Multiple lines of evidence for a hypervelocity impact origin for the Silverpit Crater

Domain: earth_science
Published: 2025-09-20
Editorial outcome: minor_revision

Abstract
An impact origin for Silverpit Crater, on the UK continental shelf, has been contested over the last two decades, with a lack of definitive evidence – traditionally petrographic evidence of shock metamorphism – to resolve the debate. Here we present 3D seismic, petrographic and biostratigraphic data, and numerical impact simulations to test the impact hypothesis. Theseismic data provide exceptional imaging of the entire structure, confirming the presence of a central uplift, annular moat, damage zone and numerous secondary craters on the contemporaneous seabed. The distribution of normal and reverse faults in the brim, and curved radial faults around the central uplift suggest a low-angle impact from the west. The pitted, flat-topped central uplift at the top chalk horizon may indicate significant devolatilization of chalk immediately following impact. Biostratigraphic data confirm that this event occurred during the middle Eocene, between 43-46 million years ago. Petrographic analysis from the reworked ejecta sequence in the nearby 43/25-1 well reveal two grains with shock lamellae, indicating shock pressures of ~10-13 GPa, consistent with results from our numerical models. This combination of data and modelling provide compelling evidence that Silverpit Crater is an exceptionally preserved hypervelocity impact structure.

Introduction
Hypervelocity impacts of asteroids and comets with Earth represent a significant hazard and are a ubiquitous process in the Solar System. Impactors larger than ~100 m diameter are capable of penetrating the atmosphere and striking Earth’s surface to form craters1. Such events are rare, with none observed in recorded history; therefore, their consequences can only be inferred from impact craters and ejecta deposits preserved in the geological record. Impact structures are relatively rare on Earth, with only ~200 confirmed impact craters in the terrestrial record2,3. Marine-impact craters are even more rarely preserved, despite over 70% of the Earth’s surface being covered with water, with only ~33 confirmed or probable marine-impact craters identified4.

Most terrestrial impact structures are poorly preserved as those exposed at the Earth’s surface are modified by weathering, erosion and tectonic deformation. Buried craters are often better preserved, but these are difficult to investigate without high-resolution seismic imaging and/or drill cores. The only impact crater on Earth that is fully imaged with 3D seismic is the ~9 km diameter Nadir Crater offshore West Africa5,6. Other craters identified on 2D seismic have either been deformed by later tectonic processes (e.g. Mjolnir7) or are too large to be imaged by 3D seismic datasets (e.g. Chesapeake8, Chicxulub9). This limits our understanding of the near-surface processes that occur during and shortly after an impact event, which likely vary depending on the environment of impact and the physical properties of the target stratigraphy.

Traditionally, proposed craters are only confirmed as hypervelocity impact structures on the basis of shocked mineral phases—principally quartz or feldspar—and associated evidence from physical samples10,11. Shocked minerals only form at extreme transient shock pressures that cannot be replicated by other terrestrial processes, even deep within the lithosphere. The Nadir Crater is the only example where seismic data alone has been used to show ‘beyond a reasonable doubt’ an impact origin6, showing that there are exceptional cases in which high-quality geophysical imaging allows near-diagnostic structural characteristics to be identified and other epigenetic mechanisms to be conclusively ruled out6.

The Silverpit Crater is situated in the Silverpit Basin in the southern North Sea, UK (Fig. 1). The basin has a complex tectonic history, having experienced multiple periods of rifting and uplift throughout the late Palaeozoic, Mesozoic and Cenozoic12,13. At the Silverpit Crater location, Triassic and Jurassic evaporites, carbonates and clastic sedimentary rocks overlie a mobile Zechstein evaporite sequence (Fig. 2). The Jurassic and Triassic sediments are partially eroded across the area by the Base Cretaceous Unconformity (BCU), which is overlain by Cretaceous limestones and chalk, and Paleogene marine mudstones. The Triassic to Paleogene section has been folded into a series of gentle 10–100 km scale, NW-SE trending anticlines and synclines, which have been subsequently eroded and then buried during the Quaternary14.

The crater was identified from 3D seismic data in 2002, and proposed as a new candidate hypervelocity impact structure15. Although the seismic data available at the time only covered part of the structure, the crater was described as a 20-km-diameter, multi-ringed structure with characteristics consistent with a complex impact crater, including a circular planform morphology, concentric faults, and a proposed stratigraphic uplift below the crater floor. The crater floor was inferred to be at or just above the Cretaceous-Paleogene (K-Pg) boundary, with an age of approximately 65-60 Ma, based on extensive deformation of the Upper Cretaceous chalk below the crater floor.

More seismic data were acquired and interpreted in the following years, further constraining the geometry of the potential impact structure16. Although the structure was still only partially imaged, these data allowed the crater rim to be redefined as a much smaller ~8 km in diameter, based on the most distant inward-facing extensional fault defining a fault terrace at the top of the Cretaceous. According to this model, concentric extensional faults beyond the rim were interpreted to have formed over longer timescales (thousands of years) due to viscous relaxation of the sediments near the seabed into the crater cavity. The stratigraphic position of the crater floor was later interpreted to be shallower than previously thought, based on the presence of faults (albeit poorly imaged because of seismic multiples) cutting across the lower Paleogene interval. This led to the proposition of a much younger, Eocene, impact age17. This age was interpreted based on the correlation of seismic with unpublished biostratigraphic data from industry well 43/25-1, which penetrates the stratigraphy around 3 km to the NW of the centre of the proposed rim.

An impact origin for the crater has been disputed, with crater formation instead being explained by salt withdrawal in the deep subsurface18, or by hydrothermal venting associated with Paleogene dykes that are present across the wider Silverpit Basin19. Some of the structural evidence for an impact origin is also disputed, with the stratigraphic uplift inferred to be a seismic artefact at the boundary of several seismic surveys, where full-fold seismic coverage was absent18. These competing hypotheses were widely reported in the media, resulting in a public debate at the Geological Society of London in 2009, where those present voted “overwhelmingly” for a non-impact origin (https://www.geolsoc.org.uk/Geoscientist/Archive/December-2009/Silverpit-not-crater). Even though this was a public vote rather than a decision arrived at by an expert panel of impact specialists, this debate’s outcome, combined with a lack of new evidence, appears to have led many researchers to consider the question closed, with limited further research on subsurface data or modelling in the decade and a half since.

Here, we reassess the impact hypothesis for Silverpit Crater with geophysical data, analysis of well cuttings, and numerical modelling. We use high-resolution, pre-stack depth migrated (PSDM) 3D seismic data acquired by Northern Endurance Partnership (NEP) that offers improved imaging of the crater, and full seismic coverage of the central uplift. Samples from drill cuttings (rock fragments transported back to the surface in drilling mud) from well 43/25-1 are analysed to constrain the age of the crater biostratigraphically, and to look for evidence of shock metamorphism in the proposed proximal ejecta deposits, or in the subsurface at the time of impact. Finally, we present numerical impact simulation results to estimate the impact energy required to produce the observed crater morphology in the submarine, sedimentary target, and particularly to test models of crater modification based on observational evidence. In combination, these data show strong evidence for a hypervelocity impact origin for this structure.

Results
Seismic observations
The Silverpit Crater is here defined as a 3.2 km-diameter circular depression with its centre situated around 3 km to the southeast of the 43/25-1 well (Figs. 2 and 3). The surface is characterised by two distinct seismic reflections, herein referred to as CF1 and CF2, separated by a seismically transparent crater-fill package, sitting within the Paleogene section. These reflections are located above a zone of broad deformation which deepens and intensifies towards the crater centre. Below the margins (rim) of the crater, the chalk in particular is deformed by normal faults into an annular moat, surrounding an uplifted, anticlinal structure below the centre of the crater at both top chalk and BCU level.

The Paleogene crater floor and contemporaneous seabed shows a stepped morphology defining three concentric circular zones: a ~1.2 km diameter nested central crater, a ~3.2 km diameter crater rim corresponding to a subtle topographic peak (sensu 20) (Figs. 3 and 4), and a ~18 km diameter zone coincident with the extent of underlying deformation, referred to here as the crater brim (sensu 21) (Fig. 5). The thickness of the crater-fill package mirrors this pattern with ~60 m, ~30 m and <10 m present within the nested inner crater, crater rim and brim respectively (Fig. 4). The latter is within tuning thickness (vertical resolution) of the seismic data and may be thinner. The top of the package, CF2, is characterised by a strong “hard” reflection (downward increase in acoustic impedance, caused by an increase in velocity and/or density) and the base, CF1, by a “soft” reflection, showing that the crater-fill has a higher acoustic impedance than the overlying and underlying lithologies.

