The influence of a sudden climatic change on marine ...

Downloaded University on April 24, 2016 Journal of the Geological Society, London, from Vol. http://jgs.lyellcollection.org/ 147, 1990, pp. 365-371, 7atfigs. PrintedofinLeeds Northern Ireland

The influence of a sudden climatic change on marine deposition in the Kimmeridgian of northwest Europe P A U L B . WIGNALL’ & ALASTAIR H . R U F F E L L 2 Department of Geology, University of Leicester, Leicester LE1 7RH, U K Present address: Department of Earth Sciences, The University, Leeds LS2 9JJ, U K ’School of Earth Sciences, Birmingham University, Birmingham B152TT, U K Abstract: A sudden change from humidstyle to semi-arid style deposition markedly affected the accumulation of the Upper Kimmeridge Clay in southern England. Many of the changes appear to be related to a change in sedimentation rate at this time. Thus softground faunas are replaced by firmground faunas; diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules; and depositional gradients, recorded by lateral biofacies changes, becomes steeper. The evidence available is in accord with a decline in offshore sedimentation rates during this interval. Other changes, suchas a declinein kaolinite abundance, were more directly controlled by the ‘drying-out’ of the hinterland. Similar changes, elsewhere in the marine geological record, could be used as climatic indicators. The climatic change is part of a wider, northern hemisphere dry event which affect a broad area in the late Jurassic. The KimmeridgeClay of southern England was one of the last depositional environments to be influenced by the climatic change at this latitude.

Climate exerts a fundamental influence upon most depositional environments, primarily through the control of factors such as temperature and rainfall. The most diagnostic climatic indicators, such as coals and evaporites, are formedaroundsea level (Hallam 1984). Indeeper marineenvironments, the climate ismost easily evaluated using clay mineralogy. Kaolinitegenerallycharacterizes warmhumid climates,althoughitsrapiddeposition in marine conditions and consequent concentration in coastal areas isalso animportantcontroluponitsdistribution aridconditions, with (Griffin et al. 1968). Undermore reduced run-off, carbonatesratherthan clastics tend to accumulate in warm marine settings although once again the distance from the terrigenous source must also be considered. Otherthan changes in clay mineralogy, the effect of climatic fluctuations is rarely appreciated in offshore settings. This is primarily due to the difficulties of determining the effects amongst the multiplicity of causal factorsthat influence offshoremarinedeposition.Inthis paper,the influence of climateon the deposition of the Kimmeridge Clay of southern England is investigated as the depositionalintervalappears to havespanned a time of major climatic change(Hallam 1984;Myers & Partington pers. comm).

Upper Jurassic climates Jurassic andCretaceousclimateswerecharacterized by warm and humid conditions developed over large areas with markedly reducedlatitudinaltemperaturegradients compared to the present day (Hallam 1984, 1985). However, in thelateJurassic of the northern hemisphere, there wasa as the low latitude arid belt reversal of this pattern expandednorthwards(Hallam 1984). The cause of this change is puzzling, particularly as itbegan at a time of maximal sea level stand in the Oxfordian t o Lower

Kimmeridgian interval (Haq et al. 1987) when humid maritime climates might have been expected to increase in importance. of the climatic change varied The precise timing regionally although it appears to have reached its peak in theuppermost Jurassic-basal Cretaceous in all areas.In Soviet Central Asia, important evaporite deposits began to form in the Oxfordian, indicating the establishment of truIy arid conditions (Hallam 1984). Further west, in Poland and Germany,anhydritedeposition began in marginal marine of the Lower Kimmeridgian areas in the upper part (Dembowska 1976; Jordan 1971). It shouldbenotedthat the Kimmeridgian stage is used inthesense of British geologists in this study and that the Upper Kimmeridgian thereforecorrelates with the LowerTithonian or Lower Volgian of manyforeigngeologists. InsouthernFrance, kaoliniteis lost from the clay mineral assemblages in the lower part of the Upper Kimmeridgian (Deconinck 1987), coincidentwith thestart of a majorperiod of limestone deposition. Semi-arid conditions are inferred for the Lower Kimmeridgianby Delfaud (1979) inagreement with the spore and pollen data of Courtinat (1989)which indicates the onset of arid style deposition in the mid-Lower France. In Kimmeridgian eudoxus Zone of southern northernFrance, in the marginalBoulonnais succession (Fig. l ) , minor evaporites are notdeveloped until the Portlandian (Townson & Wimbledon 1979). The absence of majorevaporitesinFranceingeneral suggests that only semi-arid conditions, defined by the occurrence of a minor rainy season,weredevelopedduring the terminalJurassic ‘dry’ event. Semi-arid conditions also occurred in the Oxfordian to end-Jurassicbasins of western Iberia. Significant, although probably not substantial, rainfall is indicated by the occurrence of meandering rivers and deltas whilst the presence of calcretesinred-coloured floodplain deposits indicates seasonal aridity (Wilson1971;Wright & Wilson 365

