Magnitude of marine impacts: Size segregation patterns as an observational assessment.

1. Introduction

Previous studies of resurge sediments from several marine-target impact craters indicate a relationship between the sedimentology, , the target water depth, H, and the magnitude of the event, d. This offers a valuable opportunity to obtain one of these variables if the other two are known. However, the mechanisms controlling this relationship have not been unraveled yet. During the cratering process fragments from the solid target are mixed with the seawater and, after an initial turbulent phase, they get deposited inside the seafloor crater when filled up by the resurging water. We present a mathematical model for the aquatic settling process of the resurge material as a feasible explanation for the observational data.

2. Methods

2.1. Fractal model for particle settling

The impact-related sediments considered here are formed from primary fragmented material, giving rise to a distribution of particles and aggregates that can be interpreted as a cumulative function. Thus, for the study of the particle settling a fractal model is considered [1][2].

In this study, we assume that size segregation is correlated to the concentration of the solid particles [3][4] 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.

In order to analyze the size segregation effect, Herreros and Ormö [5] proposed a fractal model in two steps, considering the settling process of two types of particles: (1) Fine particles, s; and (2) coarse particles, p, with radius R_p  > R_s, taking the settling time for the coarse particles in pure water as a time reference (t_max):

Step 1: Settling of fine particles in pure water. The suspended mass fraction of fine particles, s, in pure water is calculated. At this step, the settling of the fine particles is considered independent on the solid/water concentrations.

Step 2: Settling of coarse particles in a solid/water mixture. In this case, the coarse particles, p, are assumed to be immersed in a fluid mixture whose viscosity and density are both functions of the volumetric concentrations.

According to [5], the number of settled coarse particles per unit length, i.e. the mean clast frequency , can be approximated by: 

 

where R_m is the average size of the coarse particles and l_ds and l_dp are the heights of an equivalent cylindrical volume of deposited mass.

2.2. Application to marine impacts

After a meteorite impact, the transient crater diameter, D_t, can be related to the impactor diameter, d [6]. Considering the height of excavated material [7], H_exc = D_t/10, as a solid/water mixture column, H_T = H_exc = H_s+H_w, the impactor diameter, d, can be expressed as:

where standard values for impactor’s velocity, density and impact angle have been considered [8][9]:

2.3. Mass excess

The effect of the relative fraction of external mass apported to the system, δ, of different origin to the target fragmentation, can be evaluated by just considering the new values for the volumetric concentrations:

which involves an increase in the concentration of solids.

3. Results

As shown in [5], it is possible to represent as a function of d/H_w (in the following just d/H) since both are functions of the solid/water concentrations.

As a result, we obtain a family of curves depending on two parameters: the suspended mass of solid material when all the particles with r > R_p have been settled (i.e. t= t_max), and the excess of solid material apported to the system (Figure 1).

4. Discussion

In order to understand the physics behind the relationship observed in [10], the proposed model has been applied. Figure 2 shows the model results along with the nine drill cores extracted from seven natural impact craters.

The presented model allows the physical interpretation of the observational data, providing a better understanding of the settling conditions. According to the model, the lower position curve (orange curve) represents stable settling conditions, closer to slack-water along with a null excess of mobilized material. This ideal situation, predicted by the model, is compatible with the position and characteristics of the related drill cores. However, the cases laying on the high position curve (blue curve) are those mobilizing a larger amount of solid material and with a higher depositional time for the finest particles, probably related to turbulent flow conditions during the settling process [10].

5. Conclusions

A fractal model for the study of size segregation and particle settling after an impact event in a marine target is presented. The model explains the observed tendency for nine cores obtained from seven natural impact craters, shedding light on the physical processes behind the observations.

Acknowledgements

The authors would like to gratefully acknowledge CSIC financial support for i-LINK project LINKA20203 and to the AEI for project MDM-2017-0737.

References

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[10] Ormö et al., Earth and Planetary Science Letters, Vol. 564, 116915, 2021.