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Discussion |
1 1 Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK (e-mail: j.hooker@nhm.ac.uk)
2 2 N. P. Sille & D. P. Mattey, Department of Geology, Royal Holloway University of London, Egham TW20 0EX, UK
3 3 School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK
4 4 School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK
5 5 Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
Jerry Hooker, Margaret Collinson, Stephen Grimes, Nick Sille & David Mattey write: Recognition of the Eocene–Oligocene boundary in the Hampshire Basin, UK, has been debated since naming of the Oligocene Epoch in 1854. Previously, this was because the boundary itself had not been stabilized and because the strata concerned are largely non-marine. A Global Boundary Stratotype and Stratigraphic Point (GSSP) was established at Massignano, Italy, in 1993 in marine strata. Recognition of the boundary on extinction of the planktonic foraminiferan family Hantkeninidae made boundary identification difficult in the continental realm. Correlation to marginal marine and non-marine strata is nevertheless possible via magnetostratigraphic and sequence stratigraphic studies and, importantly, biostratigraphically via dinocyst zones at Massignano (Brinkhuis & Biffi 1993; Brinkhuis & Visscher 1995). Therefore, recent publication of the magnetostratigraphy, sequence stratigraphy and orbital cyclicity of much of the Hampshire Basin Solent Group (Gale et al. 2006) is welcomed and substantially increases the number of correlation tools available in this area. Such cyclical phenomena, however, rely on absolute dating or biostratigraphy for calibration. No radiometric dates exist for the Solent Group, so biostratigraphy remains the best means of dating the succession.
There are, however, problems with the way Gale et al. (2006) have interpreted biostratigraphic markers and therefore with their placement of the Eocene–Oligocene boundary and associated events. The organisms concerned are calcareous nannoplankton (NP zones) and mammals (MP reference levels). Thus, the record by Aubry (1985) of NP22 in the Argiles Vertes de Romainville, Paris Basin, was subsequently qualified by her (Aubry 1986, p. 307) as ‘zone NP22 (not younger; possibly older: NP21?)’. This dating was based solely on the presence of rare Isthmolithus recurvus, which ranges from NP19/20 to NP22 (Aubry 1992), this being the real level of dating for the Argiles Vertes de Romainville on nannoplankton evidence. Moreover, NP22 is latitudinally diachronous (Aubry 1992). Thus, the NP22 record in the Ruisbroek Sand in Belgium, according to the dinocyst zonation (Stover & Hardenbol 1994; Vandenberghe et al. 2003), largely predates its standard low-latitude range (Hooker et al. 2004, fig. 3).
The Belgian sequence is critical because it allows the MP21 Hoogbutsel Mammal Bed, overlying the Neerrepen Sand with the Adi dinocyst zone, to be calibrated to the northwesterly, more marine succession with contiguous dinocyst zones (Steurbaut 1992). These can then be calibrated to the sequence stratigraphically and magnetostratigraphically controlled succession in Italy (Brinkhuis & Visscher 1995). Therefore, the Grande Coupure, the turnover separating MP20 from MP21, must correlate with a point early in Chron C13n, thus approximating the onset of the Oi-1 glaciation (Zachos et al. 1992). This calibration of the Grande Coupure to C13n is directly demonstrated in the Ebro Basin, Spain, where an early post Grande Coupure MP21 fauna occurs at Santpedor (Barberà et al. 2001). Therefore, the Bembridge normal polarity zone (Gale et al. 2006, pp. 403–404, fig. 3), which calibrates to mammal reference level MP19 (Hooker 1992) cannot be C13n. Consequently, neither the Eocene–Oligocene boundary nor the Oi-1 event should be as low as the base of the Bembridge Limestone Formation (Gale et al. 2006, p. 413, fig. 12).
This raises the question of the true identity of the Bembridge normal polarity zone. At Massignano, Priabona and Bressana, Italy, the next sequence boundary (SB) below that marking Oi-1 has been calibrated by Brinkhuis & Biffi (1993) and Brinkhuis & Visscher (1995) to TA4.2/4.3 of Haq et al. (1987) (= Pr2/3 of Hardenbol et al. 1998). At Priabona this SB is at the base of dinocyst zone Aal, which at Massignano is close to a normal subchron named ‘C13n2’ (Brinkhuis & Visscher 1995) or ‘short polarity excursion I’ (Premoli Silva et al. 1988). This subchron occurs late in Chron C13r (Premoli Silva et al. 1988). It has been recognized as far away as the South Atlantic and is an important marker, as the Eocene–Oligocene boundary hantkeninid extinction occurs just after (Premoli-Silva et al. 1988). It may also be represented in the continental Sarral section, Spain (Barberà et al. 2001). In the Hampshire Basin, the next clearly marked SB below that which coincides with the Grande Coupure and thus Oi-1 is that at the base of the Bembridge Limestone, the base of sequence 5 of Gale et al. (2006) (their intervening sequences 6 and 7 within the Bembridge Marls and lower Hamstead members being poorly defined). Subchron I in Chron C13r therefore provides the best fit for the Bembridge normal polarity zone.
