mesozoic mudstone compositions and the role of kaolinite … · introduction the clay mineral...

IntroductionThe clay mineral compositions of sediments are deter-mined by provenance, climate, transport, sedimentaryfacies, deposition rate and diagenesis. Depending onthese factors, stratigraphic and geographic variationscan be predicted. As mineralogy affects rock properties,such knowledge is valuable for predicting sedimentbehaviour during diagenesis and compaction and thushydrocarbon reservoir and sealing properties.

Although an overwhelming amount of mineral analysishas been carried out in connection with petroleumresearch in the offshore areas, relatively little of thisinformation has been published, and most publishedmineral data from the offshore areas are concernedwith sandstones and reservoir diagenesis. In the presentstudy the mineralogical compositions of fine-clasticsediments from the Norwegian Sea are compared basedon samples from stratigraphic cores. The purpose is todetermine the main mineral compositions and examinepossible stratigraphic or geographic variations in mine-ralogical composition.

The study is based on semi-quantitative bulk-rock X-ray diffraction analyses that have been carried out aspart of a shallow coring program at SINTEF PetroleumResearch (former IKU) during the period 1982-1992.Data from the Lower Triassic to Lower Cretaceous sedi-mentary section are discussed from core locations out-board off the Møre coast in the south to off Troms inthe north (Fig. 1). The study area thus includes boththe Norwegian Sea and the south-westernmost part ofthe Barents Sea. Stratigraphic terms refer to the BarentsSea stratigraphy (Worsley et al. 1988, Smelror et al.1998, 2001) for Troms III and Nordland VII cores andto the Norwegian Sea (Dalland et al. 1988) for theNordland VI, Helgeland, Froan Basin and Møre areas.The stratigraphic formations and their dominant litho-logies are shown in Figure 2 which compares the NorthSea and Barents Sea formations. The concept of theshallow coring program was to sample stratigraphicunits at locations with restricted burial (Rise & Sættem1994). These sites are located marginal to the presentbasin areas or at structural highs.Studies of clay mineralogy are ideally performed on thefine fraction and commonly focus on diagenetic evolut-ion. The present data are bulk rock compositions and

NORWEGIAN JOURNAL OF GEOLOGY Mesozoic mudstone composition, Norwegian Sea 61

Mesozoic mudstone compositions and the role of kaolinite weathering – a view from shallow cores in the Norwegian Sea (Møre to Troms)

Mai Britt E. Mørk, Jorunn Os Vigran, Morten Smelror, Vidar Fjerdingstad & Reidar Bøe

Mørk, M.B.E., Vigran, J.O., Smelror, M., Fjerdingstad, V. & Bøe, R.: Mesozoic mudstone compositions and the role of kaolinite weathering – shallowcores in the Norwegian Sea (Møre to Troms). Norwegian Journal of Geology, Vol. 83, pp. 61-78. Trondheim 2003. ISSN 029-196X.

Mineralogical data from Mesozoic fine clastic sediments in shallow cores offshore Møre to Troms are compiled. The purpose is to examine strati-graphic and geographic variations in rock composition as a basis for regional comparative studies and the prediction of rock properties in offshorepetroleum exploration areas. The studied successions include different depositional environments, varying from continental, paralic and shallowmarine in the Triassic – Lower Jurassic, and shallow to open marine in the Middle Jurassic to Lower Cretaceous. Stratigraphic and regional variationin mineralogical distributions reflect combinations of depositional environment, climate and provenance and we also infer variable influences fromLate Jurassic volcanism.Some of the most distinct stratigraphic mineralogical variations are: 1) change from mica/illite + chlorite + feldspar + mixed-layer clay rich compo-sitions in the Lower Triassic to kaolinite-dominated and feldspar-poor compositions in the Upper Triassic-lowermost Jurassic deposits. This coinci-des with changes from arid continental to humid continental and paralic environments. Kaolinite is the dominant clay mineral also in upper Lowerto Middle Jurassic transgressive beds, that overlie kaolin-weathered basement. 2) The amounts of kaolinite decrease while the smectite/mixed-layerclay mineral concentrations increase in the Upper Jurassic deposits and with notable mineralogical differences between the Spekk and Hekkingenformations. This records transgression and sediment mixing in response to increasing marine influences. 3) The amounts of feldspar increase (pla-gioclase reappearance) in the Lower Cretaceous Kolje Formation. This change is of regional significance in the Western Barents Sea, possibly relatedto Early Cretaceous rifting.

Mai Britt E. Mørk, Reidar Bøe, SINTEF Petroleum Research, N-7465 Trondheim, Norway. Morten Smelror, Geological Survey of Norway, N-7491Trondheim, Norway. Vidar Fjerdingstad, Norsk Hydro Produksjon, P.b. 117, 4065 Stavanger, Norway.

thus also include the silt fraction as mudstones andsiltstones are analysed in this study. Supplementary clayfraction analysis was performed on samples fromTroms III. A few samples from conglomerate matrix arealso examined. All data must therefore be considered inrelation to lithology/grain-size, facies information andburial depth.

The Mesozoic sediments are commonly fairly well con-solidated and denote times of deeper maximum burialthan their present depth below the seabed. In Norwe-gian Sea exploration wells at depths greater than 2.5-3km, smectite, mixed-layer clay and kaolinite in the pre-sence of potassium, are variably transformed to illite(Bjørlykke 1984, Bjørlykke et al. 1995), thus losing part

62 M. B. E. Mørk et al. NORWEGIAN JOURNAL OF GEOLOGY

Fig. 1. Overview map with shallow core locations and names mentioned in the text. Structural names offshore refer to Blystad et al. 1995.

of the original and early diagenetic mineral signatures.Maximum burial of the presently studied cores is,however, interpreted to be less, and the bulk compositi-ons are dominated by detrital and early diageneticminerals. The diagenesis and burial history will be discussed in greater detail elsewhere (Mørk in prep.,SINTEF unpublished data).

Below we give a simplified overview of the stratigraphicand sedimentological relations of the cores, and theselected mineral ratios are plotted stratigraphically. Themineral data are further discussed and compared in astratigraphic framework. Special attention is paid to theoccurrence, dating and depositional environment ofUpper Triassic – Lower Jurassic kaolinitic sediments.Strongly weathered granitic basement gneiss overlainby Mesozoic sediments is also represented in the cores,and its possible influence on the Mesozoic sedimenta-tion discussed.