The crater rim diameter of 3.2 km defined here is smaller than that described previously16, although it is consistent with the interpreted scale of the original transient crater by these authors. This implies that the crater size is just above the simple-to-complex transition diameter of 2 km for sedimentary crater targets on Earth22. Previous larger crater size estimates were based on a probable near-end-Cretaceous crater age, and geomorphological observations of the top Chalk Group. Although we do see some subtle radial structures at the seabed beyond our interpreted crater rim, these are related to deformation external to the crater; that is, the ‘outer limit of deformation’, as described below20. The crater depth (considered here equivalent to crater-fill thickness) to crater rim diameter (3.2-km) ratio of ~1:50, and the presence of a central uplift, is consistent with the suppressed morphology of marine and sedimentary target craters of this size that have undergone substantial gravity-drive collapse6,22.

Both the CF1 and CF2 crater floor reflections have anomalously high amplitudes relative to the background Paleogene reflectivity (Fig. 3b, e). These high amplitudes extend ~1.5 km into the crater brim, where they gradually decrease due to tuning effects and/or a decrease in acoustic impedance contrasts. Seismic amplitude and dip attribute maps of the top and base crater-fill show several other distinctive features. CF2 displays two linear features within the inner crater, in a N-S and E-W orientation, respectively, producing a cross-like morphology (Fig. 3c). We interpret this to be a conjugate set of faults that form across the central crater floor after the crater was filled, presumably because of differential compaction. There are also several small (~0.25 × 1 km) scarps situated inside, but close to the crater rim (Fig. 3c, f), that we interpret as resurge scars formed as water cascaded back into the evacuated crater (e.g. refs. 6, 23, 24). These have not been observed in previously published geophysical data from Silverpit (c.f. 17).

Between the crater rim and the outer extent of the high-amplitude fringe (~1.5 km from the rim) at the top and base crater surfaces, there are a series of individual circular depressions of up to ~150 m diameter (Fig. 3). Seismic sections across these circular features show that they are around 40 ms (~35 m) deep at the base crater reflection, but only up to 10 ms (~9 m) deep at the top crater reflection. We interpret these as secondary craters. These pits are up to 5% of the size of the primary crater (3.2 km) at Silverpit, and are thus consistent with scaling relationships for secondary craters observed on other planets and satellites25. An alternative explanation is that these could represent degassing features around the primary crater, which is discussed further below.

Immediately below the seabed contemporaneous to the crater floor is a ~18-km diameter zone of deformation which deepens and intensifies towards the centre of the crater. In the outer parts below the crater brim, concentric faulting is limited to the lower Paleogene to uppermost Cretaceous section (Figs. 2 and 3). Towards the centre of the crater, faulting extends increasingly deeper to BCU and Lower Jurassic level. No faults are observed extending down into the Triassic section. A few faults extend across and slightly above the crater-fill package, where their displacement rapidly dies out. Extension of some structures above the crater-fill is interpreted as reactivation of crater-related faults during subsequent burial and folding.

The subsurface deformation is most clearly imaged at the top of the Upper Cretaceous chalk, where three distinct sets of faults can be interpreted (Fig. 5). The dominant faults are concentric, with mainly normal faults to the north, west and south, but mainly reverse faults in the east (Fig. 2). Both normal faults and reverse faults form conjugate fault pairs, suggesting several décollement surfaces within the chalk, with a typical ~1 km spacing between fault sets. The improved image quality in the NEP seismic data shows unambiguously that most of these concentric faults extend all the way up through the Paleogene, to the crater floor horizon (cf. 15). The contrast in deformation style is likely due to the rheological contrasts between different rock types, with the chalk deforming in a more brittle manner relative to the clay-rich Paleogene sequence above.

Cutting obliquely across the concentric faults are a set of curved, generally N-S trending faults that have a concave-to-the-east morphology (Fig. 5b). These have been previously described as ‘spiral faults’ and interpreted as having formed by centripetal radial extension following impact16. Our data show that these form exclusively on the north and east side of the crater, suggesting an asymmetric (oblique) impact. The sense of displacement on these faults varies and is difficult to fully constrain as these faults interact with the concentric faults. However, these have a strike-slip component and a reverse slip component in some locations (Fig. 5c).

In the central area below the crater, the deformation style at the top of the chalk changes. The entire Chalk Group shows intense faulting and fracturing below the crater rim, with deformation so intense that no consistent fault trends can be distinguished in seismic (Fig. 2). The Chalk Group also gets progressively thinner, towards the crater floor, from about 500 m in thickness below the outer brim, to ~250 m thickness below the crater rim (Fig. 6b). Correspondingly, the top chalk reflection deepens to form a ~1 km wide moat around a ~1 km wide, central plateau (Fig. 6a), which is elevated with respect to the moat but not relative to the undeformed chalk below the outer brim. Seismic attributes across both the moat and central plateau show a pitted texture, with individual pits of up to ~0.5 km diameter, that we interpret to be caused by fluid escape and/or devolatilization (Fig. 6b).

At the BCU level, there is a marked decrease in the intensity of deformation relative to the more brittle overlying chalk. At this depth, faulting is limited to within a 2.5 km diameter zone below the crater. The main structural feature at the BCU level is a ~200 m high stratigraphic uplift, which is associated with a set of predominantly NE-SW orientated reverse faults radiating away from the uplift (Fig. 7). These radial faults have a slightly curved nature with a concave-to-the-west geometry and are consistent with those observed at other oblique complex impact craters6, 26–28. The central uplift is offset from the centre of the Paleogene crater by around 300 m (Fig. 2b). The ratio of crater diameter to stratigraphic uplift at this depth is around 1:15. Below the BCU uplift, the seismic data quality degrades, and 70 m reflections show a consistent push-down, suggesting that seismic velocities across this zone are lower than in the surrounding area. There is no evidence of a stratigraphic uplift below around 500–700 m beneath the crater floor.

Nannofossil biostratigraphy
The 43/25-1 well is located 1 km to the northwest of the Silverpit Crater rim. It penetrates the inner part of the crater brim where seismic amplitudes at the CF1 and CF2 crater floor (and contemporaneous seabed) reflections are still elevated above background values (Fig. 3). The well-to-seismic tie (Fig. 8 and S2, S3) shows that the CF1 and CF2 reflections intersect the well at around 502 and 522 ms respectively (~471 m and ~495 m measured depth, MD). 31 samples were analysed for nannofossil assemblages using standard smear slides and polarising microscopy, between 183-607 m MD of well 43/25-1 to constrain the age of the crater floor (Fig. 8). The main results are described here and are presented fully in supplementary information.

All but one of the 23 samples taken between 439 and 597 m have a well-preserved, diverse but not abundant (5-10% of grains) typical middle-Eocene nannofossil assemblage. The age range of these sediments is constrained to be between 45.95 (Base Sphenolithus furcatolithoides) and 43.06 Ma (Base common Reticulofenestra umbilicus), equivalent to Zones CNE10-12. The one smear slide barren of nannofossils in this interval was taken from the sample at depth 494 m, with a smear slide dominated by high-birefringence particles of ~2 to 15 µm diameter, in cross-polarised light (XPL) (Fig. S4) interpreted to be carbonate. In plain polarised light (PPL), this smear slide differs from the surrounding middle-Eocene sediments in having little organic matter or clay particles, and substantially less micron-scale pyrite, which is pervasive in the background of other middle-Eocene samples (Fig. S4). To test the consistency of observations from this depth, three more small (~0.1 g) cutting samples were used to make three additional smear slides. All three of these yielded middle-Eocene nannofossil assemblages similar to those samples above and below this level (Fig. S4).

