The Holocene Marine Sequence in the Ltfkken Area of Vendsyssel , Denmark

Neue Befunde, die sich weitgehend auf einen Vetgleich von Miktofossil-Daten (Diatoma­ ceae, Foraminifera und Osttacoda) stützen, bestätigen JESSENS Ansicht. Marine Verhältnisse breite­ ten sich zwischen 8000 v. h. und 5300 v. h. von Süden het votdringend im L^kken-Gebiet aus, und es erfolgte kein Zustrom von stärker salzhaltigen Wässern aus dem Westen. Widetspruchsfteie Beweise für Seespiegelschwankungen wurden jedoch nicht gefunden.


Introduction
Holocene marine sediments exposed in the cliffs north of Ltfkken in northern Denmark (figs. 1 and 2) have been the subject of several previous investigations.The deposits at Kodak Rende, Ltfkkens Blänaese and Furreby Ä (fig.2) were originally dealt with in detail by JESSEN (1899,1918,1920,1931,1936).At L0kkens Blanaese he described a 6 m section of marine clay and shelly organic mud which contained mollusc assemblages comparable to the "fjord" faunas found at present in shallow, brackish water Danish embayments (NORDMANN 1905).JESSEN concluded that some form of land barrier must have existed to the west of Ltfkken, and that marine inundation had occurred from the Limfjord to the south, via the Store Vildmose (fig.1).
Subsequent workers in the area (CHRISTENSEN 1973;KNUDSEN 1971KNUDSEN , 1973;;MÖRNER 1969;VORK 1979) have in general supported JESSEN's view.Marine conditions penetrated up into the inner ramifications of a fjord system between 8,000 and 5,000 BP.The environment was essentially both shallow and brackish throughout this period, except where the presence of freshwater sand and peat suggested intermittant exclusion of the sea from certain localities.MÖRNER (1969) proposed that at least three marine trans gressions had occurred here.This interpretation was supported by CHRISTENSEN'S (1973) diatom work.In addition she found evidence for occasional marine inundation from the west.The other microfossils (Foraminifera andOstracoda: KNUDSEN 1971, 1973;PENNEY 1984;VORK 1979) however, do not support either of these suggestions, agreeing instead with JESSEN'S (1918JESSEN'S ( , 1920JESSEN'S ( , 1936) ) findings.Two conflicting opinions have therefore arisen.These may primarily have resulted from the different methodologies used, even on the same samples.A collation of the evidence was therefore felt necessary in order to obtain an overview of the palaeoenvironmental and Holocene sea-level history of the area.To this end much of the original material was reanalysed and is compared here with the results obtained from a new section at Ltfkkens Blänaese.This site is described in more detail elsewhere (PENNEY 1984) and only a short summary is included below.The material was processed following the procedure of MELDGAARD &KNUDSEN (1979) andPENNEY (1983).An examination of the Foraminifera and Ostracoda resulted in the erection of assemblage zones reflecting palaeoenvironmental changes in the depositional history of the site (fig.3).These will be discussed in more detail at a later date.

L0kkens Blänaese
Marine conditions were first established here above a freshwater sandy gyttja (Zone OA: fig. 3) at -1.68 m.The environment was initially very shallow (< 1 m) and brackish (3-10 ppm).A progressive increase in both water depth and salinity was apparent up through zones FB, FC, OC, OD and OE (fig.3) until a protected, shallow (< 5 m), mixopolyhaline (< 20-25 ppm) environment possessing Zostera marina on a hard bottom was established (Zones FD, FF & OF: fig.3).This correspond approxi mately with a shift from biogenic to minerogenic sedimentation at +0.15 m.Three radiocarbon dates have been obtained from a little above the base of the biogenic sediments (PENNEY 1984).Two of Quercus wood gave ages of 7,810 ± 115 BP (K-2451) and 7,950 ± 120 BP (K-3316).The third, a sample of shells (C.edule and M. edulis)

