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ERICH DRAGANITS CHRISTOPH JANDA 《Boreas: An International Journal of Quaternary Research》2003,32(2):436-442
Subaqueous, bowl-shaped depressions found in a Himalayan pond formed in an abandoned river channel in the Lingti Valley (Spiti, NW India) are spring pits (Quirke 1930 ). The occurrence of the spring pits is restricted to the western end of the pond, where coarse-grained, highly permeable alluvial fan material continues below lacustrine mud deposits. The spring pits formed by active vertical discharge of ground water from an underlying artesian alluvial fan aquifer, confined by the overlying fine-grained lacustrine sediments. The aquifer is continuously recharged by down-slope ground-water flow in the alluvial fan. These small artesian springs are comparable with much larger artesian springs described in the literature and a similar mechanism of formation is proposed. Some similarities in their shapes and mechanisms of formation may indicate that spring pits represent small, nearshore examples of pockmarks. The differences of these features, formed by persistent fluidization from short-lived seismic liquefaction processes, are discussed and the utility of the structures for palaeo-environmental and palaeo-ground-water interpretation is evaluated. 相似文献
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CHRISTOPH SPÖTL 《Sedimentology》1990,37(5):837-845
Complex aluminium phosphate-sulphate minerals of the hinsdalite group have been identified as early diagenetic precipitates in Late Permian sandstones of the Northern Calcareous Alps, western Austria. According to their chemical composition they can be regarded as solid solutions between woodhouseite, svanbergite, crandallite and goyazite. The model proposed for their origin involves the dissolution of detrital apatite in a low-pH environment and subsequent precipitation of aluminium phosphate-sulphate minerals, which clearly pre-date syntaxial quartz cementation of the sediment. Their occurrence probably has been overlooked in other sandstones showing diagenetic apatite dissolution. The recognition of such minerals could provide a significant insight into early phosphate diagenesis. 相似文献
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Dark grey, bituminous dolostones interbedded with marine-derived anhydrite horizons occur in the Triassic Reichenhall Formation of western Austria. Fossils are rare and indicate a hostile, hypersaline depositional environment. The dolomites are finely crystalline, fairly stoichiometric, well ordered and non-ferroan. Closely spaced samples (94 in total) of individual dolomite units have been analysed for their carbon and oxygen isotopic composition. The data indicate surprisingly low δ18O values (-5.7 to -2.1%0 PDB), whereas the δ13C values are comparable to the contemporary Triassic seawater (+0.2 to +2.6%0 PDB). Sedimentological evidence, including (i) lack of any evidence for extensive dissolution, (ii) distinct oxygen and carbon isotope ratios of individual dolomite units, (iii) covariance of carbon and oxygen isotopes within some dolomite layers and (iv) inclusions of celestite in dolomite, indicates a nearly closed system after early diagenesis. Combining this information with water-rock interaction calculations suggests that the lightest oxygen isotope compositions are the result of freshwater influx into the basin during very early dolomite formation. A secondary factor may be dolomite recrystallization at elevated temperatures during burial. 相似文献
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JORIE CLARK A. MARSHALL McCABE CHRISTOPH SCHNABEL PETER U. CLARK STEWART FREEMAN COLIN MADEN SHENG XU 《Boreas: An International Journal of Quaternary Research》2009,38(1):111-118
A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic 10 Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8±1.0 10 Be kyr to 20.9±1.3 10 Be kyr, with an uncertainty-weighted mean age of 19.4±0.3 10 Be kyr (19.4±1.2 kyr accounting for production rate uncertainty). Our results confirm one previous 10 Be age obtained from the moraine, with the combined data ( n =7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3±0.3 10 Be kyr (19.3±1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated 14 C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (≥20–≤18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS ∼19.5–20 kyr ago. 相似文献
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Magnesite diagenesis in redbeds: a case study from the Permian of the Northern Calcareous Alps (Tyrol, Austria) 总被引：1，自引：0，他引：1
The Upper Permian Gröden Formation of the Northern Calcareous Alps (Austria) is composed of alluvial fan and playa lake sediments that were deposited in intramontane basins. A conspicuous feature of these redbeds is the abundance of magnesite in the form of nodules and discrete layers in mudstones as well as intergranular cement in sandstones. Sedimentological observations indicate that the bulk of these carbonates formed during early diagenesis and were probably syndepositional. Petrographically, most magnesites consist of micrite or, less commonly, microspar. An early non-ferroan magnesite is post-dated by later stage ferroan magnesites. Nodules consisting of recrystallized, sparry magnesite were observed only at one location. The general absence of relics of a non-magnesite precursor mineral and the occurrence of shrinkage features suggest that the fine grained magnesites formed by transformation of a hydrated magnesium carbonate mineral, e.g. hydromagnesite. Carbon, oxygen, sulphur and strontium isotope ratios in conjunction with sedimentological criteria support a model of (hydro)magnesite precipitation in an inland playa lake system, which was fed by run-off from the surrounding hinterland. The scarcity of evaporites and the dominance of magnesite over calcite and dolomite suggest that the playa lake brines were low in sulphate and had high Mg/Ca ratios. The source for the high magnesium concentrations is thought to be the weathering of Devonian dolostones and associated massive magnesite deposits in the catchment area. 相似文献