Based on the well-to-seismic tie, the top crater floor reflections intersect the well at ~475-492 m. This positions its within the CNE10-12 nannofossil zone, showing that the age of the crater is between 43–46 Ma. The anomalous, barren sample at 494 m is near the base of this interval. The presence of a diverse middle-Eocene nannofossil assemblage in samples above and below the crater floor horizon provide clear evidence for an open marine setting, although these samples cannot be used to constrain the palaeo-water depth.

Petrography and shocked minerals
Samples from the 421–610 m depth interval were investigated every ~6 m and analysed for petrography and potential deformation microstructures. Rock samples in the Paleogene are predominantly shales, with varying amounts of quartz, feldspars and clay grains. Petrographic results for these samples are summarised in Supplementary Information. We focus here on evidence for shock metamorphism in these samples, found exclusively in the 463–494 m interval, in the section equivalent to, and immediately stratigraphically above, the Silverpit crater floor (~492 m).

We found two grains of 40 to 80 µm size at 463 and 494 m depth that contain straight lamellae (Figs. 8 and 9). These lamellae are decorated with fluid inclusions, are strictly parallel, and have a spacing that ranges between 1 and 7 µm. One of the host grains is quartz (463 m), and one is potassium feldspar (k-felspar) (494 m). The quartz grain at 463 m shows at least two different crystallographic orientations of the lamellae along {10-13} and {10-14} in different domains of the crystal, which might indicate that the grain shows Dauphine-type twinning. Lamellae orientations are consistent with shock pressures of 10–13 GPa10. A third orientation could not be indexed. The k-feldspar grain at 494 m displays parallel lamellae decorated with fluid inclusions, and the lamellae have a slightly lower bulk density suggesting amorphization, consistent with shock metamorphism29. They are neither perthitic exsolution phenomena nor cleavage planes. The lamellae fulfil the characteristics of shock lamellae and are interpreted as decorated Planar Deformation Features (PDFs).

The presence of such shocked minerals at the stratigraphic equivalent to the crater floor provides strong independent evidence for an impact origin for Silverpit Crater. The presence of shocked minerals above the crater floor is discussed further below.

Impact modelling
Reconnaissance simulations of the Silverpit impact were performed using a simplified target structure and vertical trajectory to constrain impactor parameters and timescales of different phases of deformation (Fig. 10). A five-layer target was used to represent the major rheologic divisions, including a nominal 100-m water layer, 300-m layer of weak, ductile Paleogene clay sediments, 600-m of brittle chalk, 300-m layer of ductile mudstone and underlying Triassic sediments (Fig. 10A). The simulation that produced the best match to the morphology and subsurface structure of the inner basin used an impactor diameter, speed and density of 160 m, 15 kms−1 and 3300 kg m-3, respectively. An impact speed of 15 kms−1, which is slightly lower than the average impact speed on Earth of 20 kms−1, was selected to account for atmospheric deceleration. The simulation replicates the major structural observations of the inner basin observed in the seismic data, including the approximate morphology of the Top Chalk; the central uplift in the Jurassic mudstone and the envelope zone of pervasive damage (lighter shading in chalk layer). Concentric faults are not replicated in the wider brim, especially in the Paleocene, likely because of the oversimplification of the material model used to represent the weak, water-saturated, and partially lithified sediments in the Paleocene and chalk (cf. 5).

The impact simulation forms a 1-km deep, 3-km wide transient crater within 12 s, which is lined with highly shocked clay and chalk sediments (Fig. 10B, C). Shock pressures in the chalk central uplift range from 1 GPa at a radius of 1 km to 20–30 GPa or higher near the top of the chalk and inside a 500-m radius. Particle tracking shows that the ejecta, with a range of shock pressures up to ~30 GPa, is derived entirely from the Paleogene clay, which is almost entirely excavated above the chalk inside the crater. Collapse of the crater, which enlarges the transient crater diameter by >30%, begins with uplift of the weaker mudstone and overlying chalk (30 s, Fig. 10D), which is subsequently covered by inward collapse of the overlying weaker and more mobile clay (60 s, Fig. 10E). An extended period (minutes) of resurge then floods the crater and erodes the rim (Fig. 10F). At the location of the 43/25-1 well (~2.5-km radius), proximal ejecta, with a range of shock pressures, lands after about 30 s, settling through the water column as a density current and generating a large amplitude, breaking rim-wave tsunami.

Methods
The Silverpit Crater has been assessed on the previously interpreted PGS Southern North Sea MegaSurvey and a high-resolution 3D seismic dataset acquired in 2022 on behalf of the Northern Endurance Partnership (NEP). This Greater NEP 3D seismic survey was acquired in water depths of 21–93 m. Acquisition parameters include 4×400 cubic inch sources and 9×3 km streamers with a separation distance of 50 m (ref. 31). The shot interval was 6.25 m, with a sample rate of 2 ms. Bin dimensions are 6.25x6.25 m for acquisition and 12.5x12.5 m for processing, with a nominal fold of 80. The record length for the survey is 4000 ms. Data were processed using a proprietary Pre-Stack Depth Migration (PSDM) sequence including full-waveform inversion (FWI).

The Pre-stack Depth Migrated (PSDM) data, acquired and processed using modern techniques, provides a significant uplift in data quality compared to previously available post-stack time-migrated datasets, which focused on pre-Zechstein imaging. Notably, the NEP data has greater frequency bandwidth, improved fault imaging and fewer multiples across the zones of interest (Fig. 2 and S1). The combined datasets provide 100% coverage of the crater (within the rim, sensu 20) and 95% coverage of the entire damage zone associated with the crater. The NEP data were interpreted in the time rather than depth domain, for seamless interpretation with older Megamerge data, in particular when correlating the surfaces with the 43/25-1 well, which is beyond the limit of the NEP data. Data in the time domain were generated using the same velocity model as that used to depth-migrate the PSDM data.

Seismic interpretation was carried out using Schlumberger Petrel 2020 software, including horizon mapping, structural element mapping, velocity analysis and attribute analysis.

The 43/25-1 well was drilled in 1985 by British Gas. The petrophysical data and drilling reports were made available by the North Sea Transition Authority (NSTA) (https://ndr.nstauthority.co.uk/). The well was drilled for a deeper (Palaeozoic) target with a relatively limited log suite in the shallow section. The samples in the interval of interest are from drill cuttings, with some inherent possibility of contamination from borehole collapse during drilling, which could lead to shallower (younger) samples mixing with those from the assigned depths. Caliper log data and biostratigraphic results suggest that such contamination is absent or minimal. Nevertheless, there is some outstanding depth uncertainty due to mixing in the drilling mud in the borehole, between the subsurface and the shakers on the drilling rig, where the samples are collected.

The high-amplitude reflections (CF1 and CF2) interpreted to represent the seabed contemporaneous to the crater floor were correlated northwest of the crater rim, intersecting the 43/25-1 well at 502 and 522 ms TWT. Time/depth pairs derived from borehole seismic data (check shots) were available for 43/25-1, but the shallowest measurement available was at 914 m, within the Chalk Group. A time-depth model for the well based on this check shot data alone suggests that the CF1 reflection intersects the well at ~580 m MD (measured depth) (Fig. S2). However, extrapolation of this trend to the surface results in a significant overestimation of shallow velocities. To constrain this further, four shallow check shots from within the Cenozoic stratigraphic section were spliced in from the 43/24-3 well, 5 km to the northeast. A time-depth model incorporating these check shots with those already measured at 43/25-1 results in a shallower estimate of the CF1 horizon, at ~540 m. The velocity model was further refined by generating a synthetic seismogram at the well location, using wireline log datasets. The manual correlation of this synthetic to adjacent seismic traces from the PGS MegaSurvey resulted in a shallower estimate of the CF1 crater floor horizon, at ~495 m. We consider this last value to be the most likely depth for the crater-floor equivalent seabed.

The well-tie shows a good match at all key stratigraphic levels, and a fair character match across the Paleogene section (Fig. S3). There is some uncertainty on the seismic well-tie and exact intersection with the crater due to lack of check shot data at the crater level and limited sonic data above the crater to incorporate low frequency components into the synthetic.