FORAMINIFERA
OSTRACODA °/oo :r DEPTH  from the same horizon, was considered too old (8,870 ± 125 BP: K-3317) as there was evidence for recrystallization (PENNEY 1984).The two wood dates cannot be used as sealevel index points owing to their allochthonous condition, but do suggest a minimum age for marine inundation at about 8,000 BP.
Slightly deeper (perhaps up to 7 m), more saline conditions (? > 25 ppm) were picked out in Zone FE (fig.3), corresponding to a sea-level high of about + 9-0 m.A shallower (< 3 m), mixomesohaline (< 20 ppm) environment returned above + 1.9 m and above + 2.6 m salinity may have fallen below 15 ppm and water depth dropped to around 1 m.Laminated sands containing shell and clay laminae were found above

Borehole Data
South of L0kken the marine sequence is buried by aeolian sand and there are no coastal exposures.Palaeoenvironmental data has however, been collected from several boreholes to the east and south (CHRISTENSEN 1973;KNUDSEN 1971, and unpubl.;VORK 1979).Figure 4 depicts the stratigraphy of these boreholes (locations shown on fig.2), together with a curve of marine influence which was based on a collation of the microfossil data.+ 3.0 m (fig.3).It is probable that much of this sand was irregularly supplied to the inlet by aeolian transport.The microfossil evidence (Zones FF 2 & OG: fig. 3) indicated an environment fluctuating between 0 and 3 m in depth and from 10 to 20 ppm.A 1-2 cm thick silty clay horizon at + 3.84 m contained a very distinctive assemblage of Foraminifera (Zone FG: fig.3), resembling the faunas which characterize the upper saltmarsh zone of intertidal estuaries (CULVER & BANNER 1978;SCOTT & MEDIOLI 1978;1980).In microtidal environments these faunas are found in isolated localities at water depths of less than 25 cm where salinity can range from 5 to 20 ppm (LUTZE 1968;JENSEN 1983).This clay horizon must therefore represent the final stage of marine inundation at L0kkens Blänaese, corresponding to a sea-level of about + 4.0 m.
There is no microfossil evidence to support the suggestion of any opening between the Skagerrak and the L0kken area via Ltfkkens Blänxsee.The assemblages resembled the faunas found at the present time in protected Danish lagoons and inlets throughout the marine succession, even when the watet depth attained its maximum in Zone FE (fig.3).Neither can any indication of a sea-level fall be extracted from the data.

