Guest “Geology Rocks!” by David Middleton
Impact Crater off the African Coast May Be Linked to ChicxulubThe underwater crater, spotted serendipitously in commercial observations of seafloor sediments, is believed to have formed at roughly the same time as the famous Cretaceous-Paleogene impact event.By Katherine Kornei19 September 2022
In the world of impact craters, Chicxulub is a celebrity: The 180-kilometer-diameter maw, in the Gulf of Mexico, was created by a cataclysmic asteroid impact at the end of the Cretaceous that spelled the demise of most dinosaurs. But researchers have now uncovered another crater off the coast of Guinea that might well be Chicxulub’s cousin. The newly discovered feature, albeit much smaller, is also about 66 million years old. That’s a curious coincidence, and scientists are now wondering whether the two impact structures might be linked. Perhaps Chicxulub and the newly discovered feature—dubbed Nadir crater—formed from the breakup of a parent asteroid or as part of an impact cluster, the team suggested. These results were published in Science Advances.
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The newly discovered Nadir crater appears to have formed right around the same time as that cataclysm. Uisdean Nicholson, a sedimentary geologist at Heriot-Watt University in Edinburgh, Scotland, and his colleagues discovered the candidate crater while they were poring over observations of seafloor sediments originally collected for oil and gas exploration. The team spotted the roughly 8-kilometer-wide structure in seismic reflection imaging data obtained off the coast of West Africa. “It was pure serendipity,” said Nicholson.
The putative crater is buried under roughly 300 meters of sediments topped by 900 meters of water, and its appearance strongly suggests it was created by a hypervelocity impact, said Nicholson. For starters, it’s circular in shape, with a pronounced rim. Second, it contains a small central peak, a feature that often arises in large impact craters. And perhaps most important, there’s clear evidence of deformed sediments—caused by faulting and folding—persisting hundreds of meters below what would be the crater floor. “There’s a lot of things that suggest it’s an impact,” said Gavin Kenny, a geochemist at the Swedish Museum of Natural History in Stockholm who was not involved in the research.
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The paper, Nicholson et al., 2022, is well-worth reading and the full text is available. The crater was identified on 2d seismic surveys shot by WesternGeco (1999) and TGS (2017). I see a lot of cool geology on 3d seismic data every day, mostly related to salt tectonics., but I’ve never seen anything as cool as the very well-imaged impact crater on Figure 1B…
Figure 2 from the paper zooms in on the crater on the TGS line (black rectangle on Figure 1B) and a WesternGeco line. This is a great textbook example of integrating geological and geophysical data…
MATERIALS AND METHODS
The seismic stratigraphic framework is based on an extensive 2D seismic reflection dataset and three industry wells drilled on the southeast of the plateau (Fig. 1). The 2D data were acquired and processed by TGS and WesternGeco. The NWAAM-17 survey in Guinea and Guinea Bissau was acquired in 2017 by TGS in Guinea and Guinea Bissau; data were acquired in 2017 in water depths of 15 to 4600 meters. Acquisition parameters include a 12-km streamer length towed at a depth of 18 m, with 960 hydrophone channels of 12.5-m group length and a 25-m shot point spacing, giving a 240 full-fold data coverage. Total record length was 14 s with a 2-ms sample rate for all lines. The sound source was one 4310 in3 array towed at 8 m and firing every 25 m using a Bigshot controller system. Data were processed using a pre-stack time migration (PSTM) sequence, with a subset of data processed using a pre-stack depth migration sequence, providing data in depth and time domain. Seismic frequency at the top of the crater is around 45 Hz, and interval velocity is ~1800 m/s, giving a tuning thickness (seismic resolution) of around 10 m.
The GWG99 survey in Guinea was acquired in 1999 by WesternGeco in water depths of 150 to 4500 m. Acquisition parameters include a 6-km streamer length towed at a depth of 8 m, with 480 hydrophone channels of 12.5-m group length and a 25-m shot point spacing, giving a 120 full-fold data coverage. Total record length was 10 s with a 2-ms sample rate for all lines. The sound source was one 2000 in3 array towed at 6 m and firing every 25 m. Data were processed using a PSTM sequence, with a subset of data reprocessed in 2017 to improve imaging and reduce multiples and noise. Seismic frequency at the top of the crater is around 45 Hz, and interval velocity is ~1800 m/s, giving a tuning thickness (seismic resolution) of around 10 m.
Seismic interpretation was carried out using Schlumberger Petrel 2020 software, including horizon mapping, structural element mapping, velocity analysis, and seismic-to-well ties. Lithological, petrophysical, and biostratigraphic data from the exploration wells were used to constrain the stratigraphic age and lithology of seismic reflections. Seismic-to-well ties were based on checkshot surveys, with the tie further improved by constructing synthetic seismograms using sonic and density log data. Structural reconstructions of the preimpact stratigraphy and the geometry of the transient crater were carried out by seismic flattening of the Top Maastrichtian and KU1 horizons, respectively. On the basis of a vertical seismic resolution of ~10 m for the top of the Nadir Crater, the age uncertainty for individual seismic reflections is estimated at 0.5 Ma for the Upper Cretaceous (700 m thick deposited over 34 Ma = 20 m/Ma, 10/20 = 0.5 Ma) and 0.8 Ma for the Paleogene (400 m thick deposited over 32 Ma = 12 m/Ma, 10/12.5 = 0.8 Ma).
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References
Kornei, K. (2022), Impact crater off the African coast may be linked to Chicxulub, Eos, 103, https://doi.org/10.1029/2022EO220454. Published on 19 September 2022.
Nicholson U, Bray VJ, Gulick SPS, Aduomahor B. The Nadir Crater offshore West Africa: A candidate Cretaceous-Paleogene impact structure. Sci Adv. 2022 Aug 19;8(33):eabn3096. doi: 10.1126/sciadv.abn3096. Epub 2022 Aug 17. PMID: 35977017; PMCID: PMC9385158.