For calcareous nannofossil biostratigraphic analysis, 31 sub-samples of the most representative lithologies were carefully selected from cuttings samples across the 183–610 m interval. Samples were prepared using the simple smear slide technique for nannofossil observation55. Examination of smear slides was completed using a transmitted-light microscope (Zeiss AxioScope at x1250 magnification) under cross-polarised light (XPL). Age determinations are based on the calibrated bioevents for low and middle latitudes56. Nannofossil taxonomy follows 57–60 as compiled in the online Nannotax3 database.

For the petrographic analysis, 27 cuttings samples were selected from the 421–603 m interval, covering the full range of crater floor equivalent estimates for the well. Cuttings were mounted on glass and embedded in epoxy, then the aggregates were sawed and polished to 30 µm thickness. Quartz grains in these samples were visually inspected for evidence of shock lamellae or other shock metamorphic features. Candidate shocked minerals were analysed using a scanning electron microscope with energy dispersive X-ray spectroscopy (SEM/EDX) to confirm mineral species and for crystallographic indexing using a U-stage. Estimated pressures for shock features are from ref. 10.

Impact simulations were performed using the iSALE shock physics code61, which solves the equations for conservation of mass, momentum, and energy using a finite-difference approach based on the SALE hydrocode. The specific physical response of a material to impact is approximated using an equation of state to describe volumetric response and a shear strength model. Impactor and target materials were approximated using the closest available analogue material model.

A simplified pre-impact target structure including the major sedimentary divisions was constructed based on seismic and drill core results from the area. A spatial resolution of a 10-m per grid cell was used to resolve the impactor and target to a 6-km radius and 1.5 km depth, with a zone of coarser resolution extending to over 13 km depth and 17 km radius. Beneath the seafloor was 300 m of ductile clay overlying 600 m of brittle chalk, and a 300-m layer of mudstone. A further layer of chalk extended beneath this to the bottom of the mesh. The sedimentary sequence was overlain by 100 m of water. A NEOS-derived equation of state tables for calcite62 and quartz63 were used to represent the chalk and clay/mudstone layers, respectively. Water-filled porosity in the sediments was neglected. To produce the enhanced mobility of rock masses during impact crater collapse (c.f. 47), we incorporated the Acoustic Fluidisation Block Model parameters used for carbonates64. As the choice of rock strength and material weakening parameters has a substantial effect on the final crater form and subsurface formation, we explored a range of potentially suitable values. The parameters used in the reconnaissance simulation that best replicated observations are given in Supplementary Information Table S2. We note the large difference in limiting strength (Ym) between the clay/mudstone and the chalk, which gives the clay/mudstone its weaker, more ductile behaviour, as well as the relatively low damaged material friction coefficient (μd) to account for water saturation of the sediments.

