Palaeogeography, Palaeoclimatology, Palaeoecology 403 (2014) 43–56

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Astronomically-calibrated magnetostratigraphy of the Lower Jurassicmarine successions at St. Audrie's Bay and East Quantoxhead(Hettangian–Sinemurian; Somerset, UK)

S.K. Hüsing a,⁎, A. Beniest a, A. van der Boon a, H.A. Abels b, M.H.L. Deenen a, M. Ruhl c, W. Krijgsman a

a Paleomagnetic Laboratory ‘Fort Hoofddijk’, Department of Earth Sciences, Utrecht University, Netherlandsb Stratigraphy/Paleontology, Department of Earth Sciences, Utrecht University, Netherlandsc Nordic Centre for Earth Evolution, University of Copenhagen, Denmark

⁎ Corresponding author. Tel.: +31 302535418.E-mail address: s.k.husing@uu.nl (S.K. Hüsing).

http://dx.doi.org/10.1016/j.palaeo.2014.03.0220031-0182/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 June 2013Received in revised form 3 March 2014Accepted 12 March 2014Available online 21 March 2014

Keywords:Lower JurassicHettangianSinemurianMagnetostratigraphyAstronomical tuningGeomagnetic Polarity Time Scale

Astronomically-calibrated magnetostratigraphies are becoming the standard in the Geomagnetic Polarity TimeScale (GPTS) because in time these records can be accurately compared for consistency. The Lower JurassicGPTS is however still poorly resolved with major inconsistencies between different records from the ParisBasin and the continental Newark andHartford basins,while only the latter have been astronomically-calibrated.Here, an independent cyclostratigraphy, astronomical calibration to the 405-kyr eccentricity cycle andmagnetostratigraphy for the lower Jurassic from coastal outcrops in Somerset, United Kingdom, covering theHettangian and lower Sinemurian Stages is presented. These results fully confirm the magnetostratigraphicpatterns derived in the terrestrial Hartford Basin, eastern USA, with a dominant normal polarity interrupted bythree short reverse chrons: H24r, H25r, and H26r. The cyclostratigraphic analysis of the marine strata fromthe St. Audrie's Bay/East Quantoxhead section is further extended and improved yielding an independent astro-nomical calibration to the 405-kyr, long eccentricity cycle and a floating astrochronology for the newmagnetostratigraphy. The astronomical tuning shows consistency with the Hartford Basin at the 405-kyr, longeccentricity time scale and minor differences at the 100-kyr, short eccentricity time scale. These inconsistencieswill have to be resolved to further improve the Geologic Time Scale 2012, in which the Hartford Basinmagnetostratigraphy forms the basis for the Lower Jurassic.The St. Audrie's Bay/East Quantoxhead composite section could serve as a unit stratotype section for theHettangian Stage.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Magnetostratigraphy and correlation to the Geomagnetic PolarityTime Scale (GPTS) constitute a standard dating tool in Earth Sciences,applicable to a wide variety of sedimentary and volcanic rock typesformed under different environmental conditions (continental, lacus-trine, marine). It is therefore the stratigraphic tool of choice to performcorrelations between continental and marine realms (e.g. Langereiset al., 2010). Magnetostratigraphy, when integrated with biostratigra-phy and especially cyclostratigraphy, allows high-resolution correla-tions all over the world, because palaeomagnetic polarity reversals cangeologically be seen as globally synchronous events.

The availability of a robust magnetostratigraphy has also becomea prerequisite for the definition of a Global boundary StratotypeSection and Point (GSSP), the so-called “golden spikes” for the variousstages of the Geological Time Scale (GTS). In the latest GTS of

Gradstein et al. (2012), the Neogene Stage boundaries and polarity re-versals are all dated by a combination of astrochronology, biostratigra-phy and magnetostratigraphy (e.g. Hilgen et al., 2000; Lourens et al.,2004; Hilgen et al., 2005, 2009, 2012). Extending such integrated astro-nomical time scale into the Paleogene and Mesozoic is in progress(Hilgen et al., 2010; Batenburg et al., 2012; Westerhold et al., 2012).The GTS for the Early Jurassic is far less robust (Gradstein et al.,2004, 2012). Magnetostratigraphic records of marine Hettangianand Sinemurian successions are rare and equivocal (Gallet et al., 1993;Yang et al., 1996). Consequently, the GSSP's for the Hettangian(Kuhjoch, Austria) and Sinemurian (East Quantoxhead, UK) are mainlydefined on biostratigraphic (ammonite) arguments (Bloos and Page,2002; Hillebrandt et al., 2007).

Magnetostratigraphic records integrated with cyclostratigraphicanalyses of continental sediments from the Newark Basin (USA) haveresulted in an Astronomically-calibrated Polarity Time Scale (APTS) forthe late Triassic (Olsen and Kent, 1996; Kent and Olsen, 1999), whichhas recently been confirmed by U/Pb dating (Blackburn et al., 2013).Magnetostratigraphic correlations to the continental APTS allowed to

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determine the astronomical duration of themarine Norian and RhaetianStages (Channell et al., 2003; Muttoni et al., 2004, 2010; Hüsing et al.,2011) and solved themain age controversies concerning the end-Triassicmass extinction (Whiteside et al., 2007; Deenen et al., 2010). Thecontinental APTS also proved to be in good agreement with themagnetostratigraphic and astrochronological data from Upper Triassicsuccessions at St. Audrie's Bay in the southwestern UK (Hounslowet al., 2004; Kemp and Coe, 2007).

The continental APTS was extended into the Early Jurassic by usingthe sedimentary successions of the Hartford Basin, USA (Kent andOlsen, 2008). Cyclostratigraphic analyses from the Lower Jurassic ma-rine successions at St. Audrie's Bay and East Quantoxhead (UK) resultedin an independent astronomical framework for the Hettangian Stage,allowing Ruhl et al., (2010) to locate the stratigraphic position of themarine defined Triassic-Jurassic and Hettangian-Sinemurian boundaryin the continental realm. Here, we present the magnetostratigraphy ofthe Hettangian and lower Sinemurian successions of St. Audrie'sBay and East Quantoxhead, which will be used to evaluate themarine–continental correlations in the recovery interval following theend-Triassic mass extinction and to develop a more robust GPTS forthe Lower Jurassic.

