Pyritized Diatoms: a Good Fossil Marker in the Upper Paleocene-Lower Eocene Sediments from the Belgian and Dieppe-Hampshire Basins
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Netherlands Journal of Geosciences / Geologie en Mijnbouw 83 (3): 173-178 (2004) Pyritized diatoms: a good fossil marker in the Upper Paleocene-Lower Eocene sediments from the Belgian and Dieppe-Hampshire Basins Y. Van Eetvelde1, C. Dupuis2 & C. Cornet3 1 Departement de geologie, F.U.N.D.P., 61 rue de Bruxelles, B-5000 Namur, Belgium, [email protected] (corresponding author) 2 Geologie fondamentale et appliquee, Faculte Polytechnique de Mons, rue de Houdain, 9, B-7000 Mons, Belgium, [email protected] 3 Departement de geologie, F.U.N.D.P., 61 rue de Bruxelles, B-5000 Namur, Belgium, N [email protected] Manuscript received: February 2004; accepted: August 2004 Abstract Assemblages of brackish and marine diatoms have been examined from Upper Paleocene-Lower Eocene strata of the Belgian Basin (Knokke well) and the Dieppe-Hampshire Basin (Saint-Josse borehole and Ailly sections). The diatoms observed are invariably preserved in pyrite as internal moulds and their siliceous skeletons are completely replaced by pyrite by epigenesis. Three major diatom assemblages have been observed which can be used to approximate the position of the recently defined Paleocene-Eocene boundary (defined by the Carbon Isotope Excursion).This isotope excursion occurs just below the strong increase in the abundance of Fenestrella antiqua and in the vicinity of the abundance peak of Coscinodiscus morsianus var. moelleri. They also allow correlations of the lithostratigraphic units of the Belgian Basin with the formations of the Dieppe- Hampshire and central North Sea Basins. For instance, investigations of diatoms recorded in the Knokke Clay Member of the Knokke well indicate that this unit corresponds to the lower units of the 'Sparnacian fades' of the Dieppe-Hampshire Basin and to the Sele Formation of the North Sea Basin. Keywords: Belgian Basin, diatoms, Paleocene-Eocene interval, Dieppe-Hampshire Basin, pyrite Introduction and re-analysed by Moorkens et al. (2000) for its micro- and mega-fossil content in the Paleocene/Eocene Several studies have shown that pyritized diatoms are boundary interval and also for its volcanic particles, well preserved and of biostratigraphic interest in the clay-mineralogy and lithostratigraphy. Pyrite casts of Paleogene sediments from the central North Sea diatoms {Fenestrella antiqua, Coscinodiscus morsianus var. Basin (e.g., Bidgood et al., 1999; Jacque &Thouvenin, morsianus and Trinacria regina) have been observed in 1975; King, 1983; Malm et al., 1984; Mitlehner, 1994, the Tienen Formation and in the Kortrijk Formation. 1996; Mudge & Copestake, 1992) and the Belgian The Tienen Formation, a lateral equivalent of the Basin (King, 1983, 1990; Moorkens et al., 2000). In mainly non-marine 'Sparnacian' s.s. succession of the the Dieppe-Hampshire Basin, pyritized diatoms are Paris Basin, includes two main lithostratigraphic units known to be present in the Thanetian and Ypresian (Steurbaut, 1998).The upper unit, the Oosthoek Sand deposits (Bolin et al., 1982; Bignot, 1983) and their Member is a probably shallow marine 'sand' bank utility as key reference markers has been partially type deposit with some dinocysts (Apectodinium established (Van Eetvelde & Cornet, 2002; Van hypercanthum zone) and diatoms observed in palyno- Eetvelde & Dupuis, 2004a, 2004b). In the Belgian logical preparations (Moorkens et al., 2000). These Basin, the Knokke well has been previously analysed diatoms, being very poorly preserved, were not iden by King (1990) who studied the Eocene sediments tified. The lower unit, the Knokke Clay Member is a Netherlands Journal of Geosciences / Geologie en Mijnbouw 83(3) 2004 173 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.219, on 26 Sep 2021 at 21:40:45, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016774600020230 brackish clay, i.e. a typical 'Sparnacian fades', with new official Paleocene-Eocene boundary, defined by some twenty four thin intercalated shell beds (com the Carbon Isotope Excursion (CLE.) (Luterbacher prising five brackish water mollusc species and six et al., 2000). brackish ostracod taxa) (Moorkens et al., 2000). No diatom flora has been observed in the Knokke Clay Methods and materials Member. Previous studies (Van Eetvelde & Cornet, 2002; Van Eetvelde & Dupuis, 2004a, 2004b) put in Samples were taken in the Paleocene-Eocene transi evidence the presence of very well preserved pyritized tional strata of me Knokke well (Fig. 1). Twenty-one diatoms in 'Sparnacian' brackish clays intercalated samples were processed, three from the unnamed with shell beds from the