To produce a crater size of the internal Silver Pit structure (~3.2 km diameter), the impactor was approximated as a 160-m diameter sphere striking the surface at a speed of 15 kms−1. A dunite equation of state and strength model was used for the projectile65, with a reference density of 3300 kg m−3. A vertical impact was assumed for computational expediency in these reconnaissance simulations. Future work will explore the effect of the impact angle, given the evidence for oblique impact in the structural deformation. These impactor properties are within the range expected for terrestrial meteors66.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have conducted a thorough study of new data from the Silverpit Crater, the origin of which has been debated. The paper presents new evidence that supports an impact origin for the crater. I recommend that paper be published after addressing the following remarks.\nIntroduction Line 29 – space between “than” and “a”.\nIn the introduction and results, the authors have stated that the diameter of this suspected impact structure is reported to be 3 km. I would recommend adding an explanation somewhere in the text about how this diameter is defined (either by previous work – and then state how that was redefined from the original 20km, or in the current work and what criteria were used to set the diameter at 3km – are these arguments new/different, or in agreement with the previously defined 3km boundary?)\nA crater fill package is mentioned a few times throughout the text, and there is some interpretation about its deposition. Is there any indication as to the material of this crater fill? It might be useful to elaborate a little bit about what is generally expected in a crater fill deposit in a complex crater, and how the observations/simulations in this study might support the interpretation of this unit as a crater fill deposit.\nReconstructing impact trajectory –\nLines 1- 5: the authors have mentioned several other craters with sedimentary targets where fault systems exist. It is unclear whether impact trajectories at these sites were inferred solely from fault systems, or if there were other lines of independent evidence. It is also unclear whether these other craters are analogous to the Silverpit crater (other than being sedimentary targets – are these also marine impacts?)\nIs there any indication of pre-existing fault systems, deformation or zones of weaknesses in the target lithology in the Silverpit crater area that could affect the manifestation of impact-related faulting? It is important to consider potential pre- existing features while reconstructing the impact trajectory from structural evidence.\nResults – Seismic observations\nLine 30 – the pits are interpreted as secondary craters, but the authors have not provided convincing evidence for that. Secondary craters have not been definitively identified on Earth and so it is understandable that evidence of such features is difficult to identify from seismic data only. However, it would be helpful to discuss more about why the authors have interpreted these features as secondary craters other than just their size.\nAdditionally, there are pitted textures identified in a different part of the crater. Is there any relationship between the “secondary craters” and these pits on the inferred central uplift? From the evidence presented in the paper, it seems that the only main difference between the pits and the “secondary craters” may be their size. How do you distinguish between these two features – could they be formed by the same mechanism?\nI suggest that the authors add a schematic figure that visualizes the crater morphology and the evidence of impact observed in this study – it will be useful to compile all the observations into a single figure.\nFigure 2 –\n“A” and “B” labels are missings.\nFigure 2A shows a profile across NNW-SSW, but the caption says the profile is across NNW-SSE.", "reviewer_2": "The manuscript from Nicholson et al. titled “The silver bullet - new evidence for a hypervelocity impact origin for the Silverpit Crater” is a well-rounded work trying to confirm the impact origin of the controversial Silverpit crater in the North Sea. This work found new evidence of shock metamorphism in quartz and K-feldspar from drill cores, and presented new seismic data of the crater, and ran new numerical simulations to model the impact events. Overall, the impact origin of Silverpit Crater is supported by the evidence shown from this work. The publication of this work will help settle the debate in impact cratering community and provide new prospective for future discoveries of buried impact craters.\nThe methods are described in detail, the data analysis and interpretation of the drill cores are decent and meet the standards in the field. The modeling code used parameters based on the seismic data and drill core observations. The figures are well- labeled and clear to read. The writing is smooth, and the logic flow is easy to follow.\nA few minor comments:\n1. How many drill wells are there? Why so few? Any more planned? Is there any other shock metamorphism evidence observed in the well and checkshots? Any impact melt? Diaplectic glass? Any leftover from the impactor?\n2. Figure 6c: what is that obvious resolution change in the left to middle lower part of the image? Like it looks suddenly a bit blurry?\n3. For easier to read from shallow to deep depth, I would suggest re-arranging the pictures in Figure 9: A, B, C in one row labeled 463.3m, and D, E, F in the second row labelled 493.8m; same suggestion for Figure S4, the 494m on top and 500m in the bottom. Also, the label A-F in Figure 9 is capital while in caption is a-f.\n4. Is it possible to identify and label other minerals in SEM image Figure 9A? Any element mapping? And if the quartz and K-feldspar can be seen from the SEM image, please consider outlining them. Also, in Figure 9E, label “Kfsp” should be placed on the crystal, or use an arrow, otherwise it looks like the black area is Kfsp.\n5. Overall, if the silver bullet is the petrographic evidence of shock metamorphism, I would suggest having a bigger proportion of it in the manuscript (better balance between seismic data, petrographic and modeling), e.g. make figure 9 into a big and beautiful one and move some of the description from supplementary into the main text.\nGood job and good luck!", "reviewer_3": "The manuscript \"The silver bullet - new evidence for a hypervelocity impact origin for the Silverpit Crater\" by Nicholson et al. reexamined the Silverpit Crater in the North Sea. Analyzing the high-resolution 3D seismic data, they confirmed that Silverpit is actually a crater, which was disputed previously. They also found that drilled samples from the 43/25-1 well present a dearth of organics at the same depth level of the crater floor, supporting an energetic event, an impact had happened. Their impact modelling, which successfully reproduces the shape of observed craters, also supports their confirmation. As such, this paper investigated the Silverpit Crater from multiple points of view. I am sure that many researchers working in the various fields will be interested in this paper. Nevertheless, I feel like some additional discussions would be valuable and enhance future research on the formation of this crater especially from the viewpoint of oblique impacts.\n1. Shocked grains from drilled sample\nThe authors identified two grains metamorphosed via the highly shocked pressure. They argued that the grains are close enough to the crater thus supporting the impact origin. The 43/25-1 well is located 1 km north-west of the crater rim (Page 5, Line 35), and they mentioned \"Shocked material deposited in the clay\" (Page 7, Line 20). Therefore, I suppose those grains could be an impact ejecta. If correct, please clarify the experienced pressure of impact ejecta from their impact model. Although the impact result shows the experienced pressure of each layer (Fig. 10), it is hard to identify the ejecta's pressure as written in the abstract (\"indicating shock pressures of ~10-13 GPa, consistent with results from our numerical models\"). Also, how can they exclude the possibility that the shocked grains are delivered by other impacts in a similar era? If I misunderstood something, please correct me.\n2. Effect of oblique impact\nThey discussed the impact trajectory using the deformation around the crater. While they further argued that the impact angle would be low, the discussions about the effect of (low-angle) oblique impacts are less. I acknowledge their great work on the reconnaissance impact simulations and the oblique impact simulations are beyond their focus. Nevertheless, it would be worth discussing its general effect. For example, the amount of impact ejecta in the uprange direction is less than those in the downrange direction. Moreover, there is a V-shaped zone of no impact ejecta in the uprange direction (e.g., Gault and Wedekind 1978, Luo et al. 2022). Since the drilled site is in the uprange direction of the suggested impact trajectory, the shocked ejecta taken from the 43/25-1 well might be the rare case, if my above comment is correct. Alternatively, even if I am wrong, the shock pressure in the uprange direction tends to be lower than that in the downrange direction (e.g., Collins et al. 2022). As such, more discussions could be better to link between the shocked minerals and impact models in terms of oblique impacts.\n3. Pits; secondary craters and/or devolatilization\nThe authors found that several pits of ~150 m located at the crater brim, and interpreted those pits as secondary craters (Page 4, Line 26-32 and Fig. 3). While they suggested that the size ratio between primary and secondary craters is comparable to other planets, I am curious if this still holds for the Silverpit Crater case. Because the ocean affected the crater formation. They discussed that the secondary craters could be free from the resurge tsunami (Page 8, Line 28-35), which might be true. However, the impact movie does not indicate this clearly, rather the formation of secondary craters might be also influenced by the ocean. On the other hand, they interpreted other large pits of 50-500 m diameter located around the crater center as a different origin, devolatilization (Page 5, 13-21 and Fig. 6). Could the secondary craters also have been formed via devolatilization? Or can they exclude this and other possibilities and validate these are sure to be the secondary craters? Please also clarify the difference/similarity between these pits and their origins.\nAdditionally, I think an ejecta of 50-100 m seems a too large block (Page 8, Line 29), given that the impactor size is 160 m in diameter and the secondary crater is up to 150 m. How do the authors estimate this block size?\n4. Formation of the concentric faults\nAs Stewart and Allen (2002) mentioned that the Silverpit Crater is a multi-ringed impact structure, the concentric faults on the chalk (Fig. 5) remind me of multiring basins on icy satellites (Valhalla on Callisto, Tyre and Callanish on Europa). I agree that \"the rheological contrasts between different rock types\" (Page 5, Line 3) could be the key. At the same time, I am wondering if this contrast could fit with the suggested mechanism of multiring basin formation (Melosh and McKinnon 1978, McKinnon and Melosh 1980); i.e., can the chalk over the mudstone correspond to the lithosphere over the asthenosphere? Are there any features shown in the impact model? Please comment on this.