Oligocene foraminifera from the Val d'Illiez formation (Haute-Savoie, France): refined biostratigraphy and paleoecological analysis

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Oligocene foraminifera from the Val d'Illiez formation (Haute-Savoie, France): refined biostratigraphy and paleoecological analysis

UJETZ, Bernhardt, KINDLER, Pascal, WERNLI, Roland

Abstract

Un affleurement de la Formation du Val d'Illiez (Lateltin, 1988) a été étudié en détail dans le but de préciser l'âge et le milieu de dépôt de cette unité, encore très controversés. La coupe étudiée présente deux faciès sédimentaires distincts: (1) à la base, un intervalle épais d'environ 40 mètres constitué de minces lits de marnes apparemment homogènes; (2) au sommet, un ensemble plus épais (60 mètres) formé de marnes schisteuses, de grès et de conglomérats, et présentant de nombreuses structures sédimentaires typiques des dépôts turbiditiques (séquences de Bouma, "flute casts", etc.). La découverte de Globigerina ciperoensis ciperoensis et de Globigerina ciporoensis gnauki permet de dater la Formation du Val d'Illiez de la partie inférieure de l'Oligocène moyen (zone à Globigerina ampliapertura, Bolli et Saunders, 1985). L'intervalle marneux inférieur contient en outre des foraminifères planctoniques, rares et mal conservés, attribuables à des espèces du Crétacé supérieur, du Paléocène supérieur, de l'Eocène moyen et supérieur (e.g. Marginotruncana, Acarinina, [...]

UJETZ, Bernhardt, KINDLER, Pascal, WERNLI, Roland. Oligocene foraminifera from the Val d'Illiez formation (Haute-Savoie, France): refined biostratigraphy and paleoecological analysis.

Revue de micropaleontologie , 1994, vol. 37, no. 4, p. 275-287

Available at:

http://archive-ouverte.unige.ch/unige:154344

Disclaimer: layout of this document may differ from the published version.

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REVUE DE MICROPJ\LEoNTOLOGIE Vol. 3 7, n° 4 , decemhre 1994, pp. 2 75-287

OLIGOCENE FORAMINIFERA

FROM THE VAL D'ILLIEZ FORMATION (BAUTE-SAVOIE, FRANCE) REFINED BIOSTRATIGRAPHY AND PALEOECOLOGICAL ANALYSIS

LES FORAMINIFERES OLIGOCENES

DE LA FORMATION DU VAL D'IUIEZ (BAUTE-SAVOIE, FRANCE):

PRECISIONS BIOSTRATIGRAPHIQUES ET PALEOECOLOGIQUES

by

Bernhardt UJETZ*, Pascal KINDLER* and Roland WERNLI*

ABSTRACT. - One outcrop section of the Val d'Illiez Formation ("Formation du Val d' llliez": Lateltin, 1988) was studied to define the foraminiferal biostratigraphy of this controversial unit and to interpret its depositional environment. The section consists of two main sedimentary facies: (1) a 40 m-thick lower interval of thinly bedded, apparently homogenous shales, and (2) a 60 m upper interval composed of shales, sandstones and conglomerates, displaying numerous sedimentary structures typical of gravity flow deposits (e.g. Bouma scqucncc·s, nutc C11$15, etc.). T he cnti1·c section is 11LL1;hutcd to the lower rnicld lc Olii;oc11nc, coi-rcsponding to the Globigarinn

"mplicq)(Jrlarn plunktoni<: foraminife r11I 7.0 llC (Bolli nncl Snnndcrs, 1985) bnsetl on the 1wcst111cc of Globigeri1111 ciperoensis d peroemis uurl Clobi.geri.11a 011t1c/1i1.ae11.sis [:,'1UU1ki. Rewor king wns idonlified in the l<Mcr shnly iutcr \'n l us suggcstlld by the occur-rc11ce of rnrn, pond)' p1·escr vccl lute Cretaceous, hu e: Polcocc.ne, ca l'l y Eor:cn.e und midd le E<.>ccnu plnnktonic fo rnrnin.ifon; (i.e. Mcirgi11ol1·1111ca1w, Acari11i1w, iYlorozo·uelfo, T'/1111ororali1es, 'lltrborornlio carroa:rnleusis b'l"Oup). A t:lmnge in the dcpoiilLional covin111rncn1

or

^{the Vul cl'Ulim}

Formation occurred from normal, open marine, outer neritic to upper hnthyu'I conclilions (100-SOO rn) in the lower inter val, to ver y low oxygenated, upper middle bathyal conditions (500-1 300 m) in the upper interval.

