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Lake Bosumtwi

The 2004 ICDP Bosumtwi Impact Crater Drilling Project, Ghana

Christian Koeberl University of Vienna

Bernd Milkereit University of Toronto

Jonathan T. Overpeck University of Arizona

Christopher A. Scholz Syracuse University

The 10.5-km-diameter 1.07 Ma Bosumtwi impact crater was the subject of an interdisciplinary and international drilling effort of the International Continental Scientific Drilling Program (ICDP) from July to October 2004. Sixteen different cores were drilled by DOSECC at six locations within the lake, to a maximum depth of 540 m. A total of about 2.2 km of core material was obtained. 

Introduction and Geological Setting

Bosumtwi is one of only four known impact craters associated with a tektite strewn field. It is a well-preserved complex impact structure that displays a pronounced rim and is almost completely filled by the 8 km diameter Lake Bosumtwi. The crater is excavated in 2 Ga metamorphosed and crystalline rocks of the Birimian System; it is surrounded by a slight near-circular depression and an outer ring of minor topographic highs with a diameter about 20 km. The goal of the integrated drilling, rock property and surface geophysical study was to study the three-dimensional building blocks of the impact crater (delineate key lithological units, image fault patterns, and define alteration zones).

Lake Bosumtwi Drilling

Figure 1 – Location map with ICDP boreholes and seismic profile shown in Fig. 2.

Paleoclimatic Studies at Bosumtwi

Owing to its impact origin, Bosumtwi possesses several important characteristics that make it well suited to provide a record of tropical climate change. In order to gain greater insight into the role of the tropics in triggering, intensifying and propagating climate changes, scientific drilling for the recovery of long sediment records from Lake Bosumtwi was undertaken. Five drill sites (Fig. 1) were occupied along a water-depth transect in order to facilitate the reconstruction of the lake level history. At these five drill sites, a total of 14 separate holes were drilled. Total sediment recovery was 1,833 m. For the first time the GLAD lake drilling system (a system specifically constructed for drilling at lakes) cored an entire lacustrine sediment fill from lakefloor to bedrock.

 

The complete 1 Ma lacustrine sediment fill was recovered from the crater ending in impact-glass bearing, accretionary lapilli fallout. This accretionary lapilli unit represents the initial post-impact sedimentation and provides an important age constraint for the overlying sedimentary sequence. The initial lacustrine sediment is characterized by a bioturbated, light-gray mud with abundant gastropod shells suggesting that a shallow-water oxic lake environment was established in the crater. Future study of the earliest lacustrine sediment will address important questions related to the formation of the lake and the establishment of biologic communities following the impact. Most of the overlying 294 m of mud is laminated thus these sediment cores will provide a unique 1 million year record of tropical climate change in continental Africa at extremely high resolution. The shallow water drill sites consist of alternating laminated lacustrine mud (deepwater environment), moderately-sorted sand (nearshore beach environment) and sandy gravel (fluvial or lake marginal environments). These sediments preserve a record of major lake level variability that will greatly advance the present Bosumtwi lake level histories obtained from highstand terraces and shorter piston cores. All of the drill sites will be used to produce a basin-scale stratigraphic framework for the crater sediment fill.

Geophysics and Impact Results

The two impactite cores, LB-07A and LB-08A, were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. Samples from these cores have been studied by more than a dozen different research teams from around the world. The first set of peer-reviewed papers resulting from this work has just been published in a special issue of the journal “Meteoritics and Planetary Science”, which contains 27 papers on various aspects of the impact and geophysical studies at Bosumtwi.

At both impactite holes, drilling progressed through the impact breccia layer into fractured bedrock. LB-07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB-08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke-dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift.

Major results include the complete petrographic and geochemical record of the impactite fill. Major surprises in this regard include the, in comparison to suevites outside of the crater, small impact melt component and the record of shock metamorphism in the clast content of suevites, which are both at variance with earlier numerical modeling results. Also, impact-related hydrothermal overprint seems to be limited. These examples suffice to illustrate that further research on impact structures, accompanied by numerical modeling, is required to solve these and other problematics. Impactite geochemistry has also revealed that the impact breccias and the Ivory Coast tektite compositions can be satisfactorily modeled as mixtures of the known Birimian-age target rocks (metasedimentary rocks and granitoids). The geophysical studies, calibrated against the petrophysical data retrieved from the drill cores, have allowed to develop much improved three-dimensional models for the crater volume.