Kodals Rende
MÖRNER (1969: 381) dated both the arrival (7,065 ± 135 BP: carbonate removed) and withdrawal (5,315 ± 100 BP) of marine conditions from this site (fig.2) and corre lated the sequence with his PTM-2, PTM-3 and PTM-4 shorelines for the Kattegat.He did not however, carry out a comprehensive examination of the deposits between the dated levels.CHRISTENSEN (1973) found oligohalobion diatom floras both a little above and below MÖRNER's dated horizons.She identified two periods of marine inundation in the intervening sediments, separated by a peak in the Fragillaria curve at + 5.4 m, which was considered evidence for a regression.In addition, the appearance of "North Sea" diatoms in the floras at + 5.05 m and + 6.25 m was interpreted as indicating the presence of an opening to the west on at least two occasions.These conspicuous floras were not complemented by reciprocal changes in the Foraminifera and Ostracoda.KNUDSEN (unpubl.)examined the Foraminifera from the same samples as CHRISTENSEN and found mixomesohaline/mixopolyhaline faunas throughout, with no evidence for either an opening to the west or a fall in sea-level.The former must therefore have been of such an ephemeral nature as to have caused no alteration in the sediments and mictofaunas.In addition, the possibility that "North Sea" diatoms were occasionally blown in cannot be overlooked.The Fragillaria peak may have resulted from increased freshwater runoff rather than from a fall in sea-level.A mixooligohaline environment was first recorded in B. 1 above -6.0m (fig.4).This was overlain by freshwater sand and clayey gyttja.The latter was originally described as a peat and interpreted as indicating a regression (KNUDSEN 1971: 133).This reversion to non-marine deposition could however, have resulted from the rapid buildup of a sand bar and subsequent shift of stream-course at the mouth of a river, and may not therefore, have required a fall in sea-level.
Marine conditions returned to B. 1 at -3.4 m and thereafter reached the remaining boreholes (fig.4).This must be the same event as was recorded at Ltfkkens Blänasse and dated to 8,000 BP.The sea had reached Kodals Rende by about 7,000 BP (MÖRNER 1969).The same progression of palaeoenvironments as was recorded at L0kkens Blänaese occurred in the boreholes and there was a corresponding succession of diatom floras in B.6 (CHRISTENSEN 1973).Eventually a typical "fjotd" environment was established over the whole area, inferring shallow waters (< 5 m), mixomesohaline-mixopolyhaline salinities (si.17-20 ppm) and a hard bottom of sandy mud on which grew a thick sward of Zostera marina.Although this area was evidently less isolated than the inlets to the west, no conclusive evidence could be found for any direct connection to the Skagerrak.That slightly deeper (5-10 m), more saline (> 25 ppm) conditions occasionally existed was apparent from the microfossil content of certain levels.This was noted at -1.0 to 0.0 m in B. 1, + 1.0 to + 2.0 in B.4, B. 6 and at Ltfkkens Blänaese and between +3.0 and + 4.0 in B. 3. The sea never exceeded about + 9.0 m.Shallower conditions returned in the upper pan of the marine sequence.Here the palaeoenvironmental picture was unfortunately blurred by a fluctuating input of wind blown sand.B.6 was completely smothered shonly aftet water depths attained their maximum (fig.4), whereas marine deposition continued up to at least + 4.2 m in B. 1 and B. 3.This resulted in a complicated discordancy of microfossil assemblages in each borehole -as it did Ltfkkens Blänaese.
A clay lamination at + 3.75m in B.6 contained a "North Sea" diatom assemblage and was interpreted by CHRISTENSEN (1973) as a storm surge deposit.This horizon was devoid of Foraminifera.A few normal marine microfossils (i.a. Ammonia beccarii batava and Callistocytbere littoralis) were observed at + 3.6 m in B. 1 and B. 2. Their presence might be interpreted as corresponding to the same event.These two species were however, noted at several levels in the boreholes, particularly where water depths peaked (fig.4), whilst other marine species (i.a. Cibicides lobatulus and Pontocytbere elongatd) which would have been introduced if any breach to the west had occurred, were absent.Their presence here cannot therefore, be considered incontrovertable evidence for an opening to the Skagerrak.Moreover, the aforementioned variability in sediment input in the upper part of the marine sequence may render this altitudional similarity coincidental.Radiocarbon and pollen dates have not been attempted on any of the borehole material.It is nonetheless apparent from the microfossil data (curve of marine influence on fig.4), that sedimentation rates were not consistant over the area under question.In addition, localized erosion and aerial discrepancies in postdepositional compaction could have occurred.The resultant palaeoenvironmental picture did not therefore, allow for direct cross-correlation between the boreholes on either an altitudional or zonal basis.(1969) constructed shoreline and shoreline displacement diagrams for Vendsyssel based largely on his own observations.Fitting this data into his model for Holocene sea-levels in the Kattegatt, he postulated that three transgressions (i.e. posi tive movements of sea-level) had occurred across the area (PTM-2, PTM-3, PTM-4).The data presented above does not support this interpretation.There are no clear trends in the marine intensity curves on fig. 4 which might imply a fluctuating sea-level.If these did occur, factors such as sediment supply, local accretion rates, erosion and consolida tion as well as the tolerance ranges of the individual species, all combined to obliterate the evidence.More importantly, evidence for minor fluctuations in sea-level was not forthcoming from the more isolated localities (eg.Kodals Rende and L^kkens Blänaese) where the microfossil faunas would have been more sensitive to minor changes in both water depth and salinity.There is therefore no incontrovertable evidence to support MÖRNER'S (1969) proposal for a fluctuating sea-level in the L0kken area.Both JESSEN'S (1899, 1936) and MÖRNER'S (1969) estimates for a maximum sea-level high of between + 9-0 m and + 9-5 m can however, be ratified.

Fig. 4 :
Fig. 4: Stratigraphy of the Holocene deposits in the Ltfkken area.The curves to the tight of the stratigraphic columns depict the intensity of marine influence (less intense to the left = shallower, less saline; more intense to the right = deeper, more saline) on the basis of the mictofossil data.