RESUME. - Un affleurement de la Formation du Val d'Illiez (Lateltin, 1988) a ete etudie en detail clans le but de preciser !'age et le milieu de depot de cette unite, encore tres controverses. La coupe faucl ice prescnti: d1:ux facies sedimentaires distincts: (1)

a

^la

base, un intervalle epais d'environ 40 metres constitue de minces !its de mnrncs apparcmmcnt homogenes; (2 ) au sommet, un ensemble plus cpuiS (60 metres) ronnc d e mnrm~o schistcusCS, I.le grCs Cl de co11glontOl'll t.S, IH j)l'CSentunt de no rnhn"t5C~ Structurus scdimcotail'cs typicrucs <l e.~ depots turliiditiques (Scl1ucmccs do Bouma, " nutc 1:11sts", etc.). Lu dccouvcrti: de Clobigel'i11.11 ci11ero1msis ciperoa11sis ot de Clobigarina ciporol!llsis g11au/.·i pe,.mct de elater In Formnti<>n du Val d 'Illfo·.i: 1!r. la purtie inferic1.11·e clc l'O.ligoe<'!n1i moycn (zone i\

Clobigeritut <WtfJlhtperltLra, Bolli cl S:mnd c r~ , J 985 ). lJintcr volk· rnam eux infcricur conlicnt 0 11 outrc ''"~ fora mir1 i1Crcss planctoniqu cs.

rares et mnl consc:rvl'!). uttrilm ablcs 3 des cspccos du Cretacc supt'.irieur, du PaJ(,oconc u1.1p.Jric11r, cJc l'Eor.ene mo)•cn 1ll supcricur (e.g.

Mnrgi11otrum:a na, llcari11 i1w , Mo rozouelki , P/(l.llorolnlitcs, groupc de 1\irborouilia carroa.::11le11sis). Cct nsscmblugc hfacroclitc im liquc

l('Ue, Im d6pil de son llSfJCCl homogcnc, l'intl!l'VnlJc infcrielll' l'Csulte egulernC'llt d'une resedimentation. L'analyse micropaleonl(>[ogique indiquc un changr.ment d'cn,•iro110cr:ncnt pendont le de11iit. de lu Formation du Val d 'Illiez. En effet, les sediments de l'intcn ulle inferieur se sont accumules dans des conditions de mer ouverte, dans une zone neritique externe

a

bathyale superieure (profondeur 100

a

500 m}, alors que ceux de l'intervalle superieur se sont deposes dan s la zone bathyale moyenne (profondeur 500-1 300 m), en milieu tres peu oxygene.

Key-words : Fora1ninifera - Biostratigraphy - Paleoecology - Oligocene - Reworking.

Mots-cles: Foraminiferes - Biostratigraphie - Paleoecologie - Oligocene - Remaniement.

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* Universite de Geneve, Departement de Geologic et Paleontologie, 13, rue des Mara!chers , CH-1211, Geneve 4, Suisse.

INTRODUCTION REGIONAL SETTING

The Bellevue section (Lambert coordinates : X908'100-Yl40'950, alt. 980 m) is situated 20 km SE of Geneva in the Voirons Massif located in the Chablais Prealps, Haute-Savoie, France (Fig. la-le). According to previous regional studies, this massif is composed of four vertically stacked tectonic units (from bottom to top, Fig. 1 b) : (1) autochtonous molasse, (2) subalpine (thrusted) molasse, (3) prealpine melange (Plancherel, 19 7 9) which includes slices of the Val d'Illiez Formation and (4) the Gurnigel Nappe of South Penninic origin (Caron, 1976; Caron et al., 1980; Stuij- venberg, 1979). This nappe includes the "Gres des Voirons" (Voirons sandstones) which are petro- graphically different from the "Gres du Val d'Illiez"

(Val d'Illiez sandstones, hereafter) (lvaldi, 1989).

The studied section exposes a 100 m-thick slice of the Val d'Illiez Formation carried along the basal

LEGEND

thrust plane of the Gurnigel Nappe. The Bellevue outcrop is similarly positioned to the Cucloz section in the Swiss Prealps (Mornod, 1949; Weidmann et al., 19 8 2 ). The Bellevue exposure was first studied by Lombard (19 3 7) who focused on the petrography of the sandstones and conglomerates and suggested a Priabonian age from the occurrence of Nummulites fabiani. Based on sedimentological observations, Carletti (1987) concluded that sediments from the Bellevue section were deposited in a deltaic environ- ment. The same author also found reworked foraminifers from one washed sample, but he did not study the foraminiferal assemblage in detail.

LITHOLOGY

The "Formation du Val d'Illiez" (Lateltin, 1988;

Val d'Illiez Formation hereafter) includes a succes- sion of marls, shales, sandtones and conglomerates and is thought to represent an early Oligocene filling phase of the North Apine foreland basin (Homewood et al., 1986 ). This unit has been studied by several

j'. '. '.;

l

Molasse Basin

B

S<!lli111~11ta1y cover of external massifs roads (fig. C)

ITITill1

lliiliillJ

~

Gumigel Nappe Ultrahelvetics and Prealpine Melange

ffi]

w

basement of external massifs Middle and.

Upper Prealps thrust planes (figs. B & C)

Fie. 1. - Geographical and geological setting of the Bellevue section.

Situation geographique et geologique de l'affleurement de Bellevue.