\nHere are some other minor comments.\nPage 6, Line 11: Fig. S5 is missing, or just a typo of S4?\nPage 9, Line 43: How about comparing with terrestrial complex craters, not extraterrestrial ones?\nPage 12, Lines 4-5: Please state the mesh size. It should be fine enough to resolve the layer and the diameter of 160 m (Line 20). Relating to this, adding the impactor’s properties in Table S2 might be better.\nReferences:\nCollins et al. 2022: https://doi.org/10.1038/s41467-020-15269-x\nGault and Wedekind 1978: https://ui.adsabs.harvard.edu/abs/1978LPSC....9.3843G/abstract\nLuo et al. 2022: https://doi.org/10.1029/2022JE007333\nMcKinnon and Melosh 1980: https://doi.org/10.1016/0019-1035(80)90037-8\nMelosh and McKinnon 1978: https://doi.org/10.1029/GL005i011p00985", "reviewer_4": "This a very well written paper which clearly demonstrates that the Silverpit structure is an impact crater. The seismic data are of very high quality and provide convincing evidence for the impact origin even without the complementary shocked mineral evidence. The authors might want to consider the following comments prior to a decision on acceptance.\nGENERAL COMMENTS\nA crater diameter of 3 km is close to the limit separating simple craters from complex craters. The presence of a central uplift in the seismic data suggests the Silverpit crater falls into the category for complex craters and that a transient crater would have been present directly after impact that would have been somewhat smaller than the present day final diameter. Some discussion on the scaling relationships between simple and complex craters and the implications of crater type on the seismic interpretation would have been useful.\nAccording to the authors PSDM was applied to the seismic data. However, all vertical sections through the seismic volume are in TWT. Is this a mistake or have the seismic data been converted back from depth to time?\nReflector vs. reflection: These terms seem to be used interchangeably in the MS. In my view, a reflection is something which is observed on a seismic section while a reflector is something which is present in the sub-surface. You may want to consider using these definitions in the Results and Discussion sections.\nSPECIFIC COMMENTS\nPg 3, lines 13-15: “Although the structure was still only partially imaged, these data allowed the crater rim to be redefined as a much smaller 3 km in diameter, with some evidence of radial faults on one side of the central uplift.” This sentence is confusing, is the crater 3 km in diameter or is it 3 km smaller in diameter than the 20 km mentioned earlier on line 7? From Figure 1 and further reading of the MS, I see it is the former, but it would be better to be more clear here.\nFigure 2b: What does the “300 m offset” label in the figure refer to? It is explained later in the discussion, but please explain in figure caption.\nPg 3, line 29: Change “rather thana decision” to “rather than a decision”.\nFigure 5: It would be useful to be consistent with the colors for the faults. For example; green – reverse, red – normal; for this figure and the others in the MS. Also what does the dotted line in c) represent?\nPg 6, lines 29-30: “in different domains of the crystal, which might that the grain additionally shows Dauphine-type twinning” Something is missing in this sentence. Also, you could already here note that the presence of shocked quartz above the CF2 reflector will be discussed later.\nFigure 8: CF1 and CF2 appear to be incorrectly labeled in the AI panel.\nPg 7, lines 1-3: It appears you are assuming the impact to be vertical, not at an angle. I also interpret this to be the case looking at Figure 10 (symmetrical). You should state this here. I note later that you write the modeling assumed a vertical impact, but better to make the point at this stage.\nPg 8, line 24: Change “10.f” to “10f”.\nChristopher Juhlin, Uppsala Univeristy\n2025-04-09"}, "author_response": "Point-by-point response\nWe thank the four reviewers for their excellent comments and questions. These have all been\naddressed and we provide point-by-point responses to each comment below. Reviewers’\ncomments are in black and our responses are in blue.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors have conducted a thorough study of new data from the Silverpit Crater, the origin of\nwhich has been debated. The paper presents new evidence that supports an impact origin for\nthe crater. I recommend that paper be published after addressing the following remarks. We\nthank the reviewer for these positive comments.\nIntroduction Line 29 – space between “than” and “a”. Done\nIn the introduction and results, the authors have stated that the diameter of this suspected\nimpact structure is reported to be 3 km. I would recommend adding an explanation somewhere\nin the text about how this diameter is defined (either by previous work – and then state how that\nwas redefined from the original 20km, or in the current work and what criteria were used to set\nthe diameter at 3km – are these arguments new/di(cid:431)erent, or in agreement with the previously\ndefined 3km boundary?) The section in the introduction has been expanded to clarify that the\ncrater was still interpreted by Stewart et al. (2005) to be at the Top Cretaceous, and at 8 km in\ndiameter (the 3 km in diameter referred to the diameter of the crater immediately after impact,\nas they also propose post-impact collapse to the present geometry). We have also expanded\nthe description of the rim in the Results section to explain this more clearly, and how it di(cid:431)ers\nfrom that described in this paper in particular.\nA crater fill package is mentioned a few times throughout the text, and there is some\ninterpretation about its deposition. Is there any indication as to the material of this crater fill? It\nmight be useful to elaborate a little bit about what is generally expected in a crater fill deposit in\na complex crater, and how the observations/simulations in this study might support the\ninterpretation of this unit as a crater fill deposit. We think that this crater fill is likely dominated\nby resurge-deposits – sorted breccia or clastic sediment transported back into the crater during\nthe resurge event. We have added a couple of additional sentences to the relevant section in\nthe discussion (Tsunami resurge and secondary craters), including the fact that we do not\nexpect to see significant melt lense/suevite, based on the seismic observations.\nReconstructing impact trajectory –\nLines 1- 5: the authors have mentioned several other craters with sedimentary targets where\nfault systems exist. It is unclear whether impact trajectories at these sites were inferred solely\nfrom fault systems, or if there were other lines of independent evidence. It is also unclear\nwhether these other craters are analogous to the Silverpit crater (other than being sedimentary\ntargets – are these also marine impacts?). These observations are inferred to represent oblique\nimpacts based on numerical model results which replicate this structure. This has been\nclarified in the text. These craters are not all marine craters, but that is not critical here – such\nstructures can also occur in crystalline rock as well.\nIs there any indication of pre-existing fault systems, deformation or zones of weaknesses in the\ntarget lithology in the Silverpit crater area that could a(cid:431)ect the manifestation of impact-related\nfaulting? It is important to consider potential pre-existing features while reconstructing the\nimpact trajectory from structural evidence. There is no indication of pre-existing structures in\nthe target stragraphy, at least not above the Zechstein salt, which is well below the maximum\ndepth of deformation. We have added a sentence to clarify this at the beginning of this section.\nResults – Seismic observations\nLine 30 – the pits are interpreted as secondary craters, but the authors have not provided\nconvincing evidence for that. Secondary craters have not been definitively identified on Earth\nand so it is understandable that evidence of such features is di(cid:431)icult to identify from seismic\ndata only. However, it would be helpful to discuss more about why the authors have interpreted\nthese features as secondary craters other than just their size. We acknowledge that it is\nchallenging to fully document the evidence for this, in a short paper format. However, we think\nthat the evidence for these as secondary craters is compelling, including: (1) no seismic\nevidence for ‘bottom-up’ deformation below the craters, unlike the pits in the central uplift (see\nbelow); (2) these are consistent with the expected size for secondary craters in extraterrestrial\nstructures; (3) a lack of a compelling alternative origin for these (see below) and (4) consistent\nsize (parent to secondary crater ratio) as observed on secondary craters on the moon. We have\nadded some additional explanation of this in the text, but we have also acknowledged\nalternative origins for these (devolatilization of water from clastic sediments). See also\ncomments in response to Reviewer 3, below.\nAdditionally, there are pitted textures identified in a di(cid:431)erent part of the crater. Is there any\nrelationship between the “secondary craters” and these pits on the inferred central uplift? From\nthe evidence presented in the paper, it seems that the only main di(cid:431)erence between the pits\nand the “secondary craters” may be their size. How do you distinguish between these two\nfeatures – could they be formed by the same mechanism? The two sets of features are\nmorphologically quite di(cid:431)erent, and occur in areas with very di(cid:431)erent pressure/temperature\nconditions during impact, so we think that these are genetically di(cid:431)erent. The features\ninterpreted as secondary craters, outside the crater, are widely dispersed, extending at least 3\nkm radially from the centre of the crater. The subsurface temperature in this area is, according\nto our numerical models, not su(cid:431)icient to generate major devolatilzation of carbonates in this\narea. By contrast, those around the central uplift and crater floor are associated with significant\nseismic disturbance of the chalk and Paleogene clastics (so evidence of ‘bottom-up’\ndeformation), and are much larger and more tightly clustered. We have clarified this in the\nmanuscript, including a new paragraph at the end of the Discussion acknowledging that this\nneeds to be tested further. We have also made a new version of Fig. 6a to display the central pits\n(that we infer are caused by devolatilzation) more clearly.\nI suggest that the authors add a schematic figure that visualizes the crater morphology and the\nevidence of impact observed in this study – it will be useful to compile all the observations into\na single figure. This would be good to add but we have already reached the figure limit (10 max)\nfor Nat Comms. This can be added in future papers on this structure though.