2. Sections and geological setting

Excellently exposed sections of the Lower Jurassic Blue Lias For-mation can be found at the cliffs of St. Audrie's Bay and EastQuantoxhead, located on the west Somerset coast on the southernside of the Bristol Channel Basin (Fig. 1). The St. Audrie's Bay section(51°10′54.70″N/3°17′09.79″W) is extensively studied, especially be-cause it comprises one of themost complete Triassic–Jurassic boundarytransitions in an open marine environment (Warrington et al., 1994,2008; Hesselbo et al., 2002, 2004; Deconinck et al., 2003; Hounslow

Fig. 1. The location of the St. Audrie's Bay and East Quantoxhead sections outcropping along themap of the United Kingdom and the small rectangle indicates the location of the larger map. F

et al., 2004; Korte et al., 2009; Bonis et al., 2010). The position of theTriassic–Jurassic boundary in the St. Audrie's Bay section has been iden-tified based on the first occurrence of the ammonite Psiloceras palnorbis(Warrington et al., 2008) and by stratigraphic correlation of the firstoccurrence of Cerebropollenites thiergartii pollen (Bonis et al., 2010)and the distinct shift in the organic C-isotope records (Ruhl et al.,2010) to the Triassic–Jurassic boundary GSSP in the Kuhjoch section(Hillebrandt et al., 2007; Ruhl et al., 2010). The East Quantoxhead sec-tion (51°11′27.91″N/3°14′12.25″W) is located 3 km east of St. Audrie'sBay at Limekiln Steps and contains the GSSP for the base of theSinemurian Stage (Bloos and Page, 2002). Using the lithological cyclepatterns and following earlier work, Ruhl et al. (2010) linked thesetwo sections with an approximately 10 m overlap (see Fig. 2). Com-bined, these two sections provide a continuous marine sedimentarysuccession comprising the entire Hettangian and the lower part of theSinemurian (Fig. 2; Ruhl et al., 2010), and is referred to as theSt. Audrie's Bay/East Quantoxhead composite section.

The Blue Lias Formation was deposited during a phase of rapidflooding in a sedimentary basin that was surrounded by partly emergedplatforms (Radley, 2008). The Jurassic strata are marked by distinctlimestone–shale couplets that suggest a stable allogenic forcingmecha-nism likely to be high-frequency climate control (Weedon, 1986; Smith,1989; Weedon et al., 1999). The sedimentary rhythms consist of a lam-inated black shale that grade into a dark-greymarl, and a pale-greymarlcommonly with concretionary to tabular (cemented) micritic lime-stone, which subsequently turns back into dark-grey marls and shales(Paul et al., 2008). The Blue Lias Formation has been extensively studiedin terms of bio-, chemo- and chronostratigraphy (Hallam, 1987; Smith,1989; McRoberts and Newton, 1995; Weedon et al., 1999; Hesselboet al., 2002; Deconinck et al., 2003; Hounslow et al., 2004; Mander andTwitchett, 2008; Warrington et al., 2008; Korte et al., 2009). The originof the rhythmic sedimentation has been under debate (Campos and

coast of Somerset in southwest England (figure after Ruhl et al., 2010). The inset shows aor more details around the two sites the reader is referred to Hounslow et al. (2004).

Fig. 2. The δ13CTOC and TOC proxy records for the newly extended part at the top of the East Quantoxhead section and the additional TOCmeasurements in the Triassic–Jurassic boundaryinterval are shown alongwith the previously published data of Ruhl et al. (2010). Following the spectral analysis (see Fig. 3), a 100 kyr band-pass filter was applied over the entire record,showing amatchwith the observed large-scalefluctuations associatedwith the 100-kyr eccentricity cycle forcing. The observed bundling of euxinic shales and carbonates is indicatedwithbrackets. The large brackets indicate the interpreted 405-kyr eccentricity maxima, which are calibrated to the 405-kyr eccentricity maxima of the La2010d solution (Laskar et al., 2011a).For chronostratigraphic calibration the U–Pb ages and biostratigraphic events from the Pucara Basin succession, northern Peru (Schaltegger et al., 2008; Schoene et al., 2010; Guex et al.,2012), are used. Plotted errors of the U–Pb ages are without the uncertainties of the decay constant. ETME= end-Triassic mass extinction.

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Hallam, 1979; Hallam, 1986; Weedon, 1986; Bottrell and Raiswell,1989; Smith, 1989; Paul et al., 2008). Orbital climate forcing has beenput forward as one of the potential mechanism (Weedon, 1986;Smith, 1989; Paul et al., 2008). Combined field observations and proxystudies from the St. Audrie's Bay and East Quantoxhead sectionsdemon-strated simultaneous fluctuations in black shale and limestone predom-inance and chemical proxy records (Ruhl et al., 2010). Time seriesanalysis of the proxy records shows a main periodicity between ~3.8and 5.8 m for these oscillations. Each oscillation is marked by ~3 to 5black shale horizons that are likely controlled by changes in monsoonintensity controlled by the precession cycle (Ruhl et al., 2010). The~3.8–5.8-metre-scale oscillations are interpreted as orbitally-forcedshort, ~100-kyr eccentricity cycles (Fig. 2; Ruhl et al., 2010). Conse-quently, a floating astronomical time scale was established for theHettangian and the early Sinemurian, which allowed constraining therecovery rates following the end-Triassic mass extinction as well asthe duration of the earliest Jurassic ammonites zones.

In the present study, the East Quantoxhead section of Ruhl et al.(2010) has been extended upward by 14.8m into the lower Sinemurianbucklandi ammonite zone (Fig. 2). Lithological rhythms betweenlaminated black shales, marls and limestones in the lower Sinemurianpart of the Blue Lias Formation are similar as for the Hettangian andHettangian–Sinemurian transition in this section.

3. Chemical proxy record

3.1. Methods

Formost of the St. Audrie's Bay/East Quantoxhead composite sectionmeasurements of stable carbon isotopes of bulk organic carbon(δ13CTOC) and Total Organic Carbon (TOC) have been performed andpresented by Ruhl et al. (2010). Here this record is extended upwardsover the newly sampled 14.8m-interval and across the Triassic–Jurassictransition. All measurements were performed at the University ofCopenhagen (Department of Geosciences and Natural Resource Man-agement) and methods were similar to those described in Ruhl et al.(2010). δ13CTOC values are relative to PDB and the precision is basedon the measurement of one in-house standard for every two samples(with a standard deviation of 0.1‰). Blackman–Tukey power spectrawith a Bartlett window and band-pass filters were constructed usingAnalySeries (Paillard et al., 1996).

3.2. Results proxy records

Laminated black shales in the new interval are less pronounced thanin the lowest Sinemurian part of the East Quantoxhead section which isalso reflected by lower total organic carbon (TOC) values of 0.5–3.5%

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(Fig. 2). The additional measurement of TOC values across the Triassic–Jurassic transition shows relatively low values (0–1%) in the upperWestbury and Lilstock Formation and strongly increased values(2–13%) starting at the onset of the Blue Lias Formation. Extension ofthe δ13CTOC record into the lower Sinemurian shows continuously lowvalues between −27 to −29‰.