Geological frameworkRegional overview

The Mesozoic sedimentary succession in the Norwe-gian Sea was deposited during varying tectonic and cli-matic conditions. Triassic sediments include continen-tal, red beds deposited during arid climatic conditions,and deposits related to Early Triassic rifting (Swiecickiet al. 1998) and erosion (Jongepier et al. 1996). Suchconditions predict mineralogical immature sedimentcompositions. Continental environments are also indi-cated by Mid and Upper Triassic sediments, in placesassociated with deposits from marine incursions andlocal salt deposits (Doré 1992). Arid playa mudflats orlacustrine environments are also recognised (Swiecickiet al. 1998). In the Rhaetian and Lower Jurassic succes-sion deltaic and marine sand deposits are widespread,and the paralic, coal-bearing sediments of the Åre For-


Fig. 2. Time correlation of strati-graphic formations in the Norwe-gian Sea (Dalland et al. 1988)and Barents Sea (Worsley et al.1988, Smelror et al. 2001) as aframework for shallow core unitsthat are located in the NorwegianSea (Møre, Froan, Helgeland,Nordland VI cores) and western-most Barents Sea (Nordland VIIand Troms III cores).Abbreviations: Mås = Måsnykan Formation,

Sassend = Sassendalen Group.

mation (Rhaetian-Sinemurian) in the Haltenbankenarea (Gjelberg et al. 1987, Dalland et al. 1988) suggest achange to more a humid climate.

Lower and Middle Jurassic deposits indicate increasingmarine influences with time through the Tilje, Tofte, Ileand Garn formations (Dalland et al. 1988). The overly-ing Middle and Upper Jurassic comprise open marinemudstones (Melke Formation) and marine, organicrich dark shales (Spekk Formation). These are time andfacies analogous to the Fuglen and Hekkingen formati-ons in the Barents Sea, respectively (Worsley et al. 1988,Fig. 2). Upper Jurassic – Lower Cretaceous sedimenta-tion was variably influenced by rifting (e.g. Ziegler1988, Doré 1992, Løseth & Tveten, 1996, Swiecicki et al.1998). The Lower Cretaceous sediments include marineshales of the Lange Formation and the calcareous LyrFormation in the Norwegian Sea. In the northern areasa condensed section marks the base of the Cretaceous(Klippfisk Formation, Smelror et al. 1998).

Figure 3 gives an overview of the stratigraphic positionof the analysed samples. Lower Triassic sediments arecored off Helgeland and off Lofoten (i.e. Nordland VIarea). Upper Triassic – Lower Jurassic transitional bedsare studied in the same locations and also in debris flowdeposits off Møre. Mudstones in association withUpper Lower Jurassic - Mid Jurassic transgressive shal-low marine sandstones have been studied in all the coresites from Møre to Troms. The transgressive sandstonesfrom off Møre and Froan are time analogous to the

Tofte and Ile formations, whereas Bajocian - Bathoniantransgressive sandstones in the northern areas areyounger and time-equivalents to the Garn Formation(Figs. 2 & 3). Upper Jurassic- Lower Cretaceous organicrich shales are compared between the Froan Basin –Helgeland and Nordland VII and Troms III sites. TheLower Cretaceous shale formations are represented inthe Helgeland, Nordland VI and VII and Troms IIIareas.

Cored sediments

Core lithology, main mineral constituents and selectedmineral plots are summarised for each location (Figs. 4;7; 9; 11; 12; 13). The lithostratigraphic framework issummarised very briefly below as a basis for mineralo-gical comparisons in later sections. The stratigraphicand sedimentological background data refer to shallowdrilling reports and publications (Mørk et al. 1983,Bugge et al. 1984, Skarbø et al. 1988, Weiss et al. 1991,Hansen et al. 1992, Smelror et al. 1994, Vigran et al.1994, Smelror et al. 2001).

Møre: Mesozoic sediments off Møre have been analysedon the basis of four stratigraphic cores in block 6206(Smelror et al. 1994; Fig. 1). Upper Triassic and LowerJurassic conglomeratic debris of alluvial fan and proxi-mal fan delta origin (6206/02-U-03; -U-02) are uncon-formably overlain by Upper Toarcian – Aalenian deltaicmarine sandstones that include conglomerate beds (-U-02; -08; -01). The Jurassic sediments are overlainby Cretaceous marine shales and sandstones that arecorrelated to the Lange and Kvitnos formations (Smelroret al. 1994).

Froan Basin: Weathered basement overlain by trans-gressive shallow marine sandstones of Late Toarcian agehave been cored in the northern part of the FroanBasin (6408/12-U-01, Skarbø et al. 1988, Fig. 1). UpperJurassic – Lower Cretaceous sediments were cored inthe Froan Basin (6307/07-U-02; U-03), where organicrich shales of the Upper Jurassic-Lower CretaceousSpekk Formation including Rogn Formation sandstoneare overlain by the Lower Cretaceous Lange Formation.

Helgeland: A considerable part of the stratigraphy iscovered by the eleven short stratigraphic cores from thearea between Vikna and Træna off Helgeland (Figs. 1, 3and 4). The bedrock geology, and correlated ages ofthese cores, drilled in 1982, were described by Bugge etal. (1984). The interpreted Early Triassic, Griesbachian– Smithian age is based on the palynology of coreIKU82-2 published by Vigran & Mangerud (1991). Thedata from the IKU 82 -2 to -11 cores have later beencompiled and reinterpreted in a report by Vigran et al.(1994). In the northern core location (IKU82-2) LowerTriassic sediments are represented by siltstones and


Fig. 3. Stratigraphic sample overview compared for different areas.

mudstones with sand ripples, deposited in shallowmarine environments. Upper Triassic debris-flow depo-sits of continental/paralic origin are unconformablyoverlain by Pliensbachian mudstones of restricted,shallow marine environments (IKU82-4, Fig. 5). TheTriassic – Jurassic boundary beds are strongly kaolinitic(Figs. 4 and 6). Middle and Upper Jurassic silty mud-stones and claystones (IKU82-4B; IKU82-3; IKU82-3Band IKU82-5, IKU82-C) (Melke and Spekk formations,