Lake Bosumtwi Drilling1

Figure 2 – Seismic section with deep boreholes.

Deep drilling results confirmed the general structure of the crater as imaged by the pre-drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post-impact sediments (less than 1800 m/s) and the impactites (2600-3300 m/s). The impactites exhibit extremely high porosities (up to 30 vol%), which has important implications for mechanical rock stability.

The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector-magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence.

Prior to drilling, numerical modeling estimated melt and tektite production using different impact angles and projectile velocities. The most suitable conditions for the generation of tektites are high-velocity impacts (>20 km/s) with an impact angle between 30 and 50° from the horizontal. Not all the melt is deposited inside the crater. In the case of a vertical impact at 15 km/s, 68% of the melt is deposited inside the crater. Results of the numerical modeling agreed well with the then-available geophysical data, crater size, and the distribution of tektites and microtektites of the tektite strewn field. The now observed situation for breccias within and around the crater is very different from these model results. The predicted amount of melt is much higher than the meager amounts observed. Clearly, much more melt has been incorporated in the suevite ejected outside of the structure, in comparison with the low amounts measured in the within-crater suevite occurrences. The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is thus in contrast to expectations from modeling and pre-drilling geophysics, and presents an interesting problem for comparative studies and requires re-evaluation of existing data from other terrestrial impact craters, as well as modeling parameters. This research project provided important new data and improved our understanding of global change and impact processes.

Lake Bosumtwi Drilling2

Figure 3 – Bosumtwi Project Coring Team aboard DOSECC’s GLAD800 drilling platform, R/V Kerry Kelts.

 

Read more about this scientific core drilling services project at ICDP.

Acknowledgments

This work was supported by the International Continental Drilling Program (ICDP), the U.S. NSF-Earth System History Program under Grant No. ATM-0402010, Austrian National Science Foundation (project P17194-N10), the Austrian Academy of Sciences, and by the Canadian National Science Foundation. Drilling was performed by DOSECC.

References:

Koeberl C., Milkereit B., Overpeck J.T., Scholz C.A., Amoako P.Y.O., Boamah D., Danuor S.K., Karp T., Kueck J., Hecky R.E., King J., and Peck J.A.

(2007) An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi impact crater, Ghana, drilling project – An overview. Meteoritics and Planetary Science 42, 483-511.

Ferrière, L., Koeberl, C., Ivanov, B.A., and Reimold, W.U. (2008) Shock metamorphism of Bosumtwi impact crater rocks, shock attenuation, and uplift formation. Science 322, 1678-1681.

Nowaczyk N.R., Melles, M. and Minyuk, P., 2007: A revised age model for core PG1351 from Lake El’gygytgyn, Chukotka, based on magnetic susceptibility variations correlated to northern hemisphere insolation variations, Journal of Paleolimnology, 37: 65-76.

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Berger, A. and Loutre, M.F., 1991: Insolation values for the climate of the last 10 million of years, Quaternary Science Reviews, 10: 297-317.

Brigham-Grette, J. and Carter, L.D., 1992: Pliocene marine transgressions of northern Alaska: Circumarctic Correlations and Paleoclimate, Arctic, 43(4): 74-89.

Brigham-Grette, J., Melles, M., Minyuk, P. and Scientific Party, 2007: Overview and Significance of a 250 ka Paleoclimate Record from El’gygytgyn Crater Lake, NE Russia, Journal of Paleolimnology, 37: 1-16.

Cherapanov, M.V, Snyder, J.A. and Brigham-Grette, J., 2007: Diatom Stratigraphy of the last 250 ka at Lake El’gygytgyn, northeast Siberia, Journal of Paleolimnology, 37: 155-162.

