Bosumtwi Impact Structure Research Articles

Characterization of the log lithology of cores LB‐07A and LB‐08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibility

Abstract— Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock‐forming and rock‐changing processes, e.g., deformation or sedimentation.To characterize the rocks in drill cores LB‐07A and LB‐08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite.The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta‐graywacke). It was possible to correlate layers of high (shale component), intermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB‐07A and LB‐08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta‐graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB‐07A towards core LB‐08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure.

Structure and morphology of the Bosumtwi impact structure from seismic reflection data

Abstract— The Lake Bosumtwi impact structure in West Africa offers unparalleled opportunities for the study of cratering processes, as the structure is young (1.1 Myr) and minimally eroded. Because the center part of the structure is covered by Lake Bosumtwi, which is 8 km in diameter and ∼70 m deep, it is possible to use marine‐type seismic reflection techniques to obtain high‐fidelity images of the lake subsurface, including key elements of the impact structure.Eight profiles of multichannel seismic reflection (MCS) data were acquired in a radial pattern across the basin, as well as two other high‐resolution seismic reflection surveys. The MCS data show a well‐defined central uplift near the north‐central part of the lake. Observed within the annular moat surrounding the buried central uplift is a section of post‐impact lacustrine sediments more than 300 m thick. The central uplift structure has a diameter of 1.9 km and a maximum height of 130 m above the annular moat. The central uplift has an overall irregular upper surface with a small graben structure. We observe a series of normal faults that extend as much as 120 m into the sedimentary section above the central uplift. We interpret the normal faults to be a consequence of ongoing compaction of the high‐porosity materials that comprise the central uplift. The interpreted impact structure surface, defined using seismic reflection data, was combined with regional topographic data from outside the lake in the form of a digital elevation model, which provides a useful perspective of overall impact structure morphology.

The Lake Bosumtwi meteorite impact structure, Ghana—A magnetic image from a third observational level

Abstract— The Bosumtwi impact structure in Ghana is the youngest and best‐preserved medium‐sized impact structure on Earth, and because of the vast amount of prior geophysical and geological data gathered in the area, it constitutes a great natural laboratory to try to develop new geophysical interpretation and modeling techniques.During the 2004 International Continental Scientific Drilling Program (ICDP) drilling campaign at Lake Bosumtwi, we made magnetic field observations at 162 stations around the lake. This study differs from all previous magnetic surveys at Bosumtwi, which only measured the scalar portion of the Earth's magnetic field, in that we measured the full magnetic vector at each station. Acquisition of the full magnetic vector was made possible by innovative use of a borehole deviation probe, which uses a magnetic sensor for absolute orientation reference. Estimates of the magnetic vector orientation and magnitude at each observation station were derived from a series of measurements collected at 50 cm spacing over a depth range of 25 m.In this study, we report a comparison between the scalar total field intensity derived from this new survey approach with the other two previously acquired marine and airborne magnetic data sets. The scalar total magnetic intensity (TMI) computed from the vector data set compares in close agreement with the other two data sets. Some discrepancies between the data sets can be explained by differences in the distances between the sensor and the magnetic sources for the various surveys. The highlight of this study is that we demonstrate that is possible to acquire at least partial vector data with readily available instrumentation.

Geochemistry of impactites and basement lithologies from ICDP borehole LB‐07A, Bosumtwi impact structure, Ghana

Abstract— In 2004, a drilling project by the International Continental Scientific Drilling Program (ICDP) at the Bosumtwi impact crater, Ghana (1.07 Myr old and 10.5 km in diameter), obtained drill core LB‐07A, which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure. The LB‐07A core consists of three sequences: 82.29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54.88 m of so‐called lower impactite of monomict impact breccia with several suevite intercalations, and 74.53 m of meta‐graywacke and altered shale of the basement, also containing a number of suevite intercalations. Major‐ and trace‐element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein. Compositions of polymict impact breccias of the crater fill revealed by core LB‐07A are compared with the compositions of the Ivory Coast tektites and the fallout suevites. The impactites of the LB‐07A borehole appear well homogenized with respect to the silicate component, and little change in the ranges of many major‐ and trace‐element differences is seen along the length of the borehole (except for Fe2O3, MgO, and CaO contents). Much scatter is observed for a number of elements, and in many cases this increases with depth. It is proposed that any variability in composition is likely the function of clast population differences (i.e., also of relatively small sample sizes). No systematic compositional difference between polymict lithic and suevitic impact breccias is evident. An indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies. The impactites of the borehole generally show intermediate compositions to previously defined target rocks. The fallout suevites have comparable major element abundances, except for relatively lower MgO contents. The Ivory Coast tektites are generally similar in composition to the LB‐07A suevites, but broader ranges in MgO and CaO contents are observed for the LB‐07A suevites.