Spectral analysis was performed on the upper ~120 m of the δ13CTOCrecord, spanning the uppermost Rhaetian (Triassic) to lower Sinemurian(Jurassic) time interval, which includes Ruhl's et al. (2010) data becausethe extended record only is too short for this analysis. Spectral analysis ofthe TOC record started at the base of the Alsatites liasicus ammonite zoneas sample resolution in theupper P. planorbis ammonite zone is relativelylow. Power spectra for both proxy records show two distinct peaks at~3.8 to 4.1 m, and 6 to 6.3 m (Fig. 3). For the δ13CTOC record, a292–780 cm band-pass filter with a 80% confidence interval was used,whereas for the TOC record, a 331–661 cm band-pass filter with a 80%confidence interval was applied. Each filter represents both peaks inthe power spectra, which are interpreted as being related to changes inthe sedimentation rate. Thesefilters reflect the larger,metre-scalefluctu-ations observed in the proxy records and show that they respond tothe limestone and shale dominated bundles in lithology (Fig. 2). The

Fig. 3. The power spectra for the δ13CTOC and TOC proxy records show two significantpeaks at the 80% confidence level interpreted as reflecting changes in the sedimentationrate. Combined, these peaks are interpreted as representing the 100-kyr eccentricityperiodicity (see text and Ruhl et al., 2010).

periodicity of oscillations in the proxy-filters varies with the variationin thickness of the bundles of limestone and black shale predominancethat are observed in the field. The higher-frequency peaks in the powerspectra are likely related to the occurrence of individual black shales.

4. Cyclostratigraphy

Following Weedon (1986), Weedon et al. (1999) and Paul et al.(2008), Ruhl et al. (2010) interpreted the black shale–marl alternationsas dominantly controlled by precession, and the bundling of blackshale–marl couplets were linked to short, 100-kyr eccentricity (for amore detailed discussion the reader is referred to Ruhl et al., 2010). In-dependently, bundling of carbonate beds was also found at short eccen-tricity time scale. These bundles of carbonate beds were matching inabundance and stratigraphic spacing those of the black shales. Time se-ries analysis of total organic carbon, carbon isotope, magnetic suscepti-bility, and carbonate content quantitative records revealed similardominant cyclicity at short eccentricity time scale (Ruhl et al., 2010).In the depth scale, this cyclicity occurs between 3.8 and 5.8 metresthickness. Ruhl et al. (2010) filtered the short eccentricity cycle and sub-sequently counted the cycles in the TOC record to construct a floatingastronomical framework for the St. Audrie's Bay/East Quantoxheadcomposite section. They further correlated their cyclostratigraphic in-terpretation to the eccentricity pattern derived from the HartfordBasin in the eastern USA as an astronomical solution for the Triassicand Jurassic timewas not available. However, a new astronomical solu-tion (La2010; Laskar et al., 2011a) goingback in time as far as 250Ma forthe eccentricity cycle is now available. Laskar et al. (2011a) provide foursolutions, La2010a,b,c,d, where La2010a is the nominal solution, and ofwhich La2010d is considered valid over ~54 Ma (for further discussionon the four different solutions, the reader is referred to Laskar et al.,2011a,b andWesterhold et al., 2012). Laskar et al. (2011a) emphasizedthat the direct use of the eccentricity solution for time intervals olderthan 50Ma should be treatedwith utmost care. The 405-kyr eccentricitycycle is less influenced by the chaotic behavior of the solar system(Laskar et al., 2011a,b), and thus the astronomical tuning to the405-kyr eccentricity cycle is considered more robust than to the100-kyr eccentricity cycle. Therefore, the 405-kyr eccentricity cycle is fil-tered with a wide band with cutoff frequencies set at ±0.001 cycles/kyron either side of the centered frequency following Hinnov and Hilgen(2012).

Direct first-order calibration points, such as radiometric ages, for as-tronomically tuning the St. Audrie's Bay/East Quantoxhead compositesection are unfortunately not available due to the lack of ash beds inthis section. However, absolute U–Pb ages from volcanic ash layers inthe Pucara basin, Peru, for the end-Triassic extinction, Triassic–Jurassicboundary, and Hettangian–Sinemurian boundary can be used for achronostratigraphic calibration (Fig. 2). In the Peruvian succession thevolcanic ash bed (86, e.g. in Guex et al., 2012) just above the last occur-rence of Choristoceras crickmayi indicating the end-Triassic extinctionlevel (and about 5 m below the first occurrence of Psiloceras spelae)has U–Pb ages of 201.58 ± 0.17 Ma (Schaltegger et al., 2008), 201.40 ±0.18 Ma (Schoene et al., 2008), and 201.45 ± 0.16 Ma (Guex et al.,2012). The Triassic–Jurassic boundary, defined by the first occurrenceof Psiloceras spelae, is bracketed by two volcanic ash beds (90 and 100,e.g. in Schoene et al., 2010; Guex et al., 2012) and is given an age of201.31 ± 0.18 Ma by Schoene et al. (2010). The volcanic ash bed justabove the Hettangian–Sinemurian boundary, which lies just above thelast occurrence of Badouxia canadensis, has U–Pb ages of 199.53 ±0.19 Ma (Schaltegger et al., 2008) and 199.43 ± 0.10 Ma (Guex et al.,2012). Additionally, the U–Pb ages of the end-Triassic extinction of201.564 ± 0.015/0.22 Ma (Blackburn et al., 2013), and the Triassic–Jurassic boundary of 201.27–201.48 Ma (Schoene et al., 2006) fromthe CAMP basalt help constraining the chronostratigraphy. Followingthis, the duration of the Hettangian Stage ranges between ~1.45 and~2.62 Myr.

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In this paper, the Hettangian–Sinemurianmarine record is extendedupwards, and the cyclostratigraphy of the entire succession is re-examined. In the extended part of the section, two black shale bundlesare observed at about 122 and 134m (Fig. 2), whereas no clear bundlesof limestone beds are seen. This black shale bundling occurs at the samescale as the previously recognized 100-kyr eccentricity cycles (about4–5 m; Ruhl et al., 2010), with individual black shale–marl coupletsbeing precession forced. The filters of the carbon isotope and TOCrecords of the extended uppermost 15 m do not show the same largeamplitude variations as in the lower 120 m of the section. They do,however, follow the same black shale bundling.

Intermediate to the two bundles in the new extension, a larger spac-ing of pronounced black shales occurs, which could be interpreted as analternation of black shale present–black shale missing. Such a patternmimics the patterns in marl–sapropel successions from the Mediterra-nean Neogene where these are the resultant of obliquity-precession in-terference during a 405-kyr, long eccentricity minimum (Hüsing et al.,2007). Such a pattern also occurs aroundmetre level 105 in the section,which could therefore also relate to a long eccentricity minimum andfits well with the 405-kyr eccentricity minimum at about 199.2 Ma.

