Three‐dimensional model of modern channel bend deposits
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Abstract:
We present a three‐dimensional model of heterogeneous modern channel bend deposits developed through purely structure‐imitating interpolation (kriging) of hydraulic properties. This model, augmented with ground‐penetrating radar data and directional variograms, agrees with detailed observations in similar modern environments and leads to a process‐based interpretation of the presented hydraulic conductivity structure. Integration of all available information permitted delineation and characterization of the modern streambed as a distinct hydrostratigraphic unit without coring or outcrop studies. Our results imply that the modern streambed is commonly oversimplified in available analytical and numerical models of groundwater‐surface water interactions where it is assumed to be homogeneous and isotropic and characterized by a constant width and thickness. This three‐dimensional approach that integrates concepts and principles developed in sedimentology, hydrogeology, geophysics, and geostatistics has potential implications on model development of stream‐aquifer systems.Keywords:
Ground-Penetrating Radar
Coring
Outcrop
Geostatistics
Ooid
Groundwater model
Temporal variation in ooid size reflects important changes in physical and chemical characteristics of depositional environments. Two numerical models are used to evaluate the effects of several processes influencing ooid size. The first demonstrates that low supply of new ooid nuclei and high cortex growth rate each promote growth of large ooids. The second model demonstrates that high average water velocity and velocity gradient also enhance ooid growth. Several Neoproterozoic oolites contain unusually large ooids, some reaching diameters of up to 16 mm. While lower nuclei supply and higher ooid growth rate may have prevailed prior to the evolution of carbonate-secreting organisms, neither difference can explain the presence of giant ooids in Neoproterozoic deposits because Archean through Mesoproterozoic ooids rarely exceed 5 mm in diameter. In the presence of lower nuclei supply and higher growth rate, high average water velocity may have allowed growth of such large ooids. Higher average water velocity could have been due to a prevalence of carbonate ramps over rimmed shelves during Neoproterozoic time.
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