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1、Jurassic lagoonal environments and quasi-abiotic platy limestone accumulation: microbial interventions NICOLAS TRIBOVILLARD*, JEAN TRICHET?, CHRISTIAN DE FARGE? and ALAIN TRENTESAUX* *Universite Lille 1, Laboratoire de Se dimentologie et Ge odynamique, UMR CNRS 8577, Ba timent SN5, F-59655 Villeneuv
2、e dAscq cedex, France (E-mail: Nicolas.Tribovillarduniv-lille1.fr; Alain.Trentesauxuniv-lille1.fr) ?Universite dOrle ans, UMR CNRS 6531, Laboratoire de Ge ologie de la Matie re Organique, F-45067 Orle ans cedex 2, France (E-mail: Jean.Trichetuniv-orleans.fr; Christian.Defargeuniv-orleans.fr) ABSTRAC
3、T Within the regressive sequence (eustatic sea-level fall) that characterizes the French Southern Jura carbonate platform during the Kimmeridgian, a succession of three facies (called here units 13) may be observed in a palaeolagoon near Orbagnoux. Unit 1 is a shallow-water, intensely burrowed, carb
4、onate deposit; unit 2 (consisting of laminated, organic matter-rich carbonate) represents a shallow lagoonal environment, prone to stromatolite development and organic matter accumulation; unit 3 consists of platy limestones almost devoid of identifi able biota. Trace element distributions indicate
5、that depositional and early diagenetic conditions must have been reducing during deposition of unit 1 and, especially, unit 2, whereas they were much less reducing during deposition of unit 3. However, the latter does not contain body or trace fossils, with the exception of structures interpreted as
6、 calcifi ed bacteria. The reasons why the apparently normal marine environment of unit 3 did not allow a normal biota to develop are explored and lead us to propose that: (i) unit 2 may be a Jurassic counterpart of modern kopara deposits (stromatolites of French Polynesian atolls); and (ii) unit 3 m
7、ay be the result of palaeokopara reworking. This model invokes the extensive intervention of microbes in the formation of these carbonate deposits. Keywords Bacteria, Jura, Kimmeridgian, kopara, platy limestone, stromatolites. INTRODUCTION During the Late Jurassic, the area that is now the French So
8、uthern Jura was a carbonate platform, which underwent a shallowing trend from pelagic to paralic deposition (Bernier, 1984). During this relative sea-level fall, a barrier reef developed and produced an isolated shallow lagoon. When circulation was most reduced, bituminous lam- inites formed in this
9、 lagoon. Such organic-rich sediments are encountered in the vicinity of the small hamlet of Orbagnoux (Fig. 1). This potential petroleum source rock has been studied exten- sively by several authors (Riche, 1904; Gubler Bernier Bernier, 1984; Courtinat, 1989; Gorin et al., 1989; Bernier et al., 1991
10、; Tribovillard et al., 1991, 1992, 1994; Mongenot et al., 1997; Tribovillard, 1998), who have concluded that the organic-rich facies was deposited as a result of: enhanced phytoplankton production in a very quiet depositional environ- ment; the presence of cyanobacterial activity, which developed bi
11、ofi lms at the sediment sur- face; and a pronounced scarcity of reactive iron that allowed the early vulcanization of organic matter. The aim of the present paper is to examine the sediments that lie immediately above and below the bituminous laminites. A comparison between the facies of these strat
12、a and that of the bituminous laminites will help to determine which factors allowed or precluded the deposi- tion of the organic-rich facies. Sedimentology (1999) 46, 11831197 ? 1999 International Association of Sedimentologists1183 GEOLOGICAL SETTING Themostrecentandcompletestratigraphic reconstruc
13、tionoftheFrenchSouthernJura Mountains was published by Bernier (1984). The very simplifi ed scheme that follows derives mainly from this work. During Early Kimme- ridgian times, the study area (Fig. 1) belonged to a pelagic environment in which ammonite-rich limestones formed, refl ecting open-marin
14、e condi- tions (Couches a Ce phalopodes Formation). A relative sea-level fall and platform progradation led to the deposition of neritic limestones (Calc- aires de Tabalcon Formation or unit 1 of this study; Fig. 2). These massive beds contain plank- tonic organisms (globigerinid foraminifera and ra
15、diolaria) and ammonites in the lower part of the formation, which are progressively replaced by gastropods, bivalves and sponges. Rare calcareous algae (Salpingoporella, Clypeina) were observed by Bernier (1984). During the H. beckeri ammonite zone of the Kimmeridgian, the development of a reef barr
16、ier isolated a shallow lagoon in which bituminous laminites were deposited under a water column that was most probably stratifi ed (the Laminites Bitumineuses Member of the Calcaires en Plaquettes Formation or unit 2; Fig. 2). The environment was sometimes con- fi ned: levels rich in gypsum pseudomo
17、rphs are observed. During the Late Kimmeridgian, accord- ing to Bernier (1984), the depositional environ- ment deepened temporarily (a result of possible local subsidence), which led to the deposition of platy limestones (the Calcaires en Plaquettes Formation sensu stricto or unit 3; Fig. 2). Alter-
18、 native interpretations are proposed below. This mudstone unit contains very few fossils: no coccoliths have been observed, and only rare benthonic foraminifera have been identifi ed. Only in the uppermost levels, ophiurids and bivalves (with the valves still articulated) occur together with evidenc
19、e of emersion (Enay Bernier, 1984). Bernier (1984) was puzzled by the absence of benthonic life and suggested that the episodes of mud deposition were too sudden to allow benthonic populations to develop. During Portlandian times, the area was covered with very shallow carbonate facies, representing
