necessaryto completely separate compositions of greenschists and blueschistsfrom widely distributed Shuksan localities. Pressure and/ortemperature gradients of metamorphism in the Shuksan Suite mustbe too small to cause major shifts in the compositional boundarybetween greenschist and blueschist.     

Impact of lake‐level changes on the formation of thermogene travertine in continental rifts: Evidence from Lake Bogoria,Kenya Rift Valley

Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO₃ waters containing little Ca (<2 mg l^?1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO₂ degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca²⁺ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.     

Quaternary lacustrine deposits in a high-energy semi-arid mountain environment,Karakoram Mountains,northern Pakistan

Impressive Quaternary lacustrine deposits are present as terrace remnants throughout the Karakoram Mountains, northern Pakistan. They are mainly the result of damming of drainage systems during glacial advances or by catastrophic mass movement deposits. The longevity of most lakes is relatively short, in the order of years to tens of years, but sedimentation rates are extremely high as a consequence of the high sediment loads within the rivers. This results in deposits that frequently exceed 10 m in thickness. The sediments comprise dominantly planar bedded, massive and, less commonly, planar laminated, silts, comprising detrital quartz, feldspar, mica, calcite, chlorite and illite. A facies model for lacustrine sedimentation in a high-energy semi-arid high mountain region is presented, using case studies from a glacially dammed palaeolake (Glacial Lake Gilgit) and a debris-flow dammed palaeolake (Lake Serat). The rapid deposition and absence of organic material restricts the usefulness of these lacustrine sediments as proxies for palaeoenvironmental reconstruction, but they are helpful in reconstructing the former extent of glaciers and illustrating the importance of high-magnitude–low-frequency events, such as landsliding, as formative processes contributing to the evolution of the Karakoram landscape.     

Schmidt hammer exposure-age dating: developing linear age-calibration curves using Holocene bedrock surfaces from the Jotunheimen–Jostedalsbreen regions of southern Norway

Matthews, J. A. & Owen, G. 2009: Schmidt hammer exposure-age dating: developing linear age-calibration curves using Holocene bedrock surfaces from the Jotunheimen–Jostedalsbreen regions of southern Norway. Boreas , 10.1111/j.1502-3885.2009.00107.x. ISSN 0300-9483.
The approach to calibrated-age dating of rock surfaces using Schmidt hammer R-values is developed, potential errors in dating Holocene rock surfaces are estimated and limitations are assessed. Multiple sites from glacially abraded bedrock outcrops of two ages (glacier forelands deglaciated for c . 100 years and adjacent late-Preboreal terrain deglaciated for c . 9700 years) are used to analyse the variability of mean R-values and to construct linear age-calibration curves for three sub-regions in the Jotunheimen–Jostedalsbreen regions of southern Norway. Conservative potential dating errors of 246–632 years are estimated using 95% confidence intervals associated with two control points, the width of the error limits being significantly greater for the Preboreal surfaces than for the younger Little Ice Age surfaces. Substantial improvements over previous age calibrations are largely attributable to the use of multiple sites as part of a research design that has effectively controlled for geological differences between the three sub-regions. In the context of the Holocene time scale, the technique is seen as complementary to cosmogenic-nuclide dating (which currently has lower precision) and lichenometric dating (which has a lower temporal range).     

LANDSLIDE‐GLACIER INTERACTION IN A NEOPARAGLACIAL SETTING AT TVERRBYTNEDE,JOTUNHEIMEN, SOUTHERN NORWAY

A tongue‐like, boulder‐dominated deposit in Tverrbytnede, upper Visdalen, Jotunheimen, southern Norway, is interpreted as the product of a rock avalanche (landslide) due to its angular to subangular boulders, surface morphology with longitudinal ridges, down‐feature coarsening, and cross‐cutting relationship to ‘Little Ice Age’ moraines. The rock avalanche fell onto glacier ice, probably channelled along a furrow between two glaciers, and stopped on the glacier foreland, resulting in its elongated shape and long runout distance. Its distal margin may have become remobilized as a rock glacier, but a rock glacier origin for the entire landform is discounted due to lack of source debris, presence of matrix, lack of transverse ridges, and sparcity of melt‐out collapse pits. Lichenometric dating of the deposit indicates an approximate emplacement age of ad 1900. Analysis highlights the interaction of rock‐slope failures and glaciers during deglacierization in a neoparaglacial setting, with reduced slope stability due to debuttressing and permafrost degradation, and enhanced landslide mobility due to flow over a glacier and topographic channelling. Implications for the differentiation of relict landslides, moraines and rock glaciers are discussed and interrelationships between these landforms are considered in terms of an ice‐debris process continuum.     

Noble Metal Concentrations in Shoshonitic Lamprophyres: Analysis of the Weekend Dykes, Eastern Shore, Nova Scotia, Canada

Devonian, spessartite dykes, known as the Weekend dykes, onthe Eastern Shore of Nova Scotia contain panidiomorphic texturesand mineral (amphibole, clinopyroxene, and biotite) compositionstypical of shoshonitic lamprophyres. The major element and traceelement geochemistry of the Weekend dykes is also representativeof shoshonitic lamprophyres with high large ion lithophile elementconcentrations (LILE, e.g., Rb, K, and Ba) relative to the lightrare earth elements (LREE, e.g., La) and very low Nb and Ti.Only Ta concentrations are uncharacteristically high relativeto Nb and Ti. The dykes are variably evolved but many samplesshow primitive compositions with high mgnumber [>0.70, wheremg-number=Mg/(Mg+09total Fe) atomic] high MgO and Ni concentrations(>10 wt.% and 150 ppm, respectively), and low heavy REE concentrations.All noble metal concentrations (Au, Pd, Pt, Rh, Ru, and Ir)tend to be lower in evolved samples than in primitive rocks,suggesting that evolved magmas were efficiently scavenged byimmiscible sulphide globules and that shoshonitic lamprophyreplutons may hold economic promise for the platinum-group elements(PGE). Noble metal abundances in even the most primitive rocksare low compared with many basaltic rocks excepting mid-oceanridge basalt (MORB). This makes doubtful the idea that Megumagroup mesothermal gold deposits, which are spatially and temporallyassociated with the dykes, derived their gold from the lamprophyres.Models that explain PGE concentrations and Pd/Ir ratios in awide variety of mafic rocks suggest that the low noble metalabundances probably reflect metal retention in mantle sulphides,olivine, and PGE alloys at low percentages of melting. However,noble metal abundances are higher than expected if the oceanicslab was involved in the melting process or if the source regionresembled that for most MORB, which appears to have low PGE.Mantle metasomatism represents an unlikely explanation for thePGE abundances, which are equivalent to those expected in alkalinehot-spot magmas. The high PGE abundances of hot-spot magmas(compared with MORB) may be related to the incorporation ofcore materials in deep mantle plumes. Presumably the hot-spotlikemantle became part of the subcontinental lithospheric mantlebefore the subduction and transpressional event that producedthe shoshonitic magmas.