1,484 research outputs found
Volumetric budget and grain-size fractionation of a geological sediment routing system: Eocene Escanilla Formation, south-central Pyrenees
The supply of sediment and its characteristic grain-size mix are key controls on depositional facies and stratigraphic architectures in sedimentary basins. Consequently, constraints on sediment caliber, budgets, and fluxes are a prerequisite for effective stratigraphic prediction. Here, we investigate a mid- to late Eocene (41.6â33.9 Ma) sediment routing system in the Spanish Pyrenees. We derive a full volumetric sediment budget, weighted for grain-size fractions, partitioned between terrestrial and marine depositional sectors, and we quantify sediment fluxes between depocenters. The paleoâsediment routing system was controlled by syndepositional thrust tectonics and consisted of two major feeder systems eroding the high Pyrenees that supplied a river system draining parallel to the regional tectonic strike and that ultimately exported sediment to coastal, shallow- marine and deep-marine depo centers. We show significant changes in both the volume and grain-size distribution of sediment eroded from the Pyrenean mountain belt during three different time intervals in the mid- to late Eocene, which controlled the characteristics of stratigraphy preserved in a series of wedge-top basins
The time-averaged sediment discharge from source areas increased from ~250 km3/m.y. to 700 km3/m.y. over the 7.7 m.y. interval investigated. This temporal increase in sediment supply caused major westward progradation of facies belts and led to substantial sediment bypass through the terrestrial routing system to the (initially) marine Jaca Basin. The grain-size mix, measured as size fractions of gravel, sand, and fi ner than sand, also changed over the three time intervals intervals. Integration of volumetric and grain-size information from source to sink provides an estimate of the long-term grain-size distribution of the sediment supply, comprising 9% gravel, 24% sand, and 67% finer than sand. The techniques and concepts used in the Escanilla study can profitably be applied to paleoâsediment routing systems in other tectonic and climatic settings and to catchments with a range of bedrock lithology and vegetation. This will promote a better generic understanding of the dynamics of source-to sink systems and provide a powerful tool for forward stratigraphic modeling. The sediment routing system approach has the potential to contribute strongly to new models of sequence stratigraphy
Transient landscapes at fault tips
Fault growth produces patterns of displacement and slip rate that are highly variable in both space and time. This transience is most pronounced near fault tips, where alongâstrike displacement gradients vary in time as the fault array lengthens. We use a set of statistical and field observations to quantify the response of catchments and their associated fans in three large normal fault arrays to transient patterns of displacement and slip rate. Catchments near the fault tips show distinct scaling of channel slope with drainage area compared with catchments near the strike center. This scaling becomes uniform beyond about âŒ10 km from the fault tips and is therefore like footwall relief, largely decoupled from the fault displacement profile. The estimated catchment response times to a change in slip rate also vary between fault tips and strike center. The response times for tip catchments are much longer than the inferred time since fault activity began, indicating that they are unlikely to be in equilibrium with the current fault displacement field. This disequilibrium, combined with the decoupling of slopeâarea scaling from displacement, indicates that landscapes are most sensitive to fault activity near fault tips. Active faults characterized by alongâstrike variation in slip rate thus provide excellent opportunities to explore the transient response of landscapes to tectonic forcing
Nonmonotonic dependence of the absolute entropy on temperature in supercooled Stillinger-Weber silicon
Using a recently developed thermodynamic integration method, we compute the
precise values of the excess Gibbs free energy (G^e) of the high density liquid
(HDL) phase with respect to the crystalline phase at different temperatures (T)
in the supercooled region of the Stillinger-Weber (SW) silicon [F. H.
Stillinger and T. A. Weber, Phys. Rev. B. 32, 5262 (1985)]. Based on the slope
of G^e with respect to T, we find that the absolute entropy of the HDL phase
increases as its enthalpy changes from the equilibrium value at T \ge 1065 K to
the value corresponding to a non-equilibrium state at 1060 K. We find that the
volume distribution in the equilibrium HDL phases become progressively broader
as the temperature is reduced to 1060 K, exhibiting van-der-Waals (VDW) loop in
the pressure-volume curves. Our results provides insight into the thermodynamic
cause of the transition from the HDL phase to the low density phases in SW
silicon, observed in earlier studies near 1060 K at zero pressure.Comment: This version is accepted for publication in Journal of Statistical
Physics (11 figures, 1 table
Landscape evolution at extensional relay zones
It is commonly argued that the extensional relay zones between adjacent crustal-scale normal fault segments are associated with large catchment-fan systems that deliver significant amounts of sediment to hanging wall basins. This conceptual model of extensional basin development, while useful, overlooks some of the physical constraints on catchment evolution and sediment supply in relay zones. We argue that a key factor in the geomorphic evolution of relay zones is the interplay between two different timescales, the time over which the fault array develops, and the time over which the footwall catchment-fan systems are established. Results of numerical experiments using a landscape evolution model suggest that, in isolated fault blocks, footwall catchment evolution is highly dependent on the pattern and rate of fault array growth. A rapidly linked en echelon fault geometry gives rise to capture of relay zone drainage by aggressive catchment incision in the relay zone and to consequent increases in the rate of sediment supply to the hanging wall. Capture events do not occur when the fault segments are allowed to propagate slowly toward an en echelon geometry. In neither case, however, are large relay zone catchment-fan systems developed. We propose several physical reasons for this, including geometric constraints and limits on catchment incision and sediment transport rates in relay zones. Future research efforts should focus on the timescales over which fault array development occurs, and on the quantitative variations in catchment-fan system morphology at relay zones
