Marcelo H. Garcia

Late Miocene–Holocene canyon incision in the western Altiplano, northern Chile: tectonic or climatic forcing?

Abstract: Major fluvial incision (600–1000 m) affecting the Coastal Cordillera and Central Depression of northern Chile is analysed to evaluate supposed coeval uplift of the Altiplano and/or climatic changes in the Atacama Desert. The timing of the beginning of incision is constrained by the age of deposition of the Central Depression top. In the north (18–19°S), this top corresponds to fluvial gravels accumulated between 11.9 ± 0.6 Ma and 8.3 ± 0.5 Ma, which are genetically related to semiarid climate and to an eastward poorly dissected parallel drainage network that developed between 15.0 ± 0.6 and 11.2 ± 0.6 Ma; thus, gravel deposition ended at 11.9–11.2 Ma. To the south (19–20°S), the Central Depression top corresponds to c. 6 Ma alluvial deposits. Stratigraphically determined canyon ages and knickzone locations indicate that southward dissection began later and/or developed under a regime of lower erosion capacity owing to drier climate. Vertical incision rate evolution is compatible with eastward knickzone migration. Dissection required a considerable altitude difference between ancient and present-day river base levels, which was achieved predominantly by basin infill on an already partially elevated bedrock. Therefore subsequent incision would have been triggered by local semiarid climatic periods rather than by contemporaneous surface uplift. Exoreic canyons occur when climatic conditions in the catchments are arid–semiarid whereas endoreism is developed when these conditions in catchments are hyperarid.

Cenozoic tectonic evolution in the Central Andes in northern Chile and west central Bolivia: implications for paleogeographic, magmatic and mountain building evolution

A review of available stratigraphic, structural, and magmatic evolution in northernmost Chile, and adjacent Peru and Bolivia shows that in this region: (1) compression on the Paleogene intra-arc during the middle Eocene Incaic phase formed the NNE-SSW-oriented Incaic range along the present-day Precordillera and Western Cordillera, and (2) post-Incaic tectonic conditions remained compressive until present, contrasting with other regions of the Andes, where extensional episodes occurred during part of this time lapse. A late Oligocene–early Miocene peak of deformation caused further uplift. The Incaic range formed a pop-up structure bounded by two thrusts systems of diverging vergencies; it represented a major paleogeographic feature that separated two domains with different tectonic and paleogeographic evolutions, and probably formed the Andean water divide. This range has been affected by intense erosion and was symmetrically flanked by two major basins, the Pampa del Tamarugal and the Altiplano. Magmatic activity remained located along the previous Late Cretaceous–early Eocene arc with slight eastward shift. Further compression caused westvergent thrusting and uplift along the western Eastern Cordillera bounding the Altiplano basin to the east by another pop-up shaped ridge. Eastward progression of deformation caused eastvergent thrusting of the Eastern Cordillera and Subandean zone.

Nuevos restos de toxodonte (Mamalia, Notoungulata) en estratos de la Formación Chucal, Mioceno, Altiplano de Arica, norte de Chile

espanolSe describen tres piezas dentarias de un mamifero notoungulado, encontradas en estratos de la parte superior de la Formacion Chucal, Altiplano de Arica, norte de Chile (18-19°S). Los caracteres morfologicos permiten referir el material disponible a un Toxodontidae, relacionado con especimenes generalizados de Haplodontheriinae presentes en Argentina, tales como Palyeidodon (= Prototrigodon). Estos ultimos, morfologicamente mas avanzados que aquellos de Edad Mamifero Santacrucense (Mioceno Inferior), proveen evidencias suficientes para asignar al Colloncurense (Mioceno Medio temprano) la edad maxima de los depositos portadores. EnglishThree dental pieces of a notoungulate mammal, found in the strata of the upper part of the Chucal Formation, Altiplano of Arica, northern Chile (18-19°S) are described. The morphological characters allow to refer the available material to a Toxodontidae, related to generalised specimens of Haplodontheriinae as Palyeidodon (= Prototrigodon), which are present in Argentina. These specimens, morphologically more advanced than those of Santacrucian Mammal Age (Lower Miocene), give sufficient evidence to assign to Colloncuran (Early Middle Miocene) the maximum age of the deposits where they are present.