Århus et al. 1989) are interpreted as deep shelf andopen to restricted marine deposits. Cored sediments inthe southern section off Helgeland (Fig. 1) compriseUpper Jurassic black, organic rich shales (Spekk For-mation, IKU 82-7; -7B, Århus et al. 1986), and UpperCretaceous (Shetland Group, IKU 82-8). The black sha-les are overlain by a thin unit of basal Valanginian car-bonates and Upper Hauterivian red calcareous mudsto-nes (Lyr and Lange formations). Upper Cretaceous


Fig. 4. Mineralogical variation of shallow cores off Helgeland. Samples are grouped on the basis of stratigraphy and main lithological units.Abbreviations: Cc=calcite, Chl=chlorite, Crb=cristobalite, Got=goethite, Hem=haematite, Ka=kaolinite, Kf=K-feldspar, MI=mica/illite, Mx=mixed-layer clay, Pl=plagioclase,Py=pyrite, Q=quartz, Sd=siderite, Sm=smectite. Kadinite dominated stratigraphy is marked by gray box in the right colomn.

glauconitic sandstones and mudstones in the lower partof core IKU 82-8 are the youngest sediments of thecored Mesozoic succession. Down-faulted Lower andMiddle Jurassic marine sandstones with palynologicalevidence of a near-shore origin were cored at the south-eastern outcrops (IKU 82-10 and –11).

Nordland VI: In the Nordland VI area (Figs. 1 and 7)Lower Triassic debris flow deposits occur on top ofcrystalline basement gneiss (6710/03-U-03). Conglo-merates and sandstones with clast compositions similarto the underlying basement suggest a local provenance.Upper Triassic (Rhaetian) - Lower Jurassic mudstonesand sandstones (6710/03-U-01) are kaolinitic andinclude root beds, and they are interpreted as deltaplain deposits (Figs. 5; 7 and 8). The continental (para-lic) beds are overlain by Middle Jurassic shallow marinesandstones correlative to the Måsnykan Formation(Smelror et al. 2001). Pliensbachian sediments(6710/02-U-02) include delta plain and shallow marinemudstones and sandstones. The Middle Jurassic sedi-ments (6710/03-U-01) are overlain by Lower Cretace-ous claystones (Barremian-Aptian, Lange Formation).Core 6711/04-U-01 includes Upper Cretaceous marineand glauconitic mudstones.

Nordland VII: Middle Jurassic shallow marine sand-stone (Måsnykan Formation, Smelror et al., 2001)transgressively overlies kaolin-weathered basementgneiss and debris flow deposits of local origin(6814/04-U-01, Figs. 9 and 10). The Hekkingen Forma-tion is represented by Upper Jurassic (Kimmeridgian)organic rich marine shales, including a typical develop-ment of silty mudstones (Raudåte Member, Smelror etal. 2001) at the base. Core 6814/04-U-02 penetrated ayounger (Volgian to Berriasian) part of the HekkingenFormation. Lower Cretaceous marine carbonate(Valanginian – Hauterivian condensed beds) and calca-reous mudstones occur in the upper part of the core.

Troms III: The cored section in the Troms III area com-prises a relatively continous stratigraphic section fromLower Jurassic (Upper Toarcian-Aalenian) to mid Cre-taceous (Early Albian, Smelror et al. 2001; Fig. 11).Shallow marine sandstone of the Stø Formation is over-lain by marine micaceous mudstones of the FuglenFormation (7018/05-U-06) and black organic rich sha-les of the Upper Jurassic – Lower Cretaceous Hek-kingen Formation (7018/05-U-02, -U-01). The transi-tion to the Lower Cretaceous is marked by a condensedbed of Valanginian – Hauterivian age (7018/05-U-01)that may be analogous to the Klippfisk and Lyr formati-ons. The overlying dark, marine claystones are correla-ted to the Lower Cretaceous Kolje Formation (7018/05-U-01; -U-07).


Fig. 5. Lithologs of Triassic-Jurassic boundary beds in cores IKU82-4and details of core 6710/03-U-01, associated with kaolinitic compo-sitions (Figs. 4 and 7). Note presence of root structures. The depositi-onal environments are interpreted as continental and paralic. Litho-log from 6710/03-U-01 is from Hansen et al. 1992.

Fig. 6. SEM secondary electron image of vermicular kaolinite insandstone, core IKU82-4, 10.40 m.

Mineralogical dataData presentation and main minerals

The mineralogical compositions refer to semiquantita-tive, X-ray diffraction analyses based on an internalcalibration method developed at SINTEF PetroleumResearch in the 1980’ies (Appendix 1), also used byBergan & Knarud (1990) and Mørk et al. (1990). Thedata scatter is represented in plots for each area (Figs.4, 7, 9, 11,12 & 13 ), and as these data are semiquantita-tive only main mineral abundance trends and mineralpresence/absence are discussed.

As the border land areas are dominated by quartz- andfeldspar-rich plutonic and metamorphic rocks, highquartz contents in the sediment compositions could inpart reflect coarser grain-size, quartz being the mostresistant mineral. Due to different resistance to weathe-ring, presence of a few silt-sand grains could also resultin scatter in the quartz/feldspar ratio of silty or sandymudstones. In order to examine the effect of bulkgrain-size the feldspar/(feldspar + quartz) ratio hasbeen compared for claystone/mudstone and sandstonebeds in the same formations. In spite of the greater sta-bility of quartz, our data show that this ratio is higheron average in the sandstones than in the claystones-mudstones, excluding a simple relation between pos-sible quartz enrichment and grain size.

In the diagrams presented for each area the clay mine-rals are plotted as ratios of each mineral divided by thesum of the total clay minerals + feldspar. The reason forusing ratios is to minimise the dilution effect of otherminerals, including diagenetic carbonate (and visuallyfor reducing artificial scatter in the graphs due to ana-lytical errors for very low clay mineral contents). Feld-spar is included in the denominator as kaolinite may inpart have formed at the expense of feldspar (Fig. 8b),and the feldspar – clay-mineral ratio in the silt andmud fractions may give additional information ondegree of weathering and eogenesis.