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UJETZ, KINDLER and WERNLI 277

generations of geologists and has received countless local names (e.g. "Gres de Taveyannaz du Val d'Il- liez": Vuagnat, 1943; "Flysch des Dents du Midi":

Ducloz, 1944; "Formation de Cucloz" : Mornod, 1945; "Flysch subalpin" : Mornod, 1949; "Flysch parautochtone"; Badoux, 1972; North-Helvetic Flysch : Triimpy, 19 8 0 ). It may also be considered as the lowermost facies of the Lower Marine Molasse (Diem, 1986; Homewood et al., 1986; Lateltin, 1988). These deposits have been incorporated into the Alpine orogenic belt and are now found within the Helvetic nappes and as tectonic slices below the frontal thrust plane of the Prealps (Fig. 1 b-c).

Lateltin (1988) subdivided the Val d'Illiez For- mation into three members : (1) the "Marnes laminees" which consist of grey shales with rare silt or sandstone laminae of millimetric to centimetric scale, (2) the "Schistes marno-micaces" which are composed of intercalations of centimetric and decimetric-scale fine sandstones and silty marls, and (3) the Val d'Illiez Sandstones which include decimetric to plurimetric-scale beds of sandstones and conglomerates. These sandstones and conglo- merates contain characteristic radiolarite, limestone, andesite and ophiolite fragments whose origin re- mains controversial (Homewood and Caron, 1982;

Vuagnat, 1983; Lateltin, 1988; Caron et al. , 19 8 9). Thermoluminescence analysis of quartz grains suggests that the terrigeneous components of the Val d'Illiez sandstones have been altered by alpine metamorphism (lvaldi, 1989).

PREVIOUS WORK

Detailed micropaleontological studies of the Val d 'Illiez Formation are sparse and contradictory probably due to the presence of reworked material.

In the Voirons region, Van Stuijvenberg and Jan du Chene (19 81) designated a late Eocene age for the North-Helvetic Flysch based on calcareous nan- nofossil (zone NP19 or younger, Fig. 3) and paly- nological assemblages. In contrast, Weidmann et al.

(1982) recovered an early Oligocene age based on palynomorphs (Pl 9 /P2 0 a : Hochuli, 19 7 8) from the Cucloz section in the Romand Prealps. They reviewed one sample from the work of Corminbreuf (1959) which hosted a planktonic foraminiferal assemblage pertaining to zones Pl 8 -P20 (Blow, 19 6 9). From the Bornes Massif, Charollais et al.

(19 7 5) designated an early Oligocene (zone NP2 l) age based on nannofossils and Charollais et al.

(1988) defined a middle Oligocene age based on the occurrence of planktonic foraminifers in a shaly unit underlying the "Schistes marno-micaces". Mercier de

Lepinay and Feinberg (1982) also recovered nan- nofossils (zone NP2 l) from the Val d'Illiez Forma- tion in the Haute-Giffre Massif.

Lateltin (1988) interpreted the Val d'Illiez For- mation as turbidites which were deposited below the wave-base, at depths between 200 and 400 m.

Weidmann et al. (1982) and Carletti (1987) also suggested that the Val d 'Illiez Formation was deposited below the wave-base but further recog- nized deltaic sedimentation. Charollais et al. (1988) collected ostracods from the "Schistes marno-mi- caces" and deduced shallow water, lagoonal deposi- tion.

MATERIALS AND METHODS

For this study, 35 shaly samples were selected, washed and sieved for foraminiferal analysis. Sam- pling was concentrated in the lower shaly interval (Fig. 2 ). A further 2 5 indurated sandstone samples were taken for petrographic and foraminiferal thin- section analysis.

Shaly samples were crushed and placed in boiling water for 30 minutes. A small amount of Calgon (a mud dispersant) and detergent were added to cause further disaggregation. After boiling, samples were wet sieved through 1 0 0 0 µm, 12 5 µm and 6 3 µm sieves and were retained for sediment and micro- fossil examination. The > 125-1 OOO µm residues were placed on counting trays and, when possible, about 2 00-2 50 foraminifers were systematically picked from each sample residue to provide planktonic/benthic test ratios and species diversi- ties. In this study, the diversity of foraminiferal assemblages was measured by the Fisher

a

index, which takes into account the number of both planktonic and benthic species present among a cer- tain number of individuals (Murray, 1973, 1991).

Further, the percentage of reworked foraminifers was predicted by counting the number of pre-Oligo- cene, Eocene-Oligocene and strictly Oligocene planktonic forms in each assemblage. Foraminifers were identified to species level to determine assem- blage characteristics, age zonation and to interpret the depositional environment.

Secondary electron images of selected specimens were obtained using a 6 400 Scanning Electron Microscope at 15 -3 0 kV accelerating voltage and 20-50 nm spot size. These were prepared by mount- ing specimens on SEM aluminium stubs with double- sided sticky tape and coating the stubs with carbon and gold in a vacuum evaporator.

278

(m) 100

80

60

40

20

0

OLIGOCENE FORAMINIFERA FROM SAVOIE samples Key

UB56

Lithology and facies type

UB50 I'"' -- --

_.