20、 tidal fl at Fig. 1. Geological setting of the study area. Redrawn from Bernier (1984). 1184N. Tribovillard et al. ? 1999 International Association of Sedimentologists, Sedimentology, 46, 11831197 environments (Couches de Chailley and Tidal- ites de Vouglans Formations). By the end of the Jurassic,
21、the area corresponded to palustrine and peritidal environments (Purbeckian facies; Stras- ser, 1988, 1991). This study focuses on stratigraphic units 1 and 3 that frame the bituminous laminites (unit 2) in the Orbagnoux palaeolagoon, and emphasis is laid upon unit 3. The results of this study are va
22、lid for the palaeolagoon of Orbagnoux, but cannot nec- essarily be extended to the entire Jura lagoon. PREVIOUS WORK ON THE LAMINITES BITUMINEUSES (UNIT 2) This Bituminous Laminites unit has been studied extensively (Riche, 1904; Bernier X ? 86691, Y ? 211565). Jurassic lagoonal environments1185 1 ?
23、 1999 International Association of Sedimentologists, Sedimentology, 46, 11831197 1979; Bernier, 1984; Courtinat, 1989; Bernier et al., 1991), particularly in the Orbagnoux pal- aeolagoon (Gubler Gorin et al., 1989; Tribovillard et al., 1991, 1992, 1994; Mon- genot et al., 1997; Tribovillard, 1998) a
24、nd is described in much detail elsewhere (Tribovillard et al., 1991, 1992; Mongenot et al., 1997; Tribovil- lard, 1998). A few aspects are reviewed here. Basically, the Orbagnoux bituminous laminites are made up of two components: carbonate and organic matter, whatever their respective nature and or
25、igin. The carbonate content fl uctuates between around 75% and 96%. The organic content is higher in the dark laminae than in the light ones total organic carbon, (TOC) ? 447 855% compared with 065803%. Values of the hydrogen index of the kerogens are always very high (755966 mg of hydrocarbons g)1T
26、OC; Esp- italie et al., 1985). Elemental analyses performed on the isolated kerogens indicate that organic sulphur is unusually abundant (12176% of the kerogens). Within the Laminites Bitumineuses, the fol- lowing two subfacies alternate. Parallel laminated beds These are made of alternating, submil
27、limetre- scale, lighter or darker coloured laminae. These parallel,continuous,individuallayersare clustered into millimetre- or centimetre-scale laminae, the depth of colour of which depends on the dominant component in individual layers. Various macro- and microfossils are present in theparallellam
28、inae,includingammonites, benthonicforaminiferaandostracods.Some ammonite mass mortality may be observed: very thin accumulations of juvenile forms of the genus Aspidoceras, which do not disturb the laminated structure. Scanning electron microscopy (SEM) indicates that the light-coloured laminae are
29、composedexclusivelyofcoccospheresand coccoliths. The dark-coloured laminae contain fewer coccoliths and coccospheres, embedded within a matrix of structureless, gel-like, organic matter. Acid-etched samples allow the distribu- tion of the thin organic seams, which stand out in relief, to be observed
30、. The organic laminae build an anastomosed mesh-like network. Theparallellaminatedfaciesseemsto derive mostly from settling mechanisms: peri- ods of fl ourishing coccolithophorids led to the accumulation of lighter coloured laminae, con- sisting exclusively of coccoliths. These periods alternated wi
31、th episodes of organic matter accu- mulation derived from planktonic algae (Tribovil- lard et al., 1992; Mongenot et al., 1997, 1999). The exceptional quality of lamina-scale correla- tion testifi es to a remarkably calm depositional environment. Undulating lamina beds Thisfaciescomprisesirregularly
32、undulated, alternatinglight-ordark-coloured,laminae, grossly parallel to bedding. The thickness of these laminae is the same as those in the parallel laminated facies. The light-coloured undulating laminae may exhibit birds eyes and fl at fenestrae. Thin-section observations show an alternation of d
33、ark laminae, consisting of bundles of very thin seams,andlight-coloured,thicker,carbonate laminae,occasionallycontainingabundant peloids. These peloids are interpreted as being of cyanobacterial origin (Tribovillard, 1998). In the undulating laminae, no coccoliths have been observed, but SEM observa
34、tions reveal that car- bonate is present as micrite grains, which are almost equidimensional and have grain sizes of 15 mm. The undulating lamina beds must have been less affected by or, more likely, unaffected by settling mechanisms. As concluded previously by Tribovillardet al.(1992),thebiolaminat
35、ions must have resulted from self-burial processes, i.e. mat-by-mat overgrowth related to ecological changes possibly affecting water salinity, light intensity and temperature in the absence of particle settling (Gerdes et al., 1991). Throughout the laminites, some rare, authi- genic quartz grains o
36、ccur. A few dedolomitized rhombs are also interspersed within the sedi- ments. No gypsum crystals are preserved, but numerous pseudomorphs (composed of calcite) occur in the various facies. The common presence of oxygen-demanding planktonic and benthonic organisms supports the conclusion that the wa
37、ter column must have remained (almost?) continu- ously oxic, whereas the underlying sediments were constantly characterized by anoxic pore- waters. The sharp chemical interface was caused by the presence of cyanobacterial biofi lms acting as a barrier between the two contrasting environ- ments. Thes
38、imultaneousoccurrenceofabundant organic matter and reducing conditions favoured intense sulphate reduction. Hence, large amounts of sulphide were released and trapped within the sediment below the cyanobacterial barrier. As iron was limited, sulphide reacted with organic 1186N. Tribovillard et al. ?