The studied fine clastic sediments have bulk-rock com-positions that are dominated by kaolinite, mica/illiteand quartz. Other common minerals are chlorite, K-feldspar, plagioclase, mixed-layer clay mineral, pyrite,smectite and calcite. Mixed-layer clay refers to mixed-layer illite-smectite. In the data from Helgeland-82 themixed-layer clay includes illite-smectite and a variablepresence of chlorite (T.M. Rønningsland, unpublisheddata). Minerals with lower total abundance includeopal/cristobalite, 7Å- chlorite, siderite and dolomite.Goethite, zeolite, and amphibole are very rare. The rela-tive proportion and importance of each mineral maydepend on various factors such as provenance, climate,grain-size and facies. The main mineralogical changesare discussed below for different stratigraphic intervals.


Fig. 7. Mineralogical variation of shallow cores from Nordland VI. Samples are grouped on the basis of stratigraphy and main lithological units.Abbreviations as in Fig. 4.

Lower Triassic

The main constituents of Lower Triassic mudstones inthe Nordland VI area (Fig. 7), in addition to quartz, aremica/illite, chlorite and feldspar (mostly plagioclase),together with mixed-layer clay mineral. Kaolinite ispresent only sporadically. Provenance is well-constrai-ned, as these sediments are part of locally deriveddebris flows from the west of Lofoten. The associatedcoarser sediments are rich in K-feldspar and rock-frag-ments of granitic, micaceous and amphibolitic gneissesthat mainly originated by erosion of the underlyingbasement. The well-preserved detrital mineralogy andthe lack of kaolinite (except sparse diagenetic kaolinitein sandstones) are compatible with rapid depositionand/or arid environments.

A similar mineral composition is seen in the alternatingred and green, laminated mudstones and siltstones offHelgeland that were deposited in restricted shallowmarine (?lagoonal) environments (Fig. 4). Thus, themineralogical signature of the Lower Triassic mudsto-nes in these northern locations reflects a provenance offeldspathic and micaceous basement rocks, arid climateand in some cases rapid depositional mechanisms.

Upper Triassic and Lower Jurassic

Møre: The Upper Triassic – Lower Jurassic section offMøre (Fig. 13) shows a complex mineralogical evolut-ion that in part may reflect rapid deposition of a tecto-nically induced debris flow. The conglomerates includeboth well-preserved and partly weathered crystallinebasement boulders comparable to the lithologies of thenearby gneiss area (e.g. Mørk 1985, 1988). The fine-grained conglomerate matrix (mud fraction) is rich inquartz, feldspar and mixed layer clay. Smectite is enri-ched in a few laminae, and kaolinite is concentratednear an erosional unconformity (6206/02-U-03, 124.05m depth). The presence of smectite-weathered zonesmay reflect a high proportion of basic and ultrabasic

debris compositions in the lower part of the unit, andrelatively limited water circulation.

The overlying Lower Jurassic beds in the sediments offMøre (6206/02-02) show disappearance of plagioclaseand increase in kaolinite somewhere between 112 mand 99 m depth. Kaolinite is in general more abundantin the overlying Lower Jurassic unit.

Nordland VI - Helgeland: Norian-Rhaetian and indeter-minate Upper Triassic – Lower Jurassic mudstones andassociated sandstones in the Nordland VII and off Hel-geland areas are strongly kaolinitic and lack feldspar(Figs. 4 and 7) and include only subordinate mica/illite.These sediments were deposited in delta plain environ-ments, and kaolinitic compositions are associated alsowith mottled mudstones with soil and root structures(Fig. 5) and authigenic siderite. The kaolinite in thesesediments includes both detrital and authigenic crystals(see below).

The Lower Jurassic sediments in the Nordland VI areaand off Helgeland are strongly kaolinite dominated asin the underlying Upper Triassic units (Figs. 4 and 7).Feldspar is rare or absent in the mudstones, but may beabundant in the interbedded sandstones (Figs. 8b, 8cand 4). The Pliensbachian mudstones at Nordland VIoccur in sandstone-dominated shallow marine sedi-ments. Petrographic analyses of the associated sandsto-nes and conglomerate show kaolinite-replaced detritalgrains, vermicular coarse kaolinite structures, and moretypical, diagenetic kaolinite as pore-filling, random ori-ented aggregates as in the Rhaetian sandstone (Fig. 8).The vermicular and pore-filling varieties may representtwo different stages of kaolinite growth, i.e. weatheringand diagenesis.

In the Helgeland samples kaolinitic Pliensbachianmudstones occur on top of a regional unconformity.The mudstones show variations in both feldspar andkaolinite content (Fig. 4). The kaolinitic mudstones


Fig. 8. Kaolinite occurrence in Rhaetian sandstone, core 6710/03-U-01. a) SEM secondary electron image (SEI) showing coarse vermicular kao-linite which has formed by replacement of mica without recrystallisation (186.47 m). Matrix includes authigenic chlorite. Scale bar = 10µm. b)SEM backscattered electron image of disintegrated feldspar (light), partly replaced by kaolinite (dark) (186.47 m). Scale bar = 100µm c) SEMSEI showing authigenic kaolinite on the surface of K-feldspar (189.52 m). Scale bar = 10µm.

may have a local origin from erosion of the underlyingkaolinitic sediments, although kaolinitisation couldalso have been favoured by meteoric water circulationin connection with the unconformity.

Lower – Mid Jurassic (Upper Toarcian-Callovian)

Upper Toarcian (?and lower Bajocian) sediments coredin the Møre, Froan and Nordland VI areas and repre-sented by outcrops on Andøya, were deposited undersimilar shallow marine conditions related to marinetransgression. These deposits are dominated by sand-stone. The composition of the associated mudstonebeds is kaolinite- and quartz-dominated and also inclu-des K-feldspar or mica/illite (Figs. 7; 12; 13). Thesecompositions can be interpreted as mixing products ofkaolinite-weathered and less weathered feldspathic ormicaceous basement rocks. The kaolinite/feldspar ratiois highest in mudstones north in the Froan Basin wherethese overlie kaolinite-weathered basement. This sug-gests a local kaolinite origin by erosion of weatheredbasement or older kaolinitic sediments, as describedbelow in the northern area too.

