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Discussion |
1 3DLab, School of Earth, Ocean and Planetary Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, UK
2 Present address: StatoilHydro ASA, NO-0246, Oslo, Norway
3 CeREES (Centre for Research into Earth Energy Systems), Department of Earth Sciences, University of Durham, Science Labs, Durham DH1 3LE, UK
*Corresponding author (e-mail: mostynwall{at}hotmail.com)
We welcome the opportunity to further clarify aspects of our paper and challenge key elements of Owen's discussion and the inferences he makes regarding the rigour our interpretations and those of previous workers on the origin of the Silverpit structure. We deal with each of his comments in the order that he raises them.
Owen refers to the interpretation of the Silverpit structure as an impact crater as a ‘speculative hypothesis'. This is consistent with an earlier statement he has made: ‘This feature, termed the Silverpit Crater, has earlier been interpreted as a meteor impact structure (Stewart & Allen 2002), without a shred of scientific justification' (Thomson et al. 2005). The inference that the hypothesis lacks any observational basis disregards a wealth of 3D seismic analysis by Stewart & Allen (2002, 2005). The statement is at best disingenuous, as Owen chooses to ignore the thorough work of earlier workers. Although all the observations for and against a impact origin are based upon 3D seismic data and there are as yet no other independent supportive lines of evidence we would highlight the vast body of work on impact cratering and the numerous impact craters on planetary bodies that have been identified purely from their surface morphology (Melosh 1989); in other words, the absence of shocked quartz and other impact criteria does not invalidate the impact crater interpretation. We have been very careful to term the Silverpit structure a ‘probable impact crater' and will continue to do so until such unequivocal evidence is discovered.
In our paper we identify a key reflection that we term CF and that marks the upper boundary to the deformation associated with the crater. The interpretation of the significance of this reflection and its dating is the critical aspect of the paper. We propose that a seismic package (MU) is characterized by seismic multiples and these mask the evidence for deformation that is younger than determined by earlier workers (Stewart & Allen 2002, 2005). In considering the multiple energy Owen assumes that the base of the MU package is the primary reflection that the multiples above it are derived from, which is geophysically impossible. However, his assumption is wrong; we interpret the top of package MU, the reflection CF, as the primary reflection, below which are a series of peg-leg multiples. Our interpretation of MU being dominated by multiple energy is based upon other simple criteria; for instance, the reflections within it terminate abruptly at the boundary of the 3D seismic survey with the other 3D seismic surveys covering the structure, revealing reflection geometries that are geologically sensible.
Given that CF marks the upper boundary to the deformation, the dating of this reflection is critical in the dating of the Silverpit structure. There are no detailed well correlations in the region and Owen is of course correct that there is a lack of precision in using well 43/25-1. This is because of the limitations of the sample density in the commercial borehole and poor biostratigraphic control in the southern North Sea for this stratigraphic interval. We clearly state in the paper that cuttings have been used, sampled approximately every 7–10 m, and that a precise correlation cannot be closely attained. This is why an approximate date of c. 45 Ma has been attributed and we date the crater as Mid-Eocene in age.
Owen raises the question of the validity of a pseudo-depth section (our fig. 8); but this section is in two-way-time (TWT) and is not a pseudo-depth section, therefore making Owen's comments on 3D depth migration irrelevant. However, we accept that the depth scale, which is uniform despite the increased velocity of sediments with burial depth, is not accurate and is misleading. We confirm that the positioning of the key biostratigraphic markers is accurate and our dating is in no way invalidated. The commercial borehole 43/25-1 was tied to the seismic data by using clear lithological and geophysical changes at the top and base of the Cretaceous Chalk.
The Silverpit structure is circular and therefore one cannot select classic dip and strike sections that Owen proposes we should have published. The majority of sections we present are aligned with the strike of the underlying regional folds that affect all of the post-Permian Zechstein strata. The objective was to show as much of the Silverpit structure as possible in full, and seismic lines were selected to illustrate as much of the Silverpit structure and its overlying strata as possible, with presentation of the underlying Zechstein salt as a secondary consideration. However, the regional geometry of the underlying salt is illustrated in figure 3, where the folding that affects all the post-Permian Zechstein strata is clearly illustrated, and in figure 7, where a regional line through the dataset shows both a dip and a strike line.
