Review ofPalaeobotany and Palynology, 46 (1985): 33-53 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands DIATOM ANALYSIS OF AN ATLANTIC-SUBBOREAL CORE FROM SLIJPE (WESTERN BELGIAN COASTAL PLAIN) LUC DENYS University of Antwerp (RUCA), Department ofBotany, B-2020 Antwerp (Belgium) (Received April 17, 1984) ABSTRACT Denys, L., 1985. Diatom analysis of an Atlantic-Subboreal core from Slijpe (western Belgian coastal plain). Rev. Palaeobot. Palynol., 46: 33-43. Diatom analysis of a clayey AtlanticrSubboreal sequence from the western part of the Belgian coastal plain revealed the existence of intertidal conditions throughout, with the exception of a swampy lagoonal stage at the top of the basal peat. Several trans- gressive events were shown to have influenced the local palaeoenvironment. The possible relationship of these events to the Calais 11 and Calais 111 transgressions is discussed. INTRODUCTION Classically the Holocene sequence along the Belgian coast is subdivided into the Calais deposits of Atlantic--Subboreal age (overlying the Pleistocene surface directly or separated from them by a basal peat), the Subboreal- Subatlantic surface peat and the Subatlantic Dunkerque deposits'. While the soilmapping in the fifties led to a detailed examination of the youngest deposits, the underlying Calais member received relatively little attention. In contrast to the Dunkerque deposits wherein three transgressions were distinguished, the Calais sediments were generally considered to have been deposited in the course of one single transgressive event: the Flandrian trans- gression (Ameryckx, 1960; MarCchal et al., 1964; Tavernier and Ameryckx, 1970; Paepe, 1971; Ozer, 1976). Nevertheless, Halet (1931), Moormann (1951) and Tavernier and Moormann (1954) had already mentioned that this sea level rise was probably not continuous, but may well have been inter- rupted by standstills or even small regressions. In the opinion of Baeteman et al. (1974) and Baeteman (1978) the Belgian Calais member should be sub- divided into a series of consecutive transgressive and regressive intervals similar to those distinguished in neighbouring North Sea countries. Conse- quently a lithostratigraphical subdivision into four units was proposed, based on the occurence of peaty intercalations. Moreover, in most of the literature dealing with the Belgian coastal plain the main depositional environment of ' Although this tripartition has recently been critisised strongly (Baeteman, 1981a, b), we refer to it here for ease of comparison with the existing literature. 0034-6667/85/$03.30 @ 1985 Elsevier Science Publishers B.V. 34 the Calais deposits is considered to have been an intertidal sand flat. Hence the names "sables pissards" and Atlantic "wadden" deposits. Some recent publications, however, stress the importance of low energy flats, marshes and even lagoons in some parts of the area during certain periods (Baeteman, 1978, 1981a; see also Blanchard, 1906; Moormann, 1951). In order to obtain more precise information on the formation of the Calais deposits in the western part of the coastal plain and to trace possible sea-level oscillations, several cores from the area are being investigated for their diatom content. Since diatom associations are known to be very sensitive to changes in certain environmental factors such as salinity, tidal stress, hydrogen-ion concentration, trophic conditions, etc., diatom analysis, especially when combined with other techniques, may be one of the most valuable methods to determine alterations in the local sedimentary environ- ment of coastal deposits. This paper is concerned with the analysis of a sequence of uninterrupted fine-grained clastic sediments belonging to the Calais member from Slijpe in the western coastal plain. MATERIAL AND METHODS The sequence was obtained by means of an undisturbed mechanical coring. The boring, known as Spermalie 2, was located some 6 km inland from the present shore line and about 4 km from the Pleistocene outcrops (Fig.l). The entire Holocene sequence reaches a thickness of 4.32 m of which the Calais deposits, resting upon a basal peat and covered by the surface-peat, represent only 65 cm. Their lithology and chronology are displayed in Fig.2. Some granulometric data indicate an upward fining of the sediment (Table I). Estimations of the content of organic matter, sedimentary carbon- ates and biogenic opaline silica are given in Fig.3. Organic matter was deter- mined as the percentage weight loss of dried sediment after heating to 550°C in a muffle furnace, while carbonate CO2 was estimated as the weight loss hereafter by ignition at 950°C (Wetzel, 1970; Dean, 1974). By multiplying by 2.27 the CO2 content was converted into CaCO3. Biogenic opaline silica was digested by boiling with 0.5 N NaOH (Bradbury and Winter, 1976). The organic matter content