Transgressive, shallow marine sandstones of the Mås-nykan Formation in the Nordland VI and VII areas areyounger (Bathonian - Callovian) than the above-des-cribed shallow marine sandstones from off Helgeland,Møre and Froan (Late Toarcian, Smelror et al. 1994)and marine transgressive sandstones of the Stø Forma-tion in the Troms III area (Late-Toarcian - Early Bajo-cian, Smelror et al. 2001). Kaolinite is the main claymineral in the associated mudstone beds that are domi-nated by kaolinite, quartz and mica/illite. These are themain minerals also in the overlying thicker and more

homogeneous, silty mudstone successions of the Melkeand Fuglen formations.

In the Nordland VII area the shallow marine sands weredeposited on top of a 7.1 m thick debris flow unit ofcontinental, possibly lagoonal affinity (Smelror et al.2001), which largely originated from erosion of under-lying kaolinite-weathered basement. The mudstonesare strongly kaolinitic (Fig. 9), and the presence of opal(XRD identification) supports a local origin by feldsparweathering as silica is released as part of one of themain kaolinite forming reactions at the expense of K-feldspar (Bjørlykke & Aagaard 1992). Alternating sandbeds include both well preserved and kaolinitised feld-spar from the underlying basement. The weatheredbasement in this area is thus an obvious source for kao-linite in the Middle Jurassic sediments.

The kaolinite-dominated erosion products were over-lain by sediments of mixed composition showing anupward increase in mica/illite content. This trend sug-gests increasing influence from erosion of less-weathe-red micaceous metamorphic rocks, and thus dilution ofthe kaolinite weathering products with time.

Upper Jurassic - lowermost Cretaceous

The mineralogical composition of the organic-richHekkingen Formation in the north (Nordland VII andTroms III, Figs. 9 and 11) is very similar to the underly-ing Fuglen Formation. Both are dominated by kaoli-nite, quartz and mica/illite. Kaolinite appears to be gra-dually depleted up through the formation in the TromsIII area, and contemporaneously pyrite becomes morecommon.


Fig. 9. Mineralogical variation of shallow cores from Nordland VII. Samples are grouped on the basis of stratigraphy and main lithologicalunits. Abbreviations as in Figure 4.

The time analogous Spekk Formation off Helgeland(Fig. 4) consists mainly of kaolinite, mica/illite, chlo-rite and quartz. Smectite-rich bulk compositions arecommon in the uppermost part of the formation, asso-ciated with cristobalite in the stratigraphically youngestcore (IKU82-C, ?Volgian).

In the Froan Basin quartz, K-feldspar and kaolinite arethe dominant minerals in the lower part, and mixed-layer clay becomes more abundant in the uppermostJurassic and Berriasian beds. A relatively high feldsparcontent of mudstone beds within the sandstone unitmay reflect a similar local provenance of plutonic rocksas for the feldspar rich sandstones (Rogn Formation).The kaolinite ratio scatters widely and shows highvalues in some of the Late Kimmeridgian-Early Volgianbeds. Kaolinite is common also in the Middle andUpper Volgian beds where mixed-layer clay and smec-tite are abundant. Claystone beds dominated by smec-tite and mixed-layer clay occur in the Berriasian inter-val, interbedded with kaolinite, mica/illite and mixed-layer clay rich compositions. In summary, the mixed-layer clay and smectite - rich interval is confined to theMid Volgian - Berriasian part of the stratigraphy whichis also characterised by a reduction in quartz content.

The compositional difference between the Hekkingenand Spekk formations described above is discussed in alater chapter.

Lower Cretaceous

The calcareous Klippfisk Formation and the analogousLyr Formation in the southern area record a change tomore ventilated depositional conditions (Dalland et al.1988; Smelror et al. 2001) and may also be related totectonic uplift and erosion. Kaolinite and mica/illite arethe main clay minerals in the calcareous condensedbeds (Figs. 4; 9; 11). However, it is not known if thekaolinite-dominated clay mineralogy of this intervalresults from telegenesis or from renewed supply fromdetrital sources.The marine Lower Cretaceous claystone of the KoljeFormation is dominated by kaolinite, plagioclase,mica/illite, quartz, mixed-layer clay and chlorite. Thedata show a distinct increase in feldspar across the con-densed beds both in the Nordland VI and VII andTroms III areas. The mudstones in the Nordland VIarea are rich in 7Å clay minerals which are kaolinite-dominated except in the middle part of the core sectionwhich is dominated by a 7Å peak of chlorite (berthie-rine) (Fig. 7). In the southern area (Froan Basin) thecored part of the Lange Formation is rich in kaoliniteand calcite.

Supplementary clay fraction analyses in the Troms IIIarea show a change in the mixed-layer clay composition

from illite-dominated in the Jurassic shales to smectite-dominated in the upper part of the Lower CretaceousKolje Formation (Weiss et al. 1991). This may be theresult of burial diagenesis, i.e. that the lower strata mayhave experienced maximum burial temperatures withinthe illite ordering range (90-100°C, e.g. Srodon & Eberl1984 or higher, e.g. Lynch et al. 1997).

The feldspar content is generally low in the Jurassicmudstones and claystones, and shows a distinct incre-ase across the Lower Cretaceous condensed beds. Exa-mination of the data from Troms III (Fig. 14) showsthat the feldspar increase from the Jurassic formationsto the Kolje Formation is accompanied by a decrease inquartz. If quartz was enriched in the coarse fractions,this could have been due to quartz depletion due to achange in facies. However, this is not the main reason asthe data also show an increase in the ratio offeldspar/feldspar +clay. The marked feldspar increaseacross the condensed beds is thus interpreted to reflecta change in provenance to include new erosion of feld-spathic basement rocks, probably related to onset of rif-ting off Lofoten in the Early Cretaceous (Løseth & Tve-ten 1996, Smelror et al. 2001).

DiscussionUpper Triassic - Lower Jurassic kaolinite origin

The most distinct compositional change with time inthe study area is the change from feldspar andmica/illite rich mudstones in the Lower Triassic to kao-linite-dominated mudstone compositions in the UpperTriassic – Lower Jurassic. In this interval kaolinite is themost common clay mineral also in the sandstones thatalternate with the mudstones. Kaolinite may form byextensive weathering in a humid climate (Srodon1999), or by early diagenesis under conditions of mete-oric water flux to maintain low pH and to remove dis-solved alkali elements. The latter is regarded as themain mechanism also in shallow marine deltaic envi-ronments and in telogenesis (Bjørlykke & Aagaard1992).