In our paper (Wall et al. 2008) and in other publications it is clear that the underlying Zechstein salt is characterized by elongate NW–SE-trending long-wavelength folds; no circular salt features can be identified in the southern North Sea (Underhill 2004; Thomson et al. 2005; Wall et al. 2008, fig. 7). The underlying Zechstein salt clearly has a different structural trend compared with the overlying Silverpit structure. Significantly, if faults formed to accommodate salt movement in the southern North Sea, they would trace the underlying salt geometry, as exhibited in numerous salt basins and numerical models (Ge & Jackson 1998). The timing of the salt withdrawal is also much younger than the upper limit of faulted and deformed reflections, providing further proof that the Silverpit structure is unrelated to any regional salt tectonics.
The central peak (termed conical pull-up by Owen) can clearly be seen in figure 8 and is not a result of our forcing of the seismic interpretation. As pointed out by Stewart & Allen (Underhill 2004), the central peak is imaged by three different seismic surveys and is therefore unlikely to be a seismic artefact. A pull-up interpretation of the central peak can also be refuted simply because the fill of unit F, which would presumably be the cause of a pull-up, has a significantly different geometry from that of the underlying central peak. Also, if a velocity contrast in the overburden was to cause a pull-up effect, it would also have affected the top Chalk reflection and reflections directly below the central peak, which it does not.
The publication of data taken from commercial 3D seismic surveys is difficult to present in a couple of 2D figures and it is easy to question the interpretation of the data when the 3D dataset is unavailable. However, we feel that the data presented by Wall et al. (2008) are robust and, together with the publications by Stewart & Allen (2002, 2005), make a strong case for the Silverpit structure as a probable impact crater.
The Silverpit debate will continue until further evidence is found to confirm the structure as an impact structure. Points made by Owen regarding the origin of the Silverpit structure have been stated before in previous publications (Smith 2004; Underhill 2004; Stewart & Allen 2005; Thomson et al. 2005). We suggest that, for the sake of rigorous scientific debate, alternative origins for the Silverpit structure should be published in full, rather than as replies to other papers, where the scope for fully supporting alternative models with data is limited. This would allow the scientific community to make a better assessment of the merits of the alternative interpretations.
In summary, critical in our paper is the interpretation of reflection CF as the upper boundary to the deformation. Nothing in the reply by Owen invalidates the criteria we use for identifying this as the key surface or for the age dating of it. Therefore a date of c. 45 Ma is realistic. We hope that scientific drilling could in the future date the feature and draw a line under the debate on its origin.
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References |
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 Top References |
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Ge, H. & Jackson, P.A. 1998. Physical modelling of structures formed by salt withdrawal: Implication for deformation caused by salt dissolution. AAPG Bulletin, 82, 228–250.[Abstract][GeoRef]
Melosh, H.J. 1989. Impact Cratering: A Geologic Process. Oxford Monographs on Geology and Geophysics, 11.
Smith, K. 2004. The North Sea Silverpit Crater: impact structure or pull-apart basin? Journal of the Geological Society, 161, 593–602.
Stewart, S.A. & Allen, P.J. 2002. A 20-km-diameter multi-ringed impact structure in the North Sea. Nature, 418, 520–523.[CrossRef][GeoRef]
Stewart, S.A. & Allen, P.J. 2005. 3D seismic reflection mapping of the Silverpit multi-ringed crater, North Sea. Geological Society of America Bulletin, 177, 354–368.
Thomson, K., Owen, P. & Smith, K. 2005. Discussion on the North Sea Silverpit Crater: impact structure or pull-apart basin? Journal of the Geological Society, London, 162, 217–220.
Underhill, J.R. 2004. On an alternative origin for the ‘Silverpit crater'. Nature, 426, doi: 10.1038/nature02476.
Wall, M.L.T., Cartwright, J. & Davies, R.J. 2008. An Eocene age for the proposed Silverpit Impact Crater. Journal of the Geological Society, London, 165, 781–794.
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