Gale et al. (2006, p. 407) claimed a maximum SB incision value of 15 m at the base of their sequence 5, suggesting to them that this represented a best fit for the major sea-level fall coincident with Oi-1. However, this ignores a more important sea-level fall at the base of the Nematura Bed, Hamstead Member, evidenced by reworking of rooted soil clasts and absence of lowstand and early transgressive systems tracts (Hooker et al. 2004), which calibrates well with the biostratigraphy.
The 2
positive shift in freshwater 
18O values, recorded in rodent tooth enamel by Grimes et al. (2005) between the Osborne Member and the Bembridge Limestone, has been misinterpreted by Gale et al. (2006, p. 413) as representing a cooling event coincident with their position of the onset of the Oi-1 glaciation. Grimes et al. (2005) clearly demonstrated that this shift represents a warming in summer season temperatures. This is because temperature shifts are dependent upon changes in both the local water 18O values and the carbonate proxy 18O values. Between the Osborne Member and the Bembridge Limestone the isotopic shift in the local water value is greater than that in the three carbonate proxies, indicating a rise in temperature. Moreover, cooling between the Osborne Member and the Bembridge Limestone (Gale et al. 2006, p. 413) was only one of the interpretations by Sille et al. (2004) of increased charophyte gyrogonite volume, another being warming, through increased photosynthesis and calcium carbonate secretion, consistent with the isotope results.
By equating the Bembridge normal polarity zone with Chron C13r subchron I, the Eocene–Oligocene boundary in the Hampshire Basin, UK, should lie above this normal interval, more than 4.5 m above the base of the Bembridge Marls. This means that the Insect Limestone, an important source of late Palaeogene insects and plants and usually regarded as earliest Oligocene, which lies within the Bembridge normal polarity zone, should instead be dated as latest Eocene. Therefore, the Eocene–Oligocene boundary in the Hampshire Basin is best placed around the middle of the Bembridge Marls Member, whereas the Grande Coupure and contemporaneous Oi-1 glaciation are situated immediately beneath the Nematura Bed of the overlying Hamstead Member.
26 June 2006
Andy Gale, Jenny Huggett & Ewan Laurie write: We thank Hooker et al. for their interesting and useful review of the arguments pertaining to the position of the Eocene–Oligocene boundary in the Isle of Wight succession and elsewhere. However, some points they make and issues they raise, particularly those referring to the paper by Hooker et al. (2004), need further discussion.
The stratigraphical arguments of Hooker et al. (2004, fig. 6) supporting the placement of the Eocene–Oligocene boundary around the Nematura Bed in the Upper Hamstead Member in the NE Isle of Wight are based on two lines of evidence. The first and better supported of these concerns the mammal biostratigraphy, which Hooker (1987, 1989, 1992) and Hooker et al. (2004) have documented in detail. Precisely horizoned collecting has allowed the identification of mammal zones MP19, 20 and 21 in the Hampshire Basin succession; MP19 is recorded from the Bembridge Limestone, MP20 from the Bembridge Marl, and MP21 from the Upper Hamstead Member, immediately above the Nematura Bed and overlying brackish shell beds. The Grande Coupure, a major turnover in mammal faunas long associated with terrestrial concepts of the Eocene–Oligocene boundary, can now be located to a 4 m gap in the mammal record over an interval including the Nematura Bed and overlying shell beds (Hooker et al. 2004). However, the GSSP defining the base of the Oligocene is placed in deep marine strata at Messignano in Italy, and is defined by the extinction of the planktic foraminiferan Hantkenina (Premoli Silva et al. 1988). This in turn can be related to the base of magnetochron C13n, and a major heavy oxygen isotope event called Oi-1a within the nannoplankton zone NP21 (Zachos et al. 1996). To establish a first-order correlation between the land mammal record and these marine events, Barberà et al. (2001) investigated the magnetostratigraphy of the Ebro Basin in Spain, where the Grande Coupure and the MP zones had been identified.