Literature from the North Sea basin and from East Gre-enland give evidence of climatic changes from arid andsemiarid in the Triassic to more humid towards the endof the Triassic and in the Lower Jurassic (Hallam 1984and 1985, Hurst 1985, Pearson 1990, Knarud & Bergan1990, Bjørlykke & Aagaard 1992, Clemmensen et al.1999). This implies a change to conditions that favourkaolinite growth around the Triassic-Jurassic boundary.In contrast, illite, or depending on local provenance,smectite, is the dominant detrital clay mineral in LowerTriassic sediments (Pearson 1990, Hurst 1985). Kaoli-nite appearance in shales around the Triassic – Jurassicboundary in Britain was even considered as a possible


stratigraphic marker (Pearson 1990). The climatic changehas been correlated with northward movement of Pang-aea, and was probably also influenced by the geographicspread of the epicontinental sea (Hallam 1984).

In-situ kaolinite weathering of the Scandinavian base-ment is exemplified by kaolinite/quartz assemblages ontop of granodiorites and granites in Bornholm (Callisen1934, Almeborg et al. 1968) and Andøya (north Norway,Dalland et al. 1975, Sturt et al. 1979). Sturt et al. (1979),

on the basis of K-Ar dates, suggested a Carboniferousage for the weathering on Andøya. This interpretationwas adopted by Dalland (1981), who suggested that theCarboniferous weathering surface was later uplifted andexposed for erosion in the Jurassic. This model was laterdisputed, and the Carboniferous ages regarded as arti-facts (mixed ages) caused by Ar-inheritance from pre-cursor mica (Løseth & Tveten 1996). The present resultsgive additional evidence of a Mesozoic age for theabove-metioned weathering (see below).


Fig. 10. SEM backscattered electron image of weathered feldspar-rich gneiss and overlying wacke sandstone, 6814/04-U-01. Scale bar = 100µm.a) Least weathered gneiss with original feldspar (left) enclosed in weathered matrix (178.20 m). b) Close-up of weathered matrix showingintergrowths of kaolinite (dark) and chlorite (light stripes). c) Heavy minerals (light) enriched in kaolinitic matrix (dark) in debris-flow sand-stone (169.82 m).

Fig. 11. Mineralogical variation of shallow cores from Troms III. Samples are grouped on the basis of stratigraphy and main lithological units.Abbreviations as in Fig. 4.


Fig. 12. Mineralogical variation of shallow cores from the Froan Basin. Samples are grouped on the basis of stratigraphy and main lithologicalunits. Abbreviations as in Fig. 4.

Fig. 13. Mineralogical variation of shallow cores off Møre. Samples are grouped on the basis of stratigraphy and main lithological units. Abbre-viations as in Fig. 4.

The Mesozoic stratigraphy of Andøya (Dalland 1981) iscorrelated to that of Troms III and Nordland VII (fig. 5in Smelror et al. 2001). As in Nordland VI the weathe-red basement on Andøya is overlain by kaolinitic sedi-ments followed by transgressive Jurassic sandstones.The kaolinitic clays, separating the basement gneiss andthe oldest sandstones of the transgressive Ramså For-mation at Andøya, contain palynomorphs dating thekaolin pit at Andøya as having been formed during LateToarcian to Aalenian time. Accordingly the formationof the clay initiated deposition of the HestbergetMember on Andøya and was followed by kaolinite-richsediments higher in the Member for which Manum etal. (1991) suggested a Bajocian age.

In the present study we describe kaolinite-weatheredbasement below Jurassic sediments both in the sout-hern (Froan Basin) and the northern (Nordland VII)areas. The kaolinitic beds and related beds show paly-nomorph assemblages of variable diversity, but themajority of the stratigraphically longranging fossils arederived from a terrestrial vegetation. The abundance ofpalynomorphs in the individual samples vary and theyhave different potential for confident correlation anddating. The Late Triassic to Early Jurassic floras areregionally rather uniform because of the wide climaticzones of the Triassic to Jurassic.

In all these areas kaolinite-weathered basement andlocally overlying kaolinitic sediments are overlain bytransgressive, shallow marine sandstones of Late Toar-cian – Aalenian (Froan Basin and Andøya) and Batho-nian-Callovian age (Nordland VII, Måsnykan Forma-tion). The kaolinite-weathered surface (Dalland 1974,1975, Dypvik 1979) was eroded as late as in the lateEarly - Middle Jurassic, giving a minimum age for onsetof kaolinite-weathering. The studied cores from Nord-land VI and Helgeland give evidence that supportsextensive in-situ early diagenetic kaolinite growth inUpper Triassic and Lower Jurassic sandstones andmudstones. The thickests, relatively pure kaoliniticsediments cored are perhaps from off Helgeland. Thekaolinite formation there was in response to both weat-hering and diagenesis, possibly at various places in thedepositional system.

Absence of kaolinite in Lower Triassic deposits and inbasement overlain by Lower Triassic deposits supportsa post-early Triassic age for the main kaolinite weathe-ring. However, this is not an unambiguous argument,as even in a humid and tropical climate the degree ofweathering may vary strongly. Observations from pre-sent day tropical climate (Savage et al. 1988, Savage &Potter 1991) have documented that weathering maycontinue during sediment transport, and that sedimentcomposition preservation depends on the residencetime prior to burial, i.e. deposition rate.

In summary, compilation of mineralogical data fromshallow stratigraphic cores offshore mid Norway showsa characteristic enrichment of kaolinite in Upper Trias-sic – Lower Jurassic clastic sediments. The most kaoli-nite rich compositions are seen in lagoonal to shallowmarine sediments. However, these sediments cannot beconfidently related to one stratigraphic level. Minimumages for the kaolinite weathering are given by the trans-gressive Bajocian-Bathonian sandstones, and in placesby Pliensbachian fine clastic sediments. The preferredconclusion is that conditions were favourable for kaoli-nite growth by weathering and early diagenesis at vari-ous times during the Rhaetian – Pliensbachian interval.Kaolinite in the Upper Toarcian-Aalenian and ?Bajo-cian shallow marine deposits may reflect erosion of theolder kaolinite weathering products as well as early dia-genesis.