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the Kimmeridge Clay suggests abundant freshwater run-off (Brookfield 1973;Hallam 1984). Higher in the succession, in the Portlandian, kaolinite disappears from the clay mineral fraction (Quest 1985),indicating dryconditions.Thus,it would appear that the climatic change occurred somewhere within the KimmeridgeClay Formation. Palynological and gamma ray spectrometry data of Myers & Partington (pers. comm.) indicates a substantial and sudden climatic change in the middle of the Upper Kimmeridge Clay (bed 44 of the hudfestoni Zone of Gallois & Cox, 1974). Examination of manysedimentological,geochemical and palaeontological data across this boundary reveals that many aspects of these offshore mudrocks were influenced by the climatic change.

Kimmeridge type section

W.1. Location of the Kimmeridgian sections around Boulogne, in northern France, and the Kimmeridge Claystratotype in southern England. Inset shows the Dorset coastal outcrop of bed 44, the sample localityfor this study.

1987).Similar red beds and anhydrite have been recorded further to the north, in the Celtic Sea area (Millson 1987). TheUpper Jurassic climatic record of theCentral Atlantic is well known (Chamley et d . 1983; Hallam 1986). Initially, humid conditions in the Callovian are reflected by a high kaolinite influx, although the close proximity of the source area was probably alsoa contributoryfactor.The progressive loss of kaolinitethrough the Oxfordian and Kimmeridgian parallels the decreasing rainfall recordedin western Europe. The timing of the climatic change in southern England is more difficult to ascertain on facies groundsalone. The of presence of the Abbotsbury Ironstone towards the base

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reshwater teps Stone Band

The magnificent coastal cliff sections at Kimmeridge Bay in Dorset provide an excellent stratotype for the Upper ammeridge ‘lay. The hud‘estoni ‘One occurs at the headland of Rope Lake Head and in the cliffs to the east (Fig. 1); all samples for this study were obtained from this section.

Indicators of climatic change within the Kimmeridge Clay Clay mineralogy A series of samples were analysed from across the climatic boundary spotlighted by Myers & Partington (pers. comm.). The sampleswere mostly takenfromthepredominant mudstone lithology although a number of organic-rich shales were also invesigated. Illite and, to a lesser extent, kaolinite dominated the clay mineral assemblages: theirrelative abundance was calculated as the kaolinite/illite ratio derived from their peak heights on the XRD traces of glycolated

“ , 4 y , ~ ~ i c Shale ~~~h

0

Mudstone CoccolithLimestone Dolostone

15 metres

VI

L

v w

: 2

z

Bed Name Kaolinite/lllite Ratio

Fig. 2. Variations in the kaolinite :illite ratio from the top of the wheutleyensb Zone to the middle of the pectinam Zone in the Upper Kimmeridge Clay. Samples fromRope Lake Head (cf. Fig. 1). Clay mineral data is derived from XRD analysis of mudstones only in order to minimize any lithological control.