The Lake Bosumtwi impact structure in Ghana: A brief environmental assessment and discussion of ecotourism potential

Abstract— Lake Bosumtwi is a natural inland freshwater lake that originated from a meteorite impact. The lake is becoming a popular tourist attraction in Ghana and has the potential to be developed as an ecotourism site in the future. However, there have been some unregulated human activities and unplanned infrastructure development, and there are increased levels of pollutants in the lake water. In order to make ecotourism at Lake Bosumtwi successful in the long term, the Lake Bosumtwi Development Committee has been formed to ensure that local people are empowered to mobilize their own capacities. It has been realized that an important criterion required to develop ecotourism in a socially responsible, economically efficient, and environmentally viable way is to foster a constructive dialogue between the local people and tourists about the needs of the indigenous people.

Impactites as a random medium—Using variations in physical properties to assess heterogeneity within the Bosumtwi meteorite impact crater

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.

Petrography and shock‐related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana

Abstract— Rock magnetic and magnetic mineralogy data are presented from the International Continental Scientific Drilling Program (ICDP) drill cores LB‐07A and LB‐08A of the Bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact‐related remagnetization mechanism. Basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided. Magnetic susceptibility (150–500 × 10−6 SI) and natural remanent magnetization (10−3−10−1 A/m) are generally weak, but locally higher values up to 10.6 × 10−3 SI and 43 A/m occur. Sixty‐three percent of the investigated rock specimens show Q values above 1 indicating that remanence clearly dominates over induced magnetization, which is a typical feature of impact structures. Ferrimagnetic pyrrhotite is the main magnetite phase, which occurs besides minor magnetite and a magnetic phase with a Curie temperature between 330 and 350 °C, interpreted as anomalous pyrrhotite. Coercive forces are between 20 and 40 mT. Brecciation and fracturing of pyrrhotite is a common feature confirming its pre‐impact origin. Grain sizes of pyrrhotite show a large variation but the numerous stress‐induced nanostructures observable by transmission electron microscopy (TEM) are assumed to behave as single‐domain grains. We suggest that the drilled rocks lost their pre‐shock remanence memory during the shock event and acquired a new, stable remanence during shock‐induced grain size reduction. The observed brittle microstructures indicate temperatures not higher than 250 °C, which is below the Curie temperature of ferrimagnetic pyrrhotite (310 °C).

Petrographic studies of “fallout” suevite from outside the Bosumtwi impact structure, Ghana

Abstract— Field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the Bosumtwi crater rim. Suevite occurrence there is known from an ˜1.5 km2 area; its thickness is ≤15 m. The present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous Bosumtwi ejecta deposits.Here we discuss the petrographic characteristics of drill core samples of melt‐rich suevite. Macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock (granite, graywacke, phyllite, shale, schist, and possibly slate) clasts up to about 40 cm in size. Shock metamorphic effects in the clasts include multiple sets of planar deformation features (PDFs), diaplectic quartz and feldspar glasses, lechatelierite, and ballen quartz, besides biotite with kink bands. Basement rock clasts in the suevite represent all stages of shock metamorphism, ranging from samples without shock effects to completely shock‐melted material that is indicative of shock pressures up to ˜60 GPa.

Bosumtwi impact structure, Ghana: Geochemistry of impactites and target rocks, and search for a meteoritic component

Abstract— Major and trace element data, including platinum group element abundances, of representative impactites and target rocks from the crater rim and environs of the Bosumtwi impact structure, Ghana, have been investigated for the possible presence of a meteoritic component in impact‐related rocks. A comparison of chemical data for Bosumtwi target rocks and impactites with those for Ivory Coast tektites and microtektites supports the interpretation that the Bosumtwi structure and Ivory Coast tektites formed during the same impact event. High siderophile element contents (compared to average upper crustal abundances) were determined for target rocks as well as for impactites. Chondrite‐normalized (and iron meteorite‐normalized) abundances for target rocks and impactites are similar. They do not, however, allow the unambiguous detection of the presence, or identification of the type, of a meteoritic component in the impactites. The indigenous siderophile element contents are high and possibly related to regional gold mineralization, although mineralized samples from the general region show somewhat different platinum‐group element abundance patterns compared to the rocks at Bosumtwi. The present data underline the necessity of extensive target rock analyses at Bosumtwi, and at impact structures in general, before making any conclusions regarding the presence of a meteoritic component in impactites.