Apart from black shale bundles, the co-occurrence of two bundles ofpronounced black shales separated by either less pronounced blackshales or marl intervals lacking black shales can be observed (Fig. 2;these are indicated by larger brackets at metre levels 45, 62, 77, 97,and 110). This pattern seems to fit forcing by the 405-kyr, long eccen-tricity cycle of precession-forced black shale–marl couplets and thetwo bundles of black shale are thus related to 405-kyr, long eccentricitymaxima (Fig. 2). Following this, the precession-obliquity interferencepatterns recognized at metre levels 55, 70, 90, 105 and 117 thusdo occur during long eccentricity minima and are calibrated to the405-kyr, long eccentricity minima at about 200.3, 200, 199.5, 199.2,198.75 Ma. Calibrating the eccentricity pattern observed in theSt. Audrie's Bay/East Quantoxhead composite section to the filtered405-kyr eccentricity of La2010d (Laskar et al., 2011a) for the time inter-val 202 to 198 Ma, yields a good overall match between black shalebundles and 405-kyr eccentricity maxima and minima.

The astronomical calibration of the lowest 30 m in the St. Audrie'sBay section (from 0 to 30 m; Fig. 2) remains less robust because of thelack of clear bundling of the euxinic shales and limestones. The uncer-tainties in the uppermost 15 m, where bundles of black shales havebeen recognized, might only be resolved by further additional proxyinvestigation, but might in part also relate to uncertainties in theastronomical solution (Laskar et al., 2011a,b). The here proposedcyclostratigraphicmodel and astronomical tuning to the 405-kyr eccen-tricity cycle fit well with the U–Pb calibrated duration of the HettangianStage and fall well within the errors of the ages of the Triassic–Jurassicand Hettangian–Sinemurian boundaries and the end-Triassic extinctionevent (Schoene et al., 2006, 2010; Schaltegger et al., 2008; Guex et al.,2012; Blackburn et al., 2013). However, the uncertainties of the U–Pbages in some cases can be as large as 400 kyr taking the uncertaintiesof the decay constant into account (e.g. Schoene et al., 2010), which,in turn, means that the astronomical calibration to the astronomical so-lution can be shifted by one 405-kyr long, eccentricity cycle. The herepresented cyclostratigraphic model suggests a duration of ~1.7 Myrfor the Hettangian Stage.

5. Palaeomagnetic results

5.1. Methods

Oriented samples have been taken using a gasoline-powered drill ata resolution of at least 2 samples per precession cycle, which is aboutevery 50 cm for a 1 m-precession cycle. In most parts of the compositesection the sample resolution is significantly higher, with up to 5 sam-ples per precession cycle, which have thus been sampled at 10 to20 cm intervals (i.e. in the interval between 15 and 30 m). The

St. Audrie's Bay section has been sampled from just below theTriassic–Jurassic boundary up to the double limestones at the strati-graphic level of 81 m. The stratigraphy above this level has beensampled in the East Quantoxhead section (see Fig. 2). A total of 403samples have been analyzed for palaeomagnetic purpose.

Magnetic properties were analyzed by carrying out high-tempera-ture thermomagnetic runs and isothermal remanent magnetization(IRM) acquisition. The high-temperature thermomagnetic runs wereperformed in air on a modified horizontal translation-type Curie Bal-ance (Mullender et al., 1993) using bulk sediment. A total of 12 sampleswere selected representing each lithology (marl, limestone, and euxinicshale) from the St. Audrie's Bay and East Quantoxhead sections. All sam-ples were heated starting at room temperature in air by using an oscil-lating field between 150 and 300 mT. While heating several discretetemperature stepswere selected at which the samplewas cooled beforeheating to the next selected temperature step, up to a maximum tem-perature of 700 °C. The discrete temperature steps were selected inorder to discriminate thermal behavior (unblocking) from chemicalalteration (increase in magnetization). All samples were cooled backto room temperature.

IRM acquisition curves were measured on 8 samples from theSt. Audrie's Bay section and 8 samples from the East Quantoxhead sec-tion. These samples were selected from different lithologies throughoutthe composite section. All samples were measured using a robotizedSQUID magnetometer (noise level 1–2×10−12 Am2) with 56 fieldsteps applied up to 700 mT. The resultant IRM acquisition curves wereanalyzed by component analysis of cumulative log-Gaussian (CLG)curves (Kruiver et al., 2001), where every IRM curve was decomposedinto a number of CLG curves. These can be individually characterizedby their saturation isothermal remanent magnetization (SIRM), rema-nent acquisition coercive force (B1/2), and the dispersion parameter(DP).

All 403 samples have been subjected to thermal and/or alternatingfield demagnetization. Temperature increments between 20 and 60°have been applied in a magnetically-shielded furnace to a maximumtemperature of 300 °C. Following the observation of thermal alterationduring the thermomagnetic runs (see Discussion below) and to testthe onset of thermal alteration, thermal demagnetization was carriedout to discrete temperature steps in the interval of 150 °C to 300 °C.Subsequently, these samples have been subjected to alternating field(af) demagnetization avoiding thermo-chemical alteration and thus theformation of a lab-introduced magnetization. The resultant natural rem-anentmagnetization (NRM)was subsequentlymeasured on a horizontal2G Enterprises DC SQUID magnetometer (noise level 3 × 10−12 Am2).

NRM directions were determined using principal component analy-sis (Kirschvink, 1980) and plotted in orthogonal demagnetization plots(Zijderveld, 1967). The declination and inclination angleswere calculat-ed for each component before and after bedding tilt correction (thebedding orientation varied significantly throughout the section andwas therefore measured frequently).

5.2. Rock magnetism

5.2.1. Thermomagnetic runsThe results of the thermomagnetic runs from both subsections are

very similar (Fig. 4). For all samples, the heating curve is characterizedby a decrease in total magnetization from room temperature up to atleast 360 °C, in some samples up to 400 °C. The hyperbolic shape ofthe heating curve is related to the dominant presence of paramagneticminerals. On the other hand, the significant decrease of about 20 to55% can be attributed to unblocking of the iron sulfide greigite and/orpyrrhotite. At this temperature point, the magnetization rapidly in-creases up to about 500 °C (480 to 520 °C interval), which is followedby a steady decrease to 580 °C. This thermal behaviour can be attributedto the thermal breakdown of iron sulfide, in this case of pyrite between360 and 400 °C, which leads to the formation of the iron oxide

Fig. 4. Representative examples of thermomagnetic runs of samples from the St. Audrie's Bay/East Quantoxhead composite section from all three lithologies; marl, limestone and euxinicshale. The red curve is the heating curve, indicated with the arrow pointing to the right, and the black and thicker curve is the cooling curve, indicated with the arrow pointing to the left.The main decrease in total magnetization occurs up to the temperature interval of 360 to 400 °C and can be attributed to the presence of the iron sulfide greigite and/or pyrrhotite.The subsequent increase and decrease up to 580 °C are related to the formation of the iron oxide magnetite. The small inflection at about 700 °C indicates the presence of hematite.