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samples. The use of a ratio avoids the problem of having to quantify the absolute amount of kaolinite; a calculation that cannot bedone with any precision. Internalexperimental consistency is maintained by measuring thesamepeak heights in all traces. The results of the clay mineral analysis of the mudstone horizons revealsomesurprisingfluctuations (Fig. 2). The rapid and steady decline of kaolinite towards the middle of the hudlestoni Zone is strongevidenceinfavour of an aridity increase at thistime.Abovethislevel,however, kaolinite abundance does not remainuniformly low as might beexpectedbutundergoes a curiousseries of 'saw-tooth' fluctuations (Fig. 2). The pattern appears unrelated to any otherchangesseen in the sediments and itsorigin isa mystery. Rhythmicalternations of organic-rich shalesand mudstones occur on much too fine a scale to be related. Substantial fluctuations in water depth, related to regional sea level changes, were occurring during this interval. Bed 44 of the hudlestoni Zone is a regional lowstand marked by a downward shift of coastal onlap in proximal settings away from the Dorset coast,whilst the succeeding basal pectinatus Zone sedimentsrecord much deeperconditionsinthis region (Wignall 1990). Haq et al. (1987) record a major type 1 sequenceboundary at the time of the mid-hudlestoni Zone.However,these possible eustaticsea level changes occur on much a coarser scale thanthe clay mineral fluctuations and they appear to be unrelated. Indeed, if the clay mineral signature was influenced by the position of the shorelinethen the regressive conditions of bed 44 should havesupplied more,ratherthan less, kaolinite tothe Kimmeridge Bay area. Clay mineral analysiswas alsoundertakenon a lesser number of organic-rich shales from below and above bed 44. Perhaps the most surprisingchangeobserved was in the proportion of quartz. Shales from beneath bed 44 are often silt laminated (e.g. Wignall 1989, fig. 8) whilst XRD traces of shales from above bed 44 are totally devoid of quartz peaks, a highly unusual occurrence even for mudrocks. The higher shalebeds are much more coccolith-rich and in a number of beds coccoliths are the major component (Fig. 2). Whilst it is possible that the terrigeneous quartz component was diluted by the coccolithinflux thiscannot

account for its totalabsence.Terrigenoussedimentinput appears to have been very low during the deposition of the organic-rich shales. Clay mineraldata are alsoavilable forthe marginal marinestrata of the Boulonnais(Deconicket al. 1983). Calculation of the kaolinite :illite ratio from their30 samples in the Kimmeridgian succession reveals a patternthat appears to principally reflect the relativewater depth of deposition and the distance from source (Fig. 3). Thus the peaks of kaolinite abundance correspond to shallow water intervals characterized by the development of shoreface sands. The hudlestoni Zone appears to be unrecorded in this region (Cope 1967). The Argiles de la CrBche contains wheatleyensis Zone ammonites whilst the overlying Argiles du Wimereux is of upper pectinatus Zone age (Cope 1967). Reworked wheatleyensis Zone ammonitesappear tobe present in thecondensed,phosphatic,pebblebed atthe base of the Argiles du Wimereux which lies on an erosion surface. No strata are therefore preserved from across the climatic boundary,however, the effect of the climatic changeisnot seen in the clay mineral data from the overlying strata as kaolinite is still present (Fig. 3). Deconinck et al. (1983) also recorded horizons of abundant of volcanism smectite which may indicate intervals (Zimmerle 1985) or humid pedogenic weathering (Chamley et al. 1983). If thelatter originis acceptedthen it is noteworthy that smectite dramatically declines in abundance immediately below the Argiles du Wimereux.

Palaeoecology Palaeoecological investigations of the Kimmeridge Clay have revealed a range of benthic associations which principally record variations of oxygen levels, substrate and environmental stability (Oschmann 1988;Wignall1990). One of the most important changes in ecological structure occurs in the mid-pectinatus Zone approximately 25 m above the climatic event indicated by the clay mineral data. The majority of Kimmeridge Clay assemblages from the central, organic-rich part of the succession are dominated by a few species of bivalves of which Protocardia morinica is

Farles . .. ..

