Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply

Abstract

-We studied a potential petroleum source rock deposited in a clastic-dominated ramp environment: the Argiles de Châtillon Formation (Kimmeridgian-Tithonian, Boulonnais area, northern France). The formation was deposited along a proximal-distal gradient on this ramp affected by synsedimentary fault movements. A sedimentological, mineralogical and geochemical study was conducted to decipher the distribution patterns of sedimentary parameters along such a depth increase over the ramp. It comes out that smectite distribution unexpectedly mimics the lateral depth evolution despite the good floatability of the mineral. It is also observed that the Argiles de Châtillon could accumulate noticeable amounts of organic matter in spite of paleoenvironmental conditions that were not specifically prone to organic matter preservation and burial (sedimentation rate, mineral particle grain size, productivity, oxygenation level...). Conversely, reactive iron, when being present in limited abundance, must have allowed sulfide ions to react with organic molecules instead of forming iron sulfides, which fostered organic matter preservation and accumulation. This protecting role of organic sulfide incorporation (additional to other favorable factors) cannot exist if reactive iron is relatively abundant. Lastly, our results (still fragmentary) suggest that smectite minerals could carry reactive iron, which would have occasionally hampered organic-matter preservation.Keywords: Kimmeridgian-Tithonian / Boulonnais / organic matter / petroleum source rocks / geochemistry / clay minerals Résumé -Importance des variations latérales discrètes dans les apports détritiques, observées dans des faciès de rampe sédimentaire riches en matière organique (Jurassique supérieur du Boulonnais). La formation des Argiles de Châtillon (Kimméridgien-Tithonien, Boulonnais) est un bon analogue géologique de roche mère pétrolière déposée en contexte de rampe clastique. Ces dépôts se sont mis en place le long d'un transect proximal-distal sur une rampe affectée par des mouvements tectoniques synsédimentaires. Pour mettre en évidence et comprendre la distribution des paramètres sédimentaires en fonction de l'évolution de la profondeur, une étude sédimentologique, minéralogique et géochimique fut entreprise. Il en ressort que la distribution de la smectite au sein du cortège argileux respecte ce gradient, malgré la flottabilité reconnue de ce minéral. Il apparaît également que les Argiles de Châtillon ont pu accumuler des quantités sensibles de matière organique malgré un contexte de dépôt peu favorable à la préservation et à l'enfouissement de celle-ci. En revanche, le fer réactif, quand il était présent en faible quantité dans les sédiments, a pu permettre aux ions sulfures de réagir avec les molécules organiques, plutôt que de former des sulfures de fer. Cette sulfuration a pu accroître la résistance de la matière organique à la dégradation microbienne, ce qui explique la bonne préservation du carbone organique. Ce rôle protecteur de la vulcanisation...

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin, Llanbedr (Mochras Farm) borehole, Wales

“…Kaolinite and, to a lesser extent, illite and chlorite, preferentially settle near shorelines, whereas smectite, given its smaller size and better buoyancy, is further exported to offshore environments (Chamley, ; Deconinck & Vanderaveroet, ; Gibbs, ; Hatem et al ., ). Given the opposite evolution of the proportions of smectite and kaolinite, it is necessary to address the question of the possible role of differential settling processes of clay minerals in the marine environment.…”

Climate-controlled organic matter accumulation as recorded in the Upper Jurassic Argiles de Châtillon Formation, a shallow-marine counterpart of the Kimmeridge Clay Formation

“…However, this interpretation must be rejected because compelling evidence from other proxies points to a different sea level scenario. Based on lithology and OM content (Proust et al 1993(Proust et al , 1995(Proust et al , 2001Herbin et al 1995;Tribovillard et al 2001Tribovillard et al , 2019Braaksma et al 2006;Hatem et al 2017), grain size distribution (Williams et al…”

“…Climate-controlled organic matter accumulation in the Argiles de Châtillon Formation 2001; Braaksma et al 2006;Hatem et al 2017), sedimentary structures such as HCS (Wignall & Newton, 2001;Hatem et al 2017;Tribovillard et al 2019) and on the presence of an erosional unconformity halfway up the ACF (Wignall & Newton, 2001), it is evident that the relative sea level scenario involves two transgressiveregressive cycles, including two maximum flooding surfaces (MFS) and one sequence boundary. The two MFS more or less correspond to the organic-rich levels of units II and V (Fig.…”

Late Jurassic – earliest Cretaceous prolonged shelf dysoxic–anoxic event and its possible causes

“…Black shale bands in this region, although bounded by TOC-depleted beds, are sometimes relatively thick (Geyssant et al 1993). According to Tribovillard et al (2001) and Hatem et al (2018), the TOC values in the black shales falls in the range 2-7% and sometimes 9%. Type II-III kerogen is represented mainly by amorphous OM interpreted as marine biomass degraded as a result of selective oxidation of metabolizable components, and partly by incorporation of reduced inorganic sulphur into lipids (Hatem et al 2018).…”