The Nadir Crater offshore West Africa: A candidate Cretaceous-Paleogene impact structure

Abstract

Evidence of marine target impacts, binary impact craters, or impact clusters are rare on Earth. Seismic reflection data from the Guinea Plateau, West Africa, reveal a ≥8.5-km-wide structure buried below ~300 to 400 m of Paleogene sediment with characteristics consistent with a complex impact crater. These include an elevated rim above a terraced crater floor, a pronounced central uplift, and extensive subsurface deformation. Numerical simulations of crater formation indicate a marine target (~800-m water depth) impact of a ≥400-m asteroid, resulting in a train of large tsunami waves and the potential release of substantial quantities of greenhouse gases from shallow buried black shale deposits. Our stratigraphic framework suggests that the crater formed at or near the Cretaceous-Paleogene boundary (~66 million years ago), approximately the same age as the Chicxulub impact crater. We hypothesize that this formed as part of a closely timed impact cluster or by breakup of a common parent asteroid.

Fig. 1.. Map and regional seismic sections showing location of Nadir Crater.

(A) Regional bathymetry map of the Guinea Plateau and Guinea Terrace showing location of 2D seismic reflection and well data used in this study. JS, Jane Seamount; NS, Nadir Seamount; PS, Porter Seamount. The white dashed line shows the NE extent of high-amplitude discontinuous seismic facies at the top Maastrichtian interpreted as ejecta deposits and associated tsunami deposits. The north-east limit of this facies closely corresponds with the Maastrichtian shelf-slope break at the landward margin of the Guinea Terrace. Inset map shows a paleogeographic reconstruction of the Atlantic near the end of the Cretaceous, ~66 Ma ago, made using GPlates software (58). Ch, Chicxulub Crater; Nd, Nadir Crater; Bo, Boltysh Crater. (B). Regional composite 2D seismic reflection profile extending from the GU-2B-1 well in the east to the deep Atlantic basin in the west, showing the structural and stratigraphic character of the Guinea Plateau and Guinea Terrace. (C) North-South seismic profile from the salt basin in the north to the Nadir Seamount, south of the Guinea Fracture Zone. Data courtesy of the Republic of Guinea and TGS.

Fig. 2.. Seismic characteristics of the Nadir Crater.

(A) Seabed depth map of crater showing seismic line locations and the mapped extent of the crater rim and damage zone. (B) W-E seismic section (pre-stack depth migration – depth domain) across the crater, highlighting the crater morphology and damage zone, and the extent of subsurface deformation. Data courtesy of the Republic of Guinea, TGS and WesternGeco. Stratigraphic key is on Fig. 1. (C) Detailed seismic stratigraphic and structural elements of the crater. KP, Cretaceous-Paleogene sequence (KP1 equivalent to Top Maastrichtian); KU, Upper Cretaceous seismic horizons. KU1 and KP1 “regionals” are schematic reconstructions of these seismic horizons before formation of the crater at the end of the Cretaceous and are used to reconstruct a conceptual model of crater formation (Fig. 5). (D) SW-NE seismic section (pre-stack time migration – time domain) across the crater, showing crater morphology and seismic facies outside the crater, including high-amplitude seismic facies sitting above a ~100-ms-thick unit of chaotic reflections, interpreted to have formed as a result of seismic shaking following the impact event. Data courtesy of the Republic of Guinea and WesternGeco Multiclient.

Fig. 3.. Numerical model results of iSALE hydrocode simulations of final crater architecture for different water depths (200, 500, 800, 1100, and 1500 m, as indicated on the figure)

Models assume a 400-m-diameter impactor with an impact angle of 90°. The water layer has been removed from each image to better highlight the final crater morphology. Model outcomes show total plastic strain (TPS) on the left and deformed lithologies on the right—the gray unit represents the assumed Cenomanian-Turonian black shale deposits as a marker horizon. Movies of full simulations are included in the Supplementary Materials.

Fig. 4.. Snapshots of numerical model results of an iSALE hydrocode simulation for 800-m target water depth.

We consider this to be the most likely water depth for the impact, based on crater morphology. Other model parameters as for Fig. 3. Model snapshots show transient crater formation and generation of a rim-wave tsunami after 3 s; rebound of central uplift and propagation of a rim-wave tsunami away from crater at 38 s; crater collapse, resurge, and central jet formation at 85 s; and further resurge at 245 s. Movies of full simulations are included in the Supplementary Materials.

Fig. 5.. Conceptual model of the impact sequence at the Nadir impact site, based on seismic observations and analog models (6, 7, 21).

(A) At time (t) = 0, the impactor hits the water surface at a velocity of ~20 km/s, initiating a rim-wave tsunami in its wake. (B) Several seconds later, the transient crater forms, as the impactor and a substantial body of water are vaporized. Impactites (melt rock and breccias) line the transient cavity. Tsunami waves propagate away from the evacuated crater. Shock waves cause substantial damage below and around the impact site, and seismic waves propagate across the plateau; (C) major uplift (~400 m relative to preimpact regional) occurs in the “rebound” crater modification phase, resulting in the formation of a raised crater peak; (D) radial collapse of the subsurface damage zone results in further modification of the crater, including the formation of terraces at the surface. Resurge of water transports substantial volume of ejecta and other sediment into the crater, deposited above the impactites.