Investigating the Lake Bosumtwi impact structure: Insight from numerical modeling

The current topography of the Lake Bosumtwi crater and some of its structural dimensions have been determined by geophysical methods. We combine these data with sophisticated numerical models to evaluate the cratering process itself (for example, melt and tektite generation) as well as to test the modeling code. The geophysical maps show some asymmetry in plan view, with the main anomaly north of the crater center. The simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions, while the late stage is modeled for the vertical impact without any asymmetry. We estimate the projectile size from scaling laws and then, varying material properties, reproduce a crater, which is similar to the Bosumtwi, but too deep. Bulking allows us to reconcile differences between the model results and the observed topography. Shock melt estimates are in good agreement with the Bosumtwi magnetic signature. Modeled distribution of tektites assumes an impact angle of 30°–45° and an impact direction from the N‐NE. The combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well‐suited drill sites. Besides the central uplift and the annular moat with a suggested thick breccia cover, drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact‐induced asymmetries.

Geology and geochemistry of shallow drill cores from the Bosumtwi impact structure, Ghana

Abstract— The 1.07 Ma well‐preserved Bosumtwi impact structure in Ghana (10.5 km in diameter) formed in 2 Ga‐old metamorphosed and crystalline rocks of the Birimian system. The interior of the structure is largely filled by the 8 km diameter Lake Bosumtwi, and the crater rim and region in the environs of the crater is covered by tropical rainforest, making geological studies rather difficult and restricted to road cuts and streams. In early 1999, we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical, petrological, and geochemical studies of ejecta of the Bosumtwi impact structure. A variety of impactite lithologies are present, consisting of impact glassrich suevite and several types of breccia: lithic breccia of single rock type, often grading into unbrecciated rock, with the rocks being shattered more or less in situ without much relative displacement (autochthonous?), and lithic polymict breccia that apparently do not contain any glassy material (allochtonous?). The suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth. Twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the Bosumtwi crater. Major and trace element analyses yielded compositions similar to those of the target rocks in the area (graywacke‐phyllite, shale, and granite). Graywacke‐phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples (fragmentary matrix), with a minor contribution of Pepiakese granite. The results also provide information about the thickness of the fallout suevite in the northern part of the Bosumtwi structure, which was determined to be ≤15 m and to occupy an area of ˜1.5 km2. Present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous Bosumtwi ejecta deposits. Our studies allow a comparison with the extent of the suevite at the Ries, another well‐preserved impact structure.

High‐resolution X‐ray computed tomography of impactites

High‐resolution X‐ray computed tomography (HRXCT) is a nondestructive method used to study the interiors of opaque solid objects. Here we present the results of a first application of the HRXCT method to imaging the interior of impactites, in particular, suevites (glass‐bearing impact breccias) from the Bosumtwi (Ghana) and Ries (Germany) craters and a Muong Nong–type tektite from Thailand. The aim of these studies was to determine the three‐dimensional (3‐D) distribution of clasts within the matrix of the suevites, to test this technique with respect to its suitability for the recognition of different clast types of different densities, and to determine textural characteristics of the tektite. The main part of the study concentrated on a large suevite sample (about 10 × 7 × 5 cm) from the Bosumtwi impact structure. Target rock fragments in the Bosumtwi sample consist of greywacke and sandstone/quartzitic rocks, shale and phyllite, and granites. Another large clast component is composed of impact melt and glass fragments. Macroscopic petrography and thin section petrography were used to identify the clast types in the specimen for correlation with its HRXCT signatures. The results show that HRXCT allows the easy discrimination of the relatively frothy inclusions of glassy melt in the suevites, as they are darker than the matrix in the raw X‐ray scans and can be traced through the whole sample. Color or gray scale applications allow the distinction of at least four different clast types based on density differences. The size of the smallest discernable clasts (about 0.5 mm) is determined by the resolution of the measurements, which, in turn, is a function of scan slice thickness and field of view. In the case of the tektite sample, we were also able to image the 3‐D distribution of vesicles, possibly indicating glass flow. The HRXCT method allows us to determine the three‐dimensional distribution of clast populations in impact breccias by image processing techniques and to quantify their abundances in volume percent.

Seismic investigation of the Lake Bosumtwi impact crater: preliminary results

The Lake Bosumtwi impact crater in Ghana, West Africa, has a diameter of 10.5 km and is one of the youngest ( 1.07 Ma ) well-preserved large craters on Earth. It has a total dynamic range of topography of more than 400 m , and it is the source crater of tektites and microtektites of the Ivory Coast strewn field. The crater was excavated in early Proterozoic rocks. According to its size, the Bosumtwi impact crater should be a complex impact structure, with a central peak. Multichannel seismic (MCS) reflection and wide angle data, using Ocean-Bottom-Hydrophones (OBHs), were acquired in order to investigate the structure's subsurface, image the presumed central uplift and determine the thickness of impact-related formations and the post-impact sediments. An integrated interpretation of the seismic data sets, and modelling and inversion of the OBH data yield an initial 2D velocity-depth model, which shows indications for a central peak feature. Due to the relatively low seismic velocity ( 3.0 km s −1 ) of the corresponding layer, the top of the uplifted structure is interpreted to consist of allochthonous breccia. The central peak has a width of ∼1.8 km and a maximum height of 120 m above the top of the breccia away from the center. Fracturing may be responsible for the relatively low velocity of 3.8 km s −1 in the crater floor. The post-impact sediments covering the crater structure are 180– 300 m thick. The apparent crater depth, defined as the difference between the original target surface and the top of the breccia layer, is ∼550 m and thereby slightly deeper than some other larger complex impact structures on Earth. The results indicate that the Lake Bosumtwi impact structure provides an interesting setting for scientific drilling of a young large impact crater and will be supplemented by complimentary recent geophysical (potential field) and possibly future drilling studies.

