Earth hummocks in north-east Okstindan,northern Norway: Morphology,distribution and environmental constraints

Gurney, S.D. & Hayward, S. 2015. Earth hummocks in north-east Okstindan, northern Norway: Morphology, distribution and environmental constraints. Norsk Geografisk Tidsskrift–Norwegian Journal of Geography. ISSN 0029-1951.

Earth hummocks (also termed pounus or thúfur) are a common form of periglacial non-sorted patterned ground. The study objectives were to determine the morphology, distribution and development on slopes of earth hummocks in north-east Okstindan, Norway, an area with many hummocks but few documented accounts. The methodology involved detailed geomorphological mapping and precise measurement with a profileometer. The internal structure of the hummocks was investigated through excavations and sediment sample analyses. Fourteen sites with well-developed earth hummocks (accounting for over 650 individual hummock forms) were investigated. The sites have an average altitude of 750?m and occur on slopes with an average gradient of 7°. The hummock heights are in the range 0.11–0.52?m and their diameters 0.7–1.5?m, although coalescent forms are up to 5?m in length. The hummock morphology is characterised by a variable plan form, asymmetry with respect to upslope and downslope forms, downslope elongation, coalescence, and superimposed microtopography. The hummocks’ distribution appeared to have been controlled by the existence of a frost-susceptible ‘host’ sediment, but moisture availability and topographic position played a role. The authors conclude that differential frost heave and vegetation cover stability are critical for the hummocks’ longevity in the studied landscape.     

青藏高原东北部冬给措纳湖湖区冰缘环境探讨

冬给措纳湖位于青藏高原多年冻土区东北部,湖区高寒沼泽、冰缘环境、冻土分布及其演化在全球气候变暖条件下,有其自身特点.为探讨冬给措纳湖湖区冻土分布及冰缘环境,2009年5月9—26日,对冬给措纳湖进行了考察,发现冻融草丘、冻胀丘遗迹、寒冻裂缝、热融洼地、古冻融褶皱等现代冰缘及古冰缘现象在湖区普遍存在.湖东、西岸的沼泽湿地中,热喀斯特发育明显,形成热融洼地和冻融草丘.湖北岸阶地及草场中存在零星和岛状冻土,经钎探表明,当时融化深度为0.3～0.8m,0.4m深度处冻土有胶结冰发育.湖东冲积平原的沼泽湿地中,冻融草丘和热融洼地存在;冻胀丘遗迹、寒冻裂缝在沼泽湿地边缘泥炭覆盖地存在.湖北岸二级阶地剖面发现古冻融褶皱.     

Sedimentology in metamorphic rocks,the Willyama Supergroup,Broken Hill,Australia

In the upper greenschist to granulite grade rocks of the Willyama Supergroup at Broken Hill, Australia, earlier recognition of metamorphosed graded bedding in siliciclastic metasedimentary rocks led to interpretations of these rocks as deep-water turbidites. However, graded beds can also be deposited in shelfal environments below storm wave base. This study identified other tempestite features including wave oscillation ripples, hummocky cross-stratification and swaley cross-stratification indicating that deposition took place above the wave base of the larger storms.

Albitised metasedimentary rocks of the upper Thackaringa Group show structures such as swaley cross-stratification typical of shallow-water conditions above fair-weather wave base. Deposition of the Broken Hill Group commenced with muddy Allendale Metasediments conformable on the Thackaringa Group. The Ettlewood Calc-Silicate Member, originally a dolomitic, siliceous sediment, is interpreted as coastal sabkha indicating onset of a marine transgression. The Parnell Gneiss represents a volcanic or volcaniclastic interruption, heralding gradually increasing input of sand in the Freyers Metasediments reaching a maximum in middle Freyers Metasediments, followed by an abrupt reversion to mud, still influenced by wave action. An open marine shelf is interpreted, possibly 30 m deep (no more than 100 m) in the final stage of a developing rift. The Broken Hill Group terminated with the massive Hores Gneiss volcanic unit.

Sedimentation of the siliciclastic Sundown Group took place in similar conditions, commencing with a muddy interval overlying the Hores Gneiss. The shallowing produced by ～90 m thickness of volcanic/volcaniclastic Hores Gneiss was compensated by subsidence.

Paragon Group deposition commenced with substantial black mud, resulting from isolation from the sand supply and probably isolation from the sea. A fresh connection to the sea led first to the deposition of dolomitic carbonate (King Gunnia Calc-Silicate Member), then to deposition of parallel-laminated fine sand below wave base (upper Cartwrights Creek Metasediments), followed by ripple cross-laminated sand above wave base (Bijerkerno Metasediments). The Dalnit Bore Metasediments show abundant very thin graded silt–mud units possibly deposited below storm wave base, and thicker units of stacked wave oscillation ripples deposited above the wave base of larger storms.

The Broken Hill orebody is hosted by altered Broken Hill Group metasedimentary rocks deposited at water depths of ～30 m. Unless the ore fluid temperature was less than 150°C, it is likely that the orebody formed below the seafloor: at such shallow-water depths, the confining pressure would be inadequate to suppress boiling of hotter rising hydrothermal fluids.     

Characteristics of earth hummocks (pounus) with and without permafrost in Finnish Lapland

Finnish Lapland north of 68°30'N latitude is located in the zone of discontinuous permafrost. Two main types of permafrost have previously been found in northern Finland: palsas in the mires and frost in the bedrock on the barren fell summits. The aim of this study was (1) to investigate permafrost occurrence in the peaty earth hummocks (pounus) in several mires, and (2) model characteristics of pounus with and without permafrost.
This study showed that permafrost in Finnish Lapland occurs much more widely and commonly than was previously known. A total of 59% of the studied pounus were found to contain permafrost. Over 90% of the permafrost occurrence in the pounus was correctly classified in logistic regression modelling. The probability of permafrost in a pounu decreased with the height of vegetation, and increased with the pounu height and distance from the running stream. There were clear vegetation differences between pounus with and without permafrost. Unfrozen pounus are characterized by forest and mire species, whereas on the permanently frozen pounus the vegetation is patchier with species indicating drier conditions. Pounus provide an excellent object to study short–term and local variations in permafrost formation due to their small size. They react quickly to variation in temperature, snow depth and precipitation. We conclude that pounus can be classified as sporadic permafrost features in northernmost Europe under modern climatic conditions.     

Earth hummocks (thúfur): new insights to their thermal characteristics and development in eastern Lesotho,southern Africa

The aspect‐controlled variations in soil freezing within earth hummocks of eastern Lesotho (southern Africa) are analysed. Ground thermal data were measured for an earth hummock from late autumn to early spring in 1995 and 1996, using Tinytalk? data loggers. During 1995, ground temperatures were recorded at 15 and 20 cm depth on the hummock north, east, south and west aspects, whilst in 1996 temperatures were recorded at 1 cm, 5 cm and 10 cm on the north and south aspects. The data from 1995 indicate that soil freezing commences on the hummock southern aspects and gradually progresses towards the western and northern aspects, whilst the eastern aspect remained unfrozen throughout winter. The data from 1996 indicate that a thick snow cover almost nullifies the temperature differences between the hummock northern and southern aspects. However, given the relative absence of snow during contemporary winters, freeze intensity and duration is longest on the hummock southern and western aspects, which helps explain earth hummock deformation (elongation and coalescence) in a southwesterly direction on slope gradients ≤3°. Copyright © 2005 John Wiley & Sons, Ltd.