3D anatomy of the Cretaceous-Paleogene age Nadir Crater

Hypervelocity impacts by asteroids and comets pose significant hazards, yet empirical constraints on their consequences are limited by the small number (~200) of confirmed terrestrial impact craters and the frequent erosion of surface expressions. Complex craters, which exhibit central uplifts, collapsed rims, and annular moats, require transient target weakening (e.g., thermal softening, interstitial fluid/melt fluidization, or acoustic fluidization) to allow rebound and lateral flow during crater modification. Many known marine craters also show a broader concentric faulted ‘brim’ indicative of impact into layered, water-saturated sedimentary targets. However, most terrestrial examples are incompletely preserved or only imaged with 2D seismic, complicating interpretation of subsurface structures and sedimentary processes. The Nadir Crater on the Guinea Plateau (offshore Republic of Guinea) has an approximate Cretaceous–Paleogene (K–Pg) boundary age (~66 Ma) and was previously identified on two 2D seismic profiles as a mid-sized (~28.5 km transient estimate) complex crater with a central uplift and listric normal faults. The age similarity to the Chicxulub impact raised the possibility of multiple impacts near the K–Pg boundary. Yet limited data coverage and inadequate shallow velocity models in prior 2D surveys left uncertainties in crater circularity, true spatial extent of deformation (rim vs. brim), and detailed subsurface architecture, as well as residual doubt about an impact versus alternative genesis. This study presents new high-resolution, fully covering 3D seismic data across and beyond the Nadir Crater. The goals are to: (1) resolve the 3D morphology and internal structure; (2) assess diagnostic criteria for a hypervelocity impact origin; (3) reconstruct crater formation and modification in a marine, layered target; (4) infer impact angle and trajectory from deformation asymmetries; and (5) evaluate broader environmental consequences, including tsunami–seabed interactions and widespread liquefaction.

The paper situates Nadir within the terrestrial impact record and marine impact morphologies, drawing on: (a) crater collapse mechanics and transient weakening processes (thermal softening, interstitial fluid/melt fluidization, acoustic fluidization); (b) marine crater features such as multi-ring basins and inverted sombrero/brim morphologies; and (c) limitations of prior seismic imaging for craters like Mjølnir, Chesapeake, Montagnais, and Silverpit, where 2D coverage and post-impact tectonic/compactional overprints obscure relationships. It compares observed Nadir characteristics to confirmed craters, highlights how morphology alone can be ambiguous without shock metamorphic evidence, and argues that in rare, exceptionally imaged marine cases, high-resolution geophysical data may suffice to classify impact origin. The study references oblique impact experiments and simulations to interpret asymmetries and uprange/downrange effects, and analogues (Upheaval Dome, Spider, Matt Wilson) for radial thrust systems indicating low-angle impacts. It also draws on tsunami modeling literature for marine impacts and resurge processes, and on seismite/mass-transport deposit analogs for interpreting chaotic units and concentric ridge development beyond the brim.

• Data acquisition: The MC3D seismic survey (TGS, 2019) over the Guinea Plateau (water depths 60–4500 m) employed 12 streamers of 8.025 km length at 150 m separation; shot interval 16.7 m; sample rate 2 ms. Acquisition bin 6.25 m × 25 m; processing bin 12.5 m × 12.5 m; nominal fold 80; record length 10,000 ms.
• Processing: Proprietary Pre-Stack Depth Migration (PSDM) with tomographically derived velocity model refined from far-field well velocity profiles and depth-migrated gathers. Long offsets enabled velocity inversion beneath the crater floor without raypaths through the crater, improving confidence in detected velocity anomalies and suppressing ‘pull-up’ artifacts seen in prior 2D data.
• Interpretation: Conducted in Schlumberger Petrel 2020. Workflows included horizon mapping (e.g., Upper Cretaceous KU0–KU4, K–Pg surfaces KPg1–KPg3), structural element mapping (faults, folds, duplexes, radial thrusts), attribute analysis (amplitude and variance/continuity), depth mapping, and velocity analysis to delineate uplift, annular moat, rim, brim, and wider damage zone. Stratigraphic ages were tied using seismic correlation to wells GU-2B-1 and Sabu-1 on the Guinea Plateau based on microfossil assemblages.
• Analytical approaches: (1) Planform and depth morphology quantification (crater diameter, rim and brim delineation via concentric faulting signatures; central peak relief). (2) Velocity–porosity inference in the central uplift and surrounding target to estimate relative porosity loss. (3) Structural kinematics across depth levels to reconstruct modification sequence and infer detachment levels. (4) Symmetry analysis of radial thrust geometries and vergence to estimate impact trajectory and angle. (5) Facies/geomorphology interpretation of chaotic and reworked units (ejecta/resurge and seismite) and mapping of resurge scar and outwash features.

