A method to assess event magnitude and target water depth for marine-target impacts. Part 2: The physics behind the observations
Authors: Herreros Isabel , Ormö Jens 
Published: October 2020

AbstractWe present a mathematical model for the study of the aquatic settling process of resurge sediments after a marine-target impact with cratering of the seafloor. Fragments from the solid target are mixed with the seawater. After an initial turbulent phase the suspended solid particles begin to settle. Previous studies of resurge sediments in drill cores from several craters indicate a relationship between the sedimentology, the target water depth, and the magnitude of the event [1]. Here we investigate the physics behind the relationship.1. IntroductionIn this study, we assume that size segregation is correlated to the concentration of the solid particles [2,3] giving rise to the different settling patterns observed in the core samples. The density and viscosity of the solid-water mixture depend on the solid concentration causing a different settling velocity of the particles. For low concentrations, velocities of coarse particles are higher and fine particles are excluded from the lower part of the sediment while for high values of the concentration the settling of the coarse clasts become hindered by the higher concentration of fine grains in the solid-water mixture, thus reducing the packing of coarse fragments at the bottom of the sediment column.2. Mathematical modelFor the study of the size segregation during the particle settling, a fractal model is proposed [4-10]. Two types of particles are considered: (1) Fine particles, s, with radius Rs < Rp, and (2) Coarse particles, p, with radius Rp < RpL, being RpL the upper limit for the fractal behavior. The fractal model developed by Filgueira et al. [6,7] is applied in two steps, taking the settling time for the coarse particles in pure water, tmax, as a time reference:- Step 1: Settling of fine particles with Rs < r < Rp for tmax in pure water: At this step, the settling of the fine particles is calculated, independently on the solid-water concentrations.- Step 2: Settling of coarse particles with Rp < r < RpL for tmax in a solid-water mixture: The coarse particles are assumed to be immersed in a fluid mixture whose density and viscosity depend on the solid concentration. The deposited mass fraction of coarse particles for tmax is calculated at this step, and therefore, the number of settled coarse particles per unit length, i.e. the mean clast frequency , can be approximated.3. Impact crateringAfter a meteorite impact, the transient crater diameter, Dt, can be related to the impactor diameter, d [11]. Considering a column of excavated material as a solid-water mixture column of height H = Hexc = Hw + Hsol, and taking into account that Hexc &#8776; Dt /10 [12], the impactor diameter, d, can be written as a function of the solid concentration.4. ResultsAs explained in previous sections, the mean clast frequency, , and the impactor diameter, d, can both be written in terms of the solid concentration. Therefore it should be possible to represent as a function of the impactor diameter over the depth of the water layer, d/Hw (Figure 1). This function indicates that for small values of d/Hw, i.e. either for small impacts and/or deep water bodies, the sediment pattern will show a higher number of coarse particles per unit length than for the case of very large impacts or very shallow water bodies (or both). This seems to be directly related to the solid concentration which is obviously greater in the case of large impacts and/or shallow seas.To test the function trend obtained with the presented model, published data from five drill cores have been studied. These data have been presented for four natural impact craters, namely Lockne, Tv&#228;ren, Flynn Creek (2 cores) and Wetumpka [1]. All the drill cores are from the deeper parts of the craters (e.g. moat). The values of d/Hw have been obtained performing numerical computations with the code iSALE-2D [13-15]. Figure 1 shows the model result (blue curve) along with the five drill cores&#8217; data sets (dark dots). The model reproduces very well the trend based on observational data.Figure 1: Mean clast frequency vs impactor diameter over the water depth.5. ConclusionsA fractal model for the study of size segregation and particle settling after an impact event in a marine target is presented. The model assumes that the coarse particles will develop a different settling velocity depending on the volumetric concentration of fine particles, giving rise to a variety of size segregation patterns depending on the solid-water mixture properties. The model is able to reproduce the experimental tendency for five cores from four impact craters, for which the core samples were extracted in the deepest parts. However, a larger sample of craters would be desirable in order to further validate the proposed model.AcknowledgementsThe authors would like to gratefully acknowledge CSIC financial support for i-LINK project LINKA20203 and the Spanish State Research Agency (AEI) for project MDM-2017-0737.References[1] J. Orm&#246; et al. EPSC-2020, id. EPSC2020-923.[2] J. Spearman, A.J. &#160;Manning. &#160;Ocean Dynamics 67, 465&#8211;483, 2017.[3] P. Snabre et al. Rheol Acta 48, 855&#8211;870, 2009.[4] S.W. Tyler, S.W. Wheatcraft. Soil Sci. Soc. Am. J. 56, 362&#8211;369, 1992.[5] M. Bittelli et al. Soil Sci. Soc. Am. J. 63, 782&#8211;788, 1999.[6] R.R. Filgueira et al. Soil Sci. Soc. Am. J. 67, 1703&#8211;1706, 2003.[7] R.R. Filgueira et al. Geoderma 134, 327&#8211;334, 2006.[8] B.B. Mandelbrot. &#160;New York: Freeman &amp; Co., 468, 1982.[9] D.L. Turcotte . J. Geophys. Res. 91, 1921&#8211;1926, 1986.[10] P. Baveye et al. Soil Sci. Soc. Am. J. 62, 1469&#8211;1470, 1998.[11] B.C. Johnson et al. Icarus 271, 350&#8211;359, 2016.[12] H.J. Melosh. Oxford Monographs on Geology &amp; Geophysics. Oxford University Press Inc, USA, 1989.[13] A. Amsden et al. Los Alamos National Laboratories Report, LA-8095:101p. Los Alamos, New Mexico: LANL, 1980.[14] K. W&#252;nnemann et al. Icarus 180, 514-527, 2006.[15] G.S. Collins et al. Meteoritics and Planetary Science 39, 217-231, 2004.