Tt"" .,,.,

_L

UB30 i:=a·:.····~···(·'' UB27:::i ;:

UB24

UB15 UB14 UB13 UB12

UBll

UBlO

UB9 UBS UB7 UB6 UBS UB3

UBl

0

^sandstone

f ^~~~~

conglomerate

II

^shale

Distribution of planktonic forams

II

^Oligocene planktonic forams

~

Eocene-Oligocene planktonic forams

0

Planktonic

forams Foraminiferal species diversity

25 50 ^% 75

I

^{2 4 6 8}

^a II

Pre-Oligocene

planktonic forams

!¥itil

Indeterminate

FIG. 2. - Simplified log of the Bellevue section illustrating lithology and sedimentary facies types, and the foraminiferal composition of some key samples. See text for description of Facies types 1-5.

Coupe simplijiee de l'affleurement etudik illustrant I.a lithologie, les facies sedimentaires et le contenu de que/,ques echontillons importants.

Voir le texte pour une description des facies sedimentaires 1 a ^5.

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UJETZ, KINDLER and WERNLI 279

RESULTS

SEDIMENTOLOGY AND PETROGRAPHY

The Bellevue section consists of the following five sedimentary facies : (I) thinly bedded shales, (2) alternation of thin shale and sandstone beds, (3) amalgamated sandstone and conglomerate beds, ( 4) conglomerate beds, and (5) massive sandstones beds (Fig. 2 ) .

The base of the section is characterized by the presence of homogenous grey shales that form I to 2 cm thick beds (I). Although siltstone and sand- stone intervals are clearly absent, muscovite-rich laminae can be observed within these beds. The thinly bedded shales ("marnes gris souris"; Carletti, I 9 8 7) do not present any field evidence of rework- ing. The thinly bedded shales are unconformably overlain by an alternation of thin sandstone and shale beds (2 ). It is still unclear whether this un- comformity results from purely erosional processes (channel) or from synsedimentary faulting. This sec- ond facies ("alternances schisto-greseuses"; Carletti I 9 8 7) can be interpreted as a distal turbidite deposit on the basis of truncated Bouma sequences (Ta-b) and load structures. An oblique erosional sur- face separates the thin shale and sandstone beds from overlying amalgamated sandstone and conglo- merate beds (Ta/Ta-b) formed by successive turbid- itic events (3 ). Sole marks are also common in this facies. The remaining part of the outcrop es- sentially consists of conglomerate and massive sand- stone beds ( 4 and 5). Conglomerate beds usually exceed I m in thickness and possess a lower erosional surface. These beds are composed of a sandy matrix and pebbles of variable size (0 .5- 5 cm), although a few mud clasts reach up to I m in length. This facies corresponds to the submarine fan lithofacies A of Howell and Normark (I982) and probably results from debris-flow deposition.

The massive sandstone beds may be up to 3 m thick and include large clasts along their basal scour sur- face. Thin shaly layers may occur between massive beds. This facies is similar to submarine fan facies B (Howell and Normark, I 982) and could reflect hy- draulic grain flow processes. The uppermost interval of the Bellevue section consists of amalgamated sand- stone and conglomerate beds (3 ).

The sandstone beds from the Bellevue section essentially contain quartz and feldspar grains, igneous lithoclasts and fragments of andesite and diabase in a calcitic cement. Preliminary petro- graphic analysis demonstrates that the percentages of basic volcanic debris are similar to the Val d'Illiez sandstones studied by Sawatzki (I 97 5 ).

EVIDENCE OF REWORKING

In this study, we used the planktonic forami- niferal zonations established by Caron (I 9 8 5 ), Toumarkine and Luterbacher (I 98 5 ), Bolli and Saunders (I985) for the Cretaceous, Paleocene- Eocene and Oligocene-Holocene respectively. It is possible to determine the presence of reworked planktonic foraminifers using the known strati- graphic ranges derived from these zonations. In contrast, the first and last occurrences for Tertiary smaller benthic foraminifers have not yet been globally well defined in the literature. Therefore, reworked Tertiary smaller benthic foraminifers are more difficult to recognize and are presumed rather on the degree of preservation and the occurrence of coexisting reworked planktonic foraminifers.

Evidence of reworked material from the Bellevue section is suggested by the presence in some of our samples of rare, poorly preserved Marginotruncana, Acarinina, Morozovella, Planorotalites and Tur- borotalia cerroazulensis species, from the late Cretaceous, late Paleocene-early Eocene, and middle-late Eocene (Pl. I, fig. I 7 -2 0 ; Pl. 2, fig. I- 4 ). These reworked specimens are generally fer- ruginised and orange-yellow in colour, as opposed to non-ferruginised dark coloured, pyritised, autoch- thonous tests.

The percentage of reworked planktonic foraminifers is minimal in all of the samples. How- ever, the highest percentages were observed in the lowermost samples of the Bellevue section. We cal- culated that the percentage of obviously reworked forms decreases from about I 5 % at the base of the section (UBI) to 0 % towards the top of the thinly bedded shales (UBI3) (Fig. 2). The degree of preservation also correspondingly increases.

Subsequently, in overlying samples (UBI3, UBI4, UBI 5 ), no pre-Oligocene forms were encountered.