Kaolinite/smectite variations in Upper Jurassic - lower-most Cretaceous shales

Upper Jurassic organic rich mudstone compositionsshow both stratigraphic and geographic differences incomposition. The Hekkingen Formation in NordlandVII - Troms III is dominated by kaolinite andmica/illite with minor mixed-layer clay. The Spekk For-mation off Helgeland is smectite-dominated in Vol-gian- Berriasian beds. The Spekk Formation in theFroan Basin is dominated by smectite/mixed-layer clay,and bentonites of smectite and smectite mixed-layerclay occur in the Berriasian section. There is an overalldecrease in kaolinite upwards in the organic rich for-mations.

A decline in kaolinite abundance is described also inthe time equivalent Upper Kimmeridgian Clay of sout-hern England (Wignall & Ruffel 1990). This observa-


Fig. 14. Stratigraphic variation in the feldspar/(feldspar + quartz)ratio and comparison with the stratigraphic variation of felds-par/(feldspar + clay), feldspar and quartz, Troms III area. Note dis-tinct feldspar increase from the Hekkingen to the Kolje Formation.

tion, in combination with several other sedimentologi-cal and paleontological criteria, were attributed to achange from humid to semi-arid depositional conditi-ons in the Upper Jurassic. This may have been part of anotable spread of aridity in southern Eurasia in theLate Jurassic, which Hallam (1994) related to orograp-hic effects.

In the present study we suggest that the smectite andsmectite-mixed-layer clay mineral beds in the Froan-Helgeland cores may rather have their source in con-temporaneous volcanism (bentonites) or erosion ofvolcanic or basic rocks. But, although the Late Jurassic,including Volgian time was a period with rifting at the

Atlantic margin, (Swiecicki et al. 1998), there is so farno other documentation of volcanic activity in this areain the Late Jurassic.

Late Jurassic – Early Cretaceous volcanism is mentio-ned from the North Sea (Latin et al. 1990) and Svalbard- Barents Sea (Parker 1967, Burov et al. 1977, Smith etal. 1976, Dalland & Thusu 1977, Kelly 1988). However,the dates are not very precise, and to our knowledgethere are no such dated volcanics in the Norwegian Sea.Smith & Ritchie (1993) concluded that Jurassic mag-matism in the Central North Sea predated the majorepisode of Late Jurassic – Early Cretaceous lithosphericextension.


Fig. 15. Summary diagram showing main mineralogical changes of mudstones and claystones in a stratigraphic and geographic scheme. Abbre-viations as in Figure 4.


Hansen & Lindgreen (1989) and Lindgreen (1991)noted probable volcanic smectite/illite occurrences inthe Middle-Upper Volgian Farsund and Mandal forma-tions in wells 2/7-3 and W1 in the Central Graben ofthe central North Sea. Units of smectite-richillite/smectite in Kimmeridgian mudstones of theHareelv Formation were interpreted as indicators ofvolcanism in the East Greenland rift basins (Lindgreen& Surlyk 2000). In the Barents Sea, in the area of theMjølnir meteorite impact structure (7430/10-U-01)Dypvik & Ferrell (1998) found an increased abundanceof smectite (up to 30 wt%) in Jurassic-Cretaceousboundary beds. They assumed the smectite to representseawater-altered impact glass from the ejecta blanketmaterial. Similar beds have not been observed in theNordland VII - Troms III areas.

Our preliminary conclusion is that the mineralogicaldifference between the western Barents Sea and Norwe-gian Sea cores in part reflect differences in provenanceand most likely with a greater degree of volcanic inputin the latter. The presence of bentonite beds gives sup-port for contemporaneous volcanic activity. Differencesin thermal and burial history could also explain someof the mineralogical differences, supported by evidenceof deeper burial of the Troms III units. However, wenote that the smectitic beds on the Trøndelag Platformare also very poor in K-feldspar, which would berequired to produce illite-mixed-layer clay from smec-tite. Thus, distinct primary mineralogical differencesmay have existed between the Spekk and Hekkingenformations. One may further speculate if this is relatedto differences in facies, climate, provenance and/orinfluences from volcanism, which may be the subject offurther research.

Lower Cretaceous compositional change

The feldspar and feldspar/kaolinite contents of themudstones and claystones show a distinct increaseupward in the stratigraphy from the Lower Cretaceouscondensed beds to the Kolje Formation in the Nord-land VI and VII and Troms III areas. A similar changein feldspar content is also seen elsewhere in the BarentsSea, including the Nordkapp Basin (unpublished data).This is interpreted to reflect a change in provenance toinclude increased erosion of feldspathic rocks.

Elsewhere, in England and France Rufell & Batten(1990) and Hallam et al. (1991) related changes inmineral compositions to phases with an arid climate inJurassic-Cretaceous boundary beds and in the Barre-mian-Aptian. The changes included lowering of thekaolinite content in such beds. The Kolje Formation is,on the contrary, rather rich in kaolinite and with nokaolinite depletion relative to the underlying formati-ons. The present data do not allow conclusions to be

drawn regarding climatic changes. In the present studyof Cretaceous sediments smectite appears more abun-dant in the overlying Upper Cretaceous ShetlandGroup, including the Kvitnos Formation.

ConclusionsThe main changes in mineral composition of Meso-zoic claystones and mudstones are compared strati-graphically and between different areas (Fig. 15). Someof the most distinct mineralogical changes in the strati-graphic succession in the Norwegian Sea and the south-western Barents Sea can be summarised as follows:

1. Change from mica/illite + chlorite + feldspar +mixed-layer clay rich compositions in the Lower Trias-sic to kaolinite-dominated compositions in the UpperTriassic-lowermost Jurassic.

2. Persistence of kaolinite as the dominant clay mineraland similar mineralogical composition in the Lowerand Middle Jurassic deposits studied (shallow marinesandstones and marine mudstones).

3. Occurrence of kaolinitic weathered basement belowtransgressive shallow marine sandstones of late Early –Middle Jurassic age.