\nFigure 2 –\n“A” and “B” labels are missings. These labels have been added.\nFigure 2A shows a profile across NNW-SSW, but the caption says the profile is across NNW-\nSSE. The caption has been changed to NNW-SSW.\nReviewer #2 (Remarks to the Author):\nThe manuscript from Nicholson et al. titled “The silver bullet - new evidence for a hypervelocity\nimpact origin for the Silverpit Crater” is a well-rounded work trying to confirm the impact origin\nof the controversial Silverpit crater in the North Sea. This work found new evidence of shock\nmetamorphism in quartz and K-feldspar from drill cores, and presented new seismic data of the\ncrater, and ran new numerical simulations to model the impact events. Overall, the impact\norigin of Silverpit Crater is supported by the evidence shown from this work. The publication of\nthis work will help settle the debate in impact cratering community and provide new prospective\nfor future discoveries of buried impact craters. We thank the reviewer for these positive\ncomments and agree that this should resolve the debate around the origin of Silverpit.\nThe methods are described in detail, the data analysis and interpretation of the drill cores are\ndecent and meet the standards in the field. The modeling code used parameters based on the\nseismic data and drill core observations. The figures are well-labeled and clear to read. The\nwriting is smooth, and the logic flow is easy to follow.\nA few minor comments:\n1. How many drill wells are there? Why so few? Any more planned? Is there any other shock\nmetamorphism evidence observed in the well and checkshots? Any impact melt? Diaplectic\nglass? Any leftover from the impactor? There are only two exploration wells drilled within the\nvicinity of the crater, within the outer limit of the damage zone, both shown on figure 1. These\nare old oil exploration wells, drilled long before an impact origin of the structure was proposed.\nThe 43/24-3 well sits within the shallow seismic sequence that is almost completely obscured\nby multiples, thus defining the depth of the crater floor is very di(cid:431)icult (and it may have been\nentirely eroded by the Quaternary glaciation). Even the 43/25-1 well sits outside of the crater\nrim, so no impact melt, etc., although this could potentially be present in small amounts in the\nejecta. And there are only cuttings (rock chips) available from every 15 feet or so, so the\nchances of finding impact evidence was very small, a priori. We note that we are now\nconsidering proposing a scientific ocean drilling expedition to drill and recover continuous core\nfrom this crater in a more optimal location in the coming years, to test some of the hypotheses\nthat we have developed in this paper. This may allow some of these impactites to be recovered\nas well.\n2. Figure 6c: what is that obvious resolution change in the left to middle lower part of the image?\nLike it looks suddenly a bit blurry? This is the boundary between the NEP3D and PGS\nMegamerge survey. This was mentioned in the figure caption, but is now added to the figure for\ngreater clarity.\n3. For easier to read from shallow to deep depth, I would suggest re-arranging the pictures in\nFigure 9: A, B, C in one row labeled 463.3m, and D, E, F in the second row labelled 493.8m;\nsame suggestion for Figure S4, the 494m on top and 500m in the bottom. Also, the label A-F in\nFigure 9 is capital while in caption is a-f. This has been done.\n4. Is it possible to identify and label other minerals in SEM image Figure 9A? Any element\nmapping? And if the quartz and K-feldspar can be seen from the SEM image, please consider\noutlining them. Also, in Figure 9E, label “Kfsp” should be placed on the crystal, or use an arrow,\notherwise it looks like the black area is Kfsp. Figure 9 has been rearranged and labels added to\nhighlight the shocked minerals.\n5. Overall, if the silver bullet is the petrographic evidence of shock metamorphism, I would\nsuggest having a bigger proportion of it in the manuscript (better balance between seismic data,\npetrographic and modeling), e.g. make figure 9 into a big and beautiful one and move some of\nthe description from supplementary into the main text. We added some text to this section, but\nhave had to be conservative with this, as we still have to keep the overall paper relatively short\nfor Nature Communications. Likewise, we need to keep figure sizes as small as possible,\nwithout compromising legibility or clear presentation of the necessary evidence.\nGood job and good luck! Thank you!\nReviewer #3 (Remarks to the Author):\nThe manuscript \"The silver bullet - new evidence for a hypervelocity impact origin for the\nSilverpit Crater\" by Nicholson et al. reexamined the Silverpit Crater in the North Sea. Analyzing\nthe high-resolution 3D seismic data, they confirmed that Silverpit is actually a crater, which was\ndisputed previously. They also found that drilled samples from the 43/25-1 well present a dearth\nof organics at the same depth level of the crater floor, supporting an energetic event, an impact\nhad happened. Their impact modelling, which successfully reproduces the shape of observed\ncraters, also supports their confirmation. As such, this paper investigated the Silverpit Crater\nfrom multiple points of view. I am sure that many researchers working in the various fields will\nbe interested in this paper. Nevertheless, I feel like some additional discussions would be\nvaluable and enhance future research on the formation of this crater especially from the\nviewpoint of oblique impacts. We thank reviewer 3 for these positive comments, and address\ntheir concerns below.\n1. Shocked grains from drilled sample\nThe authors identified two grains metamorphosed via the highly shocked pressure. They argued\nthat the grains are close enough to the crater thus supporting the impact origin. The 43/25-1 well\nis located 1 km north-west of the crater rim (Page 5, Line 35), and they mentioned \"Shocked\nmaterial deposited in the clay\" (Page 7, Line 20). Therefore, I suppose those grains could be an\nimpact ejecta. If correct, please clarify the experienced pressure of impact ejecta from their\nimpact model. Although the impact result shows the experienced pressure of each layer (Fig.\n10), it is hard to identify the ejecta's pressure as written in the abstract (\"indicating shock\npressures of ~10-13 GPa, consistent with results from our numerical models\"). The shock\npressure in the ejecta was stated previously in the Results section on impact modelling, but we\nhave re-organised this for greater clarity, and to explicitly state that shock pressures in the\nejecta reach up to 30 GPa.\nAlso, how can they exclude the possibility that the shocked grains are delivered by other\nimpacts in a similar era? If I misunderstood something, please correct me. This can never fully\nbe excluded, but shocked grains are so rare in nature that other impacts are not necessary to\ninvoke here. We know that these grains are at, and stratigraphically slightly above, the Eocene\nseabed just outside the crater. This evidence is much more robust than for other confirmed\nterrestrial craters. For example, the Hiawatha impact structure appears as a confirmed crater in\nimpact databases (e.g. Impact Earth: https://impact.uwo.ca/map/), confirmed by shocked\nminerals in glacial outwash deposits several kms away from the crater, with no stratigraphic\ncorrelation. Nevertheless, we added a sentence to this discussion to acknowledge that\nderivation from another impact event is a possibility (albeit unlikely).\nKenny, G.G., Hyde, W.R., Storey, M., Garde, A.A., Whitehouse, M.J., Beck, P., Johansson, L.,\nSøndergaard, A.S., Bjørk, A.A., MacGregor, J.A. and Khan, S.A., 2022. A Late Paleocene age for\nGreenland’s Hiawatha impact structure. Science Advances, 8(10), p.eabm2434.\n2. E(cid:431)ect of oblique impact\nThey discussed the impact trajectory using the deformation around the crater. While they\nfurther argued that the impact angle would be low, the discussions about the e(cid:431)ect of (low-\nangle) oblique impacts are less. I acknowledge their great work on the reconnaissance impact\nsimulations and the oblique impact simulations are beyond their focus. Nevertheless, it would\nbe worth discussing its general e(cid:431)ect. For example, the amount of impact ejecta in the uprange\ndirection is less than those in the downrange direction. Moreover, there is a V-shaped zone of no\nimpact ejecta in the uprange direction (e.g., Gault and Wedekind 1978, Luo et al. 2022). Since\nthe drilled site is in the uprange direction of the suggested impact trajectory, the shocked ejecta\ntaken from the 43/25-1 well might be the rare case, if my above comment is correct.\nAlternatively, even if I am wrong, the shock pressure in the uprange direction tends to be lower\nthan that in the downrange direction (e.g., Collins et al. 2022). As such, more discussions could\nbe better to link between the shocked minerals and impact models in terms of oblique impacts.\nThis is a point we had considered but not developed in detail in the manuscript. Ejecta is indeed\ntypically distributed downrange and lateral to the crater following oblique impacts, with an\nuprange “forbidden zone” of no ejecta. However, in marine impacts, the tsunami resurge and\nsubsequent seiching redistributes this sediment, including in the uprange direction. We have\nextended this section by adding several sentences to explain this process.\n3. Pits; secondary craters and/or devolatilization\nThe authors found that several pits of ~150 m located at the crater brim, and interpreted those\npits as secondary craters (Page 4, Line 26-32 and Fig. 3). While they suggested that the size ratio\nbetween primary and secondary craters is comparable to other planets, I am curious if this still\nholds for the Silverpit Crater case. Because the ocean a(cid:431)ected the crater formation. They\ndiscussed that the secondary craters could be free from the resurge tsunami (Page 8, Line 28-\n35), which might be true. However, the impact movie does not indicate this clearly, rather the\nformation of secondary craters might be also influenced by the ocean. On the other hand, they\ninterpreted other large pits of 50-500 m diameter located around the crater center as a di(cid:431)erent\norigin, devolatilization (Page 5, 13-21 and Fig. 6). Could the secondary craters also have been\nformed via devolatilization? Or can they exclude this and other possibilities and validate these\nare sure to be the secondary craters? Please also clarify the di(cid:431)erence/similarity between these\npits and their origins. We have added an additional paragraph at the end of the Discussion\nsection to explain why these features are likely to be genetically di(cid:431)erent. See also the response\nto a similar point by Reviewer #1.\nAdditionally, I think an ejecta of 50-100 m seems a too large block (Page 8, Line 29), given that\nthe impactor size is 160 m in diameter and the secondary crater is up to 150 m. How do the\nauthors estimate this block size? We acknowledge that the value of 50-100 m for individual\nblocks seems too large (at least at the high end of this range) – thank you for pointing this out.