Non-marine mollusc biozonation of quaternary deposits in the Netherlands

T. Meijer
RGD - Rijks Geologische Dienst

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ABSTRACT Dutch Quaternary, starting with the Pretiglian first cold stage, may be subdivided by means of non-marine molluscs into four main (Oppel) zones. The upper boundary of Zone 1 coincides approximately with the top of the Olduvai Polarity subzone which also corresponds with the beginning of the Eburonian cold stage. Zone 2 ends approximately with the Jaramillo Polarity subzone, and the upper boundary of zone 3 is in the Saalian cold stage. Zone 4 covers the period after the Saalian. Additional studies are required to refine the Zonation.

The Netherlands are situated in the southeastern part of the North Sea basin. Until the regression, which was paIynologically dated in the Late Tiglian, marine sedimentation dominated in the lowermost Quaternary in most parts of the country. After the retreat of the sea, the basin was mainly filled with river and eaolian deposits. Most important rivers are the rivers Rhine, Meuse and Scheldt and some eastern rivers. Afterwards, fluviatile sedimentation was locally interrupted by the sea at least four times caused by eustatic rise of sea-Ievel related to interglacial stages: the last being our Holocene stage. The other interglacials are the so-called Cromerian-IV, the Holsteinian and the Eemian.

Two other important interruptions are of glacigenic nature and occurred in the Northern part of the country. They are attributed to glacial advances in certain parts of the Elsterian and Saalian glacial stages.

Biostratigraphical subdivision of Dutch Quaternary beds is mainly based on pollenanalysis. Moreover, there are contributions from marine molluscs. A vertebrate zonation does not exist in the Netherlands, research on this subject has just been started.

The technique of pollenanalysis revealed up till now about 17 stages of warm temperate climate after the first major cooling of the Pretiglian cold stage, which started about 2.3 million years ago.

This research comprises the revision of the most important old Dutch collections of Quaternary non-marine molluscs. Moreover, the faunas obtained from recently cored borings and collected in important outcrops discovered in the past few years were studied. Altogether fossil molluscs from about 500 localities have been studied. About 20 species appeared to be new to science. It was tried to assign every mollusc-datum an age in terms of the Dutch palynological biozonation.

The oldest non-marine fauna has been found near the village of Tegelen in a claypit where the stratotype of the Tiglian is situated. The Tiglian-Complex consists of 3 warm phases (pollenzones T-A, T-C3 and T-C5), alternating with some cooler phases, all falling within the Matuyama Magnetic Zone; the T-C5 interval coincides with the Olduvai Magnetic subzone. In Tegelen the oldest Tiglian-A is missing; only the two warm zones of the Tiglian-C are present.

The sediments deposited during the T-C3 interval contain in Tegelen an oxbow freshwater fauna. Among the species found , Valvata goldfussiana and Neumayria crassitesta are the more significant. The younger fauna of the Tiglian-C5 interval is an al most terrestrial one with many wood land species. Important occurrences are new, at present extinct species of the genera: Soosia, Helicigona, Monachoides and so on. In this very bed a vertebrate fauna occurs, which has been correlated with the fauna of the Hungarian site Villány-3. So this fauna belongs to the Mimomys Superzone of the Villanyian vertebrate stage.

Elements of both molluscan faunas, known as the "Tegelen-fauna", often occur together and have been found in a number of borings in the South of the country in Tiglian Rhine deposits. In total 26% of the "Tegelen-fauna" is extinct now, from which 18% became already extinct in the Eburonian cold stage following the Tiglian. An extinction on this scale does not take place any more in younger stages which makes this level the most well-documented boundary in Quaternary non-marine mollusc zonation. The Tiglian Interglacial Complex is therefore considered to be the first non-marine Quaternary Malacozone of the Netherlands. The remaining part of the Early Quaternary comprises the Waalian and Bavel Interglacial Complexes. The Bavel Interglacial has been discovered some years ago, and coincides at least partly with the Jaramillo Magnetic Subzone which has formerly erroneously thought to coincide with the upper part of the preceding Waalian interglacial.

Although some faunas of Waalian age are known, we are not weil informed about this period. Perhaps Tournouerina belnensis has its last appearance in the lower part of this interglacial.

Sediments bearing rich mollusc faunas of Bavel-interglacial age have been found well exposed in the stratotype of this interglacial. The faunas contain some 'endemics' not yet found elsewhere, among which are new species of the genera Pseudemmericia, Planorbarius and Segmentina. Remarkable is the westernmost fossil occurrence of Fagotia acicularis. Other important species are: Neumayria crassitesta, Tanousia runtoniana, Potomida littoralis, Corbicula fluminalis and Pisidium clessini. Last appearances are recorded from: the ancient form of Lithoglyphus naticoides and an undescribed species of Vitrina.

I will consider the Waalian and Bavel Interglacial Complexes to be the second non-marine Malacozone.

After the Bavel Interglacial Complex there is a large span of time from which almost nothing is known malacologically up till now. This period comprises at least 3 interglacials which are enumerated as the first three warm temperate stages in the so-called Cromerian Complex. The boundary between the Magnetic Zones Matuyama and Brunhes has been detected in the glacial following the Cromerian-I Interglacial stage.