4. Kaolinite decrease and smectite/mixed-layer clayincrease in the Upper Jurassic deposits. Notable mine-ralogical differences exist between the organic richUpper Jurassic Spekk and Hekkingen formations. TheSpekk Formation in the Froan Basin and off Helgelandis rich in smectite and mixed-layer clay minerals parti-cularly in its upper part, whereas mica/illite and kaoli-nite are the most abundant minerals up throughout theHekkingen Formation in theTroms III and NordlandVII areas.

5. Feldspar increases (plagioclase reappearance) abovethe condensed Lower Cretaceous Klippfisk Formation.A general mineralogical resemblance is seen betweenLower Cretaceous and Lower Triassic sediments, exceptthat kaolinite is part of the detrital mineral composi-tion in the former.

Change (1) coincides with changes in facies and climatefrom arid continental to humid continental and deltaicconditions with kaolinite growth in delta plain envi-ronments. Change (2) and observation (3) recordtransgression and increasing marine influences, whichimply mixing of sediments from different sources. Kao-linite may have been derived from reworking and localinfluences from weathered basement as well as earlydiagenetic kaolinite growth. Change (4) may reflectkaolinite dilution due to increasing marine influences

and mixing with other sources. This may also be attri-buted to a change from humid to semi-arid depositio-nal conditions, which followed the regional spread ofaridity in southern Eurasia in the Late Jurassic. Thesmectite ± cristobalite association in the Volgian in thesouthern area is interpreted in terms of volcanic influ-

ences. Change (5) which is most pronounced in thenorthern areas records an increase in sediment supplyfrom basement areas associated with transgression afterthe period of low sedimentation responsible for theLower Cretaceous condensed beds. This renewed sedi-mentation may be related to Early Cretaceous rifting.


Acknowledgements. -The present study is based on both published andunpublished results from four IKU shallow drilling projects in theperiod 1982-1991 and some later follow-up studies. Tor Martin Røn-ningsland was responsible for X-ray diffraction analyses in the initialshallow drilling project (Helgeland 82). We express our acknowledge-ments to all colleagues that contributed in the drilling projects and toHenning Dypvik (University of Oslo) for detailed referee comments.

Mineral Nordland VI and VII, Troms III, Helgeland 82Vøring, Møre

Quartz 4.26 Å *1.0 4.26 Å * 1.0

(3.34 Å * 0.2 if very low quartz)

K-feldspar 3.24 Å *0.5 3.24 Å * 0.5

Plagioclase 3.19 Å *0.5 3.18 Å * 0.5

Chlorite 4.74 Å * 3 * 0.7 14 Å * 0.7

Kaolinite 7Å * 0.7 – Chlorite = 7 Å * 0.7

(7Å-(4.7Å *3)) *0.7

Mica/illite 10Å *1.0 (mica) or 10 Å * 1.4

10Å *1.4 (illite)

Mixed-layer clay 10 to 14 Å * 0.55 11 Å * 1.0

Smectite (14 Å - 4.7 Å) * 0.35 14 – 17 Å * 0.35 Expansion after

ethylene glycol treatment.

Calcite 3.04 Å * 0.25 3.03 Å * 0.5

Siderite 2.79 Å * 0.25 2.79 Å * 0.5

Dolomite/ankerite 2.89 Å - 2.90 * 0.20 (2.88-2.89) Å * 0.5

Pyrite 2.71 Å * 0.7 2.71 * 0.5

Amphibole 8.4 Å * 0.4

Gypsum 7.56 Å * 1.0

Clinoptilolite 8.99 Å * 0.6

Phillipsite 8.14 Å * 1.25

Opal C-T 4.05 Å * 1

Goethite 4.18 Å * 1

Zeolite 9.00 Å * 0.5

Romboclase 9.12 Å * 1

Apatite 2.78 Å * 0.3

Marcasite 2.71 Å * 0.7

Haematite 2.69 Å * 0.5

Halite 2.82 Å * 0.1

Talc 9.33 Å * 0.15

Table A1. Parameters for mineral quantification based on X-ray diffraction analyses using peak-area multiplied by empirical weight factors. Å = lattice spacing measured in Ångstrøm.


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Appendix 1:Analytical method, semiquantitative X-ray diffraction analysis.

Approximately 10 grams of each sample were ground in ethanol in anagate mortar until all solid particles were in the silt fraction or finer.The samples were then dried at 40°C and mounted as unoriented pow-der on sample holders. The samples were analysed by a Phillips X-raydiffractometer at the following conditions: range(2θ) 2°-52°, mono-chromatic CuKα radiation, 40kV, 30mA, and speed 2°/minute. Themineralogical composition was determined by interpretation of cha-racteristic reflections on the X-ray diffractograms. The quantificationof each mineral was based on the product of the peak area (peak heightmultiplied by peak width measured at half peak height) and a weighedfactor relative to quartz for the reflections shown in Table A1. Thequantification keys are experimentally determined from former experi-ence with Quaternary and pre-Quaternary samples at IKU. The smec-tite content is calculated as the difference between two peaks (14Å and4.7Å). Any possible Na-smectite will be classified and quantified asmixed-layer clay, as the main reflection of Na-smectite overlaps withmixed-layer clay. The Fe-rich chlorite berthierine is observed in a fewlocations. Its 7Å peak overlaps with kaolinite, but is distinguished fromkaolinite by lack of 14Å reflection. The mineral contents refers toweight percentages with an error probably not less than ±5% for mainminerals (estimated for 50% quartz).

Identification and quantification of minerals occurring in minoramounts (<2%) are frequently based on a single main reflection, andare therefore uncertain. The main reflection of dolomite interfers witha K-feldspar reflection, and low values of dolomite may have little sig-nificance, especially in samples rich in K-feldspar.

Troms III-fine fraction: The fine clay-fraction (<0.5µm) was separatedin sedimentation cylinders according to Stoke’s law and mounted onmillipore filters (oriented samples) after treatment with MgCl2. Sam-ples were analysed in the 2θ range 2°-35°, and re-analysed after satura-tion with glycol.

Helgeland 82 data: Samples were analysed using a slightly differentapproach (see also Table A1). Samples were run from 2°2θ to 36°2θ,2°/minute goniometer speed. The samples were crushed fine in a mor-tar and suspended in water, before being sieved on a millipore filter,and mounted on a glass slide. The lower limit of detection was approxi-mately 5%.


mesozoic mudstone compositions and the role of kaolinite … · introduction the clay mineral...

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