\nWe have replaced this with a smaller estimate of up to 5-50 m diameter, based on scaling\nrelationships from other studies and calculations from the Earth Impacts E(cid:431)ects Programme.\nWe retain a large range for this as the impact/ejection velocity for these secondary craters is\nhighly uncertain. Similar sized craters from the moon (see reference to Singer et al., 2020 in the\nrevised version) are inferred to be caused by ejected blocks of around 3 m. However, this\nassumes an ejection speed of 2.7 km/s. The likely ejection speed in the case of Silverpit is likely\nmuch lower, at around 100 m/s at a range of 1 km from the primary crater. Adopting\nconventional crater scaling theory gives an estimate for blocks of up to 1/3 of the crater size, or\na maximum of 50 m. We retain a large range because of the uncertainty over impact velocity for\nthese secondary craters.\n4. Formation of the concentric faults\nAs Stewart and Allen (2002) mentioned that the Silverpit Crater is a multi-ringed impact\nstructure, the concentric faults on the chalk (Fig. 5) remind me of multiring basins on icy\nsatellites (Valhalla on Callisto, Tyre and Callanish on Europa). I agree that \"the rheological\ncontrasts between di(cid:431)erent rock types\" (Page 5, Line 3) could be the key. At the same time, I am\nwondering if this contrast could fit with the suggested mechanism of multiring basin formation\n(Melosh and McKinnon 1978, McKinnon and Melosh 1980); i.e., can the chalk over the\nmudstone correspond to the lithosphere over the asthenosphere? Are there any features shown\nin the impact model? Please comment on this. We agree that the unique terrestrial structure of\nSilverpit could have important implications for other extraterrestrial multi-ring impact\nstructures, particularly for those on icy satellites (e.g. Tyre, Callanish on Europa), although we\nnote that these may be genetically di(cid:431)erent to multiring basins on rocky planets (e.g. Valhalla)\ndescribed primarily by Melosh and McKinnon (1978) and McKinnon and Melosh (1980). We note\nalso that, in the case of Silverpit, the rheology is more complex, in that we have relatively\nunconsolidated sediment over competent chalk, over mudstone. We have added a short\nsection to discuss this in more detail in the first part of the discussion, but note that we cannot\nextend this discussion significantly in a short-format paper.\nHere are some other minor comments.\nPage 6, Line 11: Fig. S5 is missing, or just a typo of S4? Apologies, this was a typo of S4 – now\ncorrected.\nPage 9, Line 43: How about comparing with terrestrial complex craters, not extraterrestrial\nones? It is not clear what this refers to. Page 9, Line 43 states “Assuming an original thickness of\nbetween 500 m (Fig. 6b) to 600 m (the thickness in 43/25-1; Fig. S2)…”. However, we have\nremoved the reference exclusively to Earth on the first line of this paragraph, as this statement\nabout complex craters is valid for both terrestrial and extraterrestrial craters.\nPage 12, Lines 4-5: Please state the mesh size. It should be fine enough to resolve the layer and\nthe diameter of 160 m (Line 20). Relating to this, adding the impactor’s properties in Table S2\nmight be better. The grid dimensions have been added in the Methods section.\nReferences:\nCollins et al. 2022: https://doi.org/10.1038/s41467-020-15269-x\nGault and Wedekind 1978: https://ui.adsabs.harvard.edu/abs/1978LPSC....9.3843G/abstract\nLuo et al. 2022: https://doi.org/10.1029/2022JE007333\nMcKinnon and Melosh 1980: https://doi.org/10.1016/0019-1035(80)90037-8\nMelosh and McKinnon 1978: https://doi.org/10.1029/GL005i011p00985\nReviewer #4 (Remarks to the Author):\nThis a very well written paper which clearly demonstrates that the Silverpit structure is an\nimpact crater. The seismic data are of very high quality and provide convincing evidence for the\nimpact origin even without the complementary shocked mineral evidence. The authors might\nwant to consider the following comments prior to a decision on acceptance. We thank Reviewer\n4 for these positive comments and have addressed their comments below.\nGENERAL COMMENTS\nA crater diameter of 3 km is close to the limit separating simple craters from complex craters.\nThe presence of a central uplift in the seismic data suggests the Silverpit crater falls into the\ncategory for complex craters and that a transient crater would have been present directly after\nimpact that would have been somewhat smaller than the present day final diameter. Some\ndiscussion on the scaling relationships between simple and complex craters and the\nimplications of crater type on the seismic interpretation would have been useful. Thank you for\nthis observation. We have noted that according to our new interpretation of the crater size,\nSilverpit is just above the simple-to-complex transition and that in this case a central uplift and\nsubstantial gravity-driven collapse are expected. We also now note in the modelling section that\nthe transient crater is enlarged by about 30% during crater collapse (prior to resurge).\nAccording to the authors PSDM was applied to the seismic data. However, all vertical sections\nthrough the seismic volume are in TWT. Is this a mistake or have the seismic data been\nconverted back from depth to time? Data were available in both time and depth domains.\nHowever, we used the time domain PSDM data for this interpretation, as the 43/25-1 well is not\nwithin the NEP PSDM survey area. This means that the well-to-seismic tie was carried out with\nthe older Megamerge data in the time domain. The Methods section has been expanded to\ndiscuss this.\nReflector vs. reflection: These terms seem to be used interchangeably in the MS. In my view, a\nreflection is something which is observed on a seismic section while a reflector is something\nwhich is present in the sub-surface. You may want to consider using these definitions in the\nResults and Discussion sections. We agree with the reviewer and have changed ‘reflector’ to\n‘reflection’ throughout when describing the seismic observations.\nSPECIFIC COMMENTS\nPg 3, lines 13-15: “Although the structure was still only partially imaged, these data allowed the\ncrater rim to be redefined as a much smaller 3 km in diameter, with some evidence of radial\nfaults on one side of the central uplift.” This sentence is confusing, is the crater 3 km in diameter\nor is it 3 km smaller in diameter than the 20 km mentioned earlier on line 7? From Figure 1 and\nfurther reading of the MS, I see it is the former, but it would be better to be more clear here. We\nacknowledge that this was not su(cid:431)iciently clear in the submitted version. In fact, the reported\ncrater size in previous publications is larger than 3 km. This has been clarified, and expanded\nslightly to emphasise that the authors (Stewart et al. 2005) still considered the crater to be\nCretaceous in age, and the final crater diameter to be 8 km. See also the response to Reviewer 1\non a similar point.\nFigure 2b: What does the “300 m o(cid:431)set” label in the figure refer to? It is explained later in the\ndiscussion, but please explain in figure caption. This has now been added to the figure caption.\nPg 3, line 29: Change “rather thana decision” to “rather than a decision”. Done\nFigure 5: It would be useful to be consistent with the colors for the faults. For example; green –\nreverse, red – normal; for this figure and the others in the MS. Also what does the dotted line in\nc) represent? We have been consistent with the colours of faults on the seismic profiles, with\nnormal faults shown in black, and reverse faults in red. However, in figure 5b, we wanted to\nhighlight the di(cid:431)erent fault families clearly: concentric normal faults, concentric reverse faults\nand concave-downrange faults, the latter which have a component of strike-slip motion as well.\nWe do not show every individual fault here in detail.\nThe dotted line below the red reverse faults in c has been removed to avoid ambiguity.\nPg 6, lines 29-30: “in di(cid:431)erent domains of the crystal, which might that the grain additionally\nshows Dauphine-type twinning” Something is missing in this sentence. Also, you could already\nhere note that the presence of shocked quartz above the CF2 reflector will be discussed later.\n‘Indicate’ has been added here. A short paragraph has been added to round o(cid:431) this section, also\nreferring to the later discussion on the grains above the CF2 reflector, as proposed.\nFigure 8: CF1 and CF2 appear to be incorrectly labeled in the AI panel. Yes, this has been\ncorrected.\nPg 7, lines 1-3: It appears you are assuming the impact to be vertical, not at an angle. I also\ninterpret this to be the case looking at Figure 10 (symmetrical). You should state this here. I note\nlater that you write the modeling assumed a vertical impact, but better to make the point at this\nstage. This is already mentioned in line 2 “performed using a simplified target structure and\nvertical trajectory to constrain impactor parameters”\nPg 8, line 24: Change “10.f” to “10f”. Changed\nChristopher Juhlin, Uppsala Univeristy\n2025-04-09"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors present convincing new evidence and a detailed study of the Silverpit structure, a crater with a disputed impact origin. The new results are significantly in favor of an impact origin for the structure, and will confirm a new addition to the terrestrial impact cratering record. The methods, results and discussion are sound, and well presented, with high quality figures. The authors have satisfactorily addressed the suggested revisions from the previous version, and I recommend the manuscript be accepted for publication.\nBest Regards,\nDr. Neeraja Chinchalkar", "reviewer_2": "I'm happy with the revised manuscript and I would love to see it published.", "reviewer_3": "I thank the authors for fully addressing all of my comments in the revised manuscript. I recommend this paper for publication.", "reviewer_4": "I happy with the revised manuscript. I only have the following two comments:\n1. Line 480: I think \"in particular to correlated the surfaces with the 43/25-1 well\" should be \"in particular when correlating the surfaces with the 43/25-1 well\"\n2. Line 481: I think the authors should add a sentence stating what velocity field was used to convert the PSDM image to a PSTM image. I assume it was the velocity field used for PSDM."}, "author_response": "Point-by-point response\nWe thank the editor and four reviewers for their positive review of the second version of our\nmanuscript. The final point-by-point responses are below:\nREVIEWER COMMENTS\nReviewers 1, 2 and 3 have not requested any changes.\nReviewer #4:\n1. Line 480: I think \"in particular to correlated the surfaces with the 43/25-1 well\" should be \"in\nparticular when correlating the surfaces with the 43/25-1 well\" Yes, this was not clear and has\nbeen modified accordingly.\n2. Line 481: I think the authors should add a sentence stating what velocity field was used to\nconvert the PSDM image to a PSTM image. I assume it was the velocity field used for PSDM. This\nhas now been added."}]

Decision letter