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magnetite, as reflected in the steady increase in magnetization and itsbreakdown at magnetite's Curie Temperature of 580 °C. The presenceof pyrite in the Blue Lias Formation has been shown previously(e.g. Bottrell and Raiswell, 1989). Unfortunately, this thermal behav-iour of pyrite swamps any other characteristics of magnetic mineralsbelow 600 °C potentially originally present in the sample.

In most samples a small decrease in magnetization is observed afterheating above 600 °C, which is indicative of either the presence of he-matite, or further thermal oxidation of the pyrite to magnetite system.Upon cooling the samples back from the maximum temperature of700 °C to room temperature, the total magnetization increases fromaround 580 °C to room temperature, reaching a total magnetizationthat is significantly higher than the initial magnetization of the sample.This can be attributed to the formation of new magnetic minerals, inparticular magnetite, during the thermal treatment.

5.2.2. Isothermal remanent magnetization (IRM) acquisition componentanalysis

All IRM acquisition curves are fitted with three components(Fig. 5). The first component is introduced in the low field intervalwith coercivity forces (B1/2) between 23 and 32 mT and small DPsof about 0.24 log10 mT. This component contributes about 30% tothe total IRM. Its small DP values and coercivity forces suggest amagnetite-like magnetic carrier. A second component is fit with largercoercivity forces of about 65 mT and small DPs of about 0.23 log10 mT.This component could also be related to fine-grainedmagnetite, howev-er, theweakmagnetization – in particular shown in the thermomagneticruns – precludes the sole presence ofmagnetite. An alternative interpre-tation is that these values are indicative of greigite being the magneticcarrier, and the high coercivity values and narrow distribution parame-ter might indicate a biogenic origin (Vasiliev et al., 2008; Hüsing et al.,2009). This component contributes about 60% to the total IRM and isthus the main magnetic component. Finally a third component is fit inthe high field interval with coercivity forces varying between 350 and950 mT and DPs between 0.25 and 0.4 log10 mT. The large spread in

values can be explained by the fact that we have measured up to700 mT only and magnetic carriers with coercivities higher than thisvalue are thus not well described. It is likely that this component isgoethite or hematite, but its contribution to the total IRM is very smallwith only 10%.

5.3. Demagnetization

The initial NRM intensities (at room temperature) of all samples areextremely low (ranging between 0.04 and 0.009 mA/m) and are evenlower after the first temperature step at 80 °C (as low as 0.004 mA/m).These NRM values are very close to the detection limit of the magne-tometer. Nevertheless, the progressive thermal demagnetization ofNRM yielded in about 65% of all samples good quality demagnetizationdiagrams. The remaining 35% sampleswere samples thatwere empty attemperatures starting at 175 °C (Fig. 6g), samples that showed unreal-istic combinations of declination and inclination angles (Fig. 6h; sam-ples as shown in g and h are represented with a cross in Fig. 8),samples that show gyroremanence starting around 35 mT (Fig. 6i),and samples with a declination close to the present-day field (Fig. 6j,samples as shown in i and j were included in the polarity interpretationand are therefore represented with an open dot in Fig. 8). Using thegood quality demagnetization diagrams only lead to the identificationof three components in most samples. The first component is revealedin the temperature interval between room temperature and 80 °C andhas random orientations (Fig. 6). This component is interpreted as ran-dom viscous component. A low-temperature component (LT compo-nent) unblocks at temperatures between 80 and about 180 °C. This isfollowed by a high-temperature component (HT component) that, inmost cases, thermally decays towards the origin until the maximumtemperature of 300 °C (Fig. 6b, e, and f) and in some cases continuesto decay during alternating field demagnetization up to a maximumof 100 mT (Fig. 6a and c). The direction of the HT component does notchange over the temperature\af interval 175 °C to 100 mT. This fact isused to justify calculating NRM directions in the case that the thermal

Fig. 5. Representative examples of isothermal remanent magnetization (IRM) acquisition curves that are fitted with three components: Component 1 represents a magnetite-like carrier (30% contribution), component 2 is (biogenic) greigite (60%contribution) and component 3 is hematite or goethite (10% contribution). 49

S.K.Hüsing

etal./Palaeogeography,Palaeoclimatology,Palaeoecology

403(2014)

43–56

a) b)

c) d)

e) f)

g) h) i) j)

Fig. 6. a)–f) Representative examples of Zijderveld Diagrams showing normal (a, b, and f) and reverse (c, d, and e) polarity. Examples are shown with tilt correction and in stratigraphiccoordinates. g) and f) are examples of unreliable (plotted with a cross in Fig. 8) data, i) shows an example of gyromenanence (plotted as white point in Fig. 8), and j) is an example wherethe isolation of the primary component and normal overprint component remained difficult (plotted as white point in Fig. 8). See text for further description and discussion.

50 S.K. Hüsing et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 403 (2014) 43–56

demagnetization was possible to temperatures of 240 to 260 °C only(Fig. 6d). Above this temperature range directions became randomand NRM intensities increased. Since this high-temperature/high-af(HT) component revealed normal and reverse polarity, it is consideredstable and of primary origin.

The difference in angle between the LT and HT component is rela-tively small, but well visible (Figs. 6a, b, g and 7). The difference be-tween the mean tilt-corrected HT component (Fig. 7a) and the meanof the LT in stratigraphic coordinates (Fig. 7d) is 16.5° in declination,with the LT component being significantly steeper (71.5°) than the HT

a) b)

c) d)

Fig. 7. Equal area projection of the high-temperature (HT) and low-temperature (LT) components tilt-corrected and in geographic coordinates. The present-day field declination andinclination are plotted with a red diamond. d) The LT component in geographic coordinates is considered present-day field overprint, whereas a) the tilt-corrected HT component isconsidered the stable ChRM of primary origin.

51S.K. Hüsing et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 403 (2014) 43–56

component (tilt-corrected: 59.4°; stratigraphic coordinates: 59.1°;Fig. 7). The inclination of the LT component is somewhat steeper thanthat of the present-day field values at the location (Dec = 357.75°;Inc = 66.6°). The declination and inclination angles of the tilt-corrected HT component remain positive (larger than 360°) andshallower than those of the present-day field.

The mean of the tilt-corrected normal and reverse directions ofthe HT component are not strictly antipodal (Fig. 7a). However,this non-antipodality can be explained by the poor quality of the de-magnetization data, which hampers the clear isolation of the indi-vidual components. The latter is related to overlapping blockingtemperatures of the magnetic carriers of the individual componentsand the very weak NRM intensities. Even though this data does notpass the reversal test (McFadden and McElhinny, 1990), we inter-pret this component as stable ChRM component of primary origin,whereas the LT component (Fig. 7d) is considered normal overprint,either related to the present-day field or remanence acquired in the

Brunhes ages, as has been suggested for Upper Triassic and lower-most Jurassic part of the St. Audrie's Bay section by Hounslow et al.(2004).