Exploratory study of the influence of the wake produced by acoustic doppler velocimeter probes on the water velocities within measurement volume

Acoustic doppler technique is widely used in both fields and laboratory facilities to compute the mean water velocity and to characterize the turbulence of a flow. In general they provide the three dimensional components of flow velocity in a measurement volume in the water body with fairly good spatial and temporal resolution for engineering applications. The most sophisticated devices can even gauge a velocity profile measuring the water velocity in several measurement volumes along a line. However, these devices are semi int rusive which might have, depending on the experimental setup, substantial consequences in the measurements obtained due the flow perturbation created by the probe. The goal of this paper is to explore experimentally and numerically the wake effect of the p robe on the measurement volume in order to validate the measurements provided by this kind of instruments or incorporate some corrections if needed. A computational fluid dynamic (CFD) model is used to simulate an open channel flow where the model was validated with previous experimental results. In the other hand, the laboratory measurements were conducted in an open channel flume located in the Ven Te Chow Hydrosystems Laboratory of the University of Illinois. The measurements were done using particle image velocimetry technique (PIV) producing two dimensional velocity fields around the acoustic probe measurement volume with and without the presence of the probe. The numerical and experimental ranges of Reynolds numbers (Re) tested were 3x10 6 to 1x10 7 and 1x10 4 to 5x10 4 respectively. Non dimensional contour plots showing the difference between the flow velocity and turbulent quantities with and without the probe are built. Both results show that the errors are less than 10 percent around the probe. This methodology is still under development, however it provides more insight for experimental setups and it could be applied to other acoustic doppler instruments such as the ADV (Acoustic Doppler Velocimeter) and ADCP (Acoustic Doppler Current Profiler) among others.

Exploratory Study of Oscillatory Flow over a Movable Sediment Bed with Particle-Image-Velocimetry (PIV)

Sand ripples develop spontaneously in an oscillatory flow over a movable sediment bed. The mechanics of sediment suspension in the presence of ripples is complex and its direct observation is complicated unless a technique can be used to capture the flow field and particle motion above and around the ripples. This work presents the results of an exploratory study conducted in a U-tube. The goal of the study was to use Particle-Image-Velocimetry (PIV) to capture the interaction between turbulent flow structures, suspended particles, and ripples and other bed instabilities. The PIV technique was chosen because of its potential to simultaneously determine the fluid velocity and the solid-particle velocity fields. Exploratory measurements indicate that the mechanisms responsible for the resuspension of bed particles in the presence of ripples are more complicated than anticipated. As water flows past each ripple, separation takes place at the crest of the ripple. Vortical structures generated downstream of the crest entrain sediment particles into the flow. When the flow reverses direction, each structure, along with the sediment it has suspended, is carried away from the ripples and into the outer flow field. As the structure dissipates, the sediment suspended in it is released. The PIV techniques implemented in this study provide information about velocity, vorticity, and turbulence distributions, helping to explain this complex flow field.

Acoustic Doppler Velocimeters (ADV) Performance Curves (APCs) sampling the flow turbulence