ZONES

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FORMATIONS

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Fig. 3. Kaolinite :illite fluctuations in the Kimmeridgian formations of the Boulonnais, northern France. Data from Deconinck er al. (1983). Solid vertical lines indicate intervals wheresmectite is an abundant component of the clay mineral assemblage. Comparison of the clay mineral variations with relative sea level changes, derived from facies analysis, suggests that kaolinite declines in abundance relative to illite during deeper-water intervals probably indicating the greater distance ofthe source area at this time.

Kaollnlte/llllteRatlo FACIES TYPES

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Proxlmallty

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the most common. Analysis of the functional morphology of this bivalve and the remainder of the lowdiversity fauna indicates that a shallow infaunal mode of life was dominant. The only epifaunalforms are eitheroystersattached to ammonities or small gastropods with flaredapertures(an adaptation to soft substrates). However, around the base of bed 47 (cf. Fig. 2) there is a markedfaunalchange. Pseudorhytidopilus Limpet-like gastropods of thegenus become common and at a slightly higher level free-lying serpulids and epibyssate inoceramids also appear. The infauna becomes distinctly rarer above bed 47 although P. morinica can still be found. Theecology of this new fauna is distinctly different as the majority of species resided on the sediment surface and inmost cases they were not adaptedto soft substrates. The serpulids in particular, with their narrow form, would have been particularly susceptible to foundering in soft sediment. The preservation of trace fossils shows a similar change from vaguely defined burrows, seen 44, tomore atsha1e:mudstoneinterfacesbeneathbed sharply defined examples above. Thesechanges may havebeencaused by a marked decrease in sedimentation rate leading to a firmer sediment substrate. It is possible that an increasein thecarbonate content of the sediments may also have contributed to the increase in firmness (Hattin 1986). However, this is unlikely as marly sediments first become important at the base of bed 44 (Fig. 2) and soft sedimentfaunasoccur for over 40m above this horizon. The lower pectinatus Zone appears to have been a time of crisis for a number of groups,asphytoplanktonand probablyostracodesbothundergo a turnover at this time (Rawson & Riley 1982). There is no corresponding turnover in the benthic macrofauna (Clausen -& Wignall 1990) but, as related above, soft substrate-dwelling species become much rarer.

Geochemistry The composition and diagenetic history of the Kimmeridge Clayalsoshow importantchanges within the pectinatus Zone. The majorchange is the loss of dolostones, formed at deep burial depths in the methanogenic zone (Irwin et al. 1977), andtheappearance of carbonatenodules, mostly of nucleated within the uncrushed body chambers ammonites. Dolostone formation requires substantial to survive to quantities of metabolizableorganicmatter relatively deep burial depth; thiscan be accomplishedby of organicmatterthrough the near-surface rapidburial sulphate reduction zone. Conversely, the presence of calcitic nodules requires prolonged residence time in the sulphate & Baird 1986). Thus, it is not reductionzone(Brett surprising thatdolostones andcarbonatenodulesdonot co-occur in the Kimmeridge Clay. The boundary between the two diagenatic regimes occurs at about the samelevel as the faunal change. Large pyrite nodules, which also form in thesulphatereductionzone,becomeimportant slightly earlier than the carbonate nodules, in the upper hudlestoni Zone,perhapsindicatingthe progressive establishment of the importance of this diagenetic zone. The weight percentage of organic matter also undergoes of bed 44. Beneaththe a markedchangeeitherside pectinatus Zone, many of the shale beds attain total organic 1972; carbon (TOC) values of greaterthan15%(Dunn Myers & Wignall1987), butabovethis level TOC values

c

Fig. 4. Rope.Lake Head near Kimmeridge Bay (cf. Fig. 1). The basal portion of the cliff is composed of alternations of organic-rich shales and mudstones of the lower hudlestoni Zone; the more uniform mudstonesof bed 44 comprise the centreof the cliff whilst a return to rhythmic shale-mudstone depositionis seen in the upper half of the cliff. A log of this sectionis shown in Fig.2.

rarely exceed 5% even in dark fissile shales. This change can probably be entirelyexplained by the higher carbonate content of the pectinatus Zone sediments (R. W. Tyson pers. comm.).