Given all the above-mentioned facts, the ChRMdirections have beenused for polarity interpretations only because isolating the primary(HT) component from the overprint direction remains difficult. Theresultant declination and inclination directions, whichhave large uncer-tainties, should therefore be treated with utmost care when using themfor directional purpose.

5.4. Magnetostratigraphy

Plotting the ChRM directions in stratigraphic order results in mainlynormal polarity for the St. Audrie's Bay/East Quantoxhead compositesection (Fig. 8). However, three intervals of short reverse polarity havebeen detected. The lowermost reverse interval, labeled AQ1r, (AQderives from St. Audries Bay/East Quantoxhead) is defined by only one

Fig. 8. The declination and inclination of the high-temperature (HT) component are plotted in stratigraphic order, next to the stratigraphic log of the St. Audrie's Bay/East Quantoxheadcomposite section. Black (white) points denote reliable (unreliable) ChRM interpretation, whereas the crosses represent samples, which did not yield any ChRM directions (no polarity).In the polarity column, black (white) zone indicate normal (reverse) polarity interval, and grey zones are uncertain polarity zones. The crosses in the polarity column indicate samplinggaps. The resultant polarity pattern is dominated by normal polarity (black) and characterized by three short reverse polarity intervals (white), signified AQ1r, AQ2r and AQ3r.

52 S.K. Hüsing et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 403 (2014) 43–56

sample of clearly reverse polarity, but additionally, three samples havesouth directed declination, spanning in total an interval of about 7.5 min the upper part of theHettangian in the St. Audrie's Bay section. Unfor-tunately, the interval between 73.75 and 76.6 mwas covered and couldtherefore not be sampled. The second reverse interval AQ2r is clearlydefined by a number of reverse samples over about 1.5 m in the EastQuantoxhead succession. The lower reversal boundary is located about1 m above the Hettangian–Sinemurian boundary. The uppermost andthird reverse interval AQ3r is significantly thicker than AQ2r and is

defined by a significant number of reverse samples (seven) over aninterval of about 4 m during the lower Sinemurian.

6. Discussion

6.1. Comparison with marine records

To date, the longest marine lowermost Jurassic record comes from adrill core in the Paris Basin (Yang et al., 1996). The recovered succession

53S.K. Hüsing et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 403 (2014) 43–56

ranges in age from the late Rhaetian to the early Pliensbachian(Carixian), encompassing the Hettangian and Sinemurian (Fig. 9). Thebiochronological control of the drill core remained slightly impreciseand the positions of stage boundaries thus inexact, even though Yanget al. (1996) used the combination of age diagnostic events frompalynology, ammonoides, bivalves, ostracodes, and foraminiferas.

Biostratigraphically the St. Audrie's Bay/East Quantoxhead compositesection can be correlated to the Paris Basin succession from the base ofthe Hettangian to and including the bucklandi ammonite zone duringthe early Sinemurian. In the Paris Basin succession, the Hettangian isdominated by normal polarity including several, at leastfive –dependingon where the Hettangian–Sinemurian boundary is located in thesuccession – short reverse intervals, whereas further upsection and dur-ing the bucklandi ammonite zone, themagnetostratigraphy is dominatedby a high frequency in polarity changes. The polarity pattern of the ParisBasin thus contrasts significantly that of the St. Audrie's Bay/EastQunatoxhead composite section (Fig. 9), and we will not attempt tocorrelate the short reverse intervals AQ1r, AQ2r, and AQ3r to any ofthose in the Paris Basin; mainly because of the uncertainties in thebiochronology in the Paris Basin record.

A second lowermost Jurassic record comes from pelagic sections inUmbria, Italy (Channell et al., 1984). The Sinemurian part of theSt. Audrie's Bay/East Quantoxhead section can be correlated to theSinemurian in the basal part of the Fonte Avellana section (Fig. 9).Since the Umbrian section is condensed and fossil preservation is

Fig. 9.Magneto-cyclostratigraphic correlation of the St. Audrie's Bay/East Quantoxhead composUsing the calibration to the 100-kyr eccentricity cycle the polarity pattern of the St. Audrie's Baysurprisingly goodmatch. Grey uncertainty intervals have been added in the polarity column basmuch as ~100 m thick and the H16r as much as ~200 m thick. Note that the polarity pattern ofSt. Audrie's Bay/East Quantoxhead composite section, so that a match cannot be established.section (Channell et al., 1984) however, seem to match those of the St. Audrie's Bay/East Quanthe biostratigraphic events and U–Pb ages of volcanic ash layers of the Pucara Basin section, noas shown in Fig. 2.

relatively poor, stage boundaries can, however, not be placed exact-ly. The polarity pattern in the Sinemurian part of the section ischaracterized by two short reverse intervals. These could bematching with the Sinemurian AQ2r and AQ3r resolved in the Englishsuccession.

6.2. Comparison with continental records

The Hartford Basin is a continental rift basin formed during thebreakup of Pangea in the Triassic (Kent and Olsen, 2008). Kent andOlsen (2008) present a detailed discussion on the lithostratigraphy ofthe basin fill as well as the cyclostratigraphic interpretation. Themagnetostratigraphic results from the Hartford Basin suggested thepresence of three short intervals of reverse polarity within a long nor-mal chron. Based on lithostratigraphic correlation to the Newark Basinthis long normal Chron is E24n in the astronomically tuned GPTS ofthe Newark Basin. However, in the Newark Basin GPTS no intervalswith reverse polarity within E24n have been detected.

For direct comparison of the magnetostratigraphy thecyclostratigraphy and astronomical tuning of the St. Audrie's Bay/EastQuantoxhead composite section can be correlated to that of theHartford Basin (Fig. 9). The here presented correlation diverts slightlyfrom that shown by Ruhl et al. (2010), because here, the calibrationstarted from the identification and correlation of the observed long,405-kyr eccentricity cycle. The consequences of the cyclostratigraphic

ite section with the Hartford Basin, eastern USA (Olsen et al., 2005; Kent and Olsen, 2008)./East Quantoxhead composite section can be correlated to the Hartford Basin resulting in aed on the notion of Kent and Olsen (2008) that themagnetozone H24r and H25r can be asthe Paris Basin (Yang et al., 1996) differs significantly from that in the Hartford Basin andThe two reverse intervals in the polarity pattern of the Sinemurian of the Fonte Avellanatoxhead composite section. On the left side, the astronomical calibration to La2010d andrthern Peru (Schaltegger et al., 2008; Schoene et al., 2010; Guex et al., 2012) are included

54 S.K. Hüsing et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 403 (2014) 43–56

re-examination is a shift by one ~100-kyr eccentricity cycle younger inthe upper part and two ~100-kyr eccentricity cycles in the older part ofthe section when compared to Ruhl et al. (2010; see SupplementaryFig. 1 for details).