These assemblages contain an abundance of dis- tinctly large, well preserved, stratigraphically long ranging forms and an absence of small, acarinate, poorly preserved forms.

The proportion of stratigraphically long ranging Eocene-Oligocene forms remains high in all of the assemblages. However, the quality of preservation for these forms is higher for those recovered from the samples which contain no recognizably reworked foraminifers. The percentage of foraminifers which were unidentifiable due to poor preservation, is also considerably higher in the lowermost samples further suggesting the likelihood of reworking. From the upper Val d'Illiez Sandstones interval, rare reworked larger Foraminifera (Discocyclina, Num-

280 OLIGOCENE FORAMINIFERA FROM SAVOIE mulites) and rare reworked smaller Foraminifera

(globigerinids, textulariids and rotaliids) were re- cognized in very coarse grained, sandy, and shaly horizons respectively. However, sample UB5 0 pro- vided a well preserved autochthonous foraminiferal assemblage . Foraminifers from this sample within the Val d'llliez Sandstones and the uppermost three samples (UB13, UB14 and UB15) of the lower thinly bedded shales interval were used to interpret the depositional environment of the Val d'Illiez For- mation (Fig. 2).

PLANKTONIC FORAMINIFERAL ASSEMBLAGES AND AGE DETERMINATION

The foraminiferal assemblages from the Bellevue section are predominantly composed of small and stratigraphically long ranging Paleogene Globige- rina, Globorotalia and Globigerinita species. The occurrence of long ranging species (Globigerina officinalis; Pl. 2, fig. 4-5 ; Globigerina eocaena, Globigerina corpulenta, Globigerina venezuelana;

Pl. 2 , fig. 6-7 ; Globigerina tripartita, Globigerina angiporoides, Globigerina praebulloides, Globoro- talia opima nana; Pl. 2, fig. 8-9, Catapsydrax dis- similis) emphasizes the difficulties in accurately dating Paleogene sediments. Some of these taxa exist throughout the Eocene and continue through to the end of the Oligocene. Globigerina officinalis is the most common species which was found in large pro- portions in all samples throughout the Bellevue sec- tion. The percentage of planktonic forms in foraminiferal assemblages from samples UB 13, UB14 and UB15 are high, exceeding 80 % (Fig. 2).

Planktonic foraminifers from these samples are also

distinctly larger in comparison to partially reworked assemblages in underlying horizons (UB1-UB12 ).

The percentage of planktonic foraminifers in sample UB50 from within the Val d'llliez Sandstones is rela- tively low (20 %) (Fig. 2).

Sediments throughout the Bellevue section are attributed to the Globigerina ampliapertura planktonic foraminiferal P2 0 zone (Bolli and Saunders 1985, Fig. 3 ). This age is determined by the concurrent occurrence of Globigerina ciperoen- sis ciperoensis, Globigerina ouachitaensis gnauki (Pl. 2, fig. 12-13) and Globigerina ampliapertura (Pl. 2, fig. 14-15) in sample UB50, positioned near the top of the section, within the Val d'Illiez Sandstones. Globigerina ciperoensis ciperoensis, Globigerina "ciperoensis" (Pl. 2, fig. 16-17), Glo- bigerina ouachitaensis gnauki, Globigerina oua- chitaensis ouachitaensis (Pl. 2, fig. 10-11) and Globigerina ampliapertura were also recovered from samples UB5, UB6, UB8, UB14 and UB15 towards the base of the outrop ; therefore confining the entire section to the Globigerina ampliapertura zone. That is, the lowermost part of the middle Oligocene fol- lowing Bolli and Saunders (1985 ). It should be noted that this foraminiferal zone is considered as the base of the early Oligocene by Haq et al. (198 7) (Fig. 3 ).

BENTHIC FORAMINIFERAL ASSEMBLAGES

Benthic foraminifers represent a small percentage of the total foraminiferal assemblage in samples UB13, UB14 and UB15, which were taken from the uppermost 10 metres of the thinly bedded shales. Uvigerina (Pl. 1, fig. 1-2), Bolivina and

PLATE 1

a = apertural view (vue aperturale), I = lateral view (vue laterale), p = peripheral view (vue peripheri,que), s = spiral view (vue spirale), u = umbilical view (vue ombilicale).

1. Uvigerina sp., 1, UB15, X 140 11. Nonion sp., I, UB15, x 200 2 . Uvigerina sp. , a, UB15, x200 12. Nonionella sp., p, UB50 , X 250 3. Bulimina sp., I, UB50, X 160 13 . Nonionella sp., u, UB50, X 200 4. Ru.limina sp., a. UB50, x 400 14. Nonionella sp., s, UB50 , X 200

5 . Bulimina pupoides, l, UB50 , X 140 15. Chilostomella cylindroides, 1, UB50, x 140 6. Bulimina pupoides, a, UB50, X 140 16. Chilostomella cylindroides, a, UB50, x 180 7 . Cibicides sp., I, UB15, x 120 17. Marginotruncana sp., 1, UB3, X 140 8 . Cibicides sp., s, UB15, x 105 18. Margirwtruncana sp., p, UB3, X 170 9. Cibicides sp., p, UB15, X 130 19 . Morozovella aequa, p, UB8, X 220 10. Nonion sp., p, UB15, X 250 20 . Morozovella aequa, u, UB8, X 120

