Impactites as a random medium—Using variations in physical properties to assess heterogeneity within the Bosumtwi meteorite impact crater Item Type Article; text Authors L'Heureux, E.; Milkereit, B. Citation L'Heureux, E., & Milkereit, B. (2007). Impactites as a random medium—Using variations in physical properties to assess heterogeneity within the Bosumtwi meteorite impact crater. Meteoritics & Planetary Science, 42(45), 849-858. DOI 10.1111/j.1945-5100.2007.tb01080.x Publisher The Meteoritical Society Journal Meteoritics & Planetary Science Rights Copyright © The Meteoritical Society Download date 29/09/2021 13:33:38 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final published version Link to Item http://hdl.handle.net/10150/656712 Meteoritics & Planetary Science 42, Nr 4/5, 849–858 (2007) Abstract available online at http://meteoritics.org Impactites as a random medium—Using variations in physical properties to assess heterogeneity within the Bosumtwi meteorite impact crater Elizabeth L’HEUREUX* and Bernd MILKEREIT University of Toronto, Department of Physics, 60 Saint George Street, Toronto, Ontario, Canada, M5S 1A7 *Corresponding author. E-mail: [email protected] (Received 03 August 2006; revision accepted 12 January 2007) Abstract–The recent drilling of the Bosumtwi impact structure, Ghana, has provided a unique opportunity to study the petrophysical properties of a young, well-preserved impact crater. The damage induced by impact results in extensive fracturing and mixing of target materials. We discuss here a means of using sonic velocity and density logs from two boreholes through the Bosumtwi crater fill and basement to estimate the degree of heterogeneity and fracturing within the impacted target, in order to understand the discrepancy between the large impedances derived from the log data and the nonreflective zone of impactites observed in seismic sections. Based on an analysis of the stochastic fluctuations in the log data, the Bosumtwi impactites are characterized by vertical scale lengths of 2–3 m. From the resolution of the seismic data over the crater, horizontal scale lengths are estimated at <12 m. The impactites therefore fall within the quasi-homogeneous scattering regime, i.e., seismic energy will propagate through the medium with little disruption. Scale lengths as small as these are observed in the fractured basement rocks of impact structures, whereas non-impact related crystalline environments are characterized by scale lengths an order of magnitude larger. Assuming that the high-frequency fluctuations observed in the log data are more sensitive to fracture distribution than petrology, this suggests that the small scale lengths observed within impact structures are characteristic of impact-induced damage, and could be used to estimate the extent of fracturing undergone by the rocks at any depth below an impact structure. INTRODUCTION The target of the Bosumtwi impact was the Proterozoic metasedimentary and metavolcanic rocks of the Birimian Petrophysical logs provide essential ground-truthing for Supergroup (Jones et al. 1981) that are locally intruded by the verification and interpretation of geophysical data. For the granitoid bodies (Koeberl and Reimold 2005). Samples of study of meteorite impact structures, these data provide a way various target rocks have an average density of 2680 kg/m3 of comparing the physical characteristics of structures in (Plado et al. 2000), and wide-angle seismic data indicate different parts of the world, characterized by different velocities of >5000 m/s (Scholz et al. 2002). Until recently, morphologies and target rock types. In this paper we present only two samples of impactite (suevite ejecta) collected from the results of an analysis of the newly acquired petrophysical outside the structure (Plado et al. 2000) had been measured to log data from the Bosumtwi impact structure in Ghana, drilled give an average dry density of 2370 kg/m3 and porosity of in 2004 by the International Continental Scientific Drilling 26%. Program (ICDP). The Bosumtwi structure (10.5 km in In 1999, 2000, and 2004, seismic refraction and diameter) is one of the youngest impact structures in the world, reflection data were collected over the Bosumtwi crater to with an age of 1.07 Myr (e.g., Koeberl et al. 1997; Koeberl and delineate its subsurface structure (Karp et al. 2002; Scholz Reimold 2005; Koeberl et al. 2007). Given that the crater is et al. 2002; Scholz et al. 2007). The interpretation of the almost entirely filled by Lake Bosumtwi (8 km in diameter), it Bosumtwi seismic data (Fig. 1) (Scholz et al. 2002) and is also one of the best-preserved impact structures of its type velocity model (Karp et al. 2002) shows a thick package of due to the thick layer of overlying lacustrine sediments. Its post-impact lacustrine sediment as well as the crater relatively small size and emplacement in crystalline target structure below, including a well-defined central uplift, and a