There is notmuch freedomconcerning the correlation of the 100-kyreccentricity cycles in the UK to the Hartford record, because theobserved 405-kyr eccentricity maxima in the UK record all need to becorrelated to the 405-kyr eccentricity maxima in Hartford as shown inFig. 9 relative to the Triassic–Jurassic boundary in each section. Forexample, if the two 100-kyr eccentricity maxima in the St. Audrie'sBay/East Quantoxhead composite section at 75 and 80 mwould be cor-related higher than done in Fig. 9, the 100-kyr eccentricity cycle at 98min the St. Audrie's Bay/East Quantoxhead composite sectionwould fall ina 400-kyr eccentricity minimum, which is not in agreement withthe cyclostratigraphicmodel. Astronomical corroboration of the correla-tion should come from observing the very long eccentricity cycle(1.6–2.4 Myr time scale) at the same position. A minimum of thiscycle is observed in Hartford at 1200–1400 m, for which there is nogood evidence in the St. Audrie's Bay/East Quantoxhead compositesection (Fig. 9).

Based on this cyclostratigraphic correlation (Fig. 8), the sampled in-terval in the Hartford Basin can be correlated with the Hettangian andlower Sinemurian in the St. Audrie's Bay/East Quantoxhead compositesection (Fig. 8). The correlation indicates a very good overall match be-tween the polarity pattern of the St. Audrie's Bay/East Quantoxheadcomposite section and the Hartford Basin. Following the end-Triassicmass extinction, most of the Hettangian is dominated by normal polar-ity. The first reverse interval in the St. Audrie's Bay/East Quantoxheadcomposite section AQ1r correlates to the same 100-kyr eccentricitycycle and lithological bundle as H24r in the Hartford Basin. The secondreverse interval AQ2r correlates to the lower half of one 100-kyr eccen-tricity cycle younger than H25r. The top of H25r is, however, uncertain.The base of the longer AQ3r, starts in the top of the 100-kyr eccentricitycycle that correlates with H26r, which seems to extend furtherdownwards.

As the timing of reversals is considered to be synchronous, thereasons for the observed slight offsets must lie with the stratigraphyand/or astronomical calibration of either section. Even though thecyclostratigraphic model seems to fit very well with the 405-kyr long,eccentricity curve, the discrepancy of one 100-kyr eccentricity cyclefor AQ2r with respect to H25r in the Hartford Basin might be relatedto uncertainties in the 100 kyr, short eccentricity cycle of the astronom-ical solution (Laskar et al., 2011a,b), and the astronomical calibration ofthe observed 100-kyr eccentricity black shale bundles. Further, theremight be stratigraphic uncertainties in the St. Audrie's Bay/EastQuantoxhead composite section associated with the overlap betweenthe two subsections. On the other hand, the Hartford Basin compositesection could have uncertainties in the depth scale because it has beenconstructed of various subsections cored in the basin and individualoutcrop sites (Kent and Olsen, 2008). This might lead to small strati-graphic gaps or duplications, which will result in a slight inaccuratenumber of lithological cycles. In turn, this could have significant influ-ence on the modeled precession envelope and thus the number of100-kyr eccentricity cycles for the astronomical tuning and calibration.

The poor resolution of AQ1r hampers estimating the definite dura-tion of this reverse interval. The durations of AQ2r and AQ3r are wellconstraint and can be calculated based on the 100-kyr eccentricity-scale correlation to the Hartford Basin. These durations are very similarto those of H25r and H26r of the Hartford Basin record. As a conse-quence of the astronomical calibration and palaeomagnetic correlation,the position of the Hettangian–Sinemurian boundary can be placed inthe Hartford Basin just below the base of the short reverse intervalH25r, about 15 to 30 kyr. According to the new cyclostratigraphic inter-pretation, the correlated positions of the Triassic–Jurassic boundary andETME in the Hartford Basin are shifted by two 100-kyr eccentricity cy-cles younger and the Hettangian–Sinemurian boundary in the Hartford

Basin by one 100-kyr eccentricity cycle younger then previously shownin Ruhl et al. (2010).

6.3. Revised time scale for the Early Jurassic

In the standard Geological Time Scale 2004 (GTS; Gradstein et al.,2004), the magnetostratigraphy for the Hettangian and Sinemurian isgiven only by a possible schematic pattern, based on a compilationfrom Austrian sections (Steiner and Ogg, 1988; Gallet et al., 1993), theNewark Basin (Kent et al., 1995; Kent and Olsen, 1999), and the ParisBasin (Yang et al., 1996). However, the magnetostratigraphy has yetnot been verified. In the newly published Geological Time Scale 2012(Gradstein et al., 2012) the pattern of the Geomagnetic Polarity TimeScale (GPTS) has been updated based on the magnetic polarity patternof the Hartford Basin (Kent and Olsen, 2008) and the astronomical cal-ibration of Ruhl et al. (2010). This pattern is now supported by thehere presented palaeomagnetic results of the St. Audrie's Bay/EastQuantoxhead composite section.

The ages and duration of the individual polarity intervals in the St.Audrie's Bay/East Quantoxhead composite section have been calculatedby a) counting the observed 100-kyr eccentricity cycles and b) using the405-kyr eccentricity calibration to the astronomical solution (Table 1). Adifference of 170 to 190 kyr between these ages can be partly explainedby using an age of 201 Ma for the Triassic–Jurassic boundary, instead ofan age between the U–Pb dated ash layers with ages of 201.26/201.29Ma and 201.35/201.36 Ma (Schaltegger et al., 2008; Schoene et al.,2010), and partly by the uncertainties associated with the U–Pb ages.

The calculated ages and duration of the individual polarity intervalsat St. Audrie's Bay/East Quantoxhead section are very similar to those inthe Hartford Basin (Table 1). The largest discrepancy is for the reversepolarity intervals AQ3r and H26r, respectively, where H26r is morethan twice as long as AQ3r.

6.4. The St. Audrie's Bay/East Quantoxhead section a Hettangian referencesection and potential unit stratotype section for the Hettangian Stage

In this paper, we have presented a robust magnetostratigraphy forthe entire Hettangian and early Sinemurian for the St. Audrie's Bay/East Quantoxhead section. In combination with the previously pub-lished biostratigraphy (Warrington et al., 1994, 2008; Bloos and Page,2002), cyclostratigraphy and (floating) astronomical calibration (Ruhlet al., 2010 and this paper), an integrated stratigraphy is now providedfor the St. Audrie's Bay/East Quantoxhead composite section. Theastronomically-calibrated ages for the reversal boundaries (Table 1)should be incorporated into the next GPTS. The position of the top ofAQ1r is less reliable due to the sampling gap in this interval.