Dieppe-Hampshire Basin, a Sand Unit, ten from the Knokke Clay Member, two lateral equivalent of the Knokke Clay Member of the from the Oosthoek Sand Member, three from the Knokke well. Knowing that sedimentary and environ Zoute Silt Member and three further samples from mental conditions prevailing during the deposit of the the Orchies Clay Member. Knokke Clay Member and the 'Sparnacian' brackish Separation of the pyritized diatoms from the sedi clays were the same, it is strange that no diatoms have ments (clays, silts, shell beds, sands and marls) was been observed in the Knokke Clay Member. Therefore carried out as described by Van Eetvelde & Cornet during this study, samples of theTienen Formation of (2002). Samples, once crushed were first treated with the Knokke well have been re-analysed for their diatom HC1, and secondly with tetrasodiumdiphosphate to content. disperse the argillaceous component. Residues, after In this study, we have examined the biostratigraphic a granulometric sorting by sieving, were placed into potential of pyritized diatom assemblages in the Late bromoform which permits a density separation. The Paleocene-Early Eocene strata from the Belgian Basin pyritized fraction, including the diatoms, settles down (Knokke well) (Fig. l).The assemblages recorded were and can be collected. Diatoms were mounted on slides also correlated with those observed in the central and counted under a microscope in transmitted and North Sea Basin and the Dieppe-Hampshire Basin reflected lights. Diatoms were also examined with (Saint-Josse borehole and Ailly sections) (Fig. l).The Scanning Electron Microscope (Philips XL-20) to other purpose of this work is the establishment of confirm their identification. correlation of pyritized diatom assemblages with the Fig. 1. Location map of Knokke, Ailly and Saint-Josse. Distribution of Paleogene facies in the southern part of the North Sea Basin (Modified after Dupuis et al., 1998). 174 Netherlands Journal of Geosciences / Geologie en Mijnbouw 83(3) 2004 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.219, on 26 Sep 2021 at 21:40:45, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016774600020230 Results and discussion Basin (Bidgood et al., 1999; King, 1983; Malm et al., 1984; Mitlehner, 1994, 1996; Mudge & Copestake, The diatoms recorded in the sediments from the 1992). Despite pyritization, diatoms recorded in the Belgian Basin are often preserved as internal moulds, Knokke sediments are recognizable; brackish and constituted by pyrite crystals, of the space between marine species were observed but no fresh water the two siliceous frustules which comprise the test of specimens were encountered. the original specimen. The diatoms could be also com Using our methodology especially developed to pletely pyritized by epigenesis; no unaltered siliceous extract pyritized diatoms (Van Eetvelde & Cornet, individuals were observed. By X-ray microanalyses of 2002), a diatom assemblage is recorded in the Knokke the pyritized valves, it is proved that the original silica Clay Member of the Knokke well, previously believed of the frustule of the diatoms is completely replaced to be barren of diatoms (Dupuis et al., 1990; Moorkens by pyrite and that the internal moulds are only con et al., 2000). The diatom flora is rich and includes stituted by pyrite crystals. These two particular preser Actinoptychus senarius (a few), Coscinodiscus morsianus vation styles of diatoms have already been observed in var. morsianus (common), Coscinodiscus morsianus var. the Dieppe-Hampshire Basin (Bignot, 1983; Bolin et moelleri (abundant), Odontella spp. (few), Hemiaulus al., 1982; Van Eetvelde & Cornet, 2002; Van Eetvelde elegans (few), Trinacria excavata (few) and Trinacria & Dupuis, 2004a, 2004b) and the central North Sea regina (few). Biostratigraphy North Sea basin Belgian and Paris basins Calcareous Planktonic Diatoms (this work) nannoplankton microfossils Mitlehner Martim(.971)King(.983) (1996) Diatom bioevents Mudge & Copestake NPll NSP5 pa (1992) Z M7 Bioevent w u NSP4b o NP10 pq M6 Bioevent NSP4 NP9 NSP4a u O old classical definition of the Paleocene-Eocene (P-E) boundary occurrence of Fenestrella antiqua m Carbon Isotope Excursion, C.I.E., new official P-E boundary _ - . _ _ occurrence of Coscinodiscus morsianus var. moelleri Lithostratigraphy Diatom assemblages j^ France Central Belgium North Sea reCt^T8 Saint-Josse Ailly Argile glauconieuse D3 du Phare d'Aillv Chateau de la Bruyere Argile brune a concretions i Fig. 2. Comparison of Argile et sablons D2 published latest Paleocene barren in and earliest Eocene North Sea diatoms Mont-Hulin Sables fauves 3 Argile glauconieuse biozonations with diatom D2 des Craquelins assemblages recorded in the Sables et Argiies