46th Lunar and Planetary Science Conference (2015) 2778.pdf MODELING EJECTA DISTRIBUTION AND MODIFICATION AT MONTURAQUI CRATER, CHILE: A MULTI-SOURCE APPLICATION OF ARCGIS. K. Rathbun1 and I. Ukstins Peate1, 1Dept. of Earth and Environmental Sciences, 121 Trowbridge Hall, Univ. of Iowa, Iowa City IA 52242 USA ([email protected]) Introduction: Monturaqui Crater is a small, welloriginal ejecta thickness were made following the preserved, simple crater located near the southern edge equations of Melosh  assuming a flat pre-impact of the Salar de Atacama basin in northern Chile surface and circular crater morphology. In order to qualitatively estimate paths of material removal, hy(2355’41” S, 6815’42” W). The crater is sub-circular drology tools in ArcMap were used in conjunction with with a preferential NW-SE elongation (370 m E-W, the 2-m DEM, and erosion rate estimates from various 350 m N-S, 34 m average depth) and the southern rim peer-reviewed sources were applied to assess a miniis 10-15 m higher than the northern .The target is mum value for eroded volume. Estimates of erosion comprised of Ordovician granitic basement cut by sevrates in the Atacama Desert vary, but a commonly cited eral 1 – 2 m wide mafic dikes and overlain by thin (0 – figure is <0.1 m Myr-1 [e.g., 7]. 5 m), discontinuous Pliocene Tucucaro ignimbrite . Distribution and modification of ejecta: Use of Granite and ignimbrite are both exposed in the crater the ASTER TIR and IKONOS images for mapping walls. Ejecta consist of unshocked and shocked granite ejecta beyond the rim area was ultimately impractical and ignimbrite with small volumes of dark impact melt due to their low resolution. However, it is likely that rich in Fe and Ni. Previous field mapping collected most of the granitic fragments within the continuous GPS locations for the dark impact melt, which is prefejecta blanket is granitic ejecta. Samples collected beerentially located on the southern and eastern flanks of yond the rim show evidence of shock metamorphism the crater and is discontinuous . The impact event  and a few samples of ignimbrite from the surroundthat formed Monturaqui 663 ± 90 kya  restructured ing area exhibit a frothy texture not commonly seen pre-existing drainage patterns and the ejecta blanket elsewhere in the Tucucaro. Field mapping in proximity has been subsequently dissected by numerous channels to the rim shows distinct populations of granite and that are currently inactive. Although located within the ignimbrite. The 2-m DEM generated in this project hyperarid part of the Atacama Desert, the area is subshows that most areas of higher elevation are comjected to infrequent and low-volume precipitation prised of granite. This also corresponds to higher albeevents that facilitate erosion . Monturaqui is of indo areas in IKONOS imagery. Dark impact melt occurs terest due to its uneven distribution of granitic and dark only on the SE flank of the crater. It has been hypotheimpact melt ejecta. The distribution could be a primary remnant of the impact event or a result of preferential erosion. Field observations suggest much of the ejecta have been removed  although this has yet to be confirmed by studying the stratigraphic profile at the site. The objective of this study is to digitize and combine existing data to assess the evolution of ejecta distribution around Monturaqui since its emplacement. Data and Methods: Due to the isolated nature of the field area, a remote sensing approach was combined with field-based data previously collected. Multispectral and panchromatic imagery at 1-m and 4-m resolution were acquired from IKONOS for use as a base map and to digitize geographic features. The global 10-m resolution digital elevation model (DEM) and multiple thermal infrared (TIR) images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) onboard Terra were obtained to model topography and map surFig. 1. Lithologies at Monturaqui superimposed on a 2 m DEM of the site. face ejecta. A 2-m resolution DEM of the local Warmer colors are higher in elevation than the blues. On top, pale yellow is granarea was generated by digitizing contours in ite, bright green is the mafic dikes, the red line denotes the crater rim, and uncolored areas are ignimbrite. After Cukierski . ArcMap from existing maps and calculations of 46th Lunar and Planetary Science Conference (2015) 2778.pdf sized that this could be directly related to the trajectory 196.  Melosh H.J. (1989) Impact Cratering: A geoof the projectile . logic process, Oxford University Press, 245 pp.  The ejecta blanket has been dissected by new Nishiizumi K. et al. (2005) EPSL, 237,499-507.  drainage patterns in the past 663 kyr that seldom reacKloberdanz C. (2010) Geochemical analysis of the tivate due to the hyperarid nature of the Atacama. All Monturaqui impact crater, Chile. M.S. thesis, Univ. of of the fine-grained ejecta at the surface has been blown Iowa, 188 pp.  Cukierski D. (2013) Textural and away although fine-grained material is present a few compositional analysis of Fe-Ni metallic spherules in centimeters below the surface . It is uncertain if this impact melt from Monturaqui Crater, Chile. M.S. thematerial is impact-related or not and further analysis sis, Univ. of Iowa, 135 pp. must be done to confirm its origin. Modeling of flow paths in ArcMap suggests larger material may have been removed via two primary channels on the NW and SE sides of the crater, although this only represents paths of highest probable flow accumulation and is only an approximation since it is based on a DEM. Numerous smaller channels that originate on the crater flanks are clearly visible in satellite images. The northern channel dissects an area with little granite but granite is abundant in the area of the southern channel. It remains unclear whether the uneven distribution of granitic ejecta is a primary artifact or the result of preferential erosion. Assuming there were approximately 4 – 5 m of ejecta deposited within the continuous ejecta zone, several meters of ejecta could still persist given the Atacama’s extremely low rates of erosion, and particularly concerning the granitic fragments. If the <0.1 m Myr-1 rate represents a minimum value, and not accounting for grain size distributions, <7 cm depth of material, or ~54,000 m3, could have been removed from the continuous ejecta blanket alone. Future work should generate a larger field map of the ejecta and determine its actual thickness and distribution. Conclusions: The uneven distribution of granitic ejecta cannot be easily explained without further study. Dark impact melt is present only on the SE flank but this is more likely related to the trajectory of the projectile preferential than erosion. Previous field observations of ejecta thickness do not agree with calculated estimates and further work must be done to accurately map ejecta distribution and measure its actual thickness. References:  Ugalde H. et al. (2007) Meteoritics & Planet. Sci., 42, 2153-2163.  Ramirez C. and Gardeweg M. (1982) Carta Geologica de Chile, Hoja Toconao.  Ukstins Peate I. (2014) pers. comm.  Ukstins Peate I. Fig. 2. (top) Ejecta thickness contours calculated after Melosh  and modeled in et al. (2010) LPS MMX, Abstract #2161.  ArcMap. The current model does not account for topography or the lithologic distribuHaug E.W. et al. (2010) Geomorph., 121, 184tion of ejecta. (bottom) A Flow Accumulation model from ArcMap superimposed on calculated ejecta zone limits. Darker lines represent areas of higher probable flow accumulation.
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