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Clastic features in recent halite deposits are observed along the beaches of an artificially dammed part of the Dead Sea. These features include halite oolites (termed halolites in this paper) and ripples. Halite precipitates initially either at the brine surface or on the floor. It is suggested that moderate increase of wave agitation shifts the balance towards brine-surface crystallization, and keeps the growing halite grains in constant motion. In this way rippled structures are formed. A further increase of wave energy leads to the growth of coated halite grains. The accumulation of the various halite grains along the beach, to form soft rippled floor and oolitic beach ridge is brought about during shoreward winds. During calm periods the bulk of the halite crystallizes directly on the floor. It develops into a hard crust which assumes the morphology of the substrate, including the ripple forms. 相似文献
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OLIVER SASS 《Geografiska Annaler: Series A, Physical Geography》2010,92(3):375-391
Lichenometric measurements using Rhizocarpon ssp. were carried out on 20 talus slopes in the cirques of the Finstertal valley (Austria) at an elevation of 2300–3000 m a.s.l. The aim was to assess activity patterns on selected slopes and between the slopes of the study area, to find evidence of rockfall pulses in the last centuries and to calculate rockwall retreat rates. A calibration curve was derived from five sites of known age and adapted to the prevailing size of talus boulders. We measured the five largest lichens on more than 300 boulders and the percentage coverage of Rhizocarpon‐free clasts on more than 1000 test fields. Most of the investigated talus cones are characterized by moderate rockfall supply, with the apex being more active than the talus foot and moderate redistribution by avalanche and debris flows. Considerably enhanced activity was found under rockwalls influenced by permafrost, particularly on the north faces at an elevation of >2600 m a.s.l. At currently moderately active sites, boulder falls seem to have been slightly more frequent in the late nineteenth and first half of the twentieth century. In positions where permafrost is expected in the rockwalls, a weak maximum in the late nineteenth century and highly active present‐day conditions were found, the latter being assigned to current permafrost melt. Rockwall retreat rates derived from lichen coverage are between 400 and 1500 mm/ka which is in good concurrence with talus volume assessments, but higher than the rates derived from direct rockfall measurements. The rates derived from lichen coverage have to be taken with caution as the effects of debris redistribution are hard to quantify. 相似文献
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The log-probability plots of some shallow to deep Atlantic sediments are analysed and compared with simulated mixtures of log-normal populations. The plots consist characteristically of two smoothly connected segments which display different slopes. The point of maximum curvature (PMC) between the two segments is shown to reflect the granulometric parameters (mean and sorting) and quantitative proportions of two mixed overlapping populations. This understanding is contrary to the concept which relies heavily on the break points of cumulative plots as indicating truncation of grain size distributions and therefore reflecting hydrodynamic processes. The particular behaviour of the cumulative plots from the Atlantic indicates fining of the two component populations with depth and distance from the shore, accompanied by an increasing dominance of the finer population. This behaviour is compatible with sorting processes which operate in sediments transported through individual grain movements, rather than through mass transportation or turbidity currents. 相似文献
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Travertines in the northern Hula Valley, Israel 总被引：1，自引：0，他引：1
This study describes the field and petrographic relationships of a widespread deposit of Pleistocene travertines in the northern Hula Valley, northern Israel. The travertines interfinger with conglomerate deposits and basaltic lava flows. Field relationships and radiometric dating indicate that the travertines accumulated intermittently over the past million years, and their formation virtually stopped 25 000 years ago. The travertines are characteristically highly porous. Some of the pores, as well as some spar-filled voids, preserve the shapes of stalks and leaves. The abundance of plant material suggests that photosynthesis, rather than bacterial or abiogenic processes, was the main mechanism which induced carbonate precipitation. The reasons for travertine accumulation in the past and for the cessation of its formation today are ascribed to differences in the palaeogeographic setting. In the past, water flow is viewed as having been mostly sluggish, and a widespread and shallow sheet of water was formed. The lush vegetation, combined with the relatively long residence time of the water in the area, led to increased efficiency of the calcium carbonate precipitation. Today, by contrast, water flows rapidly in gorges, precipitating only a small fraction of its load of dissolved calcium carbonate. The conjectured change in hydrological conditions is ascribed to rejuvenation of faulting activity in the Late Quaternary. 相似文献
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