Cyclicity The small scale rhythms of organic-rich shale and mudstone thatcharacterize the KimmeridgeClay of England,are developed both above and below the climatic event (Fig. 4). However, the thickness of the rhythmsapparently varies substantially eitherside of bed 44 (Fig. 5). Beneath this horizon, organic-rich shale-mudstonerhythmsrangefrom

B.

A.

N= 23

N=133 l

10

I RHYTHM

THICKNESS (metres]

Fig. 5. Variation in shale:mudstone rhythm thickness A. beneath bed 44 B. above bed 44.

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the mid-eudoxzu Zone to the lower hudlestoni Zone (see Fig. 3 for Kimmeridgian zones). The couplets cluster around a 1m thick modal average whilst above bed 44 substantially fewer rhythmsareavailableformeasurementbutthese show a bimodal peak at 0.5 m and 2 m (Fig. 5). However, statistical analysis, using the Kolmogorov-Smirnov test, reveals that these differences are not significant at the 5% level. Facies variability Kimmeridge Clay deposition was marked by widespread mudrock deposition, albeit with substantial thickness variations caused by syn-sedimentary subsidence; individual beds can commonly be traced across the full width of the present day outcrop, a distance of 400 km (Cox & Gallois 1981). Such lateral persistence clearly attests to low depositional gradients. Only detailed biofacies analysis indicates the presence of subtle variations in water depths and other parameters such as oxygen level (Wignall 1989, 1990). However,abovebed 44 theseminorchanges give way to much more significant lateralbiofaciesvariations suggesting more significant depositionalgradients.Basinal sectionscontainthegastropodsandepibyssate bivalves noted above whilst the more proximal facies are dominated by a distinctly differentfauna of byssate bivalves and pectinids, particularly Camptonectes and Musculus. A decrease in run-off, caused by the climatic change, may have ensuredthatsedimentationrateswerenolongerableto keep pace with subsidence in thecentres of the basins leading to a more substantial depositional slope and therefore a more distinct seriesof biofacies belts.

Discussion The majority of thepalaeoecologicaland geochemical changes can be interpreted as a response to a reduction in sedimentation rate produced by a decrease in run-off (Fig. 6). It is perhaps significant that the post-hudlestoni Kimmeridgian interval in the Boulonnais was also a time of reduced sedimentation. The thick shoreface sandstones below this interval are replaced by condensed,reworked phosphaticnodulehorizons which record much of the depositionalhistory of thearea(Cope 1967; Townson & Wimbledon 1979). The various geological parameters listed above appear to have changed at differenttimes, ranging fromthe lower hudlestoni Zone to the mid-pectinatus Zone (Fig. 7). This suggests that, under an increasingly arid climate, a series of critical threshold values were crossed causing the observed pattern of abrupt, stepwise changes. The actual onset of semi-arid conditions in the Kimmeridge Clay hinterland was probably coincident with the decline of pollen diversity and kaolinite abundance in the mid-hudlestoni Zone. The delayed reaction of the other parameters may reflect a period, perhaps of the order of a hundredthousandyears, when abundant clay matter was still being supplied to the basins fromadeep soilprofile formed under humid conditions. In fact the loss of vegetation mayinitially haveincreasedtherate of supply until all the soil had been eroded, whence slow sedimentation regimes became established. The overall pattern of change in western Europe appears to have been that of northward contraction of humid-style deposition, characterized by the deposition of large amounts of kaolinite-bearing muds. By the lower Upper Kim-

Humid

Semi -arid

Sof t ground fauna

AEROBIC ZONE (mms)/ SULPHATE REDUCTION ZONE (e10m)

h T:=]

Pyriteframboids formed

METHANOGENIC ZONE (-1 km) Dolostones formed Fig. 6. Interpretative summaryof the variations of the palaeoecological and geochemical parameters under the different climatic regimesof the Kimmeridge Clay.

Firmgroundfauna (gastropodsand serpulids) in occolith-rich mud Carbonatenodules (nucleated on ammonites) and pyritelenses formed

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