The capability of Acoustic Doppler Velocimeters (ADV) to resolve flow turbulence is analyzed by mean of a new tool denoted ADV Perfomance Curves (APCs). These curves can be used to define optimal flow and sampling conditions for turbulence measurements using an ADV. To generate these curves, a conceptual model is developed which simulates both, different flow conditions (flow component) and the instrument operation (instrument component). Different scenarios (ranges of flow conditions and sampling frequencies) are simulated using the conceptual model to generate several synthetic time series of water velocity and corresponding sampled signals. For the sake of comparison, the main turbulence statistics parameters of the synthetically generated sampled and non-sampled time series are plotted in dimensionless form; these plots are called APCs. The performance of the developed tools is validated using experimental results. Using the APCs a new criterion is proposed to perform ADV measurements with good resolution of the flow turbulence. In cases where this criterion can not be satisfied these curves can be used to make the corrections. Introduction Acoustic Doppler Velocimeters are capable of reporting accurate mean values of water velocities in three directions (Kraus et al.,1994; Lohrman et al., 1994; Voulgaris et al.,1998) even in low flow conditions (Lohrman et al.,1994). On the other hand, the ability of this instrument to accurately resolve flow turbulence is still uncertain (Barkdoll, 2002). Lohrman et al. (1994) argue that the ADV resolution is sufficient to capture a significant fraction of the flow turbulent kinetic energy (TKE), but they identify the Doppler noise as a problem that causes the TKE to be biased high. Most of the research related to the capability of an ADV to resolve turbulence (specifically TKE and spectra) has focused on the definition of this noise level present in the signal and how it can be removed (Lohrman et al.,1994; Anderson et al., 1995; Voulgaris et al., 1998, Nikora et al., 1998, Lemmin et al.,1999 and Mc Lelland et al., 2000). However, little attention has been dedicated to evaluating the filtering effects of the sampling strategy (spatial and temporal averaging) on the turbulent parameters (moments, spectra, autocorrelation functions, etc.). Only Voulgaris et al. (1998) discusses some issues related with the effects of the spatial averaging. As an initial approach, it can be argued that the ADV’s ability to resolve the turbulence will depend on the flow conditions. The objective of this paper is to introduce novel tools, denoted here ADV Perfomance Curves (APCs), which can be used to define optimal flow and sampling conditions for doing turbulence measurements using an ADV. These tools are validated analyzing water velocity signals sampled with ADVs in 10 different experimental setups and flow conditions at the University of Illinois at Urbana–Champaign, USA. Description of ADV operation An ADV measures three-dimensional flow velocities using the Doppler shift principle and consists, basically, of a sound emitter, three sound receivers and a signal conditioning electronic module. The emitter of the instrument generates an acoustic signal that is reflected back by sound-scattering particles present in the water (assumed to move at the water velocity). This scattered sound signal is detected by the instrument receivers and used to compute the signal Doppler phase shift with which the radial flow velocity component is calculated. The ADV uses a dual pulse-pair scheme with different pulse repetition rates (McLelland et al., 2000). The radial velocities vi (i=1, 2, 3) are computed using the Doppler relation. This process of sampling the radial velocities for the three receiver is done by ADV as a whole with frequency fS (equal to 1/T), which is between 100 and 263 Hz depending on the velocity range and the user-set frequency. Then, the radial velocities computed from each receiver are converted to a local Cartesian coordinate system (ux, uy, uz) using a transformation matrix that is determined empirically (through calibration) by the manufacturer (McLelland et al., 2000). During the time it takes to make a three-dimensional velocity measurement (T) the flow may vary, however, these high frequency variations are smoothed out in the process of signal acquisition and cannot be captured by the instrument. The direct implication of these features is that the Cartesian flow velocity represents an averaged value, over an interval time T, of the real flow velocity. In this sense T can be though as the instrument response time, and the process of acquisition itself can be seen as an analog filter with cut-off frequency 1/T (or fS). Two main conclusions can be drawn from the considerations above. The first one is that energy in the signal with frequency larger than fS is filtered out (acquisition process acts as a low-pass filter). The second one is related with aliasing of the signal. Since the velocity signal is sampled at a frequency fS the largest frequency that can be resolved by the instrument is fS/2 (Nyquist theorem, see Bendat et al., 2000). This indicates that energy in the frequency range fS/2<f< fS is folded back in the range 0<f< fS/2. The level of aliased energy will depend on the flow characteristics. After the digital velocity signal is obtained (with frequency fS), the instrument performs an average of N values of this signal producing a digital signal with frequency fR = fS /N, which is the ADV’s user-set frequency with which velocity data is recorded. This averaging process is a digital non-recursive filter (Bendat et al., 2000; Hamming,1983). The consequences of the digital treatment of the signal are analyzed in the following section. Conceptual model A conceptual model is developed here to evaluate the performance of the ADV based on the turbulence characteristics of the flows to be measured with this instrument. The model includes two components, the instrument and the flow components, which simulate both the instrument operation (based on the previous description of how the ADV works) and the power spectrum of flow velocities associated with different flow conditions, respectively. A description of each component is presented next. Instrument component For the purpose of our research, the ADV can be conceptually modeled as a twomodule linear system. The first module is the data acquisition module (DAM). This module encompasses the sound emitter and receivers, the analog to digital converter (ADC), which works at frequency fS, and the computation of flow velocities from the acquired signal. The second module is the data preprocessing module (DPM), which encompasses the averaging of the digital velocity signal that produces data at the user-set frequency fR. The DAM produces a digital signal from the flow velocity, i.e. the input is the flow velocity and the output is a digital signal with frequency fS. This module is modeled through the sampling of synthetic water velocity series produced in the flow component of the conceptual model. The DPM basically performs a time averaging of the output of the DAM in order to produce data at the user-set frequency fR. The output is the water velocity digital signal that the ADV users receive. The low-pass filtering of the signal (in the DPM) has implications in the computation of the spectrum and moments from the signal. The quantification of this effect is discussed later in this paper. Flow component Synthetic water velocities signals need to be generated to represent different ranges of flow conditions. These signals must have turbulence characteristics that resemble realistic conditions. In order to accomplish that a three dimensional model power spectrum is adopted which include all the turbulence characteristics for specified flow conditions. The model power spectrum used in this paper was proposed by Pope (2000). The input parameters of the model are energy-containing-eddy length scale, L, and Kolmogorov length scale η (which can be estimated from the value of the rate of dissipation of turbulent kinetic energy, ε, of the flow). The three dimensional energy-spectrum function, E(κ) predicted by Popé s model is given by ) ( ) ( ) ( 3 / 5 3 / 2 κη κ κ ε κ η f L f C E L − = where C = constant, κ is the wavenumber and fL and fη are shape functions defined as