The last interglacial of this complex, the Cromerian-IV, commonly correlated with the East Anglian Cromerian sensu stricto, bears scarcely known non-marine mollusc faunas. They are much alike the faunas present in the next-following interglacial stage: the Hoisteinian. This period is characterised by the last appearances of Viviparus diluvianus, Neumayria crassitesta and Pisidium clessini, which are all extinct at present, and the elsewhere still living species Valvata naticina. Moreover, this is the last interglacial in which presently extinct freshwater species do occur.I have provisionally included these two interglacials in the third non-marine Malacozone.

In the Saalian glacial stage pollenanalyses have revealed some relatively short warm temperate oscillations, taking place before the most extreme cold phase of this stage. The fauna of Belvédère, found together with a mammalian fauna and a palaeolithic chert tool industry, is attributed to one of these Intra-Saalian warm oscillations. A rich fauna with a total amount of 72 species has been found. Among them are the following species: Cochlicopa nitens, Spermodea lamellata, Discus rotundatus, Aegopinella nitidula, Zonitoides sepultus, Helicopsis striata, Bithynia troscheli and Corbicula fluminalis.

Well dated Eemian faunas are almost entirely restricted to the late-interglacial phase (according to pollenanalyses) and mostly occur together with marine molluscs. The genesis of these beds, deposited during the regression of the Eemian sea is still not entirely understood. Important terrestrial elements in these faunas are: Clausilia pumila, Discus ruderatus, Orcula dolium, last species being "endemic" for the Eemian fauna Perforatella bidentata, etc. All these species do not reappear in our country after this period. Neither do the freshwater species Bithynia troscheli and Corbicula fluminalis. Contrary to these, Theodoxus fluviatilis appears in the Eemian interglacial stage for the first time.

Most Holocene faunas are deposited in pure freshwater facies. The relative abundance of Bithynia tentaculata and Valvata piscinalis is striking. Apart from the characteristic Eemian species mentioned above, the resemblance of Holocene and Eemian faunas is very obvious. As a rule pure freshwater faunas can not be separated from each other.

The intra-Saalian warm oscillation of Belvédère, the Eemian and Holocene Interglacial stages together form the fourth non-marine Malacozone, which is marked by a practically complete absence of presently extinct species.

As one can see only interglacial faunas have been used to establish this zonation. Cold-resistant faunas have been found, too, in Late as well as Early Ouaternary glacial stages. Among them Pupilla- and Columella-faunas are most common. Although more is known about Late Quaternary cold faunas they are throughout the Ice Age the same. Next cold-resistant species found iq our country may be mentioned: Succinea elegans schumacheri, Columella columella, Vertigo genesi, Vertigo parcedentata, Pupilla loessica, Pupilla muscorum densegyrata, Vallonia tenuilabris, Gyraulus acronicus, Gyraulus rossmaessleri, Pisidium conventus, Pisidium lilljeborgi, Pisidium obtusale lapponicum and Pisidium stewarti.

Obvious developments through Quaternary time only take place at demanding elements like woodland and purely fluviatile species present merely in periods with a mild climate. Actually it is a regressive development: every cold stage took its toll of the most sensible part of the fauna. Placing the moments of extinction of these species in the more or less established pollen zonation is the first step to an independent Dutch non-marine molluscan zonation.

The zonation presented here is, as already stated before, mainly based on results of pre-existing investigations which were not primarily meant for this purpose. It is a preliminary framework suitable for further research.

ZONATION SCHEME:

ZONE 1 (pollenzonation: Tiglian Interglacial and first interstadials of the Eburonian Glacial stage; Vertebrate zonation: Mimomys-Superzone of the Villanyian Stage):

L.A.D.: Viviparus glacialis (WOOD), Soosia schlickumi sp. nov., Helicigona tigliensis sp. nov., Helicigona capeki (PETRBOK)?, Monachoides magnifica sp. nov. (all extinct). About 11 other extinct species have also their L.A.D. in this zone.

L.A.D., extinct: Valvata goldfussiana (WÜST),Tournouerina belnensis (DELAFOND and DEPÉRET), last species perhaps only occurring in the Waalian (and older), Lithoglyphus naticoides auct., Retinella (s. 1.) russeli sp. nov., Vitrina (s.l.) sp. nov.
L.A.D., not extinct: Fagotia acicularis (FÉRUSSAC), Perforatella bidentata (GMELIN) large farm, Potomida littoralis (CUVIER).

L.A.D., extinct: Viviparus diluvianus (KUNTH), Neumayria crassitesta (BRÖMME), Pisidium clessini NEUMAYR.
L.A.D., not extinct: Valvata naticina MENKE, Belgrandia marginata (MICHAUD), last species only in the Holsteinian.

L.A.D., extinct: Zonitoides sepultus LOZEK (only Intra-Saalian).
L.A.D., not extinct: Bithynia troscheli PAASCH, Corbicula fluminalis (MÜLLER), Orcula dolium (DRAPARNAUD), last species only occurring in the Eemian, Clausilia pumila PFEIFFER, Discus ruderatus (HARTMANN), Perforatella bidentata bidentata (GMELlN) little form.
L.A.D.: Theodoxus fluviatilis (LINNÉ), in the Eemian.

Complete lack of extinct species. Valvata piscinalis (MÜLLER), Bithynia tentaculata (LINNÉ) and Discus rotundatus (MÜLLER) are very abundant in this subzone.