39、 1999 International Association of Sedimentologists, Sedimentology, 46, 11831197 molecules; this early vulcanization of organic matter is an effi cient agent of organic matter preservation(SinningheDamste et al.,1989; Tegelaar et al., 1989). SAMPLES AND METHODS The section studied is located alongsi
40、de the Dorche Torrent, in the small hamlet of Orb- agnoux, on the right bank of the Rho ne River (FrenchmapIGN,Seysselsheet,1:50 000; X ? 86691,Y ? 211565).Thesampleswere studied using light microscopy and scanning electron microscopy (SEM) of gold-coated mate- rials (Folk Fig. 3C and D). These unob
41、trusive laminae resemble individual cryptalgal laminae, as if better development of a (cyanobacterial) biofi lm had been hampered or prevented. No fossils, not even coccoliths, have been observed, whatever the scale of observation. However, SEM reveals abundant carbonate bodies of 1 mm length, which
42、 may be interpreted as calcifi ed bacteria (Fig. 4A and B). These seem to have been the only life form present, together with cysts of dinofl agellates (see below). Acid etching did not reveal additional features. Mineralogy Units 1 and 2 do not contain any mineral phases otherthancalcite(ironsulphi
43、de,claymineralsand terrigenous quartz grains are all absent). Unit 2 occasionally contains calcite pseudomorphs after gypsum. The platy limestones of unit 3 are pre- dominantlycarbonate(9196%),butcontainsmall quantities of clay minerals: illite, illitesmectite mixed layers and kaolinite. The diversi
44、ty of the clay mineral assemblage indicates a detrital rather thanadiageneticorigin.Thin-sectionobservations also demonstrate the presence of scattered, tiny pyrite specks. Geochemistry Table 1 summarizes the chemical composition of the samples studied. The data concerning unit 2 come from Mongenot
45、(1998). Jurassic lagoonal environments1187 1 ? 1999 International Association of Sedimentologists, Sedimentology, 46, 11831197 Major elements The facies of the three units consist of almost pure carbonate; consequently, aluminosilicate- associated elements are at low concentration. The Al, Fe, K and
46、 Ti contents, representative of the terrigenous fraction of the sediment, are highest (although generally very low) in the platy limestones, in agreement with the presence of clay minerals. Part of the silica has a biogenic origin, as evidenced by the presence of calcifi ed sponge spicules. Trace el
47、ements For convenience, unit 2 is described fi rst. Relative to unit 3, the bituminous laminites show consis- tently high ratios for Cu/Al, Mo/Al, Ni/Al, V/Al and Zn/Al and low values for Mn/Al. For com- parison, Table 1 also lists mean values for average carbonate sedimentary rocks (Turekian Mo/Al
48、reaches lower values, and Cu/Al together with Ni/Al reach higher values. Ba/Al falls within the range of values of the light-coloured parallel laminae but shows lower values, relative to the other facies of unit 2. Cd/Al exhibits higher values. Fig. 3. (A and B) Views of a typical sample from unit 1
49、 (sample 20), showing abundant, recrystallized shell fragments and spicules. Scale bar is 2 mm and 025 mm long for A and B respectively. (C and D) Views of a typical sample from unit 3 (sample 3). (C) Close-up of faint laminated structures, which appear to be failed attempts at stromatolite growth. Scale bar ? 2 mm. (D) View of the rock matrix showing abundant fi ne-grained land-derived organic debris (minute black dots). Scale bar ? 025 mm. 1188N. Tribovillard et al. ? 1999 International Association of Sedimentologists, Sed