Effect of adding fat to feedlot rations
Fat is added to commercial feedlot rations as a concentrated energy source and to reduce dustiness and wear of feed processing machinery. We added fat at varying levels ( 0 to 6% of the ration) to study effects from fat and the influence of a surface-active additive. Two hundred 700-pound steers were allotted to 40 pens of 5 each all fed 135 days on the rations show in Table 12, according to the schedule shown in Table 13
Timing and patterns of debris flow deposition on Shepherd and Symmes Creek fans, Owens Valley, California, deduced from cosmogenic 10Be
Debris-flow fans on the western side of Owens Valley, California, show differences in their depths of fan head incision, and thus preserve significantly different surface records of sedimentation over glacial-interglacial cycles. We mapped fan lobes on two fans (Symmes and Shepherd Creek) based on the geometry of the deposits using field observations and high-resolution Airborne Laser Swath Mapping (ALSM) data, and established an absolute fan lobe chronology by using cosmogenic radionuclide exposure dating of large debris-flow boulders. While both fans and their associated catchments were subject to similar tectonic and base level conditions, the Shepherd Creek catchment was significantly glaciated while that of Symmes Creek experienced only minor glaciation. Differences in the depth of fan head incision have led to cosmogenic surface age chronologies that differ in the length of the preserved depositional records. Symmes Creek fan preserves evidence of exclusively Holocene deposition with cosmogenic 10Be ages ranging from 8 to 3 ka. In contrast, the Shepherd Creek fan surface was formed by late Pleistocene and Holocene debris-flow activity, with major deposition between 86-74, 33-15, and 11-3 ka. These age constraints on the depositional timing in Owens Valley show that debris-flow deposition in Owens Valley occurred during both glacial and interglacial periods, but may have been enhanced during marine isotope stages 4 and 2. The striking differences in the surface record of debris-flow deposition on adjacent fans have implications for the use of fan surfaces as paleoenvironmental recorders, and for the preservation of debris-flow deposits in the stratigraphic record
External and intrinsic anchoring in nematic liquid crystals: A Monte Carlo study
We present a Monte Carlo study of external surface anchoring in nematic cells
with partially disordered solid substrates, as well as of intrinsic anchoring
at free nematic interfaces. The simulations are based on the simple hexagonal
lattice model with a spatially anisotropic intermolecular potential. We
estimate the corresponding extrapolation length by imposing an elastic
deformation in a hybrid cell-like nematic sample. Our estimates for
increase with increasing surface disorder and are essentially
temperature--independent. Experimental values of are approached only when
both the coupling of nematic molecules with the substrate and the anisotropy of
nematic--nematic interactions are weak.Comment: Revisions primarily in section I
Clusters of Galaxies: New Results from the CLEF Hydrodynamics Simulation
Preliminary results are presented from the CLEF hydrodynamics simulation, a
large (N=2(428)^3 particles within a 200 Mpc/h comoving box) simulation of the
LCDM cosmology that includes both radiative cooling and a simple model for
galactic feedback. Specifically, we focus on the X-ray properties of the
simulated clusters at z=0 and demonstrate a reasonable level of agreement
between simulated and observed cluster scaling relations.Comment: 7 pages, 4 figures, accepted for publication in Advances in Space
Research (proceedings of the COSPAR 2004 Assembly, Paris
Resistivity of a Metal between the Boltzmann Transport Regime and the Anderson Transition
We study the transport properties of a finite three dimensional disordered
conductor, for both weak and strong scattering on impurities, employing the
real-space Green function technique and related Landauer-type formula. The
dirty metal is described by a nearest neighbor tight-binding Hamiltonian with a
single s-orbital per site and random on-site potential (Anderson model). We
compute exactly the zero-temperature conductance of a finite size sample placed
between two semi-infinite disorder-free leads. The resistivity is found from
the coefficient of linear scaling of the disorder averaged resistance with
sample length. This ``quantum'' resistivity is compared to the semiclassical
Boltzmann expression computed in both Born approximation and multiple
scattering approximation.Comment: 5 pages, 3 embedded EPS figure
Structure, Transport and Magnetic properties in LaSrCoRuO
The perovskite solid solutions of the type
LaSrCoRuO with 0.25 x
0.75 have been investigated for their structural, magnetic and transport
properties. All the compounds crystallize in double perovskite structure. The
magnetization measurements indicate a complex magnetic ground state with strong
competition between ferromagnetic and antiferromagnetic interactions.
Resistivity of the compounds is in confirmation with hopping conduction
behaviour though differences are noted especially for = 0.4 and 0.6. Most
importantly, low field (50Oe) magnetization measurements display negative
magnetization during the zero field cooled cycle. X-ray photoelectron
spectroscopy measurements indicate presence of Co/Co and
Ru/Ru redox couples in all compositions except = 0.5.
Presence of magnetic ions like Ru and Co gives rise to additional
ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the
observed negative magnetization.Comment: Accepted for publication in J. Magn. Magn. Mate
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