rocks make it of particular interest for the study of fracturing breccia/melt layer underlain by uplifted basement at a depth and impact-induced porosity. of approximately 450 m. The lack of reflectivity in the 849 © The Meteoritical Society, 2007. Printed in USA. 850 E. L’Heureux and B. Milkereit Fig. 1. Depth-migrated seismic section through the Bosumtwi impact structure (Karp et al. 2002). The data have a dominant frequency of 100 Hz and bandwidth of 50–150 Hz. The total depth of the section is approximately 450 m. Hard-rock drill holes LB-07A and LB-08A are indicated by black lines. Locations of the boreholes and the seismic line within the crater are indicated in the inset map. The outside edges of the profile are obscured by strong reflections from the lake bottom and sediments, as well as multiple reflections (for a complete description of the seismic data and interpretation, see Scholz et al. 2007). breccias is evident, however, data from a wide-angle PETROPHYSICAL DATA refraction experiment (Karp et al. 2002) show that this is not due to a wave propagation problem. The data exhibit a To better understand the effect of impact on crystalline characteristic feature observed over many impact structures target rocks, two hard-rock boreholes were drilled into the around the world: seismically nonreflective, or transparent, flank of the central uplift (LB-08A) and in the surrounding impactites. This effect is often observable, for example, trough (LB-07A) of the Bosumtwi impact structure by the through the disruption of pre-impact features such as over ICDP (Fig. 1). Core samples were recovered from both the Ries crater in Germany (Angenheister and Pohl 1969), boreholes, including polymict lithic impact breccias and bounded by impact-related reflectors such as the thrust suevites (Koeberl et al. 2007). For simplicity, we refer to boundary features in the Chicxulub impact structure these lithologies, including fractured basement rocks, as the (Morgan et al. 2000), or through the differences in reflective “impact unit” of the Bosumtwi structure. P-wave velocities character between post-impact sediments and fractured sampled at 0.1 m were picked from first arrivals of the sonic basement rocks such as those at Bosumtwi. The cause of this borehole measurements (minimum source-receiver spacing of feature lies in the heterogeneous nature of the medium, 1 m) from 309–544 m in borehole LB-07A, and from something that can be quantified through dimensional 228–452 m in LB-08A (Fig. 2). Density values were extracted parameters that describe the dominant scale of the variations from gamma-gamma measurements on core samples at an in physical properties. irregular sampling rate (Morris et al. 2007). To completely describe and quantify the degree of The two boreholes provided a total of 433 m of core from heterogeneity in a random medium such as the Bosumtwi the Bosumtwi impact unit, and 460 m of petrophysical log impactites and fractured basement rocks, we need to assess data. Core samples are highly fractured and weakly the statistical distribution of its physical properties, namely compacted, and show variable porosities between 1 and 40% densities (U) and seismic velocities (Vp). This can be done (Brown et al. 2006; Schleifer et al. 2006). The logs of with borehole petrophysical logs and seismic data, which borehole LB-08A show high spatial frequency fluctuations in provide a measure of this heterogeneity both in the vertical the data below ~380 m, where large sections of graywacke and lateral directions. We describe here the recently acquired dominate (Fig. 2). The impact unit from both boreholes has Vp and density logs from two boreholes in the Bosumtwi much lower seismic velocities and densities than the pre- structure, and use these data to characterize the impactite/ impact target rocks; however, in general, average velocities fractured basement interval and relate its physical properties and densities are higher in LB-08A than in LB-07A to those predicted by seismic data. This will, in addition, (3044 m/s and 2564 kg/m3 in LB-08A versus 2313 m/s and allow us to constrain the level of damage induced in target 2333 kg/m3 in LB-07A). The lower physical property values rocks due to the impact event. in LB-07A are likely a result of the greater abundance of high Impactites as a random medium 851 Fig. 2. Lithology, density, velocity, impedance, and synthetic seismograms for boreholes LB-07A (left) and LB-08A (right) in Bosumtwi. Lithological plots adapted from Coney et al. (2007) and Ferrière et al. (2007). porosity breccias and suevite in the impactite material within Xia and Ahrens (2001) defined the lowermost extent of the the trough. Given that central uplifts are thought to be created damage zone in terms of the attenuation of a propagating through movement of intact blocks of fractured target rocks shock wave and the critical pressure required to fracture target (see, for example, Melosh 1989) during the formation of the rocks.