• "

::v t=l < c::

s

t'l

t'l ...i ,N ::>:: 1::1 L-'l

2

~ (=) ::0

1::1 r ~ :::::: 0 "O

" "

;;... c.. t"" i:::::l' 0 ~ ('; 2 ::v ...i 0 t"" 2

c

0

S3

t=l < 0

r .,,

-J ~ *""

282 OLIGOCENE FORAMINIFERA FROM SAVOIE

HAQ et. al BOLLI and SAUNDERS MARTINI

31- 32- 33--- 34- 35-

3~

37- 38-

(MA)

1987 1985 1971

(Planktonic Foraminifers) (Calcareous Nannofossils)

w Globorotalia opima opima P21/N2

z w w z w S. predistensus

0 z ⁰

0

<

⁰

CJ ::i ^CJ

::i w ^::i

0 ^Cl. 0

> ^:::> w ..J a: ..J

a: ^Q

< ^Q Globigerina ampliapertura P20/N1

w

:E

H. reticulata

E. subdisticha w z >z ..Jw w Cassig. chipolensis/ P19

z

<

a:o

w ₀

z

^<o Pseudohast. micra S. pseudoradians

0 wS?

0 P18

w m ^..J

w < ⁰

I-

a:

:3

^Cl. _EOCENE ^LATE Turborotalia cerroazulensis P17 I. recurvus

FIG. 3. - Bolli and Saunders (1985) planktonic foraminiferal zonation was used for this study.

This zonation differs chronostratigraphically from the Haq et al. (1987) time scale. Martini's (1971) nannofossil zonation is given for comparison. Modified after Lateltin (1988).

NP23

NP22 NP21 NP20 NP19

Dons cette etude, fl 011$ ILtlQ flS utilise '" zo11.(ttio11 cle Forami11ife1·es plrmt:to11iq11.es pmposce por Bolli al S(lrt.nders ( 1985 ).

Po"r l'i111e.runlle de temp.< cmrsi<ltirii, ce11,e zo11atioll n c correspon<l pas e:1:a ctame11t 11. I.a c/1ro110/ogic cle lla.q et al. (J 98 7 ).

La zo11at.i.on de Martini (1971 ), basee sur los 11.amiofo.1silcs, esr. pr6se1114e <I titre c/a comparai$011. O'oprus l.«teltin (1988), modifiii.

PLATE 2

a = apertural view (vu.e aperturale), 1 = lateral view (vue latiirale), p = peripheral view (vu.e periphiirique), s = spiral view (vue spirale), u = umbilical view (vue ombilicale).

1. Acarinina nitida, u, UB9 , X 18 5 2. Acarinina spinuloinjlata, p, UBS, X 170 3. Acarinina spinuloinflata, u, UB8, ^X 160 4. Globigerina officinalis, u, UB15, X 175 S. (;lohigerina officinalis, p, UB15, X 200 6. Globigerina venezu.elana, p, UB5, X 140 7. Globigerina venezu.elana, u, UB5 , X 12 0 8. Globigerina opima nana, p, UB50, ^X 160 9. Globigerina opima nana, u, UB50, X 120

10. Globigerina ouachitaensis ouachitaensis, p, UB15, X 250 11. Globigerina ouachitaensis ouachitaensis, u, UB5 0 , X 2 5 0 l ?. (;/,-,hie;~ri.nn. mur.r.hitrum.~~~ gna11ki, u, UB5 0, x 15 5 13 . Globigerina ouachitaensis gnauki, p, UB50, X 190 14. Globigerina ampliapertura, p, UB6, X 140 15 . Globigerina ampliapertura, u, UB6, ^X 125

16 . Globigerina "ciperoensis" giant individual, u, UBI 5, X 100 17 . Globigerina "ciperoensis" giant individual, p, UB15, x l05

< c ::::! '-< trl trl ....,

2

,N trl :;:: t:I ~ trl ;::; ;:!: r 171

n

::::! ::::! 0 "' :i "O "-;J> r ::;;] t=:l' Fl 0 ::::! :z :z -"I

t:

0 r 0 ~ ~ < 0 ~

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-J :z

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284

Cibicides (Pl. 1 , fig. 7 -9) commonly occur in these samples. Rare specimens belonging to the Nodo- sariidae (Nodosaria, Fissurina, Nonion, Eponides and Bulimina) were also recovered.

In shaly horizons within the Val d'Illiez Sand- stones, rare and poorly preserved rotaliids, miliolids and textulariids were recovered. One altered speci- men of Nummulites was identified in thin section from sample UB2 4 . Rare reworked individuals of Discocyclina were identified in thin section from coarse sandstone and conglomerate beds (samples UB27, UB30, UB56).

Sample UB50 contains a rich benthic forami- niferal assemblage of low diversity. Specimens are pyritised and well preserved, suggesting the absence of reworking. Most tests have been partly removed probably as a result of post-depositional dissolution.