• Diagnostic 3D morphology and structure confirm a hypervelocity impact origin:
  • Near-circular crater rim diameter ~9.2 km; area ~67 km². Brim diameter ~22–24 km defined by concentric normal faults.
  • Central peak evident on KPg1–KPg3; relief decreases upward: ~40 m (KPg1), ~20 m (KPg2), ~10 m (KPg3).
  • Depth-to-diameter and scaling metrics consistent with complex impact craters: crater depth:diameter ~1:40; central uplift diameter:crater diameter ~1:5; stratigraphic uplift:crater diameter ~1:22.
• Seismic velocity anomaly beneath crater floor:
  • Velocities elevated by up to ~500 m/s within the central uplift relative to equivalent stratigraphic levels outside, implying ~10–15% porosity reduction and higher bulk density. The anomaly extends to ~3 km depth (~1.8 km below crater floor); no uplift at ~4 km depth, confirming previous 2D ‘pull-up’ artifacts.
• Subsurface deformation architecture:
  • Shallow levels (e.g., KU3, originally ~100–150 m below seabed) show intense folding in the central uplift, reverse faults in the annular moat, and widespread concentric normal faults in the brim. Extensional duplexes indicate lateral transport toward the crater.
  • Deeper levels (KU2–KU0; originally ~500–900 m below seabed) exhibit a well-developed annular moat and central uplift divided into an intensely deformed core and a peripheral zone with radial thrusts (throws up to ~250 m; cumulative displacement up to ~500 m). Extension at shallow levels is accommodated by a detachment near KU1.
• Planform geomorphology beyond the rim:
  • Large arcuate resurge scar east of the crater (concave west) indicating powerful resurge capable of seabed interaction ~20 km from the rim at ~800 m paleo-water depth.
  • High-variance, fan-like facies SE of rim interpreted as outwash; highest amplitudes at KPg3 south and west likely mark thicker ejecta or tsunami-reworked deposits.
  • Subtle concentric ridges outside rim in north formed within an extensive chaotic unit interpreted as a seismite/mass-transport deposit between KU4 and KPg1.
• Regional damage zone beyond brim:
  • Extensive shallow deformation south, west, and north of crater (rectangular fault blocks west–southwest; tightly spaced NW–SE domino faults to north with conjugates; concentric faults up to ~5 km beyond brim). Minimal deformation in an eastern ‘sheltered zone’ (uprange). Faulting likely driven by seismic shaking, dewatering/volume loss, and lateral transfer toward the unconfined plateau margin; strain accommodated on a detachment near KU1 (likely black shales).
• Impact angle and trajectory:
  • Bilateral symmetry of central uplift and concave-to-east imbricated radial thrusts with consistent west–southwest vergence indicate downrange transport, implying a low-angle oblique impact from the ENE (trajectory ~80° azimuth) with angle likely <30°.
• Crater modification and resurge infill:
  • KPg1 marks crater geometry immediately post-modification (depth ~230 m), KPg2 records impact breccia/suevite emplacement (depth ~130 m), and KPg3 records resurge infill (depth ~70 m). Resurge reduces apparent depth-to-diameter from ~1:40 to ~1:130, suppressing marine crater relief.
• Alternative origins excluded:
  • No evidence for bottom-up processes (e.g., salt or magmatic diapirism, volcanic vents). Integrated 3D structural/stratigraphic/velocity evidence uniquely supports impact.

The 3D seismic volume provides an unprecedented, high-confidence geophysical case for classifying Nadir as a hypervelocity impact crater. Quantitative morphology, internal structure, and velocity anomalies match established impact crater scaling and architecture. The observed asymmetric deformation—deeper annular moat uprange, west–southwest-verg­ing radial thrusts—combined with a distinct ‘sheltered zone’ uprange supports a low-angle oblique impact from the ENE. This reduces ambiguity from inherited basement fabric and demonstrates that features closest to dynamically weakened central uplift are most diagnostic for trajectory reconstruction. The study refines a multi-stage model for marine impacts: transient excavation; early central uplift formation via rebound and inward flow under transient weakening; rim collapse forming annular moat and terraces; then centripetal, concentric inward flow of shallow target sediments forming the brim; followed by resurge that emplaces sorted suevite and suppresses crater relief. The extensive chaotic seismite outside the rim, concentric ridge formation, and resurge scar/gullies attest to strong tsunami–seabed interaction and widespread liquefaction/shallow deformation over areas of ~10²–10³ km², underscoring marine impact hazards to subsea infrastructure. These observations imply that in rare, pristine, rapidly buried marine/sedimentary targets with complete high-resolution 3D coverage, geophysical imaging alone can be sufficient to confidently diagnose an impact origin, even without drill-confirmed shock metamorphism. This has implications for re-evaluating candidate marine craters (e.g., Silverpit, Praia Grande) using modern 3D seismic and for improving hazard assessments of oblique marine impacts.

High-resolution 3D seismic imaging of the Nadir Crater resolves its full morphology and subsurface architecture, providing compelling geophysical evidence for a hypervelocity impact origin at near K–Pg age. The crater exhibits a ~9.2 km rim, ~22–24 km brim, a stratigraphically uplifted central structure with diagnostic velocity/porosity anomalies, and a pronounced asymmetry consistent with a low-angle ENE oblique impact. Beyond the brim, an extensive shallow damage zone and a large resurge scar/gullies reveal strong tsunami–seabed interactions and widespread liquefaction. A refined multi-stage model for marine impact crater evolution is proposed, demonstrating that resurge profoundly suppresses crater relief (depth-to-diameter reduced from ~1:40 to ~1:130). Nadir serves as a natural laboratory to test and calibrate marine impact processes, trajectory indicators, and hazard consequences. Future work should include precise geochronology, high-resolution numerical modeling of oblique marine impacts with realistic target properties, and ocean drilling to recover shock metamorphic indicators for ultimate confirmation. Broader application of complete 3D imaging may enable confident identification of additional buried marine impact craters.

• Absence of core samples and shock metamorphic indicators means ultimate confirmation of impact origin awaits ocean drilling; conclusions rest on exceptionally diagnostic geophysical evidence.
• Interpretation of asymmetries and damage patterns is influenced by inherited structural fabric (reactivated NW–SE faults), though central uplift features mitigate this.
• The approach is most applicable to marine/sedimentary targets that were rapidly buried and pristinely preserved; deeper or tectonically modified craters may remain ambiguous with geophysics alone.
• Velocity–porosity inferences assume porosity as the dominant control on seismic velocity; compositional or diagenetic variations could contribute locally.
• Confidentiality constraints limit access to raw SEG-Y and well data (though interpreted surfaces are publicly archived).