Dowsett, H.J., 2007: The PRISM paleoclimate reconstruction and Pliocene sea-surface temperature. In: Williams, M., et al., (Eds.) Deep-Time Perspectives on Climate Change: Marrying the Signal from Computer Models and Biological Proxies: The Micropaleontological Society, Special Publications, The Geological Society, London, 459-480.

Forman S.L., Pierson J., Gomez J., Brigham-Grette J., Nowaczyk N.R. and Melles, M., 2007: Luminescence geochronology for sediments from Lake El´gygytgyn, northwest Siberia, Russia: Constraining the timing of paleoenvironmental events for the past 200 ka, Journal of Paleolimnology, 37: 77-88.

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Glushkova, O.Yu. and Smirnov, V.N., 2007: Pliocene to Holocene geomorphic evolution and paleogeography of the El’gygytgyn Lake region, NE Russia, Journal of Paleolimnology, 37, 37-47.

Haug, G.H., et al., 2005: North Pacific seasonality and the glaciation of North America 2.7 million years ago, Nature, doi:10.1038, 1-5.

Juschus O., Preusser F., Melles M. and Radtke U., 2007: Applying SAR-IRSL methodology for dating fine-grained sediments from Lake El´gygytgyn, north-eastern Siberia, Quaternary Geochronology, 2: 137-142.

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Lisiecki, L.E. and Raymo, M.E., 2005: A Pliocene-Pleistocene stack of 57 globally distributed benthic ∂18O records, Paleoceanography, 20: PA1003, doi:10.1029/2004PA001071

Lozhkin, A.V., Anderson, P.M., Matrosova, T.V. and Minyuk, P.S., 2007: The pollen record from El’gygytgyn Lake: implications for vegetation and climate histories of northern Chukokta since the late middle Pleistocene, Journal of Paleolimnology, 37: 135-153.

Melles, M., Brigham-Grette, J., Glushkova, O.Yu., Minyuk, P.S., Nowaczyk, N.R. and Hubberten, H.W., 2007: Sedimentary geochemistry of a pilot core from Lake El’gygytgyn – a sensitive record of climate variability in the East Siberian Arctic during the past three climate cycles, Journal of Paleolimnology, 37: 89-104.

Miller, G.H. and Brigham-Grette, J. (lead authors) and 17 contributing authors, in press: Temperature and Precipitation history of the Arctic, Chapter 4, IN, Past Climate Variability and change in the Arctic and High Latitudes, CCSP Synthesis and Assessment Product 1.2, US Govt Climate Change Program. 201 pgs.

Minyuk, P.S., Brigham-Grette, J., Melles, M.M., Borkhodovev, V.Yu., Glushkova, O.Yu., 2007: Inorganic geochemistry of El’gygytgyn Lake sediments (northeastern Russia) as an indicator of paleoclimatic change for the last 250 kyr, Journal of Paleolimnology, 37: 123-133

Niessen, F., Gebhardt, A.C. and Kopsch, C., 2007: Seismic investigation of the El’gygytgyn impact crater lake (Central Chukotka, NE Siberia): preliminary results, Journal of Paleolimnology, 37: 49-63.

Nolan, M. and Brigham-Grette, J., 2007: Basic Hydrology, Limnology, and meterology of modern Lake El’gygytgyn, Siberia, Journal of Paleolimnology, 37: 17-35.

Nolan, M., Liston, G., Prokein, P., Huntzinger, R., Brigham-Grette, J. and Sharpton, V., 2003: Analysis of Lake Ice Dynamics and Morphology on Lake El’gygytgyn, Siberia, using SAR and Landsat, Journal of Geophysical Research, 108(D2): 8162-8174.

Nowaczyk et al., 2002: Magnetostratigraphic results from impact crater Lake El’gygytgyn, northeastern Siberia: A 300 kyr long high resolution terrestrial paleoclimatic record from the Arctic, Geophysical Journal International, 150: 109-126.

Nowaczyk N.R., Melles, M. and Minyuk, P., 2007: A revised age model for core PG1351 from Lake El’gygytgyn, Chukotka, based on magnetic susceptibility variations correlated to northern hemisphere insolation variations, Journal of Paleolimnology, 37: 65-76.

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