Further, the absence of pre-Oligocene planktonic foraminifers in this sample suggests that the benthic foraminifers have not been rewo1·ked. Bulimina sp.

(Pl. 1, fig. 3-4 ), Bulimina. pupoides (Pl. 1, fig. 5-6), Chilostomellu cylindroicles (Pl. 1, fig. 15-16) and Nonionella sp. (Pl. 1, fig. 12-14) were retrieved from this residue. Bulimina pupoides (Pl. 1, fig. 5-6) specimens represent approximately 9 0 % of the entire foraminiferal assemblage from UB50.

These specimens are small, smooth-walled and non- ornamented.

FORAMINIFERAL SPECIES DIVERSITY

Foraminiferal assemblages from the thinly bedded shales (UB13, UB14 and UB15) show relatively high foraminiferal species diversities (a 7 , a 3 .5 , a 7 .5) in comparison to sample UBS 0 (a 1.5) from the upper Val d'Illiez Sandstones interval (Fig. 2 ).

DEPOSITIONAL ENVIRONMENT

SEDIMENTOLOGICAL EVIDENCE

Sedimentology is of limited help for interpreting the depositional environment of the Val d'Illiez For- mation. Sedimentary structures observed in the Bel- lfwnr. Rr.r-tion, within the sandstone and con~omerate

beds (e.g. Bouma sequences, flute casts),' indicate deposition by turbiditic cunents but do not yield fw·ther precision regarding the depositional environ- ment. The vertical succession of sedimentary facies from the lower shaly iuterval to the upper Val d'Il- liez Sandstones interval, can be interpreted as a

shallowing-upward sequence deposited in a deltaic environment (from pro-delta to delta front : Carletti, 198 7 ). However, the same succession of facies also occurs in deep-sea fans, where coarse middle and upper fan elastics prograde over fine-grained mate- rial deposited on the lower fan (Howell and Nor- mark, 1982). The absence of wave or tide-induced sedimentary structures as well as the lack of con- tinental debris seem to indicate deep-sea fan sedi- mentation.

MICROPALEONTOLOGICAL EVIDENCE

Analysis of the environmenta l couclilions (e.g.

bathymctt·y, salinity uncl dissolved oxygen levels) is based on the distribution palterns and functional morphology of both planktonic and benthic foraminifers. The distribution of modern benthic foraminifers provide analogues which may be used to reconstruct the depositional conditions of Caino- zoic sediments (Murray, 1983 ).

The high foraminiferal species diversities occur- ring in assemblages from the upper part of the lower shaly interval (samples UB 13 , UB 14 and UB 15) are indicative of open marine conditions with normal levels of salinity ( 3 2 -3 7 %0) and oxygen (> 1 ml/I) (Murray, 19 7 3, 1991) (Fig. 2 ). Further, the high percentage (> 8 0 % ) of large planktonic foraminifers in the thinly bedded shales interval supports open marine conditions with normal salinity ancl oxygen levels on the upper continental slope (Mm-ray 1976; Boltovskoy and Wright, 1976) (Fig. 2).

The presence of Uvigerina, a well studied bathyal benthic foraminifer and the absence of miliolids and other well defined neritic benthic foraminifers further suggests that deposition most' probably occurred at depths greater than those from the continental shelf. The occurrence of Globigerina ciperoensis ciperoensis and large (- 5 0 0 µm) Globigerina "ciperoe11sis" (Pl. 2, fig. 15-1 7), which possibly represent a new taxon, may indicate warm water conditions (Spezzaferri and Premoli Silva, 1991).

The extremely low foraminifernl species diversity (a 1 .5) recorded from sample UBS 0 in the upper Val d'Illiez Sandstones interval of the Val d'Illiez Formation could indicate a depositional environ- ment restricted by stagnation of bottom waters or with shallowing (Haynes, 1981) (.l"ig. ~ ). However, the presence of small (high sm·face to volume ratios) elongate, uniserial to triserial .benll1ic foraminlicL·s in this sample most likely results from low oxygen conditions rather than shallowing (Bernhard, 1986;

Kaiho, 1991; Sen Gupta and Machain-Castillo,

~

'

UJETZ, KINDLER and WERNLI 285

1993). Also the presence of large planktonic foraminifers, which occupied the water column over- lying the oxygen minimum zone, further suggests the possibility of a deep marine environment (Murray, 1976).

The Bulimina and Chilostomella cylindroides specimens retrieved from sample UB5 0 are smooth- walled and non-ornamented. It has been commonly recognized that pores in the test wall of calcareous foraminifers act as conduits for gas exchange. It follows that foraminifers with large and numerous pores are strategically adapted for low oxygen en- vironments (Perez-Cruz and Machain-Castillo, 1990). However, Verhallen (1991) suggested that a porous internal passage or "central canal" helps gas exchange in anaerobic-tolerant, non-costate, smooth- walled species of Bulimina from Mediterranean Neogene sediments.