decreased rapidly above the basal peat to a level of about 10%. From --373 cm on a gradual increase up to 19% just below the surface peat was observed. The carbonate content was highest just above the basal peat (+ 12%) and generally lower than 5% throughout the remaining part of the sequence. The biogenic silica content varied from 1 to 8% and showed no general agreement with the apparent corrosion of the diatom frustules. Investigation of digested sediment showed that nearly all diatom material was dissolved with exception of some highly resistant central nodules of Diploneis bombus. Sponge spicules appeared visibly unaffected, while sflicoflagellates were too scarce in the material to affect the percentages. Incomplete dissolution of diatom frustules using this method was also observed by Digerfeldt (1975). 35 N t • SLIJPE NIEUWP'OORT % (~)CORE "SPERMALIE 1.kfft Fig.1. Situation of the coring site. Area with outcropping Pleistocene coversands shaded. For diatom analysis the core was sampled in thin slices every three to four cm. Organic matter was removed following the method of Van der Werff (1955). Carbonate particles were dissolved using 1 N HC1. Only the coarser sand grains were removed by decanting carefully, the risk of altering the fossil assemblages being otherwise substantial (MSlder, 1943; Simonsen, 1962; Schrader, 1973). Permanent slides were made from the residue using the mounting medium Cumaron (nrD: 1.65) and 40 X 24 mm cover glasses. A Leitz Orthoplan microscope equipped with differential interference optics according to Nomarski was used for their examination. Following a qualitative survey, counts of at least 400 valves were made under high power magnifica- tion. Fragments were counted using the method described by Denys (1984). Percentage frequencies were calculated for the distinguished taxa (Table II, Fig.4). Similarity between assemblages from successive levels was calculated by means of Stander's index of similarity, SIMI (McIntire and Moore, 1977, p.358; Fig.4). Groups of taxa were made according to their ecological 36 CM T.A.W. CM BELOW SURFACE BROWN PEAT WITH PIECES OF WOOD (SURFACE PEAT) - 37 348- - 39 350-- ~ • • 4= - 49 360-- 4860 ~ 70 y. B.P, ! 3865 - 3385 y. B,C. BLUE-GREY CLAY WITH VIVIANITE SPOTS AND IRPA 518 (WOOD) - 59 370- SMALL PEATY PARTICLES AT THE TOP I I - 69 380-- / i - 79 390- i ...... 5650 -+ 75 y, B.P. DARK GREY CLAYEY SILT 4685 - 4405 y. B.C. I IRPA 519 (PEAT) BLUE-GREY CLAYEY SILT WITH VIVIANITE T..• SPOTS AND RHIZOMES PHRAaMITES -99 410-- ~' • ° - 102 413-- CLAYEY PHRAGMITES PEAT (BASAL PEAT) Fig.2, Lithology and '4C chronology of the Calais deposits. TABLE I Results of the granulometric analysis (% d.w. ) Grain size Depth (cm) (urn) --360 --381 --400 2000--55 1.1 0.4 1.6 55--40 1.7 10.2 14.7 40--20 7.2 32.4 19.9 20--10 1'2.9 13.7 12.2 10--5 13.4 8.3 14.7 5--2 14.7 7.3 11.8 2--0 42.1 27.8 25,1 preferences to obtain a number of ecological spectra. Hereby the percentages of taxa belonging to several groups at the same time were divided evenly among these groups. The halobion system of Van der Werff and Huls (1957-- 1974) was applied to reconstruct the palaeosalinity (Fig.4). A generalised salinity curve was constructed using the formula: S = 7PM + 6BM B + 5pB M + 37 I I .+,I I II// I o.+ 349 - J s z 357 - 365 - 373 381 393 - 400 - 408 - 412 -- 414 -- ----| ~'0 ,o' go 8'o ,00 , 3 5 Fig.3. Sediment composition and apparent relative diatom corrosion (1 = little, 5 = marked). 4pB + 3pBF + 2pFB + PF, wherein PM = the relative frequence of the group of marine taxa (M = marine; B = brackish; F = fresh-water), etc. Depending on the extent of the salinity range of the taxa, the categories euryhaline, + euryhaline and others were applied (Fig.4). Planktonic, epontic and benthic diatoms were distinguished, while Fragilaria and Paralia were considered separate (Fig.5). The ratios of assumed allochtonous to autochtonous components were calculated as in Beyens and Denys (1982): F-value = % allochtonous fresh-water component/% autochtonous salt-water compo- nent; M-value = % allochtonous salt-water component/% autochtonous fresh- water component (Fig.4). Following the classification of Simonsen (1962), pseudampotiphilous (intertidal), ampotixenous (subtidal) and tide-indifferent 38 TABLE II Percentage composition of the diatom assemblages. DEPTH ( ¢11 be]ou suH'sce )349 353 357 361 365 369 373 377 381 385 389 393 397 400 404 408 412 415 NIA~BER OF VALVES COUNTED 400 #BO 460 ~?0 470 ~40 ~20 4zO ~0 #10 470 410 411 ~10 480 4?9 ~59 640 AoA. bz~lP[,p~a ............ - 0.2 + 0.3 0.2 + 0.2 0.2 0.2" 0.5 0.2 + + + + 0.8 0.2 Aoh. b. Yam. ~n't;ea.~'~ .. - + Aoh. ~mZ4.o~Z4= .......... - - - 0.5 + + 0.2 0.4 0.2 0.6 Aoh. G~C~ VaZ,. ~uz,,amo. - - - - + Ao¢'t~,o~. oo~ ...... - + + - + + + 0.2 + + + + + + + A~'L='~mOp. ~=,.wzm4.1~a ........ + 0.9 2.0 0.7 0. u, 1.6 1.2 3.6 2.0 ~.2 I.