Lake Bonneville marl provides a stratigraphic record of lake history preserved in its carbonate minerals and stable isotopes. We have analyzed the marl in shallow cores taken at three localities in the Bonneville basin. Chronology for the cores is provided by dated volcanic ashes, ostracode biostratigraphy, and a distinctive lithologic unit believed to have been deposited during and immediately after the Bonneville Flood.A core taken at Monument Point at the north shore of Great Salt Lake encompasses virtually the entire Bonneville lake cycle, including the 26.5 ka Thiokol basaltic ash at the base and deposits representing the overflowing stage at the Provo shoreline at the top of the core. Two cores from the Old River Bed area near the threshold between the Sevier basin and the Great Salt Lake basin (the main body of Lake Bonneville) represent deposition from the end of the Stansbury oscillation ( 20 ka) to post-Provo time ( 13 ka), and one core from near Sunstone Knoll in the Sevier basin provides a nearly complete record of the period when Lake Bonneville flooded the Sevier basin (20–13 ka).In all cores, percent calcium carbonate, the aragonite to calcite ratio, and percent sand were measured at approximately 2-cm intervals, and 18O and 13C were determined in one core from the Old River Bed area. The transgressive period from about 20 ka to 15 ka is represented in all cores, but the general trends and the details of the records are different, probably as a result of water chemistry and water balance differences between the main body and the Sevier basin because they were fed by different rivers and had different hypsometries. The Old River Bed marl sections are intermediate in position and composition between the Monument Point and Sunstone Knoll sections. Variations in marl composition at the Old River Bed, which are correlated with lake-level changes, were probably caused by changes in the relative proportions of water from the two basins, which were caused by shifts in water balance in the lake.This is the second paper in a series of papers published in this issue on Climatic and Tectonic Rhythms in Lake Deposits. 相似文献
An original theoretical model has been devised to simulate mass flow over hill slopes due to gravitational sliding. The sliding mass is discretized into a sequence of contiguous blocks which are subjected to gravitational forces, to bottom friction and to surface resistance stresses that are generally negligible for subaerial flows, but are relevant for submarine slides. The blocks interact with each other while sliding down the hill flanks because of internal forces that dissipate mechanical energy and produce a momentum exchange between the individual blocks, yet conserving the total momentum of the mass. Internal forces are expressed in terms of interaction coefficients depending on the instantaneous distance between the block centers of mass, which is a measure of the deformation experienced by the blocks: the functional dependence includes three parameters, namely the interaction intensity ¯, the deformability parameter and the shape parameter , by means of which a wide range of interaction types can be fully accounted for. The time integration is performed numerically by solving the equations for the block velocities and positions at any time ti by means of the block accelerations at the previous time ti-1, and by subsequently updating the block accelerations, which allows to proceed iteratively to the following times. The model has been tested against laboratory results available from literature and by means of several numerical experiments involving a simplified geometry both for the sliding body and the basal surface, with the purpose of clarifying the influence of the model parameters on the slide dynamics. The model improves the performance of the existing kinematic models for slides, moreover preserving an equivalent numerical simplicity. Future applications and possible improvements of this model are suggested. 相似文献
Although the general aspects of oolitic depositional systems are well documented, seascape‐scale (≈103–106 m2) patterns of oolitic shoals and the details of processes acting on them are not well understood or quantified. To begin to fill this basic gap in understanding, this paper describes the morphology and hydrodynamics of Lily Bank, a Modern tidally dominated Bahamian ooid shoal. In this study, integrating remote sensing imagery with quantitative, geo‐located bathymetrical, hydrological and granulometric data in a Geographic Information System documents geomorphic and sedimentological patterns and facilitates interpreting these patterns in the context of the processes operating in this system. The results of these analyses reveal that parabolic bars up to several kilometres in wavelength and several metres in height form a common morphologic motif, although there is considerable variation on that general theme. The seascape‐scale configuration of bars and superimposed sedimentary structures is closely linked to spatial patterns of tidal movements, and includes the presence of mutually evasive flood and ebb channels. Sedimentologically, bars are neither homogenous nor random bodies; instead, granulometric parameters such as sorting and percentage mud vary systematically, as shaped by hydro‐geomorphic controls. The best sorted, coarsest ooids are on bar crests, whereas the finest grains are found in the lower energy, deeper interior and flanking regions. In short, results clearly document hydrodynamic‐bathymetrical influences on these ooid shoals and their granulometry, linkages akin to siliciclastic analogues. Sedimentological, hydrodynamic and geomorphic observations are consistent with a conceptual model for the formation of parabolic bars in which initial irregularities in non‐parabolic bars are enhanced through their effect of focusing flow. Constricted flow leads to higher flow velocities, tidal flow velocity asymmetries, differential net sediment transport and growth of bathymetrical highs. This bathymetrical divergence creates separate paths for flood‐ and ebb‐tides, facilitating emergence of better‐developed parabolic forms. The resultant parabolic geometries and component bedforms appear to be either in dynamic equilibrium with both ebb‐ and flood‐tide flows, or evolving toward that state. In exploring patterns and processes within carbonate shoals, this study illustrates some of the first documented insights on quantitative details of morphology and dynamics and in the links between geomorphic framework and grain‐size and sorting in an oolitic carbonate system. Assuming a continuity of processes between ancient and modern, the insights from this shoal provide information on possible facies geometries and on the characteristics of grains and depositional porosity of analogous facies within ancient ooid shoals. 相似文献
We report here a multiphase mineral inclusion composed of quartz, plagioclase, K-feldspar, sapphirine, spinel, orthopyroxene, and biotite, in porphyroblastic garnet within a pelitic granulite from Rajapalaiyam in the Madurai Granulite Block, southern India. In this unique textural association, hitherto unreported in previous studies, sapphirine shows four occurrences: (1) as anhedral mineral between spinel and quartz (Spr-1), (2) subhedral to euhedral needles mantled by quartz (Spr-2), (3) subhedral to anhedral mineral in orthopyroxene, and (4) isolated inclusion with quartz (Spr-4). Spr-1, Spr-2, and Spr-4 show direct grain contact with quartz, providing evidence for ultrahigh-temperature (UHT) metamorphism at temperatures exceeding 1000 °C. Associated orthopyroxene shows high Mg/(Fe + Mg) ratio ( 0.75) and Al2O3 content (up to 9.6 wt.%), also suggesting T > 1050 °C and P > 10 kbar during peak metamorphism.
Coarse spinel (Spl-1) with irregular grain morphology and adjacent quartz grains are separated by thin films of Spr-1 and K-feldspar, suggesting that Spl-1 and quartz were in equilibrium before the stability of Spr-1 + quartz. This texture implies that the P–T conditions of the rock shifted from the stability field of spinel + quartz to sapphirine + quartz. Petrogenetic grid considerations based on available data from the FMAS system favour exhumation along a counterclockwise P–T trajectory. The irregular shape of the inclusion and chemistry of the inclusion minerals are markedly different from the matrix phases suggesting the possibility that the inclusion minerals could have equilibrated from cordierite-bearing silicate-melt pockets during the garnet growth at extreme UHT conditions. 相似文献