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CHRISTINA KRAJEWSKI LOTHAR DRESEN CHRISTOPH GELBKE HORST RÜTER 《Geophysical Prospecting》1989,37(7):717-751
The possibilities for reconstructing seismic velocity distributions containing low-velocity anomalies by iterative tomographic methods are examined studying numerical and analogue 2D model data. The geometrical conditions of the model series were designed to generalize the geometrical characteristics of a typical cross-hole tomographic field case. Models with high (30%) and low (8%) velocity contrasts were realized. Traveltimes of 2D ultrasonic P-waves, determined for a dense net of raypaths across each model, form the analogue data set. The numerical data consists of traveltimes calculated along straight raypaths. Additionally, a set of curved-ray traveltimes was calculated for a smoothed version of the high-contrast model. The Simultaneous Iterative Reconstruction Technique (SIRT) was chosen from the various tomographic inversion methods. The abilities of this standard procedure are studied using the low-contrast model data. The investigations concentrate on the resolving power concerning geometry and velocity, and on the effects caused by erroneous data due to noise or a finite time precision. The grid spacing and the source and receiver patterns are modified. Smoothing and slowness constraints were tested. The inversion of high-contrast analogue model data shows that curved raypaths have to be considered. Hence, a ray-tracing algorithm using velocity gradients was developed, based on the grid structure of the tomographic inversion. This algorithm is included in the SIRT-process and the improvements concerning anomaly localization, resolution and velocity reconstruction are demonstrated. Since curved-ray tomography is time-consuming compared with straight-ray SIRT, it is necessary to consider the effects of grid spacing, ray density, slowness constraints and the 相似文献
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Experiments were carried out in a 10 × 6 m basin to simulate turbidity currents generated by the Horgen Slumping Events of 1875. The conditions for kinematic similarity were satisfied and the experiments gave further insight into the mechanics of transport triggered by the Horgen slumps. The experimental turbidity currents laid down thick deposits on a subaqueous fan, and thin sheets of turbidite on the floor of the elongate basin through longitudinal transport, comparable with the simulated deposits in Lake Zurich. It is concluded that longitudinal transport is a general phenomenon of turbidity currents. 相似文献
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The aftermath of the Carnian carbonate platform demise: a basinal perspective (Dolomites, Southern Alps) 总被引：1，自引：0，他引：1
In the Dolomites of northernmost Italy the carbonate‐platform growth came to a standstill late in the Early Carnian (Late Triassic). The response to this shutdown of shallow‐water carbonate production in the interplatform basins is largely unknown because erosion has removed most of the soft basinal sediments, giving rise to today's scenic landscape of the Dolomites. Mapping in the central part of the Dolomites and newly available core material has recently revealed a well‐preserved succession of basinal rocks within the Heiligkreuz Hospiz Basin (ital. Ospizio di Santa Croce Basin). In this paper, the regional depositional nature of arrested carbonate platform production is reconstructed by tracing its sedimentological record across the slope and into the basin. The uppermost St. Cassian Formation, the time‐equivalent basinal rocks to the prograding carbonate platforms, is overlain by the Heiligkreuz Formation, whose basal succession was deposited in a restricted and oxygen‐depleted environment immediately post‐dating the platform demise. The succession consists mainly of mudrocks, marlstones, and peloidal packstones, with abundant low‐diversity ostracod and pelecypod fauna and early diagenetic dolomite. C and O isotope values of the basal Heiligkreuz Formation, post‐dating platform demise, average + 2·4 and ? 2·4‰, respectively, and largely overlap the isotopic composition of St. Cassian carbonates. A shift toward slightly lower δ13C values in the Heiligkreuz Formation may reflect incorporation of isotopically depleted C released during bacterial sulphate reduction in the Heiligkreuz sediments. Sedimentological, palaeobiological and geochemical indices suggest that near‐normal marine conditions persisted long after the shutdown of shallow water carbonate‐platform growth, although there are clear indications of severely reduced oxygen levels in the restricted Heiligkreuz Hospiz interplatform basin. The Early Carnian platform demise induced a distinct switch in the locus of carbonate production from the shallow‐water platform and slope to the basin floor and a decrease in the availability of dissolved oxygen in the basinal waters. It is inferred that anoxia extended at least temporarily to the top of the carbonate slope, as indicated by the onlap of normal‐marine mounds by dark marlstones of the basal Heiligkreuz Formation. 相似文献