We report Rb-Sr, Sm-Nd and mineralogical data for leached whole rock, < 2 btm and < 0.1 #m clay size fractions of lower-to mid-Tertiary shales spanning the smectite/illite transition, from a single deep well in the Texas Gulf Coast. The abrupt transition at 2400 m, from smectite-to illite-dominated assemblages is accompanied by a marked increase in 87Sr/S6Sr from 0.708 to 0.711 in exchangeable sites. Leached authigenic illite from the < 0.1 /~m fraction, sampled from below the smectite/illite transition, defines a Rb-Sr isochron age of 34.8 _+ 2.0 Ma. These data are consistent with a single episode of authigenic illite precipitation over a depth range of > 1200 m, independent of the duration of burial. The exchangeable Sr in the clay has remained an open system and has undergone isotopic exchange since that time. Minimum fluid/rock ratios of 0.2 to 2 are modeled from Sr isotopic data, consistent with rock-dominated, open system diagenesis. Leachates and residues of the < 0.1/zm fraction, below the smectite/illite transition appear to have achieved Nd isotopic equilibrium during clay diagenesis, and show no signs of subsequent exchange. The clay residues have low 147Sm/144Nd (< 0.085), indicating that authigenic illite has a lower Sm/Nd ratio than its bulk rock protolith. Conversely the coexisting exchangeable REE inventory has elevated Sm/Nd, as reflected by leachate 14VSm/144Nd as high as 0.188. This Sm/Nd fractionation, in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid-rock interaction in older rocks.
We report Sm-Nd and Rb-Sr data for the fine fractions of Lower Paleozoic argillaceous rocks from Wales, UK, and New York, USA, spanning the range of low-grade metamorphic conditions from the diagenetic zone (zeolite facies) to the epizone (greenschist facies). In all cases, leaching of the fine fractions results in a high 14'Sm/ 144Nd (0.09-0.29) acid-soluble component and a complementary low '47Sm/ '44Nd (0.05-O. 14) residual component. The observed fractionation is an ancient feature related to diagenesis, burial, and metamorphism. The magnitude of Sm-Nd fractionation between leachates and residues, as well as the resulting Sm-Nd ages, vary as a function of grain size and metamorphic grade. Uncleaved Welsh mudrocks of the diagenetic zone yield Sm-Nd leachate-residue ages of 453-484 Ma, in agreement with their Llanvimian to Caradocian biostrati~aphic ages, whereas higher grade rocks of the anchizone and epizone yield Sm-Nd ages as young as 413 Ma. These ages are transitional between the time of deposition and the time of regional deformation related to the Acadian Orogeny at 390 Ma. Distinct convex-upward rare earth element (REE) patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock, forcing a depletion of middle REEs on the subsequently formed diagenetic phyllosihcates. The amount of organic matter present and the extent of later prograde reactions are probable modifiers of this ~~ionation process. Ordovician and Devonian elastic rocks associated with the Trenton and Onondaga limestones of New York yield single-sample and multi-sample Sm-Nd isochron ages that agree well with their biostratigraphic ages of 454 Ma and 390 Ma, respectively. The REE fractionation observed in shale leachates of the Ordovician Utica Formation is related to Ca/Mg of the bulk rock, and hence to the composition of the diagenetic carbonate cement. In all cases the Sm-Nd system remained closed subsequent to the peak of diagenesis or metamorphism, including the North American rocks that show no evidence of being isotopically reset during widespread remagneti~tion of the subjacent limestone units in the late Paleozoic.
The interaction between surface water and groundwater during flood events is a complex process that has traditionally been described using simplified analytical solutions, or abstracted numerical models. To make the problem tractable, it is common to idealize the flood event, simplify river channel geometry, and ignore bank soil heterogeneity, often resulting in a model that only loosely represents the site, thus limiting its applicability to any specific river cross‐section. In this study, we calibrate a site‐specific fully‐integrated surface and subsurface HydroGeoSphere model using flood events for a cross‐section along the South River near Waynesboro, VA. The calibration approach presented in this study demonstrates the incorporation of fining direction regularization with a highly parameterized inversion driven by natural stimuli, to develop several realistic realizations of hydraulic conductivity fields that reflect the depositional history of the system. Specifically, we calibrate a model with 365 unique material zones to multiple flood events recorded in a dense well network while incorporating possible fining sequences consistent with the depositional history of the riverbank. Over 25,000 individual simulations were completed using calibration software and a cloud platform specifically designed for highly parallelized computing environments. The results of this study demonstrate the use of fining direction regularization during model calibration to generate multiple calibrated model realizations that account for the depositional environment of the system.
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