New remains of toxodont (Mamalia, Notoungulata) in the strata of the Chucal Formation, Miocene, Altiplano of Arica, northern Chile.

Se describen tres piezas dentarias de un mamifero notoungulado, encontradas en estratos de la parte superior de la Formacion Chucal, Altiplano de Arica, norte de Chile (18-19°S). Los caracteres morfologicos permiten referir el material disponible a un Toxodontidae, relacionado con especimenes generalizados de Haplodontheriinae presentes en Argentina, tales como Palyeidodon (= Prototrigodon). Estos ultimos, morfologicamente mas avanzados que aquellos de Edad Mamifero Santacrucense (Mioceno Inferior), proveen evidencias suficientes para asignar al Colloncurense (Mioceno Medio temprano) la edad maxima de los depositos portadores.

Characterizing a December 2005 Density Current Event in the Chicago River , Chicago, Illinois

During the winter months, the Chicago River in Chicago, Illinois is subject to bi dire ctional flows , and density currents are thought to be responsible for the se flow variations. This paper presents detailed field measurements using three acoustic Doppler current profiler instruments and simultaneous water -quality measurements made during December 2005 . Observations indicate that the formation of density currents within the Chicago Riv er and density differences are mostly due to salinity differences between the North Branch and the main stem o f the Chicago Rive r, wh ereas temperature difference does not appreciably affect the creation of density currents. Sources of higher water temperat ure, conductivity, and salinity values should be addressed in future studies.

New methodology to subtract noise effects from turbulence parameters computed from ADV velocity signals

The Acoustic Doppler Velocimeter (ADV) signals are affected by Doppler noise which is intrinsic to the Doppler measurement technique. It has the c haracteristics of white noise and its integral effects must be subtracted from some turbulence parameters. New tools are introduced in this paper to evaluate the relative importance of the noise energy on the total energy as well as to define the characteristic frequency in the measured energy spectrum where noise energy is more important than turbulent energy (flat plateau). To develop these tools a model for the power spectrum is adopted which allows the analysis of a range of flow conditions. Results sho w that noise energy can have an important contribution on the total measured energy, mainly in low energy flows. However in those cases, the noise energy level can be defined from the spectrum because the characteristic frequency is smaller than the Nyquist frequency. After the noise energy level is defined, corrections to the rest of the turbulent parameters (length and time scale, convective velocity, etc) must be performed.