Sediments in oxygen minimum zones support a low-diversity, high-dominance foraminiferal fauna, typically with 2 or 3 species forming up to 80 % of the foraminiferal assemblage. Generally a rela- tively small number of opportunistic species become conspicuously dominant in bathyal oxygen minimum zones. These species can utilize the trophic resource of increased organic loads generally associated with dysoxia (See Gupta and Machain-Castillo, 1993 ). As most species are unable to tolerate low oxygen envi- ronments, the reduced competition allows a few adapted species to colonize this type of environment.

Thus, high dominances of a few small, hyaline, thin- walled, unornamented species (e.g. Bulimina) are typical of bathyal oxygen minimum zones (Sen Gupta and Machain-Castillo, 1993 ).

Faunas of oxygen minimum zones (oxygen

< 0 .1 ml/I) at around 1 OOO m water depth consist mainly of elongate-conical species belonging to Bulimina (Ingle and Keller, 1980). Luterbacher (19 84) studied Paleogene sediments from the south- ern Pyrenees and showed that large numbers of Bulimina were found only in turbidite facies where they occur mainly in the proximal facies at bathyal depths. Miiller-Merz and Oberhansli (1991) also recognized bathyal faunas containing high percent- ages of buliminids from the South Atlantic ocean.

Quinterno and Gardner (19 8 7) have also studied upper middle bathyal assemblages (500-1 300 m) from northern California with well developed oxygen minima (< 0.5 ml/I dissolved oxygen). These assem- blages were characterised by abundances of Bolivina, Bulimina, Buliminella and Uvigerina and closely resemble the benthic foraminiferal assem- blage in sample UB5 0. In addition, the presence of Nonionella specimens in sample UB50 may indicate normal marine salinity (Murray et al., 1989).

Reduced bottom water supply has also been recorded in the North Atlantic during the middle Oligocene (Miller and Katz, 1987). Dohmann (19 91) reco1:ded a similar low oxygenated, middle bathyal, foraminiferal assemblage consisting of Chilostomella, Bulimina and Praeglobobulimina spe- cies in lower Oligocene sediments from the Bavarian region of the Molasse Basin. Rogl (pers. comm.

19 9 3) has also recorded similar assemblages within thick dysaerobic shales from the Austrian Molasse Basin and western region of the Carpathian arc.

These sediments may be correlated with the Val d'Il- liez Formation at Bellevue, indicating that a low oxy- genated sea also existed in the Southwestern corner of the North Alpine foreland basin during the lower- middle Oligocene.

Low oxygen events occurred coincidentally with episodes of oceanic warming and sluggish deep-sea circulation (Kaiho, 1991 ). The presence of Globige- rina ciperoensis ciperoensis within the Bellevue sec- tion may indicate oceanic warming during deposition (Spezzaferri and Premoli Silva, 19 91 ).

The composition of the studied foraminiferal assemblages suggests that deposition was carried out by turbiditic currents on the continental slope con- tradictory to a shallow water deltaic environment as interpreted by Weidmann et al. (1982) and Car- letti (1987).

CONCLUSIONS

The Val d'Illiez Formation can be attributed to the lowermost part of the middle Oligocene, corres- ponding to the Globigerina ampliapertura fora- miniferal planktonic P2 0 zone based on the presence of Globigerina ciperoensis ciperoensis and Globigerina ouachitaensis gnauki (BOLLI and SAUNDERS, 1985).

The occurrence of reworking in the lower part of the thinly bedded shales interval is suggested by the abundance of poorly preserved late Cretaceous, late Paleocene, early Eocene and middle Eocene planktonic foraminifers. Resedimentation is only revealed by detailed micropaleontological analysis.

The Val d'llliez Formation was deposited on the continental slope by turbiditic currents with a change in the paleoecological conditions occurring upsequence. High foraminiferal species diversities, high planktonic to benthic ratios (> 80 % ), large, well preserved planktonic foraminifers and Uvige- rina specimens were recorded from the upper part of the lower shaly interval, suggesting that deposition

286 OLIGOCENE FORAMINIFERA FROM SAVOIE occurred in outer neritic to upper bathyal depths

(100-500 m) with normal oxygen and salinity levels.

A low foraminiferal species diversity, low planktonic to benthic foraminiferal ratio and a dominance of small, smooth-walled Bulimina and Chilostornella specimens recorded from a sample within the upper Val d'Illiez Sandstones interval suggest that depo- sition, at least in part, occurred in an oxygen minimum zone at middle bathyal dephts (500 -1 300 m). The presence of Globigerina ciperoen- sis ciperoensis may indicate oceanic warming and sluggish deep-sea circulation in the northwest alpine foreland basin during the middle Oligocene.

ACKNOWLEDGEMENTS

We would like to thank Professor Jean Charollais (University of Geneva) for his help in the field.

Special thanks are extended to Dr. Fred Rogl (Natural History Museum of Vienna), Dr. Michel Moullade (University of Nice) and Dr. David Haig (University of Western Australia) for useful criti- cisms of the manuscript. Sincere thanks to Rosanna Martini (University of Geneva) for producing SEM photos and Fred Viard (University of Geneva) for aid in photo development. This research has been kindly supported by the Swiss National Science Foundation, research project no. 21-37335-93.

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