Long and continuous marine sections, in particular in the Jurassic,are very rare. Therefore, sections containing stage boundaries have sofar been used to define GSSPs for Jurassic Stage boundaries. TheHettangian GSSP – the Triassic–Jurassic boundary – is defined and hasbeen ratified in the Kuhjoch section in Austria (Hillebrandt et al.,2007). The first occurrence (FO) of the ammonite Psiloceras planorbisin the basal part of the Blue Lias Formation (Warrington et al., 2008),the slightly lower FO of Cerebropollenites thiergartii pollen (Bonis et al.,2010), and the distinct shift in the organic C-isotope records (Ruhlet al., 2010) can be used as boundary markers in the St. Audrie's Bay/East Quantoxhead composite section. The Sinemurian (Lower Jurassic)Stage boundary level coincides with the first appearance of the ammo-nite genera Vermiceras andMetophioceras in the East Quantoxhead sec-tion (Bloos and Page, 2002). The position of the Sinemurian GSSP – theHettangian–Sinemurain Stage boundary – (Bloos and Page, 2002) close-ly coincides with the position of the base of the short reverse polarityinterval AQ1r, enabling global correlation.

The St. Audrie's Bay/East Quantoxhead composite section can beused as a Hettangian reference section and is an ideal candidate for aHettangian unit stratotype following the concept of Hilgen et al.

Table 1Stratigraphic positions, ages and duration of the reversal boundaries in the St. Audrie's Bay/East Quantoxhead composite section calculated by two different means; a) the observed 100-kyr eccentricity cycles and an age of 201 Ma for the Triassic–Jurassic boundary and b) the 405-kyr eccentricity tuning to the La2010d astronomical solution of Laskar et al. (2010a). Thedurations are compared to those in the Hartford Basin magnetostratigraphy (Kent and Olsen, 2008). ?uncertain position related to sampling gap. *The ages in the St. Audrie's Bay/EastQuantoxhead composite section are calculated using the here presented calibration to the 100-kyr eccentricity cycles (after Ruhl et al., 2010) and using the most recent CAMP age of201 Ma for the Triassic–Jurassic (T–J) boundary (Schoene et al., 2006). ^T–J boundary = Triassic–Jurassic boundary taken at 201 Ma; Δ = the difference in reversal age between the405-kyr eccentricity calibration to the astronomical solution and the counted 100-kyr eccentricity cycles.

St. Audrie's Bay/East Quantoxhead Hartford Basin

100-kyr eccentricity cycles & T–J boundary at 201 Ma* 405-kyr eccentricity tuning to La2010d

Stratigraphic position Position relative to theTop(m)

Base(m)

Top(Ma)

Base(Ma)

Duration(Myr)

T–J boundary(Myr)^ (Myr)^

Top(Ma)

Base(Ma)

Duration(Myr)

Δ(Myr)

Duration(Myr)

AQ4n 104.30 198.99 2.01 199.16 0.17 H27nAQ3r 104.30 100.55 198.99 199.06 0.070 2.01 1.94 199.16 199.24 0.080 0.18 H26r 0.150AQ3n 100.55 93.70 199.06 199.21 0.150 1.94 1.79 199.24 199.40 0.160 0.19 H26n 0.212AQ2r 93.70 92.35 199.21 199.26 0.050 1.79 1.74 199.40 199.43 0.030 0.17 H25r 0.043AQ2n 92.35 ?77.5 199.26 199.65 0.390 1.74 1.35 199.43 199.83 0.400 0.18 H25n 0.320AQ1r ?77.5 73.44 199.65 199.74 0.090 1.35 1.26 199.83 199.93 0.100 0.19 H24r 0.061AQ1n 73.44 199.74 1.26 199.93 H24n

55S.K. Hüsing et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 403 (2014) 43–56

(2006). The integrated stratigraphy provides the global correlationpotential for the St. Audrie's Bay/East Quantoxhead composite section.Reliable stable isotope record (Ruhl et al., 2010) enabled pinpointingthe distinct events following the end-Triassic mass extinction. Unfortu-nately, direct radioisotopic dating is missing due to the lack of volcanicash layers in the section. However, absolute ages for the Triassic–Jurassicboundary, such as themost recent CAMP basalt date and Peruvian volca-nic ash layers date of ~201 Ma (Schoene et al., 2006, 2010; Schalteggeret al., 2008) and for the Hettangian–Sinemurian boundary (~199.5 Ma;Schaltegger et al., 2008), can be used for the chronostratigraphic calibra-tion. Even though the St. Audrie's Bay/East Quantoxhead compositesection seems to be complete and continuous over the HettangianStage interval, potentially small stratigraphic uncertainties might berelated to the overlap of the two subsections.

7. Conclusions

Palaeomagnetic investigation of the St. Audrie's Bay/EastQuantoxhead composite section reveals a stable magnetic carrier of pri-mary origin. In fact, the primary signal ismost likely carried by a combina-tion of (possibly biogenic) greigite (60% contribution) and magnetite(30% contribution). The resultant magnetostratigraphy is characterizedby three short-lived reverse polarity intervals within a long normal po-larity chron. The durations of the reverse polarity intervals are ~90 kyrfor AQ1r, ~50 kyr for AQ2r and ~70 kyr for AQ3r. The base of AQ2r isabout 15 to 30 kyr after the Hettangian–Sinemurian GSSP.

The palaeomagnetic pattern differs significantly from that in theLower Jurassicmarine record in the Paris Basin. However, using the pro-posed and here slightly refined astronomical calibration of Ruhl et al.(2010), the St. Audrie's Bay/East Quantoxhead composite section canbe correlatedwith theHartford Basin succession in the eastern USA. De-spite a 100-kyr eccentricity discrepancy for the two younger reversals,the astronomically-calibrated magnetostratigraphic pattern generallymatches and confirms the incorporation of this polarity pattern includ-ing (astronomically derived) durations in the most recent GPTS(Gradstein et al., 2012). As reversal boundaries are considered to beglobally synchronous, further refinement of the Hartford Basin and theSomerset coast records is needed in order to resolve the minor differ-ences between the two records and to thus improve the GTS.

The St. Audrie's Bay/East Qantoxhead section contains the Triassic–Jurassic boundary as well as the Hettangian GSSP — the Hettangian–Sinemurian Stage boundary. In combination with its biostratigraphy,astronomical tuning to the 405-kyr, long eccentricity cycle of La2010,magnetostratigraphy, and isotope record, the section is an excellentcandidate to serve as Hettangian reference section and potentially asunit stratotype for the Hettangian Stage.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.palaeo.2014.03.022.

Acknowledgements

Dr. Mark Hounslow is gratefully acknowledged for introducing us tothe sections in 2007. Wolfram Kuerschner and Nina Bonis helped in es-tablishing the earlier stratigraphic framework. This researchwas fundedby the High Potential project “Earth's and Life's history: from Core toBiosphere” (CoBi) of the University of Utrecht. We greatly appreciatethe comments of the two anonymous reviewers and the editor FinnSurlyk, which has resulted in a significantly improved manuscript.

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