Barbera et al. (2001) obtained consistent palaeomagnetic data from multiple sections in the southeastern Ebro Basin, and were able to identify chrons 6n.3n to c15n (Late Eocene–Oligocene) and relate these to mammal zones MP19/20 to MP30 (Fig. 1). The succession of mammal zones MP19–MP21, and the position of the Grande Coupure at the MP20–21 boundary can be correlated between the Sarral section in the Ebro Basin and the Whitecliff–Hamstead sections in the Isle of Wight (Fig. 1). If this correlation is used to interpret the magnetic zones on the Isle of Wight (Gale et al. 2006), then the Bembridge normal polarity zone could be reinterpreted as C15n, and the Headon normal polarity zone as C16n (Fig. 1). Alternatively, Hooker et al. suggest that the Bembridge normal polarity zone could be a short normal subchron within C13r, identified in the South Atlantic and called C13.1n by Channell et al. (2003). In this regard, it is important to note that the position of the Isle of Wight succession in relation to C13n given by Hooker et al. (2004, fig. 3) is entirely hypothetical, as no investigation of the magnetostratigraphy was available at the time of publication.
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The second part of the stratigraphical argument presented by Hooker et al. (2004, fig. 3) is based on their identification and correlation of two inferred major hiatuses in the Isle of Wight succession, the lower at the base of the Bembridge Marls, and the higher immediately underlying the Nematura Bed in the Upper Hamstead Member. The durations of these gaps were given as 0.15 and 0.35 Ma, respectively, and Hooker et al. (2004) correlated them with sequence boundaries identified in the marine Late Eocene of Belgium and central Italy. Both the supposed magnitudes of these hiatuses and evidence for their correlation are inferential and unsupported by any data. The perceived need to find a break in this part of the succession led Pomerol (1989) and subsequently Vandenberghe et al. (2003) to interpret a minor, pedogenically modified, surface at the base of the Black Band as the major break at the base of the Oligocene. This correlation was rejected on biostratigraphical grounds by Hooker et al. (2004), who instead selected a similar minor surface higher in the succession (base of the Nematura Bed, 10 m above the Black Band) as the hiatus supposedly created by the ice-driven regression associated with Oi-1a.
The erosional surface at the base of the Nematura Bed is decidedly unimpressive, as there is no significant incision (less than 1 m), and mottled, pedogenically altered silty clays are overlain by grey estuarine clays containing a brackish water mollusc fauna (Polymesoda, etc.). It is similar to numerous other brackish transgressions identified in the Solent Group, which were related by Gale et al. (2006) to the 400 ka eccentricity cycle. Moreover, the orbitally tuned clay signal does not suggest any significant break in the continuity of the 400 ka cycles at the level of the Nematura Bed. It is important to remember that the Oi-1a sea-level fall is inferred to be at least 60–90 m in magnitude, and would be expected to create deep incision, major facies shifts, and the retreat of the shoreline far towards the shelf margin, a similar pattern to changes caused by major sea-level fall in the late Quaternary (e.g. Woodcock 2001). There is no evidence of any such events at the base of the Nematura Bed.
The lower hiatus of Hooker et al. (2004), at the boundary between the Bembridge Limestone and Bembridge Marl Formations, has been identified as a sedimentological break since Daley & Edwards (1971) recorded evidence of local erosion of the summit of the Bembridge Limestone. In the most complete Hampshire Basin succession, in Whitecliff Bay, the lithological transition from the Bembridge Limestone to the overlying Bembridge Oyster Bed is abrupt, but the continuity of the 400 ka cyclic clay signal (Gale et al. 2006, fig. 9) does not support the existence of a large hiatus.
Hooker et al. (2004) correlated the sub-Bembridge Oyster Bed and sub-Nematura Bed hiatuses to two sequence boundaries identified in the marine succession in central Italy by Brinkhuis & Visscher (1995). We fail to see how it is possible to correlate sequences from a marine Tethyan succession to a continental Boreal one in the absence of any fossil species common to both or other supporting data (magnetostratigraphy, chemostratigraphy). Much of the correlation proposed by Hooker et al. (2004, fig. 3) therefore appears to be inferential, conveniently used to support their stratigraphical argument.
In conclusion, the correlation of the continental and estuarine succession of the Solent Group on the Isle of Wight to the marine standard record remains problematic, and accurate positioning of the Eocene–Oligocene boundary requires further magnetostratigraphic investigation, to identify at least one higher normal polarity zone in the upper part of the Solent Group and thus strengthen correlation with the Geomagnetic Polarity Time Scale.
26 January 2007
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