Pronounced central uplift identified in the Bosumtwi impact structure, Ghana, using multichannel seismic reflection data

The Lake Bosumtwi impact structure is the youngest and best-preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes. Because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform, no significant backwash processes have modified the crater morphology. Not only may Bosumtwi contain the best-preserved central uplift structure on Earth, but it is the most accessible relatively large, young crater in the solar system generated in a large gravity field. There is a well-established link between the Lake Bosumtwi impact structure and the Ivory Coast tektite field, and the lacustrine sediments within the crater contain a unique 1 m.y. record of paleoclimate in the continental tropics south of the Sahel. Eight profiles of marine-type multichannel seismic reflection (MCS) data were acquired from the 8-km-diameter, similar to75-m-deep lake that fills much of the crater. These were augmented by wide-angle seismic data acquired with ocean-bottom hydrophones. MCS data reveal a well-defined central uplift near the northwest-central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m. The central uplift structure has a diameter of 1.9 km and a maximum height of 130 m above the annular moat inside the crater. An intermediate velocity layer (3200 m/s) beneath the lacustrine sediment is interpreted as fallback breccia or a breccia-melt horizon. The measured apparent depth of the crater (d(a)) is 500 m, implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships. The Bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws, perhaps because of the effects, of a large gravity field.

Petrology and geochemistry of target rocks from the Bosumtwi impact structure, Ghana, and comparison with Ivory Coast tektites

The 10.5 km diameter Bosumtwi crater in Ghana, West Africa, is the most likely source crater for the Ivory Coast tektites, as the tektites and the crater have the same age (1.07 Ma), and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks. The crater is excavated in 2.1–2.2 Ga old metasediments and metavolcanics of the Birimian Supergroup. Here we present the first integrated petrographic and geochemical study of rocks from the Bosumtwi impact crater. A variety of target rocks from the Bosumtwi impact structure were selected to represent the major rock types that have been described before, resulting in four groups: shale, phyllite-graywacke, and two different types of granites (from dispersed dikes and from the so-called Pepiakese intrusion at the northeastern side of the crater). These rocks were analyzed for their major and trace element composition and their petrographic characteristics. In addition, representative samples were also analyzed for their O, Sr, and Nd isotopic compositions. The target rocks do not show any unambiguous evidence of shock metamorphism (i.e., planar deformation features, PDFs). Distinct impact-characteristic shock effects (PDFs) were identified only in clasts within suevite-derived melt fragments. The compositional range of the target rocks is significantly wider than that of the Ivory Coast tektites, but overlap the tektite compositions. A best-fit line for the Bosumtwi crater rocks in a Rb-Sr isotope evolution diagram yields an “age” of 1.98 Ga, and an initial 87Sr/ 86Sr ratio of 0.701, which is close to results previously obtained for granitoid intrusions in the Birimian of Ghana. Our Nd isotopic data yield depleted mantle model ages ranging from 2.16 to 2.64 Ga, and ϵ Nd values of −17.2 to −25.9‰. Harmonic least-squares (HMX) mixing calculations were able to reproduce the composition of Ivory Coast tektites from a mixture of Bosumtwi country rocks that include about 70% phyllite-graywacke, 16% granite dike, and 14% Pepiakese granite. The oxygen isotopic composition of the metasedimentary rocks and granite dikes (δ 18O = 11.3–13.6‰) and the tektites (δ 18O = 11.7–12.9‰, also this work) agree fairly well, while the Pepiakese granites have lower values (δ 18O = 8.6–9.0‰), indicating that only minor amounts of these rocks were incorporated in the formation of the Ivory Coast tektites. The large variation in Sr and Nd isotopic compositions of the target rocks do not allow the unambiguous identification of distinct endmember compositions, but in both a 87Sr/ 86Sr vs. 1/Sr plot and an ϵ Sr vs. ϵ Nd diagram, the tektites plot within the area occupied by the metasedimentary and granitic Bosumtwi crater rocks. Thus, our data support the interpretation that the composition of the Ivory Coast tektites is similar to that of rocks exposed at the Bosumtwi impact structure, indicating formation during the same impact event.