Sédimentologie de la matière organique et des argiles du Kimméridgien de Normandie (région du Havre)

Abstract

In the northwestern part of the Paris Basin, the Kimmeridgian sediments are exposed along coastal cliffs especially in Boulonnais and Normandy (Fig. 1). The Boulonnais outcrops were recently revisited (Geyssant et al., 1993; Herbin et al., 1995; Proust et al., 1993, 1995), although the base of the Kimmeridgian (Baylei, Cymodoce and Mutabilis zones) is missing. These ammonite zones are however, clearly exposed in Normandy, as revealed by the recent revision of the litho- and biostratigraphy of this region (Samson et al., 1996; Fig. 2). Here we present the organic geochemistry and clay mineralogy contributions to this in multidisciplinary study of the Kimmeridgian in Nornandy. Analytical methods include microfacies examination, calcimetry, X-ray diffraction, Rock eval and Leco analyses. Description of the formations and their main microfacies. Five formations are recognized as follows (Fig. 2 and table):1 The "Calcaires coquilliers" (1 to 2 m, Baylei Zone), consisting mainly of biodetrital limestones (wackestones to packstones) and gray silty marlstones.2 The "Argiles à Deltoideum delta" (4 to 6 m, poorly exposed and related to the Baylei Zone) consisting of intensely bioturbated grayish to brownish claystones.3 The "Bancs de Plomb" (1 to 2.5 m, Cymodoce Zone) comprising two thick micritic limestone beds, intensely burrowed, separated by a claystone interbed in which decimetre-long wood fragments are common. 4 The "Marnes de Bléville" (6 to 12 m, Cymodoce Zone) which is subdivided into three members. The lower member comprises alternations of micritic limestones and marlstones; a specific sandy bed is intercalated at the base of this member. The middle part corresponds to the so-called "Calcaires à Harpagodes)", an undulating limestone unit with abundant gastropods. The upper member is poorly exposed and corresponds to marlstones. The uppermost bed is very similar to the "Bancs de Plomb" and marks the boundary between lower and upper Kimmeridgian. 5 The "Argiles d'Octeville" (34 m, Mutabilis and Eudoxus ammonite zones) are subdivided into 5 newly created members (Samson et al., 1996). The "Argiles du Croquet inférieures" are mainly of sandy claystones and palegray claystones, separated from the gray claystones of the "Argiles du Croquet supérieures" by a nodular bivalve- rich bed. Both are related to the Mutabilis sub-zone. The "Argiles d'Ecqueville inférieures" (Lallierianum sub-zone) and the "Argiles d'Ecqueville médianes" (Orthocera sub-zone) consist of gray to black claystones; a Nanogyra-rich bed is located at the top of the latter member The "Argiles d'Ecqueville supérieures" (Caletanum and Contejeani sub-zones) are dominated by laminated black claystones. Geochemical results and interpretation Analysis of the organic matter reveals a low concentration of organic carbon (<0.5%) in the Lower Kimmeridgian deposits (Baylei and Cymodoce zones, Fig. 3), whereas the Total Organic Carbon (TOC) content varies between 1 and 7 % in the Upper Kimmeridgian sediments (Mutabilis and Eudoxus zones, Fig. 6). The hydrogen index (HI) values indicate a recycled terrigenous material in the lower Kimmeridgian marIs and limestones, whereas more-orless oxidized marine organic matter is dominant in the upper Kimmeridgian black shales (Fig. 4). The varieties of facies with sedimentation breaks and evidence of storms, as well as the faunal diversity of the lower Kimmeridgian deposits, indicates shallow-water marine environments with a good renewal of oxygen in the bottom waters. Such conditions are unsuitable to the preservation of organic matter, which explains the low organic content detected. A rapid deepening (probably tectonically contralled) is recorded in the upper Kimmeridgian deposits where claystones with pelagic fauna are predominant. Such deeper environments were more suitable to the preservation of marine organic matter because of the increase of primary productivity related to the transgressive phase and the low oxygen diffusion in the claystones. This is attested by the organic richness of the "Argiles d'Octeville" with its good petroleum potential. Three organic enrichments are recognized within the "Argiles d'Octeville" (Mutabilis and Eudoxus zones), two of them are correlated with "organic belts" defined in distal environments from Yorkshire and Dorset (Herbin and Geyssant, 1993; Herbin et al., 1995). An older "organic belt" can be recognized in the Mutabilis Zone from Normandy, as previously suggested by Baudin (1992). Clay mineralogy and interpretation. The clay minerai assemblages consist mainly of various proportions of illite, smectite and kaolinite occurring together with small amounts of chlorite and and random illite/smectite mixed loyers (Figs. 5 and 9). The clay assemblages depend neither on the lithology nor on the abundance or type of organic matter. A comparison between the clay mineralogy and sequence stratigraphy shows that major changes, corresponding to changes from illite- and kaolinite-rich sediments to smectite-rich sediments, coincide with downward shifts in sea-level. This is particularly well expressed in the "Marnes de bléville" (Cymodoce Zone, Fig. 5). This smectite-rich sedimentation within shallow environments is attributed to the recycling of pedogenic smectites produced on land areas exposed after a sea level drop. Comparison between Normandy and Boulonnais (Fig. 10). A similar trend in clay mineralogy was recognized at the Orthocera/Caletanum sub-zones boundary in the Boulonnais outcrops (Deconinck et al.,1983; Proust et al., 1993) and related to a tectonic event. Nevertheless, the coeval sediments from Normandy do not show such evolution of the clay assemblages. Either the tectonic event isdiachronous or two tectonic events (Cymodoce/mutabilis and Orthocera/Caletanum boundaries) affected the northwestern part of the of the Paris Basin; the Orthocera sub-zone event, not being recorded in Normandy. The late Kimmeridgian in Normandy provided suitable condition for the expression of the "organic belts" whereas the proximal environments from Boulonnais hindered the expression of the orgonic enrichment.Conclusions : A synthetic distribution of the clay minerals and organic matter of the northwestern part of the Paris Basin is now available for the Kimmeridgian using both Boulonnais and Normandy sections. A sharp change from illite- and kaolinite-rich sediments to smectite-rich sediments is shown in the "Marnes de Bléville" (Cymodoce Zone) and three "orgonic belts" are recognized in the "Argiles d'Octeville" (upper Kimmenidgian) from Normandy.